# What is a virtual particle?

1. Sep 25, 2008

### Trip2

what is the differace between a particle and a virtual particle?

2. Sep 25, 2008

### Staff: Mentor

3. Sep 25, 2008

### Tac-Tics

A virtual particle is a particle which is not observed in an experiment, but influences the outcome.

In a Feynman diagram, these are particles that are created during an event, but are destroyed again before a measurement is taken. They are called "virtual", then, because they are never actually seen directly. They only serve to influence the final result, and to say they actually ever existed during the experiment is a question you can't answer.

4. Sep 25, 2008

### clem

A virtual particle is just a useful mathematical step in calculating QFT in perturbation theory. A VP never exists as a physical particle, which is why SP is a suitable candidate.

Last edited: Sep 25, 2008
5. Sep 25, 2008

### Count Iblis

That's not true, because there are then also processes in which SP would be the real particle. The properties of SP as a real particle are not so good. That's why the squared matrix element

|<JMC|WH>|^2

has been declining lately.

Last edited: Sep 25, 2008
6. Sep 25, 2008

### vanesch

Staff Emeritus
To the OP: the last two contributions are jokes where the smiley was forgotten

7. Sep 25, 2008

### clem

I am so conservative, I did not know how to make smileys, but now see its simple. Anyway the part of my previous post up to SP was serious.

8. Sep 25, 2008

### Trip2

so virtual particles are retrocausal?

9. Sep 25, 2008

### vanesch

Staff Emeritus
They can formally be "retrocausal", but as they don't correspond to anything physically well-defined, it's a bit strange to say that. They are part of a quantum field calculation. In that sense, you could almost ask whether the square root of two is retrocausal.
On the other hand, the line between "almost real virtual particles" and real particles is also thin. But these are not retrocausal.

10. Sep 26, 2008

### Trip2

so why is it said that they are causality violaters?

11. Sep 26, 2008

### Staff: Mentor

Who says they violate causality?

12. Sep 26, 2008

### masudr

Virtual particles are not bound by the relation

$$E^2 = p^2 + m^2.$$

This is not a problem however, as spacelike separated field operators have a commutator of zero, and therefore causality is not violated in quantum field theory.

13. Sep 26, 2008

### jnorman

virtual particles are just another of the many ways in which physicists indicate that they do not know what is actually happening.