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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 ...
Jeanbaptiste Roux's user avatar
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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 $ct$, ...
Julián Oviedo's user avatar
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Does it follow from Least Action Principle that particles do not go back in time, or do we stipulate this?

Consider the action integral, $S[\gamma] := \int L(\gamma(t),\dot{\gamma}(t),t)dt$. We can always re-write it in terms of an arbitrary curve parameter $\tau$ which need not coincide with time $t$: $$S[...
Rochelle's user avatar
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Energy Conservation in Pair Annihilation and Feynman-Stuckelberg Interpretation

The textbook "Modern Particle Physics" by Mark Thomson on p. (98) reads: In the left plot, an electron of energy $E$ emit a photon with energy $2E$ and, to conserve energy, produces an ...
Samama Fahim's user avatar
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Mathematics behind the thermodynamic arrow of time

Since the thermodynamic arrow of time is related to the second law of thermodynamics, how can I (mathematically) describe the operation of the operator $\hat{T}:t \mapsto -t$ on said law ? In other ...
crvenikupus's user avatar
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Can one derive the second law of thermodynamics from von Neumann's equation?

My idea is that from the von Neumann equation i$\hbar\dot{\rho}(t) = [H,\rho (t)]$ one can derive that $\dot{\sigma}(t)\geq 0$, where $\sigma(t):= -$tr $ \rho(t) $ log $\rho(t)$. However, I end up at ...
Tommy Harmon's user avatar
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Understanding orthochronous, proper and improper Lorentz transformations

The Lorentz group has four connected components that can be characterized as follows: $\det A = 1$ $\det A = -1$ $A^0_0 = 1$ $A^0_0 = -1$. I think I understand the third and fourth components well, ...
CBBAM's user avatar
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Does Einstein's Relativity contradict the Arrow of Time? [closed]

If according to Einstein spacetime is relative & not absolute, and the order of events can change depending on the observer's frame of reference, does this contradict the idea of the Arrow of Time/...
Anuj Manoj Shah's user avatar
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1 answer
157 views

Why did the Big Bang happen first?

As far as I know, the laws of physics are time-reversal invariant, which means there is no preferred direction of time. The arrow of time emerges with entropy which is a property of macrostates, not ...
John Smith's user avatar
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Why thermodynamic arrow of time will not reverse during the big crunch (considering our universe is above the critical mass)?

As the question states, Why thermodynamic arrow of time will not reverse during the big crunch (considering our universe is above the critical mass)? The doubt arised because I thought the ...
Arjun's user avatar
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Causation and radiation

Electromagnetic radiation phenomena exhibit a temporal asymmetry: we observe radiation coherently diverging from a radiating source, such the light emitted by a star, but we do not observe radiation ...
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Reversed time in Norton's dome

"Norton's Dome is a thought experiment that exhibits a non-deterministic system within the bounds of Newtonian mechanics. " A ball rolled to the top can reach it in finite time with zero ...
blademan9999's user avatar
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1 answer
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Making sense of particles going backwards in time [duplicate]

Physicists sometimes talk about particles going backwards in time. Help me make sense of this. I thought things don't "go" in any particular direction in time. They just "are" ...
user371157's user avatar
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Why is any real process which proceeds through non-equilibrium states necessarily irreversible?

As per the title, why is any real process which proceeds through nonequilibrium states necessarily irreversible? The question came up when reading Callen's definition of "reversible process" ...
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Why is causal order not related to directionality of time?

Hans Reichenbach argues for the causality and causal chain to define a topological coordinative definition of time order. Here is an excerpt from his textbook, The Philosophy of Space and Time, Dover(...
Ashwin Balaji's user avatar
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Why does time reversibility imply that there is no arrow of time fundamentally?

The fundamental laws of physics (classical and quantum) are time-reversal invariant. This symmetry is often taken to imply that, at the microscopic level, there is no preferred direction of time, no ...
John Smith's user avatar
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Could our physical universe be time-irreversible?

Suppose that in our universe, there were two initial states which lead to the same outcome state after one time step. How could we possibly remember which of the two steps we came from if the ...
AlgebraicsAnonymous's user avatar
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Possibility of anti-time

I was just wondering if every particle has an anti-particle (or so I've read/heard), are there other things which could be candidates to have an anti- "form" as well? My example is focused ...
David Motlagh's user avatar
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1 answer
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Would an observer moving backwards in time observe the same laws of physics? [duplicate]

This is a major curiosity of mine, I've grown familiar with the notion of spacetime and while I know space and time aren't directly interchangeable I've still come across thoughts of changing ...
JohnIsBueno's user avatar
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1 answer
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Arrow of time as the result of a feedback loop that stabilizes an inheret unstable reversible time [closed]

I am searching to find some literature on idea i am having, regarding the "arrow of time". The main idea is an expansion of the main theme of an Asimov sci-fi novel, where he postulates ...
Grigoris L. 's user avatar
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Can we think of entropy and time as the same thing? [duplicate]

My assumption is that entropy is a change in the distribution of energy in one direction. The universe is a system going from low to high entropy. Can time be described without the change of entropy? ...
hermancain's user avatar
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Does it make physical sense to assign an entropy to a microstate?

In physics, the entropy is related to the volume of microstates associated with a macrostate. So entropy is a property of a macrostate. However, does it make sense to assign an entropy to a microstate?...
yalis's user avatar
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Consequence of entropy increasing with time [duplicate]

Our bodies are a highly ordered collection of atoms. Entropy is a measure of disorder. Since the entropy of the universe increases with time, does that imply that humans will necessarily become ...
Shoes's user avatar
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Is the 2nd law of thermodynamics more of a likelihood than a law? [duplicate]

Given enough time, even extremely unlikely events are certain to happen. This includes spontaneous reduction in disorder. As an example, given enough time, all the particles of a gas in a container ...
Ritesh Singh's user avatar
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Why do we need an explanation for the arrow of time?

I'm reading Sabine Hossenfelder's new book Existential Physics, where she explains that because fundamental laws are time-reversible, we need an explanation for the arrow of time. Why is that exactly? ...
qleguennec's user avatar
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If in principle all processes are $TPC$ symmetric then what causes the direction of time?

Let's assume all processes are TPC symmetric. Be it the breaking of an egg (just reverse all momenta of all particles involved), the evolution or collapse of the wavefunction (a big if but let's ...
Gerald's user avatar
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1 answer
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Do irreversible thermodynamic processes CONSTITUTE time or do they MOVE IN time? [closed]

Time can be associated with irreversibility. A broken egg can't reassemble. Most, if not all, processes are irreversible and this is associated with time going in one direction. Ŕemarkably, living ...
Gerald's user avatar
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1 answer
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Could the passage of time be sinusoidal? [closed]

As we know the universe is moving towards an equilibrium, or high state of entropy. From what I understand, we call this the flow of time. When google announced their time crystal, they call it "...
Daniël van den Berg's user avatar
3 votes
2 answers
231 views

Weinberg's proof of Gibbs' $H$-theorem

I'm trying to understand Weinberg's explanation of a general $H$-theorem he attributes to Gibbs (Foundations of Modern Physics, p. 35), but I'm having trouble with the mathematical modeling. The goal ...
Yotam Feldman's user avatar
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1 answer
154 views

Can interacting QFT be formulated in terms of the path integral or hamiltonian of a relativistic particle? [duplicate]

I read that free QFT need not be formulated in terms of fields. One can derive the same propagator as the path integral of the single free particle action $\int d{\tau} \eta _{\mu \nu} x^{•\mu}x^{•\nu}...
Ryder Rude's user avatar
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Why are positrons traveling backward in time in Feynman diagrams?

We draw the positron as if it's traveling backward in time. Why? The momentum of the positron is drawn opposite to the time-direction of the diagram. I don't see this having any effect on the ...
Ryder Rude's user avatar
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1 vote
1 answer
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Confusion with time symmetry - pendulum example

If the classic nonlinear pendulum with friction equation is reviewed as example, in Wolfram-Alpha it can be seen it have decaying solutions as expected: $$\ddot{x}+2\cdot 0.021\,\dot{x}+0.2\sin(x)=0, ...
Joako's user avatar
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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 ...
sleep's user avatar
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1 answer
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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.
Quinten C's user avatar
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9 answers
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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 ...
reductionista's user avatar
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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 ...
Ghufran Fazal's user avatar
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0 answers
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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 ...
foggy's user avatar
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4 answers
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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? ...
MindWalk's user avatar
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2 answers
81 views

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 (...
Tachyon's user avatar
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5 answers
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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 ...
MatterGauge's user avatar
2 votes
1 answer
169 views

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 ...
Michael Curtis's user avatar
2 votes
1 answer
314 views

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'...
user404's user avatar
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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 ...
Hans's user avatar
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5 answers
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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 ...
Pingou's user avatar
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23 votes
13 answers
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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 ...
Amr's user avatar
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-1 votes
2 answers
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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 ...
Jase's user avatar
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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 ...
mebaker's user avatar
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4 votes
2 answers
310 views

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 ...
Damian Birchler's user avatar
13 votes
2 answers
1k views

Are chaotic systems the same as dissipative systems in inverse time?

Lyapunov exponents define whether a system expands or contracts in phase space and can be used to determine whether a dynamical system is chaotic, conservative, or dissipative. If the volume expands ...
J.Galt's user avatar
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Did a particle really seem to go backwards in time during a collision experiment at LHC?

I distinctly remember the news of a weird particle which seems to have gone backwards in time .Probably a Higgs singlet It was probably a high energy collision between proton and proton . A particle ...
Swapnil B's user avatar

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