New version of double-slit experiment

[universecode]

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?

 

[ZapperZ]

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?

When physicists propose an experiment, they have to not only outline what needs to be done and measured, but also the physics that is of interest here. The physics, i.e. what is it that we can verify or learn, is what drives the pursuit of the experiment. We build and conduct various experiment because the physics tells us what we will measure, and why such an experiment is significant, or adds to the body of knowledge. It is very seldom that we can just do and experiment just for the heck of it or just to see what happens.

You have not described the significant physics as the outcome of doing such an experiment. What exactly are we trying to look for here, and how does it add to the body of knowledge?

Secondly, doing this in vacuum is highly unnecessary. Light interaction in air, at least in a typical optics lab, is negligible.

Thirdly, you need to investigate a bit what has been done with a number of interferometer, including the Mach-Zehnder interferometer interferometer, which is a more sophisticated and controllable version of the double slit experiment.

Zz.

 

[universecode]

You have not described the significant physics as the outcome of doing such an experiment. What exactly are we trying to look for here, and how does it add to the body of knowledge?
Zz.

Well, I would have thought the purpose of such experiment is quite obvious, unless of course there is a solid theory that can confirm the results without performing such experiment, which is what I would like to find out.

QED states that probability of a photon hitting an electron is a square of the sum of amplitudes for all physically possible independent paths a photon could take to reach the detector electron.
Amplitude itself for a given energy photon is a vector whose direction essentially depends on just the length of each such path or some imaginary timing required to travel that path.

Experiments confirm that it is not possible to tell which path a photon actually takes.
At the same time experiments confirm that QED is correct which implies that photon in fact does know the length of every possible path to assign a probability to every existing electron it may hit next. This then implies the following:

Every emitted photon knows exactly the location of each electron (and any other particle it interacts with) in the universe and it just hits randomly one of them with some probability according to its relative location with respect to all such particles existing in the universe.

Assuming the above is a possible explanation for photon's behaviour i.e., that each photon somehow knows the location of every particle in the universe, when does it acquire such
knowledge:

a) at the point of emission?
b) or its knowledge about the world is constantly updated as it travels?

Hence the reasoning for the above mentioned experiment:
1. If the above experiment produces the same expected interference pattern in both cases it could mean that photon acquires knowledge about all existing particles it interacts with at the point of emission and it doesn’t even travel at all (hence another reason why we cannot establish which path it takes because it doesn't actually take any).
There are just two discrete events:
a photon is emitted and then it gets absorbed by a randomly selected particle after some time. The
selection of which particle will absorb the photon is performed at the point of emission and the fact that there is a delay between emission and absorption creates an illusion of space traveling which in reality does not happen.

2. If the above experiment produces a shifted interference pattern, at what point does the photon update its knowledge about the world? What if the double-slit barrier is shifted immediately before photon can reach the barrier?

 

[ZapperZ]

I'm puzzled. What does this have anything to do with "QED"? This is more of a basic QM/QFT/path integral stuff. And what's with the issue with photon hitting an electron? What does this have to do with the 2-slit experiment?

Your understanding of what is going on is faulty. A photon does not "... somehow knows the location of every particle in the universe..." What physics did you use to get that? You made several illogical jump to connect superposition of paths to that conclusion.

The starting premise of your understanding is wrong. This renders subsequent "derivation" of that understanding to be false as well.

Zz.

 

[universecode]

And what's with the issue with photon hitting an electron? What does this have to do with the 2-slit experiment?
Zz.

ok, thanks, please walk me through then.

What is a screen that shows the interference pattern? It is made of atoms that change some property (which we can then observe) because photon is absorbed by one of the electrons in one of those atoms, is this not correct?

Then, what is the interference pattern itself? It is just a collection of light and dark areas - light areas are atoms which absorbed our photons and dark areas are atoms where our photons have not arrived. Is this correct?

 

[ZapperZ]

ok, thanks, please walk me through then.

What is a screen that shows the interference pattern? It is made of atoms that change some property (which we can then observe) because photon is absorbed by one of the electrons in one of those atoms, is this not correct?

Then, what is the interference pattern itself? It is just a collection of light and dark areas - light areas are atoms which absorbed our photons and dark areas are atoms where our photons have not arrived. Is this correct?

You seem to think the "detector" itself has any bearing on the superposition of path of the photon? Since when? And where is the physics for this?

I can also send two supercurrents through 2 separate superconducting loop and then detect the superposition and interference pattern of currents on an oscilloscope. Where is the "screen" there? This is what we use in SQUID devices!

The phenomenon is more GENERAL that what you seem to think.

You also avoided answering my question on what this has anything to do with QED etc.

Zz.

 

[bhobba]

Every emitted photon knows exactly the location of each electron (and any other particle it interacts with) in the universe and it just hits randomly one of them with some probability according to its relative location with respect to all such particles existing in the universe.

Where you got that from beats me - its not true BTW.

The reason for probabilities in QM is a deep issue and requires a thread of its own.

But a deep theorem, called Gleason's theorem, is of relevance to the issue:
http://en.wikipedia.org/wiki/Gleason's_theorem

Thanks
Bill

 

[bhobba]

You also avoided answering my question on what this has anything to do with QED etc.

I suspect he got it from reading Feynman's - QED - The Strange Theory Of Light And Matter.

To the OP this is a general feature of quantum weirdness - its not peculiar to QED.

What is a screen that shows the interference pattern? It is made of atoms that change some property (which we can then observe) because photon is absorbed by one of the electrons in one of those atoms, is this not correct?

Then, what is the interference pattern itself? It is just a collection of light and dark areas - light areas are atoms which absorbed our photons and dark areas are atoms where our photons have not arrived. Is this correct?

As to how an observation does its magic, in modern times decoherence has a lot to say about the issue:
http://www.ipod.org.uk/reality/reality_decoherence.asp

Thanks
Bill

 

[universecode]

You seem to think the "detector" itself has any bearing on the superposition of path of the photon? Since when? And where is the physics for this?

Let's come back to this and superposition later in the discussion

I can also send two supercurrents through 2 separate superconducting loop and then detect the superposition and interference pattern of currents on an oscilloscope. Where is the "screen" there? This is what we use in SQUID devices!

The phenomenon is more GENERAL that what you seem to think.

I understand that this applies to other particles including electrons as you are describing in superconductivity but here we are talking about photons only because photons and electrons are different classes of particles and mechanisms for their respective interferences may well be slightly different - is there a theory that proves that they must be the same?

You also avoided answering my question on what this has anything to do with QED etc.

According to QED the appearance of interference pattern is not due to wave-particle duality and waves interfering with themselves - it is all about probability of finding a photon at a given place (which can only be determined by its absorption by an electron etc.), hence interference pattern is just a picture showing probability distribution given by our photons to the screen area. This probability distribution can be calculated according to QED formulas and in the mentioned experiment would depend only on relative positions of all the equipment in the experiment, is this correct?

 

[universecode]

Every emitted photon knows exactly the location of each electron (and any other particle it interacts with) in the universe and it just hits randomly one of them with some probability according to its relative location with respect to all such particles existing in the universe.

Where you got that from beats me - its not true BTW.

Thanks, Bill, that's my assumption which is exactly what I am trying to falsify with the proposed experiment.
Could you please explain why it is not true?

 

[ZapperZ]

Let's come back to this and superposition later in the discussion

And this is your fatal flaw because you do not see that the superposition principle is responsible for the interference effect.

I understand that this applies to other particles including electrons as you are describing in superconductivity but here we are talking about photons only because photons and electrons are different classes of particles and mechanisms for their respective interferences may well be slightly different - is there a theory that proves that they must be the same?

Yes, if we are talking about the appearance of interference. The same principle is responsible for all the quantum interference.

According to QED the appearance of interference pattern is not due to wave-particle duality and waves interfering with themselves - it is all about probability of finding a photon at a given place (which can only be determined by its absorption by an electron etc.), hence interference pattern is just a picture showing probability distribution given by our photons to the screen area. This probability distribution can be calculated according to QED formulas and in the mentioned experiment would depend only on relative positions of all the equipment in the experiment, is this correct?

This is not QED! Show me these "QED formulas".

The screen is irrelevant. I can, instead, replace the screen with an antenna that picks up the intensity of the photons at various locations. Same thing!

Zz.

 

[Born2bwire]

I understand that this applies to other particles including electrons as you are describing in superconductivity but here we are talking about photons only because photons and electrons are different classes of particles and mechanisms for their respective interferences may well be slightly different - is there a theory that proves that they must be the same?

The electrons operating in superconductivity behave the same way as photons. The Cooper pairs become bosons and interact just like photons. It is the effective change to bosons that gives rise to superconductivity.

Thanks, Bill, that's my assumption which is exactly what I am trying to falsify with the proposed experiment.
Could you please explain why it is not true?

Because that is not what the theory is stating. The idea that a path integral sums up the probability that a particle will follow a specific path is a mathematical abstraction, it is not the actual physics that are occurring. For example, path integrals can be used to define relativistic quantum theory (QFT), but the paths that would be included in the path integral are ones that break relativity by requiring speeds over c. This is not in conflict because we are not saying that the particle is actually moving across such a path. The path is something we cannot determine, we are merely saying that the mathematics looks like the combination of paths but that is not the actual physical description.

 

[bhobba]

Could you please explain why it is not true?

Well, to start with, the position of an unobserved particle, or any property for that matter, is not something a quantum object has.

QM is a theory about the results of measurements/observations (without going into exactly what such is which is a difficult issue requiring its own thread). When not observed the only property it has is this thing called a state which in the formalism is a lot like probabilities. This is what the formalism says - but we have all these interpretations as well that assume various things for various reasons. Trouble is there is no way to experimentally distinguish them so its anyone's guess which is correct.

I suspect you have read Feynman's beautiful QED book. In it he espouses the so called path integral approach. Rest assured mathematically its exactly the same as the usual approach me and Zapper have been talking about. Technically its what is known as a hidden variable theory (the path is the hidden variable) but of a very non trivial type. But that is just bye the bye - there is nothing in it that changes any of the stuff said in this thread - its just a different approach.

Thanks
Bill

 

[bhobba]

According to QED the appearance of interference pattern is not due to wave-particle duality and waves interfering with themselves - it is all about probability of finding a photon at a given place (which can only be determined by its absorption by an electron etc.), hence interference pattern is just a picture showing probability distribution given by our photons to the screen area. This probability distribution can be calculated according to QED formulas and in the mentioned experiment would depend only on relative positions of all the equipment in the experiment, is this correct?

QED is an example of what's known as a Quantum Field Theory (QFT). QFT is quantum principles applied to fields and is in fact a deeper theory than standard quantum mechanics (QM). It fixes up an issue with QM and relativity. In QM position is an observable, and time a parameter. But relativity tells us they should be treated on the same footing. QFT gets around this by making position and time both parameters, which more or less implies everything is a field. QED is the QFT of the electromagnetic field and electrons.

In fact many of the conceptual issues that people get confused with in QM are easier to grasp in QFT and a guy has written a nice little book about that:
https://www.amazon.com/dp/B004ULVG9O/?tag=pfamazon01-20

The Kindle version is dirt cheap and is probably worth getting a hold of.

But returning to some of the issues you raised, in bog standard QM the interference pattern is 'not due to wave-particle duality and waves interfering with themselves' but, just like you say 'it is all about probability of finding a photon at a given place'.

The technical detail of that assertion can be found here:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

This is one of the issues with the standard way QM is taught - they do it semi historically and use matter waves, wave particle duality and other stuff that preceded the full development of quantum mechanics to motivate the theory - but do not go back and show how that theory explains the stuff that motivated it. In 1927 Dirac came up with the Transformation Theory which is what generally goes under the name of QM today and did away with all that other stuff like wave particle duality and matter waves.

Much better to start with QM's conceptual core IMHO than that semi historical approach - less stuff to unlearn:
http://www.scottaaronson.com/democritus/lec9.html

Thanks
Bill

 

[Jilang]

Hi there. I am interested in your proposed experiment as the Feynman-Wheeler absorber theory would suggest that every particle "knows" in advance where it is going to end up. I proposed a similar experiment a few months ago that involved modulating the screen to compensate for the different path lengths. With regards to your original question I don't know if such an experiment has been done before, but this site might help. It's pretty mind-blowing!
http://www.science.gov/topicpages/d/double+slit+experiment.html#

 

[universecode]

Because that is not what the theory is stating. The idea that a path integral sums up the probability that a particle will follow a specific path is a mathematical abstraction, it is not the actual physics that are occurring. For example, path integrals can be used to define relativistic quantum theory (QFT), but the paths that would be included in the path integral are ones that break relativity by requiring speeds over c. This is not in conflict because we are not saying that the particle is actually moving across such a path. The path is something we cannot determine, we are merely saying that the mathematics looks like the combination of paths but that is not the actual physical description.

Thanks, well, if mathematics agrees with all experiments, why not use it as the basis point for deriving the reality?
What I am interested in is what known theory/experiment refutes the following assumptions that, at least in the case of photons:
a) at the point of emission a photon knows where all particles in the universe are;
b) it constructs a probability distribution assigning a probability to each such particle;
c) then it draws randomly from that probability distribution;
d) and then it gets absorbed by the particle drawn at the previous step after some time;
e) from the point of other observers there is a delay between emission and absorption which creates an illusion of space traveling photon because observers can change their own state meanwhile;
f) there is a universal clock (like in a classical computer) that allows photons to construct the above mentioned probability distribution based on relative positions of all particles and this clock is the basis for the speed of light limit.

I perfectly understand that these assumptions are quite far fetched, hence I am looking step by step for concrete evidence that can falsify them.

 

[universecode]

Hi there. I am interested in your proposed experiment as the Feynman-Wheeler absorber theory would suggest that every particle "knows" in advance where it is going to end up. I proposed a similar experiment a few months ago that involved modulating the screen to compensate for the different path lengths. With regards to your original question I don't know if such an experiment has been done before, but this site might help. It's pretty mind-blowing!
http://www.science.gov/topicpages/d/double+slit+experiment.html#

Thanks, I'll try to find and read your proposed experiments... just one thing - I don't assume that every particle knows in advance where it is going to end up - that would certainly disagree with experiments. My assumption is that at least photon (this may be slightly different for other particles) knows where all the particles are, constructs a probability distribution to the whole universe and ends up being absorbed by another particle randomly drawn from the constructed probability distribution.

 

[bhobba]

What I am interested in is what known theory/experiment refutes the following assumptions that, at least in the case of photons:
a) at the point of emission a photon knows where all particles in the universe are;

No known experiment has ever been able to show a particle has the property of position when not being observed. Thus its not possible for particles to know where other particles are since they do not have the property of being in a particular place.

QM does not assign any properties like position, momentum, etc, even the paths of the path integral approach, to a particle independent of observation. It only has this funny property called the state that encodes the statistical outcomes of observations, if you were to observe it.

Such is not commonsensical, and plenty of people have tried to evade it in some way. There are various conjectures like Bohmian Mechanics, but no one has ever been able to figure out a way to experimentally tell if such are true one way or another.

I mentioned that before, but for some reason you didn't get it.

I gave a link to the conceptual core of QM - did you read it? If so exactly what is your issue with it?

Thanks
Bill

 

[bhobba]

Thanks, well, if mathematics agrees with all experiments, why not use it as the basis point for deriving the reality?

Maybe that's because no one can agree what 'reality' is in the first place - think about it. But don't post about it here because its really philosophy which is off topic.

Physics is basically a mathematical model. Generally the mathematics is considered to describe reality, whatever that is. Some like Roger Penrose actually think the mathematics IS the reality. I used to think that until I saw an interesting talk by Murray Gell-Mann:


Thanks
Bill

 

[bhobba]

I perfectly understand that these assumptions are quite far fetched, hence I am looking step by step for concrete evidence that can falsify them.

Ok let's cut to the chase.

Can you derive the QM axioms from your assumptions.

To me its not that they are far fetched, its they, to me at least, don't even make sense. But if they do, and they are in some way valid you should be able to rigorously derive QM from them.

As an example of what's required here is such a derivation from the assumption of stochastic fluctuations at about the Plank scale:
http://arxiv.org/pdf/quant-ph/9508021.pdf

I seem to recall recall reading somewhere it had been disproved experimentally - which is exactly what you want ie theories subject to experimental verification.

Thanks
Bill

 

[universecode]

I gave a link to the conceptual core of QM - did you read it? If so exactly what is your issue with it?

Thanks, Bill, I haven't read yet any of the links provided but will do, so thank you for the suggestions.

I seem to recall recall reading somewhere it had been disproved experimentally - which is exactly what you want ie theories subject to experimental verification.

Is that not what I am trying to do?
I suggested a variation of the well known experiment to establish a very simple fact - whether photon "decides" which particle it will be absorbed by:
at hypothetical point of emission
or hypothetical point of absorption
or somewhere in between during its flight.
Can anyone give an answer what the result will be? or can someone tell me that such is experiment is impossible to perform as suggested?

 

[phinds]

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.

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.

 

[universecode]

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?

It would have to be done independently - is it not possible, even theoretically, to create a monochromatic source which emits a photon every time interval T+-tdelta, have synchronised clocks between the source and the barrier, the barrier located far enough so it would take more than 2*tdelta for a photon to reach the barrier and shift the barrier at intervals T?

How fast is that signal going to move?

That's the whole point - the experiment has to be designed such as no signals are used.

This, of course, is totally aside from the concept of moving the barrier in the amount of time you are talking about.

Sure enough, such experiment is technologically difficult but could it be done in theory?

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

It is certainly a thought experiment at this stage until we either find definite proof that it is impossible because it contradicts known experiments or reach the conclusion that it may be theoretically possible.

 

[bhobba]

Can anyone give an answer what the result will be? or can someone tell me that such is experiment is impossible to perform as suggested?

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.

Exactly - since photons travel at the speed of light, and any signal is limited to that speed its simply not possible.

Now if you would like to propose something with say electrons then it may be possible.

I will give it a bit of a think and see I come up with - but experimentation is not what I am into.

Thanks
Bill

 

[bhobba]

Ok - with electrons I have given it a bit of thought. How would you know the electron was emitted so you know when it has passed the barrier, so you can then move it? If you detect the electron when it's emitted you have localised it and it has an unknown momentum. You can't place a detector at the slit because that destroys the interference. The best you could do is move the slits at some frequency and see what happens. What I think would happen is the state of the electron just after it passes through the slits will determine the interference pattern. Any change before or after will not change anything. So basically you would get no interference pattern because the screen will be at a random position when the electron passes.

Added Later:

It just occurred to me you can have the electron emitter a long way back from the screen so that its momentum is known when it reaches the screen. That way you can detect when its emitted and hence ensure its past the screen when you move it.

The interference pattern you would get is where the slits were when it passed through the screen.

Thanks
Bill

 

[universecode]

The best you could do is move the slits at some frequency and see what happens.

Thanks, can the source and the slits be independently synchronised as I described for the photon experiment just before your post?

 

[bhobba]

Thanks, can the source and the slits be independently synchronised as I described for the photon experiment just before your post?

See what I added to my post. I think by having the emitter far enough back the electrons that reach the screen will have a definite momentum. The state of the electron would be such its position if measured could be anywhere across the beam - that's because it has a definite momentum, hence its position, if measured, is unknown. The slit will localise it so its state just after the slits will be a Dirac Delta functions where the slits were at the time it hit the slits, which you would know from when it was emitted and the known momentum. Moving the slits after it passed will make no difference - the pattern will be where the slits were when it passed through it.

Its analysis would be the same as the link I gave previously on the two slit experiment;
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

Added Later:

I meant before or after - not just after - sorry guys.

Thanks
Bill

 

[universecode]

See what I added to my post. I think by having the emitter far enough back the electrons that reach the screen will have a definite momentum. The state of the electron would be such its position if measured could be anywhere across the beam - that's because it has a definite momentum, hence its position, if measured, is unknown. The slit will localise it so its state just after the slits will be a Dirac Delta functions where the slits were at the time it hit the slits, which you would know from when it was emitted and the known momentum. Moving the slits after it passed will make no difference - the pattern will be where the slits were when it passed through it.

Apologies for perhaps not thinking about it properly before asking but what would be the point of moving the slits after the particle has passed through them?

 

[bhobba]

Apologies for perhaps not thinking about it properly before asking but what would be the point of moving the slits after the particle has passed through them?

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.

I meant before or after - you choose - sorry for not being clear.

Thanks
Bill

 

[Jilang]

universecode, I assume you have seen this before which is said to represent the path of the particles in the two slit experiment,
http://scienceblogs.com/principles/2011/06/03/watching-photons-interfere-obs/
If I am understanding the real intent of the proposed experiment it would be much simpler to modulate the screen (making it wavy if you like) to compensate for the path differences. I don't think this experiment has been performed. Would you expect the pattern to change?

 

[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.