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

paddycobbett used Ask the Experts™
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:

.. 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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Whether or not you are remembering the results will not affect the results. Measuring can, but recording the measurements cannot. If that is indeed what the professor says, then something else was in place that he was missing.

Another group has successfully been able to perform the double slit experiment with new types of detection that doesn't affect the photons.
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how about i made the software decide completely randomly on a per photon basis whether or not to record which slit it passed through?
the resulting pattern would be the sum of the pattern from the recorded photons, and the pattern from the unrecorded photons.

if the program recorded which slit every photon passed through and then immediately deleted that data entry?
Deleting the data entry would have no effect.
The speaker's thesis, I believe, is related to the theme in the great Fringe TV series - that conciousness plays an integral role in our perception of reality. But as much as I enjoy the Fringe series, I do not think of it as science, just science fiction.

The speaker is not well versed in quantum mechanics. He couldn't even remember the term for the photo-electric effect, but thanks to the audience participation, he was able to continue the lecture.

Enjoy the "Counciousness - The Endless Frontier" seminars if you like, but I advise not taking seriously any scientific conclusions that you may hear from them. The lower diagram showing just two points (instead of a diffraction pattern) is wrong. What you get when detecting which slit the entity goes through is the superposition of two Gaussian Distribution patterns - one distribution pattern for one slot, and one for the other slot.
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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.
The split experiment was first performed by Young in 1807. Sunlight was allowed to go through a single pinhole (P1) which then fell on two pinholes (P1,P2), slightly apart. According to Huygen's principle P acts as a point source of spherical waves. P1 and P2 then act as a second source of spherical waves which spread out an interfere causing the famous patterns. P1 and P2 must be equi-distant from P, and then the intensity will be at a point X whose distance from P1 and P2 differ by pl (p times lambda) where p is an inter
ger and lambda the wavelength. For a given p, the point X describes a hyperboloid of revolution having P1 and P2 as foci.

The original experiment, using sunlight, gave very weak fringes and indeed only near the center of the screen. Better results are obtainable with monochromaic light and with parallel slits rather than pinholes, the locus is now a cylinder and there will be symmetry about any plane perpendicular to the three slits. The first slit is nowadays replaced by a tightly bundled laser or an electron gun.

The mathematics of the phenomenen are interesting, and well known, and gives the intensity formular of 2a²(1+cosd) where a is the initial amplitude of the light and d (the delta) is equal to 2pi*(S2P-S1P)/l where S2P is the distance from slit 2 to the point on the screen and S1P the distance from S1. Thus the distance between the two slits also has an effect on the pattern like the wavelength and the distance to the screen.

A more complicated analysis will also show that the widths of the slits also play a role.

The classical analysis shows that energy is conserved, the minimum intensity being zero, the maximum 4a² the average being 2a² which is simply the sum of the intensities due to the disturbances from slit 1 and slit 2 added together. It assumes that a "cancellation" occurs, which according to the mathematics it does, but in reality there is a redistribution of energy in space since the principle of conservation of energy is not violated, and we know of no "negative" energy.

The quantum mechanical analysis assumes that the psi function is modified by a "slit operator" which alters the spacial probability of the observable. Strictly speaking there is no concept of anything going anywhere. This would be the Copenhagen interpretation. A problem arises with a quantum electrodynamical interpretation with a particle called a photon. Basically each path that the photon would take from the source to the observation point (here laser to the screen) must be considered, however silly, and a probability assigned to that path (of which there would be an infinite number). The total spacial probability is then evaluated and this gives the observed result. This doesnt't quite tally with individual particles, if such things are actually observable, which experiments with electrons actually suggest.

The classical wave theory approach is very tempting, since the mathematics involved (electrodynamics) matches that of aquanautics, ie: the waves on the sea have the same sort of maths - generation, scattering etc., as the "etheral" ones. The problem against this being of course the Compton and the photoelectric effects, which require the quantisation of the energy involved.

The particle approach works well for high energy physics, namely the wave like effects generated by electrons and such.

The problem as I see it, lies in the assumption that the photon has to go through one or the other slit. There is an assumption that it can't go through two slits simultaneously. This has something to do with the fact that the photon is a boson, like some molecules and pi mesons, which gives it certain statistical properties (or rather that the properties match those of particles obeying Bose-Einstein statistics) and that it is small. There is however a fundamental difference, the "particle" is massless.

On the other hand, when we consider the wave theory, we don't actually see the electromagnetic waves travelling though space like we see waves on the surface of the sea. What we do see is the interaction of the energy at a point of measurement and from the points of interaction we extrapolate a wave like behavior.

I think this is consistant enough to postulate an interaction according to gauge theory, ie: an interaction theory based on Feynman electrodynamics. Clearly when a photon interacts with an electron the "properties" of the photon (energy and phase) play a role. What we cannot tell is how long the interaction takes - since the only way we have of measuring time is by such interactions. We also assume that the interaction is point like.

I think that this can all be explained if one assumes that the photon, being massless, is not point like but extends over a volume which is dependant on its energy level or basically frequency. In fact considering that electromagnetic radiation consists of two vibrating energy vectors - electrostatic and magnetic - at right angles to each other and orthogonal to the direction of propagation, this is not too way out.

The fact that no interference pattern emerges when the slits are too big or too wide apart or the frequency is very high, shows that the photon on "going" through the slit definitely interacts with it. This interaction must alter some property of the photon. Since the color does not change it has not altered the momentum. It might alter the phase, or it might alter some hidden property like energy density, assuming that the photon is not a point like particle, but its domain of influence being of "macroscopic" dimensions.

The real problem with a hidden property like energy density is Bell's theorem and the tests in QM for it. That would tend to rule out such considerations. But an alteration of phase alone will not account for the spacial separation.

In conclusion of this speculation I would contend that the photon goes through both slits and interacts with them on the way, in such a manner that the energy density is changed which results in the patterns on the screen.
>> 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.
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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?


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.

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