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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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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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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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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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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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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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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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 $H$-...
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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?
Suhas B Mahesh's user avatar
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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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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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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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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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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 ...
Swapnil Bhatia's user avatar
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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?
Arpit Yadav's user avatar
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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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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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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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Entropy and time travel

Suppose that time travel is possible. Can then we circumvent the law of increasing entropy (second law of thermodynamics) transferring entropy from the future to the past?
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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 ...
Nipun Lamba's user avatar
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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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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 ...
Dheeraj Verma's user avatar
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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 ...
Jack's user avatar
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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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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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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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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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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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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
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1 answer
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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 ...
sanyam sharma's user avatar
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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 ...
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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 ...
quantCode's user avatar
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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 ...
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1 answer
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Does conservation of information mean that the direction of causality is arbitrary? [duplicate]

If it is the case that the information content of the universe is conserved, and the past can be constructed from a complete knowledge of the future just as easily as vice versa, then is there any ...
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Is QFT time symmetric, and how is it implemented?

In electromagnetism, while the Maxwell equations are time symmetric, there is a choice to restrict solutions specifically to retarded potentials, imposing a time direction on the equations. And in QFT,...
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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 ...
2 votes
2 answers
679 views

Why does time always flow forward?

According to the BBC Earth " Physics says that any event in our day-to-day lives could happen in reverse, at any time".Then why can't we just turn time backwards?
Srinath Pulaverthi's user avatar
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5 answers
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Books about arrow of time

Are there physics books about arrow of time? The concept of time is so vague, at least to me, it seems may not exist. When I think special relativity, time gets as real as other 3 spatial dimensions. ...
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Is there really a direction of time?

Laws of physics are (almost) time symmetric, so a time-reversed description of a physical process is as qualified as the original one. What's the reason then, that in reality one version seems to ...
Eric's user avatar
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How would certain situations hold up in time-reversal symmetry

According to time-reversal symmetry, if I reversed time, the laws of physics would still hold up, so if I dropped a ball to the ground, then in reverse order all of the light and sound and frictional ...
Mason's user avatar
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Direction of time in an insulated room

I am puzzled with thought experiment that resembles/is version of Bolzmann's brain-hyphothesis. I could explain it in following way: Let’s assume that we have isolated system full of some stuff, let’...
Juho Salminen's user avatar
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0 answers
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What determines time flow? (arrow of time) [closed]

I've been reading up on the arrow of time and there is one thing that is being omitted from every explanation that I've found: time flow. I am rewording this question to make it clearer. From the ...
Alex's user avatar
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Is black hole formation reversible if physics law holds even in time reverse?

As we know many situations still fulfill physics law if time is reversed, such as particle collision. But how about black hole formation? Suppose a star is turning to a black hole and starts to have ...
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