Is Wave-Particle Duality Really Real? An Analysis of the Double Slit Experiment

[conway]

Yes, I generally had trouble understanding what you meant with that kind of statement.

 

[Phrak]

Just to add to my prior post: I think the situation could probably be formulated into an inequality using logic from Bell's Theorem.

You do realize that Bell's inequality refutes quantum mechanics, right?

 

[jtbell]

Bell's inequality refutes quantum mechanics

How so?

 

[Fredrik]

"Contradicts" would probably be a better word than "refutes".

 

[Phrak]

Yeah. 'Refutes' is not the right word.

How so?

Should Bell's inequality be true, some predictions of quantum mechanicanics are incorrect.

Violations of Bell's inequality could support quantum mechanics.

To be sure, quantum mechanics predicts statistical results that would violate Bell's inequality. Quantum mechanics predicts that Bell's inequality should be violated, and with particular statistical results.

And, to be sure, it's a common error to invert the meaning of the inequality. Bell could just as easily have inverted the inequality to it's inverse, and we'd be free of confusion--though I think he initially believed it would be found experimentally supported and 'action at a distance' found false.

 

[mintparasol]

There is NO chance that a particular photon polarized at 0 degrees could pass through both a +45 polarizer and a -45 degree polarizer in the quantum mechanical view. The operative formula is the COS^2(L-R) rule, where L-R=90 degrees so that the result is 0 and there is no interference.

On the other hand, in the classical perspective, there IS a chance that any particular photon polarized at 0 degrees could pass through both a +45 polarizer and a -45 degree polarizer. Do you see why? The rule is different because the probability is resolved independently for each slit, unlike in the quantum view in which it is the relative angle of the L and R slits is important. So now you get COS^(L-0)*COS^(R-0) and there should be some interference because the result is .25 which is >0.

Well at least you are thinking about it...

I would be interested in a demonstration of a classical wave effect which eliminates all interference when partial polarizers (or classical equivalent) are present in a setup similar to the double slit. You use the example of audio, and I think a careful consideration of your analogy will demonstrate that the interference will NOT be eliminated after all - as it would need to be to match light in a double slit.

Of course there is a lot more to the story on the quantum side anyway. If light were waves (only) then a lot of things would be different (in contradiction to experiment - see for example: http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf" ). If light polarization operated as you imagine, things would be different with entangled particle pairs (in contradiction to experiment).

You may not be aware of all of the experiments out there (who is?), but you might want to at least ask before talking about the emperor's clothes. The double slit is just one piece of the puzzle.

Ok, I've printed off that paper for further reading, thanks!

I'm just wondering if we could arrange some mechanism to put our bass wave +45 degrees out of phase at the left slit and -45 degrees out of phase at the right slit, would the math work in the same way for soundwaves? Would there be any interference detectable rear of the slitted screen?

 

[conway]

But sound isn't polarized. Its a longitudinal wave while light is a transverse wave. In any case, there is no interference with light if the waves from the two slits are polarized at 90 degrees to each other. This is predicted by classical physics exactly the same as QM and does not represent any kind of mystery.

 

[DrChinese]

You do realize that Bell's inequality refutes quantum mechanics, right?

I am sure we agree that a violation of a Bell inequality refutes classical realism, and is consistent with QM. I simply think of the inequality as a boundary condition on (local) realism.

 

[Frame Dragger]

I am sure we agree that a violation of a Bell inequality refutes classical realism, and is consistent with QM. I simply think of the inequality as a boundary condition on (local) realism.

I have to say that's a good way of putting it. I've been doing a good deal of reading on deBB, and a refresher on The TI, and more and more it seems that dBB and a few other theories are simply the ones that have not been swept aside by modern experimental evidence. dBB basically hinges on the QM being in violation of Bell's Inequalities, and not classical realism. The argument of (mechanical) bias in experiments such as a "2 channel" test, keeps dBB alive unlike most other theories which required LHV's.

However, the switch from locality to non-locality in dBB seems logically contrived for the sole purpose of maintaining a particular interpretation of reality as something other than emergent phenomena from a quantum soup of superpositioned probabilities. That doesn't make it wrong, and from everything I'm reading dBB can't be refuted right now and if nothing else it does a very good job of highlighting the huge gap in explaining how the microscopic and macroscopic worlds combine to form what we experience as reality in SQM. I think that the TI is probably on the right track, but it's hard to say that those theories, or MWI are more contrived than dBB.

Given all of that, I still fall on the side of SQM for the sake of the results it provides and predictions it makes. Bell expected "Spukhafte Fernwirking" to be experimentally disproved, and instead it is now experimentally observed on a regular basis.

In my view, one of the ways this debate can be settled would be the emergence of even a VERY rudimentary computer capable of using qubits to complete an algorithm. Failure would mean little, but success in line with expectations of SQM would be pretty damned confirmatory. The problem with modern approaches is that they still use Classical mechanics that exploit quantum behaviour... which can be explained by theories such as dBB. Go figure.

The bottom line is that Bell kind of draws the line between the Classicist and SQM view of reality. It's a boundary in the common usage and not just the scientific term of art.

 

[DrChinese]

I have to say that's a good way of putting it. I've been doing a good deal of reading on deBB, and a refresher on The TI, and more and more it seems that dBB and a few other theories are simply the ones that have not been swept aside by modern experimental evidence. dBB basically hinges on the QM being in violation of Bell's Inequalities, and not classical realism. The argument of (mechanical) bias in experiments such as a "2 channel" test, keeps dBB alive unlike most other theories which required LHV's.

However, the switch from locality to non-locality in dBB seems logically contrived for the sole purpose of maintaining a particular interpretation of reality as something other than emergent phenomena from a quantum soup of superpositioned probabilities. That doesn't make it wrong, and from everything I'm reading dBB can't be refuted right now and if nothing else it does a very good job of highlighting the huge gap in explaining how the microscopic and macroscopic worlds combine to form what we experience as reality in SQM. I think that the TI is probably on the right track, but it's hard to say that those theories, or MWI are more contrived than dBB.

...

The bottom line is that Bell kind of draws the line between the Classicist and SQM view of reality. It's a boundary in the common usage and not just the scientific term of art.

I agree with what you are saying. dBB gets by because the context is non-local. Another interesting set of interpretations is the Time Symmetric group (including Relational BlockWorld RBW), in which a future context is allowed to influence the present. These have the "benefit" of being local, contextual (non-realistic) and time symmetric. Of course you swap one assumption for another, so whether the result is net better is a matter of preference.

 

[Frame Dragger]

I agree with what you are saying. dBB gets by because the context is non-local. Another interesting set of interpretations is the Time Symmetric group (including Relational BlockWorld RBW), in which a future context is allowed to influence the present. These have the "benefit" of being local, contextual (non-realistic) and time symmetric. Of course you swap one assumption for another, so whether the result is net better is a matter of preference.

This is true, and possibly experimentally verifiiable. If phonons can be detected emerging from a sonic event horizon it would be a step towards showing that a process like Hawking Radiation might exist... and that hinges on virtual particle annihilation (or lack thereof)... which is also a kind of single particle along a worldline view, but not in one temporal "direction". I suppose we'll have to wait and see.

 

[eaglelake]

In the book he makes an example of the double slit experiment we all did in high school and says that if we fire one photon at the slitted screen, we'll get an interference pattern on the rear screen.

If you fire one photon through the slits you will get one dot on the second screen. A single photon is always detected as a single particle (a dot), never as a wave. In fact, the single dot doesn’t reveal any wave properties. It is only when you detect many photons does the distribution of scattered photons begin to look like an interference pattern. There are sites on the web that show how the interference pattern is built up one photon at a time.

I am bothered that an author would make such a statement, but apparently it is all too common. No experiment has ever shown a particle behaving like a wave. The point is this – a quantum particle is not a wave.

 

[Frame Dragger]

If you fire one photon through the slits you will get one dot on the second screen. A single photon is always detected as a single particle (a dot), never as a wave. In fact, the single dot doesn’t reveal any wave properties. It is only when you detect many photons does the distribution of scattered photons begin to look like an interference pattern. There are sites on the web that show how the interference pattern is built up one photon at a time.

I am bothered that an author would make such a statement, but apparently it is all too common. No experiment has ever shown a particle behaving like a wave. The point is this – a quantum particle is not a wave.

Annnnd... you're wrong. In fact, even when passing C60 or Rubidium atoms (one at a time) there is interference consistant with passage through both apertres of the experiment. There are sites on the web that show how 8th dimension lizards run the country... that doesn't make them accurate. Interference patterns of the type you describe are not consistant with a purely particle-theory of light. Light's wave-like properties are as well established as its particle-like properties.

As for experiment with photons... you'rre wrong again? I don't know what else to say... you can in fact set up simple controls for these experiements which eliminate interence by more than specifically polarized photons passng throuhg the apertures.

EDIT: Addition: "A quantum particle is not a wave." True. Photons are quanta which have wave and particle -like properties. They are neither waves, NOR particles and feel free to call them "quanta" not "quantum particles". That latter is... meaningless and semi-redundant.

 

[eaglelake]

Annnnd... you're wrong. In fact, even when passing C60 or Rubidium atoms (one at a time) there is interference consistant with passage through both apertres of the experiment. There are sites on the web that show how 8th dimension lizards run the country... that doesn't make them accurate. Interference patterns of the type you describe are not consistant with a purely particle-theory of light. Light's wave-like properties are as well established as its particle-like properties.

As for experiment with photons... you'rre wrong again? I don't know what else to say... you can in fact set up simple controls for these experiements which eliminate interence by more than specifically polarized photons passng throuhg the apertures.

EDIT: Addition: "A quantum particle is not a wave." True. Photons are quanta which have wave and particle -like properties. They are neither waves, NOR particles and feel free to call them "quanta" not "quantum particles". That latter is... meaningless and semi-redundant.

No need to be a wiseguy! If you do not understand something, just ask for a clarification.

I responded to the statement, “if we fire one photon at the slitted screen, we'll get an interference pattern on the rear screen.”, which is not true. A single photon is always detected as a single dot on the detection screen. A single photon is never smeared over the screen in an interference pattern, as a wave would be. If you want to see wave effects then you must repeat the same experiment many times.

The original statement by mintparasol referred to a single photon, while you are talking about, “passing C60 or Rubidium atoms (one at a time)”, which is about MANY atoms. “One at a time” means more than one! You seem to imply that the interference pattern is built up “one (atom) at a time”. I agree! That is exactly what I said. So, where is the disagreement?

The basic question is, “ When does a photon, or any quantum particle, look like a particle and when does it look like a wave”? We either observe one or the other, but never both at the same time. (Bohr’s complimentarity principle)

The answer is – if you detect a single particle it looks like a particle. It looks like a wave only after you have detected many of them. [1] (and, then, only in special circumstances.)

Best wishes.

[1] A. Tonomura, et al, Amer. J. Phys., 57, 117-120 (1989)

 

[Frame Dragger]

No need to be a wiseguy! If you do not understand something, just ask for a clarification.

I responded to the statement, “if we fire one photon at the slitted screen, we'll get an interference pattern on the rear screen.”, which is not true. A single photon is always detected as a single dot on the detection screen. A single photon is never smeared over the screen in an interference pattern, as a wave would be. If you want to see wave effects then you must repeat the same experiment many times.

The original statement by mintparasol referred to a single photon, while you are talking about, “passing C60 or Rubidium atoms (one at a time)”, which is about MANY atoms. “One at a time” means more than one! You seem to imply that the interference pattern is built up “one (atom) at a time”. I agree! That is exactly what I said. So, where is the disagreement?

The basic question is, “ When does a photon, or any quantum particle, look like a particle and when does it look like a wave”? We either observe one or the other, but never both at the same time. (Bohr’s complimentarity principle)

The answer is – if you detect a single particle it looks like a particle. It looks like a wave only after you have detected many of them. [1] (and, then, only in special circumstances.)

Best wishes.

[1] A. Tonomura, et al, Amer. J. Phys., 57, 117-120 (1989)

No... you missed my point entirely. The experiment involving Rubidium showed a single atom having a unique wavefront just as a single photon does. The fact that it takes multiple passes (as you say, a buildup) to make the pattern visible is a limitation of our detection methods. If one could image a photon more exactly there would be a wavefront causing an interference pattern, visible or not. The dual nature of the quanta seems pretty clear. That's a limitation of the experimental apparaturs, but it's clear from the distribution... built over time as you say... that each individual photon, atom, etc, while observed at any given time to be particle or wave -like... has both properties at all times.

 

[Mentz114]

FrameDragger:

The fact that it takes multiple passes (as you say, a buildup) to make the pattern visible is a limitation of our detection methods.

I don't see how. A particle detector is designed to detect particles and that's what it does. What apparatus would you use to detect a 'Rubidium wave' ?

 

[Frame Dragger]

FrameDragger:


I don't see how. A particle detector is designed to detect particles and that's what it does. What apparatus would you use to detect a 'Rubidium wave' ?

The same way you do with buckyballs (C60) a la http://www.users.csbsju.edu/~frioux/two-slit/c60-slit.htm

 

[Mentz114]

The same way you do with buckyballs (C60) a la http://www.users.csbsju.edu/~frioux/two-slit/c60-slit.htm

That doesn't answer the question. The note you linked to shows the wave function that models the experiment. The wave function gives us probabilities of detecting the particle at certain locations. If you send one particle through, you detect one at the screen. If you send a 100 particles through one by one, they will make a pattern like the square of the wave function at the screen. The particle goes through one slit or the other but we cannot find out (even in principle) which slit, and still get the interference pattern building up.

I enjoyed that little note.

For more on single-photon interference look up Hong-Ou-Mandel.

 

[Dmitry67]

I wanted to remind that the balance between particle and wave behavior is interpretation-dependent.

For example, in BM there are real particles, just guided by the wave.
In MWI, there are only waves (and dots we see on the screen are the result of quantum decoherence)

 

[Frame Dragger]

That doesn't answer the question. The note you linked to shows the wave function that models the experiment. The wave function gives us probabilities of detecting the particle at certain locations. If you send one particle through, you detect one at the screen. If you send a 100 particles through one by one, they will make a pattern like the square of the wave function at the screen. The particle goes through one slit or the other but we cannot find out (even in principle) which slit, and still get the interference pattern building up.

I enjoyed that little note.

For more on single-photon interference look up Hong-Ou-Mandel.

Understood, clearly this was a misinterpretation on my part. However that does not eliminate the fact that in the absence of observation even a single photon must have properties which that experiment allows to build into an a clear interference pattern.

The distribution of many (as you say) photons allows for the interference pattern to be observed, but presumably both properties ARE always present. Even when light appear to travel as a wave, it is subjected to the effects of gravity, and when it appear to register as a single particle on a screen, it clearly has wave-like properties. The pattern at the end of the experiment can be predicted based on the arrangement of the test, and the material being tested, if not the position of each individual "strike" on the screen.

The belief the the pattern is simply a function of many particles acting without wave-like behaviour flies in the face of experimental evidence, but is compatible with non SQM or dBB theories. Even dBB postulates a pilot wave to explain experimental evidence.

Dimitry67: Well, it's interpretation dependant in the case of MWI and some others, but that is a conjecture and not really a mechanical theory. Right or wrong, it's purely ad hoc. dBB... well.. that's a separate theory that emerged before SQM, and not just a separate interpreation of QM behaviour. I still think it's wrong and somewhart contrived, but to relegate it to an interpreation of the theory it rejects is probably unfair.

 

[mintparasol]

Thanks to all who've replied here. I think it's kind of cool that a question from someone like myself who has very little understanding of the technical aspects of QM can get such a lively debate going!
I've been reading and re-reading the thread and some of the links posted here and I think I have a slightly better understanding of what's going on here than I had at the beginning.
Thanks again
ad

 

[Frame Dragger]

Thanks to all who've replied here. I think it's kind of cool that a question from someone like myself who has very little understanding of the technical aspects of QM can get such a lively debate going!
I've been reading and re-reading the thread and some of the links posted here and I think I have a slightly better understanding of what's going on here than I had at the beginning.
Thanks again
ad

Well, your question is at the heart of one of the major unsolved questions in modern physics. It was an interesting question, and bound to lead to some complex ruminations on the subject. And fireworks... musn't forget the fireworks lol.

 

[Mentz114]

FrameDragger:

The belief the the pattern is simply a function of many particles acting without wave-like behaviour flies in the face of experimental evidence, but is compatible with non SQM or dBB theories. Even dBB postulates a pilot wave to explain experimental evidence.

But SQM is saying that the pattern is determined by the wave-function otherwise there would be no interference pattern. SQM does not say that the particles behave like classical particles.

I still think it's wrong and somewhart contrived, but to relegate it [dBB] to an interpreation of the theory it rejects is probably unfair.

Saying that dBB is 'wrong' is somewhat wild and unconsidered. The same wave-function is used to calculate the trajectories and the same predictions are made. I don't think you know enough about dBB to say these things.

If you reject all this - do you think a Rubidium atom can somehow pass through both slits at the same time ?

 

[Frame Dragger]

FrameDragger:

But SQM is saying that the pattern is determined by the wave-function otherwise there would be no interference pattern. SQM does not say that the particles behave like classical particles.


Saying that dBB is 'wrong' is somewhat wild and unconsidered. The same wave-function is used to calculate the trajectories and the same predictions are made. I don't think you know enough about dBB to say these things.

If you reject all this - do you think a Rubidium atom can somehow pass through both slits at the same time ?

I realize (and have stated a couple of times) that wave-particle duality in SQM isn't simply classical waves and particles, but something else combining properties of both. I realize that the distribution on the screen is a result of the wave function of each non-classical particle. That said, depending on which theory and interpretation you subscribe to, yes... the rubidium atom, C60, or photon has wave-like properties that interefere with itself unless you place a detector in the path of the aperture.

It may be that an individual particle or atom interferes with itself, but that's impossible to confirm or refute right now as far as I know. The emergence of the pattern is the result of properties that you find in a single photon, and in some interpretations that would seem to indicate that a single particle in QM behaves like a wave at that point.

From what I've been reading of dBB (and a lot of it in the last few days after a rightous and right chastisement from Zenith) doesn't change my view of the theory. It still strikes me as last gasp of Classicism, but I've said all of this before. That said, it's the only respectable theory to survive as far as SQM re: Bell's Theorem. Personally, I don't believe the inverse of the inequalities is the case, and I think that experimental evidence found and analyzed over the next decade will eliminate it. If not, as I've stated before, and SQM fails to deliver then people will be open to different theories of the microscopic.

AS for the question about the rubidium atom, no, I don't believe one atom passes through two slits simultaneously, but it's certainly possible. What I believe has little impact on reality, and yes, I realize how ironic that statement is. I DO believe that a counterintuitive reality shouldn't be terribly shocking, and entanglement is surely as counterintuitive as it gets. Alas, there it is, with one simple, but unpalatable explanation, and dBB with a far more palatable image, but now depends on non-local hidden variables. I'm sorry, but I see that as retreat in the face of new thinking, not just academic pressures or the lack of popularity of a given theory.

 

[Mentz114]

Frame Dragger,

Fair enough. Quantum phenomena are certainly weird and trying to find intuitive meaning behind the equations is probably futile.

 

[wofsy]

I am just learning physics. Correct me please but it seems like the whole thing makes sense when you look at the quantum amplitudes as evolving according to a particular stochastic process that is similar to a Markov process.The amplitude for finding a particle in a state is the sum of the conditional amplitudes that it will land it that state given that it is in the possible previous states. If these amplitudes were real numbers rather than complex this would be a Markov process.

From this point of view, interference is not really a wave interference but rather a linear combination of conditional amplitudes. this is similar to a Markov process where the probability of finding a particle in a particular state is a sum of conditional probabilities.

Any stochastic process depends upon its initial conditions. The single and double slit are two different initial conditions.

The double slit has two initial states that generate the possible future states
according to this Markov like process of amplitudes.The single slit has only one. A triple slit would have three initial states and give a even different amplitude combinations.

When one measures which slit the particle has come out of you actually reduce the problem to a one slit case so there is no amplitude contribution any longer from the other slit. The stochastic process of amplitudes has a new initial condition that is one slit rather than two. Thus there is no interference.

This point of view explains why you see interference even when you send one particle at a time through the slits. A single particle evolves according to this Markov process of amplitudes. Measuring where the particle hits a detecting screen is much like measuring where a dust particle in a fluid will hit a barrier. Measuring many particles will produce a distribution of positions just as with dust particles. Different initial conditions will produce different distributions.

From what I can gather from reading, it seems that the Shroedinger wave equation should be called the Shroedinger diffusion equation. In fact, formally the Shroedinger equation for a free particle is a complex heat equation and just as the heat equation can be derived from a continuous Brownian motion, the Shroedinger equation can be derived form a continuous stochastic process of complex amplitudes.

The incredible thing about all of this to me is that there really is nothing else to say. That is the way it is. The classical picture of the dynamics of a particle is just inapplicable here. This quantum particle is not a particle at all but something different and ordinary usual concepts are not appropriate for it.

I would add that in Brownian motion we actually imagine a dust particle bouncing around randomly. In QM it seems that there there is no physical object bouncing around but merely a mathematical formalism that gives the right answer - every time. I think this is why Einstein disliked Quantum Mechanics whereas the theory of Brownian motion was his idea.

 

[Frame Dragger]

Frame Dragger,

Fair enough. Quantum phenomena are certainly weird and trying to find intuitive meaning behind the equations is probably futile.

I agree with literally every word in that quote. Maybe whatever supplants SQM/dBB/etc and GR will be more elucidating.

 

[Mentz114]

The amplitude for finding a particle in a state is the sum of the conditional amplitudes that it will land it that state given that it is in the possible previous states. If these amplitudes were real numbers rather than complex this would be a Markov process.

From this point of view, interference is not really a wave interference but rather a linear combination of conditional amplitudes. this is similar to a Markov process where the probability of finding a particle in a particular state is a sum of conditional probabilities.

OK, but if the amplitudes are governed by a wave function, then there is path dependent phase, and so interference will take place when you sum the amplitudes from the different paths. This can only happen with waves, so a wave phenomenon is happening. You've just rephrased it in path integral terms.

 

[wofsy]

OK, but if the amplitudes are governed by a wave function, then there is path dependent phase, and so interference will take place when you sum the amplitudes from the different paths. This can only happen with waves, so a wave phenomenon is happening. You've just rephrased it in path integral terms.

I was trying to point out that this is really a diffusion process. Waves that satisfy the wave equation are not diffusion processes.

The wave function does not satisfy the wave equation but rather a complex heat equation. I guess you do get waves when you look at stationary solutions in the presence of potentials.

 

[Mentz114]

I was trying to point out that this is really a diffusion process. Waves that satisfy the wave equation are not diffusion processes.

The wave function does not satisfy the wave equation but rather a complex heat equation. I guess you do get waves when you look at stationary solutions in the presence of potentials.

I don't know if we're talking about the same thing here. Solutions to the Schroedinger equation for realistic situations are always of the form

<br /> \psi(x,t)=Ae^{\frac{i}{\hbar}(Et+px)<br />

where E and p are the energy and momentum. This is a wave even for a free particle.

 

[wofsy]

I don't know if we're talking about the same thing here. Solutions to the Schroedinger equation for realistic situations are always of the form

<br /> \psi(x,t)=Ae^{\frac{i}{\hbar}(Et+px)<br />

where E and p are the energy and momentum. This is a wave even for a free particle.

To me, the broglie wave is a trivial case. The general Fourier transform is a super position of formal de Broglie waves and can no longer be looked at as a wave - for instance a de broglie wave has a definite momentum whereas a general Fourier transform does not -unless you want to take the mathematical view that any function that can be written as a Fourier transform is really a superposition of waves. There are some simple cases where you get a finite number of de Broglie wave glued together along their boundaries. But this picture is not right for complicated wave functions.


My point still stands that the wave equation is not satisfied here but rather a different kind of equation altogether.

 

[Mentz114]

wofsy,
you've probably got an interesting case to argue but this thread is not the place, so maybe you could start a new one about diffusion.

 

[Frame Dragger]

I don't know if we're talking about the same thing here. Solutions to the Schroedinger equation for realistic situations are always of the form

<br /> \psi(x,t)=Ae^{\frac{i}{\hbar}(Et+px)<br />

where E and p are the energy and momentum. This is a wave even for a free particle.

@wofsy: ...And not to put to fine a point on it, but that is a central tenant of SQM. If you're (Wofsy, not Mentz114) advocating a different theory that's one thing, but if you believe your description is in line with QM... it simply isn't. In fact, if you want an example of just such a momentum distribution graph, the link I provided earlier for Mentz about C60 is a visual of just this equation in action.

 

[Mentz114]

To be more precise, the plane-wave solution I quoted above won't be enough for most cases, but a linear combination of plane waves makes a nice wave-packet.

Just nit-picking ...

 

[wofsy]

I agree that this is no place to argue about QM formalism. I was just trying to explain the double slit experiment in terms of the dynamics of QM laws. The stochastic process view helped me a lot.

All I was saying is that the Shroedinger equation is not the wave equation. The wave equation involves the second derivative in time as well as space. The Shroedinger equation only uses the first derivative of time. This is a huge difference. When one uses only the first derivative you get entirely different dynamics. Instead of wave propogation you get stochastic processes. For instance, the heat equation only uses the first derivative in time.

It was revealing for me to read in Feynmann's Lectures On Physics that the Shroedinger equation falls simply out of the Markov like process of amplitude evolution. The hidden reality here was the stochastic process of complex amplitudes just as the hidden reality in heat flow is Brownian motion. This hidden reality generalizes to all QM situations such as spin or chemical bonds. In these cases,wave-particle duality does not apply.

For me, thinking about what is really meant by a wave here has been key for trying to understand the theory. It seems that simple wave packets - a finite number of superposed de Broglie waves - can be thought of as waves formally since they look like waves with localized amplitudes - but for the life of me I do not know what it means to think of de Broglie waves as particles and then superpose these particles to get another particle. That just isn't right. However with standard linear waves such superposition makes total sense.

 

[Frame Dragger]

To be more precise, the plane-wave solution I quoted above won't be enough for most cases, but a linear combination of plane waves makes a nice wave-packet.

Just nit-picking ...

"...just this equation" as in, "This very equation" not, "This equation alone". ;)

 

[mintparasol]

Well, your question is at the heart of one of the major unsolved questions in modern physics. It was an interesting question, and bound to lead to some complex ruminations on the subject. And fireworks... musn't forget the fireworks lol.

The lack of consensus is illuminating (pardon the pun!)

I had been under the impression that QM models were as immutable as, say, special relativity or the laws of thermodynamics

 

[Frame Dragger]

The lack of consensus is illuminating (pardon the pun!)

I had been under the impression that QM models were as immutable as, say, special relativity or the laws of thermodynamics

That definitely depends on who you ask, but I doubt that anyone here is filled with a deep and abiding certainty about QM,dBB,etc... etc... and when you get into the realm of interpretations, well... the only concesus is probably forced. I think everyone here has done a fine job of advocating a viewpoint, but that is all that most people can claim. QM and GR both cry out for either unification, or a new theory to replace them (crazy unlikely in the case of GR at least ;) ). The question of what will emerge from an understanding of how the world of the very small is 1.) unlike the world we observe, but from which that world emerges 2.) Like the world we observe, but for various reasons appears to be unlike 3.) OTHER... is unsolved and open for debate.

That's a pretty gross simplification on my part, but that's the kicker... without the math none of these theories make much sense using Classical analogues past the introductory period. The fact that terms such as "Observer" and "Information" take on new meanings as they become terms of art in the field, doesn't help for some people either. That's a work in progress for you though!

 

[DrChinese]

I had been under the impression that QM models were as immutable as, say, special relativity or the laws of thermodynamics

The mathematical formalism itself is essentially immutable (of course that could change) and has remained in place for over 80 years. What changes is the mapping of the formalism to underlying mechanical processes, something which is not strictly required for any theory. This "mapping" is the source of the debate and confusion, just in case that point was not clear from the above.

And by the way, there is a similar debate raging about relativity and its interpretations. Although the one about QM is more well known and tends to have more *robust* debate.

 

[Frame Dragger]

The mathematical formalism itself is essentially immutable (of course that could change) and has remained in place for over 80 years. What changes is the mapping of the formalism to underlying mechanical processes, something which is not strictly required for any theory. This "mapping" is the source of the debate and confusion, just in case that point was not clear from the above.

And by the way, there is a similar debate raging about relativity and its interpretations. Although the one about QM is more well known and tends to have more *robust* debate.

To be fair to those scientists who stake their careers and reputations on various theories... it's easy to see politics stifling science in hindsight, but when your *** is on the line... not so easy. So, in some cases the debate is spirited for the sake of retaining one's viewpoint or standing, and sometimes it's spirited because the math says very strange things about the universe that we as humans do not see in our everyday lives (and recognize as such at least).

GR and SR have plenty of debate, including ideas such as treating time as separate from space. Einstein's theories however, have had the benefit of experiments which refute some counterclaims and support it. Time dilation, gravitational lensing, and more have been directly observed. By its very nature, QM defies that same degree of precision in the absence of new thinking, math, and technology.

Want to test GR? Make some really great telescopes and wait for the right time (or make some really good gyros and lasers in the extreme). Want to test SQM? Build the Large Hadron Collider and cross your fingers. You see the problem... ;)

 

[eaglelake]

No... you missed my point entirely. The experiment involving Rubidium showed a single atom having a unique wavefront just as a single photon does. The fact that it takes multiple passes (as you say, a buildup) to make the pattern visible is a limitation of our detection methods. If one could image a photon more exactly there would be a wavefront causing an interference pattern, visible or not. The dual nature of the quanta seems pretty clear. That's a limitation of the experimental apparaturs, but it's clear from the distribution... built over time as you say... that each individual photon, atom, etc, while observed at any given time to be particle or wave -like... has both properties at all times.

I repeat, no experiment has ever “showed a single atom having a unique wavefront just as a single photon does.” ( A single photon doesn’t either). The detection of a single particle is seen as a single dot on the screen and no wave properties can be discerned from it! The quantum experiment does not reveal any wavefront for a single particle. I assume that the wave you refer to is the state function, which is a probability amplitude. It is defined in a linear vector space and no one has ever observed it in 3-space. The results of an experiment are always visible to us. Your interpretation sounds like deBroglie-Bohm, and that’s OK, but it is speculation about “what is really happening”. Both the quantum theory and experiment are silent on such things.

Further, Bohr’s complementarity principle is widely accepted and considered as a fundamental tenet of quantum mechanics; we never observe both particle and wave properties at the same time. The experiment does not reveal “both properties at all times.”

All we know for certain is that the double slit experiment yields an angular distribution of scattered particles that has maxima and minima, which we identify as constructive and destructive interference. Quantum mechanics was invented, in part, to explain such interference effects in particle scattering.

Best wishes.

 

[Galap]

To be fair to those scientists who stake their careers and reputations on various theories... it's easy to see politics stifling science in hindsight, but when your *** is on the line... not so easy. So, in some cases the debate is spirited for the sake of retaining one's viewpoint or standing, and sometimes it's spirited because the math says very strange things about the universe that we as humans do not see in our everyday lives (and recognize as such at least).

GR and SR have plenty of debate, including ideas such as treating time as separate from space. Einstein's theories however, have had the benefit of experiments which refute some counterclaims and support it. Time dilation, gravitational lensing, and more have been directly observed. By its very nature, QM defies that same degree of precision in the absence of new thinking, math, and technology.

Want to test GR? Make some really great telescopes and wait for the right time (or make some really good gyros and lasers in the extreme). Want to test SQM? Build the Large Hadron Collider and cross your fingers. You see the problem... ;)

There do exist alternative theories to GR, for example the Brans Dicke theory (http://en.wikipedia.org/wiki/Brans–Dicke_theory), which is also consistent with observations. The only reason it doesn't get much attention is that GR to most people just simply makes sense in and of itself and doesn't have 'disturbing' or 'wierd' elements like QM does.

Now because GR makes sense does it mean that Brans Dicke theory is unneccesary? No. Any theory is acceptable until experimental evidence shows it to be untrue. That's why we still use QM and GR: because of any two theories out there, they have probably given us the best predictions of anything. The only problems are that QM 'doesn't make sense' and that the two don't mesh very well.

 

[mintparasol]

Ok, I've read some of the links posted here and have gleaned a little more understanding of what's going on here by some of your replies. I won't pretend to understand the math but think I have a better understanding of what's going on here in layman's terms.

Basically, as I see it, we see no evidence of the wave property of light from the firing of a single photon. We fire it thru the slits and it hits the detector screen and is detected at a point. We have no way of determining in advance where it will hit the detector screen. It is only after firing a whole lot of photons, either all at once, or one at a time, that the wave nature of light is revealed to us by means of an interference pattern at the detector.
This means to me at least, that we get a more meaningful view of the properties of light by considering the properties of many photons rather than the properties of a single photon.
It's interesting that the time factor (i.e. whether you fire the photons all at once or one at a time) makes no difference to the result of the experiment.

 

[Frame Dragger]

Ok, I've read some of the links posted here and have gleaned a little more understanding of what's going on here by some of your replies. I won't pretend to understand the math but think I have a better understanding of what's going on here in layman's terms.

Basically, as I see it, we see no evidence of the wave property of light from the firing of a single photon. We fire it thru the slits and it hits the detector screen and is detected at a point. We have no way of determining in advance where it will hit the detector screen. It is only after firing a whole lot of photons, either all at once, or one at a time, that the wave nature of light is revealed to us by means of an interference pattern at the detector.
This means to me at least, that we get a more meaningful view of the properties of light by considering the properties of many photons rather than the properties of a single photon.
It's interesting that the time factor (i.e. whether you fire the photons all at once or one at a time) makes no difference to the result of the experiment.

You now understand the SQM interpreation of the experiment. I'd call the thread a rousing success! Differences in whether the experiment reveals the wave nature of light, or if it is the result of an ensemble, or pilot wave... you have the actual details of the mechanics down pat.

Finally, remember that if you CAN add an additional measuring device, it doesn't matter if you use the data or not. The fact that you COULD have by deploying more observation means that you can't see evidence of duality.

 

[mintparasol]

You now understand the SQM interpreation of the experiment. I'd call the thread a rousing success! Differences in whether the experiment reveals the wave nature of light, or if it is the result of an ensemble, or pilot wave... you have the actual details of the mechanics down pat.

Finally, remember that if you CAN add an additional measuring device, it doesn't matter if you use the data or not. The fact that you COULD have by deploying more observation means that you can't see evidence of duality.

Hmmm, is it right to say then that we observe individual photons as particles and we find wave properties only when we measure the properties of many photons emanating from the same source. To me, this clears up the 'mystery' of duality completely

 

[Mentz114]

This means to me at least, that we get a more meaningful view of the properties of light by considering the properties of many photons rather than the properties of a single photon.

Yes, Maxwell's equations are the classical way. Light is waves.

It's interesting that the time factor (i.e. whether you fire the photons all at once or one at a time) makes no difference to the result of the experiment..

Agreed. This is what led Feynman to say that the photon interferes with itself.

In the deB-B pilot wave model, the trajectory depends only on the initial conditions and there's no randomness except there.

[Edit : I posted simultaneously with the post above ...]

 

[Frame Dragger]

Hmmm, is it right to say then that we observe individual photons as particles and we find wave properties only when we measure the properties of many photons emanating from the same source. To me, this clears up the 'mystery' of duality completely

It leads you to an Interpretation probably. Mentz is offering the De-Broglie Bohm Pilot Wave Interpretation, I'm for SQM. They are on equal empirical footing, if not political footing (not their fault).

The more I learn about dBB, the more I find myself on the fence. I'm no convert, but it strikes me as a guess on par with SQM.

 

[DrChinese]

Basically, as I see it, we see no evidence of the wave property of light from the firing of a single photon.

I would say that is true with the double slit setup itself. However, there are other setups that show the wave nature of light on a single particle basis. I am thinking of certain special interferometer setups, for example (assuming that you accept that an interferometer shows wave effects). Not sure if that is relevant to your thinking, but thought I would mention it, see Figure 2 of this:

Non-local generation of entanglement of photons which do not meet each other

 

[mintparasol]

Thanks again to everyone, this thread has really helped my understanding.

I have some more questions, I hope you don't mind!

:- If I conduct the double slit experiment by firing one photon at a time, do I find that every now and again, a photon is 'blocked' by the dark part of the slitted screen or do all the photons always find a way thru the slits to the detector screen?

:- Is there any relationship between the frequency/wavelength of light emitted by a particular atom and the physical size/circumference of said atom?

:- I'd be grateful if someone could spend the time explaining exactly how the protons are generated and focussed down the tube in the double slit experiment. Also, to help refresh my memory of how photons are emitted from atoms in the first place. I understand it's to do with the excitement of atom-bound electrons and that the electron emits a photon as it 'jumps' from one state to another but could do with a refresher course!

Thanks again,
ad

 

[huffypuffy]

J12345 - are you attributing consciousness to particles? Bohr would just say that the measurement is the collapsing of the wave function, and that's the easy way out that physicists have followed for decades. You miss the subtle questions that we need to start asking again.

No man. Cause the device is used a day prior to the decision. which means for the statement "The measurement collapses the wave function" to be true, As soon as the device measured the gate, the wave function would collapse at the instant the device measured and the result would be set in stone. But that's not the case. our (human) measurement breaks down the wave function. Devices have been seemingly seen to collapse the function in sophisticated experiments. But we don't know if it is due to our influence now do we? We can think we had nothing to do with it. But A human built it.

Like global warming programs were no good cause the guy writing the program must agree with the hypothesis or the program won't give the results they created it to give.

Now if someone can set up an experiment that could show a human is not needed, you still can't ignore the fact that human observation breaks down the wave function as proven in the original experiment.

Does anyone know where the results are from the original delayed decision portion of the experiment? Before wheeler expanded on it years later. I really do smell a rat.