What is meant by “heat death”?
This is a topic
that came up in class, but since it was only vaguely related to what
we were actually talking about I did not take the time to explain it
very well. Heat death is a theory about the ultimate fate of the
universe. The second law of thermodynamics states that entropy
increases over time in any closed system. In other words, available
energy is always going to decrease unless there is energy being
added. The earth has life, violent weather, and so on because the
sun is constantly pouring energy into it, so the earth itself is not
really a closed system. Meanwhile, however, the sun is slowly using
up all of its energy, and one day it will burn out. Taken as a
whole, the solar system is essentially a closed system (if you ignore
the light, heat, and radiation that reaches us from the deep
stretches of outer space). Taken as a whole, then, the universe
itself is a closed system. As far as we know, the universe is using
up energy and nobody out there is putting new energy into it.
Therefore, the theory goes, the universe will eventually use up all
the energy that is available to do work, and it will die cold and
dark.
The notion of the
universe as a closed system that is burning out raises an interesting
question, related to the most interesting question that humans have
ever asked: if the universe is constantly getting colder, and it has
been in existence forever, why hasn't it burned out already? The
obvious answer is that the universe has not existed forever.
Scientists typically identify the Big Bang as the event that brought
the universe into being. At the moment of the Big Bang event, the
universe was at maximum energy. As time passes, the energy is used
up, and it is decreasingly available to do work.
But what happened
before the Big Bang? In a way, this is a paradoxical question. Time
began at the Big Bang, and so there is no sense talking about what
was there before. That does not stop people from trying to
understand what could have happened. One popular theory is that
there was a universe that existed before. It started with a Big Bang
of its own, expanded and expanded, but then gravity slowly brought
the expansion to an end, and then pulled that universe back together
until it made a gravitation singularity, that then resulted in our
Big Bang. According to this theory, the same fate awaits our
universe, the universe that it is thus created, and so on forever.
Even though this
theory is well known, possibly because it was popularized by Carl
Sagan's Cosmos book series
and television show, there are a couple of serious problems with it.
First, there does not seem to be enough mass in the universe to get
it to collapse back on itself. Maybe “dark matter,” which cannot
be detected like other forms of matter, exists in sufficient
quantities to allow the “Big Crunch” to take place. Second, the
expansion of the universe is not actually slowing down, but
accelerating! This is said to be the effect of something called
“dark energy,” which scientists do not understand well. Nobody
knows whether the universe is going to continue to expand faster and
faster, all we have to go on is our observations, since the principle
is poorly understood.
These
two problems with the “Big Crunch” scenario leaves the question
of how the universe has so much energy to begin with. Is there
another principle that works against entropy? Some physicists
believe in a steady state theory, which says that the universe as a
whole does not operate by the rules of the second law of
thermodynamics. There are various forms of this theory, but the
proponents have not been able to observe phenomena that convincingly
demonstrate their theories.
The
“Big Crunch” theory connects to an interesting time travel
paradox that I have been thinking about recently. Say that there is
a giant war that makes it hard for the planet to sustain life. A few
people, animals, and plants survive and find a way to eke out a
living. Eventually civilization is rebuilt, and technology advances
to a point where time travel is possible. A time traveler is
convinced that it would be better if the catastrophic war had never
happened, so she travels back in time to warn humanity about the
cataclysmic effects of their military conflicts. Instead of doing
what people usually do (continue on the road to self-destruction,
only to realize after the fact that they had been foolish), they heed
the warnings and destroy their extreme weapons.
The
effect of this disarmament is that all the events that led to the
birth of the time traveler and the creation of the time machine are
erased. Of course this is a paradox, because now the warning can
never come. But let's pretend that it doesn't happen. What happens
instead is that the stretch of history from the time of the war up to
the when the time traveler makes her trip becomes a type of
“hypothetical loop.” That effect that all of those events have
on the course of history is that it led to the time traveler, but
other than that all of those events, people, everything that was
built, made, dreamed, thought, and done is completely erased.
Now
some people will speculate that history will continue now in two
trajectories. The people who were left behind by the time traveler
will continue in their own “universe,” while her arrival in the
past creates a new trajectory that is, in effect, a different
“universe.” Regardless of what happens to those people,
everything that happens after she leaves is completely unknowable to
the people she travels to save. Not only is it unknowable, though,
it actually is a future that does not exist for those people who
averted war. They eliminated that future when they chose peace over
war.
This
connects to the “Big Crunch” theory because, if this universe is
crushed into nothingness to become a new universe, then everything
that happens and in our future is a “hypothetical loop.” Not
only will our existence not matter to the people of the next
universe, there is no meaningful way in which we exist to those
people at all, not even as a piece of their past. Time stopped and
started again.