EVEN a child knows you can’t weigh a ray of light. But that may not always have been the case. Physicists have finally come up with an explanation for one of their most counter-intuitive ideas – that for a tiny fraction of a second following the big bang, light itself had mass.
The idea was born a few years ago, when Tomislav Prokopec of Heidelberg University in Germany and Ola Törnkvist of Imperial College, London, tried to explain why galaxies throughout the Universe are surrounded by magnetic fields. They suggested that the fields might be remnants of photons with mass that existed during the Universe’s initial period of rapid expansion.
But no one could explain how photons might acquire mass. Most particles are thought to get their mass from an as yet undiscovered particle called the Higgs boson. The idea is that a sea of Higgs bosons fills all of space and drags on particles travelling though it, making it harder to accelerate them. Those particles affected most are the heaviest, while photons are immune and so have no mass.
Advertisement
Now the researchers, along with Richard Woodard at the University of Florida, have come up with a way photons could, after all, have mass (Physical Review Letters, vol 89, p 101301). They say the secret lies in the vacuum energy that permeates all of space.
According to quantum theory, a vacuum is not really empty: instead it’s full of pairs of particles created from nothing. Normally these particles collide and annihilate each other immediately after they form. But in the first fraction of a second after the big bang, the Universe is thought to have exploded outwards incredibly fast, a period called inflation. For pairs of particles that can feel the pull of inflation, the rapid expansion of space would have pulled them so far apart they wouldn’t have been able to annihilate each other, and would have filled space.
The Higgs boson can’t affect photons, but these charged particles can. It would have taken more energy than normal to create a photon amid this sea of particles. And the particles would have dragged on the photons. In effect, the photons had a mass of about a hundred-billionth of a gram each. After inflation stopped, the extra energy associated with this mass would have created magnetic fields that evolved into the fields that exist today.
“It’s a fascinating example of a purely quantum mechanical effect occurring on cosmic dimensions”, says Woodard.