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Time’s Almost Reversible Arrow

quantamagazine.org

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Re: Time’s Almost Reversible Arrow

#11

Post by empiricus »

koverstreet wrote: post #6
mrfusion wrote: post #2

Wouldn'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.

So if I understand correctly, time reversal means that all the object speeds get inverted, but the fields remain the same. If you change the sign of the field (gravity becomes repulsive) then things will explode very quickly :)

Re: Time’s Almost Reversible Arrow

#12

Post by blaze33 »

maho wrote: post #7

This 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…

As I understand it, the universe actually started in a heat-death state (very small temperature differences, homogeneous and isotropic), it's just that it is a very hot heat death state, not the usual cold one where everything freezes.

Then it is the universe expansion that made the entropy per volume decrease, actually allowing complex systems to evolve. The expansion still has to occur faster than the entropy increase rate but I read somewhere that the universe total entropy did not increase that much since the Big Bang.

Re: Time’s Almost Reversible Arrow

#13

Post by maho »

mrfusion wrote: post #8
koverstreet wrote: post #6

Earlier quoted context omitted.

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?

Interstellar gas/dust forming a planet increases its temperature, and it radiates thermal photons out into space. For the planets to fly apart, you would have reverse the photons as well, and aim them super precisely, and define the initial state of the planet super precisely, and... It's essentially a problem of entropy. All weirdness of macroscopic time-reversal can be reduced to the weirdly low entropy of the big bang.

Re: Time’s Almost Reversible Arrow

#14

Post by empiricus »

mrfusion wrote: post #8
koverstreet wrote: post #6

Earlier quoted context omitted.

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?

Lets consider only the moment before the collision of the small planet pieces: these are all accelerating to the center of mass. If you revert time, you still have a lot of pieces, with high speed, but these are going away from the center (and getting slowed by the gravity). After collision, it gets more tricky, because the kinetic energy of a body will be lost (mostly transformed in thermal energy) and it will be less intuitive to imagine how to revert that.

Re: Time’s Almost Reversible Arrow

#15

Post by brudgers »

The modern history of T invariance begins in 1956.

Or 1781, when Critique of Pure Reason was published. Kant proposed that time was an empirical fact based on experiencing events in time being a precondition of all human experience. The tradeoff for a well reasoned non-skeptical position was that we give up claims to "really really" know how things really are independent of human experience...i.e we give up claims to potential omniscience.

That we normally take "knowledge" to mean human knowledge, doesn't change that this is what we mean. Or to put it another way, what would time be for a computer with knowledge or as the article mentions for creatures that experience time backward?

Re: Time’s Almost Reversible Arrow

#17

Post by jsprogrammer »

mrfusion wrote: post #10

What s the current thinking on being able to run Schrödinger's cat backwards? How can time be reversed on something in an indeterminate state?

The indeterminacy may not be real. It's just that you don't know the state. I don't believe there is a rule that says you get to know everything.

Re: Time’s Almost Reversible Arrow

#18

Post by pdpi »

empiricus wrote: post #11
koverstreet wrote: post #6

Earlier quoted context omitted.

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.

So if I understand correctly, time reversal means that all the object speeds get inverted, but the fields remain the same. If you change the sign of the field (gravity becomes repulsive) then things will explode very quickly :)

Nothing gets reversed at the equation level. It's just that every equation with a dt in it gives you reversed results with a negative dt.

Re: Time’s Almost Reversible Arrow

#19

Post by roberto »

maho wrote: post #7

This 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…

> 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.

This one thing I don't understand about the 2nd law of thermodynamics: given enough time won't the system go back to a more organized state simply by chance? And in that case, wouldn't the total entropy be reduced?

Re: Time’s Almost Reversible Arrow

#20

Post by danielsju6 »

mrfusion wrote: post #8
koverstreet wrote: post #6

Earlier quoted context omitted.

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?

You'd see radiation hitting the planet warming the mantel, complex chemicals breaking down & releasing heat, all warming the planet significantly to the point where it was a molten ball, and finally lots of instability where large explosions were throwing pieces of it into space.

Basically take the movie and play it backwards.

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