169 votes
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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106 votes

What is time, does it flow, and if so what defines its direction?

What do physicists mean by time? We’ll start with the easy question what do physicists mean by time. Note that it’s easy to get mixed up between the concepts of time and the flow of time. When non-...
88 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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51 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, ...
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  • 10.9k
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 ...
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41 votes
Accepted

In reverse time, do objects at rest fall upwards?

The direction of the gravitational force would not change under time reversal. Your object would feel a force downward, just as it does usually. It might be easier to imagine you had a movie of an ...
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  • 11.4k
33 votes

How would the laws of nature behave if we reversed time?

Good question. Let's first consider the ball falling immediately before it hits the table. Neglect friction with the air for simplicity. The ball has a velocity in the downward direction. If we ...
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  • 1,256
29 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 ...
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  • 4,103
27 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 $...
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  • 8,982
26 votes

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 ...
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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 ...
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21 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 Ben Crowell and acknowledged in other answers, that ...
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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 ...
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  • 53.4k
20 votes

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 ...
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  • 300
19 votes

What is time, does it flow, and if so what defines its direction?

What is time? As Einstein said, time is what clocks measure. And if you take a look at what a clock actually does, if you open up a clock and take a cold scientific look at the empirical evidence, ...
16 votes

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 ...
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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)...
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15 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

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 ...
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12 votes

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. ...
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  • 39.1k
11 votes
Accepted

Doesn't entropy increase backwards in time, too?

The reasoning in the question is correct. If you have a box with gas particles placed in half of a box but otherwise uniformly random and with random velocities then it is overwhelmingly likely that ...
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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 ...
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11 votes

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 ...
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  • 222k
10 votes

In reverse time, do objects at rest fall upwards?

One of the problems you will encounter is causality. Imagine you have a ball resting on the ground. Without already knowing how it behaved in the past you cannot uniquely define the next frame of ...
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10 votes

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 ...
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9 votes

Why do we remember the past but not the future?

All you have direct access to at any moment is the macrostate of your brain at that moment. A (backward) memory is an inference from that state to what the macrostate of the world was at some time ...
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  • 12.5k
9 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 ...
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  • 65.7k
9 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 (...
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  • 4,265
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 ...
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