Time’s Almost Reversible Arrow
quantamagazine.org
Time’s Almost Reversible Arrow
1–10 of 82 posts
Re: Time’s Almost Reversible Arrow
#2Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean time can't run in reverse.
Also information might be lost in black holes. Wouldn't that also prohibit it reversing time?
Re: Time’s Almost Reversible Arrow
#3Great read on some of the history and current efforts in particle physics by a leader of the field (Frank Wilczek).
I was surprised to read such a well written article on quanta magazine (I've often been disappointed there) but it made sense when I saw who the author was. It read more like what might be published in a general audience Nature or Science article.
Re: Time’s Almost Reversible Arrow
#4Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean time can't run in reverse. Also information might be lost in black holes. Wouldn't that also prohibit it reversing time?
Yeah. Totally what I was thinking. Other than that though, solid theory.
Re: Time’s Almost Reversible Arrow
#5Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean time can't run in reverse. Also information might be lost in black holes. Wouldn't that also prohibit it reversing time?
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Re: Time’s Almost Reversible Arrow
#6Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean time can't run in reverse. Also information might be lost in black holes. Wouldn't that also prohibit it reversing time?
No.
Consider what it would look like if you ran the solar system in reverse - all the planets and moons would go backwards in their orbits, but gravity still works exactly the same. The equations of mechanics don't care about time.
And yes, information being lost in black holes would be an issue, if true - that's why that problem has been studied so much recently.
Re: Time’s Almost Reversible Arrow
#7This was a great read! But I wish the article would have mentioned how tiny the microscopic time-reversal effects are, compared to macroscopic time reversal. The microscopic effects are too small to explain why we have such a clear direction of time.
Our direction of time is defined by the observation that entropy (or "chaoticness") always increases with time. If you mix orange juice with water, you will not see the result unmix itself again, even though the interactions between the individual molecules are perfectly reversible.
In fact, you can build a toy-universe in your head, where all interactions are time-reversible, and you would still observe that the chaotic-ness increases with time, unless you start with a completely random initial state (in which the chaoticness stays roughly constant). This increase in chaoticness is comletely unsurprising: It's a simple consequence of the fact that there are much, much more "states" in which a system looks chaotic, than states in which it looks ordered. Therefore, a system transitioning from state to state is much more likely to be in one that looks chaotic, and very, very unlikely to ever go back to a state that is non-chaotic.
The real mystery, for me, is: Why was the initial state of our universe so completely non-chaotic? Why did the universe not start out in a state that is essentially equal to the heat-death [1]? The microscopic processes that are not time-reversible might have something to do with this, but it's not clear at all how they can solve this mystery.
[1] https://en.wikipedia.org/wiki/Heat_death_of_the_universe
Re: Time’s Almost Reversible Arrow
#8Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean time can't run in reverse. Also information might be lost in black holes. Wouldn't that also prohibit it reversing time?
No. Consider what it would look like if you ran the solar system in reverse - all the planets and moons would go backwards in their orbits, but gravity still works exactly the same. The equations of mechanics don't care about time. And yes, information being lost in black holes would be an issue, if true - that's why that problem has been studied so much recently.
But if gravity isn't reversed how would planet formation be reversed? What would allow the matter in a planet to fly apart?
Re: Time’s Almost Reversible Arrow
#9Wouldn't gravity have to be repulsive in a reversed time? Doesn't that mean time can't run in reverse. Also information might be lost in black holes. Wouldn't that also prohibit it reversing time?
https://en.wikipedia.org/wiki/Arrow_of_time
If you go to the 'overview' section, there is a nice snippet explaining all this. It points out that gravity works fine in a universe that is running backwards, however entropy is decreasing instead of increasing.
Re: Time’s Almost Reversible Arrow
#10What s the current thinking on being able to run Schrödinger's cat backwards? How can time be reversed on something in an indeterminate state?