Life on Earth is based on long chains of carbon molecules.
In this final section we will address the questions of whether of not there
is likely to be life elsewhere in the Universe, and how we might discover it if
it did exist.
It is important to distinguish between life, which is the biochemical process, and intelligent life, which is something that we could potentially interact with.
As of now, we have found no evidence that life currently exists anywhere.
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Astronomers do not take reports of UFOs seriously.
Lights in the sky almost always turnout to be Venus, or weather balloons.
The scientist always feels the need to go after the simplest explanation, whereas many people prefer the mystery.
Much more likely to get onto TV if you say you were seduced by an alien than if you say you saw the lights of a police helicopter.
There are also outright lies, ignorance and deception, especially by conspiracy theorists
Reports of little green men are not taken seriously by scientists: why would they behave in the way that they do? And why do they have two eyes, two ears and a mouth?
Remember Carl Sagan's dictum: "Extraordinary claims require extraordinary evidence"
A crucial feature of life is the ability to reproduce itself.
The variety of life functions is directly related to the very large number of different ways carbon atoms can combine together. The information is stored in DNA molecules in a code which was broken in the 1950s.
Life on Earth is based on long chains of carbon molecules.
C-C-C-C-C-C C-O-C=N-H etc with other atoms attached to them .
The chemistry of carbon is much richer than that of any other element.
We have established that the elements are the same elsewhere in the Universe, so the laws of chemistry will be the same as well. We therefore suspect that life elsewhere will be based on carbon, but we should not discount the possibility that there is some other form of life. Silicon chemistry is not out of the question, and there is the interesting question of whether things like computer "viruses" could actually be a form of life.
The important thing about carbon chemistry is that it needs an intermediate temperature range to flourish. And water helps a great deal to. We also need a source of energy to foster the chemical reactions. For these reasons we think that planets or moons are the most likely places for life to develop
How did life start?
Some people dodge the question and say it was God. The scientific approach is to see if we can come up with an explanation that does not require such a large adjustment to the laws of nature.
As science progresses more and more of life's processes can be explained by chemical reactions. It is natural to explore the possibility that life began this way as well.
Experiments have been done to simulate the early conditions in the Earth's atmosphere. We are not too certain about the early conditions, but as believe that the young Earth's surface was hotter because of increased volcanoes and increased heat from the Sun. There was also a lot of lightning and less oxygen.
Chemistry experiments done by Miller and Urey in the lab used a mixture of
methane,. water, ammonia, heat and electrical discharges in a flask for several
weeks. After that time they found a mixture of chemicals including amino acids,
which are the building blocks of proteins. But they did not find a
self-replicating system.
There is disagreement about significance of experiments. Some people take the presence of amino acids as a sign that life might have evolved given more time (A billion years or so?). Others say that it would took another enormous step to get those simple building blocks in to a sequence that can reproduce itself. Our present genetic system using DNA is very sophisticated and we have not yet come up with a theory of how it evolved. Maybe there was an earlier system that came into being more easily which then evolved into our present one.
If we look at the geological record we estimate that life began on Earth in
the form of very simple organisms about 3-4 billion years ago - when the Earth
was a lot younger than it was now. (Earth formed 4.6 billion years ago) Early
life was very primitive and all in the ocean.. Life on land is only about 400
million years old (One tenth of the Earth's history). Human history only 1-2
million years, civilization a few thousand years old.
Our ability to think about communicating is perhaps 100 years old (rockets, radio)
Main point is that there has been unintelligent life for a long time, but intelligent life for a very short time.
If we look on planets in the Solar System there is not much hope of finding life there. Life was looked for on Mars in 1976 when a lander craft analyzed the soil for signs of life-like chemistry. None seen, though the current explorers are still looking for signs that there might have been life in the past..
Venus is another possibility, but its high temperature makes it inhospitable
Europa (moon of Jupiter) is now considered the most likely possibility because it may have liquid water under its icy surface.
Everywhere else in the solar system is very unlikely to harbor life.
Does not mean life cannot exist, only that life like ours cannot.
We have said that our best bet for life is on planets, but planets around other stars are very hard to find.
What we can do is to make estimates of the number of planets that might exist in the Galaxy and make estimates of how many might spawn life, but the answers are uncertain by factors of millions. Some astronomers are "optimistic" that life may be be relatively common, but others argue that the chances are so low that we are probably the only life form in the Galaxy.
At the moment we simply do not know enough to be able to estimate the probability of life at all accurately.
Contact with another civilization by direct space travel is very unlikely
Farthest distance humans have traveled is 400,000 km, to the Moon
Farthest distance a robotic spacecraft has gone is 12 billion km, to leave the solar system. Took over 20 years
It is 10,000 times farther to nearest star.
However the Voyager spacecraft carried messages to a potential discoverer.
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Watch the movie "Contact"; one of the best recent science fiction movies.
Electromagnetic waves are the fastest possible way of communicating, so it is far more likely that contact will be made by radio than by physical travel.
Of all types of electromagnetic waves, radio waves are best for traveling through the galaxy for technical reasons.
Although we have only had radio technology for 100 years we are already capable of sending and transmitting information between the Earth and other nearby stars.
Messages have been sent into space, towards nearby stars, in case they have an inhabitable planet.
There are modest SETI (Search for Extra-Terrestrial Intelligence) under way using radio telescopes
Also a SETI institute in California that is the focus for scientific searches for extraterrestrial life
You can also take part in this search
SETI@home project
What are we looking for:?
Maybe the sounds of civilization: TV, radio etc
You could learn a lot about the Earth just from its random signals. Doppler shifts would give you periods of year and day, and you could even make a rough map of the Earth from the locations of transmitters as they move around the Earth.
Or maybe definite signals that are being sent to Earth.
Why would anyone send us signals?
In the last 60 years we have started to give out strong radio waves from the Earth. (Radars, TV transmissions). These signals have reached 60 light years into space, and passed hundreds of stars. If anyone is watching we may have been noticed. If our signals were seen by a star 10 light years away, we could get a signal back 20 years after we sent it.
Right now any star within 30 light years could have picked up our signals and sent us a reply.
So one plan is to watch the nearby stars within our "information bubble" The number of these increases every year.
If we get a message, should we answer?
The problem is that if aliens exist they are probably very very much more advanced than us.
The bare bones history of earth is, very simply: Life for 4,000,000,000 years , civilization for 4,000 years
Suppose there were two other planets where life evolved almost exactly the same way as Earth, and formed at the same time.
Suppose Earth-plus evolved 10% faster than us, and Earth-minus evolved 10% slower than us
Then Earth-minus would be 400 million years behind us, and Earth-plus would be 400 million years ahead.
We would not detect Earth-minus, but Earth-plus would have had civilization for 400,000,000 years - a hundred thousand times longer than us. There would be no contest: we would lose!
And this is life that is only 10% different from us?
So much for 3-foot high aliens with mouths and ears!
Some people have extended this argument to say that if aliens exist some of them should have become so advanced that they should have populated the galaxy and solved problems of interstellar space travel.
If they are so smart why haven't they colonized the galaxy.
Maybe they don't exist, Or maybe we are them!