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Is quantum mechanics a complete theory of nature? 
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#1
Mar612, 10:13 AM

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The wave function represents all that can be known about a quantum system, but that usually means that we only know the energy. In the case of entanglement we know the energy but not the momentum (e.g. angular momentum) of its components. When one component of an entangled system (one spin up and one down) is measured the wave function collapses and we immediately know the spin of the other particle with a speed exceeding that of light. However, if we knew how the momentum of the quantum system was distributed to begin with we could describe the system without a need for measurement and entanglement would not be an issue. So based on the inability of quantum theory to specify momentum it seems to me that quantum theory is incomplete. And due to the uncertainty principle a complete theory is impossible.



#2
Mar612, 10:41 AM

Sci Advisor
PF Gold
P: 5,441

A. Einstein, B. Podolsky, N. Rosen: "Can quantummechanical description of physical reality be considered complete?" Physical Review 41, 777 (15 May 1935) http://www.drchinese.com/David/EPR.pdf 


#3
Mar612, 01:05 PM

PF Gold
P: 821

You might find this information of use  http://www.perimeterinstitute.ca/New...eory_complete/



#4
Mar612, 01:14 PM

P: 59

Is quantum mechanics a complete theory of nature?
Thanks for that link. I had of course read about the EPR experiment but never seen the original.
I also followed the link to the philosophical discussion of the same question which I think is a reasonable way to formulate an answer. 1. quantum mechanics is the most complete theory/description of nature that we have. 2. nature itself is the only complete description IOW our descriptions of nature will always be inadequate, and understandably so 


#5
Mar612, 01:35 PM

P: 1,583

nortonian, you may already know this but EPR is by no means the end of the story. Long after the EPR paper, J.S. Bell proved a theorem in quantum mehanics that poses some challenges to Einstein's view. Here is a good explanation of Bell's proof which is relatively easy to understand. Once you understand Bell's theorem, you can try to puzzle out the philosophical implications concerning quantum mechanics.



#6
Mar612, 02:57 PM

P: 59

N. Herbert's description is excellent. The best I've seen. Thanks.
He concludes: After almost a century of contact with nature's peculiar quantum way of doing business we are still lacking a quantum world view that does justice to our new knowledge of the way the world really works. 


#7
Mar612, 04:26 PM

PF Gold
P: 698

I always wonder if physicists aren't repeating Lord Kelvin's "predictions" that never materialized:



#8
Mar612, 04:33 PM

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#9
Mar612, 09:18 PM

P: 461

funny how those two clouds obscured a vast mountain range.



#10
Mar712, 10:12 PM

P: 59

Lugita, I have had time to ponder on Nick Herbert's description of Bell's Theorem in your link and I have some ideas I would like to share with anyone out there whose interested to see if they make sense. In the example he uses a calcite crystal to separate a beam of light into two beams of oppositely polarized light. Photodetectors are then used for two purposes: to “count” the photons in each beam and to detect the polarization of the beam. Since you are already familiar with it I won't go into detail. I don't think the thought experiment he uses is a good one. Photons are bosons meaning that more than one can occupy the same state. One of the consequences is that photon bunching occurs in light beams and they are detected as coincidences when separated by beam splitters (BrownTwiss effect). According to the BrownTwiss effect when Herbert uses a calcite crystal to divide a light beam into two beams polarized at 90 degrees and measures photon coincidences he is actually dividing bunches into smaller bunches and is detecting and comparing bunches not photons. When you change the polarization of the detector (its angle) whether you detect a photon bunch may depend partially upon the size of the bunch. I also question his interpretation of detection properties. How can you define a photon to be a detection event without looking at the properties of a detector? The time required to register a single detection event by a photodetector is on the order of 109 seconds, and single photons have periods on the order of 1012 seconds. By that measure there could be thousands even hundreds of thousands of "photons" in a single event.



#11
Mar712, 11:19 PM

PF Gold
P: 1,376

I will try to explain. Let's say you place two detectors right after PDC source in two outputs. Now you measure how many single detections you have and how much of them are paired with detections in other detector. For detector you have parameter called quantum efficiency (QE) that says (in %) how many photons you can detect with this detector. If you calculate rate between single detections and paired detections using this QE parameter it agrees very well with observed rate. And second thing is that if you increase detector's QE than rate of paired detections increases as well so that for QE=100% you would have ~100% paired detections and practically no unpaired single detections. You might want to look at this as well: Singlephoton detector characterization using correlated photons: the march from feasibility to metrology 


#12
Mar812, 09:56 AM

P: 59

In the bunching model you can keep on splitting a beam until it can't be detected and you will still have coincidences in the beams because you can never detect all of the bosons in an energy state. A detection event includes all the photons in an energy state, not just one. 


#13
Mar812, 10:18 AM

P: 59

Going back to my original post I think that quantum theory can include a description of its own incompleteness if we will only recognize that.



#14
Mar812, 11:37 PM

PF Gold
P: 1,376

If so then I guess my answer is something like that: I do not know but any viable alternative makes no difference (at this time). "It also requires us to recognize that there is a payoff between detector efficiency and signalnoise discrimination." This indeed seems to be the case for SPAD detectors. But it turns out this is not a general rule for any detector: NIST Detector Counts Photons With 99 Percent Efficiency: “When these detectors indicate they’ve spotted a photon, they’re trustworthy. They don’t give false positives,” says Nam, a physicist with NIST’s Optoelectronics division. “Other types of detectors have really high gain so they can measure a single photon, but their noise levels are such that occasionally a noise glitch is mistakenly identified as a photon. This causes an error in the measurement. Reducing these errors is really important for those who are doing calculations or communications.” Always ready to explain why I think that quantum entanglement has local realistic explanation. 


#15
Mar912, 03:39 AM

P: 231

Compare for example with the discussions on Gödel's theorem with respect to quantum mechanics. Here, complete means that within the set of axioms used, there are true statements that cannot be proven true. With such a definition, there are strong indications (if not proofs) that any physical theory, and therefore also quantum mechanic, cannot be "complete", because it's not compatible with "consistent", which seems to be a required property. 


#16
Mar912, 08:49 AM

P: 59

There is another paper on detection event vs. photon by Marshall http://www.mendeley.com/research/myt...vertedphoton/ which specifically addresses parametric down conversion.



#17
Mar912, 10:37 AM

Sci Advisor
PF Gold
P: 5,441

Yes, it is always technically possible that there are 2 photons being detected at EXACTLY the same time at both detectors and masking as 1, but this is farfetched (and meaningless) in the extreme. There is no evidence of any effect like this at all. So the idea of this occurring at the calcite splitter is not viable. Unless, of course, you want to make up some new ad hoc physics. See for example: http://people.whitman.edu/~beckmk/QM.../Thorn_ajp.pdf Observing the quantum behavior of light in an undergraduate laboratory J. J. Thorn, M. S. Neel, V. W. Donato, G. S. Bergreen, R. E. Davies, and M. Beck While the classical, wavelike behavior of light ~interference and diffraction! has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light ~i.e., photons! is much more difficult. For example, while wellknown phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with singlephoton counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of secondorder coherence between the outputs to be g(2)(0)50.017760.0026, which violates the classical inequality g(2)(0)>1 by 377 standard deviations. 


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