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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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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What is the difference between “time symmetry violation” and “time reversal”?

I am getting confused by the various terms used in the literature. There is "time (reversal) symmetry" that is violated in experiments and there is "time reversal". For context, I ...
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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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Why does the number of accessible microstates always increase?

If we consider two systems with number of accessible microstates A and B and internal energies of E(A) and E(B) that are exchanging an small amount of heat Q from A to B we get the following: A total ...
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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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Where does David Albert coin the term “cognitively unstable”, attributed to him by Sean Carroll?

In his paper, Why Boltzman Brains Are Bad, Sean Carroll says the following: The randomly-fluctuating universe scenario is therefore self-undermining, or as Albert has characterized similar situations ...
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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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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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Is reversal of velocity always equivalent to reversal of time?

Let us imagine there is a container full of small particles which are allowed to collide with each other and the container walls (in 2D). If I initialise the system with given velocities and positions ...
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Second law of thermodynamics in general relativity

I recently came across some posts in Stack Exchange, which say that simultaneity doesn't have any meaning in General Relativity (see Is simultaneity well-defined in general relativity?). I came up ...
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What does the time reversibility of the laws of physics mean for causality?

Does the fact that the fundamental laws are symmetric with respect to direction of time show that causation does not exist? Since causality always requires the cause to precede the effect, but laws of ...
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Increase in entropy and integer factorization (How much work does one have to do to revert “irreversible” processes?)

This is not necessarily a direct physics question, but I am hoping, that by posting it here, someone can give a hint from the physics perspective. It is known, albeit not proven, that integer ...
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Question about a mirror Universe

In the article We've seen signs of a mirror-image universe that is touching our own in the popular magazine “New Scientist” (which describes a real scientific theory though, though in a popular form, ...
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Why do we perceive time?

Since everything (at least what we observe) is travelling through the time in the same direction and speed why do we even perceive the time? Of course, there is the time dilatation, but it is ...
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Antiparticles travelling backward through time [duplicate]

Could someone please explain to me Feynman and Wheeler's theory of waves acting forward and backward in time? I have read about their idea of an antiparticle behaving as a particle travelling ...
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How does the direction of time work with timeless wave functions?

In thermodynamical theory, if we have a set of states for example: A) gas all in top left corner of box. B) gas spread out near the left. C) gas spread out to fill entire box. We can give the ...
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Arrow of time and CPT symmetry

It's been a few years since my physics degree. But I've been wondering: if you look at pictures of collisions at CERN, isn't it obvious which way time flows - simply on a probability basis? The ...
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Is reversing the arrow of time that same as going backward into the past?

I read an article today that somwhat confused me. Is this the same as time travel...which I have always thought was a contradiction. Are they talking about reversing law of thermodynamics? In which ...
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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 ...
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Graviton particles travelling backwards

I know that time travelling backward particles are anti-matter particles but what about graviton time travelling backward?
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Are Mirrored Universes With Opposing Directions of Time Theoretically possible? [closed]

I found an article that talks about this here, entitled "Big Bang May Have Created a Mirror Universe Where Time Runs Backwards". I know any notion of backwards time is probably a dead horse in this ...
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Physics of time running backwards

Although it would seem weird to analyze physical phenomena when time runs backwards, it seems to have a logical sense, at least for me: Entropy would tend to decrease: two balls having energy ...
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What goes wrong, theoretically, when we reverse time?

(Please bear with me if this is a stupid question; I'm not a physicist, just a curious student.) I know that Noether's Theorem links symmetries to conserved quantities: the fact that the laws of ...
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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 ...
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Brouwer fixed point theorem

How does the Brouwer fixed point theorem (which provides a solution to the photo camera paradox of a causal loop) generalise to show that objects in closed time-like curves do not violate the second ...