Photons as virtual particles in High energy

[da_steve]

Hi,

So i can accept that photons are virtual particles for the electromagnetic force but i have a question.

Considering two stationary point charges. There are photon-like particles exchanged between them to produce the force? If so then placing a double slit between them should create some interference pattern (or even if there was just one and measuring behind it). I know it isn't from maxwells (potential is scalar field) but wouldn't the photons interfere if they were the force carriers?

Just confused :/

Thanks!

 

[Vanadium 50]

Your mistake is in thinking virtual particles mean "little ping-pong balls moving back and forth". Virtual particles are not real (indeed, the term is used in opposition to "real particles"), so they don't have trajectories or wavelengths. Indeed, you don't even have a fixed number of them. So you cannot ascribe the behavior of real photons to them.

 

[nikol]

If I can add something to the discussion, you can thing this way: in order to see interference pattern you need a lot of photons(e.g. a beam) with the same wavelength (e.g.energy since for light $E=\frac{hc}{\lambda}$ to go trough the slits, so this is making the experiment impossible, since the virtual photons can have any energy (wavelength).

 

[Fields]

What is really making the experiment impossible is the infinitessimally short amount of time they exist. Have we even detected any virtual particles yet? Let alone virtual photons?

 

[Vanadium 50]

Let me say it again. Virtual particles are not real. You can't see them or count them, and they don't have wavelengths.

 

[Fields]

Let me say it again. Virtual particles are not real. You can't see them or count them, and they don't have wavelengths.

That is your belief, and you are entitled to it. It is my belief they are real, in that they really do exist. This is not religion any more than the Higgs Boson was a religion before it's discovery.

 

[Vanadium 50]

My belief comes from studying physics for many years, earning a PhD in it, and practicing it professionally for two decades.

Where does your belief come from?

 

[Simon Bridge]

That is your belief, and you are entitled to it. It is my belief they are real, in that they really do exist. This is not religion any more than the Higgs Boson was a religion before it's discovery.

It is not a matter of belief.
The detected Higgs boson, your example, was a real one. The virtual Higgs boson is just that: virtual. i.e. it is not real. It exists only as a step in a calculation process. It is important not to confuse the map for the territory.

It is not useful to think of charges firing virtual photons at other charges - it gives you the wrong physics. Which is what you have discovered. You should also consider how an electric field, if it comes from actual photons getting exchanged by charges, could possibly give you attractive forces.

The photon that is mediating the EM interaction does not have the same sort of reality as the photons that make up, say, a laser beam.

You are welcome to go on believing they are - but I don't think that would be an accepted/mainstream belief. Which makes it out of bounds as a topic for this forum.

Unless you can produce an accepted source saying otherwise of course.
For a list of accepted sources, please see the forum rules.

 

[tiny-tim]

It is my belief they are real, in that they really do exist.

(we are talking about virtual particles in the context of "mediating" a force)

Fields, do you mean on-shell virtual particles, or off-shell virtual particles exist (and "mediate" a force)?

(You presumably don't believe they both exist)

This is not religion any more than the Higgs Boson was a religion before it's discovery.

The Higgs Boson was predicted as part of a scientific theory, based on equations.

The turtles on which the universe stands are predicted as part of a religious theory, not based on equations.

There is nothing in the equations to support your idea that virtual particles "mediating" a force exist in actual space-time.

EDIT: just remembered … i like comparing this with epicycles, the imaginary circles in space whose centres rotate around larger circles, and which explained (very accurately) the motion of the planets (before we knew they were ellipses) …

we now know that the epicycles have no actual existence: they are only a mathematical trick for approximating a planet's orbit (very like a Fourier series)

however, actual epicycles existing in space would at least be consistent with the equations, since the equations actually describe where they are!

it would therefore be possible to believe scientifically in their existence until observation confirmed or disproved it

however, that is not the case with virtual particles "mediating" a force … the equations do not describe where they are!​

 

[Bill_K]

The following quote was posted by Naty1, I don't have the original source:

"There is not a definite line differentiating virtual particles from real particles — the equations of physics just describe particles (which includes both equally). The amplitude that a virtual particle exists interferes with the amplitude for its non-existence; whereas for a real particle the cases of existence and non-existence cease to be coherent with each other and do not interfere any more. In the quantum field theory view, "real particles" are viewed as being detectable excitations of underlying quantum fields.." -- Carlo Rovelli

 

[Vanadium 50]

Bill, this line of argument is unhelpful. Like pornography, "I know it when I see it" - real particles and virtual particles behave differently. As there is a real qualitative distinction between real and virtual behavior, despite the fact that treating them in the same mathematical framework is sometimes convenient, it is not helpful to clear up misunderstandings like the OP's.

 

[tiny-tim]

The following quote was posted by Naty1, I don't have the original source:

it comes from wikipedia: http://en.wikipedia.org/wiki/Virtual_particles#Properties

(but the first sentence has been changed since Naty1 copied it)

no source is given!

 

[Andy Resnick]

This is a tempest in a teapot: Virtual particles are so named because their (brief) existence is limited by their violation of mass-energy conservation (Eisberg and Resnick, 'Quantum Physics', pg 634). Alternatively, intermediate states have a conserved 3-momentum but the energy is not; these are said to be virtual states (Landau and Lifshitz v.4, pg 312). Virtual particles disobey the relationship p^2 = m^2 satisfied by real particles (ibid.). L&L have a whole section (79) in that volume dedicated to the issue. An alternative name for virtual particles is 'off-mass-shell' particles (Gross, 'Relativistic Quantum Mechanics and Field Theory', p247). Similarly, virtual transitions (transitions between virtual states) occur in nonlinear optical processes (Boyd, 'Nonlinear optics', pp 6, 229 and Yariv, 'quantum electronics') such as two-photon processes. Jackson has a lengthy section dedicated to 'virtual quanta'.

 

[tiny-tim]

Hi Andy!

… Virtual particles are so named because their (brief) existence is limited by their violation of mass-energy conservation (Eisberg and Resnick, 'Quantum Physics', pg 634).
Alternatively, intermediate states have a conserved 3-momentum but the energy is not; these are said to be virtual states (Landau and Lifshitz v.4, pg 312).
Virtual particles disobey the relationship p^2 = m^2 satisfied by real particles (ibid.). L&L have a whole section (79) in that volume dedicated to the issue. An alternative name for virtual particles is 'off-mass-shell' particles (Gross, 'Relativistic Quantum Mechanics and Field Theory', p247).

Aren't those three books referring to three different (and incompatible) versions of virtual particle?

The first is the "energy-borrowing" creation of particles out of nothing (except energy "borrowed" from the vacuum): these are not involved in "mediation" of forces (the subject of this thread).

The second is virtual particles of the first kind (as in Feynman diagrams of the first kind): their masses are correct ("on-shell"), they exist in position space, and energy is conserved at collisions but 3-momentum isn't.

The third is virtual particles of the second kind (as in Feynman diagrams of the second kind): their masses are all wrong ("off-shell"), they exist in momentum space (ie the maths doesn't even suggest they exist in actual space), and 4-momentum (including energy) is conserved at collisions.

 

[Bill_K]

Going from "virtual particles of the first kind" to "virtual particles of the second kind" is just a change of basis, tiny-tim, like we do all the time in quantum mechanics. A photon state in momentum space is a linear combination of photon states in position space, and vice-versa. That does not mean they are two different kinds of particles, or that they exist in different spaces! :uhh:

In the Coulomb example that the OP questioned, the field in position space is 1/r. Take the Fourier transform and you see that this can be written as a linear combination of photons of well-defined momentum k, with amplitude 1/k². Virtual particles are typically integrated over. Does not imply a lack of reality on their part.

 

[tiny-tim]

Going from "virtual particles of the first kind" to "virtual particles of the second kind" is just a change of basis, tiny-tim, like we do all the time in quantum mechanics.

sorry, I'm not understanding this

a "first kind" virtual particle has a 3-momentum p (and an "on-shell" energy √(p² + m²)),

and all calculations involve triple-integrals: ∫∫∫ d³p

a "second kind" virtual particle has a 4-momentum q (and is "off-shell"),

and all calculations involve quadruple-integrals: ∫∫∫∫ d⁴q

that isn't a change to a different basis in the same space, it's to a different space (usually called momentum space)

where in position space is any momentum space virtual particle? (or where is it centred?) … the concept of position doesn't seem to apply in momentum space calculations

moreover (and i should have mentioned this in my last post), quantum field theory says that "first kind" virtual particles have creation and annihilation operators, but "second kind" don't … again, that's not merely a change in basis

finally, if "first kind" actually exist, then they have a 4-momentum, and it's on-shell; if "second kind" actually exist, then they have a 4-momentum, and it's off-shell … how can these be the same real particles?

 

[Vanadium 50]

None of this is helping the OP.

 

[Simon Bridge]

None of this is helping the OP.

Good point.
The thread has been hijacked hasn't it?

Actually we have not heard from OP (daSteve) since the start.

The original question is about whether you can get interference effects in electrostatic fields.
The argument is that perhaps you aught to because photons mediate the field, and you get interference effects with photons.

 

[Crazymechanic]

But then has anyone got an interference pattern with an static charge ? As long as I can tell no, so if they are not physically real then why should they be theoretically?

 

[PICOGRAV]

What happened to my post?

 

[Simon Bridge]

Since nobody else jumped in...
To my knowledge - nobody has seen interference at slits for the electric field due to a static charge.
It does not even set off sensitive photodetectors.

The charge does not act like a source of light.

The theory is just math that gives good predictions. It does not have to match reality every step of the way... only at the endpoint. You've heard Feynman on this subject right? [1]

The weird thing here is that the virtual particles have corresponding real particles.
That just means that there is a step in the math for working out interactions that looks a lot like the math for exchanging particles. But not exactly like it.

For a decent picture you probably have to go into the field theory.

The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle. A particle is a nice, regular ripple in a field, one that can travel smoothly and effortlessly through space, like a clear tone of a bell moving through the air. A “virtual particle”, generally, is a disturbance in a field that will never be found on its own, but instead is something that is caused by the presence of other particles, often of other fields.
$\qquad$ --Strassler, M. (2011) [2]​

... though there are also debates about whether the fields of Field Theory are real or the particles are real. Just search through PF for examples.
It can get philosophical very fast.

You'll find plenty of sources which appear to take the position that virtual particles are a case of a real particle that violates conservation of energy for such a short time the Universe doesn't notice. For instance:

these high-energy force-carrier particles may exist if they are short lived. In a sense, they escape reality's notice.
$\qquad$-- Barnett et al. [3]​

... which can be confusing.

A useful rule of thumb is that it is not real unless you can detect it (at least in principle) - the rest is maths.

That should provide enough for others to disagree with ... I would hope that people will provide their own citations to back up their comments ;)

-------------------------

[1] Feynman R. (1979) Nuts, Counting, and Physics - excerpt from the Douglass Robb Memorial Lectures given at the University of Auckland. The complete set [url=http://vega.org.uk/video/subseries/8]are compulsory viewing for anyone contemplating particle physics.

Just because, in QED, you have to compute a probability amplitude for every possible path does not mean that one particle actually travels every possible path. OTOH it is possible to exploit the idea to great effect.

[2] Strassler M. (2011) Virtual Particles: What are they?
Matt Strassler is a visiting scholar at Harvard University, and until recently a full professor at Rutgers University.

[3] Barnett et al (no date) http://pdg.web.cern.ch/pdg/cpep/unc_vir.html
The "Particle Adventure" resources are sponsored by the Particle Data Group of the at Lawrence Berkeley National Laboratory under the aegis of Michael Barnett.

 

[Evo]

What happened to my post?

Read your private messages.

 

[da_steve]

Thanks, everybody. Not sure i understand all your answers but enough to get the gist.