Where Was the Observer Detector in the Double-Slit Experiments?

[Doc Al]

Hi, my question is:
If there is no function, there couldn't be an interference, is this true ?

Huh? Who says there's no wavefunction? You just asked about its collapse, so I suppose you realize that there is a wavefunction.

Perhaps you can rephrase your question.

 

[SDetection]

Perhaps you can rephrase your question.

OK , I meant:
In the double-slit experiments, when there was an observer to determine which slit the electron passed through, that made the wave function collapse and the electron acted as particle, and there wasn't any interference at the detector, right ?.
But if the detector itself is an observer, this should also make the electron act as a particle, but there was actually an interference, which means the wave function didn't collapse.
Am I right ?, If so ,How could this happen ?

 

[Doc Al]

OK , I meant:
In the double-slit experiments, when there was an observer to determine which slit the electron passed through, that made the wave function collapse and the electron acted as particle, and there wasn't any interference at the detector, right ?.

Realize that performing the double-slit experiment in such a way that identifies the slit through which the particle passes requires a physically different experiment than the ordinary one. In that case, the usual interference pattern is destroyed.

But if the detector itself is an observer, this should also make the electron act as a particle, but there was actually an interference, which means the wave function didn't collapse.
Am I right ?, If so ,How could this happen ?

Again, I don't really know what you're asking. When the particle is detected at the screen, the experiment is over. Once the particle is detected, the previous wavefunction that could be used to predict (probabilistically) the location where the particle would hit the screen is irrelevant.

 

[Fredrik]

Why the function didn't collapse at the time of detection ?. couldn't the detector be considered an observer ?

The "detection" in the double-slit experiment is the interaction (between the particle and the screen) that leaves a mark on the screen. So yes, the detector (i.e. the screen) must be considered an observer.

Ok, Why the function didn't collapse at the time of detection ?

It does. (Assuming an interpretation in which there is a collapse).

Hi, but if the function collapses at the time of detection, how could be there an interference ?

The interference is between the different paths from the emission event to the detection event, not between different runs of the same experiment.

OK , I meant:
In the double-slit experiments, when there was an observer to determine which slit the electron passed through, that made the wave function collapse and the electron acted as particle, and there wasn't any interference at the detector, right ?.
But if the detector itself is an observer, this should also make the electron act as a particle, but there was actually an interference, which means the wave function didn't collapse.
Am I right ?, If so ,How could this happen ?

This is a different question than before. An "observer" at one of the slits would be a detector that can signal if a particle passed through it. In this case, the detection is the interaction (between the particle and the detector) that causes the signal.

 

[SDetection]

The interference is between the different paths from the emission event to the detection event, not between different runs of the same experiment.

Thanks, now I get it.

 

[SDetection]

OK, here is my next question:
Is there a possibility that an electron could alter the state of any particle in its way from the emitter to the detector ?

 

[Fredrik]

Yes, definitely. That's actually the main reason why the photons in the standard double-slit experiment have to go through the slits.

 

[SDetection]

Yes, definitely.

OK, is this why the electrons take different paths in their way from the emitter to the detector ?. I don't think it's a mechanism of the emitter itself, right ?.

Thank you for your time.

 

[Fredrik]

OK, is this why the electrons take different paths in their way from the emitter to the detector ?.

No, it's just the reason why you can ignore all the paths from the emission event to the detection event that don't go through the slits. Technically you have to add up the contributions from all paths, but the contribution from a path that goes through a region filled with matter is going to be extremely close to zero because of interactions between the particle and the matter in the region.

I don't think it's a mechanism of the emitter itself, right ?.

Right. It's just how matter behaves. It's a pretty weird way to behave, but nature doesn't care what we think is weird.

 

[sokrates]

OK , I meant:
In the double-slit experiments, when there was an observer to determine which slit the electron passed through, that made the wave function collapse and the electron acted as particle, and there wasn't any interference at the detector, right ?.
But if the detector itself is an observer, this should also make the electron act as a particle, but there was actually an interference, which means the wave function didn't collapse.
Am I right ?, If so ,How could this happen ?

By the time you measured the wavefunction and "collapsed it" it has already interfered with different components of itself.

The interference pattern has already formed.

And the collapse of the wavefunction at the detectors is precisely the reason why they always come in "quantized" amounts. You always hear a single click in a Geiger counter. Not a weak click. Always one.

 

[SDetection]

No, it's just the reason why you can ignore all the paths from the emission event to the detection event that don't go through the slits. Technically you have to add up the contributions from all paths, but the contribution from a path that goes through a region filled with matter is going to be extremely close to zero because of interactions between the particle and the matter in the region.

But when the electrons were going though the slits, they were still interacting vertically with other free particles, as the slits were only horizontally narrow, right ?

 

[Matterwave]

@SD: the usual way we think about the double slit is in 2 dimensions, the third dimension isn't really thought about very much. But just imagine instead of a slit, a very small hole through which the particles can pass through.

So, I think an equally interesting question is, can you set up a standard double slit experiment with PHOTONS (not electrons) and get them to NOT interfere? I'd imagine shining a light wouldn't do the trick this time...

 

[SDetection]

@SD: the usual way we think about the double slit is in 2 dimensions, the third dimension isn't really thought about very much. But just imagine instead of a slit, a very small hole through which the particles can pass through.

So, I think an equally interesting question is, can you set up a standard double slit experiment with PHOTONS (not electrons) and get them to NOT interfere? I'd imagine shining a light wouldn't do the trick this time...

Yeah!, I was going to suggest that , as it's more practical because photons unlike electrons hit the detector at one position.
If no interference happens , then the HUP is violated, right ?

 

[SDetection]

...photons unlike electrons hit the detector at one position.

Maybe because photons don't spin ?.

 

[Fredrik]

Photons have spin 1, not 0. (Electrons have spin 1/2). And what's this about photons hitting "the detector at one position"? They certainly don't hit the same spot every time. A laser beam can hit a specific spot if you fire it directly at the screen, but if you put a screen with a single slit (or just a tiny hole) between the emitter and the target screen, the area on the target screen that gets hit by photons spreads out. The smaller you make the hole, that bigger you make the area that gets hit by photons. It's hard to explain why. Feynman's book "QED: The strange theory of light and matter" explains it pretty well, but I don't think I can explain it in a post here.

 

[DrChinese]

Maybe because photons don't spin ?.

As Fredrik says, photons do spin. Both photons and electrons - in fact pretty much any nuclei as well - will exhibit interference in a suitable double slit format. (Obviously, the slits must be spaced/sized appropriate for the relevant wavelength. For particles with a rest mass, you will use a wavelength proportional to the total mass/energy.)

The rule for interference in these setups is essentially: interference appears in the absence of the possibility of which-slit information. Experiments have even been done on molecules as big as fullerene (that's 60 carbon atoms).

 

[SDetection]

Photons have spin 1, not 0. (Electrons have spin 1/2). And what's this about photons hitting "the detector at one position"? They certainly don't hit the same spot every time.

I meant photons can be more concentrated at a tiny hole than electrons.

A laser beam can hit a specific spot if you fire it directly at the screen, but if you put a screen with a single slit (or just a tiny hole) between the emitter and the target screen, the area on the target screen that gets hit by photons spreads out. The smaller you make the hole, that bigger you make the area that gets hit by photons. It's hard to explain why. Feynman's book "QED: The strange theory of light and matter" explains it pretty well, but I don't think I can explain it in a post here.

So, you're saying that this experiment was also done using a tiny hole , and there was also interference/spread of photons ?.

 

[Vanadium 50]

I meant photons can be more concentrated at a tiny hole than electrons.

Why do you think this?

 

[SDetection]

Why do you think this?

Tell me first whether it's true or false, and after that I will tell you why I think so .

 

[Cthugha]

So, I think an equally interesting question is, can you set up a standard double slit experiment with PHOTONS (not electrons) and get them to NOT interfere? I'd imagine shining a light wouldn't do the trick this time...

Sure, just use incoherent light and a double slit with standard dimensions.

 

[Fredrik]

I meant photons can be more concentrated at a tiny hole than electrons.
...
So, you're saying that this experiment was also done using a tiny hole , and there was also interference/spread of photons ?.

What experiment? Are you asking if people have tried shooting photons through a tiny hole? Of course they have. (And yes, the beam spreads out).

I don't know if it's true or not that it's easier to focus a beam of photons towards a tiny hole than a beam of electrons, but it's not really relevant for the double slit experiment. I would guess that it's much easier to focus a beam of electrons, since their shorter wavelength should enable you to use a screen with hole in it to focus the beam. (If the size of the hole is just small enough to spread out a laser beam, it wouldn't spread out an electron beam significantly because their wavelengths are shorter).

 

[SDetection]

What experiment? Are you asking if people have tried shooting photons through a tiny hole? Of course they have. (And yes, the beam spreads out).

Using one photon at a time ?. Is there a reference for that ?.
Thanks.

 

[ZapperZ]

Using one photon at a time ?. Is there a reference for that ?.
Thanks.

This is no longer in the realm of exotica or needing "references" because it has become so common, it is done in undergraduate physics labs.

See this for example:

http://ophelia.princeton.edu/~page/single_photon.html

Zz.

 

[SDetection]

This is no longer in the realm of exotica or needing "references" because it has become so common, it is done in undergraduate physics labs.

See this for example:

http://ophelia.princeton.edu/~page/single_photon.html

Zz.

I'm sorry, this experiment is not clear for me. Was it done using vertical slits or tiny round holes ?.

 

[ZapperZ]

I'm sorry, this experiment is not clear for me. Was it done using vertical slits or tiny round holes ?.

Sorry, I was referring to slits.

Would round holes make any difference in what you're after?

Zz.

 

[SDetection]

Sorry, I was referring to slits.

Would round holes make any difference in what you're after?

Zz.

Yes, It's my last try to find Einstein's hidden variable .

 

[ZapperZ]

Yes, It's my last try to find Einstein's hidden variable .

And the round holes can do this but the the slits can't?

Zz.

 

[SDetection]

And the round holes can do this but the the slits can't?

Zz.

Yes, I think so. I won't rest until I do my best to clear Einstein's name. If only I can make a photon go though a tiny round hole without changing its momentum, then it's over for the HUP!.

 

[Hootenanny]

I won't rest until I do my best to clear Einstein's name, If only I can make a photon go though a tiny round hole without changing its momentum, then it's over for the HUP!.

Well, good luck with that ...

 

[DrChinese]

Yes, I think so. I won't rest until I do my best to clear Einstein's name. If only I can make a photon go though a tiny round hole without changing its momentum, then it's over for the HUP!.

Now that's funny!