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University of Iowa News Release


Oct. 7, 2011

UI astronomer, colleagues find clock to test Einstein's limit on speed of light

Einstein's Theory of Special Relativity and its speed limit on the velocity of light seem to be under special scrutiny by scientists these days.

And so it is that a University of Iowa researcher and his colleagues have published a paper in the Oct. 7 issue of the journal Science about a discovery that may make it possible to test whether the speed of light really is constant under all conditions.

Philip Kaaret, professor in the UI College of Liberal Arts and Sciences Department of Physics and Astronomy, says that the discovery was made while using the Arizona-based telescope VERITAS (Very Energetic Radiation Imaging Telescope Array System) to detect gamma rays being emitted by a pulsar -- a dense neutron star.

In brief, the Science paper discusses the unexpected discovery of pulsed gamma rays at energies higher than 100 billion electron volts -- higher than current theory explains -- emanating from the Crab pulsar, one of the most powerful pulsars in gamma rays. Rotating about 30 times per second, the Crab pulsar is located in the Crab Nebula some 6,500 light years from the Earth.

Kaaret says the discovery also makes possible a clock to test fundamental physics.

"Beyond helping understand pulsars, these results will also let us carry out an experiment I suggested back in 1999 to test if the speed of light really is constant at very high energies," Kaaret says. "Variations in the speed of light are expected when Einstein's theory of gravity is extended to include the effects of quantum mechanics, but only at very high energies.

"The pulses from the Crab are locked to its spin and come once every 33 thousandths of second. The pulsar spin is a clock, like a frantically accelerated second hand on a watch, that lets us time the gamma-rays very precisely," he says.

The pulses of gamma rays coming from the Crab pulsar at different energy levels can be compared to two different cars -- a race car and a passenger car -- rolling side-by-side down a race course and trying to match the speed limit. If both are successful, they should reach the finish line at exactly the same time. Similarly, Einstein's theories predict that gamma rays, regardless of their energy levels, should move no faster than the speed of light.

"By checking if the pulses at the highest energies come at the same time as the pulses at lower energies, we can see if the high energy photons are slowed, or possibly even sped up, along the way," he says.

"With VERITAS, we see no evidence that the speed of light changes. There are no problems with Einstein here. And, because the Crab pulsar just keeps on pulsing, we can get better and better limits as we keep observing," Kaaret says.

The Science paper comes in the wake of a Sept. 22 announcement that a group of European physicists working at CERN, the European Center for Nuclear Research, had timed a burst of subatomic particles called neutrinos breaking the limit on the speed of light as set down by Albert Einstein in 1905. Understandably, their results have been met with significant skepticism and await experimental verification by other researchers.

The Oct. 7 Science paper can found at

The 95 coauthors of the Science paper include scientists from 26 institutions involved in the VERITAS collaboration. There are three corresponding authors.

Corresponding author and postdoctoral researcher Nepomuk Otte is one of five coauthors at the University of California, Santa Cruz. Corresponding author and graduate student Andrew McCann is one of seven coauthors at McGill University. Corresponding author and research scientist Martin Schroedter is one of seven coauthors at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics.

VERITAS is funded by the U.S. Department of Energy Office of Science, the National Science Foundation, the Smithsonian Astrophysical Observatory, the National Sciences and Engineering Research Council of Canada, Science Foundation Ireland and, in the U.K., by the Science and Technology Facilities Council.

STORY SOURCE: University of Iowa News Services, 300 Plaza Centre One, Suite 371, Iowa City, Iowa 52242-2500

MEDIA CONTACTS: Media: Gary Galluzzo, 319-384-0009,; Program: Philip Kaaret, 319-335-1985,