BEING 4 minutes late doesn't usually cause physicists around the world to fizz with excitement - but it's a different matter if the latecomer is a photon, and its tardiness could indicate a breakdown of relativity on cosmic scales. What's more,
this delay could provide us with our first hints of quantum gravity at work, and thus be a unique way of testing string theory.
Last month, the
MAGIC gamma-ray telescopecollaboration based on La Palma in the Canary Islands announced that they had
measured a 4-minute time difference between the arrival of high and low-energy gamma rays released at the same time in a flare from the Markarian 501 galaxy, some half a billion light years away.
According to Einstein's theory of special relativity, both sets of photons should have arrived simultaneously, and the team is controversially claiming that the discrepancy is due to the first detected effects of quantum gravity.
Theories of quantum gravity - which attempts to shoehorn gravity into quantum mechanics - predict that space-time fluctuates rapidly on so-called Planck scales of about 10-35 metres. These fluctuations could have slowed down high-energy gamma-rays, causing MAGIC's observed time delay, says team member Nick Mavromatos at King's College London. Mavromatos and his colleagues have developed
a model for quantum gravity that is based on an unconventional version of string theory, which they say predicts the 4-minute delay exactly. Alternative quantum gravity models based on standard versions of string theory can't explain the effect, he says.
Not surprisingly, the announcement prompted a burst of high-energy activity on physics blogs, as arguments raged over whether or not the results are real. "If true, it would be a Nobel prize-winning discovery," says Subir Sarkar, an astrophysicist at the University of Oxford.
For the two best-understood string-theory models,the news is not good. "If the result is correct it would be earth-shaking," says string theorist Joe Polchinski at the University of California in Santa Barbara. "If MAGIC is right then [conventional] string theory is wrong."
Amid the excitement, there is still a big question mark hanging over MAGIC's time delay. The collaboration
cannot rule out the straightforward explanation that the high-energy and low-energy gamma-ray photons were emitted at different times at the source, admits team member Dimitri Nanopoulos at Texas A&M University, College Station. But he stands by his team's interpretation.
"Nature would be playing a dirty trick on us, if it was doing something strange at the source that created the exact time delay that our theory predicts,"he says. The team plan to analyse more gamma-ray flares to check whether they also display the same effect.
Even if the time delay does turn out to have another explanation, the result is still important, says Sarkar. "Theories of quantum gravity are so mathematical, people think you can only get at the right one by looking for the most aesthetically pleasing theory," he says. "Now MAGIC has confirmed that gamma rays can probe the scales where quantum gravity might kick in, and that in itself is exciting."
Like Polchinski, string theorist Leonard Susskind at Stanford University in Palo Alto, California, believes that ultimately the time delay will be explained simply. "Most of the time the results go away," says Susskind. "But once in a very great while they stick."
