Are "virtual particles" a fact?

[gib65]

Are "virtual particles" a fact? I mean, from what I understand, virtual particles exist for such a short period of time, their existence cannot be tested. So as scientists, what's our reason for believing in them? Is it because they are theoretically necessary in order to explain certain phenomena, and if so, are they really necessary or just the best explanation we've come up with?

[Gilded]

Well yes, they are necessary in most (or all?) mainstream models and fit there quite well too, to explain stuff like Casimir effect etc. But you can't detect virtual particles directly, as they're more like particle-like abstractions, that show some similarity to "real particles". However, I recall there being some relativistic phenomena where virtual particles are observed as real (which of course contradicts what I just said... ?).

 

...I think I'll just go and sit in the corner and wait for swansont or Martin.

[timo]

I assume you know virtual particles in the context of Feynman diagrams: Feynman diagrams are at the very basic level nothing but pictorial representations of mathematical terms (addends in a Taylon expansion). So in that sense they are exactly as "real" as the x in the Taylor expansion of sin(x) = x + O(x²). They can also be tested in a similar way. Just plug in several small x and you´ll notice that the result is roughly x. So in that sense the x is real, necessary for sin(x) and can be tested.

[Meir Achuz]

Are "virtual particles" a fact? I mean, from what I understand, virtual particles exist for such a short period of time, their existence cannot be tested. So as scientists, what's our reason for believing in them? Is it because they are theoretically necessary in order to explain certain phenomena, and if so, are they really necessary or just the best explanation we've come up with?

Virtual particle are mathematical constructs in momentum space in a perturbation calculation, usually of scattering. They don't exist in any real physical sense.

[gib65]

I assume you know virtual particles in the context of Feynman diagrams.

 

I'm embarassed to say, but no I don't . Nor am I familiar with Tayler expansions. I'm so out of the loop.

 

Are you saying that virtual particles are real in the same sense that mathematical formulas are real?

[swansont]

I'm embarassed to say, but no I don't . Nor am I familiar with Tayler expansions. I'm so out of the loop.

 

Are you saying that virtual particles are real in the same sense that mathematical formulas are real?

 

It's more of a representation issue. You can expand any function is a geometric series, and then use just the first term or two under specific conditions, e.g. sin x = x when x is small (and you can define small mathematically here). So the Feynman diagrams are valid because the Taylor expansion is. "Factual" is a different question.

 

I think you can play the "what is real/factual and what is a model" game without going to virtual particles. Are phonons a fact, or are they just a representation of a vibrational mode of a lattice? Are photons a fact, or just a representation of the EM vibrational mode of space?

 

One answer to all of these questions is "nature behaves as if they are." So within the context of our models, they are all facts.

[5614]

OK, now I don't know how much you do know about Feynman diagrams, so I'll start at the basics. (excuse my Paint skills in advance!)

 

The following shows a simple Feynman diagram. It shows an electron (e-) and a positron (e+) coming together and annihalating, this produces a photon (the squiggly line).

 

However this high energy photon can have more energy than that of an electron and positron combined. Consequently there is no reason why that photon could not just randomly turn into an electron/positron pair. This pair will then annihalate each other very quickly, as shown in this Feynman diagram:

 

They are known as virtual as they do not exist as an input nor as an output to the annihalation process shown in the Feynman diagram. However who's to say that this process only happens once? In this diagram:

the resultant photon splits into an electron/positron pair twice and before the annihalation the electron interacts with itself via a virtual photon.

 

I can't remember what process this was originally noticed with, however initially calculations of some process were quite accurate, however not precise. When the calculations included one loop (a virtual particle or pair) the calculation became more precise. With a 2nd loop it became even more accurate etc.. The calculations are however extremely complex and require supercomputers to compute.

 

This is a good proof that virtual particles to exist, as including them in equations allows us to get answers which agree with experimental data.

 

=====

 

There is one other experiment that is very relevant, and I think this is one of my favourite experiments. It's called the Casimir effect.

 

Imagine two plates parallel to each other and very close to one another. These are uncharged, in a vacuum and we can ignore gravity, so you would have thought there was no force between the two plates. However there is. It's very small, but noticable. This is known as the Casimir effect.

 

The explanation of this force is that around the two plates you have a 'normal' amount of virtual particles. However between these two plates there is a very little area for virtual particles to exist in. Very simply there is not enough room for 'big' virtual particles to pop into existence between the two plates. As there are less virtual particles between the plates relative to the outside of the plates there is a pressure difference, which means that the plates feel a very small force pushing them together.

 

A great visual of this (found through Google Images, hosted on wikimedia) is this:

 

There's a good analogy of the Casimir effect here:

http://en.wikipedia.org/wiki/Casimir_effect#Analogies

(the 2nd para, about ships in the ocean)

[spikerz66]

thank you 5614 for the diagrams i did not know what they were at all and now i have a pretty good idea about the casimir effect also. (im trying to catch up on these things )

[swansont]

The explanation of this force is that around the two plates you have a 'normal' amount of virtual particles. However between these two plates there is a very little area for virtual particles to exist in. Very simply there is not enough room for 'big' virtual particles to pop into existence between the two plates. As there are less virtual particles between the plates relative to the outside of the plates there is a pressure difference, which means that the plates feel a very small force pushing them together.

 

The explanation that works for me is that you exclude photon modes with the plates. Since they are conducting, a standing wave between them has to have a node at the position of the plate, and waves can't exist with a half-wavelength longer than the separation. So the energy density between the plates is lower than outside of them, giving you an attractive force.

[Severian]

excuse my Paint skills in advance!

 

Jaxodraw is your friend: http://jaxodraw.sourceforge.net/

[NeonBlack]

1 quick question:

Since a photon with sufficient energy may spontaneously become an electron positron pair (or any other fundamental particle/antiparticle presumably), which quickly annihilates, why isn't the speed of higher energy photons slower since this process must take some time?

[insane_alien]

1 quick question:

Since a photon with sufficient energy may spontaneously become an electron positron pair (or any other fundamental particle/antiparticle presumably), which quickly annihilates, why isn't the speed of higher energy photons slower since this process must take some time?

 

well, the photons break down in to two smaller photons by this process, say we have a high energy photon that breaks down into a electron-positron pair, these then attract each other and annihilate, this usually leads to 2 seperate photons of lower energy(~half of the origional) but while this would slow it down, it only occurs at the particle stage, when its still a photon it still goes at c.

[swansont]

1 quick question:

Since a photon with sufficient energy may spontaneously become an electron positron pair (or any other fundamental particle/antiparticle presumably), which quickly annihilates, why isn't the speed of higher energy photons slower since this process must take some time?

 

If it's a real photon, and actually forms the particle/antiparticle pair, you get two photons out of the annihilation,

 

If it's forming a virtual pair, any photon will do this; it's duration is limited only by the Heisenberg Uncertainty Principle. I think you can think of it as the cost of being a photon - it's already priced into the speed being c. i.e. the speed being c already includes the fact that the virtual pairs will form.

[GutZ]

Finally I get what the squiggly line was...It's so obvious now...I thought it was based on a timeline.

[Severian]

1 quick question:

Since a photon with sufficient energy may spontaneously become an electron positron pair (or any other fundamental particle/antiparticle presumably), which quickly annihilates, why isn't the speed of higher energy photons slower since this process must take some time?

 

That is actually a very good question. To rephrase it a little (if I may), you are actually asking why the insertion of a virtual electron-positron pair in a loop does not contribute to the mass of the photon. Generally one would expect that it does, and thereby change the speed of the particle (so your intuition is exactly correct).

 

However, in this case the mass of the photon is protected by the electromagnetic U(1) gauge invariance. Any mass term would break the gauge invariance, so are not allowed. In other words, the theory is set up in such a way that the contribution to the mass from this effect is zero (although it does change the electromagnetic coupling).

[Ragib]

Virtual Particles can never be experimental verified. However, they provide a very good interpretation of Black Hole Evaporation, also known as Hawking Radiation. There are of course other interpretations, but virtual particles are eqivalent and useful in the mathematics, espically when dealing with Quantum Electrodynamics.

[5614]

swansont: I like the standing wave and nodes explanation, thanks.

 

Severian: I've seen programs like that before and I was searching for one before I posted, but couldn't find it so gave up and used Paint, thanks for that though, it'll definately help in the future.

 

Finally I get what the squiggly line was...It's so obvious now...I thought it was based on a timeline

Nice to see you understand now, and I'm not entirely sure what your timeline confusion was, but some Feynman diagrams do have a time axis. So if you go back and look at my first diagram there are no direction arrows, the diagram has no reference to time. Whereas my 2nd and 3rd diagrams have arrows which represent the flow of time. In those diagrams the x-axis represents time. All the particles have an arrow representing which way in time they flow. Photons do not need an arrow, as time, in a photon's frame, is meaningless, as they travel at c.

 

Furthermore in my diagram I've shown a positron as an arrow moving forward in time and labelled as e+. However Feynman showed, mathematically, that a positron moving forward in time is identical to an electron moving backwards in time. So instead of representing the positron as I did, I could have drawn the arrow pointing to the left (backwards), and labelled it e-.

 

Now I've got a question regarding time in Feynman diagrams. Some show time on the x-axis, some on the y-axis and some have no reference to time, so what is the correct way of drawing a Feynman diagram?