by Alok Jha
But an organ to see with or limbs to move around with are probably both universal features. “Limbs have evolved independently in different creatures—an octopus has tentacles but they do the same job but with very different structure.” DNA is most likely a parochial feature of Earth, but the idea of evolution of species by natural selection is almost certainly a universal.
What about intelligence? This is not uncommon here on Earth and has also developed in lots of different ways. There are many intelligent creatures on this planet—octopuses, dolphins and whales are all bright. Even mantis shrimps are surprisingly good at solving puzzles if they have to get to their food.
But intelligence would not be enough for us to discover aliens living on another planet. For that we would need an ability for mass cooperation, the capability to develop technology. “What would give us the possibility of communicating with other planets is not intelligence as such, it’s our ability to store anybody’s bright ideas in a form that the rest of the culture can access and use,” says Stewart.
Humans today are individually no smarter than previous generations for a good many thousands of years, possibly hundreds of thousands of years. But collectively, our culture can achieve things that were inconceivable 100 years ago. Extelligence, as Stewart calls it, started with the invention of speech and writing, got going with printing, and is now running riot with the Internet.
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SETI ESTIMATES
10,000 detectable civilizations in our galaxy contact in the next 30 years
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Once a species is extelligent, lots of things become possible—they might transcend biology. “If you’re going to talk about intelligent life, for me the important thing is timescales,” says Seth Shostak, a senior astronomer at SETI. Humans went from using radio waves for communication to launching spacecraft in less than 70 years; in 100 years we might have cracked artificial intelligence. “Intelligent biology can engineer its own successor and can do it very quickly when you’re talking about interstellar communication and interstellar travel. You’ve already gone beyond the confines of a three-pound brain sitting in a skull on top of your neck.”
While aliens are so out of our experience, guessing motives and intentions if they ever got in touch seems to be, at best, a stab in the dark. The physicist Paul Davies has argued that alien brains, with their different architecture, would no doubt interpret information very differently to ours. What we think of as beautiful or friendly might come across as violent to them, or vice versa.
“Lots of people think that because they would be so wise and knowledgeable they would be peaceful,” says Stewart. “I don’t think you can assume that. I don’t think you can put human views onto them and that’s a dangerous way of thinking. Aliens are alien. If they exist at all, we cannot assume they’re like us.”
What are the chances?
The father of SETI, Frank Drake, reckons that our detection of the first extraterrestrial signal might only be 30 years away, thanks to increasing computing power and the ability to sift more data from more stars ever more quickly. Speaking to Scientific American in 2011, he guessed that the number of detectable civilizations in our galaxy right now was about 10,000. It is just a matter of time before we stumble across one.
Future generations of ground-based telescopes will also help in the search, such as the proposed European Extremely Large Telescope (with a 30-meter main mirror). This could be operational by 2030, and would be powerful enough to image the atmospheres of faraway planets, looking for chemical signatures that could indicate life.
The SETI Institute is also getting an upgrade, with the continued construction of the Allen Telescope Array. When it has its full complement of 300 dishes scanning the skies, it could examine 1,000 star systems in a couple of years. Shostak is confident that as telescope technology keeps improving, SETI will find an ET signal within the next two decades. “The bottom line to that is that we will have looked at another million star systems in two dozen years. If this is going to work, it will work soon.”
Is he worried about Hawking’s warning about hostile aliens? “This is an unwarranted fear,” he says. “If they’re interested in resources, they have ways of finding rocky planets that don’t depend on whether we broadcast or not. They could have found us a billion years ago.”
Anyway, if we are genuinely worried about shouting in the stellar jungle, he says, the first thing to do is shut down the BBC, NBC, CBS and all the radars at airports. Those broadcasts have been streaming into space for years, and the oldest is already more than 80 light years from Earth. If we are worried about attracting the attention of passing aliens, we’re too late to have stopped them watching every episode of Big Brother or The Jerry Springer Show. It’s hard to guess whether those broadcasts would attract or repel an alien race.
Death of the Sun
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The Sun hangs in the sky, the source of almost all our energy. It lights up our planet, gives us life, allows plants to thrive and makes the world go around. But it will also, one day, blow the Earth apart.
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We are in a lucky mid-phase of the Sun’s life, a time when its pleasant yellowness is just warm enough to keep our oceans liquid and there is the right amount of radiation to keep plants and animals alive.
In 5 billion years from now, the Sun will emerge from its pleasant middle age and start to get bigger. And hotter. It will eject material, swallow up Mercury and Venus and cause untold damage as it begins its slide toward death. On the Earth, we will have a ringside seat to the destruction, and long after all life has been killed, the remnants of our planet will get involved in the show itself.
The life cycle of stars
Before the Sun, there was a vast cloud of swirling dust and gas. Over millions of years, the hydrogen atoms began to move closer together under the force of gravity and the cloud started to warm up. As it reached 10 million kelvins, the hydrogen began to fuse and the star began to shine. Our Sun began its life fusing hydrogen, and this is how it will spend 90 percent of its total existence. At present, it is around halfway through the 10 billion years during which a star of its size will normally burn hydrogen.
In around 5 billion years, the Sun will run out of hydrogen to burn, because all of its core will be fused into helium. At this point, the core of the star cannot support itself and it begins to collapse. As it shrinks, it also heats up, and eventually a shell around the shrinking core becomes hot enough to start fusing the hydrogen atoms there. This process will cause the outer layers of the Sun to expand to several tens of times its current diameter. Because the surface at this stage is so far away from the source of the energy, it will cool to around 3,500 kelvins, around half the temperature of the star today, and glow red.
Meanwhile, the helium core continues to contract, and when it reaches around 100 million kelvins, the atoms begin to fuse into carbon and oxygen. The outer surface of the star will become hotter once again, glowing blue and white.
When the helium runs out and the core is entirely composed of carbon and oxygen, the star will once again start to contract and warm up. A shell of helium outside the core starts to fuse and the star once again swells to hundreds of times its current diameter. At this stage it is called a “symptotic giant branch” star. After around 30 million years, the remnant core will once again start to shrink and heat up.
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On the Earth, we will have a ringside seat to the destruction, and long after all life has been killed, the remnants of our planet will get involved in the show itself.
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Stars follow different lives depending on their initial mass. The biggest stars will burn brightest, for the shortest time and eventually collapse into black holes. Smaller stars, such as our Sun, will end as tiny, super-dense white dwarfs.
But a star that starts out the size of the Sun will never get hot enough at its core to start fusing carbon atoms or oxygen atoms there. Instead, the outer layers of the star b
egin to cool and reach temperatures where electrons can bind themselves to the free-floating atomic nuclei, forming neutral atoms. The outer envelope of the star continues to blow off into space within a relatively sudden 100,000 years, and this halo will contain the raw materials for the formation of planets in future, including carbon, oxygen, neon, sulfur, sodium, argon and chlorine.
These remnants of low-mass stars, called planetary nebulae, are some of the most beautiful objects seen in space, as the hot core of the star lights up the surrounding gas clouds, producing vivid fluorescent colors against the blackness of space. Scientists have been moved to give them names, such as the Cat’s Eye, the Starfish Twins, Blue Snowball, Eskimo, and the Ant, and they are among the most popular pictures taken by the Hubble space telescope.
The core of the star, meanwhile, will contract for another few thousand years until the electrons become degenerate, which means they cannot be pressed any further together. At this stage, the surface of the remnant has a temperature of around 10,000 kelvins and the object becomes a white dwarf. It contains over half the mass of the Sun packed into a hot (1 million kelvin) ball the size of the Earth. A teaspoon of this object would weigh a ton on Earth.
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There is virtually no chance that anything living here will survive. If, somehow, something managed to cling on, it would be faced with a cold, dark, impoverished world without any water. It could not survive long.
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Slowly, over billions of years, this white dwarf radiates away energy and cools down further. Finally, when it can shine no longer, all that will be left of the Sun is a lump of ash known as a black dwarf.
What happens to the Earth?
The vast changes in store for the Sun do not bode well for our planet. Even if the Earth manages to survive the first expansion of our star, there is virtually no chance that anything living here will survive. If, somehow, something managed to cling on, it would be faced with a cold, dark, impoverished world without any water. It could not survive long.
“Planetary nebulae are a glimpse into the future of our own solar system,” says Bruce Balick, an astronomer at the University of Washington. “When the Sun reaches the eleventh hour of its life, it will swell to the size of Earth’s present orbit, causing Mercury and Venus to burn up like giant meteors. Earth will escape this fate because the Sun will have blown out some of its material, weakening its gravity so that our planet slips into a new, larger orbit.”
The burned-out husks of Mercury and Venus will find themselves orbiting the Sun from inside the red giant. On Earth, if we have survived, sunrises and sunsets will take hours, and at noon, the gigantic red Sun will take up half the sky. The oceans, along with the atmosphere, will boil away into space. “The view won’t be very different than that within a kiln,” says Balick. “The intense radiant heat will transform the surface to a thick layer of pottery.” In all, a biblical view of hell.
Balick does however see one up side to the rather compromised situation: the Earth would be able to witness from the inside the formation, millions of years after the red giant, of a planetary nebula. “The Sun will eject its outer layers in an extreme version of the present-day solar wind,” he says. “Eventually the red behemoth will be stripped to its core, which will quickly settle down as a white dwarf star. Lit by this blue-hot pinprick, objects on Earth will cast sharp-edged, pitch-black shadows; sunrise and sunset will take no longer than an eyeblink. Exposed rock will turn to plasma as ultraviolet radiation from the dwarf destroys all molecular bonds, coating the surface with an eerie iridescent fog, constantly lifting and swirling. As the dwarf radiates away its energy, it will fade into a cold, dark cinder. Thus, our world will end first in fire, then in ice.”
The white dwarf would rise in the Earth’s sky as an intense spot of light, 100 times brighter than the Sun is today but no bigger than Venus. The light from this star would bake the Earth’s rocks and rip apart the molecules on the surface, creating a new atmosphere of free electrons.
What can we do?
It seems there is little we humans can do in the face of such stellar inevitability. In 2001, however, scientists at the University of Michigan came up with an idea for a global-scale project that could shift the Earth’s orbit by small amounts by engineering close passes to asteroids and comets. The gravitational attraction of these bodies would pull the Earth into an incrementally higher orbit around the Sun. Over billions of years, this might be far enough outside today’s orbit to save us from the worst ravages of the Sun’s red giant phase. It’s a dangerous idea, though, because it would take thousands if not millions of close passes with big rocks in space to move our orbit. And engineers would have to make sure that none of these rocks ever hit the Earth, or else they would cause a different type of doomsday event (see p.137). If it somehow worked, it would only save the Earth for a short while, until the Sun eventually dimmed into nothingness.
Our best option, then, is to leave the Earth and colonize a planet far from here. We certainly have enough time to invent, test and build the required technology. There is also enough time to identify and reach the closest planets at sub-light speeds, though each trip might take several generations. It sounds far-fetched, perhaps, but it is one of the inevitable things about nature that stars, and therefore their planets, have a finite lifetime. Like it or not, we cannot stay on the Earth forever.
Galactic Collision
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There are some parts of our cosmic ballet that sound far too big to think about, things we just have to watch while resigning ourselves to their effects. The Sun expanding and engulfing the Earth is one event that will mark a turning point for our species. Another phenomenon that will happen at around the same time sounds on the surface far more violent: the wholesale collision of our galaxy with another.
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Look at the sky, and virtually all of the galaxies we can see are speeding away from us, an echo of the Big Bang that reverberates to this day in the expansion of space and the universe. All of them, that is, except the galaxy M31, which is moving toward us. At its current speed of 120 km/s (75 miles per second), M31 (also known as Andromeda) will reach the Milky Way some time in the next 5 billion years. If we have managed to live that long, this cosmic smash might well finish us off.
Our galactic neighborhood
The Milky Way is part of a loose collection of a few dozen galaxies, called the Local Group. From a human scale, calling it a collection might seem odd, given that the galaxies themselves are separated by up to 100 times their own diameters. But, bound and governed by gravity, these cosmic objects are jostled around their vast backdrop. The galaxies move slowly into and out of each other’s paths, and over the full lifetime of a typical galaxy—estimates range from 10 billion to 20 billion years—a collision is inevitable.
In our Local Group, the Milky Way and Andromeda are the biggest characters, roughly the same size as each other at around 270 billion times the mass of the Sun. Andromeda is the smaller cousin, a swirling mass of several billion stars that is currently 2.5 million light years away.
Until 20 years ago, astronomers had doubts about whether galaxies would ever collide, given the huge spaces between them. But a combination of improved observation and better computer models that can simulate galactic motions has revealed that collisions are not only possible, they are probably more common than anyone thought. According to astronomers Joshua Barnes, Lars Hernquist and Francois Schweizer, the available evidence suggests that colliding galaxies often merge into a new kind of object. “We have become increasingly convinced that such collisions control the evolution of many galaxies and lead to the formation of a variety of peculiar objects, possibly including the distant and extraordinarily luminous quasars.”
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The timescales over which the collisions will occur are unimaginably vast.
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How close are we to a collision? What would happen?
Even in the most violent encounter, it
is unlikely that the stars themselves will actually hit each other, given the vast spaces between them. In many large galactic clusters, pairs of galaxies are thought to come toward each other at high speeds—thousands of kilometers per second—and pass right through each other, apparently causing no damage in the process. “Oddly enough, if the same galaxies were to approach at a velocity of only a few hundred kilometers per second, they would violently disrupt one another and probably would merge within a few hundred million years,” say Barnes, Hernquist and Schweizer in an article for Scientific American. “Such seemingly paradoxical behavior reflects the fact that galactic interactions are governed by gravitational forces. The slower an encounter between two galaxies, the more time there is for gravity to produce huge, disruptive tides and the greater the resulting damage.”
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CURRENT SPEED OF ANDROMEDA
434,500 km/h
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But that does not mean the components of the galaxies feel nothing in the process. Just as the Earth’s oceans feel the attraction of the Moon in the creation of the tides, so the passing galaxies will create a gravitational tide as they pass each other.