Output Stream of Consciousness

In the course of pursuing other questions, I ran across something in particle physics that I didn't know.

(Okay, okay, you can list what I don't know about in text and fill up a 40 Gb drive, but I digress)

I had always heard that gamma rays can produce particles "out of thin air"...

The explanation I had seen was that there is a seething cauldron of virtual particles that exist for very short periods, and that the gamma radiation could give these virtual particles, which were 'borrowing energy from the vacuum to exist', real existence.

I thought this could occur anywhere.

This turns out not to be the case.

Turns out that gamma rays will only produce particles if they are close to a charged particle, be it a nucleus, electron, or a whole atom (which may be neutral in total, but has charge separation).

There's a strong body of evidence that implies it never happens any other way.

So what's so special about charged particles that gamma rays won't produce an electron-positron pair?

Perhaps slightly more to the point, can anyone clear up why, if there are 'virtual particles everywhere', that they won't come to life from radiation without matter present?

Most of the produced particles in particle accelerators happen near the matter being collided or from decay products, besides. Would any of them occur without nearby matter?

Here's an interesting thread on the subject over on Physics Forums.

I get the feeling that our view of reality in 40 years is going to be profoundly different...