BEING 4 minutes late doesn鈥檛 usually cause physicists around the world to fizz with excitement 鈥� but it鈥檚 a different matter if the latecomer is a photon, and its tardiness could indicate a breakdown of relativity on cosmic scales. What鈥檚 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 telescope collaboration 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鈥檚 theory of special relativity, both sets of photons should have arrived simultaneously, and the team is controversially .
Theories of quantum gravity 鈥� which attempts to 鈥� 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鈥檚 observed time delay, says team member Nick Mavromatos at King鈥檚 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鈥檛 explain the effect, he says.
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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. 鈥淚f true, it would be a Nobel prize-winning discovery,鈥� says Subir Sarkar, an astrophysicist at the University of Oxford.
For the two best-understood string-theory models,the news is not good. 鈥淚f the result is correct it would be earth-shaking,鈥� says string theorist Joe Polchinski at the University of California in Santa Barbara. 鈥淚f MAGIC is right then [conventional] string theory is wrong.鈥�
Amid the excitement, there is still a big question mark hanging over MAGIC鈥檚 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鈥檚 interpretation. 鈥淣ature 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,鈥� 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. 鈥淭heories of quantum gravity are so mathematical, people think you can only get at the right one by looking for the most aesthetically pleasing theory,鈥� he says. 鈥淣ow MAGIC has confirmed that gamma rays can probe the scales where quantum gravity might kick in, and that in itself is exciting.鈥�
Like Polchinski, string theorist Leonard Susskind at Stanford University in Palo Alto, California, believes that ultimately the time delay will be explained simply. 鈥淢ost of the time the results go away,鈥� says Susskind. 鈥淏ut once in a very great while they stick.鈥�
