# Questions tagged [arrow-of-time]

A concept related to the asymmetry of time, usually related to the second law of thermodynamics, which says that entropy always either increases or stays the same.

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### On time arrow and coordinate change in General relativity

This may be a silly question but I would like to have things cleared up once and for all in my head. I will take the example of a Schwarzschild black hole as a solution to vacuum Einstein Field ...
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### Unidirectionality of Time in Spacetime

I have a question regarding the dimension of time. We all know that an event in spacetime is defined by a point $${x}^{u} = (ct, x, y, z) .$$ The only component that breaks the symmetry is $ct$, ...
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### In-principle, can we precisely simulate physics in reverse?

A naïve question, I'll admit, about entropy and the various arrows of time. If we have as input the position or momentum (the wave function, I suppose) of every particle, would it then be possible to ...
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### Is antimatter just matter inverted in an higher dimension? [closed]

This question arose in me after watching https://youtu.be/mmtLgYVEuJs?t=394. The link has a time in it so it takes you to the part I am talking about in the video.
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### Time is the only dimension that has an arrow, and the only dimension which contributes an opposite sign to the metric. Is that just a coincidence?

Time is different from space in these two seemingly independent ways. One of them is generally believed to have to do with special boundary conditions at the beginning of time. But if you knew nothing ...
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### What is entropy in the concept of arrow of time? [duplicate]

When I first started to study arrow of time, I first heard the word Entropy and after watching and reading internet resources I can't get what is this in the concept of arrow of time. So, please tell ...
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### Has the possibility of time travel been proven or demonstrated? [duplicate]

This study by Lesovik et al., 2019 seems to claim they have either sent information or an electron back in time. What is the current status of the idea of time travel (at least on the scale of ...
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### Time's Arrow and QM

Something puzzles me. It is sometimes said that the fundamental equations of physics are time-reversible, creating the problem of time's arrow. But... isn't Schrodinger's wave equation time-dependent? ...
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### Gravitational attraction correlation to time [closed]

Could the fact that gravity is only attractive, and not repulsive, have anything to do with the fact that time only moves forward? EDIT: I am asking specifically about gravity, and not other forces (...
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### How do we explain the motion of a time-reversed emptying balloon in vacuum?

Imagine a balloon filled with air, let loose in outer space. There is an imbalance of pressure pushing to the insides of the balloon, and this imbalance is the motor of motion. There is no pressure in ...
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### What does the arrow of time and entropy say about the universe and repetition?

This question What is the relationship between how time is viewed in thermodynamics and how time is viewed in general relativity? is close to what I was wondering, but it didn't get into repetition ...
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### If antiparticles are particles moving back in time, would messages from the future be possible?

So, here's the idea: if Feynman's interpretation of antiparticles is true and antiparticles are particles moving back in time, then they are carrying with them information from the future, no? If that'...
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### Could Matter Go Backwards in Time?

In the real world, it seems that traveling backwards in time is impossible, but do we have a theorem in physics that would imply this fact? Some people (including Feynman) describe antiparticles as ...
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### Reversibility of the arrow of time

I often read in physics vulgarisation books about how paradoxal it is that the time seems to go only one direction, as entropy grows with time, and nobody has ever seen a broken cup repair itself and ...
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### Why is the second law of thermodynamics not symmetric with respect to time reversal?

The question might have some misconceptions/ sloppy intuition sorry if that's the case (I'm not a physicist). I seem to have the intuition that given a system of $N$ charged particles in 3D space ...
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### A 'better' definition of time [closed]

I have been annoyed by official definitions of time from many Google searches and decided after listening to Sean Carroll's podcast to write my own. Here is what I did so far: What is time? A state ...
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### Is a lack anti-matter related to the arrow of time?

Mathematically speaking, we know that antiparticles can be equivalently thought of as particles travelling backwards in time. We also know there is a distinct lack of anti-particles in the universe ...
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### Does an irreversible thermodynamic process always increase the entropy?

I know that if over the a given thermodynamic process the entropy of the universe increases, then that process is irreversible. But does the opposite also hold true, i.e. do all irreversible processes ...