The Double Slit Experiment Explained

[Polymath257]

He should as a professor of physics. (Matt O'Dowd) His stuff is right at the edge of what I can watch as "edutainment". Anything more in depth would require work. but I guess it goes wide over the head of most of this audience.

Fair enough. I'm a math professor who has also done the PhD qualifying exams in physics. I was working for a second PhD in physics, but my responsibilities in the math department increased and my advisor died, so that didn't happen. I still keep a keen interest in this stuff, including the mathematical details.

 

[Kenny]

And if you are seeing them, then *you* are the observer.

More generally, it is NOT consciousness that is required to 'collapse' a wave function, but merely interaction with a complex environment. The environment serves as the 'observer'.



Expectations have nothing to do with this. In fact, the first people to observe some of these effects were quite surprised by them. the actual results go against the intuitions most people initially have. Even Einstein's intuition on these matters was *wrong*.

hmmmm.. they said they still don't understand how it happens... how is it that you do know?

 

[Heyo]

Right. The entanglement correlates the results of the two sides. But there is only *one* wave function and that wave function works for the entire apparatus.

Next, *every* detector collapses that wave function to some degree, but potentially not all the way. So, a measurement can give a definite value at one point, be correlated, but still not give a definite value at another point.

Next, if you look at only the D0 detector, and don't worry about the results in the others, the result is that you get an interference pattern. The 'paradox' comes when you correlate the results at D0 and those at the other detectors.

So, you get a detection at D0. What can still happen? Well, we don't have 'which slit' information, so we only know there was a detection at D0. Nothing else. The 'collapsed' wave function after that detection is *still* a superposition of wave functions from the two slits.

Now, detectors D3 and D4 *only* detect from a single slit. So, if they are hit, the component of the wave function that is relevant is from that slit and that slit only. Those detectors collapsed the wave function to give that information. And that means that when we *select out* those detections at D0 that correspond to detections at D3 or D4, we won't see an interference pattern: the component of the wave function that would produce it is selected away *after the experiment is run*.

On the other hand, detections at D2 do NOT have 'which slit' information, which means that the component of the wave function after that still has component from both slits and so will show an interference pattern. The D2 collapse is to an interference state.

One crucial thing here is that, just like in the Aspect experiment, things at D0 look random: there is an interference pattern. It is only when you put the information together that you see these correlations. Everything happens in a forward time direction and through propagation of probabilities and correlations through the wave function. Furthermore, each measurement 'collapses' the wave function to some degree, but not completely.



Right. QM is a non-causal, but local theory. it is also not a realist theory: particles do NOT have definite properties at all times: they have *probabilities* of different values for properties. And those probabilities can be *correlated* by entanglement.

The wave functions propagate locally, but the value of any measurement is undetermined ahead of time, although *correlations* are. The correlation between the two sides of the apparatus is made at that initial prism that splits the beams and is propagated to both sides. When a measurement is made, we know some *part* of the wave function, but not all of it.

Still trying to wrap my head around it.
Suppose the following experiment: same setup but with a very elongated path for the delayed choice / quantum eraser part. (Bouncing photon off of a mirror in Mars orbit / sending them through a Einstein-Bose-Condensate etc.)
Replace half-silvered mirrors at D3/D4 by LCD mirrors that can switch state through an external choice (e.g. a quantum random generator).
Send a burst of photons.
Depending on the observed pattern (interference / non interference) predict the choice that will be made for the LCD mirror.
Will that work?

 

[Heyo]

hmmmm.. they said they still don't understand how it happens... how is it that you do know?

Who is "they"?
I don't know of any reputable physicist who doesn't think that any interaction with a complex system collapses the wave function. In fact, most point out that it is a misunderstanding to think that a conscious observer is necessary.

 

[Kenny]

Who is "they"?
I don't know of any reputable physicist who doesn't think that any interaction with a complex system collapses the wave function. In fact, most point out that it is a misunderstanding to think that a conscious observer is necessary.

Just quoting the video.

 

[Heyo]

Just quoting the video.

Can you be a bit more precise, please? Who is saying what in which video (at which timestamp)?

 

[Polymath257]

Still trying to wrap my head around it.
Suppose the following experiment: same setup but with a very elongated path for the delayed choice / quantum eraser part. (Bouncing photon off of a mirror in Mars orbit / sending them through a Einstein-Bose-Condensate etc.)
Replace half-silvered mirrors at D3/D4 by LCD mirrors that can switch state through an external choice (e.g. a quantum random generator).
Send a burst of photons.
Depending on the observed pattern (interference / non interference) predict the choice that will be made for the LCD mirror.
Will that work?

That is a very good question, but no it won't work. There is *never* an interference pattern at D0 alone ( I was wrong above--sorry). The interference pattern comes from *correlations* between D0 and the detectors D3 and D4. To get these, we have to know the results at D0 *and* that other detector and compare timing. But, without knowing which other detector has been hit and knowing only D0 results, we *never* have an interference pattern.

On the other side, once D0 has a detection, the probabilities of going through the other detectors is fixed. The 'no information' detectors will have no interference when correlated with D0 and the 'information' detectors will have interference, *but cancelling interference* when correlated with D0.

This is ultimately because there *is* which-side information in the wave function because we separate the slit information on the other side. And *that* separation is determined soon after going through the splitter.

See Delayed-choice quantum eraser - Wikipedia which has a good treatment.

 

[halbhh]

Actually, if there was an universal observer or "knower", there wouldn't be any interference. Interference only happens when nobody is looking.
Ergo, there is no observer.

God* disproved by QM.

(God*, in this case, defined as universal observer.)

heh heh Plenty of imagined versions of God, including some surprisingly simplistic versions made up by putative Christians also, have been easily disproved.

It's of course a more actual God that would be another matter entirely.

Which though some might ask? I'd go with listening to someone that knows more, the teacher Jesus, because an Einstein is going to do better at understanding what they are talking about than just whatever random individual or preacher you happen to hear. There's a reason we know names like Lao Tzu, Einstein, Jesus. It's not an accident they became known around the world. So, if you want to learn more about the actuality of God, the real, you'd listen to Jesus, just like for a physics topic you'd really prefer a well spoken physicist to explain it.