In late August 2007, an Associated Press article put forth the claim that scientists were no more than 10 years away from creating artificial life -- and possibly as few as three. Could such a thing be possible? Scientists have made tremendous strides in decoding human and animal genomes, synthesizing DNA and cloning. Creating artificial, functioning biological organisms seems to present a tremendous leap beyond any of these abilities. But some of the companies and researchers involved in the quest for artificial life believe that ­the 10-year time frame is possible. Not only that -- they say that the development of wet artificial life (as it's often called) will radically affect our views of biological life and our place in the universe. ­

Artificial life could exist as soon as 10 years from now, according to an Associated Press article published in 2007. Learn about artificial life.

­The claims about the impending invention of artificial life may be a tad bold. Among the skeptics is Francis Collins, head of the Human Genome Project, says the 10-year time frame is too ambitious. Even so, the prospe­ct of artificial life holds a lot of ap­peal, and we'll take a look at it in this article.
Wet artificial life is not a modified or genetically engineered organism. It's life created entirely from basic parts. But as we saw in our article about weird life, scientists do­n't have a rigorous, standardized definition of what life is. Even so, biologists have some basic ideas about which qualities artificial life needs to possess in order to be considered alive.


­First, artificial life needs to have DNA or genetic code. It also needs to be able to reproduce and to pass on its genetic code. The life form then needs somewhere to place its genetic code, a protective casing or membrane, similar to a cell wall, that keeps the DNA and other parts together. The cell wall should also allow for normal biological processes to be carried out. In other words, it should be permeable enough to allow for the absorption of nutrients and relatively impermeable against pathogens. Once its basic parts are together, the organism should be self-sustaining: It should eat and metabolize food. Finally, the life form needs the ability to repair itself and to adapt and evolve.
Developing some of these characteristics presents many challenges to researchers. But one Harvard scientist predicted (in that same AP article) that by early 2007, great advances would be made in creating cell membranes [source: Associated Press]. Keeping an artificial organism alive for more than a few minutes or a few hours is also a challenge, though scientists can focus on strengthening the organisms after some of the initial hurdles are overcome.
To create DNA, some scientists advocate placing nucleotides (the building blocks of DNA) inside the cell casing. The nucleotides could somehow be combined to form DNA. That in itself might pose a challenge, as enzymes may be required to assemble the nucleotides, which might violate the "basic parts" rule for creating artificial life.
On the next page, we'll take a look at more challenges that stand between scientists and artificial life. We'll also consider this question: Will artificial life forms get out of control?

Artificial Life Concerns and Challenges

© Photographer: Jan Kaliciak Agency: Dreamstime.com Some scientists believe that using enzymes to combine nucleotides into DNA wouldn't result in truly artificial life.

Science fiction books and movies are filled with examples of out-of-control machines, viruses, artificial organisms and artificial intelligences. These fictions represent the worst possible outcome, some people say, in "playing God." Some scientists offer the reassurance that by the time artificial organisms are actually created, more mechanisms would be in place to control them.
It's also important to remember that the wide range of diverse and complex organisms on Earth represents the product of almost four billion years of evolution. Even if the 10-year time frame is correct, scientists in 2017 won't be working with artificially created toxic plants, predatory animals or unstoppable viruses. Early synthetic life forms will be rather simple organisms of a few cells or less. In fact, more danger likely lies in the abuse of genetic engineering techniques to modify existing viruses to make them highly contagious or virulent.
To those who say that scientists don't have the right to "play God," advocates often say that creating artificial life is a natural extension of humanity's desire for progress and discovery. Research into artificial life may yield insights into some of biology's fundamental processes, though again, science fiction depictions of artificial creations run amok has likely not helped the case for artificial life.
Since there is some dispute about what defines both life and artificial life, we may see several premature claims of success from biologists. What would qualify as a success? Does it have to be a functional, complex, self-replicating organism, or would a simple bit of artificially created, self-replicating genetic code suffice? How basic must the ingredients be that are combined to create the organism? Francis Collins says that scientists would be "cheating" by using enzymes, which are themselves derived from life forms [source: PBS].
In what may represent an important first step, some scientists have already produced artificial viruses, but they did so by synthetically reproducing DNA of known viruses. They then injected this DNA into cells that weren't synthetically formed.
Once an artificial organism is created, how (and for how long) will it live? Collins believes that artificial life should be able to survive in a basic environment, perhaps in a simple sugar solution, without humans providing complicated chemicals [source: PBS]. Others might say that, at least at first, making some sort of microbe or organism that can survive briefly qualifies as a success -- even if it requires a lot of outside control or monitoring.
At the very least, some of the initial claims regarding artificial life will face significant scrutiny. In the coming years, expect an ongoing debate about what defines life, both "real" and artificial.
For more information about artificial life and other related topics, please check out the links on the next page.

WATCH VIDEOS

­­

life

Are we 10 years away from artificial life?

In late August 2007, an Associated Press article put forth the claim that scientists were no more than 10 years away from creating artificial life -- and possibly as few as three. Could such a thing be possible? Scientists have made tremendous strides in decoding human and animal genomes, synthesizing DNA and cloning. Creating artificial, functioning biological organisms seems to present a tremendous leap beyond any of these abilities. But some of the companies and researchers involved in the quest for artificial life believe that ­the 10-year time frame is possible. Not only that -- they say that the development of wet artificial life (as it's often called) will radically affect our views of biological life and our place in the universe. ­

Artificial life could exist as soon as 10 years from now, according to an Associated Press article published in 2007. Learn about artificial life.

­The claims about the impending invention of artificial life may be a tad bold. Among the skeptics is Francis Collins, head of the Human Genome Project, says the 10-year time frame is too ambitious. Even so, the prospe­ct of artificial life holds a lot of ap­peal, and we'll take a look at it in this article.
Wet artificial life is not a modified or genetically engineered organism. It's life created entirely from basic parts. But as we saw in our article about weird life, scientists do­n't have a rigorous, standardized definition of what life is. Even so, biologists have some basic ideas about which qualities artificial life needs to possess in order to be considered alive.


­First, artificial life needs to have DNA or genetic code. It also needs to be able to reproduce and to pass on its genetic code. The life form then needs somewhere to place its genetic code, a protective casing or membrane, similar to a cell wall, that keeps the DNA and other parts together. The cell wall should also allow for normal biological processes to be carried out. In other words, it should be permeable enough to allow for the absorption of nutrients and relatively impermeable against pathogens. Once its basic parts are together, the organism should be self-sustaining: It should eat and metabolize food. Finally, the life form needs the ability to repair itself and to adapt and evolve.
Developing some of these characteristics presents many challenges to researchers. But one Harvard scientist predicted (in that same AP article) that by early 2007, great advances would be made in creating cell membranes [source: Associated Press]. Keeping an artificial organism alive for more than a few minutes or a few hours is also a challenge, though scientists can focus on strengthening the organisms after some of the initial hurdles are overcome.
To create DNA, some scientists advocate placing nucleotides (the building blocks of DNA) inside the cell casing. The nucleotides could somehow be combined to form DNA. That in itself might pose a challenge, as enzymes may be required to assemble the nucleotides, which might violate the "basic parts" rule for creating artificial life.
On the next page, we'll take a look at more challenges that stand between scientists and artificial life. We'll also consider this question: Will artificial life forms get out of control?

Artificial Life Concerns and Challenges

© Photographer: Jan Kaliciak Agency: Dreamstime.com Some scientists believe that using enzymes to combine nucleotides into DNA wouldn't result in truly artificial life.

Science fiction books and movies are filled with examples of out-of-control machines, viruses, artificial organisms and artificial intelligences. These fictions represent the worst possible outcome, some people say, in "playing God." Some scientists offer the reassurance that by the time artificial organisms are actually created, more mechanisms would be in place to control them.
It's also important to remember that the wide range of diverse and complex organisms on Earth represents the product of almost four billion years of evolution. Even if the 10-year time frame is correct, scientists in 2017 won't be working with artificially created toxic plants, predatory animals or unstoppable viruses. Early synthetic life forms will be rather simple organisms of a few cells or less. In fact, more danger likely lies in the abuse of genetic engineering techniques to modify existing viruses to make them highly contagious or virulent.
To those who say that scientists don't have the right to "play God," advocates often say that creating artificial life is a natural extension of humanity's desire for progress and discovery. Research into artificial life may yield insights into some of biology's fundamental processes, though again, science fiction depictions of artificial creations run amok has likely not helped the case for artificial life.
Since there is some dispute about what defines both life and artificial life, we may see several premature claims of success from biologists. What would qualify as a success? Does it have to be a functional, complex, self-replicating organism, or would a simple bit of artificially created, self-replicating genetic code suffice? How basic must the ingredients be that are combined to create the organism? Francis Collins says that scientists would be "cheating" by using enzymes, which are themselves derived from life forms [source: PBS].
In what may represent an important first step, some scientists have already produced artificial viruses, but they did so by synthetically reproducing DNA of known viruses. They then injected this DNA into cells that weren't synthetically formed.
Once an artificial organism is created, how (and for how long) will it live? Collins believes that artificial life should be able to survive in a basic environment, perhaps in a simple sugar solution, without humans providing complicated chemicals [source: PBS]. Others might say that, at least at first, making some sort of microbe or organism that can survive briefly qualifies as a success -- even if it requires a lot of outside control or monitoring.
At the very least, some of the initial claims regarding artificial life will face significant scrutiny. In the coming years, expect an ongoing debate about what defines life, both "real" and artificial.
For more information about artificial life and other related topics, please check out the links on the next page.

WATCH VIDEOS

­­

Ever since humans acknowledged the enormity of the universe, we have intuited that life must exist somewhere, either in our galaxy or some galaxy far, far away. If the­ universe contains billions of galaxies, and if each galaxy contains billions of stars, and if a fraction of those stars have Earth-like planets, then hundreds -- maybe even thousands -- of alien civilizations must exist across the cosmos. Right?

Milky Way Image Gallery

Photo © ESO
In June 2008, European astronomers discovered three super Earths orbiting what they thought was a solo star. The discovery was good news for the possibility of life elsewhere in the universe. See more Milky Way pictures.

For a while, science contented itself with the logic alone. Then, in 1995, astronomers located the first planets outside our solar system. Since then, they've detected nearly 300 of these extra-solar planets. Although most are large, hot planets similar to Jupiter (which is why they're easier to find), smaller, Earth-like planets are beginning to reveal themselves. In June 2008, European astronomers found three planets, all a little larger than Earth, orbiting a star 42 light-years away [source: Vastag].


­These discoveries have served as an affirmation for those involved with the search for extraterrestrial intelligent life, or SETI. Harvard physicist and SETI leader Paul Horowitz boldly stated in a 1996 interview with TIME Magazine, "Intelligent life in the universe? Guaranteed. Intelligent life in our galaxy? So overwhelmingly likely that I'd give you almost any odds you'd like."
And yet his enthusiasm must be tempered by what scientists call the Fermi Paradox. This paradox, first articulated by nuclear physicist Enrico Fermi in 1950, asks the following questions: If extraterrestrials are so common, why haven't they visited? Why haven't they communicated with us? Or, finally, why haven't they left behind some residue of their existence, such as heat or light or some other electromagnetic offal?
Perhaps extraterrestrial life isn't so common after all. Or perhaps extraterrestrial life that gives rise to advanced civilizations isn't so common. If only astronomers could quantify those odds. If only they had a formula that accounted for all of the right variables related to extraterrestrial life. As it turns out, they do. In 1961, as a way to help convene the first serious conference on SETI, radio astronomer Frank Drake presented a formula, now known as the Drake Equation, that estimates the number of potential intelligent civilizations in our galaxy. The formula has generated much controversy, mainly because it leads to widely variable results. And yet it remains our one best way to quantify just how many extraterrestrials are out there trying to communicate.
Let's take a closer look at the equation and its implications.

 

Are We Alone? The Drake Equation

Trying to calculate the probability that extraterrestrial life exists in the universe is actually quite complicated. The universe isn't a static environment. Stars are born, they live and they die. Some stars form in association with planets. Others don't. Only some of those planets have the right conditions to support life.
Life is a tricky variable in its own right. Some planets might support complex organic molecules -- proteins and nucleic acids -- and nothing else. Other planets might support simple, single-celled organisms. And still others might support multicellular organisms, including those advanced enough to develop the technologies to travel or send signals into outer space. Finally, even organisms that have adapted extremely well to their environments don't last forever. As both the dinosaurs and the Roman Empire illustrate here on Earth, all dynasties come to an end, be it cataclysmic or otherwise.

Getty Images
Ellie Arroway, played by Jodie Foster in the movie "Contact," was consumed by the thought of life on other planets.

Fra­nk D­rake had to account for all of these variables in developing a formula to quantify the odds of finding extraterrestrial life. His first task was deciding what he wanted to calculate. First, he limited his thinking to extraterrestrials in our home galaxy -- and only those that might be capable of interstellar communication. Then he inserted a mathematical factor to account for all of the conditions required to enable such civilizations to evolve. The result is the following formula:

N = Rf_pn_ef_lf_if_cL

In this equation, N is the number of detectable civilizations in our galaxy. The other variables are described below:

• R is the rate of star formation in the galaxy
• f_p is the fraction of stars that form planets
• n_e is the number of planets hospitable to life (i.e., Earth-like planets)
• f_l is the fraction of these planets on which life actually emerges
• f_i is the fraction of these planets on which intelligent life arises
• f_c is the fraction of these planets with intelligent beings capable of interstellar communication
• L is the length of time such a civilization remains detectable

The only variable known with any degree of certainty is the rate of stellar formation, R. In the Milky Way, a typical spiral galaxy, new stars form at a rate of roughly four per year [source: Cain]. The variable astronomers feel most uncertain about is L, the length of time a civilization remains detectable. A variety of estimates have been used for L, ranging from 10 years to 10 million years.
Astronomers can make educated guesses about the rest of the variables. For example, of the nine planets in our solar system, only four are what astronomers call terrestrial planets -- those that have a solid surface. Of those terrestrial planets, only Earth supports life. If we take our solar system as representative, then we might argue that n_e equals 1/4 or 0.25. Similar guesses have been made about the other variables and, interestingly, they all end up having very similar values, usually in a range between 0.1 and 1.0. So, a typical calculation might look like this:

N = 4 x 0.5 x 0.25 x 0.2 x 0.2 x 0.2 x 3,000,000

which gives us a value of 12,000 civilizations in our galaxy.
Drake's original calculations were very close to this value for N. When he ran the numbers, he predicted that there might be 10,000 detectable civilizations in the Milky Way [source: Garber]. Carl Sagan, a leader in the SETI movement until he passed away in 1996, was even more generous when he suggested that 1 million civilizations might exist in the galaxy [source: Lemarchand]. That's a lot of ETs!
No wonder astronomers were so optimistic when they started searching diligently for extraterrestrial life in the 1960s. On the next page, we'll look at how they've conducted this search and what it has turned up.

 

Testing and Revising the Drake Equation

Armed with an estimate of the number of communicative civilizations in our galaxy, SETI scientists set out to find them. They had two basic options: face-to-face communication or long-distance communication. The former scenario required that extraterrestrials visit humans or vice versa. This seemed highly unlikely given the distances between our solar system and other stars in the Milky Way. The latter scenario involved radio broadcasts, either sending or receiving electromagnetic signals through space.

Photo courtesy of the NAIC - Arecibo Observatory, a facility of the NSF
Aerial view of the Arecibo Observatory in Puerto Rico

In 1974, astronomers intentionally transmitted a 210-byte message from the Arecibo Observatory in Puerto Rico in the hopes of signaling a civilization in the globular star cluster M13. The message contained fundamental information about humans and our corner of the universe, such as the atomic numbers of key elements and the chemical structure of DNA. But this sort of active communication has been rare. Astronomers mostly rely on passive communication -- listening for transmissions sent by alien civilizations.
A radio telescope is the tool of choice for such listening experiments because it's designed to detect longer-wavelength energy that optical telescopes can't see. In radio astronomy, a giant dish is pointed to a nearby, sunlike star and tuned to the microwave region of the electromagnetic spectrum. The microwave frequency band, between 1,000 megahertz and 3,000 megahertz (MHz), is ideal because it's less contaminated with unwanted noise. It also contains an emission line -- 1,420 MHz -- that astronomers can hear as a persistent hiss across the galaxy. This narrow line corresponds to energy transformations taking place in neutral hydrogen. As a primordial element of the universe, hydrogen should be known to all intergalactic civilizations, making it an ideal marker. Several teams from around the world have been systematically listening to stars across the Milky Way and adjacent galaxies since 1960.
Despite their collective efforts, no SETI search has received a confirmed, extraterrestrial signal. Our telescopes have picked up a few unexplained and intriguing signals, such as the so-called "Wow" signal detected by researchers at Ohio State University in 1977, but no transmission has been repeated in such a way that it provides indisputable evidence of extraterrestrial life. All of which brings us back to the Fermi Paradox: If thousands of civilizations in the Milky Way galaxy, why haven't we detected them?
Since Drake and Sagan made their estimates, astronomers have become more conservative. Paul Horowitz, who boldly guaranteed the existence of extraterrestrial life, has generated more modest results from the Drake Equation, finding that N may be closer to 1,000 civilizations [source: Crawford]. But even that figure may be too large.
In 2002, Skeptic magazine publisher Michael Shermer argued that astronomers weren't being critical enough in their evaluation of L, the length of time a civilization remains detectable. Looking at 60 civilizations that have existed on Earth since the dawn of humanity, Shermer came up with a value for L that ranged from 304.5 years to 420.6 years. If you plug these numbers into the Drake Equation, you find that N equals 2.44 and 3.36, respectively. Tweak the numbers some more, and you can easily get N to fall to one or even lower. Suddenly, the odds of hearing from an extraterrestrial life form are considerably lower.
Even the most enthusiastic SETI supporters are troubled by the lack of results produced by more than 40 years of "listening" to the cosmic airwaves. And yet most of that search has been confined to our home galaxy. Even if there are only three or four civilizations per galaxy, there are billions and billions of galaxies. This tilts the odds again in favor of finding extraterrestrial life, which is why many SETI astronomers take the same approach to their work as lottery players: You can't win if you don't play.

aliens· extra terrestrial life

What are the odds there is life in outer space?

Ever since humans acknowledged the enormity of the universe, we have intuited that life must exist somewhere, either in our galaxy or some galaxy far, far away. If the­ universe contains billions of galaxies, and if each galaxy contains billions of stars, and if a fraction of those stars have Earth-like planets, then hundreds -- maybe even thousands -- of alien civilizations must exist across the cosmos. Right?

Milky Way Image Gallery

Photo © ESO
In June 2008, European astronomers discovered three super Earths orbiting what they thought was a solo star. The discovery was good news for the possibility of life elsewhere in the universe. See more Milky Way pictures.

For a while, science contented itself with the logic alone. Then, in 1995, astronomers located the first planets outside our solar system. Since then, they've detected nearly 300 of these extra-solar planets. Although most are large, hot planets similar to Jupiter (which is why they're easier to find), smaller, Earth-like planets are beginning to reveal themselves. In June 2008, European astronomers found three planets, all a little larger than Earth, orbiting a star 42 light-years away [source: Vastag].


­These discoveries have served as an affirmation for those involved with the search for extraterrestrial intelligent life, or SETI. Harvard physicist and SETI leader Paul Horowitz boldly stated in a 1996 interview with TIME Magazine, "Intelligent life in the universe? Guaranteed. Intelligent life in our galaxy? So overwhelmingly likely that I'd give you almost any odds you'd like."
And yet his enthusiasm must be tempered by what scientists call the Fermi Paradox. This paradox, first articulated by nuclear physicist Enrico Fermi in 1950, asks the following questions: If extraterrestrials are so common, why haven't they visited? Why haven't they communicated with us? Or, finally, why haven't they left behind some residue of their existence, such as heat or light or some other electromagnetic offal?
Perhaps extraterrestrial life isn't so common after all. Or perhaps extraterrestrial life that gives rise to advanced civilizations isn't so common. If only astronomers could quantify those odds. If only they had a formula that accounted for all of the right variables related to extraterrestrial life. As it turns out, they do. In 1961, as a way to help convene the first serious conference on SETI, radio astronomer Frank Drake presented a formula, now known as the Drake Equation, that estimates the number of potential intelligent civilizations in our galaxy. The formula has generated much controversy, mainly because it leads to widely variable results. And yet it remains our one best way to quantify just how many extraterrestrials are out there trying to communicate.
Let's take a closer look at the equation and its implications.

 

Are We Alone? The Drake Equation

Trying to calculate the probability that extraterrestrial life exists in the universe is actually quite complicated. The universe isn't a static environment. Stars are born, they live and they die. Some stars form in association with planets. Others don't. Only some of those planets have the right conditions to support life.
Life is a tricky variable in its own right. Some planets might support complex organic molecules -- proteins and nucleic acids -- and nothing else. Other planets might support simple, single-celled organisms. And still others might support multicellular organisms, including those advanced enough to develop the technologies to travel or send signals into outer space. Finally, even organisms that have adapted extremely well to their environments don't last forever. As both the dinosaurs and the Roman Empire illustrate here on Earth, all dynasties come to an end, be it cataclysmic or otherwise.

Getty Images
Ellie Arroway, played by Jodie Foster in the movie "Contact," was consumed by the thought of life on other planets.

Fra­nk D­rake had to account for all of these variables in developing a formula to quantify the odds of finding extraterrestrial life. His first task was deciding what he wanted to calculate. First, he limited his thinking to extraterrestrials in our home galaxy -- and only those that might be capable of interstellar communication. Then he inserted a mathematical factor to account for all of the conditions required to enable such civilizations to evolve. The result is the following formula:

N = Rf_pn_ef_lf_if_cL

In this equation, N is the number of detectable civilizations in our galaxy. The other variables are described below:

• R is the rate of star formation in the galaxy
• f_p is the fraction of stars that form planets
• n_e is the number of planets hospitable to life (i.e., Earth-like planets)
• f_l is the fraction of these planets on which life actually emerges
• f_i is the fraction of these planets on which intelligent life arises
• f_c is the fraction of these planets with intelligent beings capable of interstellar communication
• L is the length of time such a civilization remains detectable

The only variable known with any degree of certainty is the rate of stellar formation, R. In the Milky Way, a typical spiral galaxy, new stars form at a rate of roughly four per year [source: Cain]. The variable astronomers feel most uncertain about is L, the length of time a civilization remains detectable. A variety of estimates have been used for L, ranging from 10 years to 10 million years.
Astronomers can make educated guesses about the rest of the variables. For example, of the nine planets in our solar system, only four are what astronomers call terrestrial planets -- those that have a solid surface. Of those terrestrial planets, only Earth supports life. If we take our solar system as representative, then we might argue that n_e equals 1/4 or 0.25. Similar guesses have been made about the other variables and, interestingly, they all end up having very similar values, usually in a range between 0.1 and 1.0. So, a typical calculation might look like this:

N = 4 x 0.5 x 0.25 x 0.2 x 0.2 x 0.2 x 3,000,000

which gives us a value of 12,000 civilizations in our galaxy.
Drake's original calculations were very close to this value for N. When he ran the numbers, he predicted that there might be 10,000 detectable civilizations in the Milky Way [source: Garber]. Carl Sagan, a leader in the SETI movement until he passed away in 1996, was even more generous when he suggested that 1 million civilizations might exist in the galaxy [source: Lemarchand]. That's a lot of ETs!
No wonder astronomers were so optimistic when they started searching diligently for extraterrestrial life in the 1960s. On the next page, we'll look at how they've conducted this search and what it has turned up.

 

Testing and Revising the Drake Equation

Armed with an estimate of the number of communicative civilizations in our galaxy, SETI scientists set out to find them. They had two basic options: face-to-face communication or long-distance communication. The former scenario required that extraterrestrials visit humans or vice versa. This seemed highly unlikely given the distances between our solar system and other stars in the Milky Way. The latter scenario involved radio broadcasts, either sending or receiving electromagnetic signals through space.

Photo courtesy of the NAIC - Arecibo Observatory, a facility of the NSF
Aerial view of the Arecibo Observatory in Puerto Rico

In 1974, astronomers intentionally transmitted a 210-byte message from the Arecibo Observatory in Puerto Rico in the hopes of signaling a civilization in the globular star cluster M13. The message contained fundamental information about humans and our corner of the universe, such as the atomic numbers of key elements and the chemical structure of DNA. But this sort of active communication has been rare. Astronomers mostly rely on passive communication -- listening for transmissions sent by alien civilizations.
A radio telescope is the tool of choice for such listening experiments because it's designed to detect longer-wavelength energy that optical telescopes can't see. In radio astronomy, a giant dish is pointed to a nearby, sunlike star and tuned to the microwave region of the electromagnetic spectrum. The microwave frequency band, between 1,000 megahertz and 3,000 megahertz (MHz), is ideal because it's less contaminated with unwanted noise. It also contains an emission line -- 1,420 MHz -- that astronomers can hear as a persistent hiss across the galaxy. This narrow line corresponds to energy transformations taking place in neutral hydrogen. As a primordial element of the universe, hydrogen should be known to all intergalactic civilizations, making it an ideal marker. Several teams from around the world have been systematically listening to stars across the Milky Way and adjacent galaxies since 1960.
Despite their collective efforts, no SETI search has received a confirmed, extraterrestrial signal. Our telescopes have picked up a few unexplained and intriguing signals, such as the so-called "Wow" signal detected by researchers at Ohio State University in 1977, but no transmission has been repeated in such a way that it provides indisputable evidence of extraterrestrial life. All of which brings us back to the Fermi Paradox: If thousands of civilizations in the Milky Way galaxy, why haven't we detected them?
Since Drake and Sagan made their estimates, astronomers have become more conservative. Paul Horowitz, who boldly guaranteed the existence of extraterrestrial life, has generated more modest results from the Drake Equation, finding that N may be closer to 1,000 civilizations [source: Crawford]. But even that figure may be too large.
In 2002, Skeptic magazine publisher Michael Shermer argued that astronomers weren't being critical enough in their evaluation of L, the length of time a civilization remains detectable. Looking at 60 civilizations that have existed on Earth since the dawn of humanity, Shermer came up with a value for L that ranged from 304.5 years to 420.6 years. If you plug these numbers into the Drake Equation, you find that N equals 2.44 and 3.36, respectively. Tweak the numbers some more, and you can easily get N to fall to one or even lower. Suddenly, the odds of hearing from an extraterrestrial life form are considerably lower.
Even the most enthusiastic SETI supporters are troubled by the lack of results produced by more than 40 years of "listening" to the cosmic airwaves. And yet most of that search has been confined to our home galaxy. Even if there are only three or four civilizations per galaxy, there are billions and billions of galaxies. This tilts the odds again in favor of finding extraterrestrial life, which is why many SETI astronomers take the same approach to their work as lottery players: You can't win if you don't play.

Specially by- by Cristen Conger
Do fish feel pain?
the August 2004 issue of Gourmet magazine, David Foster Wallace asked readers to "Co­nsider the Lobster." What started out as a routine assignment to Maine's annual lobster festival spiraled into a quest to understand the lobster's point of view from entrapment to the dinner table. After all, lobsters look rather pathetic in grocery store tanks, clobbering each other with banded pincers. Their reward for enduring uncomfortable quarters is death by being boiled alive. And to top it off, the sound of steam whistling through the dying lobsters' shells mimics that of a final farewell scream.
Setting aside any anthropomorphic sentiment, what's the truth? Are people who cook lobsters merciless murderers who inflict cruel and unusual punishment on their crusty captives? (Spoiler alert: Wallace doesn't give us a concrete answer.) Pain is a complex experience to measure -- especially when the subject can't self-report. Mammals, such as your dog or cat, display more obvious pain signals by breathing heavily, guarding the wounded area of their bodies and vocalizing distress. Fish, on the other hand, thrash about on the end of the line, but that's about it.
About a year before Wallace's lobster piece hit the stands, physiologist Lynne Sneddon hooked some major public attention. Sneddon published a study proposing that fish -- specifically rainbow trout -- can detect and feel pain. Previous research on whether fish feel pain focused solely on cartilaginous species like sharks. For the first time, Sneddon's work documented potential pain sensations in teleostic (bony) fish.

Seiya Kawamoto/Getty Images
Can lobsters feel the pain of that boiling water

­More precisely, Sneddon discovered the presen­ce of 58 pain receptors called nociceptors along the trout's lips. She did so by testing the effects of bee venom and acetic acid injections into the mouth area. The affected fish exhibited "anomalous behavior," such as rubbing their noses into gravel and shaking their bodies [source: Sneddon]. Since morphine appeared to ease the discomfort, Sneddon concluded that the trout's reactions weren't simply reflexive but genuine displays of pain response.
The study's reception among the fishing industry and other scientists wasn't entirely smooth sailing. One critique dismissed the findings, arguing that fish lack the brain complexity to feel pain. Others chalked it up to a natural physiological response to a threatening situation. Animal rights advocates, on the other hand, saw it as proof positive of the inhumanity of recreational and industrial fishing.
In a way, all parties were correct.

Fish Pain

Guy Crittenden/Getty Images
That trout's nociceptors are probably firing like crazy.

We know that pain is uncomfortable, but what exactly is going on in our bodies that causes us to wince and tear up? According to the International Association for the Study of Pain, pain is a twofold experience. First, you have the neurobio­logical process of our nerves communicating with the brain. Second, there's the emotional response to pain that varies from person to person. (For more details, you can read about humans' pain response in How Pain Works.)
Despite its bad reputation, pain can be a handy survival tool. Think about the sensory reaction of putting your hand on a hot stove, registering the burn and retracting your arm. In this way, pain protects us. It tells your body that you're in danger of tissue damage and need to get to safety. Nociceptor nerve cells in our bodies deliver that initial pain-related communiqué along a route through the central nervous system to the brain. Research has shown that other mammals, birds and fish also have nociceptors. The presence of those nerve cells implies that fish have the sensory capability to recognize when something is harming their bodies. Hence, fish can experience the neurological pain process.
One of the primary arguments against fish having a sense of pain is that their brains lack the structural elements, namely the neocortex, necessary for it. Although fish's brains are certainly less complex than ours, they aren't mindlessly floating around in the water. Akin to the fight-or-flight response, fish have chemicals in their bodies that are released when they're in danger [source: Chandroo, Duncan and Moccia]. Fish also possess pain nerve fiber tracts in their bodies that are similar to those that tetrapods (vertebrates with four limbs) have [source: Chandroo, Duncan and Moccia]. Overall, the scientific consensus is that fish have the anatomical requirements to demonstrate neurophysiologic and behavioral reactions to pain as a means of survival [source: Braithwaite and Huntingford].
However, little evidence exists to suggest that the fish also react emotionally to pain like humans. As another way of safeguarding our bodies, our pain response comes with a negative emotional reaction that excites the amygdala in our brains and forms a memory of the unpleasant effect of a particular stimulus. For that reason, most children will touch a hot stove only once. Fish brains do contain an amygdaloid complex, but it produces aggression rather than fear [source: Chandroo, Duncan and Moccia]. Nevertheless, fish are able to form sensory memories. For instance, trout can remember how to avoid getting caught in fishing nets months after the initial experience [source: Donkin]. So while it remains unclear whether fish genuinely suffer from pain, memories of it stick in their brains.
If fish can sense pain but don't necessarily feel it emotionally like humans, where does that leave us? The fishing industry has already instituted catch-and-release policies to reduce the number of purposeless fish deaths. Some fishermen use barbless hooks and necks that cut down a fish's struggle time before being reeled in. As for the lobster, chefs may kill them by striking them swiftly through the eyes rather than dropping them into the boiling water alive [source: Wallace].
Ether way, it's morally debatable. For some people, no amount of concrete proof of a fish's agony could tear them away from hooking a fresh bass or trout. To others, people who fish are detestable because they inflict pain onto another living being. Whichever side of the line you stand on, there's one thing easily agreed on: Whether fish, fowl or mammal, neurological pain happens to us all. It's the capacity for suffering that remains up for dispute.

amygdala· fish pain· neocortex

Do fish feel pain?

Specially by- by Cristen Conger
Do fish feel pain?
the August 2004 issue of Gourmet magazine, David Foster Wallace asked readers to "Co­nsider the Lobster." What started out as a routine assignment to Maine's annual lobster festival spiraled into a quest to understand the lobster's point of view from entrapment to the dinner table. After all, lobsters look rather pathetic in grocery store tanks, clobbering each other with banded pincers. Their reward for enduring uncomfortable quarters is death by being boiled alive. And to top it off, the sound of steam whistling through the dying lobsters' shells mimics that of a final farewell scream.
Setting aside any anthropomorphic sentiment, what's the truth? Are people who cook lobsters merciless murderers who inflict cruel and unusual punishment on their crusty captives? (Spoiler alert: Wallace doesn't give us a concrete answer.) Pain is a complex experience to measure -- especially when the subject can't self-report. Mammals, such as your dog or cat, display more obvious pain signals by breathing heavily, guarding the wounded area of their bodies and vocalizing distress. Fish, on the other hand, thrash about on the end of the line, but that's about it.
About a year before Wallace's lobster piece hit the stands, physiologist Lynne Sneddon hooked some major public attention. Sneddon published a study proposing that fish -- specifically rainbow trout -- can detect and feel pain. Previous research on whether fish feel pain focused solely on cartilaginous species like sharks. For the first time, Sneddon's work documented potential pain sensations in teleostic (bony) fish.

Seiya Kawamoto/Getty Images
Can lobsters feel the pain of that boiling water

­More precisely, Sneddon discovered the presen­ce of 58 pain receptors called nociceptors along the trout's lips. She did so by testing the effects of bee venom and acetic acid injections into the mouth area. The affected fish exhibited "anomalous behavior," such as rubbing their noses into gravel and shaking their bodies [source: Sneddon]. Since morphine appeared to ease the discomfort, Sneddon concluded that the trout's reactions weren't simply reflexive but genuine displays of pain response.
The study's reception among the fishing industry and other scientists wasn't entirely smooth sailing. One critique dismissed the findings, arguing that fish lack the brain complexity to feel pain. Others chalked it up to a natural physiological response to a threatening situation. Animal rights advocates, on the other hand, saw it as proof positive of the inhumanity of recreational and industrial fishing.
In a way, all parties were correct.

Fish Pain

Guy Crittenden/Getty Images
That trout's nociceptors are probably firing like crazy.

We know that pain is uncomfortable, but what exactly is going on in our bodies that causes us to wince and tear up? According to the International Association for the Study of Pain, pain is a twofold experience. First, you have the neurobio­logical process of our nerves communicating with the brain. Second, there's the emotional response to pain that varies from person to person. (For more details, you can read about humans' pain response in How Pain Works.)
Despite its bad reputation, pain can be a handy survival tool. Think about the sensory reaction of putting your hand on a hot stove, registering the burn and retracting your arm. In this way, pain protects us. It tells your body that you're in danger of tissue damage and need to get to safety. Nociceptor nerve cells in our bodies deliver that initial pain-related communiqué along a route through the central nervous system to the brain. Research has shown that other mammals, birds and fish also have nociceptors. The presence of those nerve cells implies that fish have the sensory capability to recognize when something is harming their bodies. Hence, fish can experience the neurological pain process.
One of the primary arguments against fish having a sense of pain is that their brains lack the structural elements, namely the neocortex, necessary for it. Although fish's brains are certainly less complex than ours, they aren't mindlessly floating around in the water. Akin to the fight-or-flight response, fish have chemicals in their bodies that are released when they're in danger [source: Chandroo, Duncan and Moccia]. Fish also possess pain nerve fiber tracts in their bodies that are similar to those that tetrapods (vertebrates with four limbs) have [source: Chandroo, Duncan and Moccia]. Overall, the scientific consensus is that fish have the anatomical requirements to demonstrate neurophysiologic and behavioral reactions to pain as a means of survival [source: Braithwaite and Huntingford].
However, little evidence exists to suggest that the fish also react emotionally to pain like humans. As another way of safeguarding our bodies, our pain response comes with a negative emotional reaction that excites the amygdala in our brains and forms a memory of the unpleasant effect of a particular stimulus. For that reason, most children will touch a hot stove only once. Fish brains do contain an amygdaloid complex, but it produces aggression rather than fear [source: Chandroo, Duncan and Moccia]. Nevertheless, fish are able to form sensory memories. For instance, trout can remember how to avoid getting caught in fishing nets months after the initial experience [source: Donkin]. So while it remains unclear whether fish genuinely suffer from pain, memories of it stick in their brains.
If fish can sense pain but don't necessarily feel it emotionally like humans, where does that leave us? The fishing industry has already instituted catch-and-release policies to reduce the number of purposeless fish deaths. Some fishermen use barbless hooks and necks that cut down a fish's struggle time before being reeled in. As for the lobster, chefs may kill them by striking them swiftly through the eyes rather than dropping them into the boiling water alive [source: Wallace].
Ether way, it's morally debatable. For some people, no amount of concrete proof of a fish's agony could tear them away from hooking a fresh bass or trout. To others, people who fish are detestable because they inflict pain onto another living being. Whichever side of the line you stand on, there's one thing easily agreed on: Whether fish, fowl or mammal, neurological pain happens to us all. It's the capacity for suffering that remains up for dispute.

INTRODUCTION

When the last time you gazed upward and marveled at the mysterious, life-giving force that is the sun?
If you believe the whole staring-at-the-sun-makes-you-go-blind thing (which is actually true), you’re probably not doing a whole lot of sun-gazing. But it’s a real marvel: The sun warms our planet every day, provides the light by which we see and is necessary for life on Earth. It can also cause cell death and make us blind. It could fit 1.3 million Earths inside its sphere [source: SpaceDaily]. It produces poem-worthy sunsets and as much energy as 1 trillion megaton bombs every second [source: Boston Globe].
All of this, and our sun is just a plain old average star, by universal standards. It’s really just proximity that makes it so special to Earth. We wouldn’t be here if the sun weren’t so close.
So, how close is the sun? And how much space does it take to hold 1.3 million Earths? And while we’re at it:

• If the sun is in the vacuum of space, how does it burn?
• What keeps all that gas from leaking into space?
• Why does the sun send out solar flares?
• Will the sun ever stop burning? (And if so, when? And what will happen to Earth and its inhabitants?)

Sunspot Image Gallery


Photo courtesy of NASA
The sun warms our planet, provides us with light and is crucial to all life on Earth. See more sunspot pictures.

In this article, we'll examine the fascinating world of our nearest star. We will look at the parts of the sun, find out how it makes light and heat, and explore its major features.
The sun has "burned" for more than 4.5 billion years. It's a massive collection of gas, mostly hydrogen and helium. Because it is so massive, it has immense gravity, enough gravitational force to hold all of that hydrogen and helium together (and to hold all of the planets in their orbits around the sun).
We say the sun burns, but it doesn’t burn like wood burns. Instead, the sun is a gigantic nuclear reactor.

 

 

The Parts of the Sun

­­

Photo courtesy SOHO consortium.
Figure 1. Basic overview of the parts of the sun. The flare, sunspots and the prominence are all clipped from actual SOHO images.

The sun is a star, just like the other stars we see at night. The difference is distance -- the other stars we see are light-years away, while our sun is only about 8 light minutes away -- many thousands of times closer.
­Officially, the sun is classified as a G2 type star, based on its temperature and the wavelengths or spectrum of light that it emits. There are lots of G2s out there, and Earth's sun is merely one of billions of st­ars that orbit the center of our galaxy, made up of the same substance and components.
T­he sun is composed of gas. It has no solid surface. However, it still has a defined structure. The three major structural areas of the sun are shown in the upper half of Figure 1. They include:

• Core -- The center of the sun, comprising 25 percent of its radius.
• Radiative zone --The section immediately surrounding the core, comprising 45 percent of its radius.
• Convective zone -- The outermost ring of the sun, comprising the 30 percent of its radius.

Above the surface of the sun is its atmosphere, which consists of three parts, shown in the lower half of Figure 1:

• Photosphere -- The innermost part of the sun's atmosphere and the only part we can see.
• Chromosphere -- The area between the photosphere and the corona; hotter than the photosphere.
• Corona -- The extremely hot outermost layer, extending outward several million miles from the chromosphere.

­All of the major features of the sun can be explained by the nuclear reactions that produce its energy, by the magnetic fields resulting from the movements of the gas and by its immense gravity.
It begins at the core.

 

 

The Sun's Interior: Core

The core starts from the center and extends outward to encompass 25 percent of the sun's radius. Its temperature is greater than 15 million degrees Kelvin [source: Montana]. At the core, gravity pulls all of the mass inward and creates an intense pressure. The pressure is high enough to force atoms of hydrogen to come together in nuclear fusion reactions -- something we try to emulate here on Earth. Two atoms of hydrogen are combined to create helium-4 and energy in several steps:

NASA/WireImage/Getty Images
A powerful solar flare erupted from Sunspot 486 on Oct. 28, 2003. The flare sent X-rays traveling at the speed of light toward Earth, causing a radio storm in the ionosphere.

1. Two protons combine to form a deuterium atom (hydrogen atom with one neutron and one proton), a positron (similar to electron, but with a positive charge) and a neutrino.
2. A proton and a deuterium atom combine to form a helium-3 atom (two protons with one neutron) and a gamma ray.
3. Two helium-3 atoms combine to form a helium-4 atom (two protons and two neutrons) and two protons.

These reactions account for 85 percent of the sun's energy. The remaining 15 percent comes from the following reactions:

1. A helium-3 atom and a helium-4 atom combine to form a beryllium-7 (four protons and three neutrons) and a gamma ray.
2. A beryllium-7 atom captures an electron to become lithium-7 atom (three protons and four neutrons) and a neutrino.
3. The lithium-7 combines with a proton to form two helium-4 atoms.

­The helium-4 atoms are less massive than the two hydrogen atoms that started the process, so the difference in mass is converted to energy as described by Einstein's theory of relativity (E=mc²). The energy is emitted in various forms of light: ultraviolet light, X-rays, visible light, infrared, microwaves and radio waves.
The sun also emits energized particles (neutrinos, protons) that make up the solar wind. This energy strikes Earth, where it warms the planet, drives our weather and provides energy for life. We aren't harmed by most of the radiation or solar wind because the Earth's atmosphere protects us.

The Sun's Interior: Radiative and Convective Zones

Sun Facts Average distance from Earth: 93 million miles (150 million kilometers) Radius: 418,000 miles (696,000 kilometers) Mass: 1.99 x 1030 kilograms (330,000 Earth masses) Makeup (by mass): 74 percent hydrogen, 25 percent helium, 1 percent other elements Average temperature: 5,800 degrees Kelvin (surface), 15.5 million degrees Kelvin (core) Average density: 1.41 grams per cm3 Volume: 1.4 x 1027 cubic meters Rotational period: 25 days (center) to 35 days (poles) Distance from center of Milky Way: 25,000 light years Orbital speed/period: 138 miles per second/200 million years

After covering the core, it's time to extend outward in the sun's structure. Next up are the radiative and convective zones.
The radiative zone extends outward from the core, accounting for 45 percent of the sun's radius. In this zone, the energy from the core is carried outward by photons, or light units. As one photon is made, it travels about 1 micron (1 millionth of a meter) before being absorbed by a gas molecule. Upon absorption, the gas molecule is heated and re-emits another photon of the same wavelength. The re-emitted photon travels another micron before being absorbed by another gas molecule and the cycle repeats itself; each interaction between photon and gas molecule takes time. Approximately 10²⁵ absorptions and re-emissions take place in this zone before a photon reaches the surface, so there is a significant time delay between a photon made in the core and one that reaches the surface.
The convective zone, which is the final 30 percent of the sun's radius, is dominated by convection currents that carry the energy outward to the surface. These convection currents are rising movements of hot gas next to falling movements of cool gas, and it looks kind of like glitter in a simmering pot of water. The convection currents carry photons outward to the surface faster than the radiative transfer that occurs in the core and radiative zone. With so many interactions occurring between photons and gas molecules in the radiative and convection zones, it takes a photon approximately 100,000 to 200,000 years to reach the surface.

The Sun's Atmosphere

We've finally made our way to the surface. Let's trace through the atmosphere next. Just like Earth, the sun boasts an atmosphere. However, the sun's is composed of the photosphere, the chromosphere and the corona.
The photosphere is the lowest region of the sun's atmosphere and is the region that we can see. "The surface of the sun" typically refers to the photosphere, at least in lay terms. It is 180-240 miles (300-400 kilometers wide) and has an average temperature of 5,800 degrees Kelvin. It appears granulated or bubbly, much like the surface of a simmering pot of water. The bumps are the upper surfaces of the convection current cells beneath; each granulation can be 600 miles (1,000 kilometers) wide. As we pass up through the photosphere, the temperature drops and the gases, because they are cooler, do not emit as much light energy. This makes them less opaque to the human eye. Therefore, the outer edge of the photosphere looks dark, an effect called limb darkening that accounts for the clear crisp edge of the sun's surface.
The chromosphere extends above the photosphere to about 1,200 miles (2,000 kilometers). The temperature rises across the chromosphere from 4,500 degrees Kelvin to about 10,000 degrees Kelvin. The chromosphere is thought to be heated by convection within the underlying photosphere. As gases churn in the photosphere, they produce shock waves that heat the surrounding gas and send it piercing through the chromosphere in millions of tiny spikes of hot gas called spicules. Each spicule rises to approximately 3,000 miles (5,000 kilometers) above the photosphere and lasts only a few minutes. Spicules may also follow along magnetic field lines of the sun, which are made by the movements of gases inside the sun.
The corona is the final layer of the sun and extends several million miles or kilometers outward from the other spheres. It can be seen best during a solar eclipse and in X-ray images of the sun. The temperature of the corona averages 2 million degrees Kelvin. Although no one is sure why the corona is so hot, it is thought to be caused by the sun's magnetism. The corona has bright areas (hot) and dark areas called coronal holes. Coronal holes are relatively cool and are thought to be areas where particles of the solar wind escape.
T­hrough telescope images we can see several interesting features on the sun that can have effects here on Earth. Let's take a look at three of them: sunspots, solar prominences and solar flares.

The Sun's Features: Sunspots, Solar Prominences and Solar Flares

Photo courtesy of NASA
After many weeks of a blank sun with no sunspots, a small new sunspot emerged on Sept. 23, 2008, marking a new solar cycle.

Of course, the spheres are graced with interesting features and activity. We'll take a look at them here.
Dark, cool areas called sunspots appear on the photosphere. Sunspots always appear in pairs and are intense magnetic fields (about 5,000 times greater than the Earth's magnetic field) that break through the surface. Field lines leave through one sunspot and re-enter through the other one. The magnetic field is caused by movements of gases in the sun's interior.
Sunspot activity occurs as part of an 11-year cycle called the solar cycle where there are periods of maximum and minimum activity.
It is not known what causes this 11-year cycle, but two hypotheses have been proposed:

• Uneven rotation of the sun distorts and twists magnetic field lines in the interior. The twisted field lines break through the surface forming sunspot pairs. Eventually, the field lines break apart and sunspot activity decreases. The cycle starts again.
• Huge tubes of gas circle the sun's interior at high latitudes and begin to move toward the equator. When they roll against each other, they form spots. When they reach the equator, they break up and sunspots decline.

­Occasionally, clouds of gases from the chromosphere will rise and orient themselves along the magnetic lines from sunspot pairs. These arches of gas are called solar prominences.
Prominences can last two to three months and can extend 30,000 miles (50,000 kilometers) or more above the sun's surface. Up­on reaching this height, they can erupt for a few minutes to hours and send large amounts of material racing through the corona and outward into space at 600 miles per second (1,000 kilometers per second); these eruptions are called coronal mass ejections.
Sometimes in complex sunspot groups, abrupt, violent explosions from the sun occur. These are called solar flares.
Solar flares are thought to be caused by sudden magnetic field changes in areas where the sun's magnetic field is concentrated. They're accompanied by the release of gas, electrons, visible light, ultraviolet light and X-rays. When this radiation and these particles reach the Earth's magnetic field, they interact with it at the poles to produce the auroras (borealis and australis). Solar flares can also disrupt communications, satellites, navigation systems and even power grids. The radiation and particles ionize the atmosphere and prevent the movement of radio waves between satellites and the ground or between the ground and the ground. The ionized particles in the atmosphere can induce electric currents in power lines and cause power surges. These power surges can overload a power grid and cause blackouts. You can learn more about solar flares by reading Could an extremely powerful solar flare destroy all the electronics on Earth? All of this activity requires energy, which is in limited supply. Eventually, the sun will run out of fuel.

The Fate of the Sun

­T­he sun has been shining for about 4.5 billion years [source: Berkeley]. The size of the sun is a balance between the outward pressure made by the release of energy from nuclear fusion and the inward pull of gravity. Over its 4.5 billion years of life, the sun's radius has gotten about 6 percent bigger [source: Berkeley]. It has enough hydrogen fuel to "burn" for about 10 billion years, meaning it has a bit over 5 billion years left, and during this time it will continue to expand at the same rate [source: Berkeley].

Photo courtesy of NASA Sun-Earth Day 2010
When our sun becomes a red giant, its radius will be about 100 times what it is now. Planetary nebulae are the remains of sunlike stars that have reached the end of their red giant stage.

When the core runs out of hydrogen fuel, it will contract under the weight of gravity; however, some hydrogen fusion will occur in the upper layers. As the core contracts, it heats up and this heats the upper layers causing them to expand. As the outer layers expand, the radius of the sun will increase and it will become a red giant, an elderly star.

The radius of the red giant sun will be 100 times what it is now, lying just beyond the Earth's orbit, so the Earth will plunge into the core of the red giant sun and be vaporized [source: NASA]. At some point after this, the core will become hot enough to cause the helium to fuse into carbon.
When the helium fuel has exhausted, the core will expand and cool. The upper layers will expand and eject material.
Finally, the core will cool into a white dwarf.
Eventually, it will further cool into a nearly invisible black dwarf. This entire process will take a few billion years.
So for the next several billion years, humanity is safe -- in terms of the sun's existence, at least. Other debacles are anybody's guess.

chromosphere· coronal holesening· limb dark· sun· The Sun;s Atmosphere· The Sun;s Interior: Radiative and Convective Zones

How the Sun Works?

INTRODUCTION

When the last time you gazed upward and marveled at the mysterious, life-giving force that is the sun?
If you believe the whole staring-at-the-sun-makes-you-go-blind thing (which is actually true), you’re probably not doing a whole lot of sun-gazing. But it’s a real marvel: The sun warms our planet every day, provides the light by which we see and is necessary for life on Earth. It can also cause cell death and make us blind. It could fit 1.3 million Earths inside its sphere [source: SpaceDaily]. It produces poem-worthy sunsets and as much energy as 1 trillion megaton bombs every second [source: Boston Globe].
All of this, and our sun is just a plain old average star, by universal standards. It’s really just proximity that makes it so special to Earth. We wouldn’t be here if the sun weren’t so close.
So, how close is the sun? And how much space does it take to hold 1.3 million Earths? And while we’re at it:

• If the sun is in the vacuum of space, how does it burn?
• What keeps all that gas from leaking into space?
• Why does the sun send out solar flares?
• Will the sun ever stop burning? (And if so, when? And what will happen to Earth and its inhabitants?)

Sunspot Image Gallery


Photo courtesy of NASA
The sun warms our planet, provides us with light and is crucial to all life on Earth. See more sunspot pictures.

In this article, we'll examine the fascinating world of our nearest star. We will look at the parts of the sun, find out how it makes light and heat, and explore its major features.
The sun has "burned" for more than 4.5 billion years. It's a massive collection of gas, mostly hydrogen and helium. Because it is so massive, it has immense gravity, enough gravitational force to hold all of that hydrogen and helium together (and to hold all of the planets in their orbits around the sun).
We say the sun burns, but it doesn’t burn like wood burns. Instead, the sun is a gigantic nuclear reactor.

 

 

The Parts of the Sun

­­

Photo courtesy SOHO consortium.
Figure 1. Basic overview of the parts of the sun. The flare, sunspots and the prominence are all clipped from actual SOHO images.

The sun is a star, just like the other stars we see at night. The difference is distance -- the other stars we see are light-years away, while our sun is only about 8 light minutes away -- many thousands of times closer.
­Officially, the sun is classified as a G2 type star, based on its temperature and the wavelengths or spectrum of light that it emits. There are lots of G2s out there, and Earth's sun is merely one of billions of st­ars that orbit the center of our galaxy, made up of the same substance and components.
T­he sun is composed of gas. It has no solid surface. However, it still has a defined structure. The three major structural areas of the sun are shown in the upper half of Figure 1. They include:

• Core -- The center of the sun, comprising 25 percent of its radius.
• Radiative zone --The section immediately surrounding the core, comprising 45 percent of its radius.
• Convective zone -- The outermost ring of the sun, comprising the 30 percent of its radius.

Above the surface of the sun is its atmosphere, which consists of three parts, shown in the lower half of Figure 1:

• Photosphere -- The innermost part of the sun's atmosphere and the only part we can see.
• Chromosphere -- The area between the photosphere and the corona; hotter than the photosphere.
• Corona -- The extremely hot outermost layer, extending outward several million miles from the chromosphere.

­All of the major features of the sun can be explained by the nuclear reactions that produce its energy, by the magnetic fields resulting from the movements of the gas and by its immense gravity.
It begins at the core.

 

 

The Sun's Interior: Core

The core starts from the center and extends outward to encompass 25 percent of the sun's radius. Its temperature is greater than 15 million degrees Kelvin [source: Montana]. At the core, gravity pulls all of the mass inward and creates an intense pressure. The pressure is high enough to force atoms of hydrogen to come together in nuclear fusion reactions -- something we try to emulate here on Earth. Two atoms of hydrogen are combined to create helium-4 and energy in several steps:

NASA/WireImage/Getty Images
A powerful solar flare erupted from Sunspot 486 on Oct. 28, 2003. The flare sent X-rays traveling at the speed of light toward Earth, causing a radio storm in the ionosphere.

1. Two protons combine to form a deuterium atom (hydrogen atom with one neutron and one proton), a positron (similar to electron, but with a positive charge) and a neutrino.
2. A proton and a deuterium atom combine to form a helium-3 atom (two protons with one neutron) and a gamma ray.
3. Two helium-3 atoms combine to form a helium-4 atom (two protons and two neutrons) and two protons.

These reactions account for 85 percent of the sun's energy. The remaining 15 percent comes from the following reactions:

1. A helium-3 atom and a helium-4 atom combine to form a beryllium-7 (four protons and three neutrons) and a gamma ray.
2. A beryllium-7 atom captures an electron to become lithium-7 atom (three protons and four neutrons) and a neutrino.
3. The lithium-7 combines with a proton to form two helium-4 atoms.

­The helium-4 atoms are less massive than the two hydrogen atoms that started the process, so the difference in mass is converted to energy as described by Einstein's theory of relativity (E=mc²). The energy is emitted in various forms of light: ultraviolet light, X-rays, visible light, infrared, microwaves and radio waves.
The sun also emits energized particles (neutrinos, protons) that make up the solar wind. This energy strikes Earth, where it warms the planet, drives our weather and provides energy for life. We aren't harmed by most of the radiation or solar wind because the Earth's atmosphere protects us.

The Sun's Interior: Radiative and Convective Zones

Sun Facts Average distance from Earth: 93 million miles (150 million kilometers) Radius: 418,000 miles (696,000 kilometers) Mass: 1.99 x 1030 kilograms (330,000 Earth masses) Makeup (by mass): 74 percent hydrogen, 25 percent helium, 1 percent other elements Average temperature: 5,800 degrees Kelvin (surface), 15.5 million degrees Kelvin (core) Average density: 1.41 grams per cm3 Volume: 1.4 x 1027 cubic meters Rotational period: 25 days (center) to 35 days (poles) Distance from center of Milky Way: 25,000 light years Orbital speed/period: 138 miles per second/200 million years

After covering the core, it's time to extend outward in the sun's structure. Next up are the radiative and convective zones.
The radiative zone extends outward from the core, accounting for 45 percent of the sun's radius. In this zone, the energy from the core is carried outward by photons, or light units. As one photon is made, it travels about 1 micron (1 millionth of a meter) before being absorbed by a gas molecule. Upon absorption, the gas molecule is heated and re-emits another photon of the same wavelength. The re-emitted photon travels another micron before being absorbed by another gas molecule and the cycle repeats itself; each interaction between photon and gas molecule takes time. Approximately 10²⁵ absorptions and re-emissions take place in this zone before a photon reaches the surface, so there is a significant time delay between a photon made in the core and one that reaches the surface.
The convective zone, which is the final 30 percent of the sun's radius, is dominated by convection currents that carry the energy outward to the surface. These convection currents are rising movements of hot gas next to falling movements of cool gas, and it looks kind of like glitter in a simmering pot of water. The convection currents carry photons outward to the surface faster than the radiative transfer that occurs in the core and radiative zone. With so many interactions occurring between photons and gas molecules in the radiative and convection zones, it takes a photon approximately 100,000 to 200,000 years to reach the surface.

The Sun's Atmosphere

We've finally made our way to the surface. Let's trace through the atmosphere next. Just like Earth, the sun boasts an atmosphere. However, the sun's is composed of the photosphere, the chromosphere and the corona.
The photosphere is the lowest region of the sun's atmosphere and is the region that we can see. "The surface of the sun" typically refers to the photosphere, at least in lay terms. It is 180-240 miles (300-400 kilometers wide) and has an average temperature of 5,800 degrees Kelvin. It appears granulated or bubbly, much like the surface of a simmering pot of water. The bumps are the upper surfaces of the convection current cells beneath; each granulation can be 600 miles (1,000 kilometers) wide. As we pass up through the photosphere, the temperature drops and the gases, because they are cooler, do not emit as much light energy. This makes them less opaque to the human eye. Therefore, the outer edge of the photosphere looks dark, an effect called limb darkening that accounts for the clear crisp edge of the sun's surface.
The chromosphere extends above the photosphere to about 1,200 miles (2,000 kilometers). The temperature rises across the chromosphere from 4,500 degrees Kelvin to about 10,000 degrees Kelvin. The chromosphere is thought to be heated by convection within the underlying photosphere. As gases churn in the photosphere, they produce shock waves that heat the surrounding gas and send it piercing through the chromosphere in millions of tiny spikes of hot gas called spicules. Each spicule rises to approximately 3,000 miles (5,000 kilometers) above the photosphere and lasts only a few minutes. Spicules may also follow along magnetic field lines of the sun, which are made by the movements of gases inside the sun.
The corona is the final layer of the sun and extends several million miles or kilometers outward from the other spheres. It can be seen best during a solar eclipse and in X-ray images of the sun. The temperature of the corona averages 2 million degrees Kelvin. Although no one is sure why the corona is so hot, it is thought to be caused by the sun's magnetism. The corona has bright areas (hot) and dark areas called coronal holes. Coronal holes are relatively cool and are thought to be areas where particles of the solar wind escape.
T­hrough telescope images we can see several interesting features on the sun that can have effects here on Earth. Let's take a look at three of them: sunspots, solar prominences and solar flares.

The Sun's Features: Sunspots, Solar Prominences and Solar Flares

Photo courtesy of NASA
After many weeks of a blank sun with no sunspots, a small new sunspot emerged on Sept. 23, 2008, marking a new solar cycle.

Of course, the spheres are graced with interesting features and activity. We'll take a look at them here.
Dark, cool areas called sunspots appear on the photosphere. Sunspots always appear in pairs and are intense magnetic fields (about 5,000 times greater than the Earth's magnetic field) that break through the surface. Field lines leave through one sunspot and re-enter through the other one. The magnetic field is caused by movements of gases in the sun's interior.
Sunspot activity occurs as part of an 11-year cycle called the solar cycle where there are periods of maximum and minimum activity.
It is not known what causes this 11-year cycle, but two hypotheses have been proposed:

• Uneven rotation of the sun distorts and twists magnetic field lines in the interior. The twisted field lines break through the surface forming sunspot pairs. Eventually, the field lines break apart and sunspot activity decreases. The cycle starts again.
• Huge tubes of gas circle the sun's interior at high latitudes and begin to move toward the equator. When they roll against each other, they form spots. When they reach the equator, they break up and sunspots decline.

­Occasionally, clouds of gases from the chromosphere will rise and orient themselves along the magnetic lines from sunspot pairs. These arches of gas are called solar prominences.
Prominences can last two to three months and can extend 30,000 miles (50,000 kilometers) or more above the sun's surface. Up­on reaching this height, they can erupt for a few minutes to hours and send large amounts of material racing through the corona and outward into space at 600 miles per second (1,000 kilometers per second); these eruptions are called coronal mass ejections.
Sometimes in complex sunspot groups, abrupt, violent explosions from the sun occur. These are called solar flares.
Solar flares are thought to be caused by sudden magnetic field changes in areas where the sun's magnetic field is concentrated. They're accompanied by the release of gas, electrons, visible light, ultraviolet light and X-rays. When this radiation and these particles reach the Earth's magnetic field, they interact with it at the poles to produce the auroras (borealis and australis). Solar flares can also disrupt communications, satellites, navigation systems and even power grids. The radiation and particles ionize the atmosphere and prevent the movement of radio waves between satellites and the ground or between the ground and the ground. The ionized particles in the atmosphere can induce electric currents in power lines and cause power surges. These power surges can overload a power grid and cause blackouts. You can learn more about solar flares by reading Could an extremely powerful solar flare destroy all the electronics on Earth? All of this activity requires energy, which is in limited supply. Eventually, the sun will run out of fuel.

The Fate of the Sun

­T­he sun has been shining for about 4.5 billion years [source: Berkeley]. The size of the sun is a balance between the outward pressure made by the release of energy from nuclear fusion and the inward pull of gravity. Over its 4.5 billion years of life, the sun's radius has gotten about 6 percent bigger [source: Berkeley]. It has enough hydrogen fuel to "burn" for about 10 billion years, meaning it has a bit over 5 billion years left, and during this time it will continue to expand at the same rate [source: Berkeley].

Photo courtesy of NASA Sun-Earth Day 2010
When our sun becomes a red giant, its radius will be about 100 times what it is now. Planetary nebulae are the remains of sunlike stars that have reached the end of their red giant stage.

When the core runs out of hydrogen fuel, it will contract under the weight of gravity; however, some hydrogen fusion will occur in the upper layers. As the core contracts, it heats up and this heats the upper layers causing them to expand. As the outer layers expand, the radius of the sun will increase and it will become a red giant, an elderly star.

The radius of the red giant sun will be 100 times what it is now, lying just beyond the Earth's orbit, so the Earth will plunge into the core of the red giant sun and be vaporized [source: NASA]. At some point after this, the core will become hot enough to cause the helium to fuse into carbon.
When the helium fuel has exhausted, the core will expand and cool. The upper layers will expand and eject material.
Finally, the core will cool into a white dwarf.
Eventually, it will further cool into a nearly invisible black dwarf. This entire process will take a few billion years.
So for the next several billion years, humanity is safe -- in terms of the sun's existence, at least. Other debacles are anybody's guess.

Do you ever get the feeling that time is just dragging on? You might be working in the office, sitting at your desk at school during a long lecture or waiting for the doctor to see you, but when you look up at the clock, you could swear the 15 minutes it took for the long hand to move a quarter of an hour were really twice as long. No matter how much you squirm and fidget, time is taking its sweet time in getting to the future.

Evening Standard/Getty Images
A student at Enfield School in London wonders why time doesn't just pick up the pace. Is time a concrete, immutable concept, or does it actually change?

On the other hand, sometimes it can feel like time moves too quickly. Deep, engaging conversations with friends and loved ones can last for several hours but make you feel like time swept by in minutes. You can wake up right when the alarm goes off in the morning but somehow still end up running late for work. You're left throwing your hands up, wondering what happened to all of that lost time.
Time is a strangely contradictory concept. Many of us think of it as a concrete way of describing how long an event takes to unfold. And why wouldn't we, when we have fancy gadgets like watches? Modern technology has given us clocks, which help us measure time precisely. Atomic clocks, which measure the resonance frequencies of atoms, are even better at telling time. When someone standing still then walks 10 paces forward, we can easily measure with a stopwatch the number of seconds it took from the beginning of that short journey to its end.

But time doesn't always feel precise to us. When you bring two different people into the equation, especially if they don't have any watches, getting them to agree on their experience of time becomes increasingly difficult. So is time as simple as we think it is, or is it more fluid and relative? How is time connected to space?

Absolute Time and Relative Time

Daniel Berehulak/Getty Images
A copy of Newton's "Philosphiae Naturalis Principia Mathematica" at the Science Museum Library and Archives in Swindon, England.

When physicist and philosopher Isaac Newton completed his "Philosophiae Naturalis Principia Mathematica" in the late 17th century, he led a scientific revolution that changed the way people viewed the world. In the work, he laid out several concepts that would become the basis for classical physics. Among the important theories Newton introduced were the laws of motion that govern the way objects move through space, including the law of universal gravitation, and the foundation for calculus. In other words, most people consider Newton a genius, and scientists still apply his ideas to everyday circumstances.

Newton included in the "Principia Mathematica" a scholium, or an appendix of explanatory notes, and in it he defined several important principles, including the idea of absolute time. Although he understood that clocks weren't perfect and measuring time was subject to human error, Newton believed in an absolute time that was similar to a universal, omnipotent God-like time, one that was the same for everyone, everywhere. In other words, someone standing at the North Pole on Earth would experience time the same way as someone standing on Mars.
Newton's view on time kept it separate from space. When Albert Einstein introduced his Theory of Relativity in the early 20th century, however, he suggested that time wasn't separate from space but connected to it. Time and space combined to form space-time, and everyone measures his or her own experience in it differently because the speed of light (300,000 km per second) is the same for all observers. In other words, if all observers have to agree on the speed of light being 300,000 km per second, then they can't agree on the time it takes for other objects to travel relative to them.

Einstein also suggested that space-time wasn't flat, but curved or "warped" by the existence of matter and energy. Large bodies in space-time, like the Earth, aren't just floating in orbit. Instead, imagine an apple resting on a stretched out blanket -- the weight of the apple warps the sheet. If the Earth is an apple, then we can imagine the Earth's blanket as space-time.

This means that someone moving through space-time will experience it differently at various points. Time will actually appear to move slower near massive objects, because space-time is warped by the weight. These predictions have actually been proven. In 1962, scientists placed two atomic clocks at the bottom and top of a water tower. The clock at the bottom, the one closer to the massive center of the Earth, was running slower than the clock at the top. Einstein called this phenomenon time dilation.

A further explanation of the bending of space-time and time dilation came in the form of a thought experiment called the twin paradox, devised in 1911 by French physicist Paul Langevin. If one twin lives at the foot of a mountain and the other lives at the top, the twin closer to the Earth will age more slowly. He or she would turn out younger than the other twin, though by a very small amount. If you sent one twin in a spaceship accelerating close to the speed of light, however, he or she would return much younger than the other twin, because high acceleration and large gravitational masses are the same in relativity. Of course, no one's gone so far as to send somebody's twin into high-speed orbit, but scientists proved the hypothesis true in the '70s by sending an atomic clock into orbit. It returned to Earth having run much slower than grounded atomic clocks [source: Europhysics News].

Fox Photos/Getty Images
So you're telling me if you send me up there at high speeds, I'll appear younger than Jimmy when I come back down? Where do I sign up?!

So next time you're late for work or want the weekends to last longer, make sure you stay close to the ground and accelerate as much as possible. Boring lectures and waiting areas in doctor's offices, on the other hand, should be spent in the topmost room  of high towers. For lots more information on physics and the nature of time, see the next page in a timely fashion.

in future· time travelling

Does time change speed?

Do you ever get the feeling that time is just dragging on? You might be working in the office, sitting at your desk at school during a long lecture or waiting for the doctor to see you, but when you look up at the clock, you could swear the 15 minutes it took for the long hand to move a quarter of an hour were really twice as long. No matter how much you squirm and fidget, time is taking its sweet time in getting to the future.

Evening Standard/Getty Images
A student at Enfield School in London wonders why time doesn't just pick up the pace. Is time a concrete, immutable concept, or does it actually change?

On the other hand, sometimes it can feel like time moves too quickly. Deep, engaging conversations with friends and loved ones can last for several hours but make you feel like time swept by in minutes. You can wake up right when the alarm goes off in the morning but somehow still end up running late for work. You're left throwing your hands up, wondering what happened to all of that lost time.
Time is a strangely contradictory concept. Many of us think of it as a concrete way of describing how long an event takes to unfold. And why wouldn't we, when we have fancy gadgets like watches? Modern technology has given us clocks, which help us measure time precisely. Atomic clocks, which measure the resonance frequencies of atoms, are even better at telling time. When someone standing still then walks 10 paces forward, we can easily measure with a stopwatch the number of seconds it took from the beginning of that short journey to its end.

But time doesn't always feel precise to us. When you bring two different people into the equation, especially if they don't have any watches, getting them to agree on their experience of time becomes increasingly difficult. So is time as simple as we think it is, or is it more fluid and relative? How is time connected to space?

Absolute Time and Relative Time

Daniel Berehulak/Getty Images
A copy of Newton's "Philosphiae Naturalis Principia Mathematica" at the Science Museum Library and Archives in Swindon, England.

When physicist and philosopher Isaac Newton completed his "Philosophiae Naturalis Principia Mathematica" in the late 17th century, he led a scientific revolution that changed the way people viewed the world. In the work, he laid out several concepts that would become the basis for classical physics. Among the important theories Newton introduced were the laws of motion that govern the way objects move through space, including the law of universal gravitation, and the foundation for calculus. In other words, most people consider Newton a genius, and scientists still apply his ideas to everyday circumstances.

Newton included in the "Principia Mathematica" a scholium, or an appendix of explanatory notes, and in it he defined several important principles, including the idea of absolute time. Although he understood that clocks weren't perfect and measuring time was subject to human error, Newton believed in an absolute time that was similar to a universal, omnipotent God-like time, one that was the same for everyone, everywhere. In other words, someone standing at the North Pole on Earth would experience time the same way as someone standing on Mars.
Newton's view on time kept it separate from space. When Albert Einstein introduced his Theory of Relativity in the early 20th century, however, he suggested that time wasn't separate from space but connected to it. Time and space combined to form space-time, and everyone measures his or her own experience in it differently because the speed of light (300,000 km per second) is the same for all observers. In other words, if all observers have to agree on the speed of light being 300,000 km per second, then they can't agree on the time it takes for other objects to travel relative to them.

Einstein also suggested that space-time wasn't flat, but curved or "warped" by the existence of matter and energy. Large bodies in space-time, like the Earth, aren't just floating in orbit. Instead, imagine an apple resting on a stretched out blanket -- the weight of the apple warps the sheet. If the Earth is an apple, then we can imagine the Earth's blanket as space-time.

This means that someone moving through space-time will experience it differently at various points. Time will actually appear to move slower near massive objects, because space-time is warped by the weight. These predictions have actually been proven. In 1962, scientists placed two atomic clocks at the bottom and top of a water tower. The clock at the bottom, the one closer to the massive center of the Earth, was running slower than the clock at the top. Einstein called this phenomenon time dilation.

A further explanation of the bending of space-time and time dilation came in the form of a thought experiment called the twin paradox, devised in 1911 by French physicist Paul Langevin. If one twin lives at the foot of a mountain and the other lives at the top, the twin closer to the Earth will age more slowly. He or she would turn out younger than the other twin, though by a very small amount. If you sent one twin in a spaceship accelerating close to the speed of light, however, he or she would return much younger than the other twin, because high acceleration and large gravitational masses are the same in relativity. Of course, no one's gone so far as to send somebody's twin into high-speed orbit, but scientists proved the hypothesis true in the '70s by sending an atomic clock into orbit. It returned to Earth having run much slower than grounded atomic clocks [source: Europhysics News].

Fox Photos/Getty Images
So you're telling me if you send me up there at high speeds, I'll appear younger than Jimmy when I come back down? Where do I sign up?!

So next time you're late for work or want the weekends to last longer, make sure you stay close to the ground and accelerate as much as possible. Boring lectures and waiting areas in doctor's offices, on the other hand, should be spent in the topmost room  of high towers. For lots more information on physics and the nature of time, see the next page in a timely fashion.

Peter Macdiarmid/Getty ImagesIn the BBC TV series "Dr. Who," the Tardis -- the Doctor's machine for traveling through time and space -- is disguised as a blue police box. As a Time Lord, The Doctor lives outside of time -- his life has no clearly definable past, present and future.

Science fiction has thoroughly covered the topic of time travel, starting with H.G. Wells' "The Time Machine" in 1895 and continuing right up to modern movies like "Déjà Vu" starring Denzel Washington. But physicists have also explored the nature of time and the plausibility of time travel for more than century, beginning with Albert Einstein's theories of relativity. Thanks to Einstein, scientists know that time slows as moving objects approach the speed of light. Gravity also slows time. This means that, in one sense, all of us can already consider ourselves time travelers in a limited way because we experience a tiny time warp (a difference of only nanoseconds) when we, for example, take a flight on an airplane. But physicists who study time travel today search for plausible ways to create a time warp large enough to allow noticeable travel into the past or future.


In his book "How to Build a Time Machine," physicist Paul Davies writes, "The theory of relativity implies that a limited form of time travel is certainly possible, while unrestricted time travel -- to any epoch, past or future -- might just be possible, too." This astonishing statement begs an important question: If time travel did indeed become a reality, how would it affect our world as we currently experience it?

First, it's important to realize that building a time machine would likely involve enormous expense, and the sheer complexity of such an apparatus would mean only a limited group of time travelers would have access to it. But even a small group of "astronauts" traveling through time and space could conceivably have a tremendous impact on life as we know it today. The possibilities, in fact, seem almost infinite.

Let's begin by assuming that it's possible to create a complete loop in time travel -- that time travelers could travel back into the past and then return to the future (or vice versa). Although scientists view traveling to the future as a much less problematic proposition than traveling to the past, our daily lives wouldn't change much if we could only send time travelers backward or forward in time, unable to recall them to the present. If we could, in fact, complete this loop of time travel, we can conjure up an incredible array of possible effects

Possibilities and Paradoxes of Time Travel

Imagine sending a time traveling astronaut 100 years into the future. The time traveler could witness technological advancements that we can only dream of today, much as people at the turn of the 20th century likely couldn't imagine the items we take for granted in 2010, such as iPods or laptop computers. The time traveler could also gain insight into medical advancements, such as new medicines, treatments and surgical techniques. If the time traveler could bring this knowledge backward in time to the present, the time from which he or she came, society could effectively leap forward in terms of its technical and scientific knowledge.

The futuristic time traveler could also bring back knowledge of what lay ahead for the world. He or she could warn of natural disasters, geopolitical conflicts, epidemics and other events of worldwide importance. This knowledge could potentially change the very way we operate. For example, what if a time traveler journeyed into the future and literally saw the effects that automobiles would eventually have on our planet? What if the time traveler witnessed an environment so polluted and damaged that it's unrecognizable? How might that change our willingness to use alternative forms of transportation?

Imagine that time travel became less restricted and more available to a larger population. Perhaps travel into the future would be exploited for personal gain. A futuristic time traveler could draw on knowledge of the stock market to guide his or her investment decisions, effectively using the granddaddy of all insider information to amass a fortune. Militaries might rely on time travel to gain valuable knowledge about the enemy's positioning and resources in future battles. Terrorists could use time travel to scout out the scenes of future attacks, allowing them to carefully plan with precise knowledge of future conditions.

Consistent Versus Inconsistent Causal Loops Physicist Paul Davies gives a good example of a consistent causal loop in his book "How to Build a Time Machine." A mathematics professor uses a time machine to travel forward in time, where he discovers a new theorem. He returns back to the time he came from and gives one of his particularly gifted students the idea for that theorem. The student goes on to publish the theorem, and it turns out that it was this very student's work that the professor perused during his journey to the future. The narrative here is consistent. On the other hand, with the grandfather paradox, a time traveler goes back in time and kills his grandfather. But if the time traveler's grandfather dies before the time traveler is born, how can he or she exist at all? And if the time traveler doesn't exist, how could he or she travel back in time to kill granddad?

The potential effects seem equally limitless in terms of the less likely possibility of time travel into the past. History books would no longer be based solely on exhaustive research and interpretation of ancient materials. Time travelers could resolve historical debates and verify how things did or didn't happen in the past. Imagine how different our understanding of the world might be if we could say definitively, for example, whether Moses actually parted the Red Sea or whether Lee Harvey Oswald acted alone in killing John F. Kennedy. A journey into the past could prove or disprove religious beliefs or result in face-to-face encounters with people such as Jesus, Buddha, Napoleon or Cleopatra -- or even the time traveler's former self. Perhaps time travelers could even bring back from the past things that had been lost, such as extinct species or dead and long-forgotten languages.

But here it's very important to raise the issue of self-consistent narratives and paradoxes. The concept of self-consistent narratives tells us that anything a time traveler would alter or affect in the past would have to remain consistent with the future from which he or she journeyed. Changing the past would effectively change the future, creating a causal loop. But such causal loops would only pose inherent problems if changes to the past resulted in a future different from the one the time traveler came from.

But perhaps the question of how time travel would affect life as we know it goes deeper than even a discussion of potential paradoxes and causal loops. Perhaps a discussion of specific effects of consequences on life as we know it makes little sense when faced with something that could change everything about the way in which we perceive our world.

Time Travel Turned Total Mayhem

As physicist Paul Davies describes it, unrestricted time travel -- meaning time travel that could form a complete loop to both the past and future -- would ultimately lead to total mayhem. In his words, "Time travel opens a view of the world that is a sort of madhouse where the rational order of things would no longer work. Under those circumstances, it's very hard to see how ordinary human life could continue."

In a world where the relationship between past, present and future is turned on its head, we would transcend the things that define our lives today. We would lose our notion of how time works, which could be so fundamentally damaging to our worldview that we would no longer care as much about the things that matter to us today: work, finances, making plans with friends and family, shopping -- you name it. These things just wouldn't be relevant in this crazy new world because we'd have a newfound preoccupation with simply making sense of a world without a set chronology -- we wouldn't know the order in which things occur.

It may be beside the point, then, to talk about resolving historical debates, saving endangered species or gaining technological, financial or military insight because those things might very likely fall by the wayside in the strange world that would follow the advent of unrestricted time travel.

As Davies makes clear, none of this fallout would occur from one-way travel. Hitching a one-way ride to the future or even the past (assuming we stick with self-consistent narratives) wouldn't cause this kind of profound reordering of the world as we currently experience it. But closing that loop of travel could be, in a word, disastrous.

Davies points out that science fiction normally focuses on the novelty aspect of time travel. But according to him, "It's not a novelty or a curiosity, it's something that strikes at the very rational basis of how we live and function. It's really hard to imagine that anything could be the same again." In his view, unrestricted time travel could change life as we know it so dramatically that we wouldn't even recognize it. Because chronology would have no meaning, we couldn't easily tell if something happened before or after, was a cause or an effect, and we would lose the ability to predict rationally the outcomes of our actions. In essence, it would be as though we had all gone insane.

These sobering potential effects of time travel have caused some scientists to wonder whether a principle exists in nature that would actually prevent unrestricted time travel, such as Stephen Hawking's "chronology protection hypothesis." This type of "theory of everything" might provide a scientific explanation as to why we could never unhinge the universe as we know it by making unrestricted time travel a reality. Scientists have yet to discover such a theory, but hearing Davies' take on the frightening effects of time travel makes one hope that they find it soon -- even if it means that we won't ever know for sure who killed JFK.

in future· time travelling

How would time travel affect life as we know it?

Peter Macdiarmid/Getty ImagesIn the BBC TV series "Dr. Who," the Tardis -- the Doctor's machine for traveling through time and space -- is disguised as a blue police box. As a Time Lord, The Doctor lives outside of time -- his life has no clearly definable past, present and future.

Science fiction has thoroughly covered the topic of time travel, starting with H.G. Wells' "The Time Machine" in 1895 and continuing right up to modern movies like "Déjà Vu" starring Denzel Washington. But physicists have also explored the nature of time and the plausibility of time travel for more than century, beginning with Albert Einstein's theories of relativity. Thanks to Einstein, scientists know that time slows as moving objects approach the speed of light. Gravity also slows time. This means that, in one sense, all of us can already consider ourselves time travelers in a limited way because we experience a tiny time warp (a difference of only nanoseconds) when we, for example, take a flight on an airplane. But physicists who study time travel today search for plausible ways to create a time warp large enough to allow noticeable travel into the past or future.


In his book "How to Build a Time Machine," physicist Paul Davies writes, "The theory of relativity implies that a limited form of time travel is certainly possible, while unrestricted time travel -- to any epoch, past or future -- might just be possible, too." This astonishing statement begs an important question: If time travel did indeed become a reality, how would it affect our world as we currently experience it?

First, it's important to realize that building a time machine would likely involve enormous expense, and the sheer complexity of such an apparatus would mean only a limited group of time travelers would have access to it. But even a small group of "astronauts" traveling through time and space could conceivably have a tremendous impact on life as we know it today. The possibilities, in fact, seem almost infinite.

Let's begin by assuming that it's possible to create a complete loop in time travel -- that time travelers could travel back into the past and then return to the future (or vice versa). Although scientists view traveling to the future as a much less problematic proposition than traveling to the past, our daily lives wouldn't change much if we could only send time travelers backward or forward in time, unable to recall them to the present. If we could, in fact, complete this loop of time travel, we can conjure up an incredible array of possible effects

Possibilities and Paradoxes of Time Travel

Imagine sending a time traveling astronaut 100 years into the future. The time traveler could witness technological advancements that we can only dream of today, much as people at the turn of the 20th century likely couldn't imagine the items we take for granted in 2010, such as iPods or laptop computers. The time traveler could also gain insight into medical advancements, such as new medicines, treatments and surgical techniques. If the time traveler could bring this knowledge backward in time to the present, the time from which he or she came, society could effectively leap forward in terms of its technical and scientific knowledge.

The futuristic time traveler could also bring back knowledge of what lay ahead for the world. He or she could warn of natural disasters, geopolitical conflicts, epidemics and other events of worldwide importance. This knowledge could potentially change the very way we operate. For example, what if a time traveler journeyed into the future and literally saw the effects that automobiles would eventually have on our planet? What if the time traveler witnessed an environment so polluted and damaged that it's unrecognizable? How might that change our willingness to use alternative forms of transportation?

Imagine that time travel became less restricted and more available to a larger population. Perhaps travel into the future would be exploited for personal gain. A futuristic time traveler could draw on knowledge of the stock market to guide his or her investment decisions, effectively using the granddaddy of all insider information to amass a fortune. Militaries might rely on time travel to gain valuable knowledge about the enemy's positioning and resources in future battles. Terrorists could use time travel to scout out the scenes of future attacks, allowing them to carefully plan with precise knowledge of future conditions.

Consistent Versus Inconsistent Causal Loops Physicist Paul Davies gives a good example of a consistent causal loop in his book "How to Build a Time Machine." A mathematics professor uses a time machine to travel forward in time, where he discovers a new theorem. He returns back to the time he came from and gives one of his particularly gifted students the idea for that theorem. The student goes on to publish the theorem, and it turns out that it was this very student's work that the professor perused during his journey to the future. The narrative here is consistent. On the other hand, with the grandfather paradox, a time traveler goes back in time and kills his grandfather. But if the time traveler's grandfather dies before the time traveler is born, how can he or she exist at all? And if the time traveler doesn't exist, how could he or she travel back in time to kill granddad?

The potential effects seem equally limitless in terms of the less likely possibility of time travel into the past. History books would no longer be based solely on exhaustive research and interpretation of ancient materials. Time travelers could resolve historical debates and verify how things did or didn't happen in the past. Imagine how different our understanding of the world might be if we could say definitively, for example, whether Moses actually parted the Red Sea or whether Lee Harvey Oswald acted alone in killing John F. Kennedy. A journey into the past could prove or disprove religious beliefs or result in face-to-face encounters with people such as Jesus, Buddha, Napoleon or Cleopatra -- or even the time traveler's former self. Perhaps time travelers could even bring back from the past things that had been lost, such as extinct species or dead and long-forgotten languages.

But here it's very important to raise the issue of self-consistent narratives and paradoxes. The concept of self-consistent narratives tells us that anything a time traveler would alter or affect in the past would have to remain consistent with the future from which he or she journeyed. Changing the past would effectively change the future, creating a causal loop. But such causal loops would only pose inherent problems if changes to the past resulted in a future different from the one the time traveler came from.

But perhaps the question of how time travel would affect life as we know it goes deeper than even a discussion of potential paradoxes and causal loops. Perhaps a discussion of specific effects of consequences on life as we know it makes little sense when faced with something that could change everything about the way in which we perceive our world.

Time Travel Turned Total Mayhem

As physicist Paul Davies describes it, unrestricted time travel -- meaning time travel that could form a complete loop to both the past and future -- would ultimately lead to total mayhem. In his words, "Time travel opens a view of the world that is a sort of madhouse where the rational order of things would no longer work. Under those circumstances, it's very hard to see how ordinary human life could continue."

In a world where the relationship between past, present and future is turned on its head, we would transcend the things that define our lives today. We would lose our notion of how time works, which could be so fundamentally damaging to our worldview that we would no longer care as much about the things that matter to us today: work, finances, making plans with friends and family, shopping -- you name it. These things just wouldn't be relevant in this crazy new world because we'd have a newfound preoccupation with simply making sense of a world without a set chronology -- we wouldn't know the order in which things occur.

It may be beside the point, then, to talk about resolving historical debates, saving endangered species or gaining technological, financial or military insight because those things might very likely fall by the wayside in the strange world that would follow the advent of unrestricted time travel.

As Davies makes clear, none of this fallout would occur from one-way travel. Hitching a one-way ride to the future or even the past (assuming we stick with self-consistent narratives) wouldn't cause this kind of profound reordering of the world as we currently experience it. But closing that loop of travel could be, in a word, disastrous.

Davies points out that science fiction normally focuses on the novelty aspect of time travel. But according to him, "It's not a novelty or a curiosity, it's something that strikes at the very rational basis of how we live and function. It's really hard to imagine that anything could be the same again." In his view, unrestricted time travel could change life as we know it so dramatically that we wouldn't even recognize it. Because chronology would have no meaning, we couldn't easily tell if something happened before or after, was a cause or an effect, and we would lose the ability to predict rationally the outcomes of our actions. In essence, it would be as though we had all gone insane.

These sobering potential effects of time travel have caused some scientists to wonder whether a principle exists in nature that would actually prevent unrestricted time travel, such as Stephen Hawking's "chronology protection hypothesis." This type of "theory of everything" might provide a scientific explanation as to why we could never unhinge the universe as we know it by making unrestricted time travel a reality. Scientists have yet to discover such a theory, but hearing Davies' take on the frightening effects of time travel makes one hope that they find it soon -- even if it means that we won't ever know for sure who killed JFK.

What if you fell into a black hole?

When Alice falls into the rabbit hole, she experiences a host of "curiouser and curiouser" phenomena. She gets smaller, she gets bigger, she almost drowns in a sea of her own tears and she meets an assembly of strange creatures. As it turns out, falling into a black hole may rival the strange experiences described by Lewis Carroll in his classic fantasy. Unlike Alice, however, who emerges from her adventures no worse for the wear, a person who journeys into one of the universe's most mysterious objects is not likely to fare so well. To understand why, it will help to define what a black hole is and how it interacts with the universe around it.

The Unfolding Universe: Supermassive Black Holes

Astronomers have only observed black holes indirectly, yet they can paint a clear picture, albeit one based on circumstantial evidence. They feel certain, for example, that black holes indicate regions where matter has been compressed into an infinite density. The gravity of such a region is so strong that nothing can escape its inexorable pull, not even light. As matter and light fall into the superdense region, X-rays and other forms of electromagnetic energy get blasted into space. It's this cosmic ejecta that alerts astronomers to the existence of these mysterious monsters.
Like monsters we can see, black holes come in different sizes. Astronomers measure this using something known as the Schwarzschild radius. That radius describes the size of the event horizon, the spherical boundary of a black hole. The greater the object's mass, the larger its event horizon and the larger its radius. Regardless of how massive it is, a black hole's center point is what astronomers call a singularity -- a place where matter is infinitely dense.
Of course, you'll have to find one of these objects if you're going to fall into it. We'll search for one next.

 

 

Back in Black

Astronomers now believe black holes lie at the centers of most galaxies. They classify these beasts as supermassive black holes because each one may have a maximum mass equivalent to billions of suns. Stellar black holes are the remains of dying stars that have collapsed upon themselves. They're smaller than their supermassive cousins -- the smallest is about 12 miles (19 kilometers) across -- and may be quite common in any galaxy [source: Wethington]. The closest stellar black hole to our solar system is Cygnus X-1, which is about 6,000 light-years away.

This illustration shows a galaxy with a supermassive black hole at its core. That galactic bad boy at the center is shooting out jets of radio waves.
NASA/JPL-Caltech

Let's say you go to your travel agent and book a one-way flight to an exotic, five-star black hole. You check your luggage, board the rocket and enjoy the ride, trying hard to ignore the screaming kid in the seat behind you. What you see as you approach your final destination depends on what's around the black hole. A companion star located nearby could have its light and gases stripped away and drawn into the neighboring gravity well. Black holes in this situation will often be encircled by an accretion disk, a ring of hot, luminous gas with transitory bright spots similar to solar flares. Even if a black hole resides by itself in a dark corner of the universe, it gives off strong bursts of radiation that your ship's sensors will be able to detect. The bottom line: You'll know exactly when you're getting close.
The captain knows, too, so he or she stops the spacecraft just beyond the black hole's gravitational pull. You'll make the rest of the trip using the jet packs mounted on your spacesuit. You don the suit, exit the vessel and start zipping toward your destiny. Eventually, the black hole's gravity seizes you and begins to reel you in. As you get closer, you orient yourself so you're falling feet-first toward the event horizon.
Brace yourself. On the next, things get really interesting on our black hole journey.

Falling to Pieces

Here's where it starts to get bad. Tidal forces -- so named because similar gravitational forces between the moon and the Earth cause ocean tides -- increase dramatically as the distance between you and the black hole shrinks. This means the gravity acting on your feet is much stronger than the gravity acting on your head. As a result, your feet begin to accelerate much faster than your head. Your body stretches out, not uncomfortably at first, but over time, the stretching will become more severe. Astronomers call this spaghettification because the intense gravitational field pulls you into a long, thin piece of spaghetti.

{Yeah, spaghetti is all fun and games until you're the spaghetti.
Ableimages/Lifesize/Thinkstock}

When you start feeling pain depends on the size of the black hole. If you're falling into a supermassive black hole, you'll begin to notice the tidal forces within about 600,000 kilometers (372,822 miles) of the center -- after you've already crossed the event horizon [source: Bunn]. If you're falling into a stellar black hole, you'll start feeling uncomfortable within 6,000 kilometers (3,728 miles) of the center, long before you cross the horizon [source: Bunn].
Either way, spaghettification leads to a painful conclusion. When the tidal forces exceed the elastic limits of your body, you'll snap apart at the weakest point, probably just above the hips. You'll see your lower half floating next to you, and you'll see it begin to stretch anew as tidal forces latch onto it. The same thing happens to your torso, of course, until each half snaps a second time. In a matter of seconds, you're a goner, reduced to a string of disconnected atoms that march into the black hole's singularity like ants disappearing into a colony.
It's not a great way to die, but there's one consolation: In space, no one can hear you scream.

Sources

• Broderick, Avery E. and Abraham Loeb. "Portrait of a Black Hole." Scientific American. December 2009.
• Bunn, Ted. "What would happen to me if I fell into a black hole?" Berkeley Cosmology. September 1995. (July 20, 2011) http://cosmology.berkeley.edu/Education/BHfaq.html#q3
• Cain, Fraser. "Maximizing Survival Time Inside the Event Horizon of a Black Hole." Universe Today. May 10, 2007. (July 20, 2011) http://www.universetoday.com/1605/maximizing-survival-time-inside-the-event-horizon-of-a-black-hole/
• HubbleSite Special Feature. "Black Holes: Gravity's Relentless Pull." 2005. (July 20, 2011) http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q16.html
• Malone, Noreen. "What happens if you fall into a black hole?" Slate. (July 20, 2011) http://www.slate.com/id/2199664/
• Plait, Phil. "Tyson spaghettifies!" Bad Astronomy. Jan. 13, 2009. (July 20, 2011) http://blogs.discovermagazine.com/badastronomy/2009/01/13/tyson-spaghettifies/
• Rees, Martin. “Universe: The Definitive Visual Guide.” Dorling Kindersley Limited. 2008.
• Wethington, Nicholos. "Black Hole on Earth." Universe Today. Aug. 29, 2009. (July 20, 2011) http://www.universetoday.com/38434/black-hole-on-earth/

 

 

 

 

 

 

 

 

 

 

 

 

black hole· what is balck hole

What if you fell into a black hole?

What if you fell into a black hole?

When Alice falls into the rabbit hole, she experiences a host of "curiouser and curiouser" phenomena. She gets smaller, she gets bigger, she almost drowns in a sea of her own tears and she meets an assembly of strange creatures. As it turns out, falling into a black hole may rival the strange experiences described by Lewis Carroll in his classic fantasy. Unlike Alice, however, who emerges from her adventures no worse for the wear, a person who journeys into one of the universe's most mysterious objects is not likely to fare so well. To understand why, it will help to define what a black hole is and how it interacts with the universe around it.

The Unfolding Universe: Supermassive Black Holes

Astronomers have only observed black holes indirectly, yet they can paint a clear picture, albeit one based on circumstantial evidence. They feel certain, for example, that black holes indicate regions where matter has been compressed into an infinite density. The gravity of such a region is so strong that nothing can escape its inexorable pull, not even light. As matter and light fall into the superdense region, X-rays and other forms of electromagnetic energy get blasted into space. It's this cosmic ejecta that alerts astronomers to the existence of these mysterious monsters.
Like monsters we can see, black holes come in different sizes. Astronomers measure this using something known as the Schwarzschild radius. That radius describes the size of the event horizon, the spherical boundary of a black hole. The greater the object's mass, the larger its event horizon and the larger its radius. Regardless of how massive it is, a black hole's center point is what astronomers call a singularity -- a place where matter is infinitely dense.
Of course, you'll have to find one of these objects if you're going to fall into it. We'll search for one next.

 

 

Back in Black

Astronomers now believe black holes lie at the centers of most galaxies. They classify these beasts as supermassive black holes because each one may have a maximum mass equivalent to billions of suns. Stellar black holes are the remains of dying stars that have collapsed upon themselves. They're smaller than their supermassive cousins -- the smallest is about 12 miles (19 kilometers) across -- and may be quite common in any galaxy [source: Wethington]. The closest stellar black hole to our solar system is Cygnus X-1, which is about 6,000 light-years away.

This illustration shows a galaxy with a supermassive black hole at its core. That galactic bad boy at the center is shooting out jets of radio waves.
NASA/JPL-Caltech

Let's say you go to your travel agent and book a one-way flight to an exotic, five-star black hole. You check your luggage, board the rocket and enjoy the ride, trying hard to ignore the screaming kid in the seat behind you. What you see as you approach your final destination depends on what's around the black hole. A companion star located nearby could have its light and gases stripped away and drawn into the neighboring gravity well. Black holes in this situation will often be encircled by an accretion disk, a ring of hot, luminous gas with transitory bright spots similar to solar flares. Even if a black hole resides by itself in a dark corner of the universe, it gives off strong bursts of radiation that your ship's sensors will be able to detect. The bottom line: You'll know exactly when you're getting close.
The captain knows, too, so he or she stops the spacecraft just beyond the black hole's gravitational pull. You'll make the rest of the trip using the jet packs mounted on your spacesuit. You don the suit, exit the vessel and start zipping toward your destiny. Eventually, the black hole's gravity seizes you and begins to reel you in. As you get closer, you orient yourself so you're falling feet-first toward the event horizon.
Brace yourself. On the next, things get really interesting on our black hole journey.

Falling to Pieces

Here's where it starts to get bad. Tidal forces -- so named because similar gravitational forces between the moon and the Earth cause ocean tides -- increase dramatically as the distance between you and the black hole shrinks. This means the gravity acting on your feet is much stronger than the gravity acting on your head. As a result, your feet begin to accelerate much faster than your head. Your body stretches out, not uncomfortably at first, but over time, the stretching will become more severe. Astronomers call this spaghettification because the intense gravitational field pulls you into a long, thin piece of spaghetti.

{Yeah, spaghetti is all fun and games until you're the spaghetti.
Ableimages/Lifesize/Thinkstock}

When you start feeling pain depends on the size of the black hole. If you're falling into a supermassive black hole, you'll begin to notice the tidal forces within about 600,000 kilometers (372,822 miles) of the center -- after you've already crossed the event horizon [source: Bunn]. If you're falling into a stellar black hole, you'll start feeling uncomfortable within 6,000 kilometers (3,728 miles) of the center, long before you cross the horizon [source: Bunn].
Either way, spaghettification leads to a painful conclusion. When the tidal forces exceed the elastic limits of your body, you'll snap apart at the weakest point, probably just above the hips. You'll see your lower half floating next to you, and you'll see it begin to stretch anew as tidal forces latch onto it. The same thing happens to your torso, of course, until each half snaps a second time. In a matter of seconds, you're a goner, reduced to a string of disconnected atoms that march into the black hole's singularity like ants disappearing into a colony.
It's not a great way to die, but there's one consolation: In space, no one can hear you scream.

Sources

• Broderick, Avery E. and Abraham Loeb. "Portrait of a Black Hole." Scientific American. December 2009.
• Bunn, Ted. "What would happen to me if I fell into a black hole?" Berkeley Cosmology. September 1995. (July 20, 2011) http://cosmology.berkeley.edu/Education/BHfaq.html#q3
• Cain, Fraser. "Maximizing Survival Time Inside the Event Horizon of a Black Hole." Universe Today. May 10, 2007. (July 20, 2011) http://www.universetoday.com/1605/maximizing-survival-time-inside-the-event-horizon-of-a-black-hole/
• HubbleSite Special Feature. "Black Holes: Gravity's Relentless Pull." 2005. (July 20, 2011) http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q16.html
• Malone, Noreen. "What happens if you fall into a black hole?" Slate. (July 20, 2011) http://www.slate.com/id/2199664/
• Plait, Phil. "Tyson spaghettifies!" Bad Astronomy. Jan. 13, 2009. (July 20, 2011) http://blogs.discovermagazine.com/badastronomy/2009/01/13/tyson-spaghettifies/
• Rees, Martin. “Universe: The Definitive Visual Guide.” Dorling Kindersley Limited. 2008.
• Wethington, Nicholos. "Black Hole on Earth." Universe Today. Aug. 29, 2009. (July 20, 2011) http://www.universetoday.com/38434/black-hole-on-earth/