Double Slit Experiment AGAIN!

[confused2]

Sorry I can't get the youtube links to work without adverts .. the link does come (eventually)

I stumbled on a girl doing what I think is a fairly well presented double slit 'thing'. This might not be new to others so a quick run through (she's glacial). Laser. Two slits. Shows double slit interference. Polarizing filters at 90 degrees before each slit which identifies which slit the light came through .. Shows no interference. NOW .. put a polarising filter at 45 degrees AFTER the slits .. and show that the interference comes back.

'Normal' double slit interference
The big bright patches are due to the width of each slit. The narrower fringes are due to the distance between the slits - this is how we know which is which.
https://youtu.be/sAm7iVdAvTA?t=128

Interference pattern with polarising filters at 90 degrees .. no double slit interference
https://youtu.be/sAm7iVdAvTA?t=478

Interference pattern due to width of slit with one slit covered
https://youtu.be/sAm7iVdAvTA?t=517

Here she puts a polarising filter at 45 degrees AFTER the slits and gets the double slit interference back.
https://youtu.be/sAm7iVdAvTA?t=755

Full video here:
https://www.youtube.com/watch?v=sAm7iVdAvTA

Bearing in mind that I am an 'armchair physicist'..
I've looked through the comments so I'll try to pre-empt some of them.
This is a single photon experiment but with lots of them. Individual photons only interact with themselves - it is possible to entangle photons but that has nothing to do with what we're seeing here. The brightness of a pixel in a camera (or eye) is the result of the number of photons detected and has fuck all to do with waves.

[Secular Sanity]

Bearing in mind that I am an 'armchair physicist'..
I've looked through the comments so I'll try to pre-empt some of them.
This is a single photon experiment but with lots of them. Individual photons only interact with themselves - it is possible to entangle photons but that has nothing to do with what we're seeing here. The brightness of a pixel in a camera (or eye) is the result of the number of photons detected and has fuck all to do with waves.

I'm guessing that you don't agree with that comment.

Polarization of light has to do with which direction it's waving.

https://youtu.be/FyNgs_viwBg?si=-SN-oLsBWz39XZ4x&t=3842

I love her videos, BTW. She has another one on whether light slows down in water or not. I have an old iPhone...no LiDAR.

https://youtu.be/yP1kKN3ghOU?si=l4ASoApwLeR90O4P

[confused2]

Polarization of light has to do with which direction it's waving.

Comparing a wave analysis with a quantum analysis is (best I can think of) like farming and biology. Neither is wrong but they're looking at different levels of the same thing.

She used polarising filters at 90 degrees to mark which slit the photon had passed through. Once the photon is marked the interference effect vanishes. A wave analysis would also predict that the interference would vanish but for a very different reason (the 90 degree polarisations simply don't interfere). Always (almost) a quantum effect will lead to a wave effect which is generally much easier to explain and understand.

Search "Dirac a photon only interferes with itself" .. yes you can get magic crystals but that isn't what is going on here. If you know or simply accept that a photon only interferes with itself then the double slit experiment is a trip into wonderland. Props to the lady for marking and then restoring the superposition with a filter some way after the slits. I see it but do not understand it - I think she might have touched very lightly on .. shut up and calculate.

There's a lab demo supplier that gives a graph of a double slit using single photons
[ https://www.teachspin.com/two-slit ] and (not surprisingly) it shows the same effect as your waves and their intensity but as photons counted.
There's a vid. too..
https://youtu.be/7y5RJHiDOK8?t=303
Edit .. the point isn't that you CAN see the interference with single photons - the point is that it is ALWAYS single photon interference.

---

The speed of light in water .. argh! Spoiler .. so other folks don't nearly burst a blood vessel at the end - she doesn't know. It is interesting to look at what she tests so not really a waste of time. I'd suggest a thread for it but I've got nothing she hasn't already tried (I'm not going down the Huygens route again). I think she mentioned she was a research 'something' .. if she can't find out it is possible nobody knows.

[Secular Sanity]

Polarization of light has to do with which direction it's waving.

Comparing a wave analysis with a quantum analysis is (best I can think of) like farming and biology. Neither is wrong but they're looking at different levels of the same thing.

She used polarising filters at 90 degrees to mark which slit the photon had passed through. Once the photon is marked the interference effect vanishes. A wave analysis would also predict that the interference would vanish but for a very different reason (the 90 degree polarisations simply don't interfere). Always (almost) a quantum effect will lead to a wave effect which is generally much easier to explain and understand.

Search "Dirac a photon only interferes with itself" .. yes you can get magic crystals but that isn't what is going on here. If you know or simply accept that a photon only interferes with itself then the double slit experiment is a trip into wonderland. Props to the lady for marking and then restoring the superposition with a filter some way after the slits. I see it but do not understand it - I think she might have touched very lightly on .. shut up and calculate.

There's a lab demo supplier that gives a graph of a double slit using single photons
[ https://www.teachspin.com/two-slit ] and (not surprisingly) it shows the same effect as your waves and their intensity but as photons counted.
There's a vid. too..
https://youtu.be/7y5RJHiDOK8?t=303
Edit .. the point isn't that you CAN see the interference with single photons - the point is that it is ALWAYS single photon interference.

"and has fuck all to do with waves."

I guess I’m misunderstanding something here. Why is he saying that it has nothing to do with waves?

The photon does interfere with itself as its wavefunction spreads out and interacts with different parts of itself in the single slit creating a diffraction pattern. The single-slit diffraction pattern is a result of this wave-like nature.

The key difference is that the double-slit experiment shows the interference of the wave from two different sources (the two slits). The waves from each slit can either add together (constructive interference → bright spots) or cancel each other out (destructive interference → dark spots).

And with the TeachSpin thingy, when there’s two slits the photon passes through both, and its wave function interacts with itself, but you shouldn't get fewer total clicks just because of interference. You might get fewer clicks at specific spots, because the wavefunction has zero probability in some regions. If your detector happens to be sitting on a dark fringe (destructive interference), you’ll record fewer clicks — maybe even none.

It must only be measuring clicks at one position on the interference screen, you're only seeing how many photons land there, not how many land overall, right?

Both vertical and horizontal polarizations can partially pass through a 45° polarizer.

So, they both have a 50% chance of passing through the 45° polarizer, and if they do, they both come out looking identical: polarized at 45°. No way to tell if they came from the vertical slit or the horizontal one. So, she’s essentially creating another double slit.

[confused2]

And with the TeachSpin thingy, when there’s two slits the photon passes through both, and its wave function interacts with itself, but you shouldn't get fewer total clicks just because of interference. You might get fewer clicks at specific spots, because the wavefunction has zero probability in some regions. If your detector happens to be sitting on a dark fringe (destructive interference), you’ll record fewer clicks — maybe even none.

It must only be measuring clicks at one position on the interference screen, you're only seeing how many photons land there, not how many land overall, right?

Back in the day I looked at one of their sample results and added the total detections with one, the other and then both slits open - it did add up to the 'right' number - they don't vanish or 'cancel out' - you're right they just hop over to another place.

[with the 45° polarizer] .. she’s essentially creating another double slit.

I  think you have it there - I couldn't see that - thanks.

Whether you call or think of a wavefunction as a wave (I don't) isn't really important - it is what it is. 

[Secular Sanity]

And with the TeachSpin thingy, when there’s two slits the photon passes through both, and its wave function interacts with itself, but you shouldn't get fewer total clicks just because of interference. You might get fewer clicks at specific spots, because the wavefunction has zero probability in some regions. If your detector happens to be sitting on a dark fringe (destructive interference), you’ll record fewer clicks — maybe even none.

It must only be measuring clicks at one position on the interference screen, you're only seeing how many photons land there, not how many land overall, right?

Back in the day I looked at one of their sample results and added the total detections with one, the other and then both slits open - it did add up to the 'right' number - they don't vanish or 'cancel out' - you're right they just hop over to another place.

[with the 45° polarizer] .. she’s essentially creating another double slit.

I  think you have it there - I couldn't see that - thanks.

Whether you call or think of a wavefunction as a wave (I don't) isn't really important - it is what it is. 

I think it works well when looking at the double-split, superposition, tunneling, etc., and when you’re trying to understand how probability amplitudes interfere. Maybe even when solving the Schrödinger equation—it literally looks like a wave equation.

Might not work if you think it’s a physical wave in space like water or sound waves or if you assume it’s spread out like a classical wave even after a measurement.

Another thing though, she tossed out the Many Worlds’ interpretation. Obviously, she’s smarter than I, so…I could be wrong, but I don’t think any of these interpretations are necessary to explain what's happening in the quantum eraser experiment (or similar polarization-based demonstrations). The phenomenon only requires the standard quantum principles of superposition and interference.

Interesting though. Thanks for bringing it up.

Good day to you, C2.

[confused2]

Here's Pi's response to my prompting..

In a sense, you could say that before detection, there are infinite "Schrödinger's cats" (or wave functions) representing the different possible locations where the photon could be detected on the screen. Each of these "cats" is a superposition of states, with the interference pattern encoded within.
When the photon is detected, all of these "cats" (or wave functions) collapse into a single "Schrödinger's cat" (or wave function), representing the actual location of the photon on the screen. So, it’s as if all the infinite possibilities are narrowed down to just one at the moment of detection."