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.