Does time exist at Quantum level?

[Gaz1982]

There seems to be some talk of Entanglement "defeating" the speed of light. But in these discussions people talk about c in terms of time frames - it takes Light X to travel Y ... etc

If the Quantum universe simply doesn't facilitate such measurements then does that go someway to explaining what we perceive to be instantaneous.

I'm interested in what time really means at a Quantum level and how it can be meaningfully measured.

 

[Nugatory]

If the Quantum universe simply doesn't facilitate such measurements then does that go some way to explaining what we perceive to be instantaneous.

Not really... It just wraps the same problem in different words...

I send two particles out in opposite directions. They're entangled so that if we measure one of them to be spin-up then a measurement of the other one will necessarily be spin-down. I let them fly away from me for an entire year, and then someone a full light-year away to my left measures the spin of the left-moving particle and finds it to be spin-up. A naive picture says that this measurement causes the wave function of the entire system of two entangled particles to collapse so that the right-moving particle two light-years away immediately becomes spin-down. Experiments confirm that the universe really does behave in a way that is consistent with this naive picture:
1) The two particles' spin will always be opposite, no matter how far apart they are when the first measurement is made.
2) The spin of the two particles is determined by the first measurement; it is not possible that the two particles were just launched with opposite spins from the start. (The experiments that show this have spawned many threads in their own right - google for "Bell's Theorem").

These experiments haven't been done at distances of light-years (hard to set up a lab on alpha centauri) but they have been done at distances of meters and kilometers.

 

[bhobba]

Time is no different in QM - its something we parameterise our theories with and it's measured the same way.

What you are talking about is the entanglement thing.
http://www.johnboccio.com/research/quantum/notes/paper.pdf

As you can see if it is instantaneous is open to question.

Suppose I put a red slip of paper in an envelope and a green slip in another then mix them up, keep one, and send another to the other side of the universe. I open the envelope and see green. Immediately I know the other is red. No communication took place - we simply have a correlation. Entanglement may simply be like that, its a bit different as the above on Bells Theorem shows, in that its not the same as classical correlations, but it's still a correlation.

In fact in what's called Quantum Field Theory (QFT) we have something called the cluster decomposition property which basically says for uncorrelated systems sufficiently separated regions behave independently. This is the notion locality in QFT. Note the key word - uncorrelated. Entangled systems are correlated. This leaves up in the air if locality is even applicable for entangled systems. Personally I don't think it is, but mine is very much a minority view.

Thanks
Bill

 

[atyy]

Time in quantum mechanics is classical, just like space. Quantum field theory assumes a classical spacetime. So time is just what your classical clock on the wall reads.

There is a second answer in which time is position - you treat the clock as a quantum system, and the time the clock reads is the position of its hands when you look at the clock.

But even if you have a quantum clock in the box with Schroedinger's cat, since spacetime and your own measurement apparatus is classical, you can still have a classical clock outside of the box.

 

[Gaz1982]

"Suppose I put a red slip of paper in an envelope and a green slip in another then mix them up, keep one, and send another to the other side of the universe. I open the envelope and see green. Immediately I know the other is red. No communication took place - we simply have a correlation. Entanglement may simply be like that, its a bit different as the above on Bells Theorem shows, in that its not the same as classical correlations, but it's still a correlation."

Could you expand on this a bit?

Do you have any doubts about Bell's inequality?Thanks

 

[bhobba]

Could you expand on this a bit? Do you have any doubts about Bell's inequality?

There isn't anything to expand. Its simply a correlation - but of a different type to classical ones like the red and green slips - as Bells Theorem shows.

I have no doubts at all about Bells Theorem or its implications. Under my view as simply a correlation it violates naive reality and is ambivalent to if its local or not. In fact from the cluster decomposition property, which is the careful statement of locality in QFT, correlated systems are excluded. Hence its even up for grabs if locality is a concept that's even applicable. But if you reject naive reality its of no consequence.

Thanks
Bill

 

[Jimster41]

Good question.
I'd love to know how wrong-er-ish these question-like non-answers are?

Enforcement of Super Selection Rules occurs FTL
Enforcement of Super Selection Rules is non-local
Enforcement of Super Selection Rules is non-temporal (in our space-time at least) - this is the same thing as saying it is "non-local"
QM symmetries are non-local, non-temporal structures (in our space-time at least)

I keep thinking the answer is somehow that there is an aspect of the QM reality we inhabit, that is not subject to time, as all other aspects of that reality are. Whether that means the aspect is utterly non-temporal (which defies definition I think), or just more like alt-temporal seems open, but it does contain information that matters in our space-time.

I was under the impression this was just what the ADS/CFT Correspondence and "Holographic Universe" models are trying to capture.

 

[microsansfil]

A naive picture says that this measurement causes the wave function of the entire system of two entangled particles to collapse so that the right-moving particle two light-years away immediately becomes spin-down. Experiments confirm that the universe really does behave in a way that is consistent with this naive picture:
1) The two particles' spin will always be opposite, no matter how far apart they are when the first measurement is made.
2) The spin of the two particles is determined by the first measurement; it is not possible that the two particles were just launched with opposite spins from the start. (The experiments that show this have spawned many threads in their own right - google for "Bell's Theorem").

These experiments haven't been done at distances of light-years (hard to set up a lab on alpha centauri) but they have been done at distances of meters and kilometers.

How do you confirm the absolute simultaneity at distances of meters and kilometers by experiment when we know that in the framework of relativity the simultaneity is relative ?

Regards
Patrick

 

[Jimster41]

On one hand if I had to guess, I'd say the experimenters who painstakingly set these tests of quantum mechanics up can calculate the possible effects of Einstein's theory of General Relativity (and the "Relativity of Simultaneity") over the distances the experiments are performed, accounting for the relative velocities of the pieces of the experiment, and accounting for the curvature of space-time due to the presence of mass, and rule those out as factors in the experimental outcomes.

On the other hand, as I understand it, the idea of exactly what Einstein's space-time looks like and what "simultaneity" and "distance" mean to a photon (and other quanta) which are the subject of these experiments is not exactly clear - It can be predicted very accurately to a point, but there is not yet a widely accepted theory that formally unifies Quantum Mechanics and General Relativity. This seems to me consistent with your objection, and sort of exactly the mystery.

 

[bhobba]

How do you confirm the absolute simultaneity at distances of meters and kilometers by experiment when we know that in the framework of relativity the simultaneity is relative ?

Because its usually tests involving ordinary QM which is based on the Galilean transformations.

Thanks
Bill

 

[morrobay]

How do you confirm the absolute simultaneity at distances of meters and kilometers by experiment when we know that in the framework of relativity the simultaneity is relative ?

Regards
Patrick

Consider A and B detectors separated by one km with photon entangled source on AB axis
Detector A is .4 km from source and detector B is .6 km from source.
Measurement at detector A at t₀ and measurement at detector B at t₁
And also .2 km/c < 1 km/c

 

[morrobay]

Consider A and B detectors separated by one km with photon entangled source on AB axis
Detector A is .4 km from source and detector B is .6 km from source.
Measurement at detector A at t₀ and measurement at detector B at t₁
And also .2 km/c < 1 km/c

Δt for t₁ - t₀ = 6.6 * 10^-7 sec with both observers stationary. In order for both measurements to be
simultaneous for an observer in a moving frame S' 1 km from detector A overhead detector B. That is, Δt' = 0
That observer would have velocity 5.94 * 10⁷ m/sec. From Δt' = γ( Δt.- vΔx/c²)
Then solve for v with x = 1000 m

 

[Nugatory]

How do you confirm the absolute simultaneity at distances of meters and kilometers by experiment when we know that in the framework of relativity the simultaneity is relative ?

You're right, and that's why I used the word "naive". I'm describing the experiment from the point of view of an experimenter who is at rest with respect to both the source and the detectors, and the detections are only simultaneous using that frame. If we consider the experiment as described by an observer who is moving relative to the parts of the experiment, then the two detections are not simultaneous.

Depending on their speeds, some observers will find that detector A triggered before detector B and others will find that B triggered before A. That creates something of a problem for the naive explanation that says that the "first" detection determines the result of the "second".

 

[yoron]

Bhobba, you write "In fact in what's called Quantum Field Theory (QFT) we have something called the cluster decomposition property which basically says for uncorrelated systems sufficiently separated regions behave independently. This is the notion locality in QFT. Note the key word - uncorrelated. Entangled systems are correlated. This leaves up in the air if locality is even applicable for entangled systems. Personally I don't think it is, but mine is very much a minority view."

The mystery to me is that probability of the 'spin' to be either up or down (50/50 probability) with the opposite 'photon' assumingly knowing it instantaneously. to me it's not the correlation that confuse me, but the way it is assumed to always 'know' a proper choice, of two. Time definitely exist, I can guarantee that one personally :) but how it correlate is interesting. Naively I'm thinking of it as a static space, when trying to see it. One where every probability is counted in, with time becoming what makes a universe (always 'Locally defined') so to speak..

Better expressed, in such a universe it's the limits and constants that's interesting, because they should form the logic we measure on. And that is definitely interesting to me.
==

This is a simplified description of a 'spin' naturally, then again, it's perfectly enough for me :)

 

[Jimster41]

Because its usually tests involving ordinary QM which is based on the Galilean transformations.

Thanks
Bill

In other words an incomplete and approximate theory of gravity. Not Einstein's?

 

[darkchild]

I'm interested in what time really means at a Quantum level and how it can be meaningfully measured.

I think that those two things — the meaning of time and the way in which it is measured — are always the same thing in the context of all branches of physics.

 

[bhobba]

with the opposite 'photon' assumingly knowing it instantaneously.

That's your problem right there. You are using your classical intuition to read things into it the theory doesn't say.

QM is a theory about observation - you can't 'assumingly' anything beyond that.

Think back to the red and green slips - assumingly they they know it instantaneously as well - but of course there is nothing of the sort going on - you simply have correlated them. Same in QM with entanglement - but its different to classical correlations.

Thanks
Bill

 

[bhobba]

In other words an incomplete and approximate theory of gravity. Not Einstein's?

No. Standard QM is not relativistic just like classical mechanics is not relativistic. Because of that they obey the Galilean Transformations:
https://en.wikipedia.org/wiki/Galilean_transformation

Physically it is the Lorentz Transformations with the maximum speed C taken to infinity ie there is no maximum speed ie its not local. In fact, if you look at Chapter 3 of Balrentine you will see in QM all the dynamics follows from that alone. Right at its very foundations its non local. For locality to be an issue in QM you need to go to relativistic QM - which is called Quantum Field Theory (QFT). In QFT locality is defined by the Cluster Decomposition Property which more or less forces you to exclude correlated systems like EPR. Of course you can play around with definitions to get a different view. It needs to be pointed out the definition of locality in discussions of Bell is pretty similar to a loose statement of the Cluster Decomposition property, so its not really anything Earth shattering.

Thanks
Bill

 

[Ilja]

Time is no different in QM - its something we parameterise our theories with and it's measured the same way.

No, its different. In classical mechanics, Newton has distinguished true time and apparent time (as measured by humans) simply out of priniciple, because they are philosophically different. In QM, they are really different. There is no operator for time measurement, and every clock goes, with some non-zero probability, even backward in time.

Under my view as simply a correlation it violates naive reality and is ambivalent to if its local or not.

I would like to object against phrases like naive reality. This suggests a solution of the problem consists of some sort of sophisticated notion of reality or so. But what is used in Bell's theorem is an extremely weak notion of reality - the EPR criterion of reality - which does not have a meaningful possibility to be weakened. Thus, to save Einstein causality from falsification, you simply have to give up realism. And causality too - because you have to give up Reichenbach's principle of common cause.

Essentially, you preserve nothing in this way. Giving up realism means giving up any attempt to find a realistic model, a realistic explanation of the world. Giving up causality means to give up the search for causal explanations of observed correlations. The "weaker form" of realism is simply solipcism, the "weaker form" of causality would be not to care anymore about causal explanations of correlations.

So, nor the notion of realism used in Bell's proof is "naive", nor the notion of causality based on Reichenbach's common cause.

 

[bhobba]

No, its different.

I don't agree. In both cases its a parameter. Time is what a clock measures. Newton had many many misconceptions. But that is only to be expected - in the intervening centuries much has been learned.

Giving up realism means giving up any attempt to find a realistic model, a realistic explanation of the world.

Yes - giving up that things exist independent of observation means - well you give up things exist independent of observation. Its how you react to it that's the issue - you obviously react quite alarmingly to it. I, and many others, don't.

Thanks
Bill

 

[Ilja]

I don't agree. In both cases its a parameter. Time is what a clock measures.

Time is in both theories a parameter. So far, ok. In QM, it is the parameter t which appears in the Schrödinger equation.

But in QM this parameter is not what clocks measure. What clocks measure is different. I repeat, for every clock there is a non-zero probability that it goes sometimes backward in time. So, whatever clocks measure, it is something different, not time (at least not the parameter of the Schrödinger equation).

That's a theorem proven in Unruh W., Wald R., Time and the interpretation of canonical quantum gravity, Physical Review D Volume 40 issue 8 1989

 

[bhobba]

But in QM this parameter is not what clocks measure. What clocks measure is different. I repeat, for every clock there is a non-zero probability that it goes sometimes backward in time.

Would that be the same kind of probability all the atoms in the room will move in the same direction and levitate me out of the chair?

Thanks
Bill

 

[atyy]

Time is in both theories a parameter. So far, ok. In QM, it is the parameter t which appears in the Schrödinger equation.

But in QM this parameter is not what clocks measure. What clocks measure is different. I repeat, for every clock there is a non-zero probability that it goes sometimes backward in time. So, whatever clocks measure, it is something different, not time (at least not the parameter of the Schrödinger equation).

That's a theorem proven in Unruh W., Wald R., Time and the interpretation of canonical quantum gravity, Physical Review D Volume 40 issue 8 1989

In Copenhagen, since there is a classical observer, presumably he can have a classical clock too, so that classical time only goes forward. The time observed on a quantum clock could go backwards, but the classical clock should be ok.

 

[bhobba]

In Copenhagen, since there is a classical observer, presumably he can have a classical clock too, so that classical time only goes forward. The time observed on a quantum clock could go backwards, but the classical clock should be ok.

Its a conceptualisation of course. Real clocks may have a probability of going backwards but like the atoms lining up to levitate me its of no practical concern, hence people don't worry about it in models.

Thanks
Bill

 

[Ilja]

Would that be the same kind of probability all the atoms in the room will move in the same direction and levitate me out of the chair?

The theorem gives no numbers, thus, the probability may be low, but there is no big number of atoms involved, so I would not hope for such astronomically small numbers. Anyway, this is a conceptual question, a question of principle, thus, the actual numbers would be quite unimportant.

In Copenhagen, since there is a classical observer, presumably he can have a classical clock too, so that classical time only goes forward. The time observed on a quantum clock could go backwards, but the classical clock should be ok.

Anyway, the classical clock can be described quantum mechanically too. And if in this case a difference appears, then the quantum description is the more accurate and has to be preferred. So I don't think this helps - except if one wants to find an excuse to ignore this point.

The ideology "time is what clocks show" makes no sense. Known already to Newton, in QM a proven theorem, and in relativity it is even worse - the time between two events becomes unmeasurable simply because different clocks, even ideal clocks, give different results if they have moved differently between the events.

 

[atyy]

Anyway, the classical clock can be described quantum mechanically too. And if in this case a difference appears, then the quantum description is the more accurate and has to be preferred. So I don't think this helps - except if one wants to find an excuse to ignore this point.

At least in Copenhagen, it seems we should have an excuse to ignore this point. If the clock is quantum, then we need a classical observer to observe the quantum clock, and the classical observer will have a classical clock.

The ideology "time is what clocks show" makes no sense. Known already to Newton, in QM a proven theorem, and in relativity it is even worse - the time between two events becomes unmeasurable simply because different clocks, even ideal clocks, give different results if they have moved differently between the events.

At least classically, time is what "ideal" clocks show does make sense by definition. Time is what makes the laws of physics true. Then if the laws are physics are true, we can define an ideal clock as that which shows time.

 

[Ilja]

At least classically, time is what "ideal" clocks show does make sense by definition. Time is what makes the laws of physics true. Then if the laws are physics are true, we can define an ideal clock as that which shows time.

"By definition" does not help you, because it may follow that there simply are no ideal clocks, thus, the definition defines nonexisting things.

Which is what happens in relativity - where the idea that "time is what clocks measure" makes the "twin paradox" really paradoxical, while from the Newtonian point of view it is simply the difference between mathematical, true time which flows by itself and the human attempts to measure it, which are anyway inaccurate.

 

[yoron]

Maybe Bhobba. but I still get stuck on how it 'know' to correlate. Either one have to assume that the correlation is done by ones manipulation of light at some stage, meaning us 'forcing' it by our manipulation to correlate, then timing doesn't matter as it is a manipulation done, due to some stage of the creation of this experiment, creating a condition. Or it is correlated. If it is correlated, then the universe we measure on only becomes a part of what exist to me. With both cases seeming to have very little, or rather nothing, to do with time.
=

The reason for my wondering is this probability it has of either being up or down in a final measurement. Would there only be 'up' then this idea I have of it only being a part we can measure on would fall away, I think :) Also, to me it gives 'time' a real importance, a reality and no 'illusion', it being this way

 

[bhobba]

Maybe Bhobba. but I still get stuck on how it 'know' to correlate.

Its what the math says must happen by the definition of an entangled system.

Thanks
Bill

 

[Jimster41]

No. Standard QM is not relativistic just like classical mechanics is not relativistic. Because of that they obey the Galilean Transformations:
https://en.wikipedia.org/wiki/Galilean_transformation

Physically it is the Lorentz Transformations with the maximum speed C taken to infinity ie there is no maximum speed ie its not local. In fact, if you look at Chapter 3 of Balrentine you will see in QM all the dynamics follows from that alone. Right at its very foundations its non local. For locality to be an issue in QM you need to go to relativistic QM - which is called Quantum Field Theory (QFT). In QFT locality is defined by the Cluster Decomposition Property which more or less forces you to exclude correlated systems like EPR. Of course you can play around with definitions to get a different view. It needs to be pointed out the definition of locality in discussions of Bell is pretty similar to a loose statement of the Cluster Decomposition property, so its not really anything Earth shattering.

Thanks
Bill

Thanks Bill I learned something there, about the organization (and history) of the theories.

as I understand it reality is relativistic though. The speed of light really is nowhere infinity. We didn't know this at one time now we do. So it seems important to distinguish between ways an engineer might get by (using their ragged copy of Ballentine) when trying to calculate something "good enough for who it's for" and ways of explaining what is happening at the bottom of things - which seems more what the OP was asking.

I'm just getting familiar with the surprising differences between QFT and QM. I had formed the cartoon that measurements require mixed states - at which time the Cluster Decomposition Property fails. It is only for pure state evolution? I was just following a thread on QFT, and the discussion was very much about what the limits of a photon's wave state are in space time.

Excluding Correlated systems like EPR pairs might be totally practical for calculating things (in photonics or elsewhere), but they have been observed in nature (as I understand it) and bear explaining don't they? And I think by almost all accounts they do display the kind of magnificent weirdness (but not magic) that other great scientific discoveries do. I can understand that you might be concerned that people are excited by something that is wrong, but I sometimes wonder if you think it is okay to be excited by an understanding of the quantum world that is correct, or whether somehow those states of mind do not "commute".

I have a pristine copy of Ballantine, but still trying to finish Susskind.

 

[bhobba]

but they have been observed in nature (as I understand it) and bear explaining don't they?

Yes - and QM explains it.

Here is the explanation. Consider two systems that can only exist in state |a> and |b>. If system 1 is in state |a> and system 2 in state |b> that is written as |a>|b>. Similarly if system 1 is in state |b> and system 2 in state |a> that is written as state |b>|a>. From the principle of superposition you can have a superposition of the two states such as 1/√2|a>|b> + 1/2|b>|a> and is the state I will illustrate what's going on with. Such a state is called entangled - neither system is in state |a> or |b> - in fact it turns out they are now in mixed states - but I won't go into that here. Now let's observe system 1. Because it only has two states you must get |a> or |b>. If you get |a> then system 2 must be in state |b>, and similarly if you get |b> system 2 is in state |a>. Entanglement is broken and you can see the results are correlated by the way they are entangled. There is nothing mysterious going on - its fully explained by the principles of QM.

The issue is if you have an EPR type set-up where you observe each system at any distance apart. So that means if you observe system 1 you immediately know the state of system 2 that can, theoretically, be on the other side of the universe. That's all there is to it. It's not mysterious. Its simply an application of the principles of QM.

Thanks
Bill

 

[Jimster41]

Lucid, as always, Bill. I do appreciate it. Since I find even Susskin hard going. The sentence that seems undersold is "can only be in state A or B".

Until the measurement, potentially taking place at space-like separation, it is in neither and both as you say. At the moment of measurement, what space-like process reminds it of that "can only be in..." rule?

 

[bhobba]

Until the measurement, potentially taking place at space-like separation, it is in neither and both as you say. At the moment of measurement, what space-like process reminds it of that "can only be in..." rule?

Remember I said it can only be in state |a> or |b> - that means when you observe it for |a> or |b> you must get |a> or |b>. Although I didn't mention it because its rather obvious, they are orthogonal.

Thanks
Bill

 

[Jimster41]

Remember I said it can only be in state |a> or |b> - that means when you observe it for |a> or |b> you must get |a> or |b>.

Thanks
Bill

It's one thing for you to state that rule as an axiom in some formal logic. It seems another for nature herself to state it at space like separation for real observations.

 

[bhobba]

It's one thing for you to state that rule as an axiom in some formal logic. It seems another for nature herself to state it at space like separation for real observations.

Now that's the issue isn't it. It seems strange so you try to read more into it. That's the rock bottom issue here. As I have said time and time again QM is easy if you accept it without doing that. That's what I mean by - let go. Once you do that progress is swift in understanding.

Thanks
Bill