Time Reversal and Other Curiosities
Believe it or not, I’ve been known to lie; but this is the whole truth. Look out your window tonight and you might see a few thousand stars reaching out into the great expanse. These days, many people consider cosmologists to be in the know about all the stars and galaxies out there. The actual truth is much different.
Remember time? Don’t give me that blank stare. You might remember your birth, but you will never remember your death. This time asymmetry is very irritating to physicists since physical laws don’t really care which way time flows. You might someday see a shuttle launch, but you will never see it land on its launch pad on an imploding cloud of smoke.
Part of the redemption to this mystery lies in entropy. To the uninitiated, entropy is simply the measure of the disorder of a system. That hot cup of coffee next to you is right now in a macrostate, a certain pressure and temperature. Look closer, and the atoms themselves are in their own states of position and velocity; a microstate. Entropy is the number of (actually log) of the number of microstates to a macrostate. Now mix in some cream to the coffee, and you have just raised its microstates and with its entropy.
But the entropy of a closed system will never decrease. Try as you may, the entropy of the cosmos has only been increasing with time. But if its been increasing, it had to be in a low state to begin with.
It may be difficult to comprehend, but empty space contains vast quantities of entropy, a consequence of still not fully understood gravitational microstates. This entropy is part of the quantum space-time fabric of the cosmos itself. Gravity is still awaiting its full understanding when scientists someday build a working theory of Quantum Gravity.
This inevitably brings us to initial conditions, which many cosmologist believe involved inflation; the runaway expansion of the early universe due to ultra dense dark energy. But this creates new problems in itself: as Roger Penrose of Oxford University and others have pointed out. The number of microstates in the universe is a constant, but entropy must always increase to the most stable macrostate. That is the high-entropy vacuum, but this conflicts with initial conditions. But nature has one more trick up its sleeve, quantum fluctuations: the random creation and destruction of particles in a vacuum.
Back in 2004 Jennifer Chen and Sean Carroll of the University of Chicago proposed a curious solution to the unbalanced equation of entropy. Instead of entropy increasing without bound in our universe, let it increase without bound in a multi-universe. In another universe, a quantum fluctuation would pinch off and begin another baby universe in an infinite cascade. As an added curiosity, time could flow backwards in some universes to counter-balance the increasing entropy of this universe and others. It is a brilliant yet still controversial explanation for the entropy problem.