# Several questions about the poincare recurrence time?

1. Mar 12, 2015

### dsigler924

Keep in mind I am a complete layman when it comes to physics.

Is the Poincare recurrence time the time it will take for the universe to be exactly in the state again as it is now or the time before the universe will even begin to be able to starting producing basic patterns from chaos?

What is to keep the rearrangement of patterns being like the show sliders where there would be alternative histories(but in are own reconstructed universe rather than parallel ones) before we return to the history we are familiar with? Or is it just or own history that we just experienced that can repeat?

What would keep things from being weirdly out of order like people from different eras being born together?

2. Mar 12, 2015

### Staff: Mentor

The assumptions of the PoincarĂ© recurrence time (like a finite volume) don't seem to be satisfied in our universe.

There are always random patterns in chaotic systems, that's the idea of chaotic systems. The smaller the pattern you are looking for the more frequent it will show up. As an example, rolling two "6" in a row with dice is more likely than rolling ten "6" in a row.
If we would live in such a universe, everything would happen at some point, without a specific order in time.
How do you identify those persons, if not by time of birth and actions in their environment? It does not help to be named Neil Armstrong, a person born in the old Rome won't fly to moon.

3. Mar 12, 2015

### dsigler924

I mean Neil Armstrong and Emily Dickinson are both valid configurations of matter and if we are going through every possible configuration of matter and it's locations in space there would be a situation where they are standing next to other, or got married or hated each others guts as we have to go through every possible pattern of nature no matter how weird right?

4. Mar 12, 2015

### Staff: Mentor

If everything possible happens: sure. No matter how you identify those persons with the persons in "our world".

5. Mar 12, 2015

### dsigler924

After reading about it I think I misinterpreted my own idea. I thought that the poincare recurrence time meant that you have to go through every possible configuration of particle states before you can get back to the current state.

I still would like to know if the recurrence time means that you return to the original state and history repeats itself or it's a measurement of how long it will take before all the particles states in the universe are again exactly like they are now?

6. Mar 12, 2015

### Chalnoth

The Poincare recurrence time is the time it takes for a system to return to its exact previous state. It is valid provided that the fundamental laws of physics are unitary. Most proposed fundamental laws of physics (e.g. string theory) are unitary, as it's difficult to conceive of a universe that is not unitary and yet still respects causality (that is, that later states are time-evolved realizations of earlier states). And yes, if Poincare recurrence holds for the universe, the entire history of the universe will repeat.

But the recurrence time for the universe inside our horizon is absurdly large, so much so that it is little more than a theoretical curiosity.