Exploring the Existence of Virtual Particles: A Comprehensive Analysis

[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.

 

[Demystifier]

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

 

[ansgar]

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

 

[ansgar]

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

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

 

[SpectraCat]

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

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

 

[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

 

[jtbell]

Here's a good thread on this subject from last year:

How "Real" Are Virtual Particles?

 

[Rouge72]

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

 

[Rouge72]

Here's a good thread on this subject from last year:

How "Real" Are Virtual Particles?

i get different answers in the thread.I also read a thread that virtual particle are allowed to travel FTL?!

 

[jtbell]

i get different answers in the thread.

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."

 

[Rouge72]

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."

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

 

[jtbell]

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

 

[ansgar]

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

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.

 

[Rouge72]

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

 

[Fredrik]

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

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).

 

[Demystifier]

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

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)$

 

[Born2bwire]

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)$

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

 

[Rouge72]

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).

are virtual particle's physics for math?

 

[Fredrik]

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.

 

[ansgar]

are virtual particle's physics for math?

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

 

[Demystifier]

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

I would put it this way. The same physics can be represented by several different maths. It does not seem reasonable to believe that these different maths describe different realities.

 

[Rouge72]

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.

what I am trying to find out is if they can travel FTL?

 

[Fredrik]

what I am trying to find out is if they can travel FTL?

You won't get a much better answer than what I already gave you in #15. FTL speeds are definitely included in the calculation, but there's no good reason to think that something is actually traveling along the paths in spacetime that contribute to the final result.

 

[inempty]

Can we get a physics understanding of interations without the notion of virtual particle?

 

[ansgar]

Can we get a physics understanding of interations without the notion of virtual particle?

yes but we cannot (as today) calculate anything to get numbers.

 

[Demystifier]

Can we get a physics understanding of interations without the notion of virtual particle?

I'm not sure what do you think by "understanding", but we can certainly do a calculation without them.

 

[ansgar]

I'm not sure what do you think by "understanding", but we can certainly do a calculation without them.

can we? I have only heard and learned about perturbation methods in QFT

 

[Demystifier]

yes but we cannot (as today) calculate anything to get numbers.

That's not quite correct. In some cases we can get numbers as well.

 

[Demystifier]

can we? I have only heard and learned about perturbation methods in QFT

Here are counterexamples:
- lattice calculations
- interactions with a classical background
- low dimensional fully quantum exactly solvable systems

 

[Demystifier]

Besides, we can do a perturbative calculation from the scratch, without using the technique of Feynman diagrams and Feynman rules. Without Feynman diagrams, one is not tempted to refer to certain terms in the expansion as "virtual particles".

 

[Rouge72]

You won't get a much better answer than what I already gave you in #15. FTL speeds are definitely included in the calculation, but there's no good reason to think that something is actually traveling along the paths in spacetime that contribute to the final result.

is there evidence or proof for the existence of virtual particles?

 

[SpectraCat]

is there evidence or proof for the existence of virtual particles?

Check out "The universe in a nutshell" by Stephen Hawking. Or just google the "Casimir effect". There have been attempts to measure "vacuum fluctuations", or the "zero point energy", by measuring tiny forces between uncharged metal plates in vacuum. One proposed qualitative, phenomenological explanation of these effects is that they arise because the plates create a cavity, thereby restricting the frequencies of the virtual particles that can pop into existence between them. One can show this more formally using Feynman diagrams with virtual particles, as is done in many texts.

However, it should definitely be noted that alternative explanations of the effect in terms of relativistically retarded van der Waals forces arising from the charged particles moving inside the conductors have also been demonstrated. (see http://arxiv.org/PS_cache/hep-th/pdf/0503/0503158v1.pdf)

So, while the Casimir effect is consistent with the existence of virtual particles, it evidently does not require them.