What's in the empty space around an atom?

Will Tingle took on Phil's quantum question with the help of Ben Allanach, Professor of Theoretical Physics at the University of Cambridge...

Will - Good question, Phil. Alan on the forum chimes in with a good point. He says, 'yes, or at least from the point of view of the particle. The physics model that we use uses interaction cross-sections of different particles and media. The cross section for neutrinos to anything, for example, is so small that the neutrinos universe consists of almost entirely empty space. But if we look at an atomic structure, the electrons on the periphery are whizzing around in a sort of cloud, but they can't be all places at all times, even if they do travel at the speed of light.' So, what, if anything, makes up the empty space in their momentary absence? Well, regrettably, the answer is quantum physics. Fortunately, we have the University of Cambridge's Ben Allanach to talk us through it.

Ben - Well, there is something in apparently empty space, actually, because no space is completely empty. If you look at it at the smallest distance scales, quantum field theory tells you that every species of particle has a kind of quantum jelly, let's call it, associated with it, which fills the entire space. When there's a ripple in this jelly, there's a spatially small ripple. We would measure a particle there. So if there's an electron jelly, for example, through all space. When there's a localised ripple in that electron jelly, that's where we would measure an electron. And every particle species has one of these and if you have moving ripples, if the ripples move along the electron jelly, they will tend to excite ripples in the photon jelly. So what we understand then is that moving electrons tend to produce particles of light, photons. This is a big, old, complicated theory, but what happens is when you look at the tiniest distance scales, all of these jellies actually are all wobbling about and being very unruly, so they're all sitting on the background space and time.

Will - You're saying that, when there isn't, say, a subatomic particle filling a space, there is still this almost four dimensional net, this quantum field, that holds stuff together.

Ben - You can just think of it as the potential for a particle to be there. And then when there's a ripple in it, then you've got a good probability of the particle being there.

Will - Big thanks to Phil for the question, and Ben Allanach for the answer. Next time, we're answering a question from listener David, who asks, can you extract DNA from fossils? Good question. And we'll be digging into that one next time.