Confused about virtual particles

In summary, the distinction between virtual and real particles is not based on their interaction or decay, but on the presence and location of their propagator vertices in a Feynman diagram. All particles are, in a sense, virtual and only exist within the context of interactions.
  • #1
kelly0303
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Hello! I am a bit confused about the distinction between virtual and real particles. For example a Z boson, which has a very short lifetime, in all experiments will decay to some other stable particles (i.e. it is detected through its decay). This means that it will always appear as a propagator, and as far as I understand, the propagators are not, usually, on shell. The mass of the Z was obtained by looking for a resonance peak, and that is the quoted mass in PDG for example. But what does it mean for a Z particle to not be virtual i.e. do we even have a real Z? So my question is, especially for unstable particle, which always appear as propagators in some Feynman diagram, when is it real and when is it virtual? Thank you!
 
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  • #3
kelly0303 said:
Hello! I am a bit confused about the distinction between virtual and real particles. For example a Z boson, which has a very short lifetime, in all experiments will decay to some other stable particles (i.e. it is detected through its decay). This means that it will always appear as a propagator, and as far as I understand, the propagators are not, usually, on shell. The mass of the Z was obtained by looking for a resonance peak, and that is the quoted mass in PDG for example. But what does it mean for a Z particle to not be virtual i.e. do we even have a real Z? So my question is, especially for unstable particle, which always appear as propagators in some Feynman diagram, when is it real and when is it virtual? Thank you!
Real particles appear in the 'in' and 'out' states, with only one vertex interacting with the rest of the diagram, and are on-shell. Virtual particles have both propagator vertices appearing inside the diagram and are off-shell.
If your Z boson survives long enough to leave the interaction volume then it would be deemed "real". But in actuality all particles are virtual, since interactions can occur anywhere.
 
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  • #4
Michael Price said:
Real particles appear in the 'in' and 'out' states, with only one vertex interacting with the rest of the diagram, and are on-shell. Virtual particles have both propagator vertices appearing inside the diagram and are off-shell.
Thank you for you reply! But this is what I am confused about. You can't have for a (say) Z boson a diagram where the Z boson comes 'in' or 'out' of the diagram. The basis for QFT (if I understood it well) is that at infinity the particles are free, then they interact, then they are free again at infinity. But a Z boson (or any unstable particle) can't come and go to infinity, as it decays very fast. So the Z boson can only appear as a propagator, otherwise it would mean that it traveled for a long time without decaying. So how does one define a real particle?
 
  • #5
kelly0303 said:
Thank you for you reply! But this is what I am confused about. You can't have for a (say) Z boson a diagram where the Z boson comes 'in' or 'out' of the diagram. The basis for QFT (if I understood it well) is that at infinity the particles are free, then they interact, then they are free again at infinity. But a Z boson (or any unstable particle) can't come and go to infinity, as it decays very fast. So the Z boson can only appear as a propagator, otherwise it would mean that it traveled for a long time without decaying. So how does one define a real particle?
I half answered this in an update, but let me complete it here. The interaction volume is arbitary, and the distinction between free and interacting a bit artifical. (There is no such thing as a free particle - they are just convenient fictions.). In reality all particles are virtual, and it is the so-called "real" particles that don't exist, ironically.
 

1. What are virtual particles?

Virtual particles are particles that are constantly popping in and out of existence in the quantum vacuum. They are not observable in the traditional sense, but their effects can be seen through various physical phenomena.

2. How are virtual particles different from real particles?

Virtual particles are different from real particles in that they do not have a definite mass or energy, and they cannot be directly observed or measured. They also do not follow the same laws of physics as real particles.

3. What is the role of virtual particles in quantum mechanics?

Virtual particles play a crucial role in quantum mechanics as they are a manifestation of the uncertainty principle and the concept of quantum fluctuations. They also help explain various phenomena, such as the Casimir effect and Hawking radiation.

4. Can virtual particles become real particles?

Yes, virtual particles can become real particles under certain conditions. For example, in particle accelerators, high energy collisions can create real particles from the energy of virtual particles. However, these real particles are short-lived and quickly decay back into virtual particles.

5. How do virtual particles contribute to the vacuum energy of space?

Virtual particles contribute to the vacuum energy of space by constantly popping in and out of existence. This creates a fluctuation in the energy of the vacuum, which contributes to the overall vacuum energy. However, the exact role of virtual particles in the vacuum energy is still a topic of debate in the scientific community.

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