# New version of double-slit experiment

1. Jun 10, 2014

### 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?

2. Jun 10, 2014

### ZapperZ

Staff Emeritus
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.

3. Jun 10, 2014

### universecode

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 doesnâ€™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 travelling 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?

4. Jun 10, 2014

### ZapperZ

Staff Emeritus
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.

5. Jun 10, 2014

### universecode

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?

Last edited: Jun 10, 2014
6. Jun 10, 2014

### ZapperZ

Staff Emeritus
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.

7. Jun 10, 2014

### Staff: Mentor

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

8. Jun 10, 2014

### Staff: Mentor

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.

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 [Broken]

Thanks
Bill

Last edited by a moderator: May 6, 2017
9. Jun 10, 2014

### universecode

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

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?

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?

10. Jun 10, 2014

### universecode

11. Jun 10, 2014

### ZapperZ

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

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

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.

12. Jun 10, 2014

### Born2bwire

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.

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 occuring. 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.

Last edited: Jun 10, 2014
13. Jun 10, 2014

### Staff: Mentor

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

Last edited: Jun 10, 2014
14. Jun 10, 2014

### Staff: Mentor

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/Fields-Color.../ref=tmm_kin_swatch_0?_encoding=UTF8&sr=&qid=

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

Last edited by a moderator: May 6, 2017
15. Jun 11, 2014

### 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#

16. Jun 11, 2014

### universecode

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 travelling 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.

17. Jun 11, 2014

### universecode

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.

18. Jun 11, 2014

### Staff: Mentor

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

19. Jun 11, 2014

### Staff: Mentor

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

Last edited by a moderator: Sep 25, 2014
20. Jun 11, 2014

### Staff: Mentor

Ok lets 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

Last edited: Jun 11, 2014
21. Jun 11, 2014

### universecode

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

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?

22. Jun 11, 2014

### phinds

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.

23. Jun 11, 2014

### universecode

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?

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

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

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.

24. Jun 11, 2014

### Staff: Mentor

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

Last edited: Jun 11, 2014
25. Jun 11, 2014

### Staff: Mentor

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 cant 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.