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Old 30th July 2019, 01:29 AM   #81
HansMustermann
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Originally Posted by Robin View Post
I am thinking of the one described by Feynman in "Six Easy Pieces" and also in "Path Integrals".
It's been a while, so I'm not sure exactly what his is. But I don't remember any double slit experiment on electrons that also involves a photon stream. The one that involves a photon stream is the interference experiment with light. The one involving electrons is interference of electrons.

Well, technically even the latter involves a photon field for accelerating the electrons, which really is what any EM field is, but even that is very different from a stream, and should not be between the two apertures and the screen anyway.

If you found one that involves both electrons and a stream of photons, please do link to it.

Originally Posted by Robin View Post
But let me get it clear what you are saying. You are saying that in QM every time an electron interacts with any other particle, then this causes a wave function collapse? Yes?
I'm saying that in whatever universe you're in, any interaction that is a Dirac equation happens in a specific point in time and space. (Well, within the limits imposed by uncertainty.) And it will always involve exactly the number of particles in that Dirac equation.

Even if you go multiple worlds, that wave function is still a probability function of where that interaction will be. It will NOT be a fantasy scenario in which the electron interacts a little with this atom, and a little with that other one, etc, in any meaningful sense. It's a probability for something to happen or not happen, but when it does, it's all or nothing.

Which interpretation you use might or might not call it a collapse (God knows there are even loonies which claim it's not a collapse unless a human consciousness observes it,) but it is nevertheless an all or nothing thing. In no "world" do you get 1% of the electron exciting this atom over here, and 2% of it exciting the other atom over there, and so on. What you get is a 1% probability of it being here, vs 2% of it being over there, but when it happens it is all or nothing. The WHOLE electron does that interaction.

Sure, you can keep writing wave functions from there to whatever point where you consider it collapsed, or to infinity and not deal with it at all. But at the end of the day, no matter how far you go down that route, you still have a probability of something happening, and that something is an all or nothing thing. Because that's why it's even called "quantum".


To give an analogy, I could do the same writing a function of how probable it is for me to get a certain card at a blackjack table. And I can keep writing that function to infinity, as cards are kept being pulled from the deck, and the deck gets reshuffled when empty, and so on. And it's all valid maths.

And I can call each sequence of cards -- the vector of drawn cards, if you will -- a world. And see, I'm not dealing with what card was actually drawn, I'm just doing the maths of how that function expands over time. And again, it's all valid maths too.

But at the end of the day, I'm still dealing with what happens in one world. Whether it's the only one, or just the one I happen to find myself in after the card is flipped face-up, the end result is the same. And that result is still a very discrete card value. I might have a certain probability for it to be a 9 and a certain probability for it to be a 10, but there still is no such thing as drawing a 9.87 card. And there still is no such thing as drawing just a little bit of a 9 and just a little bit of a 10. It's still an all or nothing affair.
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Old 30th July 2019, 01:40 AM   #82
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In Many Worlds every event actually happens. There is no 10% chance that this will happen and 90% chance that that will happen. Both things happen with a 100% chance. When an election cloud exists in a superposition of states with it's position spread out, the electron really is everywhere. And "measurement" or interaction doesn't cause it to be localised to some particular point, it's still everywhere.

It's not just that it's not called collapse in many worlds. It's that no collapse actually happens.
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Old 30th July 2019, 01:56 AM   #83
HansMustermann
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@Robin
Actually, turns out that it's quite easy to find the book online: https://archive.org/details/six-easy-pieces/page/n153

And yep, he's not saying anything else than I was saying.

There are in fact two experiments described there. One where just the electrons are counted, and one where he adds a light source, BUT there it turns out that the light DOES interact with the electrons. In fact, he EXPLICITLY makes the point that the distribution you get when you observe the electrons with that light source is different from the distribution when you don't. And he uses it to illustrate the issue of the uncertainty principle.

The idea that the electrons interact with the screen but not the photon stream is still just your own misunderstanding. That is NOT what Fenman is saying there.

More importantly, all over the whole chapter the point that he is making is that electrons do arrive as a whole, and there is no such nonsense as just a little bit of the electron interacting with the screen or anything else. When it does, it's the WHOLE electron.

Furthermore, whether it's with the photons from the light source or with the screen, it still all happens in a specific point of time and space, not a little bit here and a little bit there. Sure, you're still limited by the uncertainty principle, and when you lower the wavelength, your resolution is proportionally lower, but that's also a caveat I've been putting all over the place.

Moreover, if you look at the effect it has on the experiment, effectively the whole point is that if you observe the electrons, you no longer have the same wave function as if you don't. If you observe them (with those photons), you no longer get the interference pattern. If you know whether they came via slit A or slit B, you no longer get an interference between the two, because (in your world) you no longer have a wavefunction that covers both.

You can decide to call it a collapse or not when you observe the electrons that way, but effectively now they are either here or there, and no longer all over the place.
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Old 30th July 2019, 02:00 AM   #84
HansMustermann
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Originally Posted by Roboramma View Post
In Many Worlds every event actually happens. There is no 10% chance that this will happen and 90% chance that that will happen. Both things happen with a 100% chance. When an election cloud exists in a superposition of states with it's position spread out, the electron really is everywhere. And "measurement" or interaction doesn't cause it to be localised to some particular point, it's still everywhere.

It's not just that it's not called collapse in many worlds. It's that no collapse actually happens.
Yes, I know everything happens, because that's what you get when you sum up all those worlds. But in any world you happen to be writing this afterwards, it still only happened or not in one place. There's no getting around the fact that in any particular world, the electron only hit the screen in one place, because any claim to the contrary would be trivially disproved by any experimental data. Sure, when you add up all the worlds, it hit every single point on the screen, but that's a whole other issue altogether.

Edit:

Look, here's a simple example: let's take something "simple", like observing a neutrino interact with matter, via the weak force. You already know why the interaction is rare and short range, but for anyone who doesn't, short version is it involves a boson which would normally have massively more energy than the particle creating it, and uncertainty puts a certain limit on ΔEΔt. So for a massive ΔE, it can only live an incredibly short Δt, so it's very unlikely to actually hit a nucleus.

So you have a given number of incoming neutrinos per second, a number of atoms in your big tank, how many weak force interactions do you expect to see?

Well, in many worlds (as I understand it), essentially every single neutrino interacted with every single atom. Which is correct too, because when you add the probabilities for every single possibility, it has to add up to 1.

But that's not what you see in such an experiment, is it?

Why? Because you're reading your output data in just one world, not in all of them, basically.

And in that just one world, you do NOT get that 100% probability. In fact, what you get is many orders of magnitude less than 100%.

And what I'm saying is that ultimately for such an experiment or indeed any practical application, you have to take that one world slice out of it. Which will get you back to some less than 100% probabilities.

You can call it the probability to be in this world, if you will, instead of the probability for the interaction to happen, but ultimately it's the same probability. It HAS to be the same probability, or it wouldn't fit the same experimental data.
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Old 30th July 2019, 04:09 AM   #85
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Originally Posted by HansMustermann View Post
@Robin
Actually, turns out that it's quite easy to find the book online: https://archive.org/details/six-easy-pieces/page/n153

And yep, he's not saying anything else than I was saying.

There are in fact two experiments described there. One where just the electrons are counted, and one where he adds a light source, BUT there it turns out that the light DOES interact with the electrons. In fact, he EXPLICITLY makes the point that the distribution you get when you observe the electrons with that light source is different from the distribution when you don't. And he uses it to illustrate the issue of the uncertainty principle.

The idea that the electrons interact with the screen but not the photon stream is still just your own misunderstanding. That is NOT what Fenman is saying there.

More importantly, all over the whole chapter the point that he is making is that electrons do arrive as a whole, and there is no such nonsense as just a little bit of the electron interacting with the screen or anything else. When it does, it's the WHOLE electron.

Furthermore, whether it's with the photons from the light source or with the screen, it still all happens in a specific point of time and space, not a little bit here and a little bit there. Sure, you're still limited by the uncertainty principle, and when you lower the wavelength, your resolution is proportionally lower, but that's also a caveat I've been putting all over the place.

Moreover, if you look at the effect it has on the experiment, effectively the whole point is that if you observe the electrons, you no longer have the same wave function as if you don't. If you observe them (with those photons), you no longer get the interference pattern. If you know whether they came via slit A or slit B, you no longer get an interference between the two, because (in your world) you no longer have a wavefunction that covers both.

You can decide to call it a collapse or not when you observe the electrons that way, but effectively now they are either here or there, and no longer all over the place.
Long story short, it interacts with the photon without collapsing the wave function. If the wavelength of the photon is long enough it doesn't even cause decoherence.
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Old 30th July 2019, 04:18 AM   #86
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Originally Posted by HansMustermann View Post
...and there is no such nonsense as just a little bit of the electron interacting with the screen...
Would you mind telling me who has suggested that there is a little bit of the electron interacting with the screen.

I have never heard anyone suggest such a thing.

Maybe you should concentrate on what people actually say rather than what you pretend they say.
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Old 30th July 2019, 05:10 AM   #87
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It seems to be a major point that Feynman makes about the double slit experiment.

If the light has a short wavelength then you would be able to tell which slit the electron passes through, then the interference pattern disappears and you sum the probabilities as a simple sum.

But if the light has a long wave length and the flash is too diffuse to let you see which slit the electron has passed through then we are back to interfering probabilities.

Certainly it has been a long time since I read it.
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Old 30th July 2019, 05:55 AM   #88
HansMustermann
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Well, I will admit that it is an interesting point. With a bit of better explaining, we could have avoided most of the arguing.
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Old 30th July 2019, 08:28 AM   #89
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Originally Posted by HansMustermann View Post
With a bit of better explaining, we could have avoided most of the arguing.
I think that's probably true of every thread on this forum.
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