Thermodynamics of Time Travel

Many people like to imagine that it would be possible to go back in time. Maybe they want to change something that happened in the past. They don’t understand that forward motion in time as we perceive it is essentially a thermodynamic phenomenon and going backward in time is just an extremely improbable occurrence. Let me explain by considering two different scenarios.

Scenario 1: Imagine a cylinder of compressed air in a sealed room that has had all of the air removed so it is a vacuum. A valve on the tank is opened and the gas flows out of the cylinder until the pressure in the tank matches the pressure in the room. The flow of gas represents the forward motion of time. The gas particles are moving randomly, bouncing around the room, off of surfaces and each other.

Scenario 2: Next, imagine the opposite. You are sitting in a room with a compressed gas cylinder with an open valve on it. Suddenly all of the air in the room moves into the gas cylinder forming a vacuum in the room.

In both cases, there is the same amount of matter and energy in the room, so the first law of thermodynamics, the conservation of matter and energy, is obeyed. The second example violates the second law of thermodynamics. It doesn’t mean that it can’t happen. There is some chance that all of the particles will bounce off of each other in just the right way to end up in the bottle. The probability of that happening is so low that it basically can’t happen.

Imagine that someone accidentally knocks a glass off of a table and it falls to the floor and shatters with a crash. That is an example of time moving forward. We never observe all of the pieces of glass reassembling themselves and leaping back onto the table. All of the matter still exists in the broken fragments of glass. All of the energy still exists, it has been converted into sound, the impact with the floor when the glass fell, the broken bonds between the glass molecules, eventually ending up as heat. There is a chance that the energy will accumulate in just the right way to push the glass back together and force it back onto the table. It is just extremely unlikely. It is hard to imagine just how improbable something like that would be.

The improbability of the above two examples makes them impossible. Now imagine how unlikely it would be for an entire planet to go back in time, which would be the minimum necessary for time travel.