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Special relativty 1 : Quantum gravity 0

One of the key postulates of Einstein’s theory of special relativity is that the speed of light in a vacuum is constant. This means that whatever the energy of the photons making up the ray of light, its speed is always the same. It’s called Lorentz invariance, and it’s been on trial once again, courtesy of the Fermi Large Area Telescope (LAT) Collaboration.

In a paper published in Nature last month, Abdo and colleagues analysed the light coming from a distant and fleeting gamma ray burst to try and pick up any variation in the speed of its photons – and found no variation, at least down a limit.

Gamma-ray bursts (GRBs) are believed to be released during supernovae and are the brightest events occurring in the universe – despite the fact that most of them are billions of light years away from Earth. The radiation emitted during a GRB is extremely intense, typically releasing as much energy in a few seconds as the Sun will in its entire lifetime. They are good candidates for measuring a variation in light speed due to the cosmological distances the light has to travel to reach us – even tiny variations in photon speed are amplified enough to be revealed in the sharp features of the light curve emitted during the burst.

Artist's Illustration of Gamma-ray Burst GRB080319B

Illustration of a Gamma-ray Burst

Researchers at the Fermi LAT Collaboration were alerted to a particularly interesting gamma-ray burst after it was picked up by both the Large Area Telescope and the Gamma-ray Burst Monitor, which are aboard the Fermi Gamma-ray Space Telescope. This telescope is a joint project between NASA, the US Department of Energy and government agencies in France, Germany, Italy, Japan and Sweden, and is currently in low Earth orbit. A photon, with an energy of 31GeV, emitted less than a second after the start of the burst was singled out and used to find a limit for the variation of the speed of light.

In special relativity this limit should not exist, as there is no length at which the Lorentz invariance should be broken. However, some theoretical physicists think that at very small lengths it could in fact be violated. In order to formulate a “theory of everything”, they are attempting to reconcile gravitational effects with quantum mechanics and create a theory of quantum gravity. According to these theories, at the Planck scale (lengths of approximately 1.62 x 10-33 cm) quantum mechanics should interact with gravity, influencing the nature of space-time and so changing the speed of light.

In the research conducted by the Fermi LAT Collaboration, however, Lorentz invariance was found by two independent methods to hold true down to the Planck length divided by 1.2. This is a blow to some quantum gravity theories that require the fabric of space-time to be altered on small scales.

While this may be bad news for some modern day physicists, it’s good news for Einstein – after over 100 years, his theory of special relativity still stands.

Reference: Nature doi:10.1038/nature08574

Image Credit: NASA/Swift/Mary Pat Hrybyk-Keith and John Jones

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