New version of double-slit experiment

[mfb]

It is possible to do double-slit experiments where lasers form the slits. I would expect that you can move those slits quite fast - at least much faster than any mechanical slits.
Alternative: AOMs can generate frequencies up to 1GHz, moving the "slits" by 1 wavelength in 1 nanosecond or 30 cm of photon propagation.

I don't see the point of the experiment, however. Standard QM can perfectly predict a result and I don't see any proposed deviation from this result.

 

[universecode]

It is possible to do double-slit experiments where lasers form the slits. I would expect that you can move those slits quite fast - at least much faster than any mechanical slits.
Alternative: AOMs can generate frequencies up to 1GHz, moving the "slits" by 1 wavelength in 1 nanosecond or 30 cm of photon propagation.

I don't see the point of the experiment, however. Standard QM can perfectly predict a result and I don't see any proposed deviation from this result.

Thank you, mfb, let me find some information on how those slits are actually constructed by laser... if you already have a reference, I'd appreciate it.

The point of the experiment is very straightforward - if I understand correctly most QM theories postulate that outcome of experiment is created in the measurement action. I'd like to think that the outcome is created at emission. This should not change anything of what we know already, however, it would beautifully explain many other things currently unexplained.

Hence, the experiment has to be done in such a way that photon or electron does not interact with anything at all i.e., it must be in undefined state all the way from the source to the detector.
So the experiment must be constructed in such a way that we would still not be able to measure particle properties with certainty which would contradict established uncertainty principle and put the particle into defined state.

The only thing we wish to establish here is whether changing apparatus in the middle of the supposed particle's flight changes the probabilities of detection.

 

[DrClaude]

You should look up Wheeler's[/PLAIN] delayed choice experiment.

 

[universecode]

You should look up Wheeler's[/PLAIN] delayed choice experiment.

Thank you, this is very close to what I've been looking for but still not the same as I am proposing.
Having quickly read those experiments I see faults in those designs.

Very briefly - when detecting photons from distant quasars how do we know we are detecting the same photons that left quasars? Those photons encountered at least one electron on the way to us when the photon was absorbed and re-emitted hence making another decision at each point of emission, am I not right here, please someone tell me?

Similar issue with the delayed choice - double slit and a lens - well how many times a photon was absorbed and emitted inside the lens?

I would appreciate if someone could tell me I am completely wrong here.

 

[bhobba]

Very briefly - when detecting photons from distant quasars how do we know we are detecting the same photons that left quasars?

We don't, but the issue is irrelevant. In the double slit experiment we don't even know if its the same one that was emitted. It could have been picked up by some super-agency who exchanged it for another one some humongous number of for all we know. That's because all fundamental particles are indistinguishable. The reason for that is they are all excitations of the same underlying quantum field.

In many ways a lot of issues with QM disappears in QFT. The following book is dirt cheap and explains that view pretty well:
https://www.amazon.com/dp/B004ULVG9O/?tag=pfamazon01-20

Thanks
Bill

 

[universecode]

In many ways a lot of issues with QM disappears in QFT. The following book is dirt cheap and explains that view pretty well:
https://www.amazon.com/dp/B004ULVG9O/?tag=pfamazon01-20

Thanks, Bill, I'll see if that book offers something, although based on its reviews probably unlikely...
I have been reading Zee's QFT in a Nutshell by the way for quite a while now - can't say I spent much time analysing all equations but I think I understand most of the concepts presented there.

 

[universecode]

In the double slit experiment we don't even know if its the same one that was emitted.

Exactly, that's why I am trying to design an experiment that is extremely simple and preferably with photons because electrons we already sort of know keep emitting and absorbing photons (which might as well guide free flowing electron into its detected position by continuously creating new future outcome for it)

 

[Cthugha]

Similar issue with the delayed choice - double slit and a lens - well how many times a photon was absorbed and emitted inside the lens?

It was never absorbed and reemitted. If a photon gets absorbed and reemitted the direction of emission is random which gives a very different look. Have you read the manuscripts about actual implementations of the delayed choice experiments, in particular things like the Jacques et al. version (ref. 12 in the wikipedia article linked earlier) ?

 

[universecode]

Have you read the manuscripts about actual implementations of the delayed choice experiments, in particular things like the Jacques et al. version (ref. 12 in the wikipedia article linked earlier) ?

Thanks, started reading it - looks interesting...

On the other point:

It was never absorbed and reemitted. If a photon gets absorbed and reemitted the direction of emission is random which gives a very different look.

As far as I understand, it is always absorbed and re-emitted by electrons, that's how it changes the direction inside the lens and yes, it is always emitted in random direction but the fact that there is a detector nearby makes the detector the most probable location it would end up at next - if there was no detector nearby it would end up at some other location in the universe, is this not correct?

 

[Cthugha]

As far as I understand, it is always absorbed and re-emitted by electrons, that's how it changes the direction inside the lens and yes, it is always emitted in random direction but the fact that there is a detector nearby makes the detector the most probable location it would end up at next - if there was no detector nearby it would end up at some other location in the universe, is this not correct?

No, the absorption and reemission thing is like an urban legend in physics. We have a FAQ entry on that topic: https://www.physicsforums.com/showthread.php?t=511177.

If it was absorbed, the phase would get all messed up and randomized. This is not what is happening.

Loosely speaking, the em field has some effect on the charges in any material and moves them a bit and the moving charges in turn create a field which in turn is superposed with the initial field. The sum of the fields behaves like a slower field. Due to the curved surface you get the focusing. This can be explained by the Huygens-Fresnel principle.

What is important is that this is a reversible interaction, just like rotating polarization. If you undo the change - say by adding a second lens and making the beam parallel again, there is no way one could ever tell that these changes took place. No measurement has happened, phase is not scrambled up. The quantum state of all things involved has not changed to some state orthogonal to the initial state. There is no experimental way to distinguish whether you changed something or not - unless of course you perform some additional measurement at the lens and mess things up intentionally.

 

[bhobba]

I have been reading Zee's QFT in a Nutshell by the way for quite a while now - can't say I spent much time analysing all equations but I think I understand most of the concepts presented there.

That's a pretty advanced text. I have it as well but wouldn't attempt it without having a background at something like the level of Ballentine. That said I wouldn't attempt something like Peskin and Schroeder without that text first.

QM is bad enough, but QFT is HARD, really HARD.

Thanks
Bill

 

[DrChinese]

As an aside to the more meaningful comments already provided, I HAVE to comment on this aspect of your proposed experiment.

Seriously? You think there is some way to have a physical barrier start moving after a photon is emitted but BEFORE the photon reaches the barrier? What signal are you going to send to the mechanism that moves the barrier to let it know the photon has been emitted? How fast is that signal going to move? This, of course, is totally aside from the concept of moving the barrier in the amount of time you are talking about.

If this were a thought experiment that would be one thing but you seem to have proposed it as an actual physical experiment.

Actually, I think there is a way to do that (send the signal) so that a photon is confined within a small time window (perhaps a fraction of a nanosecond). Not sure if that window would do much to accomplish the OP's objective. The method would be to use one of a pair of PDC photons to herald the impending arrival of the other at the slit. The second could be delayed as much as needed. It might not work with every pair, but it would with some pairs.

 

[DrChinese]

I am trying to find a definite answer whether the following version of the double-slit experiment has ever been performed.

Calculate/observe what interference pattern should appear by emitting
photons individually one-by-one through the double-slit barrier and onto a detector
screen behind it, with all equipment being in a vacuum to make sure each photon does not interact with anything other than the barrier and the screen.

Then perform the same experiment but keep shifting the double-slit
barrier slightly after each photon has been emitted but before it is
supposed to go through the slits.

Will the interference pattern appear to look similar and in the same
location as in the original static version of the experiment or will
it be different?

Could it be that such experiment cannot be performed at all because it is not possible to tell whether and when each photon is actually emitted to be able to shift the barrier after the emission?

There are 2 issues here, already alluded to by others.

1. Your ideas about the path histories is considered "interpretation dependent". They are mostly right in a few interpretations but not supported by others.

2. The predictions of your "interpretation" are simply the same as all others in this case. If the photon's paths are not prohibited (constrained), they are allowed and interference occurs. Moving something before or after the photon goes by in a path makes no difference.

So while you might wonder as to the result, it doesn't really make for much of an experiment when a null result is predicted by every interpretation.

 

[Nugatory]

The point of the experiment is very straightforward - if I understand correctly most QM theories postulate that outcome of experiment is created in the measurement action. I'd like to think that the outcome is created at emission. This should not change anything of what we know already, however, it would beautifully explain many other things currently unexplained.

I would expect that any interpretation that determines the outcome at emission time would run afoul of Bell's theorem when applied to entangled particles. If so, the idea has already been experimentally falsified, although both the argument and the experiments are more subtle than universecode is hoping for.

 

[universecode]

Thank you everyone for comments, I am thinking about everything suggested and reading suggested references, which will take time...

Meanwhile, let me simplify the experiment to make the point even more clear.

First we don't even need a double-slit barrier - just source and detector in vacuum.

Source emits a single photon every time interval T with let's say 95% probability that emission will happen at T +- tdelta, is this possible to achieve?

The detector is located some distance away larger than 2*tdelta*c, let's call this location X.

We run the experiment for n*T time and observe how many times the detector will be hit. Number n should be sufficiently large to have statistically valid results, as the result will give us empirical probability of finding a photon at the given location X of the detector.

Then we move the detector slightly to location Y leaving the source as it was and again run the experiment for n*T time and again observe how many times the detector will be hit - this number should be different (after adjustment for statistical variation) because the distance from the source to the detector is slightly different, is this correct?

Now the third run is more complicated. We place the detector into the location of the first run X. Again run the experiment for n*T time, but at every time interval T we move the detector from X to Y and then back again at T+tdelta.
Obviously source and detector would have to be synchronised initially to ensure intervals T begin at the same time but then we must make sure no signals whatsoever are sent between the source and the detector.

Assuming all this is somehow possible to perform, what would be the number of detector hits on this third run?

 

[universecode]

No, the absorption and reemission thing is like an urban legend in physics. We have a FAQ entry on that topic: https://www.physicsforums.com/showthread.php?t=511177.

Just read this FAQ.
Well, I see a fault in the logic here.

A common explanation that has been provided is that a photon moving through the material still moves at the speed of c, but when it encounters the atom of the material, it is absorbed by the atom via an atomic transition. After a very slight delay, a photon is then re-emitted.

The whole logic is based on the assumption that it is atom as a whole absorbs a photon, which is in my understanding is incorrect.
It is the electrons that absorb and re-emit photons and it is called photon-electron scattering. Furthermore electrons exchange photons with protons and this is what keeps them in some kind of orbit around the nucleus. So the process of photon traveling inside a material depends on electrons positions and not simply on a type of atom which is what seems to the argument falsified in this FAQ.
I am afraid that I will not be accepting this explanation as a credible answer.

 

[ZapperZ]

The whole logic is based on the assumption that it is atom as a whole absorbs a photon, which is in my understanding is incorrect.
It is the electrons that absorb and re-emit photons and it is called photon-electron scattering. Furthermore electrons exchange photons with protons and this is what keeps them in some kind of orbit around the nucleus. So the process of photon traveling inside a material depends on electrons positions and not simply on a type of atom which is what seems to the argument falsified in this FAQ.
I am afraid that I will not be accepting this explanation as a credible answer.

Actually, that is incorrect. The energy levels that the electrons occupy do not appear on their own, and do not exist for an individual, isolated electron! It appears only when the electron is in the potential provided by the atom! So it is the entire atom that is responsible for all the energy state, and which is why these energy states are unique for the different types of atoms! It is how we can identify the type of elements simply by looking at the spectral lines!

Free electron can't absorb photons.

Zz.

 

[universecode]

Free electron can't absorb photons.

That's not my understanding...
What about this as just a random example?
http://en.wikipedia.org/wiki/Compton_scattering

or any other text about photon-electron scattering.

 

[Jilang]

The difference is that in an atom the electron can only have well defined energies. With a free electron you go have a continuous spectrum.

 

[ZapperZ]

That's not my understanding...
What about this as just a random example?
http://en.wikipedia.org/wiki/Compton_scattering

or any other text about photon-electron scattering.

You need to read and understand carefully. I said it cannot be ABSORBED, which is what you were arguing about! Compton scattering is when a photon and electron scatter of each other, changing their energies/momentum! After the scattering, both electron and photon go off their separate ways. The photon isn't absorbed!

Zz.

 

[universecode]

You need to read and understand carefully. I said it cannot be ABSORBED, which is what you were arguing about! Compton scattering is when a photon and electron scatter of each other, changing their energies/momentum! After the scattering, both electron and photon go off their separate ways. The photon isn't absorbed!

Zz.

As far as I understand, according to Feynman, photon-electron scattering happens via absorption and emmittion.
Please see here for example
http://www.phys.ufl.edu/~avery/course/4390/f2013/lectures/feynman_diagrams_1.pdf

or this chapter from a book:
http://books.google.co.uk/books?id=...v=onepage&q=feynman photon scattering&f=false

 

[ZapperZ]

As far as I understand, according to Feynman, photon-electron scattering happens via absorption and emmittion.
Please see here for example
http://www.phys.ufl.edu/~avery/course/4390/f2013/lectures/feynman_diagrams_1.pdf

or this chapter from a book:
http://books.google.co.uk/books?id=...v=onepage&q=feynman photon scattering&f=false

And you think this applies to photons being absorbed by atoms? Really?

You need to see how frustrating this is. I tell you that there are no lions, and you argued the point by showing me a leopard. Many of us are trying to correct the error in your understanding here, but unfortunately, it is not getting through.

There's nothing else I can do here, and you're welcome to hold on to whatever it is you are believing in.

Zz.

 

[universecode]

And you think this applies to photons being absorbed by atoms? Really?

No, what I am saying is that light propagates through material not via absorption by atoms and changing electrons' energy levels (which is the assumption made right at the beginning of that text) but I suspect it happens rather by scattering off electrons.

 

[Nugatory]

No, what I am saying is that light propagates through material not via absorption by atoms and changing electrons' energy levels (which is the assumption made right at the beginning of that text) but I suspect it happens rather by scattering off electrons.

And the difference between these two phenomena is what?

 

[DrChinese]

I am afraid that I will not be accepting this explanation as a credible answer.

First off, your "experiment" is about path histories. It is not about how photons and other atoms interact. So this is far off topic.

Second, you obviously know quite little about quantum field theory and are prone to making general newbie comments. ZapperZ is a working particle physicist that deals with these issues daily.

I might suggest that you approach instead from more of an "I still don't understand" perspective rather than "this is not acceptable to me". Getting back to the topic:

Your "outcome at emission" is just as inadequate any other mechanistic explanation. The general view is that the ENTIRE setup must be considered. That would be:

a) the source around time of emission
b) the intervening space traversed, at various times and considering its make-up
c) the detector around the time of detection
d) and yes, places where it is not absorbed can make contributions to the end result - even ones that appear to NOT be part of its path

Moving the detector before the arrival of the photon makes no difference. There is an interpretation of QM called "relational block world" that describes the space-time relationships accurately. That would be useful for you to look at. Please note that despite it being accurate, that does not make it any more "true" than any other interpretation. They all predict the same results.

 

[DrChinese]

but I suspect it happens rather by scattering off electrons.

Seriously? You "suspect"? On what would you have to base such suspicion?

You have apparently missed 100% of the point of Zz's FAQ. Light propagation through a material is best described by field effects. Thinking of it as bouncing around from electron to electron is not only wrong, it goes directly against your own comments about multiple paths.

 

[Cthugha]

but I suspect it happens rather by scattering off electrons.

Well, in lack of a better term, that is simply nonsense.

 

[universecode]

Well, in lack of a better term, that is simply nonsense.

Excellent! If idea is nonsense but no one can provide logical arguments why then it must be a step in the right direction.

If you really wish to help, please do not send me to read everything what's been written before - my life span unfortunately isn't that long.
If you are certain this is nonsense, please provide facts that can logically falsify the idea, then I will go away and verify those facts and then we will all make a conclusion whether it is nonsense or not.

 

[Jilang]

Universecode, are you suggesting that the photon that is detected is not the same one that was emitted? Would this works for electrons too? Buckyballs?

 

[Cthugha]

Excellent! If idea is nonsense but no one can provide logical arguments why then it must be a step in the right direction.

If you really wish to help, please do not send me to read everything what's been written before - my life span unfortunately isn't that long.
If you are certain this is nonsense, please provide facts that can logically falsify the idea, then I will go away and verify those facts and then we will all make a conclusion whether it is nonsense or not.

You have already been given more than enough logical arguments and have already been pointed to where you can read about that. See ZapperZ's post and the FAQ. We cannot read and understand for you.

There's nothing else I can do here, and you're welcome to hold on to whatever it is you are believing in.

I second that.