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Double Slit experiment, what would happen if i did this?

I am fascinated by the double slit experiment, and can't claim to understand it completely but am curious to know what would be observed with the experiment I would like to describe. First off, I just watched a youtube video: http://www.youtube.com/watch?v=LW6Mq352f0E

.. and the professor describes how the experiment was executed with the detectors off, and they observed an interference pattern, and then the detectors were then turned on and recorded the data (i.e which slit each photon went through) and they observed a double slit on the target screen (since it was now appearing to act as a particle). However, he goes on to say that the experiment was performed again, and the detectors had been (accidently) turned on, but it was not recording the data.. and it showed an interference pattern.. presumably because despite the detectors being on it wasn't recording and so there was no data to be observed at a future date? I am a computer programmer and so imagine I was to write the program which received input from the detectors and recorded which slit each photon went through. Ok, and imagine I introduced a control to enable/disable whether or not the data was recorded.. would i observe an interference pattern switch to 2 slits every time i disabled/enabled this control?

Ok, to take it a step further, how about i made the software decide completely randomly on a per photon basis whether or not to record which slit it passed through? What would i be likely to observe? Or in another case, imagine if the program recorded which slit every photon passed through and then immediately deleted that data entry? What could i expect to observe?
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TommySzalapski
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the experiment was performed again, and the detectors had been (accidently) turned on, but it was not recording the data.. and it showed an interference pattern.
That could only work if you kept the detectors entangled with the photons without allowing decoherence, and performed a quantum erasure.
Which is not what the speaker described.
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phoffric

>> that the photon, being massless
IIRC, Feynman wrote that in the gedenken experiment that even if an electron were shot from a very far distance to the double slits one at a time (to avoid any interaction between the projected electrons), then the interference pattern would occur, unless, of course, there was an attempt to observe which slit the electron actually went through, in which case, the expected superposition of two Gaussian patterns would emerge. Electrons obviously have mass, but are apparently have small enough mass to exhibit the same quantum interference effects as a photon.
small enough mass to exhibit the same quantum interference
quantum interference occurs with larger mass too, but the wavelength gets smaller
>>Feynman wrote ...

Yes of course, this is the famous effect or measuring the momentum more than once. The first measurement fixes the value, so to speak, subsequent measurements return the same values. I cannot remember the name of the experiment which was done with the angular momentum of electrons in the x,y and z directions, but it lays the basis for the commutability of the angular momentum operators.

>>with larger mass too, but the wavelength gets smaller

which suggests, in light of the fact that the classical analysis has intensity inversely proportional to wave length, that "massive" bodies have more energy. Is this more momentum, or more effective mass?
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Thanks you very much for all the comments, I undoubtedly have follow up comments & questions but think I need to investigate this area further first. Many thanks to all, very interesting reading.