[Harri (OP)]

On a large scale, the world we exist in and the universe our world exists in has been, and as far as I am aware is forever changing. Using a second as a measurement for instance, our universe has been changing every second since the big bang, it is changing every second as I sit here typing this, and will continue to change every second into the future. Would this be a legitimate way of describing what is meant by a past, present and future? Past, present and future = change? If so, would the same be said of the quantum world?

[geordief]

I have had ,in the past  the belief that time required motion  to exist.However I have been assured that   clocks that rely on radioactive decay show that this is not the case.(no motion)

I also wonder (from bits and pieces I have heard)  whether it may be the case in the quantum world  that all particles are indistinguishable from each other..would that rule out   "change" at that level?

So ,how would one go from  the universal sameness of the quantum realm (if that is indeed how it is  or might be) to the variety of the classical realm?

[guest47899]

in quantum, spin is angular momentum, but quantum spin is unlike angular momentum spin in a hadron vector quantity. spin for a hadron is unidirectional and relates to time in a linear frame.. quantum spin is multidirectional. This mean that it's spin carries no vector orbital properties. as such, it cannot be used to measue linear time.

[evan_au]

Past, present and future = change?

"Change" tells you that time exists.

But "change", by itself, doesn't explain the other major characteristic of time: That it seems to travel in only one direction.

For this you have to look at entropy: Time flows in the direction where entropy increases.
- You can't tell the direction of time by looking at a single particle
- You often can't tell the direction of time by looking at the interaction of 2 particles
- But by the time you look at the interaction of 3 or more (real) particles, the direction of time is usually apparent

See: https://en.wikipedia.org/wiki/Entropy_(arrow_of_time)

[Halc]

I have had ,in the past  the belief that time required motion  to exist.However I have been assured that   clocks that rely on radioactive decay show that this is not the case.(no motion)

I had responded to the posted question, but not so much to the thread title.  This is an interesting point.

Given a universe with but one unstable particle, it will decay at some point, and it would be indistinguishable if it took a year or a microsecond to do that.  Time would be meaningless.  But given two different particles, suddenly one has a measurably greater probability of decaying than the other.  Time is still sort of meaningless, but that very real probability of decay makes it not totally meaningless.  That's just two particles, and already we can see 'classic' time emerging.

[jeffreyH]

I like a good debate but the concept of time seems to be flogged to death for no reason. It is simply a mechanical means of measurement of change in one form or another. The fact that those mechanics don't always run at the same pace in different frames is not that difficult.

[geordief]

I like a good debate but the concept of time seems to be flogged to death for no reason. It is simply a mechanical means of measurement of change in one form or another. The fact that those mechanics don't always run at the same pace in different frames is not that difficult.

Yes,a fair point.

On the other hand "time" has always (I imagine) been a focal point of humanity's  thoughts....until ,amazingly we are introduced to time dilation and  the non existence of absolute time.

Small wonder we are ,on an individual basis trying to keep up with the new normal  some 100 years later.

And how many of the general public are even dimly aware what special and general relativity have meant for our understanding of what can be called the "timing as a measure of change" mechanism?

Would they care ? Would they "believe"?

[jeffreyH]

If anyone has used a stopwatch they are aware of time as a measurement of change.

[yor_on]

Depends Harri.

First of all, there is no way for an observer macroscopically to be without his local clock . So even if quantum objects would be 'time less', we have no way I know of to prove it. Our clocks always tick.

The other thing is a question of what 'time' is. Is it a 'creation' of SpaceTime, similar to decoherence, scale dependent? Or a 'property' of SpaceTime, both microscopically and macroscopically.

We will always find a past, a 'present' and a future, until we're dead.

[geordief]

If anyone has used a stopwatch they are aware of time as a measurement of change.

I can only speak for myself  but I  personally seem to feel a need to "anthroposize" time or more accurately to give it its own elemental existence (the Greeks certainly did this with their Chronos).

Naturally I assume that the "general population" shares my (outdated) instincts  and so might only  appreciate that a stopwatch only measured change if it was first suggested to them.

Perhaps ,even then they might only agree to that  in a lip-service kind of way and hold onto the notion that indeed "time is passing" as if it was a "thing" ...or maybe they would cop on more quickly than myself whom I admit to be a snail like learner at best.

[chiralSPO]

I think it is very difficult to distinguish between our own personal perceptions of time (which is subjective and based on many complicated and poorly understood biophysical and psychological mechanisms) from the the fundamental temporal dimension that our mathematical models use. Additionally, it is important to remember that even the versions of "time" invoked in our various models is not an accurate representation of whatever "time" "actually" "is"

As far as time standing still in the quantum world... don't be confused by the fact that quantum objects don't appear to follow the same rules as macroscopic objects. The rules are the same (the real world and the quantum world are the same world), we just have to remember that macroscopic objects are collections of vast numbers of elementary particles whose properties are very strongly correlated with each other. This necessarily means that the most important processes and interactions of these enormous collections will be quite different from very small collections of particles.

[Bill S]

I like a good debate but the concept of time seems to be flogged to death for no reason.

“Flogged to death” – Yes.    “For no reason” - No.

I think the reason that the subject of time, in a variety of guises, keeps appearing is twofold.

1. We tend to be with St. Augustine, when he said: “What then is time? If no one asks me, I know;
if I wish to explain it to someone who asks, I know not.”

2. We easily lose sight of the point that you make:

It is simply a mechanical means of measurement of change in one form or another.

   
A major factor is the way in which we talk and think about time.  For example: we talk of time passing, when what we probably mean is something along the lines of: change occurs continually, and therefore, we see a progression in time, which is what we use to measure that change.

I doubt that even the most inveterate pedant would suggest that the protracted version would be preferable.  As long as our thinking is adjusted appropriately, the “shorthand” version is fine.  Taking “analogies” too literally, though, tends to lead to confused thinking.

Some examples from this thread include:

  "Change" tells you that time exists.

Change tells us that change exists.  We add the concept of time.

But "change", by itself, doesn't explain the other major characteristic of time: That it seems to travel in only one direction.

The idea that time travels is, in itself, an analogy that should not be taken too literally.

For this you have to look at entropy: Time flows in the direction where entropy increases.

Doesn’t the second law of thermodynamics state that in a closed system, entropy tends to increase, or to remain constant.  In spite of the presence of the word “tends”, the assertion “it never decreases” can still be found attached to many explanations.  Could this be a source of confused thinking?  Even in a closed system, can “time progress” in the direction of decreasing entropy?

[chiralSPO]

Doesn’t the second law of thermodynamics state that in a closed system, entropy tends to increase, or to remain constant.  In spite of the presence of the word “tends”, the assertion “it never decreases” can still be found attached to many explanations.  Could this be a source of confused thinking?  Even in a closed system, can “time progress” in the direction of decreasing entropy?

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

If we have a small closed system entropy can increase or decrease depending on the initial state.

Imagine four coins that can be either H or T. At regular intervals a coin is selected at random, and flipped such that it has a 50/50 chance of being H or T.

If the system starts out HHHH (the lowest possible entropy for the system), then after the first flip, entropy has a 50% chance of increasing and 50% chance of staying the same. Eventually the system will reach a state of maximal entropy, and can then only decrease or stay the same. Ultimately, as this game continues, the system will oscillate through all possible states with all levels of entropy (with each of the states essentially being favored by entropy).

This scenario is also only valid if the coins are all equivalent, and we don't care about their relationships to each other (you can think of this as the difference between combinations and permutations). If every coin is unique, then all states are equally likely, and have identical entropy. Entropy of the system will never change even though the system clearly changes as the coins are flipped.

[chiralSPO]

PS: I think the best definition of time I have ever heard is this (roughly):

"Time is what keeps everything from happening at once"

[jeffreyH]

OK then. Are the motions of all the bodies in the solar system time reversible? Would they act in exactly the same manner if they were run backwards? We can treat these objects as point particles mathematically. How does this impact entropy?

[Halc]

OK then. Are the motions of all the bodies in the solar system time reversible? Would they act in exactly the same manner if they were run backwards? We can treat these objects as point particles mathematically. How does this impact entropy?

Motions that do not involve entropy would be reversible.  Everything would spin and orbit the other way, and it wouldn't look wrong.  But the sun would continue to burn fuel.  Tidal forces would continue to push the moon outward, not a reversal of its trend now.  Tidal friction is entropic, so it is one-way.

If you treat objects as mathematical points, then there is no combustion and no tidal drift.  The reversed solar system would be mathematically perfectly a rewind of prior state.

[evan_au]

In spite of the presence of the word “tends”, the assertion “it never decreases” can still be found attached to many explanations.

If you have a small system, like the 4 coins mentioned by ChiralSPO, then entropy will (sometimes) decrease back to the original 4H. But it will be somewhat uncommon - the most common occurrence would be to have 2H + 2T, with other combinations being less common.

If you have a larger system, like 1 million coins, the most common combination (after allowing a few million flips for it to stabilise) will be around 500,000H+500,000T; combinations like 1,000,000H will almost never occur.

Now turn that into something practical that you can pick up with your hand, like 6x10²³ atoms, and you can say that returning to the original state will practically never occur. Hence the use of the emphatic "never".

It is possible to manually flip all the coins back to the original all-H state, but this takes information and hence energy. So you can reduce entropy by expending energy, but left to itself, entropy tends to increase in macroscopic systems (at any temperature above absolute zero).

[jeffreyH]

OK then. Are the motions of all the bodies in the solar system time reversible? Would they act in exactly the same manner if they were run backwards? We can treat these objects as point particles mathematically. How does this impact entropy?

Motions that do not involve entropy would be reversible.  Everything would spin and orbit the other way, and it wouldn't look wrong.  But the sun would continue to burn fuel.  Tidal forces would continue to push the moon outward, not a reversal of its trend now.  Tidal friction is entropic, so it is one-way.

If you treat objects as mathematical points, then there is no combustion and no tidal drift.  The reversed solar system would be mathematically perfectly a rewind of prior state.

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.

[Bill S]

 

  If you treat objects as mathematical points, then there is no combustion and no tidal drift.  The reversed solar system would be mathematically perfectly a rewind of prior state. 

True, but it would not be the solar system we experience; would it?                                                                                                               

[Bill S]

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.

That answers the question: "Does QM modify GR, or vice versa; but I don't see how it answers the original question.