Arrow of Time FAQ


There’s an extraordinarily helpful discussion of the cosmological thinking about the puzzle of the direction of time in the Universe.

(The phrase “arrow of time” refers to the observation that time flows in one direction only. Physically speaking this characteristic of reality is linked (apparently) to the 2nd Law of Thermodynamics – that entropy increases over time.)

The puzzle is: that if the Big Bang Theory is correct, the Universe begins in a low entropy situation – how did it get there?

From the long article:

So what does a high-entropy state look like?

Empty space. In a theory like general relativity, where energy and particle number and volume are not conserved, we can always expand space to give rise to more phase space for matter particles, thus allowing the entropy to increase. Note that our actual universe is evolving (under the influence of the cosmological constant) to an increasingly cold, empty state — exactly as we should expect if such a state were high entropy. The real cosmological puzzle, then, is why our universe ever found itself with so many particles packed into such a tiny volume.

Could the universe just be a statistical fluctuation?

No. This was a suggestion of Bolzmann’s and Schuetz’s, but it doesn’t work in the real world. The idea is that, since the tendency of entropy to increase is statistical rather than absolute, starting from a state of maximal entropy we would (given world enough and time) witness downward fluctuations into lower-entropy states. That’s true, but large fluctuations are much less frequent than small fluctuations, and our universe would have to be an enormously large fluctuation. There is no reason, anthropic or otherwise, for the entropy to be as low as it is; we should be much closer to thermal equilibrium if this model were correct. The reductio ad absurdum of this argument leads us to Boltzmann Brains — random brain-sized fluctuations that stick around just long enough to perceive their own existence before dissolving back into the chaos.

[…]Doesn’t the collapse of the wavefunction in quantum mechanics violate time-reversal invariance?

It certainly appears to, but whether it “really” does depends (sadly) on one’s interpretation of quantum mechanics. If you believe something like the Copenhagen interpretation, then yes, there really is a stochastic and irreversible process of wavefunction collapse. Once again, however, it is unclear how this could help explain the arrow of time — whether or not wavefunctions collapse, we are left without an explanation of why the early universe had such a small entropy. If you believe in something like the Many-Worlds interpretation, then the evolution of the wavefunction is completely unitary and reversible; it just appears to be irreversible, since we don’t have access to the entire wavefunction. Rather, we belong in some particular semiclassical history, separated out from other histories by the process of decoherence. In that case, the fact that wavefunctions appear to collapse in one direction of time but not the other is not an explanation for the arrow of time, but in fact a consequence of it. The low-entropy early universe was in something close to a pure state, which enabled countless “branchings” as it evolved into the future.”

The final part of the FAQ goes on to explain, that assuming there is an explanation found for the Arrow of Time that makes sense with our observation that pretty much every other physical quantity does not show a directionality but is instead symetric, then time as we perceive it is an illusion.

Which means that to a creature not embedded in our Universe, both the future and the past would be equally accessible.

Which is a very evocative statement. Especially if you’re thinking in terms of classical theology.

Read the rest here.

The Author

Episcopal bishop, dad, astronomer, erstwhile dancer...