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New Measurement Technique Puts Elusive Virtual Particles in Relief

It turns out that measuring nothing is pretty hard.

A pair of Japanese physicists has developed a new method for probing the peculiar particle fizz that we generally know as vacuum energy, or the built-in energy of empty space. The technique measures a vacuum effect known as Delbrück scattering and may allow for sensitive tests of the theory of quantum electrodynamics. The duo's work is described this week in the Physical Review Letters.

What do we mean by vacuum energy? One of the great kicks of quantum physics—perhaps the greatest, even—is that there is no nothing. Nature really does fundamentally abhor a vacuum and this abhorrence tells us how the universe will end. Which is as nothing, or as good (dark, cold) as nothing gets.


Generally, this is the notion of vacuum energy. If we take some small cube of space and (somehow) remove all of the matter and energy from it, then there is still some left. There's nothing we can do about it—there's just no zero to be found in emptiness. Broadly, this is a consequence of the universe's rule of uncertainty, which places a limit on what we can know simultaneously of certain properties of particles. If I cannot, say, know perfectly both the position and momentum (velocity plus direction) of a particle, if follows that I can't say that some chunk of space is empty because that's just too much (too certain) knowledge.

That sounds a bit abstract, but this ban on perfect zero has a physical manifestation. This is the notion of virtual particles. Basically, because the universe can't just deal with some emptiness, it's perpetually spinning out particle/antiparticle pairs as like a perpetual low-level jitter. These are real particles with real energy, but we don't have to worry too much about them because they annihilate each other usually right away.

The phenomenon can be seen in the Casimir effect. If you take a couple of metal plates and hold them really close together, you find that the vacuum forces acting between the plates are less than those acting on the outside of the plates because you've constrained the possible particle wavelengths of the virtual particles in the gap. The result is a slight attractive force between the two plates.


This brings us to the new research. As incoming "real" particles, gamma rays in particular, meet a field of virtual particles, such as those congregating around an atomic nucleus, there's a scattering effect. They bounce off of it. This effect is known as Delbrück scattering.

If we could get some really good measurements of this form of scattering, we might learn some new things about quantum electrodynamics, or the processes by which light and matter interact. These new things might even point toward "new physics," or physics beyond the current dossier of known particles and forces. The catch is that this sort of scattering occurs in concert with three other forms of scattering, so it's hard to pick out from all of the noise.

The solution the researchers found is a combination of tweaks, particularly to the polarization of the incoming gamma rays (the particles to be scattered). Polarize photons of just the right energy in just right way and then blast the field at just the right angle and we can get a scattering effect some two times that of the other concurrent scattering effects. For new observations using this method, they predict a 1 percent accuracy over the course of 76 days.

These are still just calculations, however. The next step is experimentation with the technique and this is when we'll start really poking around the realm of new physics.