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17 April 2014 12:48 pm ,
Vol. 344 ,
Officials last week revealed that the U.S. contribution to ITER could cost $3.9 billion by 2034—roughly four times the...
An experimental hepatitis B drug that looked safe in animal trials tragically killed five of 15 patients in 1993. Now,...
Using the two high-quality genomes that exist for Neandertals and Denisovans, researchers find clues to gene activity...
A new report from the Intergovernmental Panel on Climate Change (IPCC) concludes that humanity has done little to slow...
Astronomers have discovered an Earth-sized planet in the habitable zone of a red dwarf—a star cooler than the sun—500...
Three years ago, Jennifer Francis of Rutgers University proposed that a warming Arctic was altering the behavior of the...
- 17 April 2014 12:48 pm , Vol. 344 , #6181
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Cosmic Compass Points to Light Photons
10 March 1998 7:30 pm
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.