Do virtual particles exist or not?

[Lapidus]

Conservation of energy, momentum and special relativity.

I posted a video lecture by Leonard Susskind some posts earlier, you might take look. He explains a few things about the energy-time uncertainty relation.

 

[Goldstone1]

There is no evidence for virtual particles being real, neither experimental nor theoretical. They are purely mathematical. The stability of the hydrogen atom has nothing to do with this.

You seem to be unaware that in atoms, this creation of virtual photons explains the Lamb shift observed in spectral lines.

 

[tiny-tim]

Maverick (or anybody else, of course!), how do you know so well that 'virtual' states/ processes do not appear in non-perturbative qft?

Lapidus, whatever makes you think that they do?

(remember, Professor Susskind says that they're only a "mathematical construct" )

 

[Polyrhythmic]

How and why does that follow from your (kinda bold) comparison of qft with calculus?

Because the only reason virtual particles show up in the theory is because we expand the path integral into a Taylor series. This is clearly calculus.

 

[Polyrhythmic]

You seem to be unaware that in atoms, this creation of virtual photons explains the Lamb shift observed in spectral lines.

That explanation is simply flawed. You can calculate the lamb shift, virtual particles may show up mathematically, but that doesn't tell us anything about reality.

 

[Goldstone1]

Goldstone1, virtual particles are a mathematical device coupled to a particular theory, there are no virtual particles in Lattice QFT for example.

As I explained, the Lamb Shift of the Hydrogen Spectral line is modeled by an interaction of virtual particles. If there is a small energy contribution, how can you call them mere mathematical abstractions? They are obviously quite real and tangible with real effects in our world.

 

[Goldstone1]

That explanation is simply flawed. You can calculate the lamb shift, virtual particles may show up mathematically, but that doesn't tell us anything about reality.

They show up mathematically as objects with a real energy which has been observed - need not I even mention the Casimir Effect which has an explanation of virtual particles, unless you want to adopt the new idea of it being van der waals forces, nevertheless, these ''objects'' are a ''mathematical'' representation which is physical in all its array - it's a real measured energy, so calling them ethereal is not correct.

 

[dm4b]

Well it's just that most interpretation have the fundamental flaw that they treat measurement as a classical event external to your quantum wavefunction. In reality quantum would suggest that when one makes a measurement one's "measurement Hilbert space" becomes entangled with the state vector/wavefunction of the system you were measuring.

Well, that much I understand, but I am still unable to connect the dots of how that solves the problem of the "instantaneous connection" between entangled particles.

I thought I read an article on physorg.com that mentioned a pair of scientists achieved entanglement in time, and showed that it is mathematically equivalent to entaglement in space. (But, don't quote me on that!)

I thought Penrose offered a cool view of entanglement along these lines. I'll have to see if I can't find it.

 

[Polyrhythmic]

They show up mathematically as objects with a real energy which has been observed - need not I even mention the Casimir Effect which has an explanation of virtual particles, unless you want to adopt the new idea of it being van der waals forces, nevertheless, these ''objects'' are a ''mathematical'' representation which is physical in all its array - it's a real measured energy, so calling them ethereal is not correct.

You can get all those results without introducing the concept of virtual particles.

 

[Lapidus]

Lapidus, whatever makes you think that they do?

The energy-time uncertainty relation. I thought you have watched video lecture, tiny-tim!

(remember, Professor Susskind says that they're only a "mathematical construct" )

Professor Susskind said a whole lot more than that. Maybe you might rewatch. But this time don't skip half of it!

 

[Polyrhythmic]

The energy-time uncertainty relation. I thought you have watched video lecture, tiny-tim!

It's you who should rewatch it. What he shows is that even if they existed, they were not detectable. But since they are only a mathematical construct anyways, this is just additional info.

 

[Goldstone1]

You can get all those results without introducing the concept of virtual particles.

What is quantum physics without virtual particles? They explain interactions very well. Why change something which is not broke.

 

[Goldstone1]

The energy-time uncertainty relation. I thought you have watched video lecture, tiny-tim!



Professor Susskind said a whole lot more than that. Maybe you might rewatch. But this time don't skip half of it!

Susskind, as great a scientist as he is, is one of many scientists with different views. Some see things more mathematical than others.

 

[Polyrhythmic]

What is quantum physics without virtual particles? They explain interactions very well. Why change something which is not broke.

Everything. Quantum field theory is fine, it is one of the most successful concepts in physics so far. And that without any need for virtual particles. To the contrary, once you introduce them, you have to explain unphysical and undetectable things, which is not the point of a physical theory.

 

[dm4b]

Well, in reality most physicists subscribe to the David Mermin interpretation of Quantum Mechanics which essentially consists of just one sentence:

"Shut up and calculate!"

When asked or polled they'll often just say the Copenhagen Interpretation (which is really very incorrect given the original meaning of the interpretation) because that's sort of a code word among physicists that means "*shrug* I don't really care". Similarly, a lot of physicists look at QFT as essentially the physics of Feynman Diagrams because that's how you really get any experimentally verifiable numbers out of it. So I think a lot of physicists would just off the cuff say "*shrug*, Feynman Diagram's represent real physics, why not, it doesn't really make a difference to me". And as I mentioned earlier this outlook led things astray for awhile (or so Zee claims, I'm not particularly knowledgeable about the history of QFT).

So I think the default position amongst working physicists is to just say Copenhagen Interpretation and Feynman Diagrams are real because this is sort of a code for "I really don't care about the Ontology of physics, since that way leads to madness and no remotely appliable (or publishable) results". You'd be surprised how few physicists give any thought to interpretation at all, the reason being it doesn't really make a difference and it's not going to help your career. But at the end of the day the concept of virtual particles has its origin in a mathematical crutch which has been shown to be less than perfect. It's really directly analogous to the role of perturbation in quantum mechanics, it's only good for catching perturbing potentials, if the perturbation is large (or the x^4 term of your QFT is large) your whole perturbation/virtual particle interpretations is going to be wrong, and that's known from the get go. That alone makes the Ontological notion of taking them as real to be crazy IMHO.

Thanks maverick. Yes, not surprised at all on that actually. I've always found it somewhat disappointing, because I personally love the interpretation side of things too. Seems like sort of a cop out to not try and address it, if you are after the "truth" about reality. I don't think we can get that from calculations alone. We need to ask what those calculations mean. Traditionally, that's how things were always done ... but now that things are getting tougher/murkier we seem to be shying away from that.

But, with 20 billion different interpretations to QM, who can blame them too ;-)

 

[Goldstone1]

Everything. Quantum field theory is fine, it is one of the most successful concepts in physics so far. And that without any need for virtual particles. To the contrary, once you introduce them, you have to explain unphysical and undetectable things, which is not the point of a physical theory.

Their by-products, their ''effects'' are a matter of experimental varification so you cannot say they are not detectable.

 

[Polyrhythmic]

Their by-products, their ''effects'' are a matter of experimental varification so you cannot say they are not detectable.

Those effects are also there if you completely leave out virtual particles. Nobody would miss them!

 

[dm4b]

How do you explain quantum physical static forces, such as the Coulomb force, without virtual particles?

I'd love to see a good explanation of this still too.

More generally, what is the mechanism behind the electromagnetic force.

The popular view is two electrons (or some other charged particle) exchanging virtual photons, which mediate the message. But, what is the correct way to view this, without the use of fictitious entities such as virtual particles?

I've never seen this explained well.

 

[Goldstone1]

Those effects are also there if you completely leave out virtual particles. Nobody would miss them!

I would miss them, and many quantum theories which rely on virtual particles, such as the Dirac Equation would miss them.

 

[Polyrhythmic]

I would miss them, and many quantum theories which rely on virtual particles, such as the Dirac Equation would miss them.

No quantum field theory relies on virtual particles. And regarding the Dirac equation, virtual particles have got nothing to do with it.

 

[Goldstone1]

No quantum field theory relies on virtual particles. And regarding the Dirac equation, virtual particles have got nothing to do with it.

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

 

[Polyrhythmic]

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

This is just not true. The Dirac equation describes fermionic particles and their anti-particles. The hole interpretation is outdated, and virtual particles have got nothing to do with all this.

 

[mattt]

You can compute the lamb shift and anomalous magnetic moment non-perturbatively. It has nothing to do with virtual particles.

 

[Lapidus]

It's you who should rewatch it. What he shows is that even if they existed, they were not detectable. But since they are only a mathematical construct anyways, this is just additional info.

Poly, they are by definition not directly detectable. Hence the name virtual.

But, as Prof. Susskind explains, or Prof. Randall, or anybody else with some very basic understanding of quantum physics will tell you that the time-uncertainty relation allows undetectable states/ particles to exist for short times.

And no perturbation theory mentioned.

In addition, they are necessary to turn up in the calculations to make the probability amplitudes for the measurable states come out right.

And yes, again my questions, if someone cares to answer in this very busy thread:

How do we know that 'virtual' states/ processes do not appear in non-perturbative qft?
(For example, what about all the off-shell histories in the path integral)

What physical mechanism actually forbids 'virtual' particles/ processes from happening, processes which are perfectly allowed by the laws of quantum physics?

How do you explain quantum physical static forces, such as the Coulomb force, without virtual particles?

 

[maverick_starstrider]

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

Virtual particles only appear in QFT when you essentially expand in a taylor series. If you DON'T need to expand in a taylor series they never show up. If we could solve all these integrals directly... they would never show up. They are solely an artifact of a specific APPROXIMATION scheme, of which there are known phenomena that cannot be described within this scheme. As I said, look at the equation for the first-order correction in non-relativistic (i.e. regular quantum) perturbation. It looks like this

$$\sum_{n \neq 0} \frac{\langle n \vert V \vert 0 \rangle}{E_n - E_0}$$

the

$$\langle n \vert V \vert 0 \rangle$$

COULD be interpreted as a kind of PROPAGATOR or TRANSITION AMPLITUDE. Saying that virtual particles are real is EXACTLY like saying that in reality this system ACTUALLY IS undergoing every possible transition to every higher-state through this funny, not quite right, propagation. No one thinks this way of course, and if it's not true in QM then why should it be true in QFT?

Let's be absolutely clear here. An expansion in Feynman Diagrams (i.e. using virtual particles) IS a Taylor series expansion of a non-gaussian integral. That's what it is. The little pictures help you keep the ordering of your Wick's Expansion straight. It's a visual tool to help you order your terms right.

 

[Goldstone1]

This is just not true. The Dirac equation describes fermionic particles and their anti-particles. The hole interpretation is outdated, and virtual particles have got nothing to do with all this.

Well I beg to differ... the Dirac sea is a reseviour of negative virtual energy states... have you done any work on this side of the theory?

 

[Goldstone1]

Virtual particles only appear in QFT when you essentially expand in a taylor series. If you DON'T need to expand in a taylor series they never show up. If we could solve all these integrals directly... they would never show up. They are solely an artifact of a specific APPROXIMATION scheme, of which there are known phenomena that cannot be described within this scheme. As I said, look at the equation for the first-order correction in non-relativistic (i.e. regular quantum) perturbation. It looks like this

$$\sum_{n \neq 0} \frac{\langle n \vert V \vert 0 \rangle}{E_n - E_0}$$

the

$$\langle n \vert V \vert 0 \rangle$$

COULD be interpreted as a kind of PROPAGATOR or TRANSITION AMPLITUDE. Saying that virtual particles are real is EXACTLY like saying that in reality this system ACTUALLY IS undergoing every possible transition to every higher-state through this funny, not quite right, propagation. No one thinks this way of course, and if it's not true in QM then why should it be true in QFT?

If the energy shift is measured, it is real. End of. Virtual particles are objects which have taken this identity.

 

[Goldstone1]

As Lapidus explains:

''But, as Prof. Susskind explains, or Prof. Randall, or anybody else with some very basic understanding of quantum physics will tell you that the time-uncertainty relation allows undetectable states/ particles to exist for short times. ''

This is what this energy shift is. It is a detectable difference in energy by a quantum which cannot be detected. It is is real though, because it leaves real physical effects in the world.

 

[tiny-tim]

Decouple the dirac equation into left movers and right movers, and then the theory asks where the positron comes from when it is created. It is effectively a hole in the sea, once a virtual particle. The Dirac Equation does involve the ''mathematical'' concept of virtual particles.

Goldstone1, this is complete rubbish

Even Dirac himself declared in print that the "sea" and "hole" idea should be abandoned.

(and holes never had anything to do with virtual particles, anyway … holes came in with the Dirac equation in 1928(?), and went out again when virtual particles came in)

hmmm … I'll guess that you've been looking at http://en.wikipedia.org/wiki/Virtual_particles#History" …

Paul Dirac was the first to propose that empty space (a vacuum) can be visualized as consisting of a sea of virtual electron-positron pairs, known as the Dirac sea.​

… this is simply historically wrong …

the Dirac sea was a "sea" of infinitely many electrons, and real positrons were holes in the sea: there were no virtual positrons, and no electron-positron pairs at all

And (for what it's worth) the historical article http://en.wikipedia.org/wiki/Dirac_sea" makes no mention of virtual particles! ​

 

[Lapidus]

Virtual particles only appear in QFT when you essentially expand in a taylor series.

Since I assume you are not an expert on non-perturbative quantum field theory that must just be a wild claim.

Why should there be no virtual particles or processes in non-perturbative qft? How do you know?