The Physics of Star Trek (15 page)

One should also remember that even if one derives a well-defined probability, its
interpretation can be pretty subtle. For example, the probability of any specific sequence
of eventssuch as the fact that I am sitting in this specific type of chair typing at this
specific computer (among all the millions of computers manufactured each year), in this
specific place (among all the possible cities in the world), at this specific time of day
(among the 86,400 seconds in each day) is vanish-ingly small. The same can be said for any
other set of circumstances in my life. Likewise, in the inanimate world, the probability
that, say, a radioactive nucleus will decay at the exact moment it does is also
vanishingly small. However, we do not calculate such probabilities. We ask, rather, how
likely it is that the nucleus will decay in some nonzero time interval, or how much more
probable a decay is at one time compared to another time.

When one is attempting to estimate the probabilities of life in the galaxy, one has to be
very careful not to overrestrict the sequence of events one considers. If one does, and
people have, one is likely to conclude that the probability that life formed on Earth when
it did is infinitesimally small, which is sometimes used as an argument for the existence
of Divine intervention. However, as I have just indicated, the same vanishingly small
probability could be assigned to the likelihood that the stoplight I can see out my window
will turn red while I am waiting in my car there at precisely 11:57 A.M. on June 3, 1999.
This does not mean, however, that such a thing won't happen.

The important fact to recognize is that
life did form
in the galaxy at least once. I cannot overemphasize how important this is. Based on all
our experience in science, nature rarely produces a phenomenon just once. We are a test
case. The fact that we exist proves that the formation of life is possible. Once we know
that life can originate here in the galaxy, the likelihood of it occurring elsewhere is
vastly increased. (Of course, as some evolutionary biologists have argued, it need not
develop an intelligence.)

While our imaginations are no doubt far too feeble to consider all the combinations of
conditions which might give rise to intelligent life, we can use our own existence to ask
what properties of the universe were essential or

important in our own evolution.

We first begin with the universe as a whole. I have already mentioned one cosmic
coincidence: that there was one extra proton produced in the early universe for every 10
billion or so protons and antiprotons. Without these extra little guys, matter would have
annihilated with antimatter, and there would be no matter left in the universe today,
intelligent or otherwise.

The next obvious feature of the universe in which we live is that it is old, very old. It
took intelligent life about 3.5 billion years to develop on Earth. Hence, our existence
requires a universe that accommodated our arrival by lasting billions of years. The
current best estimate for the age of our universe is between about 10 billion and 20
billion years, which is plenty long enough. It turns out, however, that it is not so easy
a priori to design a universe that expands, as our universe does, without either
recollapsing very quickly in a reverse of the big banga big crunchor expanding so fast
that there would have been no time for matter to clump together into stars and galaxies.
The initial conditions of the universe, or some dynamical physical process early in its
history, would have to be very finely tuned to get things just right.

This has become known as the “flatness” problem, and understanding it has become one of
the central issues in cosmology today. Gravitational attraction due to the presence of
matter tends to slow the expansion of the universe. As a result, two possibilities remain.
Either there is enough matter in the universe to cause the expansion to halt and reverse
(a “closed” universe), or there is not (an “open” universe). What is surprising about the
present universe is that when we add up all the matter we estimate is out there, the
amount we find is suspiciously close to the borderline between these two possibilitiesa
“flat” universe, in which the observed expansion would slow but never quite stop in any
finite amount of time.

What makes this particularly surprising is that as the universe evolves, if it is not
exactly flat then it deviates more and more from being flat as time goes on. Since the
universe is probably at least 10 billion years old today, and observations suggest that
the universe is close to being flat today, then at much earlier times it must have been
immeasurably close to being flat. It is hard to imagine how this could happen at random
without some physical process enforcing it. Some 15 years ago, a candidate physical
process was invented. Known as “inflation,” it is a ubiquitous process that can occur due
to quantum mechanical effects in the early universe.

Recall that empty space is not really empty but that quantum fluctuations in the vacuum
can carry energy. It turns out that it is possible, as the nature of forces between
elementary particles evolves with temperature in the early universe, for the energy stored
as quantum fluctuations in empty space to be the dominant form of energy in the universe.
This vacuum energy can repel gravitationally rather than attract. It is hypothesized that
the universe went through a brief inflationary phase, during which it was dominated by
such vacuum energy, resulting in a very rapid expansion. One can show that when this
period ends and the vacuum energy is transferred into the energy of matter and radiation,
the universe can easily end up being flat to very high precision.

However, another, perhaps more severe, problem remains. In fact Einstein first introduced
the problem when he tried to apply his new general theory of relativity to the universe.
At that time, it was not yet known that the universe was expanding; rather, the universe
was believed to be static and unchanging on large scales. So Einstein had to figure out
some way to stop all this matter from collapsing due to its own gravitational attraction.
He added a term to his equations called the cosmological constant, which essentially
introduced a cosmic repulsion to balance the gravitational attraction of matter on large
scales. Once it was recognized that the universe is not static, Einstein realized that
there was no need for such a term, whose addition he called “the biggest blunder” he had
ever made.

Unfortunately, as in trying to put the toothpaste back into the tube, once the possibility
of a cosmological constant is raised, there is no going back. If such a term is possible
in Einstein's equations then we must explain why it is absent in the observed universe. In
fact, the vacuum energy I described above produces exactly the effect that Einstein sought
to produce with the cosmological constant. So the question becomes, How come such vacuum
energy is not overwhelmingly dominant in the universe today?or, How come the universe
isn't still inflating?

We have no answer to this question. It is probably one of the most profound unanswered
questions in physics.

Every calculation we perform with the theories we now have suggests that the vacuum energy
should be many orders of magnitude larger today than it is allowed to be on the basis of
our observations. There are ideas, based

on exotica like Euclidean wormholes, for how to make it vanish, but none of these ideas is
firmly grounded.

Perhaps even more surprising, recent observations on a variety of scales all suggest that
the cosmological constant, while much smaller than we can explain, may nevertheless not be
zero today, and may therefore have had a measurable effect on the evolution of the
universemaking it older than it might otherwise have been, for example. This is a subject
of great interest, and in fact is occupying much of my own present research efforts.

Nevertheless, whatever the resolution of this problem, it is clear that the near flatness
of the universe was one of the conditions necessary for the eventual origin of life on
Earth and that the cosmological conditions favoring the formation of life on Earth hold
elsewhere as well.

At a fundamental microphysical level, there is also a whole slew of cosmic coincidences
that allowed life to form on Earth. If any one of a number of fundamental physical
quantities in nature was slightly different, then the conditions essential for the
evolution of life on Earth would not have existed. For example, if the very small mass
difference between a neutron and proton (about 1 part in 1000) were changed by only a
factor of 2, the abundance of elements in the universe, some of which are essential to
life on Earth, would be radically different from what we observe today. Along the same
lines, if the energy level of one of the excited states of the nucleus of the carbon atom
were slightly different, then the reactions that produce carbon in the interiors of stars
would not occur and there would be no carbon the basis of organic moleculesin the universe
today.

Of course, it is hard to know how much emphasis to put on these coincidences. It is not
surprising, since we
have
evolved in this universe, to find that the constants of nature happen to have the values
that allowed us to evolve in the first place. One might imagine, for the purposes of
argument, that our observed universe is part of a meta- universe that exists on a much
larger scale than we can observe. In each of the universes making up this meta- universe,
the constants of nature could be different. In those universes that have constants
incompatible with the evolution of life, no one is around to measure anything. To
paraphrase the argument of the Russian cosmologist Andrei Linde, who happens to subscribe
to this form of what is known as the “anthropic principle,” it is like an intelligent fish
wondering why the universe in which it lives (the inside of a fish bowl) is made of water.
The answer is simple: if it weren't made of water, the fish wouldn't be there to ask the
question.

Since most of these issues, while interesting, are not empirically resolvable at the
present time, they are perhaps best left to philosophers, theologians, or perhaps science
fiction writers. Let us then accept the fact that the universe
has
managed to evolve, both microscopically and macroscopically, in a way that is conducive to
the evolution of life. We next turn to our own home, the Milky Way galaxy.

When we consider which systems in our own galaxy may house intelligent life, the physics
issues are much more clear-cut. Given that there exist stars in the Milky Way which, from
all estimates, are at least 10 billion years old, while life on Earth is no older than
about 3.5 billion years, we are prompted to ask how long life could have existed in our
galaxy before it arose on Earth.

When our galaxy began to condense out of the universal expansion some 10 billion to 20
billion years ago, its first generation stars were made up completely of hydrogen and
helium, which were the only elements created with any significant abundance during the big
bang. Nuclear fusion inside these stars continued to convert hydrogen to helium, and once
this hydrogen fuel was exhausted, helium began to “burn” to form yet heavier elements.
These fusion reactions will continue to power a star until its core is primarily iron.
Iron cannot be made to fuse to form heavier elements, and thus the nuclear fuel of a star
is exhausted. The rate at which a star burns its nuclear fuel depends on its mass. Our own
Sun, after 5 billion years of burning hydrogen, is not even halfway through the first
phase of its stellar evolution. Stars of 10 solar massesthat is, 10 times heavier than the
Sunburn fuel at about 1000 times the rate the Sun does. Such stars will go through their
hydrogen fuel in less than 100 million years, instead of in the Sun's 10-billion-year
lifetime.

What happens to one of these massive stars when it exhausts its nuclear fuel? Within
seconds of burning the last bit, the outer parts of the star are blown off in an explosion
known as a supernova, one of the most brilliant fireworks displays in the universe.
Supernovae briefly shine with the brightness of a billion stars. At the present time, they
are occurring at the rate of about two or three every 100 years in the galaxy. Almost 1000
years ago, Chinese astronomers observed a new star visible in the daytime sky, which they
called a “guest star.” This supernova created what we now observe telescopically as the
Crab Nebula. It is interesting that nowhere in Western Europe was this transient object
recorded. Church dogma at the time declared the heavens to be eternal

and unchanging, and it was much easier not to take notice than to be burned at the stake.
Almost 500 years later, European astronomers had broken free enough of this dogma so that
the Danish astronomer Tycho Brahe was able to record the next observable supernova in the
galaxy.

Many of the heavy elements created during the stellar processing, and others created
during the explosion itself, are dispersed into the interstellar medium, and some of this
“stardust” is incorporated in gas that collapses to form another star somewhere else. Over
billions of years, later generations of starsso-called Population 1 stars, like our
Sunform, and any number of these can be surrounded by a swirling disk of gas and dust,
which would coalesce to form planets containing heavy elements like calcium, carbon, and
iron. Out of this stuff we are made. Every atom in our bodies was created billions of
years ago, in the fiery furnace of some long dead star. I find this one of the most
fascinating and poetic facts about the universe: we are all literally star children.

Now, it would not be much use if a planet like the Earth happened to form near a very
massive star. As we have seen, such stars evolve and die within the course of 100 million
years or so. Only stars of the mass of our Sun or less will last longer than 5 billion
years in a stable phase of hydrogen burning. It is hard to imagine how life could form on
a planet orbiting a star that changed in luminosity by huge amounts over the course of
such evolution. Conversely, if a star smaller and dimmer than our Sun should have a
planetary system, any planet warm enough to sustain life would probably be so close in as
to be wracked by tidal forces. Thus, if we are going to look for life, it is a good bet to
look at stars not too different from our own. As it happens, the Sun is a rather ordinary
member of the galaxy. About 25 percent of all stars in the Milky Waysome 100 billion of
themfall in the range required. Most of these are older even than the Sun and could
therefore, in principle, have provided sites for life up to 4 billion to 5 billion years
before the Sun did.