Sunday, February 15, 2009

Time and Thermodynamics

Hypothetical situation. All of the particles in the universe kick into reverse and start going backwards. For some reason every particle in the universe instantaneously reverses course. And also space begins contracting instead of expanding. Everything in the universe hits a rubberwall and bounces back 180 degrees.

So now instead of expanding, everything is on an exact "rewind" mode, and we're headed back to the "Big Bang".

The laws of physics work the same in both directions...if you solve them forward in time, you can take your answers, reverse the equations and get your starting values, right?

This is what they always go on about with the "arrow of time". The laws of physics work the same forwards and backwards in time. It's not impossible for an egg to "unscramble", it's just very very very very very unlikely. But if it did so, no laws of physics would be broken. And, in fact, if you wait long enough, it will eventually happen.

Okay, so everything has reversed direction. The actual reversal process is, of course, impossible. But after everything reverses, everything just plays out by the normal laws of physics. Only that one instant of reversal breaks the laws of physics.

TIME is still moving forward in the same direction as before. We didn't reverse time. We just reversed the direction of every particle.

So, now photons and neutrinos no longer shoot away from the sun - instead now they shoot towards the sun, which when the photons and the neutrinos and gamma rays hit helium atoms, the helium atoms split back into individual hydrogen atoms, and absorb some energy in the process. Again, no physical laws are broken, and time is moving forward.

Now, back on earth, everything is playing out in reverse as well. You breath in carbon dioxide and absorb heat from your surroundings and use the heat to break the carbon dioxide into carbon and oxygen. You exhale the oxygen, and you turn the carbon into sugars, which you eventually return to your digestive track where it's reconstituted into food, which you regurgitate onto your fork and place it back onto your plate.

Okay. So, still no physical laws broken. Entropy is decreasing, but that's not impossible, no laws of physics are being broken.

In this case, it must happen because we perfectly reversed the trajectory of every particle in the universe.

NOW. Your brain is also working backwards. But exactly backwards from before. Every thought that you had yesterday, you will have again tomorrow, in reverse. You will unthink it.

My question is, what would you experience in this case? What would it be like to live in this universe where time is still going forward, but where all particles are retracing their steps precisely?

The laws of phsyics are still working exactly as before, but because all particle trajectories were perfectly reversed, everything is rolling back towards the big bang.

In my opinion, we wouldn't notice any difference. We would NOT experience the universe moving in reverse, we would still experience it moving forward exactly as we do now...we would still see the universe as expanding even though it was contracting, we would still see the sun giving off light and energy even though it was absorbing both. In other words, we would still see a universe with increasing entropy even though we actually would live in a universe with decreasing entropy.

And why would that be the case? Because our mental states determine what is the past for us and what is the future. There is no "external arrow of time". The arrow of time is internal. The past is the past because we remember it and because the neurons of our brains tell us that it has already happened to us. The future is the future because it's unknown, and because the neurons of our brains tell us that it will happen to us soon.

If there is an external arrow of time, it is irrelevant, because as this thought experiment shows, it doesn't affect the way we perceive time. Our internal mental state at any given instant determines what is the future and what is the past for us.

In fact, you could run the universe forwards and backwards as many times as you wanted like this. We would never notice anything. We would always percieve increasing entropy. For us, time would always move forward, never backwards.

My point being, as always, that our experience of reality is always entirely dependent on our brain state. We can't know ANYTHING about the universe that is not represented in the information of our brain state at any given instant.

Forwards or backwards, it's all just particles moving around, assuming various configurations, some of which give rise to consciousness.

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