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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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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How can retardation in flows be determined?

Say that the effect of retardation of flow is to be calculated. How can it be determined? Retardation is understood to occur as a consequence of flow changes to be propagated with the speed of sound. ...
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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 ...
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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. In the volume expands ...
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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 ...
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Is the 4th dimension an infinite set of 3rd dimensions?

My understanding goes like this: an x-y plane (or flat 2-dimensional surface) is an infinite set of lines, each one dimensional. The x-y-z space (or 3d volume) is an infinite set of 2-dimensional ...
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Is time still (frozen) for a single isolated particle?

I understand entropy is called the arrow of time because it is the only physical property which always increases in an isolated system (based on 2nd law of thermodynamics). On the other hand, entropy ...
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Does time symmetry cause the matter/antimatter asymmetry?

It’s known that the fundamental laws of physics are time symmetric. In electromagnetism $t$ can be changed to $-t$ and time reversal is also valid in particle physics, as there is the CPT symmetry. ...
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Where does the irreversiblity came from if all the fundamental interaction are reversible? [duplicate]

There isn't too much to explain: We know that all fundamental forces are reversible then where does the irreversibility come from? Edit: The following is edit based on comments: Consider a block of ...
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Why does time seem to flow in only one direction? [duplicate]

According to my understanding. Since Einstein, physicists have thought of space and time as forming a four-dimensional structure known as “spacetime.” But space differs from time in some very ...
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Time reversal using entropy

We characterize the flow of time with respect to entropy where energy is going from an unstable state to a stable state. Basically from high energy to low energy. Would this mean any particle or ...
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Does gravity reverse entropy? [duplicate]

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 ...
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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 ...
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If all motion stops, does time cease to exist? Is awareness then still possible in this stae? [closed]

I mean on an entire universe level. We rotate the sun, the sun some black hole, the galaxy speeding off from some explosions in an unknown central location... On it goes. Everything moving at ...
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Is the increase of entropy a theorem?

Let us consider an isolated system of $N$ particles whose motion is determined by Newtonian mechanic, over time that stretches from $-\infty$ to $+\infty$. The system is isolated, therefore the only ...
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Can thermodynamics be considered logical?

One of the laws says that heat won't flow from cool to warm and at the same time this same theory claims that there is a finite (albeit tiny) chance that it will, because there is always such a ...
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How entropy (thermodynamics) related to time flow?

In Tenet (2020), Laura speaks about inverted bullets: Laura: It's inverted. Its entropy runs backward. So, to our eyes, its movement is reversed. We think it's a type of inverse radiation triggered ...
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Physical basis for arrow of time

If laws of physics are time symmetric, is motion forwards and backwards in time only defined in terms of increase or decrease in entropy (low entropy is past and High entropy is future)? If it is so, ...
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Simplest possible thought experiment that illustrates the difference between the past and the future?

We are led to believe that the "arrow of time" is determined by the direction in which entropy increases. Entropy is a measure of disorder, but disorder is very much in the eye of the ...
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Can quantum measurements be the origin of thermodynamic arrow of time?

We can practically consider that the microscopic interactions are symmetric with respect to time(as we can neglect weak force for many cases which is the only interaction that can violate $T$ symmetry)...
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Why is Entropy One-Directional? [duplicate]

If the laws of physics work the same forward and backward in time, why does entropy grow in one direction?
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How does $S = k\ln W$ relate to the statement that heat does not flow from cold to hot objects?

There're several ways the Second Law of Thermodynamics can be stated. I'm thinking of these two: Entropy never decreases spontaneously. and Heat does not spontaneously flow from cold to hot objects....
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Wouldn't a negative mass be going faster than $c$ according to our current models of relativity?

I heard that there are some physicists trying to figure out, out least hypothetically, how things with positive and negative mass may interact with each other. I'm really confused about how this can ...
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What could it mean to have "memories" of the thermodynamic future? [closed]

Edit -> Editing as the original question seemed generic, ambiguous and susceptible to being misconstrued for pseudoscience (astrology,etc) --- Essentially what I am saying is this : a) If you face ...
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Is there something like Anti-time? [closed]

According to mathematics, everything has its opponent. Suppose something is $x$. Let $x = 5$, then $x^2 = 25$, and again if $x^2 = 25$, then x may be positive or negative. In the same way, let $x = ...
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Obtaining the direction of flow from principle of increase of entropy?

Can we mathematically show that water flows from high-pressure region to low-temperature region and why particles flow from a region of higher chemical potential to lower on the basis of the principle ...
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If the integral of $\frac{dQ}{T}$ over an engine cycle is less than 0 , then why is entropy of the universe always increasing?

For a reversible process, we define $ dS= \frac{dQ}{T}$ , so, the integral being negative would suggest that the entropy of universe decreases with each cycle of the engine because the clasius ...
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Does Another Universe Where Time Runs Backwards Revive the Idea of a Cyclical Universe?

https://www.pbs.org/wgbh/nova/article/big-bang-may-created-mirror-universe-time-runs-backwards/ This article talks about a new theory where the big bang happened in one direction for us and the other ...
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3 votes
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Is the arrow of time only given from initial conditions?

I am not a physicist, but I have tried to sort out where the arrow of time comes from or how it fits into our physical models. Harvey Brown, a philosopher, physicist, and professor emeritus at ...
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Irreversible processes in special relativity

According to special relativity (as I understand it) simultaneity is no longer a universal concept in special relativity. Consider two events A and B which are simultaneous in reference frame 1; in ...
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What was the rationale behind Stephen Hawking's "biggest mistake"?

Albert Einstein said he made his biggest blunder, while Stephen Hawking spoke of his "biggest mistake". One can read this in this interesting book review (The Nature of Space and Time by Stephen ...
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Can we revert back a broken egg into the original one? Given that we are allowed to increase entropy in some other part of the system

Background (and much of the argument for the question) The second law of thermodynamics says(as my book states it): If a process occurs in a closed system, the entropy of the system increases for ...
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Geodesic with decreasing value of time coordinate

Does there exists an example of geodesic for an exotic space-time manifold in which even though the proper time on the geodesic is increasing but still the time coordinate of the geodesic in global ...
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Is general relativity compatible with an arrow of time?

The spacetime of Special Relativity is Minkowski's "Block Universe" where time is treated as a dimension. Does General Relativity allow for an arrow of time or "flow" of time? If yes, can this be ...
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Does an electron has entropy?....and if not then does Arrow of time apply to them?

We know that entropy is the measure of disorder , which is related to a system having constituent particles , and this entropy gives us the concept of 'the arrow of time' , i.e. time has only one ...
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What happens here when the Second Law of Thermodynamics doesn't hold true?

In the 1920s, theoretical physicists, most notably Albert Einstein, considered the possibility of a cyclic model for the universe as an (everlasting) alternative to the model of an expanding universe. ...
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Does the speed of universal expansion lead to time going backwards? [closed]

After reading that time stops at the speed of light and at event horizons I wondered if when the speed of light is exceeded does time loop backwards ... The thought struck me too that the term space-...
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Why is there an arrow of time? [duplicate]

I believe I read on the internet that quantum mechanics predicts that the fundamental laws are time invertible. The web page probably really meant one theory predicts that. Does that theory combined ...
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Is there a connection between the spreading of the wave packet of the universe and the time arrow?

Entropy of position and wave packet spreading. Why do wave packets spread out over time? Wave packet of the universe and its  spreading. Is there a connection between the spreading of the wave ...
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