Life in the Universe

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 Life in the Universe

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همسة براءة



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مُساهمةموضوع: Life in the Universe    2011-04-28, 20:56


In
this talk, I would like to speculate a little, on the development of
life in the universe, and in particular, the development of intelligent
life. I shall take this to include the human race, even though much of
its behaviour through out history, has been pretty stupid, and not
calculated to aid the survival of the species. Two questions I shall
discuss are, 'What is the probability of life existing else where in
the universe?' and, 'How may life develop in the future?'

It
is a matter of common experience, that things get more disordered and
chaotic with time. This observation can be elevated to the status of a
law, the so-called Second Law of Thermodynamics. This says that the
total amount of disorder, or entropy, in the universe, always increases
with time. However, the Law refers only to the total amount of
disorder. The order in one body can increase, provided that the amount
of disorder in its surroundings increases by a greater amount. This is
what happens in a living being. One can define Life to be an ordered
system that can sustain itself against the tendency to disorder, and
can reproduce itself. That is, it can make similar, but independent,
ordered systems. To do these things, the system must convert energy in
some ordered form, like food, sunlight, or electric power, into
disordered energy, in the form of heat. In
this way, the system can satisfy the requirement that the total amount
of disorder increases, while, at the same time, increasing the order in
itself and its offspring. A living being usually has two elements: a
set of instructions that tell the system how to sustain and reproduce
itself, and a mechanism to carry out the instructions. In biology,
these two parts are called genes and metabolism. But it is worth
emphasising that there need be nothing biological about them. For
example, a computer virus is a program that will make copies of itself
in the memory of a computer, and will transfer itself to other
computers. Thus it fits the definition of a living system, that I have
given. Like a biological virus, it is a rather degenerate form, because
it contains only instructions or genes, and doesn't have any metabolism
of its own. Instead, it reprograms the metabolism of the host computer,
or cell. Some people have questioned whether viruses should count as
life, because they are parasites, and can not exist independently of
their hosts. But then most forms of life, ourselves included, are
parasites, in that they feed off and depend for their survival on other
forms of life. I think computer viruses should count as life. Maybe it
says something about human nature, that the only form of life we have
created so far is purely destructive. Talk about creating life in our
own image. I shall return to electronic forms of life later on.

What we normally think of as 'life' is based on chains of carbon atoms,
with a few other atoms, such as nitrogen or phosphorous. One can
speculate that one might have life with some other chemical basis, such
as silicon, but carbon seems the most favourable case, because it has
the richest chemistry. That carbon atoms should exist at all, with the
properties that they have, requires a fine adjustment of physical
constants, such as the QCD scale, the electric charge, and even the
dimension of space-time. If
these constants had significantly different values, either the nucleus
of the carbon atom would not be stable, or the electrons would collapse
in on the nucleus. At first sight, it seems remarkable that the
universe is so finely tuned. Maybe this is evidence, that the universe
was specially designed to produce the human race. However, one has to
be careful about such arguments, because of what is known as the
Anthropic Principle. This is based on the self-evident truth, that if
the universe had not been suitable for life, we wouldn't be asking why
it is so finely adjusted. One can apply the Anthropic Principle, in
either its Strong, or Weak, versions. For
the Strong Anthropic Principle, one supposes that there are many
different universes, each with different values of the physical
constants. In a small number, the values will allow the existence of
objects like carbon atoms, which can act as the building blocks of
living systems. Since we must live in one of these universes, we should
not be surprised that the physical constants are finely tuned. If they
weren't, we wouldn't be here. The strong form of the Anthropic
Principle is not very satisfactory. What
operational meaning can one give to the existence of all those other
universes? And if they are separate from our own universe, how can what
happens in them, affect our universe. Instead, I shall adopt what is
known as the Weak Anthropic Principle. That is, I shall take the values
of the physical constants, as given. But I shall see what conclusions
can be drawn, from the fact that life exists on this planet, at this
stage in the history of the universe.

There was no carbon,
when the universe began in the Big Bang, about 15 billion years ago. It
was so hot, that all the matter would have been in the form of
particles, called protons and neutrons. There
would initially have been equal numbers of protons and neutrons.
However, as the universe expanded, it would have cooled. About a minute
after the Big Bang, the temperature would have fallen to about a
billion degrees, about a hundred times the temperature in the Sun. At
this temperature, the neutrons will start to decay into more protons.
If this had been all that happened, all the matter in the universe
would have ended up as the simplest element, hydrogen, whose nucleus
consists of a single proton. However, some of the neutrons collided
with protons, and stuck together to form the next simplest element,
helium, whose nucleus consists of two protons and two neutrons. But no
heavier elements, like carbon or oxygen, would have been formed in the
early universe. It is difficult to imagine that one could build a
living system, out of just hydrogen and helium, and anyway the early
universe was still far too hot for atoms to combine into molecules.

The
universe would have continued to expand, and cool. But some regions
would have had slightly higher densities than others. The gravitational
attraction of the extra matter in those regions, would slow down their
expansion, and eventually stop it. Instead, they would collapse to form
galaxies and stars, starting from about two billion years after the Big
Bang. Some of the early stars would have been more massive than our
Sun. They would have been hotter than the Sun, and would have burnt the
original hydrogen and helium, into heavier elements, such as carbon,
oxygen, and iron. This could have taken only a few hundred million
years. After that, some of the stars would have exploded as supernovas,
and scattered the heavy elements back into space, to form the raw
material for later generations of stars.

Other stars are too
far away, for us to be able to see directly, if they have planets going
round them. But certain stars, called pulsars, give off regular pulses
of radio waves. We observe a slight variation in the rate of some
pulsars, and this is interpreted as indicating that they are being
disturbed, by having Earth sized planets going round them. Planets
going round pulsars are unlikely to have life, because any living
beings would have been killed, in the supernova explosion that led to
the star becoming a pulsar. But, the fact that several pulsars are
observed to have planets suggests that a reasonable fraction of the
hundred billion stars in our galaxy may also have planets. The
necessary planetary conditions for our form of life may therefore have
existed from about four billion years after the Big Bang.

Our
solar system was formed about four and a half billion years ago, or
about ten billion years after the Big Bang, from gas contaminated with
the remains of earlier stars. The Earth was formed largely out of the
heavier elements, including carbon and oxygen. Somehow, some of these
atoms came to be arranged in the form of molecules of DNA. This has the
famous double helix form, discovered by Crick and Watson, in a hut on
the New Museum site in Cambridge. Linking the two chains in the helix,
are pairs of nucleic acids. There are four types of nucleic acid,
adenine, cytosine, guanine, and thiamine. I'm afraid my speech
synthesiser is not very good, at pronouncing their names. Obviously, it
was not designed for molecular biologists. An adenine on one chain is
always matched with a thiamine on the other chain, and a guanine with a
cytosine. Thus the sequence of nucleic acids on one chain defines a
unique, complementary sequence, on the other chain. The two chains can
then separate and each act as templates to build further chains. Thus
DNA molecules can reproduce the genetic information, coded in their
sequences of nucleic acids. Sections of the sequence can also be used
to make proteins and other chemicals, which can carry out the
instructions, coded in the sequence, and assemble the raw material for
DNA to reproduce itself.

We
do not know how DNA molecules first appeared. The chances against a DNA
molecule arising by random fluctuations are very small. Some people
have therefore suggested that life came to Earth from elsewhere, and
that there are seeds of life floating round in the galaxy. However, it
seems unlikely that DNA could survive for long in the radiation in
space. And even if it could, it would not really help explain the
origin of life, because the time available since the formation of
carbon is only just over double the age of the Earth.

One
possibility is that the formation of something like DNA, which could
reproduce itself, is extremely unlikely. However, in a universe with a
very large, or infinite, number of stars, one would expect it to occur
in a few stellar systems, but they would be very widely separated. The
fact that life happened to occur on Earth, is not however surprising or
unlikely. It is just an application of the Weak Anthropic Principle: if
life had appeared instead on another planet, we would be asking why it
had occurred there.

If the appearance of life on a given
planet was very unlikely, one might have expected it to take a long
time. More precisely, one might have expected life to appear just in
time for the subsequent evolution to intelligent beings, like us, to
have occurred before the cut off, provided by the life time of the Sun.
This is about ten billion years, after which the Sun will swell up and
engulf the Earth. An intelligent form of life, might have mastered
space travel, and be able to escape to another star. But otherwise,
life on Earth would be doomed.

There is fossil evidence, that
there was some form of life on Earth, about three and a half billion
years ago. This may have been only 500 million years after the Earth
became stable and cool enough, for life to develop. But life could have
taken 7 billion years to develop, and still have left time to evolve to
beings like us, who could ask about the origin of life. If the
probability of life developing on a given planet, is very small, why
did it happen on Earth, in about one 14th of the time available.

The
early appearance of life on Earth suggests that there's a good chance
of the spontaneous generation of life, in suitable conditions. Maybe
there was some simpler form of organisation, which built up DNA. Once
DNA appeared, it would have been so successful, that it might have
completely replaced the earlier forms. We don't know what these earlier
forms would have been. One possibility is RNA. This is like DNA, but
rather simpler, and without the double helix structure. Short lengths
of RNA, could reproduce themselves like DNA, and might eventually build
up to DNA. One can not make nucleic acids in the laboratory, from
non-living material, let alone RNA. But given 500 million years, and
oceans covering most of the Earth, there might be a reasonable
probability of RNA, being made by chance.

As DNA reproduced
itself, there would have been random errors. Many of these errors would
have been harmful, and would have died out. Some would have been
neutral. That is they would not have affected the function of the gene.
Such errors would contribute to a gradual genetic drift, which seems to
occur in all populations. And a few errors would have been favourable
to the survival of the species. These would have been chosen by
Darwinian natural selection.

The process of biological
evolution was very slow at first. It took two and a half billion years,
to evolve from the earliest cells to multi-cell animals, and another
billion years to evolve through fish and reptiles, to mammals. But then
evolution seemed to have speeded up. It only took about a hundred
million years, to develop from the early mammals to us. The reason is,
fish contain most of the important human organs, and mammals,
essentially all of them. All that was required to evolve from early
mammals, like lemurs, to humans, was a bit of fine-tuning.

But
with the human race, evolution reached a critical stage, comparable in
importance with the development of DNA. This was the development of
language, and particularly written language. It meant that information
can be passed on, from generation to generation, other than
genetically, through DNA. There has been no detectable change in human
DNA, brought about by biological evolution, in the ten thousand years
of recorded history. But the amount of knowledge handed on from
generation to generation has grown enormously. The
DNA in human beings contains about three billion nucleic acids.
However, much of the information coded in this sequence, is redundant,
or is inactive. So the total amount of useful information in our genes,
is probably something like a hundred million bits. One bit of
information is the answer to a yes no question. By contrast, a paper
back novel might contain two million bits of information. So a human is
equivalent to 50 Mills and Boon romances. A major national library can
contain about five million books, or about ten trillion bits. So the
amount of information handed down in books, is a hundred thousand times
as much as in DNA.

Even more important, is the fact that the
information in books, can be changed, and updated, much more rapidly.
It has taken us several million years to evolve from the apes. During
that time, the useful information in our DNA, has probably changed by
only a few million bits. So the rate of biological evolution in humans,
is about a bit a year. By contrast, there are about 50,000 new books
published in the English language each year, containing of the order of
a hundred billion bits of information. Of course, the great majority of
this information is garbage, and no use to any form of life. But, even
so, the rate at which useful information can be added is millions, if
not billions, higher than with DNA.

This has meant that we
have entered a new phase of evolution. At first, evolution proceeded by
natural selection, from random mutations. This Darwinian phase, lasted
about three and a half billion years, and produced us, beings who
developed language, to exchange information. But in the last ten
thousand years or so, we have been in what might be called, an external
transmission phase. In this, the internal record of information, handed
down to succeeding generations in DNA, has not changed significantly.
But the external record, in books, and other long lasting forms of
storage, has grown enormously. Some people would use the term,
evolution, only for the internally transmitted genetic material, and
would object to it being applied to information handed down externally.
But I think that is too narrow a view. We are more than just our genes.
We may be no stronger, or inherently more intelligent, than our cave
man ancestors. But what distinguishes us from them, is the knowledge
that we have accumulated over the last ten thousand years, and
particularly, over the last three hundred. I think it is legitimate to
take a broader view, and include externally transmitted information, as
well as DNA, in the evolution of the human race.

The time
scale for evolution, in the external transmission period, is the time
scale for accumulation of information. This used to be hundreds, or
even thousands, of years. But now this time scale has shrunk to about
50 years, or less. On the other hand, the brains with which we process
this information have evolved only on the Darwinian time scale, of
hundreds of thousands of years. This is beginning to cause problems. In
the 18th century, there was said to be a man who had read every book
written. But nowadays, if you read one book a day, it would take you
about 15,000 years to read through the books in a national Library. By
which time, many more books would have been written.

This has
meant that no one person can be the master of more than a small corner
of human knowledge. People have to specialise, in narrower and narrower
fields. This is likely to be a major limitation in the future. We
certainly can not continue, for long, with the exponential rate of
growth of knowledge that we have had in the last three hundred years.
An even greater limitation and danger for future generations, is that
we still have the instincts, and in particular, the aggressive
impulses, that we had in cave man days. Aggression, in the form of
subjugating or killing other men, and taking their women and food, has
had definite survival advantage, up to the present time. But now it
could destroy the entire human race, and much of the rest of life on
Earth. A nuclear war, is still the most immediate danger, but there are
others, such as the release of a genetically engineered virus. Or the
green house effect becoming unstable.

There is no time, to
wait for Darwinian evolution, to make us more intelligent, and better
natured. But we are now entering a new phase, of what might be called,
self designed evolution, in which we will be able to change and improve
our DNA. There is a project now on, to map the entire sequence of human
DNA. It will cost a few billion dollars, but that is chicken feed, for
a project of this importance. Once we have read the book of life, we
will start writing in corrections. At first, these changes will be
confined to the repair of genetic defects, like cystic fibrosis, and
muscular dystrophy. These are controlled by single genes, and so are
fairly easy to identify, and correct. Other qualities, such as
intelligence, are probably controlled by a large number of genes. It
will be much more difficult to find them, and work out the relations
between them. Nevertheless, I am sure that during the next century,
people will discover how to modify both intelligence, and instincts
like aggression.

Laws
will be passed, against genetic engineering with humans. But some
people won't be able to resist the temptation, to improve human
characteristics, such as size of memory, resistance to disease, and
length of life. Once such super humans appear, there are going to be
major political problems, with the unimproved humans, who won't be able
to compete. Presumably, they will die out, or become unimportant.
Instead, there will be a race of self-designing beings, who are
improving themselves at an ever-increasing rate.

If this race
manages to redesign itself, to reduce or eliminate the risk of
self-destruction, it will probably spread out, and colonise other
planets and stars. However, long distance space travel, will be
difficult for chemically based life forms, like DNA. The natural
lifetime for such beings is short, compared to the travel time.
According to the theory of relativity, nothing can travel faster than
light. So the round trip to the nearest star would take at least 8
years, and to the centre of the galaxy, about a hundred thousand years.
In science fiction, they overcome this difficulty, by space warps, or
travel through extra dimensions. But I don't think these will ever be
possible, no matter how intelligent life becomes. In the theory of
relativity, if one can travel faster than light, one can also travel
back in time. This would lead to problems with people going back, and
changing the past. One would also expect to have seen large numbers of
tourists from the future, curious to look at our quaint, old-fashioned
ways.

It might be possible to use genetic engineering, to make
DNA based life survive indefinitely, or at least for a hundred thousand
years. But an easier way, which is almost within our capabilities
already, would be to send machines. These could be designed to last
long enough for interstellar travel. When they arrived at a new star,
they could land on a suitable planet, and mine material to produce more
machines, which could be sent on to yet more stars. These machines
would be a new form of life, based on mechanical and electronic
components, rather than macromolecules. They could eventually replace
DNA based life, just as DNA may have replaced an earlier form of life.

This
mechanical life could also be self-designing. Thus it seems that the
external transmission period of evolution, will have been just a very
short interlude, between the Darwinian phase, and a biological, or
mechanical, self design phase. This is shown on this next diagram,
which is not to scale, because there's no way one can show a period of
ten thousand years, on the same scale as billions of years. How long
the self-design phase will last is open to question. It may be
unstable, and life may destroy itself, or get into a dead end. If it
does not, it should be able to survive the death of the Sun, in about 5
billion years, by moving to planets around other stars. Most stars will
have burnt out in another 15 billion years or so, and the universe will
be approaching a state of complete disorder, according to the Second
Law of Thermodynamics. But Freeman Dyson has shown that, despite this,
life could adapt to the ever-decreasing supply of ordered energy, and
therefore could, in principle, continue forever.

What are the
chances that we will encounter some alien form of life, as we explore
the galaxy. If the argument about the time scale for the appearance of
life on Earth is correct, there ought to be many other stars, whose
planets have life on them. Some of these stellar systems could have
formed 5 billion years before the Earth. So why is the galaxy not
crawling with self designing mechanical or biological life forms? Why
hasn't the Earth been visited, and even colonised. I discount
suggestions that UFO's contain beings from outer space. I think any
visits by aliens, would be much more obvious, and probably also, much
more unpleasant.

What is the explanation of why we have not
been visited? One possibility is that the argument, about the
appearance of life on Earth, is wrong. Maybe the probability of life
spontaneously appearing is so low, that Earth is the only planet in the
galaxy, or in the observable universe, in which it happened. Another
possibility is that there was a reasonable probability of forming self
reproducing systems, like cells, but that most of these forms of life
did not evolve intelligence. We are used to thinking of intelligent
life, as an inevitable consequence of evolution. But the Anthropic
Principle should warn us to be wary of such arguments. It is more
likely that evolution is a random process, with intelligence as only
one of a large number of possible outcomes. It is not clear that
intelligence has any long-term survival value. Bacteria, and other
single cell organisms, will live on, if all other life on Earth is
wiped out by our actions. There
is support for the view that intelligence, was an unlikely development
for life on Earth, from the chronology of evolution. It took a very
long time, two and a half billion years, to go from single cells to
multi-cell beings, which are a necessary precursor to intelligence.
This is a good fraction of the total time available, before the Sun
blows up. So it would be consistent with the hypothesis, that the
probability for life to develop intelligence, is low. In this case, we
might expect to find many other life forms in the galaxy, but we are
unlikely to find intelligent life. Another way, in which life could
fail to develop to an intelligent stage, would be if an asteroid or
comet were to collide with the planet. We have just observed the
collision of a comet, Schumacher-Levi, with Jupiter. It produced a
series of enormous fireballs. It is thought the collision of a rather
smaller body with the Earth, about 70 million years ago, was
responsible for the extinction of the dinosaurs. A few small early
mammals survived, but anything as large as a human, would have almost
certainly been wiped out. It is difficult to say how often such
collisions occur, but a reasonable guess might be every twenty million
years, on average. If this figure is correct, it would mean that
intelligent life on Earth has developed only because of the lucky
chance that there have been no major collisions in the last 70 million
years. Other planets in the galaxy, on which life has developed, may
not have had a long enough collision free period to evolve intelligent
beings.

A third possibility is that there is a reasonable
probability for life to form, and to evolve to intelligent beings, in
the external transmission phase. But at that point, the system becomes
unstable, and the intelligent life destroys itself. This would be a
very pessimistic conclusion. I very much hope it isn't true. I
prefer a fourth possibility: there are other forms of intelligent life
out there, but that we have been overlooked. There used to be a project
called SETI, the search for extra-terrestrial intelligence. It involved
scanning the radio frequencies, to see if we could pick up signals from
alien civilisations. I thought this project was worth supporting,
though it was cancelled due to a lack of funds. But we should have been
wary of answering back, until we have develop a bit further. Meeting a
more advanced civilisation, at our present stage, might be a bit like
the original inhabitants of America meeting Columbus. I don't think
they were better off for it.

That is all I have to say. Thank you for listening.
الرجوع الى أعلى الصفحة اذهب الى الأسفل
معاينة صفحة البيانات الشخصي للعضو
AZER16



نوع المتصفح موزيلا

صلي على النبي

صل الله عليه وسلم


انجازاتي
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مُساهمةموضوع: رد: Life in the Universe    2011-04-28, 21:14

thank you a lot
best wishes
..
..
..
الرجوع الى أعلى الصفحة اذهب الى الأسفل
معاينة صفحة البيانات الشخصي للعضو
همسة براءة



نوع المتصفح موزيلا

صلي على النبي

صل الله عليه وسلم


انجازاتي
لايتوفر على اوسمة بعد:

الوسام الأول


مُساهمةموضوع: رد: Life in the Universe    2011-04-28, 22:02

u are welcom
thnx
الرجوع الى أعلى الصفحة اذهب الى الأسفل
معاينة صفحة البيانات الشخصي للعضو
Roshan



نوع المتصفح موزيلا

صلي على النبي

صل الله عليه وسلم


انجازاتي
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الوسام الأول


مُساهمةموضوع: رد: Life in the Universe    2011-08-14, 20:08

بارك الله فيييييييييييييك
موضوع رائع
ومفيد
وقيم
اجمل تحية
الرجوع الى أعلى الصفحة اذهب الى الأسفل
معاينة صفحة البيانات الشخصي للعضو
 
Life in the Universe
استعرض الموضوع السابق استعرض الموضوع التالي الرجوع الى أعلى الصفحة 
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