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 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 Alpert 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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Is there an actually formal theory of memory?

After reading through (Rovelli, 2020) and (Wolpert, 1992), I am not convinced that they present a formal theory of memory, and its relation with an arrow of time (be it the thermodynamics one or any ...
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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 StackExchange, 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 ...
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Can time flow backwards? [duplicate]

Is there any equations in physics or laws that say time can flow backwards instead of forward?
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What is the relationship between how time is viewed in thermodynamics and how time is viewed in general relativity?

From my limited understanding of physics, it seems that the second law of thermodynamics, in which entropy never decreases over time in a closed system, relates to how time can only go forward i.e. ...
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Is time travel a violation of second law of thermodynamics?

According to the increase of entropy principle, the entropy of the universe is always increasing. So, does going back in time violates the second law of thermodynamics? Because entropy of the universe ...
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Confusion about entropy when applied to the whole universe. What are the macrostates?

I'm really confused about the concept of entropy when applied to the whole universe. The often hear that the universe started with very low entropy and as the entropy increases the universe will ...
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328 views

Can you reverse the arrow of time?

Has there been any attempt to reverse the arrow of time?
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What distinguishes time directions from spacial directions?

Other than the fact that one may move in only one direction in time but can move in two directions in each spacial dimension (e.g. arrow of time), are there any other physical characteristics that ...
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231 views

Will a shrinking universe have a reverse arrow of time?

I'm not a physicist so forgive me if this question is silly. I'm reading (actually listening) to Mysteries of Modern Physics: Time by Prof. Sean Carroll. I'm not sure if the concepts in this book ...
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How can we determine time flows in one direction? [duplicate]

Suppose that time could flow in either direction, forward or reverse, similar to a movie. How could our brain possibly record that time had moved backwards? Would there be any way to tell if time had ...
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1answer
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Is Time the same as The arrow of Time? [duplicate]

I asked this question here. Some doubts emerged in my mind from the comments. Here they are: Are time and the arrow of time, the same thing? All of modern physics, seems to treat time as just another ...
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Understanding time [duplicate]

Am sorry if this is a silly question. I have heard something like this from cosmologists: "There is no intrinsic arrow of time. Today, we believe that the direction of this arrow is due to our ...
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1answer
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Thermalization in Classical Mechanics: A Paradox

I was wondering to know whether there is any argument shows how a classical system (a system with a Hamiltonian and Poisson bracket) approaches its equilibrium and how the entropy increases. Newton's ...
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1answer
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Do past states of a system have lower entropy?

It's often said that the second law of thermodynamics is the only time asymmetric law in physics, namely $S(t_2) \geq S(t_1)$ if $t_2 > t_1$. But it seems to me that the concrete application of ...
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1answer
126 views

Reverse entropy, reverse causality and physical laws

I do not have any scientific education and I am far away from understanding relativity, quantum mechanics and such. I am just wondering about entropy, causality and if we could reverse these in a ...
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Does time matter?

Aside from the second law of thermodynamics it seems physics just doesn't care about the direction of time. So does time really matter? What if instead of 3 spacial and one time dimension. 3+0 or 3+2....
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Validity of Boltzmanns Equation and $H$-function theorem?

A while ago I came across a resource (which I have forgotten) on the validity of Boltzmann's equation. It talked about the fact that the Boltzmann's equation is valid at the extrema of the $H$-...