Why can't we detect virtual photons?

  • Context: Undergrad 
  • Thread starter Thread starter Sophrosyne
  • Start date Start date
  • Tags Tags
    Photons Qed Virtual
Click For Summary

Discussion Overview

The discussion revolves around the nature of virtual photons in quantum electrodynamics (QED), exploring their characteristics, detection, and the implications of their existence as force carriers between fermions. Participants raise questions about the ability to measure properties such as frequency or wavelength, the possibility of detecting virtual photons, and the conceptual understanding of particles in quantum field theory.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question how virtual photons can be identified as photons if they cannot be detected directly.
  • Others argue that virtual particles do not exist as physical entities but are merely mathematical tools used in calculations.
  • One participant suggests that while the momentum transferred can be observed, assigning a wavelength to virtual photons does not make sense.
  • Another participant mentions that under certain conditions, such as adding energy or material, real photons can be created from interactions, like bremsstrahlung.
  • There is a discussion about the classification of electromagnetic fields and whether nonpropagating fields exhibit wave-particle duality similar to propagating fields.
  • Some participants assert that wave-particle duality is an outdated concept and that modern quantum theories do not require this notion.
  • Questions are raised about how momentum is observed in interactions, with some stating it can be measured before and after the interaction.

Areas of Agreement / Disagreement

Participants express differing views on the existence and nature of virtual photons, with some asserting they are non-existent while others discuss their implications in theoretical frameworks. The discussion remains unresolved with multiple competing perspectives on the topic.

Contextual Notes

Limitations include the dependence on definitions of virtual particles and the unresolved nature of how certain properties can be measured or assigned to them. The discussion also highlights the ambiguity surrounding the interpretation of quantum field theory models.

Sophrosyne
Messages
128
Reaction score
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.
 
Physics news on Phys.org
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.
 
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.
 
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]
 
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?
 
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/
 
snorkack said:
How do you observe the momentum?
You observe the momentum before and after the interaction.
 

Similar threads

High School Virtual photons as force carriers
  • · Replies 29 ·
Replies
29
Views
3K
Undergrad Are All Photons Truly Virtual?
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Virtual Photons: The Long-Distance Repulsion of Electrons
  • · Replies 2 ·
Replies
2
Views
2K
Graduate Particles vs. Virtual Particles vs. Fields
  • · Replies 2 ·
Replies
2
Views
2K
High School Can Gluons Emit Photons?
  • · Replies 4 ·
Replies
4
Views
3K
Undergrad Why do modern X ray images have better resolution compared to older ones?
  • · Replies 6 ·
Replies
6
Views
2K
Undergrad Detecting Virtual Photons & EM Force: A Particle Physics Quandary
  • · Replies 2 ·
Replies
2
Views
4K
Undergrad What is the Waveform of Virtual Photons?
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad How Does Binding Energy Relate to Nucleon Distance in Atomic Nuclei?
  • · Replies 17 ·
Replies
17
Views
3K
Undergrad Progress In Calculating The HLbL Contribution To Muon g-2
  • · Replies 0 ·
Replies
0
Views
3K