[jeffreyH]

Well it means that time does not stand still in QM since entropy is dependent upon QM. As a side note, the equivalence principle is also established in QM. So that is no bar to reconciling QM and GR. Experimentation with neutrons have shown thus.
https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.icepp.s.u-tokyo.ac.jp/~asai/work/metaX/AJP84.pdf&ved=2ahUKEwizp8flg9ffAhXLx4UKHcgkDQ04ChAWMAZ6BAgFEAE&usg=AOvVaw0-ZK_aj8W6AjtV1pipMeOf

[Bill S]

My understanding is that entropy does not, in fact, determine the arrow of time--it merely demonstrates that there is an arrow. 

Agreed.

Entropy is just a natural outcome of statistics. The fact that it is increasing only means that the universe started in a highly ordered state, and is still much more ordered than it ultimately will (can) be.

I have some issues with this, and with your “coins” example, but it all needs time for thought, and it might not be appropriate in this thread. 

[Halc]

Therefore the quantum changes guarantee that the gravitational system is not time reversible. So that answers the original question. Time is inherent to QM. Thus QM modifies general relativity and not the other way round.

I would have said that the nature of QM means that entropy is not time reversible.  Classic objects like billiard balls on a frictionless surface exhibit time reversible behavior, but the interaction between subatomic particles do not.

Gravity has little to do with this. Gravity itself seems not to be entropic, so a gravitational system is much the classic billiard balls so long as the objects are treated as points.  Treating them as non-points opens the door for entropic effects such as friction.

[Bill S]

I’m trying to distil something I can understand from #18, #21 & #24.  I’m not there yet, but need to check my progress.

1. Orbital motion is dependent on gravity.  This motion is time reversable.  Therefore, if there were no other factors involved, the movements of the solar system would be time reversable.  These motions do not involve entropy.

2. Other factors involved include the sun burning fuel and tidal friction.  These involve QM.  They also involve entropy, which means they are not time reversable.

3.  The reasoning in point 1 is valid only if the objects in the solar system are treated as point particles, which, in reality, they are not.  Therefore, friction becomes a significant factor.

[chiralSPO]

It seems to me that there are many aspects of behavior that are completely symmetrical with respect to time, whether looking at quantum systems or billiard balls or theoretical point masses etc. Anything that is conserved with forward time will necessarily be conserved with reversed time. Where we run into trouble is where there are changes. (apologies if this is painfully obvious to the reader)

If we think about the examples of solar system or billiard balls running in reverse, in general the kinematics are superficially fine going either way--we can use simple Newtonian equations and make equally valid descriptions and predictions whichever direction time is going. There are some deviations between the simple model and the reality--some of which are the same regardless of temporal direction, but many are opposite: friction leads to anomalous decreases of velocity of an object as time progresses the usual way, but would have the opposite effect as time runs backwards.

Another way to think of friction is that in the forward direction kinetic energy tends to spread out in addition to being transferred. Similarly, while we are used to the sun as an object that continuously pours energy out in the form of light, when time is reversed then it is a sink, that attracts light and charged particles, which it pulls in as it converts helium into protons. If we consider the billiard ball scenario, it also runs into trouble when we think about the "break." Newton's equations are adequate for modeling the balls rolling around either way time runs, but as soon as they all happen to converge into a perfect triangle and transfer all their momentum and kinetic energy into the cue ball, you know it is running backwards.

I think that this time forward or reversed issue stems from the fact that our expectations, which are based on our experiences, are violated. We just have to reverse the appropriate rules. Just as it is more likely for heat to move from hot objects to colder ones, or large objects to break into smaller ones, or cause to precede effect, when we reverse time it becomes more likely for heat to move from cold objects to hot, or aggregation or many objections into one. This feels wrong because it is not what we see and know from experience.

[guest47899]

what is the length pf a photon? that depends on it's wavelength, which varies with frequency. gamma to microwave, the 2nd law of thermodynamics applies, heat entropies into a colder stabler equilibrium. "time" predicates entrophy.

the third law of thermodynamics calls absolute zero a ground state. entropy is the loss of a ground state. an excited state is any energy above the ground state. in systems with negative temperatures, absolute zero is an excited state. 

so we have a vehicle for a bijective pairing of two different sets. two different corresponding values sets of opposite entropies that pair via thermal bijection, if for only a very small "time" period/window.  negative temperatures are a ground state for excited absolute zero and positive temperatures are an excited state for ground state absolute zero.

this very small "time" period on either side of absolute zero, could best be understood as symmetery breaking. 

"Symmetry breaking can be distinguished into two types, explicit symmetry breaking and spontaneous symmetry breaking, characterized by whether the equations of motion fail to be invariant or the ground state fails to be invariant."

https://en.wikipedia.org/wiki/Symmetry_breaking


In quantum, you have a ground state, it's a vaccuum state or zero point energy state. Where or at what point does symmetry breaking occur in quantum? What is the the thermal/motion function in quantum that will produce a "below the well" vacuum state that will provide two corresponding value sets of entropy, for bijective pairing?

 

[Halc]

Good post.  Some comments.

Another way to think of friction is that in the forward direction kinetic energy tends to spread out in addition to being transferred.

Momentum tends to spead out and be transferred.  It is conserved.  Kinetic energy is just plain lost when friction is involved.  Yes, some of it is transferred, like tides transferring about 3% of kinetic energy to the moon and the other 97% lost to heat.

Similarly, while we are used to the sun as an object that continuously pours energy out in the form of light, when time is reversed then it is a sink, that attracts light and charged particles, which it pulls in as it converts helium into protons.

It seems to attract it, but if it works, it is because light just happens by chance to head that way and get there at exactly the right time to make some change that cumulates in the creation of more protons.  There is no attraction of the light going on.

If we consider the billiard ball scenario, it also runs into trouble when we think about the "break." Newton's equations are adequate for modeling the balls rolling around either way time runs, but as soon as they all happen to converge into a perfect triangle and transfer all their momentum and kinetic energy into the cue ball, you know it is running backwards.

Yes, even with no entropy involved (we assume the balls are not speeding up), we recognize order being created from disorder, which is not impossible (as pointed out above in the post about the coins), but still a freak chance just like all the photons happening to wind up going towards the sun.

I think that this time forward or reversed issue stems from the fact that our expectations, which are based on our experiences, are violated. We just have to reverse the appropriate rules.

Indeed, the people in this reverse world would know what was coming, but could only guess at the past.  They would not notice anything amiss, but we would as objective 'observers' of the progression of events in the unexpected order.

2. Other factors involved include the sun burning fuel and tidal friction.  These involve QM.

In a pure classic non-QM universe, there would still be chemistry and friction.  This is debatable since chemistry is very much a direct function of QM at the lowest levels, as is say electronics.  So while some might say that entropy is in principle reversible (no physics is broken by a collections of wooden blocks un-falling into a tower), it is debatable if this can possibly hold at the smaller scales.
Can a candle-stub really un-burn into a new candle if all particles were reversed?  I doubt that.

[evan_au]

1. Orbital motion is dependent on gravity.

That is true for low-intensity gravity, like the Solar System; the orbital ellipses predicted by Kepler and Newton are reversible.

When you get to high-intensity gravitational fields, like binary pulsars and binary black holes, some of the energy is radiated away as gravitational waves. This involves entropy (energy spreading out through the universe), and the ellipses turn into converging spirals. This is not reversible.

no physics is broken by a collections of wooden blocks un-falling into a tower

However, towers and chemistry comes down to how concentrated or diffuse the energy is, which is controlled by entropy.

The blocks at the top of the tower have a lot of gravitational potential energy in a small block. When the tower falls down, that energy is spread out into the floor, heat, air movement, chipping the blocks, etc. It is highly unlikely that the energy will become concentrated again in that one block so that all its atoms happen to jiggle upwards at the same time, lifting it back into its original position.

On the other hand, if the blocks started out laying flat on a low-friction surface (eg ice), then a small amount of energy (like random thermal currents) could jiggle them around, back into their original configuration.

Chemistry is similar: Some reactions which release small amounts of energy (eg H₂O = HO^-+H⁺) are reversible, and are in state of equilibrium. Other reactions (like 2H₂ + O₂ = 2H₂O) which release lots of energy are effectively one-way, since it is highly unlikely that the spread-out energy will reappear in such a way as to undo highly energetic reactions. 

[Halc]

When you get to high-intensity gravitational fields, like binary pulsars and binary black holes, some of the energy is radiated away as gravitational waves. This involves entropy (energy spreading out through the universe), and the ellipses turn into converging spirals. This is not reversible.

A point I had not thought of.  So much for the mathematical-point theory.
In this perfect reverse universe with all the photons magically heading back to the sun, would not the gravity waves also head back to the black hole and spiral-out the object orbiting outward?

The blocks at the top of the tower have a lot of gravitational potential energy in a small block. When the tower falls down, that energy is spread out into the floor, heat, air movement, chipping the blocks, etc. It is highly unlikely that the energy will become concentrated again in that one block so that all its atoms happen to jiggle upwards at the same time, lifting it back into its original position.

The energy of the fall would be dissipated as heat and such, but in the end it is a lot like the break of the pool balls, an unlikely chance that looks backwards.  I suppose my block tower would need to be mathematical blocks that would have no choice to bounce around after falling since heat is really hard to reverse.

[yor_on]

Interesting notion Halc " I would have said that the nature of QM means that entropy is not time reversible.  Classic objects like billiard balls on a frictionless surface exhibit time reversible behavior, but the interaction between subatomic particles do not. "

A Feynman diagram is time reversible, and Entropy is considered so too. " The Second Law of Thermodynamics allows for the entropy to remain the same regardless of the direction of time. If the entropy is constant in either direction of time, there would be no preferred direction. However, the entropy can only be a constant if the system is in the highest possible state of disorder, such as a gas that always was, and always will be, uniformly spread out in its container. The existence of a thermodynamic arrow of time implies that the system is highly ordered in one time direction only, which would by definition be the "past". Thus this law is about the boundary conditions rather than the equations of motion of our world. "
https://en.wikipedia.org/wiki/Entropy_%28arrow_of_time%29

expand on how you think when suggesting that time reversibility won't work in a quantum regime, were you thinking of 'probability', super positions?

[evan_au]

In this perfect reverse universe with all the photons magically heading back to the sun

The trick of reversing time, and seeing if it makes sense says:
- Q1: If the Solar System were reversed in time, would the planetary orbits look normal?
- A1: Yes, The planetary orbits would look pretty normal.
          But if you looked closely at the orbit of the Moon, spiralling inwards is not normal, since tides dissipate energy as heat; the Moon should spiral outwards.

- Q2: If the Sun were reversed in time, would it look normal?
- A2: No, this would look very abnormal. Photons from across our galaxy (and beyond) do not spontaneously fly back towards the Sun. Heat does not spontaneously move from cooler places (the visible surface of the Sun) to hotter places (the center of the Sun). Helium does not spontaneously break down into Hydrogen. 

- Q3: If the Earth were reversed in time, would it look normal?
- A3: No, this would look rather abnormal. Heat does not spontaneously move from cooler places (the surface of the Earth) to hotter places (the center of the Earth). Cows do not regurgitate grass. Grass does not emit photons, turning complex chemicals like chlorophyl into carbon dioxide, water and nitrates. Random vibrations of the entire Earth's crust do not synchronise and converge at a point and move huge blocks of the Earth's crust.   

So parts of this macroscopic universe like the orbits of the planets are (close to) ideal, lossless, reversible systems.
But other parts like the Sun and the Earth have a clear direction to time's arrow, as dictated by entropy, and are clearly not reversible.

A Feynman diagram is time reversible

I agree that some simple Feynman diagrams with just 2 interacting real particles can be ambiguous in the direction of time.

But I suggest that by the time you get to the interaction of 3 or more real particles, a Feynman diagram provides a pretty clear idea of the direction of time (but not an absolute guarantee).

Take the decay of a neutron in a vacuum:
n⁰ → p⁺ + e⁻ + ν_e + 0.78MeV

1) The right hand side has 3 real particles which exist at the same time. The odds against these particles just happening to come together in the same place at the same time in a vacuum to form a neutron is so unlikely that it gives a clear direction to time's arrow.

2) The reaction releases 0.78MeV of energy, which is randomly split up amongst the products, and heads off in 3 different directions into the universe. Energy does not spontaneously gather from different parts of the universe and combine at a single point. This gives a clear direction to time's arrow.

See: https://en.wikipedia.org/wiki/Free_neutron_decay
This link shows a Feynman diagram for this reaction (with an arrow of time...)

[geordief]

 I had to drop out of this thread as it became too advanced for me but does the preceding post put the matter of any posited reversibility of time to bed.?

If the existing universe cannot proceed along those lines is that all there is to say ? (except that there may be sub arenas where it remains possible.)

[Bill S]

Cows do not regurgitate grass. Grass does not emit photons, turning complex chemicals like chlorophyl into carbon dioxide, water and nitrates. Random vibrations of the entire Earth's crust do not synchronise and converge at a point and move huge blocks of the Earth's crust.

Many SF authors have painted grim pictures of the outcome of time reversal, including wallowing in such revolting ideas as “eating” in reverse, and using words such as “mouth” and “food” as terms of abuse.  However, causality is obviously a significant factor, as is the question of possible communication between two people who are travelling through time in opposite directions. 

Thinking along the lines that it is the physical world that changes, and time simply measures that change, the whole idea of these reversed actions is called into question.  If A and B are travelling in opposite directions relative to each other; A will be experience the world as changing (in her RF) in a way in which cause precedes effect. The same will apply to B, in his RF.  Thus, only if either can observe the other’s world can either have any perception of “reversed time”. 

This raises the question: Could A and B observe each other; if so, how?

[yor_on]

Evan, I'm not saying that I expect time reversal to be. That's also why I asked Halc to expand on his thoughts there. It's called a symmetry in physics and the universe seems to work on those.

" A mathematical theorem called the CPT theorem asserts that the laws of physics, in the way that physicists know how to formulate them, must be invariant under the combined operations of space reversal, time reversal, and something called charge conjugation (which I won’t go into here). For example, if we formulate a law that respects charge conjugation but violates space reversal, then it must violate time reversal. In 1964 experimental physicists found that the disintegration of a subnuclear particle called the kaon violates space reversal and charge conjugation in such a way that, according to the mathematical theorem, the disintegration must also violate time reversal.

For more than a quarter of a century, then, physicists have had indirect evidence that the laws of physics indeed violate time-reversal invariance. What physicists have been searching for in vain is direct evidence that time-reversal invariance is violated, without appealing to some fancy mathematical theorem. Physicists are a real skeptical lot and are never completely happy with mathematical theorems. Mathematical reasoning in itself cannot be wrong, but theorems have to start somewhere, with assumptions; and the assumptions that go into the proof of the CPT theorem, while seemingly reasonable to the vast majority of physicists, still make some physicists sit up at night and sweat. " http://discovermagazine.com/1992/oct/timereversal140

It's weird. Myself I think we have one arrow and that you can't reverse it and only 'stop it' observer dependently aka observing a black hole from far away. So, is it a symmetry or not?

[yor_on]

Feynman diagram was just a example Halc, although it is said that "  the crossing symmetry in quantum physics says that you can flip any Feynman space-time diagram onto its side, swapping time and space, and the presented image will present a situation having an equal probability as the original one."

This link is pretty sweet: https://www.quantamagazine.org/why-feynman-diagrams-are-so-important-20160705/

you was thinking of considering a measurement being time reversed? Effect and Cause instead of cause and effect, sort of?

When it comes to CPT " if you take the Universe and everything in it and flip the electrical charge (C), invert everything as though through a mirror (P), and reverse the direction of time (T), then the base laws of physics all continue to work the same. " which may be true, but as Evan points out if the same goes for a whole universe it would lead to very strange effects. Then again, this is not the exact same and I'm not sure if we would see cause and effect turned over as in that picture. I guess it's a equivalence to the idea of space and time being able to 'change signs' in a black hole. It's a time reversal but not one we will notice. Or maybe I'm wrong, been so long since I looked at that one.

[geordief]

If time is reversed would not cause and effect be reversed as well so that "locally there would be no difference?

I think there was recent speculation that there might have been 2 universes created at the outset  with time being reversed one vis a vis the other.

Would cause and effect not have operated just the same in either universe and would not time appear to be reversed simply from the perspective of the other universe?

Locally time ,cause and effect would operate the same (only possible) way.

[Bill S]

If time is reversed would not cause and effect be reversed as well so that "locally there would be no difference?

 

That's the only scenario that makes sense to me. 

When it comes to CPT " if you take the Universe and everything in it and flip the electrical charge (C), invert everything as though through a mirror (P), and reverse the direction of time (T), then the base laws of physics all continue to work the same."

This seems to support that line of thought.

[yor_on]

"  The event horizon of a black hole is a very weird and complicated place. According to my undergrad GR course, at the event horizon of a black hole, time becomes a space-like coordinates, and space becomes a time-like coordinate. From what I remember, a time-like coordinate is one in which you absolutely have to move forward.

Therefore, beyond the black hole horizon, it is impossible to move backwards in space (away from the black hole) in the same manner as it is impossible to move back in time outside of the horizon.

Does it also mean that it is possible to move through time in both direction (time being a space-like coordinate inside the horizon)? Probably, but then, it's still impossible to get out of the black hole, so no one outside would know!

As a side note, according to the holographic principle, information on all events occuring beyond an event horizon is encoded into the event horizon itself, such that it is irrelevant to try to understand what goes on beyond the event horizon. In fact, the very physical existence of something beyond an event horizon is questionable, since it cannot affect us any much more than the event horizon itself can. "   PhilMacKay      https://physics.stackexchange.com/questions/101979/time-reversal-in-a-black-hole


The way I read it your 'local clock' do not disappear. It 'ticks on' as usual, I severely doubt anyone finding his 'clock' to go backwards as a result of coordinate change. If it was possible so should all other functions do too, as your electro chemistry etc. Otherwise it will become a contradiction in terms as all functions of the body use a one way arrow. Even your thoughts. You can also argue that a coordinate change inside a black hole is impossible as it must be a equivalence to 'c'.