Light is very light. That is the conclusion of a table-top experiment to weigh light's fleet-footed courier, the photon. The report, appearing in a last week's Physical Review Letters, indicates the photon is slight indeed, weighing in at less than 0.00000000000000000000000000000000000000000000000002 grams. The new limit doesn't break any laws of physics--which say that the photon is massless--but the technique might provide a new way to search for new short-range forces.
Since no photon will sit still on a scale, physicists have resorted to less direct methods. If photons have mass, they reason, low frequencies of light will travel slower than higher frequencies. (Just as a "low energy" baseball travels slower than a high energy one.) A photon with mass should also slightly alter the strength of magnetic fields. But researchers have not seen light moving at different speeds, or noticed any anomalous weakening of either Earth's or Jupiter's magnetic field with distance. Those facts suggest that even if photons do have mass, it must be very tiny.
Seeking to bring photon experiments back to Earth, physicist Roderic Lakes of the University of Wisconsin, Madison, devised a new way to weigh a photon. Since photons carry the electromagnetic force, a photon with mass would imply a new weak kind of magnetism. The strange new force would be most visible around magnetic fields that stretch for large distances, such as the one that permeates our entire galaxy. So Lakes built a "cosmic compass," a delicate electromagnet dangling from a thin tungsten wire. The new force would tug on the magnet, forcing it to turn slightly as Earth spins through the cosmic magnetic field. After months of monitoring, Lakes saw not the slightest twist. The compass was still--except for some noise from students occasionally bumping the table. Since Lakes saw no evidence for the new force, he was able to knock the mass limit down a full decimal place, to 2 x 10-50 grams.
Lakes vows to keep at what he admits may be a never ending sport. If the photon does have mass, he says, one of the canons of physics "goes down the toilet," and that's reason enough to continue. Ed Williams, a physicist at National Institute of Standards and Technology, says the work is interesting and the technique sound. Lakes hopes the cosmic compass might be useful for sensing the feeble tug of other weak forces that would topple the accepted laws of physics.