New version of double-slit experiment

[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.

 

[UltrafastPED]

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

Having read this far, I think you may enjoy this analytical/philosophical presentation:

"How Presentism Contradicts Relativity": http://users.ox.ac.uk/~lina0174/kansas.pdf

PS: Relativity wins!

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

In that case you should take a few courses in quantum physics and quantum mechanics. Then you can convince yourself. If you like hands-on work you can even plan and carry out experiments - perhaps even the one you originally proposed, more or less.

One caveat: when working with single photon sources it is not possible to know the precise emission time in advance; it is inherently a random process. Similar to radioactive decay, and for the same reasons.

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.

You can spend a life time studying quantum physics! What else could you possibly want to do with your life?

 

[DrChinese]

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

Just one question: how have you survived this long?



You don't have time to read the precise explanations in FAQs and references folks are providing you, but want someone to spoon feed you material that you will regurgitate anyway. Sounds like a good deal to me.

 

[Nugatory]

You can spend a life time studying quantum physics! What else could you possibly want to do with your life?

Well, there's always general relativity... That's kinda cool too.

 

[mfb]

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?

I think you take Feynman diagrams too literal. But it does not matter here. Have a look at this diagram (from here): a photon gets destroyed, and a new photon appears - in pure vacuum!

So if you like that picture of "photon destruction and re-emission" in lenses, your proposed experiment is completely impossible, even in a perfect vacuum.

 

[universecode]

I think you take Feynman diagrams too literal. But it does not matter here. Have a look at this diagram (from here): a photon gets destroyed, and a new photon appears - in pure vacuum!

So if you like that picture of "photon destruction and re-emission" in lenses, your proposed experiment is completely impossible, even in a perfect vacuum.

Thank you, mfb, finally someone understand what I am talking about, and thank you for the links - very useful.
I knew general idea about photon disappearing-appearing in a vacuum, although this is what I'll be spending more time learning about... I just hope it is a relatively rare event and shouldn't have significant effect on the probabilities we are trying to measure, what do you think?

By the way, have you read my second version of the experiment without the double-slit, any views on that?

 

[mfb]

I just hope it is a relatively rare event

It is not a rare event, it is happening "all the time". It is a fundamental part of what we call "photon". You cannot have photons without this.
And as I said, you take those diagrams too literally. It is pointless to try to say "see, this happened here". Feynman diagrams are a way to visualize calculations in perturbation theory. They are not the actual physics, and some processes are even impossible to describe with Feynman diagrams.

By the way, have you read my second version of the experiment without the double-slit, any views on that?

Changing the experimental setup doesn't change the physics behind all that.

 

[ZapperZ]

It is not a rare event, it is happening "all the time". It is a fundamental part of what we call "photon". You cannot have photons without this.
And as I said, you take those diagrams too literally. It is pointless to try to say "see, this happened here". Feynman diagrams are a way to visualize calculations in perturbation theory. They are not the actual physics, and some processes are even impossible to describe with Feynman diagrams.

This is why this type of discussion is very frustrating, and why I asked the OP why he thinks referring to such a thing is relevant to what was being discussed. It is a common problem when parts of physics are taken out of context without any understanding on what they mean.

The fact that Feynman diagram is a means to visualize a mathematical procecure is often missed or overlooked by many who do not understand QFT.

Zz.

 

[DrChinese]

By the way, have you read my second version of the experiment without the double-slit, any views on that?

Previously answered, and answer ignored... again.

 

[universecode]

And as I said, you take those diagrams too literally.

Thanks, please understand that my objective is not to become an expert in some theory to get a job or publish generic papers etc.

Has there ever been a high level discussion arguing why taking Feynman diagrams literally is a terrible idea not worth pursuing under any circumstances?
I would like to read those and I'd appreciate if someone could point to any such discussion.

 

[mfb]

Thanks, please understand that my objective is not to become an expert in some theory to get a job or publish generic papers etc.

That's fine. Just keep in mind that several users in this thread are physicists and publish papers.

Has there ever been a high level discussion arguing why taking Feynman diagrams literally is a terrible idea not worth pursuing under any circumstances?

It just does not work. If you start with the actual physics (the calculations), this is directly obvious. If you start with Feynman diagrams, it is hard to forget that wrong idea.

Consider this, for example: for every possible process, if you have any chance to draw Feynman diagrams, you have an infinite number of Feynman diagrams. And all of them contribute to every process. So how do you find out "which diagram happened"? You cannot. Not even in principle, because "this diagram happened" is not a meaningful concept.

 

[universecode]

That's fine. Just keep in mind that several users in this thread are physicists and publish papers.

Apologies if I offended anyone somehow, didn't mean to.

It just does not work. If you start with the actual physics (the calculations), this is directly obvious. If you start with Feynman diagrams, it is hard to forget that wrong idea.

Consider this, for example: for every possible process, if you have any chance to draw Feynman diagrams, you have an infinite number of Feynman diagrams. And all of them contribute to every process. So how do you find out "which diagram happened"? You cannot. Not even in principle, because "this diagram happened" is not a meaningful concept.

Well, sure, isn't that the problem when you assume that outcome is created in the act the measurement? At which point you will have infinite number of past histories, infinite number of diagrams to consider etc.

What if you assume what I am postulating that outcome is created at the act of emission or each next junction is decided at the previous junction on the diagram?
Past history becomes irrelevant at each junction. Am I completely off the mark here?

 

[Nugatory]

What if you assume what I am postulating that outcome is created at the act of emission or each next junction is decided at the previous junction on the diagram?
... Am I completely off the mark here?

That seems like a logical enough starting point. It's intuitive; it's consistent with our experience with classical particles, even very small ones; and it's consistent with much of the semi-classical thinking (for example, the Bohr atom and the photoelectric effect) that preceded the formal development of quantum mechanics.

It's not easy (impossible in principle?) to falsify this line of thinking with experiments that involve only one particle at a time.

However, that line of thinking also suggests that if two photons are emitted in the same event, then the results of a measurement of either photon will be determined at the emission event. That experiment has been done many times, and the results are unequivocal: there is no way to describe the subsequent behavior of both particles based on their state at the time of emission.

Google for "Bell's Theorem", "EPR paradox", and "Alain Aspect".

 

[universecode]

However, that line of thinking also suggests that if two photons are emitted in the same event, then the results of a measurement of either photon will be determined at the emission event.

Thanks, may I just clarify that this event would always imply entanglement as we know it at present?

 

[Nugatory]

Thanks, may I just clarify that this event would always imply entanglement as we know it at present?

If we both understand the question that you're asking, then the answer is "yes".

But it would be well to remember that any time you hear someone speak of "two entangled particles" they're playing a bit fast and loose with the language. It would be more accurate to speak of "a single quantum system with two properties that we can measure".

 

[universecode]

If we both understand the question that you're asking, then the answer is "yes".

But it would be well to remember that any time you hear someone speak of "two entangled particles" they're playing a bit fast and loose with the language. It would be more accurate to speak of "a single quantum system with two properties that we can measure".

Exactly, thank you, I am going to do some more thinking about the Bell's theorem from another angle...

 

[DrChinese]

1. Has there ever been a high level discussion arguing why taking Feynman diagrams literally is a terrible idea not worth pursuing under any circumstances?

Nugatory already mentioned, but I will give you a counter-example to Feynman diagram. There is not one that explains how particle entanglement works. That is because such diagrams are local realistic, but QM as a whole is not.

2. Past history becomes irrelevant at each junction. Am I completely off the mark here?

Yes, off the mark. That does not actually follow from QM either. An example is entanglement swapping, in which case the decision to entangle 2 particles can be made AFTER the particles are detected. If you follow the space time diagram, both past and future are relevant to the description and effects are best described as flowing in both forward and backward time directions. (You might expect this because entanglement is a counterexample in 1.)

I do think your decision to look at Bell's Theorem from more angles is a good one. (Yes a totally predictable comment... )

 

[universecode]

I do think your decision to look at Bell's Theorem from more angles is a good one. (Yes a totally predictable comment... )

Thanks, your comments are finally useful to my objective.
Anyway, I just read your version of Bell's theorem proof to make sure we are talking about the same thing.
Are you absolutely certain about your version?

Let's have a look at the first and the main assumption:

a. ASSUME that a photon has 3 simultaneously real Hidden Variables A, B and C at the angles 0 degrees, 120 degrees and 240 degrees per the diagram above. These 3 Hidden Variables, if they exist, would correspond to simultaneous elements of reality associated with the photon's measurable polarization attributes at measurement settings A, B and C. In other words, each hidden variable gives us the answer to the question "will this photon pass through a polarizer lens set at a specific angle?" Presumably, this information is SOMEHOW encoded definitely in the photon at the time it is created (sort of like an instruction set) and does NOT depend in any way on the polarizer lens itself (which is encountered at a later time).

Well, this does not apply to my postulates! Nothing in my proposal would contradict Bell's theorem conclusions because I do not assume hidden variables as defined by Bell.
I thought I was very clear in explaining my point but I guess not, so let me repeat it:

a) at the point of emission (i.e., at each junction on Feynman's diagram) a photon knows where all particles in the universe are (whatever "location" means, if anything);
b) it constructs a probability distribution on the whole of the universe assigning a probability to each such particle;
c) it draws randomly from that constructed probability distribution;
d) and then it gets absorbed by the particle drawn at the previous step after some time.

The key point here is b) - when decision is made at the point of emission it incorporates the knowledge of the existence of future detector at that moment.
State of the particle in my proposal is obviously dependent on measurement and does not exist without it - the key difference is that it is created at emission not at the measurement itself.

 

[Nugatory]

Well, this does not apply to my postulates! Nothing in my proposal would contradict Bell's theorem conclusions because I do not assume hidden variables as defined by Bell.
...
The key point here is b) - when decision is made at the point of emission it incorporates the knowledge of the existence of future detector at that moment.
State of the particle in my proposal is obviously dependent on measurement and does not exist without it - the key difference is that it is created at emission not at the measurement itself.

So you've just been talking about a transactional interpretation all along? The interpretations that propose that the state of a system can be affected by detector settings that haven't yet been made work just fine. As with any other interpretation, they cannot be falsified by experiments because they make no predictions that differ from the predictions of any other interpretation.

 

[universecode]

So you've just been talking about a transactional interpretation all along? The interpretations that propose that the state of a system can be affected by detector settings that haven't yet been made work just fine. As with any other interpretation, they cannot be falsified by experiments because they make no predictions that differ from the predictions of any other interpretation.

Thanks, if that's what it is then I'll try to find and read what others have said about it already...

Anyway, I think it does make different predictions, at least theoretically.
Consider single photon source and two detectors as explained in my latest version of the experiment.
As far as I understand, theories predict that probability of hitting those detectors will be different dependent on their location with respect to the source, if there is absolutely nothing else in the vicinity of the experiment.

So, if we somehow could fool the nature a bit by dislocating a detector during the photon's "flight" theories would predict different probability of hitting such detector, is this correct?
I predict the probability would be the same as if nothing changed and the detector has stayed where it was all the time required for the photon to reach the detector from the source.

Obviously, as it has already been mentioned, even if this is in a vacuum with nothing around, the photon could be appearing and disappearing or interacting with vacuum infinite number of times hence adjusting its outcome according the detector dislocation.
But I would like to think this is not what's happening for the following reason. If the same experiment is left on its own with the source being switched off, would detectors keep registering photons appearing from nowhere? If all setup correctly - probably not. Hence even if there are photons appearing from vacuum they would not be relevant to our experiment.

 

[Nugatory]

So, if we somehow could fool the nature a bit by dislocating a detector during the photon's "flight" theories would predict different probability of hitting such detector, is this correct?

no. The only thing that matters is the position of the detector when the interaction happens, not the way by which the detector came to be in that position or how it moved around before the interaction.

 

[universecode]

no. The only thing that matters is the position of the detector when the interaction happens, not the way by which the detector came to be in that position or how it moved around before the interaction.

Is there a definite proof of such fact? I'd like to analyse how the conclusion was made.

 

[universecode]

If what I am postulating is even remotely correct, it would explain something that is truly mind-boggling - the true origin of the ubiquitous Normal Distribution.

It is well known that the mean of two or more random variables independently drawn from the same distribution is distributed approximately normally, irrespective of the form of the original distribution. If a distribution is constructed on the whole universe at each emission and the next step/event is decided by randomly drawing from it then it is natural that what we are observing is the mean of these processes which is found to be Normal.

Can other theories predict this?

 

[Nugatory]

Is there a definite proof of such fact? I'd like to analyse how the conclusion was made.

Bell experiments have been done with the detector angles selected and set after the emission event. The results are as predicted by QM and the same as if the detector angle was set before the emission event.

 

[universecode]

Bell experiments have been done with the detector angles selected and set after the emission event. The results are as predicted by QM and the same as if the detector angle was set before the emission event.

Thanks, have you got a link to this particular experiment, so I could analyse it in details, please?

 

[DrClaude]

Thanks, have you got a link to this particular experiment, so I could analyse it in details, please?

For instance:
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.81.5039 (if you don't have access, see http://arxiv.org/abs/quant-ph/9810080)

 

[DrChinese]

Well, this does not apply to my postulates! Nothing in my proposal would contradict Bell's theorem conclusions because I do not assume hidden variables as defined by Bell.
I thought I was very clear in explaining my point but I guess not, so let me repeat it:

a) at the point of emission (i.e., at each junction on Feynman's diagram) a photon knows where all particles in the universe are (whatever "location" means, if anything);
b) it constructs a probability distribution on the whole of the universe assigning a probability to each such particle;
c) it draws randomly from that constructed probability distribution;
d) and then it gets absorbed by the particle drawn at the previous step after some time.

The key point here is b) - when decision is made at the point of emission it incorporates the knowledge of the existence of future detector at that moment.
State of the particle in my proposal is obviously dependent on measurement and does not exist without it - the key difference is that it is created at emission not at the measurement itself.

No problem, this is consistent with a number of existing QM interpretations. Exactly as we have said in this thread any number of times. Such interpretations are not local realistic.

See post #43 for example. And #55.

 

[universecode]

Such interpretations are not local realistic.

Thanks, is it good, bad or what does it imply that I may not be aware of?

See post #43 for example. And #55.

I wish I could figure out what I should be looking at from those posts - could you please be more specific with precise arguments that relate to my line of thinking?

 

[DrChinese]

1. Thanks, is it good, bad or what does it imply that I may not be aware of?

I wish I could figure out what I should be looking at from those posts - could you please be more specific with precise arguments that relate to my line of thinking?

I don't know how to be more specific than the post itself:

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.

So your "hypothesis" matches those made by others in various interpretations which are known to not run afoul of Bell. Because to be viable, no interpretation can be "local realistic. So your ideas are fine. And as referenced by DrClaude and mentioned by Nugatory and myself, the location of the target doesn't matter except as of the time of detection.

 

[universecode]

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.

Thanks for repeating, but I still don't get what you are talking about.

Let's go step by step with simple true or false answers to avoid confusion.

Assume you have a universe (which is the same as ours but there is very little in it) and it contains only a source of photons and few detectors in random places. The source emits photons one by one which are detected by the detectors. Do you agree that (generally, apart from special equivalent locations) every detector has different probability of observing photons emitted from this source?

 

[Nugatory]

Do you agree that (generally, apart from special equivalent locations) every detector has different probability of observing photons emitted from this source?

Yes, of course. That's what QM predicts in all interpretations, that's also the classical prediction for the intensity of light, and it's a result that all experiments so far have confirmed (although of course the experiments cannot be done in this hypothetical completely isolated universe - instead we calculate and validate upper bounds on the errors introduced by not being completely isolated).

 

[universecode]

Yes, of course. That's what QM predicts in all interpretations, that's also the classical prediction for the intensity of light, and it's a result that all experiments so far have confirmed (although of course the experiments cannot be done in this hypothetical completely isolated universe - instead we calculate and validate upper bounds on the errors introduced by not being completely isolated).

Thanks, good, let's continue with our hypothetical universe and shuffle all the detectors, so they are all in different locations now. The source stays the same and continues emitting photons.
Will each detector have the same probability of detecting photons as it had before?

 

[DrChinese]

Thanks, good, let's continue with our hypothetical universe and shuffle all the detectors, so they are all in different locations now. The source stays the same and continues emitting photons.
Will each detector have the same probability of detecting photons as it had before?

Generally, the probabilities will change.

 

[universecode]

Generally, the probabilities will change.

Thanks.
Nugatory, would you confirm this answer?
You seem to be quite good at elaborating your arguments, so I just want to seal this answer before proceeding further.

 

[Nugatory]

Thanks.
Nugatory, would you confirm this answer?
You seem to be quite good at elaborating your arguments, so I just want to seal this answer before proceeding further.

So far so good...

 

[universecode]

Good, now if we return all detectors to where they were originally, will they again start observing photons each with the same probability they respectively had originally?

 

[Nugatory]

Good, now if we return all detectors to where they were originally, will they again start observing photons each with the same probability they respectively had originally?

If we've managed not to mess something up in all the moving around, yes. So far we're in accord with all interpretations of quantum mechanics and also the classical prediction for particles other than photons (and the only reason I make that exception is that there is no classical prediction for the behavior of photons).

 

[universecode]

If we've managed not to mess something up in all the moving around, yes. So far we're in accord with all interpretations of quantum mechanics and also the classical prediction for particles other than photons (and the only reason I make that exception is that there is no classical prediction for the behavior of photons).

Thanks, suppose all our detectors are around 1 light-hour away from the source and in both cases we have been observing a photon registered by one of the detectors consistently every 2 hours plus-minus 10 minutes. Would such observation contradict anything?

 

[Nugatory]

Thanks, suppose all our detectors are around 1 light-hour away from the source and in both cases we have been observing a photon registered by one of the detectors consistently every 2 hours plus-minus 10 minutes. Would such observation contradict anything?

By "both cases" you mean before we moved the detectors and after we put them back?
Not yet, as long as the source and detectors are at rest relative to one another, "around 1 light-hour away" means that the differences in the distances are small compared with ten light-minutes, "consistently" means that we ran long enough to get statistically significant results, and probably some other reasonableness assumptions that I've missed.

The non-Poisson distribution of the arrivals would be a very strong hint that that someone was turning the light on and off back at the source.

 

[DrChinese]

Thanks, suppose all our detectors are around 1 light-hour away from the source and in both cases we have been observing a photon registered by one of the detectors consistently every 2 hours plus-minus 10 minutes. Would such observation contradict anything?

We are assuming that the setup leads us to this consistent observation for the sake of the example.

 

[universecode]

By "both cases" you mean before we moved the detectors and after we put them back?

Yes

Not yet, as long as the source and detectors are at rest relative to one another, "around 1 light-hour away" means that the differences in the distances are small compared with ten light-minutes, "consistently" means that we ran long enough to get statistically significant results, and probably some other reasonableness assumptions that I've missed.

The non-Poisson distribution of the arrivals would be a very strong hint that that someone was turning the light on and off back at the source.

Thank you, excellent explanation with right assumptions.

Now, may I confirm something that I think will be very useful for anyone who might decide to read this so they can clearly understand what's going here.

The fact of switching the source on/off is completely independent from the probabilities eventually observed at each detector. Regardless of how diabolical the operator decides to behave the probabilities after sufficiently long run will tend to be the same as if there were no operator and the source was just controlled by a timer which switches the source on every two hours and then off 10 minutes after switching on?