Why can't we detect virtual photons?

  • #1
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I have heard a virtual phton as used in QED defined as being forced carriers between two fermions which last for very short periods of time.

A couple of questions about this:

1) how do we know these are photons and not some other force carrier if we cannot detect them directly?

2) can a frequency or wavelength be calculated or measured for these photons? Does it depend on, for example, how fast two electrons are approaching each other and therefore the energy required to push them apart?

3) If we put a detector between two magnets, or two charged particles, can there be any chance of catching these photon? For example, if they are interacting in the x-ray range, would putting an x-ray film between two magnets catch some of the four times going back-and-forth between these two magnets?

Thanks.
 

Answers and Replies

  • #4
34,685
10,839
1) how do we know these are photons and not some other force carrier if we cannot detect them directly?
We cannot. But we can calculate the amplitudes of photons, Z and Higgs, and at low energy the photon is by far the most important contribution. Around 90 GeV, the Z is more important, and for heavier particles, the Higgs can be a relevant contribution.
2) can a frequency or wavelength be calculated or measured for these photons?
You can observe the momentum transferred, but assigning a wavelength to virtual photons doesn't make sense.
3) If we put a detector between two magnets, or two charged particles, can there be any chance of catching these photon?
No, in the same way you cannot catch the concept of the number 3, or other mathematical tools.
 
  • #5
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Do real point-particles exist?
Well, there exists "something" and we can measure properties of that "something". Also, we have a mathematical model that predicts the outcomes of that measurements with very high accuracy. We call that "something" a particle. And just beacuse people who worked on that model called part of it (internal lines of Feynman diagrams) "virtual particles" doesn't mean we should treat it like that "something" that we take measurements of. It's obvious if you know how the model (QFT) is constructed. And there are also nonperturbative approaches where "virtual particles" don't even appear.
 
  • #6
ChrisVer
Gold Member
3,347
448
3) If we put a detector between two magnets, or two charged particles, can there be any chance of catching these photon? For example, if they are interacting in the x-ray range, would putting an x-ray film between two magnets catch some of the four times going back-and-forth between these two magnets?
give the system enough energy [or better said add matterial] and you will end up creating real photons [eg via bremsstrahlung]
 
  • #7
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You can observe the momentum transferred, but assigning a wavelength to virtual photons doesn't make sense.
How do you observe the momentum?
Look at it this way: electromagnetic field can be classified as electromagnetic waves, which propagate free of their source, and nonpropagating electromagnetic fields stuck to their sources.
Electromagnetic waves possess wave-particle duality which is observable. Emission and absorption of waves takes place in quanta that possess defined energy, momentum and spin.
Now, "virtual particles" are based on conjecture that nonpropagating fields also possess wave-particle duality and consist of "virtual particles", like propagating fields consist of real particles.
But which features of nonpropagating fields actually display quantization?
 
  • #8
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Now, "virtual particles" are based on conjecture that nonpropagating fields also possess wave-particle duality and consist of "virtual particles"
Wave-particle duality is an outdated concept and it has been discussed here many times, eg. here:
https://www.physicsforums.com/threads/why-does-wave-particle-duality-not-exist-anymore.910647/ Modern quantum theories (developed after 1924) don't need notion of any particle-wave duality. Virtual particles and what they are also has been discussed many times. Here you have 16 pages of discussion:
https://www.physicsforums.com/threads/misconceptions-about-virtual-particles-comments.865706/
 
  • #9
34,685
10,839
How do you observe the momentum?
You observe the momentum before and after the interaction.
 

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