# Destruction of virtual particles and virtual particle pairs

1. Jun 21, 2013

### Maximise24

I'm trying to understand the process that destroys particles very shortly after they "appear". I have read that they are usually annihilated by a virtual antiparticle, much in the same way as can happen to real particles. However, is this always the case? And if so, is it an intrinsic property of these particles to appear in pairs and why exactly is this?

In other words: could a virtual (anti)particle be created on its "own", without a partner, for example because of the Heisenberg time-energy uncertainty relation, simply because it exists too briefly for it to have any real energy? Apparently, a virtual pair can be split up (Hawking radiation), but can they also be created that way?

2. Jun 21, 2013

### Bill_K

Creation and absorption of any particle, virtual or not, must obey the conservation laws. A photon can be created alone, but an electron cannot because it is a negatively charged particle, and a positively charged particle (positron) must be created at the same time.

Energy must always be conserved too. Not just on the average, or over a long time, it must be exactly conserved whenever a particle is created or absorbed. If the new particle carries away energy, one of the other particles must lose the same amount to make up for it.

3. Jun 21, 2013

### phinds

FYI, there is an analogy regarding Hawking Radiation that says it is caused by virtual particle pairs popping into existance just outside the event horizon of a black hole, wherein one of the pair falls in and the other heads off into space, and the one that falls in always reduces the mass of the BH, thus evaporating it via Hawking Radiation.

BUT ... Hawking himself has said that this is JUST a way of talking about it in English and that actual particle-pair production is NOT the true reason for Hawking Radiation.

4. Jun 21, 2013

### Bill_K

It's easy to think the picture is that of an itty bitty particle being created "just outside" the horizon of a great big black hole. In fact, Hawking radiation is a global effect (so you can't say where it happens) and it only happens when the size of the particle is comparable to the size of the hole. Furthermore it's not just energy that disappears into the hole. If the emitted particle is electrically charged, the electric charge of the hole must change so as to compensate, just as if an antiparticle had fallen in.

5. Jun 21, 2013

### phinds

This would seem to clearly imply that only really tiny BHs experience Hawking Radiation. Doesn't seem to make any sense.

6. Jun 21, 2013

### Bill_K

That's correct. As the hole gets smaller, the process goes faster. For astronomical size holes the rate is quite negligible!

7. Jun 21, 2013

### phinds

So you're telling me there are particles the size of supermassive black holes? Is this some kind of joke?

8. Jun 21, 2013

### Bill_K

No no, the effect is appreciable only for SMALL black holes. And yes, large (i.e. long de Broglie wavelength, very low energy) particles.

9. Jun 22, 2013

### Staff: Mentor

With a 4-vector of (0,0,0,0)? How is that a "photon"? How is that anything?

To clarify: The "size" of a photon is given by its wavelength in that description, and the size of massive particles by their de-Broglie wavelength.

10. Jun 22, 2013

### Bill_K

The OP asked whether virtual particles have to be created in pairs. I answered that a photon can be created alone, by which I meant without creating a second particle. For example an electron can emit and reabsorb a virtual photon.

11. Jun 22, 2013

### Staff: Mentor

Okay, connected to other particles, a single photon can appear. But then you also get interactions like $W^- \to e^- \bar{\nu_e}$, where an electron appears without a positron.

12. Jun 22, 2013

### Maximise24

So, if I understand correctly, virtual particles can appear "on their own" (provided they do not break any conservation laws) in connection to other real particles, but not, say, in a vacuum, where there will always be a virtual antiparticle created with it?

13. Jun 22, 2013

### Staff: Mentor

Yes. The connection to real particles is necessary for energy/momentum conservation.

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