# Virtual photon-antiphoton pairs?

1. Apr 3, 2012

### johne1618

I have heard of the phenomenon of virtual particle-antiparticle pairs popping out of the vacuum and then back into it within a time $\Delta t \approx \hbar / \Delta E$.

Do virtual photon-antiphoton pairs pop out of the vacuum in a similar way?

I understand that antiphotons are the "same" as photons - is that right?

In that case how would the two photons annihilate? Does one have positive energy and the other negative energy?

John

2. Apr 3, 2012

### JamesOrland

No, not at all. When we say that a photon and an anti-photon are the same, we really mean it. There is no such thing as an anti-photon, because if you apply the transformations on it that would in other particles yield their counterparts, you get exactly the same you started with.

What is to say, an anti-photon is just a regular photon like any other. They wouldn't annihilate, they would just interfere with each other and then go on their ways.

And if you do assume they annihilate... annihilation produces only more photons. So you would have two photons colliding, which would produce two more photons that would just go their merry way, without minding each other too much.

3. Apr 3, 2012

### M Quack

There are no anti-photons. All photons have positive energy.

Two photons can not annihilate without creating a new particle/antiparticle pair, because of the conservation of energy.

If they have enough energy (more than 1.022 GeV in total), they can annihilate and create an electron-positron pair. If they have more energy they could also create other particles.

The electron is the lightest (=least energy) massive particle, so that is the first process that can happen.

4. Apr 3, 2012

### JamesOrland

Wait, I didn't know that. So all annihilation processes can be time-reversed?

5. Apr 3, 2012

### M Quack

Yes. It is one of the main mechanisms of how very hard gamma rays get absorbed - the second photon is usually a virtual photon of the electric field near a nucleus.

Look up "pair production" (Wiki entry is wrong because it give the impression that this can happen with a single photon)

http://www.icecube.wisc.edu/~tmontaruli/801/lect9.pdf

6. Apr 3, 2012

### JamesOrland

Interesting. But I think it's misleading of you, or it may cause some confusion, to say that they 'annihilate' because, at least as far as I know, the concept is linked to the idea of a particle/anti-particle pair generating energy when in contact, and not the other way around :P

7. Apr 3, 2012

### M Quack

Well, after the process you don't have any photons any more... Can you suggest a better word?

As far as "generating energy" goes, rest mass may be converted into energy (or the other way around). There is no energy generated in the strict sense. Total energy and momentum are conserved, along with charge, etc.

8. Apr 3, 2012

### JamesOrland

True enough, I suppose.

9. Apr 3, 2012

### M Quack

Coming back to the original question:

Yes, a pair of virtual photons can pop into existence for a short time and then disappear again. This can happen in vacuum and has even been measured in the Casimir effect.

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

10. Apr 3, 2012

Staff Emeritus
Here we go again.

There are at least three threads here where the argument that "Casmir proves virtual photons" is rebutted. One can calculate the Casimir effect without invoking virtual particles at all; therefore one cannot logically say it tells you anything about them.

11. Apr 3, 2012

### K^2

I wouldn't call that annihilation, though. They get absorbed by the created particles. In annihilation process, you are typically looking at a single world line that does a U-turn.

And yes, there has to be a virtual photon involved. Two light-cone photons aren't going to just interact in empty space to give you particle-anti-particle pairs.

12. Apr 4, 2012

### M Quack

I'll leave that discussion to the experts then.

Can you give a better example/proof of virtual particles from vacuum fluctuations?

The Cotton-Mouton effect has not yet been observed in vacuum, I believe. And I am not sure if it would prove the eistence of virtual particles from vacuum. But then again I am not an expert, as you already know.

13. Apr 4, 2012

Staff Emeritus
If you want to discuss that, I suggest you continue on one of the Casimir threads.

14. Apr 11, 2012

### johne1618

So there are no "real" antiphotons. Real photons travel on light-cones and don't "experience" time. Therefore there can't be any negative-energy photons traveling backwards in time to act like "real" antiphotons.

But if a photon is virtual then it can travel on a path off a light-cone.

If a negative-energy virtual photon travels on the time-reversed path would it then be a virtual antiphoton?

If virtual antiphotons can exist could virtual photon-antiphoton pairs be created from the vacuum?

I presume that at the edge of a black hole the gravitational field is strong enough to prevent the virtual photon-antiphoton pairs from annihilating so that one "real" photon can escape as Hawking radiation while the other "real" photon falls down the black hole.

Most elementary discussions of Hawking radiation describe particle-antiparticle pairs being pulled apart by the gravitational field. But the wavelength of Hawking radiation is of the same size as the black hole itself so it has to be of the form of photons and not massive particle/antiparticles like electron/positrons.

Last edited: Apr 11, 2012
15. Apr 11, 2012

### DrDu

The Casimir effect can conveniently be calclulated using the concept of "virtual photons". I am not surprised that there are other formulations. I can also do quantum mechanics without using imaginary numbers. Nevertheless they are a useful concept, not more and not less.

16. Apr 11, 2012

### DrDu

This particle-anti-particle concept has less to do with particles travelling backwards in time (a lousy trick of Feynman to draw funny pictures) but with the concept of charge. Photons don't carry charge, so they can can pop out in any reaction in any amount. You don't need to create them together with a particle carrying opposite charge.

17. Apr 11, 2012

### johne1618

Do virtual particles and antiparticles annihilate because their wavefunctions are mirrors of each other in every respect and therefore exactly cancel?

If an electron/positron pair pops out of the vacuum I guess they have opposite spin as well as opposite charge.

If one of the spins was altered somehow would that stop them annihilating completely if they collided?