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.
Questions tagged [arrow-of-time]
215 questions
142
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Is anti-matter matter going backwards in time?
Some sources describe antimatter as just like normal matter, but "going backwards in time". What does that really mean? Is that a good analogy in general, and can it be made mathematically precise? Physically, how could something move backwards in…
Gerard
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10 answers
What is time, does it flow, and if so what defines its direction?
This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address issues such as:
What do physicists mean by…
John Rennie
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64
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5 answers
Have researchers managed to "reverse time"? If so, what does that mean for physics?
According to press releases, researchers have reversed time in a quantum computer and violated the second law of thermodynamics. What does that mean for physics? Will it allow time travel?
Further information:
"Arrow of time and its reversal on…
Omar Einstein
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8 answers
Does the scientific community consider the Loschmidt paradox resolved? If so what is the resolution?
Does the scientific community consider the Loschmidt paradox resolved? If so what is the resolution?
I have never seen dissipation explained, although what I have seen a lot is descriptions of dissipation (i.e. more detailed pathways/mechanisms for…
propaganda
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16 answers
Can a broken egg spontaneously reassemble itself (as in the video)?
According to the fluctuation theorem, the second law of thermodynamics is a statistical law. Violations at the micro scale, therefore, certainly have a non-zero probability. However, the application of the theory, in particular the Jarzynski…
kbakshi314
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In reverse time, do objects at rest fall upwards?
I want to develop a game where time runs backwards, based on the idea that physical laws are reversible in time. However, when I have objects at rest on the earth, having gravity run backwards would mean that the objects are repulsed and falling…
Konrad Höffner
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4 answers
How would the laws of nature behave if we reversed time?
Suppose a ball falls from a certain height and reaches the ground. Later on, somehow we managed to reverse time. Now on reversing time, will the ball move upward to reach the same point from where it was fallen (as literally, reversing time means…
newera
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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 about our universe, and you were constructing…
reductionista
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25
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6 answers
Why does time not run backwards inside a refrigerator?
The arrow of time is often associated with the fact that entropy always increases. On the other side that should mean, if entropy decreases time should run backwards. But inside a refrigerator we have that situation. Entropy inside a refrigerator…
asmaier
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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 colliding (under the effect of gravitational forces…
Amr
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Is a 1D vector also a scalar?
A vector in one dimension has only one component. Can we consider it as a scalar at the same time?
Why time is not a vector, although it can be negative and positive (when solving for time the kinematics equation for example)?
Revo
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22
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2 answers
Doesn't entropy increase backwards in time, too?
In statistical explanations of entropy, we can often read about a (thought) experiment of the following sort.
We have a bunch of particles in box, packed densely in one of the corners. We assume some temperature, and with it some random initial…
isarandi
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Does gravity reverse entropy?
How I got interested:-
A few days ago I was watching a few YouTube videos about reversing entropy and how it was impossible.
But while thinking about it, it suddenly seemed like gravity reverses entropy.
My understanding of entropy:-
Entropy is the…
Rounak Sarkar
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18
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3 answers
Demystifying time-reversal symmetry in physics
Briefly, which physical theories are expected to be time reversal invariant? That is, the mapping of $t\to -t$ will not alter the physics.
Even in Classical Mechanics (CM) it is not obvious if time reversal ought to leave things invariant:
On the…
user929304
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6 answers
Why does a sign difference between space and time lead to time that only flows forward?
Ever since special relativity we've had this equation that puts time and space on an equal footing:
$$ds^2 = -dt^2 + dx^2 + dy^2 + dz^2.$$
But they're obviously not equivalent, because there's a sign difference between space and time.
Question: how…
Allure
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