Tuesday, June 19, 2007

Particles with No Mass

Geek Press points out that pysicists are searching for unparticles. What? Of course, I know very little of modern physics. Some would say that I'm better off. What did surprise me, that I missed in my two years of basic college physics and that never came up in discussions with my physics-major husband, was that photons are considered particles with no mass.

This is the way it is with particles: their mass corresponds to a particular energy, as Einstein famously formulated in E=mc2. At the energy levels that prevail in our everyday world, matter consists largely of the familiar particles that make up atoms: protons, neutrons and electrons. If energies were a thousand times greater, a new suite of particles would predominate, as is thought to have been the case in the early instants of the Big Bang. The Universe would then look very different.

Georgi's unparticle stuff doesn't behave like that, he argues in Physical Review Letters1. You can boost energy levels as much as you like, and the unparticles would look just the same. This property is called 'scale invariance'.

Light fantastic

There is already known stuff like this, Georgi points out: light, made of particles called photons. Their scale invariance is possible because photons have no mass and so can have any level of energy. Particles with non-zero mass can't show scale invariance. But that prohibition doesn't extend to 'stuff' that is not made of particles, says Georgi (he is careful not to call it 'matter', which implies particles). He says that theories of scale-invariant stuff have been understood mathematically for a long time. "But it's hard to describe this stuff because it is so different from what we are used to," he says. In particular, he notes, it can't be particulate.

So these unparticles have mass but are not matter. I'm very confused and my head is starting to hurt!

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