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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134 votes
11 answers
102k views

Is anti-matter matter going backwards in time?

Some sources describe antimatter as just like normal matter, but "going backwards in time". What does that really mean? Is that a good analogy in general, and can it be made mathematically precise? ...
Gerard's user avatar
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105 votes
10 answers
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What is time, does it flow, and if so what defines its direction?

This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address ...
64 votes
5 answers
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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 ...
Omar Einstein 's user avatar
38 votes
8 answers
6k views

Does the scientific community consider the Loschmidt paradox resolved? If so what is the resolution?

Does the scientific community consider the Loschmidt paradox resolved? If so what is the resolution? I have never seen dissipation explained, although what I have seen a lot is descriptions of ...
propaganda's user avatar
34 votes
16 answers
11k views

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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32 votes
11 answers
5k views

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 ...
Konrad Höffner's user avatar
29 votes
4 answers
6k views

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 ...
newera's user avatar
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26 votes
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
25 votes
6 answers
2k views

Why does time not run backwards inside a refrigerator?

The arrow of time is often associated with the fact that entropy always increases. On the other side that should mean, if entropy decreases time should run backwards. But inside a refrigerator we have ...
asmaier's user avatar
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23 votes
13 answers
6k views

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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22 votes
4 answers
14k views

Is a 1D vector also a scalar?

A vector in one dimension has only one component. Can we consider it as a scalar at the same time? Why time is not a vector, although it can be negative and positive (when solving for time the ...
Revo's user avatar
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22 votes
2 answers
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Doesn't entropy increase backwards in time, too?

In statistical explanations of entropy, we can often read about a (thought) experiment of the following sort. We have a bunch of particles in box, packed densely in one of the corners. We assume some ...
isarandi's user avatar
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18 votes
8 answers
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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 ...
Rounak Sarkar's user avatar
17 votes
4 answers
4k views

On the nature of the collapse of the wave function

The collapse of the wave function by measurements is one of the most mysterious properties of quantum mechanics. At what scale does the wave function collapse? What are the conditions for a collapse?...
Jack's user avatar
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17 votes
3 answers
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Demystifying time-reversal symmetry in physics

Briefly, which physical theories are expected to be time reversal invariant? That is, the mapping of $t\to -t$ will not alter the physics. Even in Classical Mechanics (CM) it is not obvious if time ...
user929304's user avatar
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17 votes
7 answers
8k views

Why do we remember the past but not the future? [closed]

The question is sometimes referred to as the "psychological arrow of time" (Hawking, 1985). Here the past is understood as a moment or time when the entropy of the universe was lower, and contrarily ...
Eric's user avatar
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17 votes
3 answers
6k views

Why does the low entropy at the big bang require an explanation? (cosmological arrow of time)

I have read Sean Carrol's book. I have listened to Roger Penrose talk on "Before the Big Bang". Both are offering to explain the mystery of low entropy, highly ordered state, at the Big Bang. Since ...
Gordon 's user avatar
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16 votes
4 answers
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Comments on entropy and the direction of time in Landau and Lifshitz's Statistical Mechanics

In Landau and Lifshitz's Stat Mech Volume I is the comment: However, despite this symmetry, quantum mechanics does in fact involve an important non-equivalence of the two directions of time. ...
Peeter Joot's user avatar
15 votes
8 answers
6k views

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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15 votes
3 answers
2k views

Is the universe deterministic when looking backwards?

Someone once told me that if, in theory, we could know the position and motion of all the particles in the universe, we could use that data to run time backwards, and work out everything that had come ...
joeytwiddle's user avatar
15 votes
2 answers
1k views

Quantum entaglement and the arrow of time

I have seen several claims to that quantum mechanics is required to explain the arrow of time which I take to mean the macroscopic irreversibility of physical systems. This is presumably to resolve ...
Daniel Mahler's user avatar
14 votes
6 answers
4k views

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 ...
Allure's user avatar
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14 votes
3 answers
442 views

What makes now special?

I recently came across the paper Relation between the psychological and thermodynamic arrows of time (arXiv). Their argument makes sense to me, however their concept doesn't seem to address what 'now'...
iRoygbiv's user avatar
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11 answers
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Where does the irreversiblity came from if all the fundamental interaction are reversible?

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 ...
Young Kindaichi's user avatar
13 votes
2 answers
1k views

Calculation of the cross section

Why, when we calculate the total cross section, we make the average other initial states and the sum over final states?
Andy Bale's user avatar
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12 votes
3 answers
956 views

Arrow of time and entropy?

The arrow of time is usually defined by the direction in which entropy increases. In a closed system, if there's a max entropy that the system can reach, does that mean time stops or at least become ...
iBe's user avatar
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12 votes
4 answers
2k views

Entropy as an arrow of time

From what I understand, entropy is a concept defined by the experimentalist due to his ignorance of the exact microstate of a system. To say the number of accessible microstates $W$ of the universe is ...
orange_soda's user avatar
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12 votes
2 answers
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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. If the volume expands ...
J.Galt's user avatar
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12 votes
2 answers
2k views

Will entropy continue to increase even if the universe begins to contract?

If the universe is heading for a big crunch, when the universe starts to collapse will entropy decrease and the arrow of time consequently reverse or not? I'm interested in the explanations, not just ...
Kenshin's user avatar
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11 votes
3 answers
402 views

Reference request for low entropy Big Bang

There is a somewhat widely accepted argument that the second law of thermodynamics exists because the universe began in a low-entropy state. I'm writing a paper that mentions this (and must be ...
10 votes
2 answers
756 views

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 ...
user581844's user avatar
9 votes
6 answers
2k views

Second law of thermodynamics and the arrow of time: why isn't time considered fundamental?

I've come across this explanation that the "arrow of time" is a consequence of the second law of thermodynamics, which says that the entropy of an isolated system is always increasing. The argument is ...
Joebevo's user avatar
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9 votes
3 answers
1k views

Physics without time

The structure of physical law as we generally recognize it today is firmly held together by the existence of time. Although physical laws can both predict and retrodict (reversible time does not break ...
docscience's user avatar
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9 votes
3 answers
747 views

Did really everything begin with a state with very low entropy?

As emphasized by Penrose many years ago, cosmology can only make sense if the world started in a state of exceptionally low entropy. The low entropy starting point is the ultimate reason that the ...
Newman's user avatar
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8 votes
3 answers
1k views

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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8 votes
3 answers
5k views

Why does the law of increasing entropy, a law arising from statistics of many particles, underpin modern physics?

As far as I interpret it, the law of ever increasing entropy states that "a system will always move towards the most disordered state, never in the other direction". Now, I understand why it would ...
Hannesh's user avatar
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8 votes
1 answer
416 views

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
8 votes
6 answers
2k views

Explaining Arrow of Time with Entropy

I just watched [some BBC show] where the host talked about Arrow of Time, where by 2nd law of thermodynamics states that "nature" is always low entropy to high entropy OR "ordered/structured" to "...
Jake's user avatar
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8 votes
1 answer
355 views

Is Quantum Mechanics time-symmetric?

In physics we have the CPT-theorem which guarantees time-reversal symmetry of dynamical evolution (although in some edge cases we will also have to reverse parity and charge). It seems clear enough ...
DPatt's user avatar
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8 votes
2 answers
1k views

Spontaneous conversion of heat into work at negative temperatures

Consider a heavy macroscopic object moving in a gas. Friction causes its kinetic energy to be converted into heat. Thermodynamically, there is (effectively) no entropy associated with the kinetic ...
N. Virgo's user avatar
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7 votes
3 answers
291 views

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
7 votes
1 answer
1k views

Are laws of gravity time symmetric?

Time symmetry is often explained by the example of orbiting objects... What I can't find an explanation for is the moment when an object enters into orbit around another object. That clearly breaks ...
Marin Bonacci's user avatar
7 votes
1 answer
818 views

Wick rotation and the arrow of time

It is well known that we can switch from a statistical system to a quantum mechanical system by a Wick rotation. Has this rotation some implication on the way the time flow? namely, this is an ...
Emanuele Luzio's user avatar
6 votes
3 answers
2k views

Is there a mechanism for time symmetry breaking?

Excluding Thermodynamic's arrow of time, all mathematical descriptions of time are symmetric. We know the arrow of time is real and we know the equations describing physics are real so is there any ...
user4884's user avatar
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6 votes
3 answers
2k views

Is there really time reversibility in physics? [closed]

First, how i got to the question. I was randomly looking at this page: http://en.wikipedia.org/wiki/Langton%27s_ant describing some sort of cellular automaton. Here is the path of this automaton ...
Asphir Dom's user avatar
6 votes
3 answers
8k views

How would we perceive time going backwards? [closed]

I haven't taken Physics in University. Lately, I've been reading about some of the branches of physics through Wikipedia. I read several times that many of the theoretical models do not explain why ...
learningtech's user avatar
6 votes
1 answer
953 views

The arrow of time and the cosmology of a black hole interior

When solving the Einstein field equations in Schwarzschild metric for an observer falling into a black hole the radial coordinate r of the black hole and time t switch roles in the equations when r<...
Tomi's user avatar
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6 votes
2 answers
212 views

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 ...
Tom's user avatar
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6 votes
4 answers
405 views

Is this an adequate resolution of the "paradox" of the arrow of time?

From time to time I encounter things on the internet about physics mysteries concerning the "arrow of time". It is held that the laws of physics at a microscopic level are the same ...
Michael Hardy's user avatar
6 votes
10 answers
768 views

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 ...
Roger Wood's user avatar
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