172
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Accepted
Have researchers managed to "reverse time"? If so, what does that mean for physics?
First of all, let's get some important 'sociological' aspects out of the way:
While website you've linked to, phys.org, tries to pass itself off as a science-journalism site, it is nothing of the ...
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89
votes
Can a broken egg spontaneously reassemble itself (as in the video)?
No, it's not possible.
See, there's a problem with the English word "possible": it's an English word. Even in the best cases it's hard to translate technical, scientific ideas into English ...
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53
votes
Can a broken egg spontaneously reassemble itself (as in the video)?
It's possible, but won't happen anywhere within even one universe lifetime, not even close. Physicists often hate saying it is technically possible because the process is so mind bogglingly unlikely, ...
- 11.7k
49
votes
Accepted
Can a broken egg spontaneously reassemble itself (as in the video)?
Up to the limits of our theoretical understanding, yes, there is nothing in principle wrong with seeing what happens in the video happen for real in the sense that you can formulate this entire ...
- 20.6k
30
votes
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?
Minkowski spacetime is a mathematical model constructed to capture aspects of the phenomena we observe. It is a product of the human imagination, like all of our models of physics.
The observed fact ...
- 21.8k
28
votes
Accepted
Why is the second law of thermodynamics not symmetric with respect to time reversal?
The arrow of time in thermodynamics is statistical.
Suppose you have a deterministic system that maps from states that can have character $X$ or character $Y$, to other states that can have character $...
- 14k
27
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Have researchers managed to "reverse time"? If so, what does that mean for physics?
They did not reverse time, they reversed the "arrow of time", meaning that time continued forward but entropy decreased a little, for a moment. Small temporary violations of the second law happens ...
- 13.1k
23
votes
Accepted
Why does a sign difference between space and time lead to time that only flows forward?
We can move back and forth in space, so why does the negative sign mean we can't move back and forth in time?
As illustrated in the answer by user4552 and acknowledged in other answers, that relative ...
- 54.7k
22
votes
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
Theoretically, it is possible, at least if by 'original state' you mean 'macroscopically identical' - if you want the microscopic state to be identical, you encounter a problem, that it is impossible ...
- 3,692
22
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Where does the irreversiblity came from if all the fundamental interaction are reversible?
There's a distinction between microscopic reversibility and macroscopic reversibility. Or if you will, a difference between something being irreversible in theory versus irreversible in practice. (Or ...
- 345
21
votes
Accepted
Does gravity reverse entropy?
A few days ago I was watching a few YouTube videos about reversing entropy and how it was impossible.
I think you might mean "decrease entropy", and it's perfectly possible for the entropy ...
- 58.1k
17
votes
Accepted
Does it make physical sense to assign an entropy to a microstate?
When being strict the answer is "no": The entropy is always a property of the ensemble not of the microstates and by definition all microstates in a microcanonical ensemble have the same ...
- 11.9k
16
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Why does a sign difference between space and time lead to time that only flows forward?
The sign that appears in the metric or line element, i.e. in
$ds^2 = -dt^2 + dx^2 + dy^2 + dz^2$
does establish a difference between space and time, but it does not, on its own, contain all of the ...
- 60.1k
15
votes
Are Mirrored Universes With Opposing Directions of Time Theoretically possible?
A little bit of digging allows one to find the original Scientific American article, which links (and I wish every science article did this) to a journal article from Physical Review Letters (preprint)...
- 1,554
15
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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
Let us first consider what exactly happens when an egg breaks. Chemical bonds are broken in the egg shell (mainly calcium carbonate) and the energy is converted to heat and sound. The interior of egg ...
- 9,468
12
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Where does the irreversiblity came from if all the fundamental interaction are reversible?
Irreversibility comes from the thermodynamics: the probability that we return to the same state in any reasonable amount of time is extremely small. In more technical terms: the entropy is increasing. ...
- 61.8k
12
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Why is the second law of thermodynamics not symmetric with respect to time reversal?
A long comment.
Thermodynamics can be shown mathematically to be an emergent theory from statistical mechanics. Its laws are observational laws, deduced from variables and their measurements, that are ...
- 235k
11
votes
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
I assume a chicken egg. A hen can create a new egg that is macroscpically identical to the old one. Elementary particles are indistinguishable so even the fact you're holding the remains of the old ...
11
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Can a broken egg spontaneously reassemble itself (as in the video)?
All these answers that say "yes it is possible ... but very very unlikely" are failing to take into consideration the limits of human knowledge itself. In dealing with something as ...
- 60.1k
11
votes
Accepted
Are chaotic systems the same as dissipative systems in inverse time?
Yes, you are missing something. Looking at the change of phase-space volume ($∇·f$), you get three categories – if you have a constant sign of $∇·f$ (more on the alternative at the end):
dissipative (...
- 6,359
11
votes
Does it make physical sense to assign an entropy to a microstate?
I would like to add a complement to the excellent Sebastian Riese's answer.
Entropy is an overloaded term. Speaking about entropy without specifying which entropy one is using is the best way to ...
10
votes
Accepted
Demystifying time-reversal symmetry in physics
First of all, it's worth explaining what a time-reversal actually is: given a PDE (such as the Schrödinger equation or Newton's equations for a classical system) which describes the dynamics of the ...
- 935
10
votes
Why does a sign difference between space and time lead to time that only flows forward?
how does a relative sign difference lead to a situation where time only flows forward and never backward? We can move back and forth in space, so why does the negative sign mean we can't move back and ...
- 105k
9
votes
Why does time not run backwards inside a refrigerator?
But should that also mean, that no arrow of time can be defined for an open system like a refrigerator? Or must we conclude that the connection between entropy and time is an illusion? If we cannot ...
- 16.4k
9
votes
Accepted
Why does time not run backwards inside a refrigerator?
The short answer is that the entropy in the system decreases only due to the fact that the outflux of entropy is larger than the local growth. But the positive local growth of entropy is what is ...
- 20.5k
9
votes
How do we explain the motion of a time-reversed emptying balloon in vacuum?
To time reverse that scenario, you would reverse the direction of every atom. You would be very precise about it so that as all the atoms in the escaping air precisely follow their trajectory in ...
- 42.6k
8
votes
Is there a mathematical relationship between time and entropy?
There is no relation in which you can find both the entropy $S$ you know from equilibrium thermodynamics (ETD) or statistical mechanics (ESM) and the time variable $t$ you know from dynamics.
The ...
- 16.4k
8
votes
Have researchers managed to "reverse time"? If so, what does that mean for physics?
It just means they can make a couple of qubits go back to the state they were originally in, and they can do this in a determinable way. In that very small universe all the EM forces, and EM forces ...
- 2,760
8
votes
Why do we perceive time?
Once you ask about perception, it is no longer physics question.
Anyway we do not percieve time. Look around and all you will see is present moment. The time is abstraction given by the memory of ...
- 5,888
8
votes
Does gravity reverse entropy?
When gas collapses to form a star, it will increase in temperature. If we ignore radiation and any interaction of the gas with its surroundings, this will be an adiabatic compression where entropy ...
- 2,197
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