Possibilty of flavor oscillation of an electron

  • #26
vanhees71
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Of coarse it's interference. It's however important to get a picture which is correct in all aspects of the phenomenon. QFT resolves also the problem with energy-momentum conservation. Thevsimplified intro-textbook are not really wrong but also not entirely right, and it leads to confusion sometimes. So it's good to havd a completely correct QFT description though it's not a simple one, but I'd not consider neutrino oscillations a simple topic anyway.
 
  • #27
Of coarse it's interference. It's however important to get a picture which is correct in all aspects of the phenomenon. QFT resolves also the problem with energy-momentum conservation. Thevsimplified intro-textbook are not really wrong but also not entirely right, and it leads to confusion sometimes. So it's good to havd a completely correct QFT description though it's not a simple one, but I'd not consider neutrino oscillations a simple topic anyway.
Energy / momentum is not conserverd in QFT for small time intervals.
 
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  • #28
PeroK
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Energy / momentum is not conserverd in QFT for small time intervals.
The Feynman diagrams have energy-momentum conservation built in (even for virtual particles). The virtual particles do not, however, stay on their mass shell.
 
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  • #29
The Feynman diagrams have energy-momentum conservation built in (even for virtual particles). The virtual particles do not, however, stay on their mass shell.
According to the Heisenberg Uncertainty energy can appear as long as it dissapears quickly.That means if we have defined the momentum with great precision and have a great uncertainty of the position of the particle , momentum can come from nowhere as long as it dissapears quickly.Quantum field theory is based on Quantum Mechanics and in Unrelativistic Quantum Mechanics those things can happen , why not in QFT?
 
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  • #30
PeroK
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According to the Heisenberg Uncertainty energy can appear as long as it dissapears quickly.
That is essentialy just a popular science sound-bite, which has no meaningful relation to the real physics - and very much out of place in an A-level thread.
 
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  • #31
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That is essentialy just a popular science sound-bite
And if you want to clear up some others misconceptions about quantum mechanics, this article (by our @Demystifier as I recall properly?) https://arxiv.org/abs/quant-ph/0609163 is very good for that purpose. Section 3 is devoted to energy-time uncertainty, that's why this paper came to my mind o0)
 
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  • #32
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According to the Heisenberg Uncertainty energy can appear as long as it dissapears quickly.
The uncertainty relations make no such statement. There is no process that violates energy or momentum conservation - not even for a short time. This is true for all types of quantum mechanics - nonrelativistic, relativistic and QFT.
 
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  • #33
The uncertainty relations make no such statement. There is no process that violates energy or momentum conservation - not even for a short time. This is true for all types of quantum mechanics - nonrelativistic, relativistic and QFT.
3:25 and watch it for 10 seconds.
 
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  • #34
PeroK
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3:25 and watch it for 10 seconds.
This is what's called "popular" science, where science is presented in a simplified form for those not willing or able to tackle university level text books, or academic papers on the subject.

This video is aimed at a audience that is interested in science but not actively studying it at university level.
 
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  • #35
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@Helena Wells can you show us a proper textbook or a peer reviewed paper that claims the same thing?
 
  • #37
Since this comes from the official youtube channel of Fermilab which has the 2nd biggest particle accelerator in the world and dr.Don Lincoln is an experimental physicists there I don't think they would post something wrong.
 
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  • #38
PeroK
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Since this comes from the official youtube channel of Fermilab which has the 2nd biggest particle accelerator in the world and dr.Don Lincoln is an experimental physicists there I don't think they would post something wrong.
It's not so much wrong, as a simplified version designed to try to explain a more complicated subject to a audience of non-physicists. In any case, such videos are not considered valid sources on PF for precisely this reason. They do not represent a valid academic presentation of the subject.
 
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  • #39
George Jones
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Since this comes from the official youtube channel of Fermilab which has the 2nd biggest particle accelerator in the world and dr.Don Lincoln is an experimental physicists there I don't think they would post something wrong.
Read carefully the except below from the text "Intoduction to Elementary Particles" by David Griffiths, who is also the author of the quantum mechanics text referenced by @PeroK in post #36.

Griffiths P1.2.jpg
 
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  • #40
PeroK
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And, to be fair to Dr Don Lincoln, when he invokes the borrowing of energy in the above video, he does accompany what he says with a slightly apologetic shrug! As if to say, "well, that's good enough for YouTube".
 
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  • #41
Ok how do you explain that photons can be massive in QED for short timespans and while the time is passing they are more likely to become massless.
 
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  • #42
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And where are you getting this from?
 
  • #44
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Well, as been said, this video is not a valid source on PF since it is a pop-sci video. I've never seen anything like that in any textbook on QED or QFT, so it would be nice if you could provide some other source of this claim (textbook or peer reviewed paper).
 
  • #45
Well, as been said, this video is not a valid source on PF since it is a pop-sci video. I've never seen anything like that in any textbook on QED or QFT, so it would be nice if you could provide some other source of this claim (textbook or peer reviewed paper).
Well ok you can google it right now 'Mass of photons in QED'.
 
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  • #46
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That's too broad and will bring up tons of pages not related to what you speciffically wrote. And what you wrote is not something that one would find in most textbooks, that is why I ask you if you have other sources of this claim. This video is not an acceptable source, and you've already been said why. If you want to learn something you should study textbooks, not fun videos on YouTube, even if they were recorded by someone from FemiLab. Even figures like Hawking wrote some very questionable things in their pop-sci books. That's why PF has this policy regarding sources. Argument from authority does not count.
 
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  • #47
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@Helena Wells: Stop this. You are using popular science descriptions on youtube and suggestions for google searches as "source" to argue with physicists - including particle physicists relying on the exact conservation of energy in their daily job. This isn't going to work.
 
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  • #48
vanhees71
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Since this comes from the official youtube channel of Fermilab which has the 2nd biggest particle accelerator in the world and dr.Don Lincoln is an experimental physicists there I don't think they would post something wrong.
"One should present physics as simple as possible, but not simpler." (free quote by Einstein). Sometimes there are legends built by popular-science writers that violate this rule, anc unfortunately the wrong legends are used again and again in the lack of a better idea. Taking "virtual particles" as real or reading Feynman diagrams as something different than an utmost efficient notation of formulas to calculate S-matrix elements is among them. I don't blame the scientists in there attempt to explain physics to the general public though. I've also no idea how to explain things without the only language we know, which is math. But still one should explain the science to the general public.
 
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  • #49
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Energy / momentum is not conserverd in QFT for small time intervals.
As previously pointed out by others QFT doesn't admit such "small time" breakings of energy-momentum conservation. I particularly find the pop-science to handle this quite silly and complicating the actual situation. While it is true that the actual reason the exact conservation is kept requires certain amount of mathematical sophistication I think it can be explained before entering more complicated perturbative situations in the free complex scalar field case given that anything concerning this conservation is sorted out in QFT even before interaction and the perturbative methods enter(and this indeed makes the math much more complicated).

So for the propagation of these quantized free fields that are going to serve later to build the classical limit(what particle physicists sometimes refer to as the tree level) in the full theory, there is what the author of a well knows QFT textbook calls a "beautiful Lorentz-invariant object", the Feynman propagator and this object involves a certain integral and this integral(written in one of the ways it can be represented) converges only in the limit in which a certain parameter #\epsilon#, that we can think of for this didactic purpose as the "small time intervals" you found in popular divulgative media, goes to zero.

So this keeps exact energy-momentum conservation already even if by itself it appears to be a "non-local" expression because it involves certain quantum tunneling properties, but in this limit this is exactly canceled out by oppositely charged fields in the relevant quantum commutation relations so the final result is fine. Of course all this gets more complicated with interaction and radiative corrections but the basic block are still the free fields and the radiative corrections at higher order are built from the tree level so this picture of exact conservation should hold.
 
  • #50
hilbert2
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There's this Arxiv document about the possibility of charged leptons converting to one another:

https://arxiv.org/pdf/0706.1216.pdf

There it's said that that the electron, muon and tau are the exact charged lepton mass eigenstates. But how do you justify this claim? Is there a possibility of even a little bit of mixing of those species in the mass eigenstates, or does something crazy result from the electroweak theory if you assume that?
 

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