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October 23rd, 2014, 11:04 AM   #1
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wave particle

I am confused..I'm always reading the statement which goes something along the lines of ' light appears to act like a wave OR and particle depending on the type of experiment one is doing'....' as if it knows the type of experiment being done on it'

but I believe the experiment has been done where, we set up light going through two slits giving us our double slit interference, at the same time at some point on the interference pattern there is a photon dectector(maybe a photoelectric device) which indeed registers incoming photons at the same time the slits produce interference

So why does light appear to 'choose' whether it behaves as a particle or wave, when according to this experiement, it is simply always 'both' at the same time. --thus wouldnt it be said that light has no choice in the matter???

Or is this experiment something I misread somewhere and in fact doesn't happen?
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October 23rd, 2014, 11:44 AM   #2
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Originally Posted by Kinroh View Post
I am confused..I'm always reading the statement which goes something along the lines of ' light appears to act like a wave OR and particle depending on the type of experiment one is doing'....' as if it knows the type of experiment being done on it'

but I believe the experiment has been done where, we set up light going through two slits giving us our double slit interference, at the same time at some point on the interference pattern there is a photon dectector(maybe a photoelectric device) which indeed registers incoming photons at the same time the slits produce interference

So why does light appear to 'choose' whether it behaves as a particle or wave, when according to this experiement, it is simply always 'both' at the same time. --thus wouldnt it be said that light has no choice in the matter???

Or is this experiment something I misread somewhere and in fact doesn't happen?
This is one of the deeper mysteries about Quantum Mechanics. It has been said that if you understand the Stern-Gerlach experiment you know all the principles of QM. A similar comment can be made about Young's double slit experiment.

It is true that if you perform an experiment to measure a wave you will get a wave. (Young's double slit for example.) If you do an experiment to measure a particle then you get a particle. (Photo-electric effect.) No one can explain why.

That's not the really weird part, though. In the double slit if we try to detect which slit the particle goes through we destroy the interference pattern on the screen. But if we detect the photon at the source and only let one photon at time into the experiment we don't immediately get and interference pattern. It's only one photon after all. But if we do this a large number of times, one photon at a time, and record where each photon hit the screen we find that we do, in fact, get an interference pattern! It can get pretty spooky. I just accept it as something we can't explain yet and hope that in the future this will be reconciled with some kind of logic we can understand.

-Dan
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October 24th, 2014, 03:08 AM   #3
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The best way to think about wave-particle duality is to stop asking the question "Is it a wave or a particle?" and ask instead "what is it?". The answer to this question is that it is a quantum particle, which follows the rules of quantum mechanics.

One of the laws of quantum mechanics is the uncertainty principle, which states that some measurements of a quantum particle cannot be made on a "fly on the wall" basis. That is, by making a certain measurement you perturb the system you are measuring in such a way that it affects other measured values. Position of a particle and momentum (or velocity) are pairs of observable quantities that are like this. The principle states that you can't measure the momentum of something precisely without introducing a larger uncertainty in the position measurement. Similarly, you cannot measure the momentum of something precisely without introducing a larger uncertainty in the position. Depending on which of these is the case depends on whether you measure particle or wave behaviours i.e. wave-particle duality is a consequence of the uncertainty principle.

-----

This is often considered to be a really strange phenomenon, but it is actually fairly sensible. Try considering the following situation: you are deaf and blind and the only way you can sense is by touch (let's forget about taste and smell here!). You are in a world where you can only determine the contents of that world by touch.

Now... imagine there is an elephant moving nearby and you want to figure out what it's doing. You actually have to go up to the elephant and touch it in order to sense that it is there. If you touch it you get a handle on where it is and if you touch it several times over a period of time, you can get a handle on it's velocity (and if you know it's mass, it's momentum too).

Now imagine that instead of trying to sense where an elephant is going you are instead trying to sense where a fly is going. Aside from the difficulty of touching a fast-moving fly, if you do touch it, it will change its speed and position considerably, whereas the elephant isn't really affected. By making the 'measurement' you have perturbed the system to behave in a certain way. This affects your measurement such that you will be uncertain about the fly's position and momentum.

Now, you may ask "is it possible to touch the fly in such a way that it won't be perturbed?" The answer is "no, by merely making a measurement you have affected the outcome of the result". The reason for this is that at the quantum level, you are limited by the options that you have; you can only make a measurement using an interaction and, at the quantum level, interactions will affect the system you are observing and you'll get your uncertainty.

Also, note that this "perturbation" manifests itself as an uncertainty. It doesn't make sense to ask the question "what is the exact change in speed and change in position caused by my measurement?" because you can never know the exact path taken by the quantum particle or the path taken by your hand! All you can say is "I tried to measure this pair of values and I am now uncertain about them". The uncertainty in each variable can be quantified however. If your uncertainty is mainly in the momentum and you know the position well, it appears to behave like a particle. If your uncertainty is mainly in the position and you know the momentum well, it appears to behave like a wave.
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Last edited by Benit13; October 24th, 2014 at 03:25 AM.
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October 29th, 2014, 04:42 PM   #4
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thanks for the replies.. I sill trying to get some kind of half decent understanding, however...

They way I have been told to look at the uncertainty principle, the way I was told it was derived was that:

Electrons diffract through a single slit and form a diffraction pattern, the smaller the slit, the greater precision you know the location of the electron (as it passes through the slit only), since as the slit gets smaller, there's less and less locations for the electron to exist, however the smaller the slit, the greater the electrons CAN diffract, so therefore there're more 'options' for the electron in terms of momentum.

If the slit Is large enough, electrons essentially don't diffract, and there momentum will be basically known, since they will all go directly ahead in one known direction, and no diffraction pattern is observed. But for this to happen the slit must be large, and therefore the electrons have many more possible 'locations' as they pass through the slit...

in this way the precise location and momentum cannot be simultaneously known precisely...

So at least as far as this logic goes, its never that the electron 'knows' whether to behave as a wave or a particle based on the type of experiment being done...it's clearly the experiment affecting the behavior of the electron, in combination with our lack of having a better way to measure the location of an electron when the slit is big, or its our lack of having a better way to predict the momentum of an electron when the slit is very small...

Knowing that the electron diffracts when the slit is small enough, why is it such surprise that sending single electrons, or I guess, equivalently, photons, over a long period of time forms a diffraction pattern.

I see that the exact mechanics are not obvious, and that that's the real question at at hand... but wouldn't you expect similar 'particles' when shot through the same slit, to all have similar behavior? thereby forming maximas at some set of coordinates, one following the other, with some kind of average, and some kind of displacement form the average, depending on...I don't know.. tiny differences in the energy and momentum of each electron/whatever tiny 'particle'???

Last edited by Kinroh; October 29th, 2014 at 05:27 PM.
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October 29th, 2014, 05:32 PM   #5
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Originally Posted by Kinroh View Post
Knowing that the electron diffracts when the slit is small enough, why is it such surprise that sending single electrons, or I guess, equivalently, photons, over a long period of time forms a diffraction pattern.

I see that the exact mechanics are not obvious, and that that's the real question at at hand... but wouldn't you expect similar 'particles' when shot through the same slit, to all have similar behavior? thereby forming maximas at some set of coordinates, one following the other, with some kind of average, and some kind of displacement form the average, depending on...I don't know.. tiny differences in the energy and momentum of each electron/whatever tiny 'particle'???
The problem, particularly in the first statement, is that electrons going into the system one by one don't have a chance to interfere with one another...it is wave interference which causes the diffraction pattern.

There are two main ways of interpreting this. The first is called the Copenhagen Interpretation, which essentially says that the electron takes all possible paths through the system at once, and thus will interfere with itself, causing the diffraction pattern. The other way is called "Many Worlds Theory" and basically states that, for any quantum "decision" that is made the Universe splits into pieces and those Universes interfere with each other, creating the diffraction pattern. Both interpretations are equally annoying from a logical standpoint.

I should mention...Many Worlds has nothing at all to do with M-branes from superstring theory. They are different concepts.

-Dan
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October 29th, 2014, 05:35 PM   #6
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I'm confused,

if we assume that there are two electrons going through the slit, and they are in this case half a wavelength pathlength difference...would we expect to then see nothing on the screen?

where do the electrons that interfere destructively go? If their marks do not show up on the diffraction pattern, and its a case of 2 electrons (or perhaps even a single electron) being half a wavelength out of sink, which causes the destructive interference in the first place, where do these electrons go when they don't appear to 'hit' the screen where the diffraction pattern appears ?

Last edited by Kinroh; October 29th, 2014 at 05:57 PM.
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October 30th, 2014, 01:33 AM   #7
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The electrons either bounce off the surface they hit or are absorbed. It's not a destructive interference, it's just that they happen to be out of phase with each other at the observation point, they are both still there, you just can't observe them.

If you play 2 sinewaves of the same frequency from 2 different places (stereo speakers for example), and move around, the volume of the sinewave will appear to go up and down, this is due to the same interference, some positions will have the sinewaves meeting out of phase with each other so you don't hear it.
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October 30th, 2014, 03:49 AM   #8
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Quote:
Originally Posted by Kinroh View Post
I see that the exact mechanics are not obvious, and that that's the real question at at hand... but wouldn't you expect similar 'particles' when shot through the same slit, to all have similar behavior? thereby forming maximas at some set of coordinates, one following the other, with some kind of average, and some kind of displacement form the average, depending on...I don't know.. tiny differences in the energy and momentum of each electron/whatever tiny 'particle'???
All objects can be assigned a de Broglie wavelength. If the width of the slit is of the order of the de Broglie wavelength, diffraction effects will start to be observed. I believe the largest object to exhibit clear quantum behaviour is a buckyball (C60 molecule), but that was a fairly old result. There are probably newer studies into this.

You've got a pretty solid understanding of wave-particle duality now! The stuff related to the Copenhagen interpretation and many worlds interpretation is all about the 'underlying mechanism', which is still being researched. There is also the "measurement problem" which refers specifically to entangled photons and the nature of information. I can recommend "Alice in Quantumland" for this... it's a brilliant book!
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October 30th, 2014, 04:54 AM   #9
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Quote:
Originally Posted by topsquark View Post
...
There are two main ways of interpreting this. The first is called the Copenhagen Interpretation, which essentially says that the electron takes all possible paths through the system at once, and thus will interfere with itself, causing the diffraction pattern...
That is not the Copenhagen Interpretation (not even according to the Wikipedia article you point at.

I will quote the preamble as it coincideds with my recollection of what the Copenhagen Interpretation is (that it is agnostic about the unobserved reallity, whatever that might be):

Quote:
The Copenhagen interpretation is one of the earliest and most commonly taught interpretations of quantum mechanics. It holds that quantum mechanics does not yield a description of an objective reality but deals only with probabilities of observing, or measuring, various aspects of energy quanta, entities that fit neither the classical idea of particles nor the classical idea of waves. The act of measurement causes the set of probabilities to immediately and randomly assume only one of the possible values. This feature of mathematics is known as wavefunction collapse. The essential concepts of the interpretation were devised by Niels Bohr, Werner Heisenberg and others in the years 1924–27.
Which is almost something that Newton might have written if he had not been dead for two hundred years.

CB
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October 30th, 2014, 08:13 PM   #10
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That is not the Copenhagen Interpretation (not even according to the Wikipedia article you point at.

CB
You are correct. My apologies and thanks for pointing that out. I had thought my statement was part of the CI. Now you've got me puzzled as to where I got the idea from. Possibly Feynman's path integrals.

-Dan
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