The not quite certain quantum physics thread that can't ever be all things quantum

a_unique_person

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a_unique_person
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https://www.forbes.com/sites/starts...ge-the-outcome-and-this-experiment-shows-how/

An interesting read. As usual I don't understand it all so no point my putting up an incorrect interpretation of what it means or even how correct it is. They seem to be claiming that the act of measuring which slit a particle going through the famous two slit experiment will stop the creation of a wave pattern. I hadn't heard of this before.

AIUI everything is actually a wave until it is forced to interact with something, then the wave collapses to a particle.
 
It seems to be a historical account of QM experiments,

as there's no theory/model of QM yet, media can make their own interpretations of what's happening.

me personally, I'm going with fields.
 
a_unique_person said:
AIUI everything is actually a wave until it is forced to interact with something, then the wave collapses to a particle.
Click to expand...
Not really. The wave just changes its configuration to a form that's more localized. But that is also just an abstraction. It's pointless trying to think about what it 'is'.

What there is are mathematical models based on observed behavior, which then predict other behavior and the observations that are to be expected. Fortunately, it seems that reality can be precisely described with simple mathematical formulas. We don't know why, and we don't really care. All that matters is figuring out the correct formulas.
 
a_unique_person said:
AIUI everything is actually a wave until it is forced to interact with something, then the wave collapses to a particle.
Click to expand...
My understanding is that when you have more than one way something could have happened (ie the electron might have passed through one slit or the other) then if there would be no way to tell which event occurred then you sum the probabilities as interfering with each other and hence get the multiple bands on the back screen, the interference pattern.

But if there would have been a way to tell which event happened (ie a stream of photons which will produce a flash when the electron passes through) then you sum the probabilities simply and not as interfering with each other and hence you get the two bands on the back screen.

So both patterns are wave patterns, but different kinds of wave patterns. Don't take that as gospel, I am no expert.
 
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a_unique_person said:
Oh, it's all fields.


A theoretical physicist tells me there are no particles.


https://www.youtube.com/watch?v=zNVQfWC_evg
Click to expand...

Aren't the words we use (field, particle, wave) just conceptual frameworks and not the thing itself?

Originally I thought of particles as very tiny billiard balls, because that's how they are typically illustrated, but they are too small to see with the naked eye, and in any case the things we see are not the thing itself but light reflected off the thing. They probably would not look like a tiny billiard ball if it was possible to actually see them.
 
When it comes to talking about fields, I always think of that video by Sean Carroll that PixieMisa linked to years ago. It was called 'The Higgs-Boson revisited'. That it was all fields made sense to me.
 
a_unique_person said:
...

AIUI everything is actually a wave until it is forced to interact with something, then the wave collapses to a particle.
Click to expand...
It is more bizarre than that. When two particles are entangled and measured one after the other, the state of the first measured particle is dependent on how the second particle will be measured. That is to say that the first particle knows how the second will be measured before the experimenters know. The entangled particle knows what will happen and acts accordingly before humans know.

Also, we are one step closer to reconciling the vying theories of the quantum and atomic (nano) worlds. Quantum Effects Observed in Large Metal
 
ServiceSoon said:
It is more bizarre than that. When two particles are entangled and measured one after the other, the state of the first measured particle is dependent on how the second particle will be measured. That is to say that the first particle knows how the second will be measured before the experimenters know. The entangled particle knows what will happen and acts accordingly before humans know.
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But, (again, as I understand it), it is not that the first particle knows, the important thing is that such a determination between competing possibilities would be possible.
 
Robin said:
But, (again, as I understand it), it is not that the first particle knows, the important thing is that such a determination between competing possibilities would be possible.
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Could you restate that? I'm not disagreeing with you, I'm just having trouble parsing what you wrote.
 
Roboramma said:
Could you restate that? I'm not disagreeing with you, I'm just having trouble parsing what you wrote.
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Say an event that could happen two ways A and B. You have a probability amplitude for each.

If you only see the end result and have no way to tell if it happened in way A or B then you sum the amplitudes and then get the probability from that.

In the double slit experiment this gives the interference pattern.

But if there is some way you could have told whether it happened by way A or way B then you find the probabilities from each amplitude and then you sum the probabilities. In the double slit experiment this gives the two bands.

Again, this is just my understanding.
 
Robin said:
Say an event that could happen two ways A and B. You have a probability amplitude for each.

If you only see the end result and have no way to tell if it happened in way A or B then you sum the amplitudes and then get the probability from that.

In the double slit experiment this gives the interference pattern.

But if there is some way you could have told whether it happened by way A or way B then you find the probabilities from each amplitude and then you sum the probabilities. In the double slit experiment this gives the two bands.

Again, this is just my understanding.
Click to expand...

I think your understanding is basically correct.

I'm not sure how it relates to what ServiceSoon wrote, though.
 
The Forbes article makes a common mistake in the description of the case when which-way information is detected. Any arrangement where the fringes are visible across a substantial width of the screen, will require slits to be of the order of the wavelength of the interfering waves. This is necessary to result in sufficient diffraction at each slit that the electrons or light arising at the two slits substantially overlap at the detection screen. If you acquire which way information then the fringes disappear to be replaced by a broad band without fringes, not by two bands (the broad band, which is the intensity envelope in the with-fringes case, is actually represented by a sinc squared function, the Fourier transform of the slit function, a top hat).
 
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