Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

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.

0
votes
0answers
35 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 ...
11
votes
5answers
3k 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 sign ...
1
vote
0answers
30 views

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 ...
1
vote
0answers
25 views

Can time flow backwards? [duplicate]

Is there any equations in physics or laws that say time can flow backwards instead of forward?
6
votes
2answers
93 views

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. ...
3
votes
1answer
190 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 ...
6
votes
2answers
119 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 ...
2
votes
1answer
90 views

Can you reverse the arrow of time?

Has there been any attempt to reverse the arrow of time?
1
vote
0answers
32 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 ...
0
votes
0answers
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. ...
2
votes
1answer
111 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 ...
0
votes
0answers
28 views

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 ...
1
vote
1answer
88 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 ...
0
votes
0answers
79 views

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 ...
0
votes
0answers
41 views

Doesn't backward in time paradox exist in complex 8-space model and others?

Penrose twistors employ complex 4-space. Multiple complex space or multiple time dimensions have been proposed before in theoretical physics. For example there is the 8-space where there is zero ...
0
votes
1answer
88 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 ...
2
votes
1answer
61 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 ...
1
vote
2answers
81 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....
4
votes
0answers
51 views

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 $H$-...
3
votes
1answer
153 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?
1
vote
0answers
162 views

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 ...
1
vote
1answer
76 views

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 ...
7
votes
1answer
138 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 ...
0
votes
0answers
43 views

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 ...
0
votes
2answers
97 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 ...
3
votes
0answers
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 ...
1
vote
2answers
122 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?
2
votes
2answers
470 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 ...
-1
votes
1answer
304 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 ...
1
vote
0answers
63 views

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 ...
1
vote
0answers
73 views

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 ...
2
votes
1answer
151 views

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 (...
0
votes
1answer
524 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 ...
1
vote
2answers
168 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 ...
-6
votes
1answer
443 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 ...
4
votes
1answer
264 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 ...
2
votes
1answer
107 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 ...
-1
votes
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 ...
3
votes
3answers
164 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 ...
4
votes
1answer
782 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 ...
19
votes
6answers
960 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 ...
6
votes
3answers
247 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 ...
1
vote
1answer
96 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 ...
14
votes
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 ...
1
vote
3answers
465 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 ...
13
votes
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 ...
1
vote
1answer
118 views

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 ...
4
votes
2answers
211 views

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,...
66
votes
5answers
7k views

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
2answers
368 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?