Ask HN: Time loop and partial blackhole? | Hacker News Reader

Ask HN: Time loop and partial blackhole?

9 pointskashyapPI4 comments

Hello. I am a 9th grade student from India.

AI was used only for grammar correction. These are my own questions.

I am confused about two concepts:

1. Second Law of Thermodynamics: It states that the entropy of an isolated system increases. Decreasing entropy requires work. Does this imply that while time dilation from General Relativity can cause time to pass slower, time can never run backwards, as that would require a spontaneous decrease in entropy?

2. General Relativity: Time passes slower in stronger gravitational fields. As a mental model, is it partially correct to visualize this as the rate of atomic vibrations/motion decreasing in strong gravity, causing clocks to tick slower?

I am not proposing a new theory. I am asking to identify flaws in my current understanding. Please explain at a high school level.

Thank you for your time.

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Are you familiar with the ScienceClic channel on YouTube? They have some really nice videos about subjects like this:

https://m.youtube.com/watch?v=NfTmy1ApCvI

https://m.youtube.com/watch?v=GQZ3R81iyE0

I second the other commenters, and recommend watching the linked videos.

General Relativity and Entropy have not yet been unified into a single theory. In the meantime, gravity always takes priority over entropy. Or in other words, physicists don't yet know how gravity could make entropy slower or faster, so gravity ignores entropy and entropy just has to deal with whatever happens within the gravitational field.

A spaceship in a strong gravity field would not know their time was any different. Someone outside the field would give them a different relative measurement. But inside the field, entropy is not slower and not faster, because the spaceship doesn't see any change in the speed of their own clocks, even if they hear from someone else that their clocks are going at a different speed.

Good luck!

I am not a physicist, and actually know nothing about physics, so take these with a grain of salt:

1. Time runs slower the faster you go, stopping when you reach c. Going back in time would require you to travel faster than c, which the laws of physics generally frown upon.

2. No, I'm not sure there's an intuitive explanation here but motion doesn't slow down in high gravity, time itself does.

1) The two are not directly dependent in that way. General relatively can, in theory, allow for some time-travel like effects, but not ones that violate causality. General relativity, and indeed most theories of physics, are actually 'time reversible', meaning they work just as well (in principle) forwards and backwards. Entropy in thermodynamics is one of the few things which directly implies there's a 'natural' direction for time.

(roughly speaking, this means, that in theory, if you were to instantaneously reverse the motion of every particle in the universe, things would start to play out backwards and entropy would start to decrease. The second law of thermodynamics is basically just saying that this is would only happen if you could do it exactly. Any other configuration of the particles will result in entropy increasing)

2) It's not directly due to the forces from the gravity. In fact, it's more the other way around: spacetime is distorted by gravity and this is what produces the apparent force on objects, as well as time dilation. (or more precisely, mass and energy cause the distortion, which causes the force we call gravity and time dilation).

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