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I read in a book that Newton's laws, the Schrödinger equation, the special and general theory of relativity, etc., make no distinction between the past and the future.

It then explains that the second law of thermodynamics does make that distinction, but later goes on to say that law is basically molecules bumping into each other, which happens according to the fundamental laws.

Then, does it follow that, at any point in time, if I could locate every particle in the universe and reverse its velocity, the universe would run "backward" and end up at the Big Bang?

AndyD
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  • Amateurs (like myself) who might attempt an answer might want to 1st look at the answer (by Emilio Pisanty) to https://physics.stackexchange.com/questions/44628/cpt-violation-and-symmetry-conservation-laws . – Edouard Mar 02 '20 at 07:06
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    The noted physicist Aron Wall has also remarked that, since time and human neurology are both thermodynamic processes, individuals in either of two causally-separated regions might experience the direction of their passage thru the time dimension identically, even if their passage thru it would appear, to an (entirely hypothetical) outside observer, to be occurring in opposite directions. – Edouard Mar 02 '20 at 07:20
  • I'd tend to guess that the aforementioned outside observer might be at, or comprise, a "node" in the quantum-mechanical view (-which seems to include a means of providing for their existence that's possibly analogous to relativistic "causal separations" as black hole horizons), however inaccessible "they" might be. Its success in technology has been remarkable. – Edouard Mar 02 '20 at 14:22

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What you are talking about is time asymmetry. Pierre Laplace produced a thought experiment called Laplace's Demon which stated essentially what you are talking about, that a vast intellect should be able to calculate the complete positions and velocities of all the particles of the universe.

When statistical mechanics was introduced, it basically eliminated this possibility by showing that it was statistically unlikely for the universe to revert to the big bang, this would be an extremely low entropy state, and due to the second law of thermodynamics, entropy must increase.

There are some issues with simply running everything backwards, take the magnetic field for example, if you took a charge and just reversed its direction, then its magnetic field would curl in the opposite direction (clockwise as opposed to counterclockwise) as opposed to curling in the same direction but backwards (clockwise to clockwise but translating backwards). This is given by the right hand rule.

There is also an issue with the amount of energy you would need to add to the universe in order to completely reverse every trajectory which is on an energetically favorable path at the moment.

benchmuser
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    When quantum mechanics is taken into account, even the experiment itself cannot be controlled since all the quantum microstates are probabilistic, and the action of reversal will be probabilistic. – anna v Mar 02 '20 at 06:29
  • Thank you! I see that you are very careful with your wording with words such as "basically" and "unlikely". Does this mean there is still a possibility? If somehow a demon had enough energy and the logistical ability to tug every particle, could he still reverse the universe to the Big Bang? – AndyD Mar 02 '20 at 07:04
  • Also, I don't get the part on magnetic fields. For example, suppose we reversed a hydroelectric dam and the electric current it had generated. Every water particle would reverse its path. The turbine would spin in the opposite direction. The electric current would run backward. The magnetic field generated by the current will curl backward. Everything that it has been affecting could also run backward accordingly. I can't see any conflicts here. – AndyD Mar 02 '20 at 07:14
  • @annav Thank you. I see. But does that mean quantum mechanics is time-asymmetric? – AndyD Mar 02 '20 at 07:18
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    No, it means that it makes it impossible to exactly reverse all variables even in theory, for example to get a neutron from the products of a neutron decay, you would need to give the exact momenta of the proton , the electron and the electron antineutrino,if done classically. But quantum mechancally you only have probable control on the momenta of the particles. To get them to make up a neutron exactly as it was when it decayed in the non reverse time, cannot be done: no control, it is quantum mechanics dice. – anna v Mar 02 '20 at 07:24
  • @AndyD, If you have a single positron coming straight at you (out of the page). By the RHR, the magnetic field goes counterclockwise, if you were to reverse the direction of the positron so that it is now going into the page, the magnetic field circulates clockwise, if it was as simple as reversing the direction, the magnetic field orientation should still maintain its counterclockwise direction as it 'reversed in time'. If you looked at the history of this positron, it would have the orientation related to its prior positions, but reversing the velocity doesn't get the same result. – benchmuser Mar 02 '20 at 19:16