The Double Slit Experiment - Monthly(ish) Mailbag #4

Jim Al-Khalili’s explanation of the double slit experiment sparked lots of questions: Jon has the answers. Would the double slit experiment work with sand? Does the detector need to make some physical signal? Does it mean that things can be in two places at once? What’s the difference between the observer effect and uncertainty principle?

[recklessroges]

I think your sign-off should be "Where we respond to your questions" (because "where we answer" presumes resolution in the viewers mind, which we can't be certain of.) Nice video. Thank you.

[finnaginfrost]

If we sent 2 (or more) particles through a 3-slit divider and only measured whether or not one or more particles passed through only one of the slits, would we partially see a particle action and partially see a waveform?

[ZeedijkMike]

1st time a actually understood the split experiment. As far as it is possible to understand anything to do with Quantum mechanics :-/
Good video.

[LowLightVideos]

1:43 - Instead of Sand, it's like a Guy and two Chicks. There's an Interference Pattern that can lead to Bedding more than the two you started with.

Science!

[SophiasIchor]

I don't think there is a wave/particle duality. There's just energy-momentum fluctuating in fields. The "appearance" of a light quanta or photon is just the spatio-temporal location where energy-momentum transfers or "moves" between fields. In this way, all is as Schrodinger said: "All is waves."

[BariumCobaltNitrogn]

I have a problem with the language of the particle "choosing" which path to take depending on if it is being observed or not, like a guilty employee at the register making change or pilfering cash. If the particle knows someone is watching, he'll choose the beige linens, but when no one is looking he goes for the black silk sheets. Are there really no better, more accurate words that can describe an experiment with counter-intuitive results? Is it better to describe the results as quantum weirdness, or spooky action at a distance than to say I don't know, if it can't be described in clear accurate language leaving out opinion and commentary. 'Observed' needs to be clarified so it is clear what mechanical function is taking place, unless you do mean "a guy in a chair with a clipboard". It's like describing light as coming out of our eyes, I'm sure someone had a problem with that concept the minute the guy said it. Coming soon: an omniscient energy field with morals.

[smokey]

Retrocausation is what the quantum eraser experiment shows. As if the object being measured knew when it was measured and how to respond not based on path but as to how it was measured... A particle or a wave. The eraser experiment shows how the object being measured can go back and change what was measured after the fact. The future effecting the past. The only way for this to occur would be an entanglement of position. As a wave, because it is spread out over more than one location, if you tried to single out a particle it would settle into one time frame in perspective to GR. Yet as a wave it is in a superpositional state in more than one location at the same time. Which means different speeds and locations within the wave. So it can be both in the past one thing and in the future another yet be the same thing at one time... When measured, of course. 😉

[badlydrawnturtle]

The uncertainty principle is a formalization of the minimum possible observer effect on any measurement of a quantum system. This whole “it's not related to observation, it's purely the information that we know - ooh!” nonsense needs to stop.

Also, for anyone wondering, the quantum eraser doesn't actually ‘erase’ anything. The effects are all in what correlations you pay attention to in the resulting data. There's no backwards causality, sorry to disappoint you.

[xnoreq]

The explanations in this video are wrong and one only needs to accept two facts to see this: e.g. "photons" propagate like a wave and can only be absorbed/emitted as quanta.
(I'm putting photons in scare quotes because saying photon or particle makes people immediately think of tiny balls of energy - that however is a wrong image.)

There is no which-slit information. The photon travels through both slits like a wave, so there can not be any such information.
In the "unmarked" setup of the experiment the wave-like propagation leads to interference, given that the detector screen has a large enough distance from the slits to give room for the interference to happen, just like with e.g. a water wave.
In the "marked" setup, perpendicular polarization filters are inserted at the slits. The photon still travels through both slits, but the filters change the light's polarization. This is what prevents an interference.
The interference can be restored by adding another polarization filter common to both slits.

Mathematically this is beautifully expressed as a superposition. In the "marked" setup the filters change this superposition (but it's still a superposition!) in such a way that interference is not possible.
Add another filter common to both slits and now the terms that prevent the interference cancel out.

[AdityaMehendale]

The "why" is more important than the "what" :/