Cosmology and Time's Arrow (nerd content.)

[skywatch]

If anyone here is interested in our attempts to understand the actual meaning of time, not only those little machines we use to count it, perhaps here's a place we can chat about such esoterica. (It's true, I'm a nerd.) I was very happy to see this article in Science News this week about new models for bi-directional time beginning at the start of the universe. It's a fascinating approach:

https://www.sciencenews.org/article/arrow-time

Basically, it's a simplified model of a gravitation-only universe that shows a singularity can form with entropy increasing in both negative and positive time directions, with the 2nd law of thermodynamics preserved in each direction along its own relative axis. Yet, the forward and backward arrows can never see each other. I think it's an elegant possibility, if only a model.

[pepperami]

I'm going to have to read this a couple of times

If I've got the basics, there is a possible alternate universe, that has reversed in times (relative to ours) since the big bang but possibly formed in the same way but as it ages it ages into our past?

[Matt]

If anyone here is interested in our attempts to understand the actual meaning of time, not only those little machines we use to count it, perhaps here's a place we can chat about such esoterica. (It's true, I'm a nerd.) I was very happy to see this article in Science News this week about new models for bi-directional time beginning at the start of the universe. It's a fascinating approach:

https://www.sciencenews.org/article/arrow-time

Basically, it's a simplified model of a gravitation-only universe that shows a singularity can form with entropy increasing in both negative and positive time directions, with the 2nd law of thermodynamics preserved in each direction along its own relative axis. Yet, the forward and backward arrows can never see each other. I think it's an elegant possibility, if only a model.

There's a simple a priori issue that bites way before one gets out into the real world that can be nicely exposed using Conway's Life. Take, say, a glider. The pattern that makes up a glider, once in place will deterministically keep going such that whatever the state at any time 'T' will determine any future states. However, there are a multitude of potential states that could have led to 'T'. In other words, even in an entirely deterministic, linear world, while the future is overdetermined the past is underdetermined. Obviously the real world is slightly more complicated than life, not to mention non linear and stochastic, (if Bell's inequality holds) but the principle remains exactly the same.

To elucidate this, imagine a simple rubber ball sitting still on the floor in a simple square room with, say, four windows. If all the physical information about the room at any given time 'T' is available to you, you will be able to predict pretty reliably what is going to happen to the ball in the next five minutes. However, from 'T' you will not be able to say what happened in the last five: did it bounce in through any particular window, has it sat there for months. You simply can't tell.

See the problem?

(if you haven't played with Life, try it, it's incredibly helpful for thinking about this sort of stuff.)

[Seriously]

So I've gotta set my watch 5 minutes forward is what you're saying?...........

[Matt]

So I've gotta set my watch 5 minutes forward is what you're saying?...........

Only if you suspect that setting your watch is the cause of the ball's arrival in the room. See also 'The Frame Problem!'

[Seriously]

So I should just imagine I've set it forward 5 mins?...

Or assume I have ?


and where is the cat at this stage?

[Matt]

So I should just imagine I've set it forward 5 mins?...

Or assume I have ?


and where is the cat at this stage?

Like anyone can ever answer that question.

[Seriously]

[Matt]

I'm not an expert but I'd say that's yesterday's tarka phall (for those who prefer their curries a little 'otter).

[Seriously]

Rather apt riddle from him


This thing all things devours:
Birds, beasts, trees, flowers;
Gnaws iron, bites steel;
Grinds hard stones to meal;
Slays king, ruins town,
And beats high mountain down.