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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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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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10k views

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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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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2answers
119 views

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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38 views

Must the whole universe have the same entropic arrow of time?

Theoretically, could it be possible for one galaxy to have low entropy in the past, but another nearby to have low entropy in the future? I understand that, cosmologically, there almost certainly are ...
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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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1answer
269 views

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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2answers
124 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 ...
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1answer
103 views

Can you reverse the arrow of time?

Has there been any attempt to reverse the arrow of time?
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37 views

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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49 views

Why is it impossible for us to reverse the clocks?

We only get older, not younger. Trees only get taller; they don't return to acorns. Our Sun only ever uses up its fuel, never returning to a cool ball of hydrogen gas. Time only goes in one direction. ...
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1answer
147 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
91 views

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
94 views

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
62 views

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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2answers
82 views

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 $H$-...
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157 views

Is there a mathematical relationship between time and entropy? [closed]

If there is a relation between time and entropy, what is it? Are there limitations for this equation? Or if there is no relationship between them, what is the current state of research?
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Redo the experiment 'Reversal of thermodynamic arrow of time'

In a recent paper "Reversing the thermodynamic arrow of time using quantum correlations" by Micadei et al, an experiment was carried out to show a reversal of time. Basically they prepare a mixed ...
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1answer
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Can two distinct quantum universes ever have the same configuration, and what does it mean for many-worlds? [closed]

First, I hear that, on a whiteboard, one may casually invert causality and run time in reverse. Next, I hear that there are interpretations of QM, like Chaitin's Great Programmer interpretation or de ...
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1answer
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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 ...
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How do we know that time is going forwards? [duplicate]

Do we know that time is going forwards? Or could it be going backwards? Maybe sideways? If it is going backwards, then does that mean that it's being used up? Or does time not exist at all and it is ...
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2answers
104 views

Does time reversal symmetry hold in systems that use the geometry (shape) of a molecule as an input parameter to determine a subsequent output? [closed]

Does time reversal symmetry hold in systems that use the geometry (shape) of a molecule as an input parameter to determine a subsequent output? A system such as a synapse of a neuron that responds to ...
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47 views

Is there a scheme for secure physical time-stamping?

Is there a physical phenomenon that could be used to record digital information in such a way that it has the following properties: it relies on some immutable physical law and does not rely on any ...
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2answers
129 views

Is entropy a function of time?

Is entropy a function of time? Since universe is expanding with time (entropy increases) and contracting on reversal of time. Can we say entropy somehow related with time in addition to state?
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2answers
603 views

Explanation of why doesn't heat flow spontaneously from a colder to a hotter body [duplicate]

The second law of thermodynamics says that heat cannot spontaneously flow from a colder to a hotter reservoir but only with the expenditure of mechanical energy. This is taken as a postulate or law in ...
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1answer
338 views

Why time always moves only in one direction? [duplicate]

My question is not whether time has a preferred direction. My question is why time apparently doesn't change direction (i.e say first 10 min toward positive direct , next 10 min toward negative ...
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What was the reasoning behind Stephen Hawking's thought that time would reverse if the Universe started to contract?

Once upon a time, Stephen Hawking had in mind that time would stop and reverse as soon as the Universe would pass from a state of expansion to a state of contraction. What was the reasoning behind ...
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0answers
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What does it mean to reverse time? [duplicate]

Experiments are done to see if a process is symmetric (or asymmetric) with respect to the reversal of time. So if a process happens if the particles move in one direction, but not the other way round ...
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1answer
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CPT-symmetric initial state

Consider a hypothetical state of the universe such that it coincides with its CPT “mirror-image”. I.e., let us reflect positions of all objects by some plane (parity inversion), reverse all momenta (...
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1answer
630 views

The Feynman diagram for Pair production

The Feynman diagram above is for pair production and at the vertex of photon materialization, the arrow representing positron is against the time axis whereas the second arrow at that vertex which I ...
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2answers
173 views

How second-order differential equations do not violate causality?

The second order differential equations are time reversible. That means: they don't distinguish the time arrow direction. There is no reason for the time to flow forward. My professor told me that ...
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505 views

Can time run backwards? [closed]

Although equations of physics do not prohibit the backward arrow of time but the movement is arrested by the law of entropy...which makes the backward movement of time less likely but it is not ...
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1answer
279 views

What exactly is the physical picture of Time Reversal Symmetry

What's the exact meaning for time reversal symmetry in classical mechanics and quantum mechanics, respectively? Why is it right for just single or just a few particles but wrong for many particles or ...
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1answer
111 views

Entropy: the arrow head of time [closed]

So recently I started reading thermodynamics and on the entropy concept. I understand that the entropy universe always increases. The entropy increases in a particular direction which is the direction ...
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2answers
85 views

Is there a better answer to this argument claiming the impossibility of time extending infinitely into the past?

My friend claimed that time cannot extend infinitely into the past. He explained: "If an event A will happen in 10 years, when do you expect it to happen? In 10 years. But if event A (such as the ...
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3answers
184 views

In a “universe” where time runs backwards, is cause/effect preserved?

Caveat: I'm a layman not a physicist, and this may be more semantics than physics. In a universe where time runs backwards (think of a movie run backwards, where a vase that is shattered on the floor ...
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1answer
848 views

Electrons moving faster than light and backward in time?

In Lawrence Krauss's book "A Universe From Nothing"; page 62 mentions that for a very short period of time, so small it cannot be measured, an electron due to the uncertainty principle can appear to ...
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6answers
994 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 ...
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3answers
251 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 regardless of ...
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1answer
100 views

How does time propagate? [duplicate]

I was wondering how time propagates through the spacetime and how its pace is slowed down by effect of gravity? Without understanding the propagation of time , I found it impossible to understand the ...
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3answers
1k views

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 ...
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567 views

Arrow of time related to heat transfer

As passage of time is related to entropy increase, and entropy variation is related to heat exchange (irreversibility), can quantity of time variation inside a system can be related to the amount heat ...
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3answers
1k 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 ...