# Virtual Particle

1. Jan 25, 2010

### Rouge72

i've been scrolling through the archives and can't get an answer if virtual particle exist or not or are real,I get different answers looking on websites.

2. Jan 25, 2010

### Demystifier

It does not exist. (This is perhaps the most frequent question on this forum and I am tired of explaining it each time.)

3. Jan 25, 2010

### ansgar

they arise from the perturbative expansion of interaction terms, thus they are only a calculation tool... just as for instance Ghost particles are.

4. Jan 25, 2010

### ansgar

you should have the answer saved in a textfile somewhere and just paste it here :)

5. Jan 25, 2010

### SpectraCat

Or even better, add the explanatory post in question to the FAQ, and direct folks there.

6. Jan 25, 2010

### Demystifier

Well, I have done something similar. I have written a paper (published in Found. Phys.) in which one section (Sec. 9.3) is devoted to this. The paper is
http://arxiv.org/abs/quant-ph/0609163

7. Jan 25, 2010

### Staff: Mentor

8. Jan 25, 2010

### Rouge72

if they don't exist,how come they have real effects?

9. Jan 25, 2010

### Rouge72

Last edited: Jan 25, 2010
10. Jan 25, 2010

### Staff: Mentor

You really expect everyone to agree about something on an Internet forum?

If you can't decide whose answers you trust more, then you'll just have to pick up a copy of Weinberg's or whosever textbook and learn quantum field theory for yourself. Then you can decide for yourself whether virtual particles meet your definition of "real."

11. Jan 25, 2010

### Rouge72

so there are going to be different opinions on this topic.

12. Jan 25, 2010

### Staff: Mentor

Because different people have different ideas of what "real" means.

13. Jan 26, 2010

### ansgar

IF we could calculate things without this power expansion, would they exists even on paper then?

The same is true for ghost particles, they are only mathematical entities which allow us to calculate stuff.

14. Jan 26, 2010

### Rouge72

another thing that blew my mind is that they are allowed to travel faster than light?

15. Jan 26, 2010

### Fredrik

Staff Emeritus
What that really means is just that when you calculate the probability amplitude you're interested in, you have to integrate the functions of the four-momenta that you're dealing with, over all of $\mathbb R^4$ rather than over the "mass shell" (the subset defined by $(p^0)^2-(p^1)^2-(p^2)^2-(p^3)^2=m^2$).

There's no good reason to think that these mathematical operations describe what "actually happens" during an interaction. QFTs (where we encounter the virtual particles) are theories of matter and interactions in the framework of QM, and virtual particles appear when you express a certain function as a series and interpret the individual terms as a description of what actually happens. Why would it be the terms that describe what actually happens and not, say, their sum? It seems even less likely that they do when we consider the fact that QM itself may not be a description of the world out there. It could be just an algorithm that tells us how to calculate probabilities of possibilities.

So to think of virtual particles as "what's actually happening" is a bit of a stretch. (It certainly could be what's actually happening, but experiments can't confirm or deny that it is).

Last edited: Jan 26, 2010
16. Jan 26, 2010

### Demystifier

Maybe the following analogy can help. Suppose that you have 1$(one dollar) in your pocket. It is real. However, mathematically, you can write 1$=1.243$+(-0.243$)
So one can use this identity to say that you actually have 1.243$and -0.243$. Of course, you really have neither 1.243$nor -0.243$. You have them only in a virtual sense. Yet, their virtual existence has a real effect. Their real effect is that you have 1$. Does it help? By the way, virtual money may have not only multi-decimal values and negative values (which probably can be comprehended at some extent), but even complex values: 1$=(1+3i)$+(1-3i)$

Last edited: Jan 26, 2010
17. Jan 26, 2010

### Born2bwire

Lord knows that enough of my money is simply imaginary :(

18. Jan 26, 2010

### Rouge72

are virtual particle's physics for math?

19. Jan 27, 2010

### Fredrik

Staff Emeritus
Not sure I understand the question, but virtual particles appear in perturbation theory calculations of probability amplitudes. You use those amplitudes to calculate the probabilities of the different possible results of your experiments. It's conceivable that there exists a better way of doing the calculations which wouldn't include virtual particles at all, but I don't think anyone knows any other way to do them.

20. Jan 27, 2010

### ansgar

math only, we can represent physics with math - but not all math can represent physics.