Why can't we detect virtual photons?

In summary: By looking at the final state particles and their properties, you can infer the momentum transferred by the virtual photon. However, virtual particles cannot be directly observed or measured. In summary, virtual photons are defined as force carriers between two fermions and can be calculated and measured through their interactions with other particles. However, they cannot be directly detected or measured and the concept of wave-particle duality does not apply to them. Virtual particles are simply a mathematical tool used in quantum field theory.
  • #1
Sophrosyne
128
21
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.
 
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  • #4
Sophrosyne said:
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.
Sophrosyne said:
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.
Sophrosyne said:
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
MathematicalPhysicist said:
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
Sophrosyne said:
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
mfb said:
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 possesses wave-particle duality which is observable. Emission and absorption of waves takes place in quanta that possesses defined energy, momentum and spin.
Now, "virtual particles" are based on conjecture that nonpropagating fields also possesses 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
snorkack said:
Now, "virtual particles" are based on conjecture that nonpropagating fields also possesses 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
snorkack said:
How do you observe the momentum?
You observe the momentum before and after the interaction.
 

1. Why can't we detect virtual photons?

Virtual photons are particles that exist for a very short period of time and are constantly popping in and out of existence. They do not have a physical presence and cannot be directly measured or detected by instruments.

2. Can we see virtual photons?

No, we cannot see virtual photons because they are not physical particles and do not interact with light or other forms of electromagnetic radiation.

3. How do we know virtual photons exist if we can't detect them?

We know virtual photons exist based on theoretical models and calculations in quantum field theory. These models have been extensively tested and accurately predict various phenomena, including the behavior of virtual photons.

4. Why do virtual photons play a role in particle interactions?

Virtual photons play a role in particle interactions because they are carriers of the electromagnetic force. They are constantly being exchanged between particles, mediating interactions between charged particles.

5. Can virtual photons be observed indirectly?

Yes, virtual photons can be observed indirectly through their effects on measurable quantities such as the electromagnetic field. However, they themselves cannot be directly observed.

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