# Does the past exist?

#### DuckAmuck

I'm not exactly sure where to put this question, so admins feel free to move it.

I have been reading about Conway's game of life, and learned about the halting problem. Basically, you can reach states in the game that are irreversible. Basically, the conditions at any point in time, in general, won't give you enough information to reproduce the previous steps. Simplest example: you have one cell, and it dies, so you are left with an empty board. That empty board can't tell you there was once one cell.

Is this also the case with reality? Are there states that systems can reach which are not reversible (as in not rewindable)? Basically a state that cannot tell you about the past of the system. If so, then is that the nail in the coffin for time travel to the past? Either that or if you go into the past, it wouldn't be "complete".

Any thoughts on this?

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#### SteamKing

Staff Emeritus
Homework Helper
Not only does the past exist, but it keeps changing all the time, as people in the present are not always satisfied with what went on in the past.

IDK about board games, but in real life, things are rarely erased so thoroughly that absolutely no trace is left behind. Most of the details might get lost, but you at least know that something happened or someone existed waaaay back when.

As far as physical processes go, things generally are not reversible. It's kinda one of the basic laws of the universe. For more discussion, see:

https://en.wikipedia.org/wiki/Entropy_(arrow_of_time)

#### DuckAmuck

Oh I know about entropy and thermodynamic irreversibility. But I think what I'm talking about might be a different kind of irreversibility.

Like if there's a process that occurs that's thermodynamically irreversible, and all you know is a final state (positions and velocities of particles), you might be able to extrapolate backwards and arrive at the initial state. But with Conway's game of life you cannot do this: the final state will not have enough information to extrapolate back to the initial state.

#### SteamKing

Staff Emeritus
Homework Helper
Oh I know about entropy and thermodynamic irreversibility. But I think what I'm talking about might be a different kind of irreversibility.

Like if there's a process that occurs that's thermodynamically irreversible, and all you know is a final state (positions and velocities of particles), you might be able to extrapolate backwards and arrive at the initial state. But with Conway's game of life you cannot do this: the final state will not have enough information to extrapolate back to the initial state.
It's the same with real life, not just board games. And a similar problem crops up in trying to predict the future, given a certain amount of information in the present to work with.

For example, Newton's laws of motion are pretty accurate and succinct about describing the motion of orbiting bodies. However succinct these laws are, once you consider more than two bodies in orbit around each other, analysis of the stability of the system, like will things keep orbiting one another for another billion years or so, becomes essentially impossible. A small change in measuring the current state of a system today can lead to catastrophic consequences down the line.

I thought a good introduction to the subject, for a general audience, was Chaos: Making a New Science by James Gleick:

https://www.amazon.com/dp/0143113453/?tag=pfamazon01-20

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