# I Why can't we detect virtual photons?

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1. Apr 29, 2017

### Sophrosyne

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.

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.

2. Apr 30, 2017

### weirdoguy

3. Apr 30, 2017

### MathematicalPhysicist

4. Apr 30, 2017

### Staff: Mentor

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.
You can observe the momentum transferred, but assigning a wavelength to virtual photons doesn't make sense.
No, in the same way you cannot catch the concept of the number 3, or other mathematical tools.

5. Apr 30, 2017

### weirdoguy

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. Apr 30, 2017

### ChrisVer

give the system enough energy [or better said add matterial] and you will end up creating real photons [eg via bremsstrahlung]

7. May 1, 2017

### snorkack

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. May 1, 2017

### weirdoguy

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: