The theoretical symmetry of physical laws under the transformation of time reversal, that is t -> -t.
Questions tagged [time-reversal-symmetry]
481 questions
65
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Do sunrises and sunsets look the same in a still image?
A question that popped into my head: if I see a picture of the sun close to the horizon, in an unknown place, can I know if it was taken at sunset or sunrise?
Do sunrises and sunsets look the same in a still image? Can one tell them apart?
AlonMln
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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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Do pear-shaped nuclei really have anything to do with time travel?
Recently (in the last week or two), various articles about pear shaped nuclei have appeared, such as this one from Science Alert and this from the BBC
The Science Alert article includes the quote
We've found these nuclei literally point towards a…
Matt
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Does Heisenberg's uncertainty under time evolution always grow?
Recently there have been some interesting questions on standard QM and especially on uncertainty principle and I enjoyed reviewing these basic concepts. And I came to realize I have an interesting question of my own. I guess the answer should be…
Marek
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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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Reflection positivity for general fields
In the Euclidean QFT obtained by "Wick-rotating" a unitary QFT, the correlation functions satisfy a property called reflection positivity, see e.g. this Wikipedia article for the case of a scalar field.
What's the correct formulation if you have…
Yuji
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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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Does light reflect if incident at exactly the critical angle?
A lot of textbooks and exam boards claim that light incident at exactly the critical angle is transmitted along the media boundary (i.e. at right-angles to the normal), but this seems to violate the principle of reversibility in classical physics. …
Michael C Price
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5 answers
How would physics explain why I can't un-fold paper?
This seems like a silly question, but I would love to see a serious answer.
It's known that almost all fundamental laws of physics are time-reversible, and time-asymmetry only comes about when we consider increases in entropy. It's easy to think…
MaximusIdeal
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Why particle hole symmetry and chiral symmetry are called symmetries?
$PHP^{-1}=-H$ (particle-hole symmetry)
and
$\Gamma H \Gamma^{-1}=-H$ (chiral symmetry)
I understand why we get the negative signs but im just a bit confused as to why such equalities mean $H$ is particle hole and chiral symmetric.
When we say $H$…
Blackwidow
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Spontaneous Time Reversal Symmetry Breaking?
It is known that you can break P spontaneously--- look at any chiral molecule for an example. Spontaneous T breaking is harder for me to visualize. Is there a well known condensed matter system which is uncontroversial example where T is broken…
23
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13 answers
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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If electrons were just positrons moving backwards in time, then shouldn't we see them coming out of black holes?
I have read this question (What would happen if I crossed the shell of a hollow black hole?):
In effect, the formerly spacelike direction inwards becomes timelike, and the singularity is in the future (and just as unavoidable as the future is for…
Árpád Szendrei
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What sort of experiment would directly test time reversal invariance?
I guess the title says it all: how could/would you experimentally test whether our universe is truly time reversal invariant, without relying on the CPT theorem? What experiments have been proposed to check this? Have any of them been performed?
I…
David Z
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Do colliding black holes violate time reversal symmetry?
Two black holes can collide and merge into one bigger black hole, but not split into two. Does this mean colliding black holes violate time reversal symmetry?
Related: Do black holes violate T-symmetry? Based on the answer to that question,…
Allure
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