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LarryS
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Why must photons, for example, that carry the EM force be virtual? Is it because they are tied to gauge invariance and that is not observable?
Thanks in advance.
Thanks in advance.
haael said:Carriers of force are not particles. They are fields.
There are configurations of fields that have changing potential. They do not obey equations of motion thus are not particles. But the gradient of potential means they extert forces.
And there are configurations of fields that have zero (mean) potential, but at the same time they obey wave equation = equations of motion. They are particles, but they don't carry forces.
A very useful analogy: you have a guitar string, attached to some point. You can take the string into your hand and pull it constantly. It will extert force. On the other hand, you can pluck the string. It will make a sound.
Pulling the string is analogous to force-carrying field. Sound (waves traveling on the string) is analogous to particles, excitations of the field.
TL;DR: Forces arise from non-wave non-constant solutions of fields, particles arise from wave solutions.
Remember: force carriers are not particles.
What's the fuss about virtual particles then? Well, they are a mathematical trick. You can express a non-wave configuration as a weighted sum of wave configurations. This is a procedure somewhat analogous to Taylor expansion or better Fourier expansion. You take a subset of configurations (wave solutions only) and use it to construct any other configuration.
The waves used in this construction are technically particle-like solutions, that's why we call them virtual particles. But they are not particles. They don't obey equations of motion. They are simply a mathematical re-expression of a non-particle field configuration.
Why are we doing this? Well, particles have a simple commutator relation. We know how to compute a commutator between two pure waves. We don't know instantly how to compute commutator between everything else. Until we express everything as a weighted sum (or integral) of particle configurations.
This is how Feynmann's quantization works.
Real photons are excitations of a field (particles) - yes.referframe said:Photons are the gauge bosons for the EM field. I understand that in QFT, all particles, including photons, are characterized as excitations of a field. But, are they still not particles in the QFT sense? Isn't that what the Second Quantization was all about?
Yes, they are particles in the QFT sense and only in the QFT sense. Note that particularly for photons any idea to think of them as particles in the classical sense is doomed to misunderstanding. Note that you cannot even define a position observable for photons!referframe said:Photons are the gauge bosons for the EM field. I understand that in QFT, all particles, including photons, are characterized as excitations of a field. But, are they still not particles in the QFT sense? Isn't that what the Second Quantization was all about?
OK, but is a force-carrying photon's "virtualness" due to local gauge invariance basically being unobservable (forgetting for the moment about the AB effect) ?vanhees71 said:Yes, they are particles in the QFT sense and only in the QFT sense. Note that particularly for photons any idea to think of them as particles in the classical sense is doomed to misunderstanding. Note that you cannot even define a position observable for photons!
I would say no.referframe said:OK, but is a force-carrying photon's "virtualness" due to local gauge invariance basically being unobservable (forgetting for the moment about the AB effect) ?
Force-carrying particles are considered "virtual" because they do not exist as physical particles in the traditional sense. They are instead considered virtual particles, which are particles that only exist for a very short amount of time and do not have all the properties of a regular particle.
Virtual force-carrying particles play a crucial role in the fundamental forces of nature. They are responsible for mediating the interactions between particles, such as the electromagnetic force between charged particles or the strong nuclear force between quarks in an atomic nucleus.
Virtual force-carrying particles differ from real particles in several ways. They have a much shorter lifespan, they do not have all the properties of a regular particle, and they cannot be directly observed or detected. They only exist as mathematical concepts to explain the interactions between particles.
We cannot observe virtual force-carrying particles directly because they only exist for a very short amount of time, on the order of 10^-23 seconds. This is much shorter than the time it takes for any type of measurement or observation to be made. Additionally, they do not have all the properties of a regular particle, making them impossible to detect through traditional methods.
Virtual force-carrying particles are integral to our understanding of the universe and the fundamental forces that govern it. They help explain the interactions between particles and provide a mathematical framework for understanding the laws of nature. Without them, many phenomena, such as the behavior of subatomic particles, would be impossible to explain.