Light without a source

  • #36
PeroK
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Wow, great. There is a lot to read and it will take some time. But I am just curious, what really happens in the process such as in Feynman diagrams? Is the answer is that we know how physically describe the whole process, and we build mathematical tools to calculate it but we do not know its true nature?
In physics generally there is no "true nature". We know what elementary processes take place: scattering, pair production etc. And we can experimentally measure their likelihood. Unless we find a way to probe deeper experimentally, then that is all we can say "really" happens.

We then construct a theory to model what we observe. This is modern particle physics, QFT etc. This theory produces complex integrals that can be expanded as a series of ever more complex terms.

In order to evaluate those terms Feynman invented a system of diagrams, essentially to make the calculations less difficult. One interpretation of those diagrams involves virtual particles.

Whether the model of virtual particle interactions is the "true nature" or only an aid to calculation is not a question physics can answer.

The Insights articles expand on this.

Personally, I believe we should remain agnostic about anything that is beyond our current ability to measure. And focus on what physics can tell us, rather than on what it can't.
 
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  • #37
wonderingchicken
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I actually hoping for non-mathematical explanations (since my blob inside the cranium can't understand overcomplicated mathematical descriptions) why vacuum fluctuations never actually happened in reality, but after reading Arnold's articles, I think that's what he actually meant. So, if I have to conclude, creation out of nothing such as particles popping out of nothing and vice versa never happened thus everything have existing sources in order to exist. Also, that also means before Big Bang there is no such thing as absolute nothing like what pop-sci said. There could be electrons and quarks, etc. before Big Bang.
 
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  • #38
bob012345
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I actually hoping for non-mathematical explanations (since my blob inside the cranium can't understand overcomplicated mathematical descriptions) why vacuum fluctuations never actually happened in reality, but after reading Arnold's articles, I think that's what he actually meant. So, if I have to conclude, creation out of nothing such as particles popping out of nothing and vice versa never happened thus everything have existing sources in order to exist. Also, that also means before Big Bang there is no such thing as absolute nothing like what pop-sci said. There could be electrons and quarks, etc. before Big Bang.
I wonder about the views expressed in these Insight articles by Arnold if it is basically the only view among the experts or are there others? It seems to suggest all these mathematical processes that appear like virtual processes are merely calculation tools to get the right answers because you don't know which physical process will happen so you sum up probabilities of various possibilities. If I understand it, QFT gives the average probability amplitude of going from one state to another over an infinite number of trials.

It also suggests that examples such as the Casimir effect and van der Waals forces are only due to the materials involves such as the plates for the Casimir effect and atoms and molecules for the van der Waals forces and nothing else. For example, if there are no vacuum fluctuations involved in the Casimir effect, it should be explainable solely on the grounds of interactions between the atoms of the plates and not on the bases of supposed excluded wavelengths of virtual EM radiation between the plates.
 
  • #39
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if there are no vacuum fluctuations involved in the Casimir effect, it should be explainable solely on the grounds of interactions between the atoms of the plates and not on the bases of supposed excluded wavelengths of virtual EM radiation between the plates.

And it is.

It seems to suggest all these mathematical processes that appear like virtual processes are merely calculation tools to get the right answers because you don't know which physical process will happen so you sum up probabilities of various possibilities.

Yes to the bolded part, but it's not that we don't know which of the processes depicted by Feynman diagrams will happen - we simply do not know what is happening during the "collision". We know only in and out states. I don't know if I recall correctly, but Feynman diagrams were invented a posteriori, after perturbation series was considered. Feynman diagrams are also used outside of QFT , in pure mathematical context of Greens functions and perturbation theory (@Orodruin wrote about it in his book).
 
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  • #40
bob012345
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And it is.
How is the Casimir effect explained without resorting to certain wavelengths of EM radiation not being allowed between the plates and there being a higher radiation pressure outside the plates? Can you provide a reference? Thanks.
 
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  • #41
weirdoguy
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First paper that comes to my mind is the one by @Demystifier:
https://arxiv.org/abs/1702.03291
Well, actually it's second, because with the first one I don't remeber authors name, and I can't find it on my computer o0)
Also, calculating the Casimir force is one of the exercises in Radovanović's Problem book in QFT and solution given by him is not reffering to virtual particles, or anything like that.

This issue has been discussed here on PF quite a number of times, so you can search for some threads about that too.
 
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  • #42
Vanadium 50
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But I think the virtual photons would include optical frequencies.
Virtual photons don't have frequencies.

Do you think this is clarifying things for the OP?
 
  • #45
hutchphd
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How is the Casimir effect explained without resorting to certain wavelengths of EM radiation not being allowed between the plates and there being a higher radiation pressure outside the plates?
I take issue with your logic here. Even if true, the fact that a particular somewhat obscure result has not been explained without a particular calculational artifice offers scant evidence in support of the larger question.
 
  • #46
bob012345
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I take issue with your logic here. Even if true, the fact that a particular somewhat obscure result has not been explained without a particular calculational artifice offers scant evidence in support of the larger question.
The Casimir effect has been studied for decades and is not really some obscure result. I have not seen it described without mentioning vacuum fluctuations. Now, here, people are saying it has nothing to do with the vacuum. So that was my question, what does it have to do with, not how can it not be vacuum fluctuations.
 
  • #47
bob012345
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Virtual photons don't have frequencies.

Do you think this is clarifying things for the OP?

If you want to move the Casimir discussion to a new thread or just not discuss it further in this one that is fine with me. I think the original OP question has been answered .
 
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  • #48
sophiecentaur
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One photon of a frequency is indistinguishable from a photon of the same frequency. Would there be any reason for wanting to identify where a photon came from - except when explaining 'unexplained' photons? That would have to be at the sharp end of experimental Physics when chasing some genuine anomalous results.

I have to question the reasoning behind many of these sorts of threads. They seem to be chasing something 'extra' before getting familiar with what's already been established. Sherlock Holmes got it right about first examining and rejecting all possibilities before considering something apparently impossible.
 
  • #49
hutchphd
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some obscure result.
Please quote me correctly. I said "somewhat obscure result". Apology accepted.
 
  • #50
bob012345
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Please quote me correctly. I said "somewhat obscure result". Apology accepted.
Retrocausal apology given.
 
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  • #52
Vanadium 50
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Bob is a good man. Bon Homme Jaffe.
 
  • #53
bob012345
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I have to question the reasoning behind many of these sorts of threads. They seem to be chasing something 'extra' before getting familiar with what's already been established. Sherlock Holmes got it right about first examining and rejecting all possibilities before considering something apparently impossible.
I think the process of examining and rejecting all possibilities is just a bit messier for some than others especially if one comes with a mind entangled in some mental wool that needs untangling first.
 
  • #54
sophiecentaur
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I think the process of examining and rejecting all possibilities is just a bit messier for some than others especially if one comes with a mind entangled in some mental wool that needs untangling first.
I have just made a comment elsewhere about the Q and A approach to self teaching. It's very easy to waste a lot of effort up blind alleys with Q and A. I very much doubt that you can untangle that mental wool on your own and put appropriate weighting to the hundreds of facts that are available.
You need to get used to taking stuff on board in the order that it's usually presented on formal courses - that method has been tried and tested.
 

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