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Nugatory

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To see any quantum effect, you need a single-particle source and a screen that records each individual impact so that the pattern builds up one dot at a time. That's well beyond the reach of any home experiment that I am aware of.

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dlgoff

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Is there any easy way to test both wave and particle behavior of light using some homemade detector ?

Here's a "one particle a time" video:To see any quantum effect, you need a single-particle source and a screen that records each individual impact so that the pattern builds up one dot at a time.

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There was a nice article in the recent issue of AJP

To see any quantum effect, you need a single-particle source and a screen that records each individual impact so that the pattern builds up one dot at a time. That's well beyond the reach of any home experiment that I am aware of.

http://dx.doi.org/10.1119/1.4960475

http://arxiv.org/abs/1602.05987

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Very interesting videos. ThanksThere was a nice article in the recent issue of AJP

http://dx.doi.org/10.1119/1.4960475

http://arxiv.org/abs/1602.05987

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I might be wrong but you only "know" the position and not the momentum. If you would try to measure it then the detector will get entangled with each photon. In order for that to happen then you need to build a detector for the slits.

You would need to do that on each individual photon. By just looking at the wall or whatever you'll see the overall position of photons in a stream of them (many). Not to be confused with actual knowledge of their momentum.

If I replied in error then any member should feel free to correct me.

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seeing the light is a matter of dispersion of photons from the water vapor (freezer) or talc particles. After the light goes through the slits the intensity is diminished due to absorption/reflection by the black plastic, and thus there are fewer photons to be dispersed by the water/talc particles, and with fewer dispersed photons there is a lower chance of the naked eye being able to detect the weak rays between the wall and the slits (beam intensity increases with the number of photons present).

the wave will only collapse if you can tell which slit the photon passed through (path information). because you are looking at more than one photon at a time, and your camera is unable to detect a single photon, you will not be able to get path information from your device, and the wave will not collapse.

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So I am in doubt whether the position and momentum uncertainty are applicable in this test

I can see quite bright fringes and ray inside the freezer using my red-light laser pointer. I wonder if I can use a more powerful laser then can I view all the rays coming out from the slits like a cone and create the fringes?

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A laser "ray" is not a single-photon state but a coherent state. At high intensities as provided by a laser pointer, it's to utmost accuracy describable by good old classical electrodynamics. The fascinating thing with lasers is, however, that they provide light with huge coherence lengths at high intensity in such sharp beams as your son can observe as described which is not achievable with "conventional" light sources. The thing that's quantum here is not the light itself, because it's very well described as a classical electromagnetic wave (in fact as a Gaussian beam), but the creation of such a coherent em. wave, which is due to stimulated emission from the active laser material:

https://en.wikipedia.org/wiki/Laser_diode

Of course, Nugatory's assessment above is completely right. To get a double-slit interference pattern with good contrast, the laser beam must be wide enough to shine over both slits. Thus a single photon "contained" in the coherent state, as a wide enough position uncertainty such as to make it impossible to say through which slit it went before hitting the screen where you observe the pattern. What you also immediately realize from this hands-on experience is that the position of a photon "contained" in the coherent state in direction of the beam is completely undetermined, it's not even definable as an observable in the first place.

I think it's great that your son and you do such experiments with the laser pointer, because it provides direct insight in what's so very wrong with the picture many popular-science concerning photons and the electromagnetic field at the quantum level (but unfortunately also some introductory QM textbooks at the intro level, where they produce the most severe confusion about QT yound students have to suffer from such outdated textbooks, which are however still even written today by just copying the bad intro sections from the old textbooks). You only have to be aware of the many flaws of such books, which provide an outdated picture of "old quantum theory" with its incomprehensible ad-hoc assumptions like wave-particle duality (which is most severely wrong for photons).

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You are recreating Young's purely classical, no quantum mechanics needed, demonstration in 1801 that light is a wave. There's an interference pattern if both slits are open, no interference pattern if either slit is closed, and the presence or absence of a

Why do people say there is no interference when one slit is closed when clearly you can measure the fringes that are mathematically well defined...and visible.

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For a double slit with the distance of the slits ##d## with a width ##b## you get (with ##k=2 \pi/\lambda \sin \theta##, where ##\theta## is the angle between the apperture and the normal to the observational plane)

$$A_{\text{double slit}}(k) \propto \int_{d-b/2}^{d+b/2} \mathrm{d} x \cos(k x) = 2 \cos (k d) \frac{1}{b} \sin \left (\frac{k b}{2} \right),$$

while for the single slit you get

$$A_{\text{single slit}}(k) \propto \int_{0}^{b/2} \mathrm{d} x \cos(k x)=\frac{1}{k} \sin \left (\frac{k b}{2} \right).$$

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Nugatory

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That's diffraction, also a wave phenomenon.Why do people say there is no interference when one slit is closed when clearly you can measure the fringes that are mathematically well defined...and visible.

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[...] You only have to be aware of the many flaws of such books, which provide an outdated picture of "old quantum theory" with its incomprehensible ad-hoc assumptions like wave-particle duality (which is most severely wrong for photons).

I thought the wave particle duality is correct. It's a wave which collapses (particle) but never both.

What makes you state otherwise?

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Interference must still occur in a single slit to cause fringes just like diffraction must occur in a double slit.That's diffraction, also a wave phenomenon.

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Nugatory

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Yes, of course - that's how classical electromagnetic waves behave. But this thread started out being about the two-slit interference pattern, not the single-slit pattern. In any case, the appearance of a single-slit diffraction pattern in OP's test setup has nothing to do with any quantum phenomena; he's sending classical electromagnetic waves through one or two slits, and getting the behavior you'd expect from classical electromagnetic waves.Interference must still occur in a single slit to cause fringes just like diffraction must occur in a double slit.

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I thought the wave particle duality is correct. It's a wave which collapses (particle) but never both.

Light can exhibit some behaviors associated with waves or particles. It could be argued, however, that light consists of neither waves nor particles, but rather something we can't picture, and don't have a word for.

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I'm a proponent of the minimal statistical interpretation of quantum theory, i.e., the interpretation you need in physics without philosophical distortions of the scientific theory. There is no wave-particle duality but only the Hilbert-space formalism a la Dirac with the probabilistic interpration of states via Born's rule. The wave-function collapse is a short-hand concept of some flavors of the Copenhagen interpretation that's heuristic at best misleading at worst. It has to be handeled with care.I thought the wave particle duality is correct. It's a wave which collapses (particle) but never both.

What makes you state otherwise?

In the double-slit experiment in this interpretation you have particles (I don't discuss photons here since they do not admit a proper particle interpretation in the literal sense, because they do not allow for a properly defined position observable, so take electrons, neutrons, or any other massive particle as an example) prepared at a pretty well defined momentum running towards a double slit, where many of them are absorbed (if you wish they "collapsed" in a quite literal sense, but there's no mystery, they are simply absorbed by the wall, and you dont' care about them anysmore) and some are going through the one or the other slits. If the momentum was defined precisely enough, due to the Heisenberg uncertainty principle this implies that the probability to go through the one or the other slit is quite similar (in the wave-mechanics language: the wave packet should be broad enough in transverse direction to cover both slits), you cannot decide through which slit an individual particle comes. Also you have no more information at which point the particle will hit the screen where it is detected than the probability given by the corresponding solution of the Schrödinger equation, applying Born's Rule. This can be verified by preparing a lot of particles in this way, getting the interference pattern on the observational screen from many events. What's called "collapse" is nothing else than the interaction of the particle with this screen, usually getting absorbed, and you don't care anymore about its further fate.

- #19

bhobba

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I thought the wave particle duality is correct. It's a wave which collapses (particle) but never both. What makes you state otherwise?

Well first you need to read a more advanced treatment of QM like Ballentine:

https://www.amazon.com/dp/9814578584/?tag=pfamazon01-20

But you can't build up to that cold. I recommend the following in the following order:

https://www.amazon.com/dp/0471827223/?tag=pfamazon01-20

https://www.amazon.com/dp/0465075681/?tag=pfamazon01-20

https://www.amazon.com/dp/0465062903/?tag=pfamazon01-20

https://www.amazon.com/dp/1118460820/?tag=pfamazon01-20

https://www.amazon.com/dp/0674843924/?tag=pfamazon01-20

https://www.amazon.com/dp/9812569715/?tag=pfamazon01-20

https://www.amazon.com/dp/9812569731/?tag=pfamazon01-20

https://www.amazon.com/dp/9812569758/?tag=pfamazon01-20

Then you can undertake Ballentine - but don't hurry - its a long hard slog.

At each stage you will find superseded ideas like the wave-particle duality replaced by more advanced ones.

Eventually you will understand a more advanced explanation of the wave-particle duality motivation - the double slit:

http://cds.cern.ch/record/1024152/files/0703126.pdf

Be warned however, as Vanhees will correctly point out, even the above is not correct:

:http://arxiv.org/pdf/1009.2408.pdf

To make matters worse even the corrected version above is not correct. Physics can be truly maddening like that. QFT in particular is badly prone to it but that is a whole new story. You might like to become acquainted with QM myths:

http://arxiv.org/abs/quant-ph/0609163

Bottom line is the whole situation is quite subtle. But you have come to the place where you get the full story - warts and all - that will take some time to get accros. There is unfortunately no short-cut if you want the truth.

For what its worth I MUCH prefer the following as a starting point to QM:

http://www.scottaaronson.com/democritus/lec9.html

But it's way non-standard - what I recommended before is usual and bog-standard - but takes quite a while to get to the 'meat' so to speak.

Thanks

Bill

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First guess is involving a sine....there the main physics is solved, now you have a reason to follow the algebra and geometry to see the quantitative details you have already solved qualitatively.

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First guess is involving a sine....there the main physics is solved, now you have a reason to follow the algebra and geometry to see the quantitative details you have already solved qualitatively.

I've solved the components and have did most of the work via Khan Academy which helped me remind a lot of stuff from elementary school. What I lack however at really basic fundamental level is the intuition for such systems.

Unfortunately I don't have the energy anymore at the end of the day to go through it after a day of work but I'm trying

I've ordered all the books from Amazon that are given to me.

Thanks a lot for your encouragements.

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