Vacuum fluctuations - possible stupid question

[Tim Gillespie]

My background is in Health Physics and as such, I have had only rudimentary instruction in quantum mechanics, so my understanding (such as it is) is largely conceptual. With that in mind, this may be a very ignorant question, so I apologize in advance.

I understand the theoretical basis for vacuum fluctuations and I understand the implications like the Casimir effect and Hawking radiation. However, one large piece of my understanding is missing. In simple terms, an electron, for example, may interact with a positronium virtual particle, which will very quickly annihilate. This is a "virtual" interaction and can only be detected indirectly by the effects mentioned above and others. So far, so good. My question is - what happens to the annihilation photons? Shouldn't the universe be awash with 511 keV photons from virtual positronium annihilation events? Clearly it is not, and the fault is with my understanding of vacuum fluctuations.

Thanks.

 

[Demystifier]

Real electron-positron pair has energy 2x511 keV's or more. But virtual electron-positron pair does not need to have that energy. Their total energy may even be zero, as one of them may have negative energy. A real particle cannot have negative energy, but a virtual one can.

That being said, I would also add that Casimir and Hawking effects cannot be well described in terms of virtual particles. Both effects are related to quantum fluctuations, but the concept of quantum fluctuations is much more general than the concept of virtual particles.

 

[PeterDonis]

My question is - what happens to the annihilation photons?

A virtual electron-positron pair doesn't have to be created from a pair of photons or annihilate into a pair of photons. If you're familiar with Feynman diagrams, imagine a diagram that's just a circle: an isolated loop consisting of one electron/positron line. Viewed as a sequence of "time slices", this looks like an electron/positron pair popping out of the vacuum, then annihilating back into the vacuum, without interacting with anything else.

 

[A. Neumaier]

Shouldn't the universe be awash with 511 keV photons from virtual positronium annihilation events? Clearly it is not, and the fault is with my understanding of vacuum fluctuations.

To improve your understanding of vacuum fluctuations, read https://www.physicsforums.com/insights/vacuum-fluctuation-myth/

 

[Grinkle]

To improve your understanding of vacuum fluctuations

I suggest 'To debunk your (mis)understanding ...' ;-)

At least it was that result for me. Where there was misconception, there is now a big question mark. I guess that is progress.