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Author Topic: Double Slit Experiment - Need an Explanation Please  (Read 5919 times)

Heronumber0

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Go easy here guys, I am a biologist and not a knowledgeable one at that!
In the double slit experiment:

Quote
The double-slit experiment consists of letting light diffract through two slits, which produces fringes or wave-like interference patterns on a screen. These interference patterns will result in projected light and dark regions that correspond to where the light waves have constructively (added) and destructively (subtracted) interfered.

The experiment can also be performed with a beam of electrons or atoms, showing similar interference patterns; this is taken as evidence of the "wave-particle duality" predicted by quantum physics. Note, however, that a double-slit experiment can also be performed with water waves in a ripple tank; the explanation of the observed wave phenomena does not require quantum mechanics in any way. The phenomenon is quantum mechanical only when quantum particles—such as atoms, electrons, or photons—manifest as waves.

http://en.wikipedia.org/wiki/Double-slit_experiment

My question to people who know about this stuff: do the particles fired at the slit actually interact with the atoms that the double slit is made in? Also if the interference pattern can be interrupted by a detector placed behind the slit, do the atoms of the detector affect the results? Also could the detector not be atomic or subatomic? Finally, could you fire something big through the slits and get the same pattern (e.g. graphite balls)?

« Last Edit: 10/07/2007 19:04:45 by Heronumber0 »


 

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Double Slit Experiment - Need an Explanation Please
« Reply #2 on: 11/07/2007 09:38:39 »
That reference only deals with part of the question

http://web.phys.ksu.edu/vqm/tutorials/matterwaves/index.html

This reference deals with the other part of the question.

The particle wave duality of matter does extend to the macroscopic but if you work out the wavelength of even tiny lumps of matter like electrons they are very short.  Thats why the electron microscope has much better resolution than an optical microscope.

In the double slit experiment the only effect that the slit has on the particles/waves is to stop them or let them through.  OK the beam spreads by diffraction or you wouldnt be able to see the interference pattern but that is not in any way dependant on the material of the slit other than its ability to stop the particles/waves.
 

Heronumber0

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Double Slit Experiment - Need an Explanation Please
« Reply #3 on: 11/07/2007 22:53:14 »
Thanks so much guys, I have read one of the sources and have looked at the simulation program in the other.  Can you please tell me why buckyballs (weighing in at 720 amu/ Da, I think) have an effect on the double slit in the same way that an elctron does? 
 

Offline JP

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Double Slit Experiment - Need an Explanation Please
« Reply #4 on: 12/07/2007 21:22:39 »
I just scanned over the article in Nature in which the buckyball experiment was performed (volume 401, p. 680): http://www.nature.com/nature/journal/v401/n6754/full/401680a0.html

Anything you shoot at a double slit setup should show an interference pattern so long as there is information available about which slit it went through.  This means it can't interact with its environment in such a way that you can tell which slit it went through. 

For the buckyballs, the way you could get information on which slit they went through is if they absorbed/emitted radiation, or collided with something enroute to the slits.  The experiment was set up in such a way that the buckyballs weren't expected to collide with any other molecules, so you couldn't get information about their path that way.  The molecules could absorb or emit radiation, however, but in order to see the buckyball with enough resolution to tell which slit it went through, it would need to absorb/emit radiation with a wavelength smaller than the distance between the neighboring slits (your resolution power is on the order of a wavelength).  It turns out that in this experiment, the radiation emitted  by the buckyballs was larger than the distance between the slits.  Finally, the buckyball was warmer than its surroundings, so it would be highly unlikely to absorb radiation.

Therefore, since you can't get any information on which slit the buckyball went through, you can see an interference pattern.
 

Offline lightarrow

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Double Slit Experiment - Need an Explanation Please
« Reply #5 on: 13/07/2007 15:03:08 »
Go easy here guys, I am a biologist and not a knowledgeable one at that!
In the double slit experiment:
Quote
The double-slit experiment consists of letting light diffract through two slits, which produces fringes or wave-like interference patterns on a screen. These interference patterns will result in projected light and dark regions that correspond to where the light waves have constructively (added) and destructively (subtracted) interfered.
The experiment can also be performed with a beam of electrons or atoms, showing similar interference patterns; this is taken as evidence of the "wave-particle duality" predicted by quantum physics. Note, however, that a double-slit experiment can also be performed with water waves in a ripple tank; the explanation of the observed wave phenomena does not require quantum mechanics in any way. The phenomenon is quantum mechanical only when quantum particles—such as atoms, electrons, or photons—manifest as waves.

http://en.wikipedia.org/wiki/Double-slit_experiment
My question to people who know about this stuff: do the particles fired at the slit actually interact with the atoms that the double slit is made in?

If you think that those particles go through one slit OR the other, you're wrong; (I tell you this way just to make you understand better, the subject is very complex). They go through BOTH at the same time.

That said, it also have to be said that there certainly is an interaction between particles and screen, but Soul Surfer has explained you that this interaction is not responsible of the "strange" behaviour of the particle; the interaction is from the particle "field" and the screen field. Proof: the pattern on the last screen depends on the distance between the two slits. If there were "particles" going through one slit or the other, how could they know the other slit's distance?
Quote
Also if the interference pattern can be interrupted by a detector placed behind the slit, do the atoms of the detector affect the results?
I don't understand your question: the detector is made of atoms; if they didn't interact with the "particle" coming out of the slits, how could it detect them?
Quote
Also could the detector not be atomic or subatomic?
The detector could even be a single electron, at the limit.
Quote
Finally, could you fire something big through the slits and get the same pattern (e.g. graphite balls)?
The largest object I know that has been diffracted going through slits is a Bose-Einstein condensate of many thousands of atoms. The whole system behaves, in specific conditions (between which: very low temperature and very low global speed), as a unique big quantum system, with global wavelenght big enough for the diffraction. A macroscopic object has a global wavelenght too little for this, so it cannot go through the slits: λ = h/p
λ = wavelenght; p = momentum; h = Planck's constant.
 

Heronumber0

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Double Slit Experiment - Need an Explanation Please
« Reply #6 on: 13/07/2007 20:05:05 »
Thanks for the answer lightarrow.  The stuff about the Bose Einstein condensate is amazing. Do you have a website or source for this information?
 

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Double Slit Experiment - Need an Explanation Please
« Reply #6 on: 13/07/2007 20:05:05 »

 

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