Is quantum mechanics a complete theory of nature?

DrChinese

This is an out of the blue comment, and I don't see any connection to the subject matter. Around here, an expert is an expert. Not an "expert".

nortonian

I agree.

There are 8 pages of historical developments and experimental discussions in the paper you cite but only two sentences are used to define what a “single photon” is. I don't question the accuracy of the experiments or that they are able to make good predictions. I question the assumptions that they begin with and the logic behind them. Can you cite something more basic?

I believe that the Marshall and Santos papers I cited do a better job of looking at fundamentals. Although they do not offer a more accurate theory they have the advantage that they reject non-locality. Will you comment on their argument that when the zero point field is used to describe the photon it is actually a classical model?

zonde

I am trying to understand your objections. Do you think that all the reasoning should start with something that we know for sure? And if we do not know for sure anything than we can do no reasoning, right?

But then I do not understand how this bunching model is better. Or maybe I do:
You believe that single-photon model somehow implies non-locality but bunching model implies locality.

Well, I do not agree. Single-photon model by itself does not conflict with local realism.
On the other hand Bell theorem applies to your bunching model just as well.

ThomasT

It seems that it's reasonable to assume that the quantum theory is an incomplete description of physical reality. And that the incompleteness of the theory, in a certain sense, can be deduced/inferred from the theory itself. But, afaik, when people speak of the completeness of quantum theory they don't mean that it's a complete description of physical reality (After all, how could anyone ascertain that -- what might it refer to?). Rather, what they mean is that the quantum theory incorporates everything that's known about reality via quantum experimental phenomena.

So, how can your OP possibly ever be definitively answered?

nortonian

For my part, I am trying to understand what a photon is, but when I look at the literature I am receiving contradictory information. If we don't know for sure what a photon is then it is ridiculous to use that model to reject locality. My objections to quantum mechanics are that the fundamentals are dealt with on a purely phenomenological basis. If you can't see it it doesn't exist. To show what I mean I have checked a well-respected source from the article you cited: R. Loudon, The Quantum Theory of Light, 3rd ed. ~Clarendon, Oxford, 2000.

“The one-photon state has the important and distinctive property that it can produce only a single current pulse in the ionization of a photodetector.”

DrChinese, If we are talking about one-photon states then when you said then I agree that you are right not because it is physically impossible, but because it was defined to be impossible.

Loudon also states the following:

“A one-photon excitation in such a mode (spatial mode) is distributed over the entire interferometer, including both internal paths.” page 2.

I understand this to mean that the one-photon state is delocalized and because it is in both arms at the same time it is a non-local definition. It should not be surprising that a non-local model would result in non-locality.

When quantum mechanics rejects a physical model such as bunching because it is viewed as incomplete they insist that a better model must give better predictions. IOW better predictions is more important than a local theory? We can have both locality and predictive power if we admit that it is impossible to know for sure what constitutes a detection event.

ThomasT do you equate reality with what we observe? IOW is there more to reality than what we observe?

DrChinese

Now you are mixing metaphors. There are a lot of ideas about what a photon is, but with none of them is there a local realistic way to explain Bell test results. So no, you will be out on your own on this objection.

As to the bunching phenomena you postulate, all you have to do is give me a specific scenario and I believe we can explain why this does not apply. Please recall that there are probably hundreds of different types of Bell tests which violate local realism, many which do not use photons at all. For example, see:

http://www.nature.com/nature/journal.../409791a0.html

Local realism is the idea that objects have definite properties whether or not they are measured, and that measurements of these properties are not affected by events taking place sufficiently far away1. Einstein, Podolsky and Rosen2 used these reasonable assumptions to conclude that quantum mechanics is incomplete. Starting in 1965, Bell and others constructed mathematical inequalities whereby experimental tests could distinguish between quantum mechanics and local realistic theories. Many experiments have since been done that are consistent with quantum mechanics and inconsistent with local realism. But these conclusions remain the subject of considerable interest and debate, and experiments are still being refined to overcome ‘loopholes’ that might allow a local realistic interpretation. Here we have measured correlations in the classical properties of massive entangled particles (9Be+ ions): these correlations violate a form of Bell's inequality. Our measured value of the appropriate Bell's ‘signal’ is 2.25 ± 0.03, whereas a value of 2 is the maximum allowed by local realistic theories of nature. In contrast to previous measurements with massive particles, this violation of Bell's inequality was obtained by use of a complete set of measurements. Moreover, the high detection efficiency of our apparatus eliminates the so-called ‘detection’ loophole.

So your model needs a little revving up to explain this. 'Cause there ain't no bunching of Beryllium.

zonde

Yes, it is ridiculous to reject locality. Point. Without any "if ... then ...". Please try to understand that. It has nothing to do with different models of photons.

Yes, I agree. They are valid objections.

nortonian

Quantum mechanics (from Loudon):
Hypothetical bunching model:
Photons are localized with a diffuse external field and a single frequency. By themselves they do not have sufficient energy to cause a detection event, but the superposition of fields of many photons leads to intensities sufficient to cause a detection event. Detections are caused by the superposed fields of photons.

It is thereby assumed that realism can be defined by a mathematical analysis of experiments. Don't you think that this is presumptuous? It seems more likely that realism is more fundamental than quantum mechanics. The problem with quantum mechanics is that it only accepts challenges to its interpretations that abide by its rules. In the Nature article there are two objections to violations of the Bell inequality: that there is a subliminal communication and that not all detections were recorded. It does not suggest what to me is the real cause, that the detection event is incorrectly interpreted. Bell's inequality is a commentary on the nature of detection events, not locality or photons. Clearly we cannot look behind the phenomena to determine the truth, but as long as that possibility exists local realism has not been disproved.

Quantum mechanics should be able to say what part of reality cannot be observed, IOW precisely define its own limitations.

The trouble with trying to prove quantum mechanics wrong is that they insist that you come up with better predictions. All one has to do is prove that the predictions are based on a superficial understanding of nature or photons or whatever. If Bell was using an incorrect model then he is proving something about quantum mechanics, not reality.

DrChinese

1. What you are describing is a classical picture and this model has been experimentally falsified, as I cited above. Also, please note that down converted photons do not qualify for the general excitation modes Loudon describes as they follow a very specific set of rules due to how they are collected. Recall that a single photon of a specific wavelength is split into 2 photons with twice the wavelength (half the frequency). Filters insure that improper frequencies are excluded. So the math doesn't work for there to be 2 on one side and 1 on the other.

2. No, it's not presumptuous in light of Bell. If you accept "elements of reality" as defined by EPR, you must reject local realism. Or not, if you simply reject everything you don't want to believe. (I can't assist on that side.)

DrChinese

This is a part of the scientific method in that it is considered a waste of time to come with with theories which are no better than existing theories. A few people are still in a similar denial over relativity, asserting that spacetime is actually flat. And yet no superior model supports that contention.

A better model is a better model. However, we now know that won't be one which is local realistic.

nortonian

1. It is not a classical model. Quantization is imposed on energy absorption by way of classical field superposition and on energy emission by way of electron transition. As far as down converted photons, it seems we have a misunderstanding. What you call a photon I am calling a detection event caused by classical superposition of fields. Quantum mechanics has defined them to be the same: photon absorption = photon emission. How do you know energy absorption and electron decay are symmetric processes? They are not simultaneous events.
2.From my post #4
From my last post:
You didn't challenge my earlier post so why now?


You believe it is not a better theory to preserve local realism. I believe it is a better theory irrespective of whether better predictions are obtained. What is wrong with striving for a theory with local realism so long as experimental predictions are obeyed? I don't think this is similar to disagreements with SR at all since curved space-time can be experimentally observed. Quantum mechanics says it is observing photons but no optical experiment can prove that.

The only reason you know that there will be no local realistic model is because it was defined to be non-local.

Cthugha

Important things first:

This is the correct trademark of identifying single photons!

That is NOT how bunching works. Bunching occurs for thermal or similar light (but not for coherent or non-classical) and a similar process occurs in situations when two photons start out in different states and can end up in the same state. It does not happen on any occasion, especially there is no tendency for these two photons to stay in the same state. Also the timescale over which bunching occurs is roughly the coherence time of the light which is typically in the picosecond to femtosecond range for SPDC and therefore pretty short.

You can always easily check whether you have a single photons state or more photons present (for a state that can be prepared repeatedly of course) by placing a beam splitter in the beam and checking whether both can fire simultaneously. For a state containing two photons, you will see them fire as photons do not have the tendency to take the same exit port of a beam splitter if they entered via the same entrance port, but are independent. If you just have 1 photon, they will never fire simultaneously. This experiment has been performed and published at least a few hundred times in the community working on single photon sources. Also you could simply use photon-number resolving detectors or two-photon absorption to show that.

This is true for some single photon states, but not for all. If you have heralded single-photon states or turnstile single photon sources, they are pretty localized.

DrChinese

Thanks for explaining this better than l could.

I thought the citation might help nortonian, but he would have to read it first.

zonde

Wrong
All one has to do is demonstrate that falsifiable prediction of theory actually fails.

Question for you: is quantum entanglement falsifiable prediction of quantum mechanics?

Even if Bell was using incorrect model we can use his theorem as baseline to analyze viability of different local realistic explanations for entanglement.

nortonian

The important things are not models. They are things like momentum and time that qm uses in an inconsistent manner. Let's get away from models of em interaction and look at the fundamental questions. A qm photon is delocalized and is integrated over an infinite number of frequencies so it does not fit the pattern of other particles. A proper model of the photon must be localized with a diffuse field of a single frequency similar to other particles. The qm model changes depending upon the energy of the photon. Sometimes it is localized sometimes not.

Another fundamental question concerns how qm deals with time. In optical experiments qm uses a single time parameter, the phase, when defining photons. A model that uses superposed photons to define detection is able to include continuous time into the energy absorption/emission process. The fields of superposed photons cause the outer electrons of photosensitive atoms to oscillate. If the superposition is of sufficient intensity the electron is forced into a higher orbital, where after a finite decay time it drops back and emits a photon. The conservation of momentum is exactly obeyed.

In the qm model the superposed fields and detection event are both included in the photon so that observation is an instantaneous event. (The path integral method does use both times but it integrates over all time a questionable practice.) The interaction does not take place in continuous time as would be necessary to include momentum in a description. If we introduce momentum into a description of photon states, how does a single photon cause excitation? It would be necessary for the photon to impact the electron with precisely the amount of energy to raise it into a higher orbital without knocking it out of the atom completely. Visualization is not possible. I have a problem with that because I am thinking, what are they hiding? Where is the momentum? The important stuff, the fundamentals, the details are skipped over.

Quantum mechanics treats time and momentum inconsistently.

Question for you: If they are not actually photons, but rather detection events then what difference does it make? Unless you give me a description that includes momentum they are not photons.

Cthugha

No.

It can be. It does not have to.

If it is non-monochromatic it can be necessary to integrate over some frequencies. This is not always necessary and that is not different from other particles.

Definitely not!

This is very wrong. You are aware that the photon concept of a single frequency which you can use as a basis for constructing em fields and the single photon state as an eigenstate of the photon number operator are different things, right?

An eigenstate of the photon number operator does not even have a well defined phase due to uncertainty.

I thought the important things are not models?

I described in my last post why that model is wrong, how it is ruled out and how it can be tested, I will not repeat that as you just ignored it before.

This is nonsense. You need to consider photon creation and annihilation operators at DIFFERENT times to describe emission and absorption events.

You know that typical detectors do not use some isolated atoms, but some solid state detector material which has a continuum of excitable states, right? Even for single atoms I do not see your problem. Photons with matching energy are absorbed. Others are not. This has nothing to do with momentum. For momentum you need to check the wavevector.

That is simply wrong.

Sorry, but I cannot help you much as you are claiming plain nonsense. The proper way is to read and understand what qm says about this and then ask questions to deepen understanding and not to make up something you do not like and then claim that this is what qm says.

zonde

We are speaking about Bell theorem right?

Then the difference is that we can not be sure that detection events will pair up perfectly with assumed perfect detectors. Basically we can not claim that fair sampling assumption holds and therefore Bell theorem does not apply.
However if detection events would pair up perfectly for efficient detection then there is no difference.