Is Classical EM Field the Same as Photon Wave Function?

[PeterDonis]

Mulling over it after reading some relativity threads, I thought nothing moved in spacetime because the worldline was the graph of it.

First, "propagate" does not mean "move in spacetime". It is just a word that is used to describe how EM fields at one spacetime event are related to EM fields at other spacetime events.

Second, you evidently did not understand what I meant about not drawing inferences from ordinary language. Stop trying to figure out how EM works by looking at definitions of the word "propagate", or "move", or any other ordinary language word. Go look at the actual math.

 

[DrChinese]

Yes - and the words "appears to" are, I think, critical here.

Alice's (local) measurement results are entirely described by the (local) reduced density operator and the (local) measurement operator - and similarly for Bob.

There's nothing in QM in which the results Alice observes are in any way affected by things Bob does (including measurements).

The notion that something Alice does in her lab affects something in Bob's lab is interpretation dependent I think.

"Appears to" = critical, yes.
Interpretation dependent, yes.
I just don't think it is clear that Alice and Bob's realities are independent as to their shared (entangled) context.

 

[bhobba]

how does electromagnetism propagate then with respect to spacetime?

The same way you propagate wrt space-time - its a meaningless question. I generally don't like to quote philosophers but Wittgenstein said it best in the Tractatus - Whereof one cannot speak, thereof one must be silent.

You exist in space-time. EM fields exist in space-time. They vary at different points in space-time. This can happen in such a way if you measure its values at various space-time points it looks like the field is propagating - but it isn't really - its just changing values is such a way propagation is a useful description as in say radio waves.

Thanks
Bill

 

[Blue Scallop]

The same way you propagate wrt space-time - its a meaningless question. I generally don't like to quote philosophers but Wittgenstein said it best in the Tractatus - Whereof one cannot speak, thereof one must be silent.

You exist in space-time. EM fields exist in space-time. They vary at different points in space-time. This can happen in such a way if you measure its values at various space-time points it looks like the field is propagating - but it isn't really - its just changing values is such a way propagation is a useful description as in say radio waves.

Thanks
Bill

They say it takes 9 minutes for the sunlight to travel to earth. You are saying it only looks like the field is propagating, but it isn't because the field is defined all over space.. so before the sunlight photons even reach the earth.. those same photon fields are already on earth?? and only the field is changing values making it looks like the sun light is travelling?

 

[Simon Phoenix]

I just don't think it is clear that Alice and Bob's realities are independent as to their shared (entangled) context.

Ultimately neither do I, but I think that's a function of the way I look at (interpret) QM, rather than something that's a function of the formalism.

At its starkest QM is a description that relates preparation procedures with measurement outcomes; it leaves the thorny question of what's 'really' going on between these extremes as a matter of personal taste. It seems to me, then, that any definition of local vs non-local, if we want it to be independent of interpretation, has to be in terms of measurement outcomes - which is something that is interpretation independent.

In these terms then it seems to me that the formalism of QM doesn't force us to view it as non-local. Measurement results 'there' are not affected by anything we do 'here'. It's only when we try to put things into an interpretative framework (for example, hidden variables) do things begin to look as if there might be some non-local character.

Having said all that the way I think about QM is in terms of states and (ideal) measurements projecting systems into new states and so my own tendency is to view QM as having a non-local character, but I also recognize that the way I look at QM, whilst it allows me to calculate measurement probabilities correctly, is somewhat dodgy

 

[Demystifier]

why are there no non gauge forces? what exact stuff in physics is violated?

Well, gravity is not a gauge force in the usual sense. Besides, the interaction with the Higgs field, which for some traditional reasons is usually not called a force, but is not less a force than other interactions, is also not a gauge force.

 

[martinbn]

You can call things whatever you like, but I think there is a sense of the word "local" that is not captured by signal speed.

Suppose that there were a pair of correlated coins such that, no matter how far away the two coins are from each other, the nth flip of one coin always gives the opposite result from the nth flip of the other coin. Other than that correlation, the coin flips seem to be perfectly random--the nth flip of each coin considered separately has a 50/50 of resulting in "heads" or "tails".

I would call such a correlation "nonlocal"; it relates distant phenomena.

I guess that would really be non-local, but is there anything that behaves this way? I mean you make measurements on the coins and they show the correlation, not that you have to prepare the pair of coins before each measurement.

 

[Blue Scallop]

Well, gravity is not a gauge force in the usual sense. Besides, the interaction with the Higgs field, which for some traditional reasons is usually not called a force, but is not less a force than other interactions, is also not a gauge force.

gauge forces = strong, weak, electromagnetic field

non gauge forces = gravity, higgs field...

what are the other non gauge forces?

 

[Demystifier]

gauge forces = strong, weak, electromagnetic field

non gauge forces = gravity, higgs field...

what are the other non gauge forces?

That's all, as far as we know.

 

[Blue Scallop]

That's all, as far as we know.

why do we treat higgs field as higgs force? why force?

 

[Demystifier]

why do we treat higgs field as higgs force? why force?

Because it interacts with other fields. In the classical limit, any interaction can be interpreted as a force.

 

[stevendaryl]

There's nothing in QM in which the results Alice observes are in any way affected by things Bob does (including measurements).

The notion that something Alice does in her lab affects something in Bob's lab is interpretation dependent I think.

I guess that would really be non-local, but is there anything that behaves this way?

The point was that local doesn't mean "no FTL signals". No, nothing actually works this way.

 

[rubi]

That is untrue. There are simply different definitions of nonlocality, and reality is certainly nonlocal by one of these well motivated definitions

No, it's completely true.
1. "Reality is non-local" is an ill-defined sentence. All physical definitions of "locality" are expressed in mathematics and hence, they can only applied to "theories about reality", but never "reality" itself. So, at best, a particular theory about reality could be non-local.
2. Every definition of locality deserves the name if and only if it implies the principle of locality, i.e. actions in one region of spacetime can only affect the events in the future light cone of that region. There exist mathematical proofs that QM is compatible with this principle. What you personally decided to call non-locality is called the violation of local realism by actual physicists. The violation of local realism does not imply the violation of the principle of locality. It is perfectly possible to maintain the principle of locality and QM (as I said, mathematical proofs exist).

 

[Simon Phoenix]

The point was that local doesn't mean "no FTL signals".

In some sense the whole issue of QM and "FTL" is a bit of a red herring - an interesting shade of red to be sure. And maybe the discussion should also take into account causality.

Clearly there are situations in which something I do 'here' affects something over 'there'. If I hit a golf ball here it will, some time later, cause a splash there - at least if I'm hitting it anyway. But in relation to QM the issue is whether a measurement I do here affects 'reality' there. I don't think QM, strictly speaking, forces us to adopt that position.

But for me the local/non-local thing isn't what's most fascinating. I think the Bell inequalities show us that classical theories are in big trouble before we even consider measurement events that are spacelike separated. Of course it's important to have the spacelike bit if we want to rule out local hidden variable theories.

If we're not worried about the spacelike bit then we could, in principle, construct a hidden variable theory that might explain the violation of the Bell inequalities that would be local - but I think any such theory would be artificial and somewhat ad-hoc. It would be hard, I think, to construct a general hidden variable theory to account for all of the possible entanglement experiment variants we might conceive - and such a theory would probably be as far removed from what we might consider 'classical' physics to be as QM is (on the surface at least).

I'm not sure I would consider the Bohm version to be the kind of theory I'm talking about here. That seems to me to be a kind of sleight of hand - it essentially takes QM as its starting point anyway and just redefines things a bit to give us this unexplained complex guiding potential - so it's a theory that's explicitly constructed to be the same as QM that ultimately doesn't explain anything.

So even without all of the endless FTL/non-local tongue-twisting I think entanglement implies that our description of the world must be very different from what is traditionally understood as classical physics.

 

[atyy]

No, it's completely true.
1. "Reality is non-local" is an ill-defined sentence. All physical definitions of "locality" are expressed in mathematics and hence, they can only applied to "theories about reality", but never "reality" itself. So, at best, a particular theory about reality could be non-local.
2. Every definition of locality deserves the name if and only if it implies the principle of locality, i.e. actions in one region of spacetime can only affect the events in the future light cone of that region. There exist mathematical proofs that QM is compatible with this principle. What you personally decided to call non-locality is called the violation of local realism by actual physicists. The violation of local realism does not imply the violation of the principle of locality. It is perfectly possible to maintain the principle of locality and QM (as I said, mathematical proofs exist).

Well, you agree with the very Bohmians the you criticize!

 

[rubi]

Well, you agree with the very Bohmians the you criticize!

No, the Bohmians I criticize want to make you believe that the violation of Bell's inequality implies that "reality is non-local" (or alternatively "quantum mechanics is non-local"). Neither statement is justified. And presumably, the motivation for making such false claims is to make the absurd consequences of BM appear inevitable. Relaxing the standards for arguments in physics in order to have less people reject ones theory is a terrible scientific practice and that's why I call these people dishonest.

 

[Demystifier]

absurd consequences of BM

What absurd consequences?

 

[stevendaryl]

Of course, this argument is invalid and the existence of perfectly local interpretations proves them wrong.

Which interpretation are you talking about here?

 

[bhobba]

They say it takes 9 minutes for the sunlight to travel to earth. You are saying it only looks like the field is propagating, but it isn't because the field is defined all over space.. so before the sunlight photons even reach the earth.. those same photon fields are already on earth?? and only the field is changing values making it looks like the sun light is travelling?

Think of a wave in a rope. People can hold it and move it up and down. Waves travel along the rope - but the rope or the people (they are stationary - only their arms move up and down or stay still) do not move in the direction of the wave. The rope simply moves up and down and those ups and downs move along the rope so we say waves are traveling along it. But the points on the rope are simply moving up and down. The same with EM waves. The height of the rope corresponds to the value of the EM field at that point. That's all there is - values at a particular point. But those values, just like the height of the rope, can form waves that seem to move through space. It isn't really - its just the values change to give the impression it is.

Thanks
Bill

 

[martinbn]

The point was that local doesn't mean "no FTL signals". No, nothing actually works this way.

OK, but what would be a theory that has no FTL signals and describes objects that behave like your coins.

 

[rubi]

What absurd consequences?

This is not a thread about BM, so I don't want to start another discussion about BM here. There are lots of things that are absurd about BM, but for the sake of not opening pandoras box, let's just stick with the example of actions at a distance.

Which interpretation are you talking about here?

I'm not talking about a specific interpretation. Any local interpretation serves as a proof for the possibility of maintaining the principle of locality in QM. Here's a list of some interpretations and the local ones are marked: https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#Comparison_of_interpretations

 

[bhobba]

gauge forces = strong, weak, electromagnetic field non gauge forces = gravity, higgs field...what are the other non gauge forces?

Gravity can be treated as a gauge force - it just usually isn't:
https://en.wikipedia.org/wiki/Gauge_theory_gravity

The answer to where gauge forces come from is deeply mathematical and comes from symmetry. For example if you try to find the combined theory of a spin 0 particle and a spin one particles (photons) then you find they obey a certain global symmetry. But we expect from SR for them to be local. If you demand them to be local - vola - you get the EM force.

The following book gives the detail:
https://www.amazon.com/dp/3319192000/?tag=pfamazon01-20

Thanks
Bill

 

[Blue Scallop]

Think of a wave in a rope. People can hold it and move it up and down. Waves travel along the rope - but the rope or the people (they are stationary - only their arms move up and down or stay still) do not move in the direction of the wave. The rope simply moves up and down and those ups and downs move along the rope so we say waves are traveling along it. But the points on the rope are simply moving up and down. The same with EM waves. The height of the rope corresponds to the value of the EM field at that point. That's all there is - values at a particular point. But those values, just like the height of the rope, can form waves that seem to move through space. It isn't really - its just the values change to give the impression it is.

Thanks
Bill

So before the first photons from a say Supernova reaches the earth... the photons EM field are already present on Earth but the height of the rope and the moving up and down occurs outside the solar system and the disturbance slowly occurs to each field region or portion and the disturbance moves at the speed of light?

 

[bhobba]

So before the first photons from a say Supernova reaches the earth... the photons EM field are already present on Earth but the height of the rope and the moving up and down occurs outside the solar system and the disturbance slowly occurs to each field region or portion and the disturbance moves at the speed of light?

Well you are mixing quantum and classical concepts, you can't do that. I was speaking classically. Quantum Mechanically the quantum EM field is everywhere. Photons are excitation's in the field. Like all things quantum we have zero idea what's going on until its observed, so speaking of a traveling photon is not a good idea.

Classically every point in space has a value of the EM field - you can even measure it with sensitive enough equipment. Now, just like the rope, if you have an EM wave and instruments along its 'path' we would see it change like the height of the robe to be like a moving wave. That's exactly what happens with a radio antenna. It sits there at some region where EM waves pass by. The EM field changes in the antenna and electrons are set in motion, you have a current and you can detect the waves. But all that's going on is the value of the EM field at the antenna changes. Nothing is moving except the electrons in the antenna due to the changing EM field where the antenna is.

Thanks
Bill

 

[Blue Scallop]

Well you are mixing quantum and classical concepts, you can't do that. I was speaking classically. Quantum Mechanically the quantum EM field is everywhere. Photons are excitation's in the field. Like all things quantum we have zero idea what's going on until its observed, so speaking of a traveling photon is not a good idea.

Thanks
Bill

Thanks for making it clear. When you mentioned "EM field".. I thought you were talking about the quantum EM field.. maybe one should say "classical EM field" vs "quantum EM 2nd quantized field" to avoid confusion... but I got your point. In the double slit experiements.. we shouldn't think of terms of traveling electrons but electron probability waves which can interfere with it. By the way, is there no chance the classical EM field is the probability wave of the photon where the probability wave becomes physical (become Bohmized)?

 

[bhobba]

By the way, is there no chance the classical EM field is the probability wave of the photon where the probability wave becomes physical (become Bohmized)?

Maybe:
http://cds.cern.ch/record/944002/files/0604169.pdf

But an expert in BM needs to comment. I am not one.

Thanks
Bill

 

[Demystifier]

By the way, is there no chance the classical EM field is the probability wave of the photon where the probability wave becomes physical (become Bohmized)?

No there isn't. Classical EM field depends on only one position. However, it contains many photons so "Bohmization" requires a wave function that depends on many positions.

 

[atyy]

No, the Bohmians I criticize want to make you believe that the violation of Bell's inequality implies that "reality is non-local" (or alternatively "quantum mechanics is non-local"). Neither statement is justified. And presumably, the motivation for making such false claims is to make the absurd consequences of BM appear inevitable. Relaxing the standards for arguments in physics in order to have less people reject ones theory is a terrible scientific practice and that's why I call these people dishonest.

Both statements are justified once you put in all the caveats, and the caveats are well known, so they should be taken automatically, rather than assuming that people disagree with you when they don't.

 

[atyy]

If Bell had used a better term instead of local/non-local, for example a term that was not already in use, then most of the discussions about Bell's theorem would not exist. What's wrong with terms like non-separable or non-factorizable?

It is a term that lacks physics. If you insist on sticking to mathematics, you lose powerful physical consequences of Bell's theorem, such as proving that the randomness of quantum mechanics cannot be overcome if faster than light signalling is not allowed.

 

[PeterDonis]

the field is defined all over space..

All over spacetime. Yes.

so before the sunlight photons even reach the earth.. those same photon fields are already on earth??

No. There is an event in spacetime where radiation is emitted from the Sun. There is another event in spacetime where that same radiation is received on Earth. Those two events are separated by a null (lightlike) interval in spacetime. The photon field has a value at each of those events, and those values are related--that relationship is what we refer to when we say the photons "propagate" from the Sun to the Earth.