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Quantum Gravity - revealed by gamma ray bursts?

Gamma ray bursts - those terrific and mysterious flashes of high-energy light now considered to be probes to the farthest reaches of the Universe and earliest moments of time - may have yet another secret to reveal: quantum gravity.

Not yet observed in nature, quantum gravity is the long-sought missing link between Einstein's General Relativity and Quantum Mechanics, the two incongruous pillars of modern physics. NASA's Fermi Gamma-ray Space Telescope (Fermi) launched in 2008 with one of its goals to detect, for the first time, the effects of quantum gravity in the speed of gamma-ray burst photons.

The gist of this is that the gamma-ray bursts that Fermi detects will be powerful enough and distant enough to see the highest of the high-energy photons traveling slightly more slowly than lower-energy photons, weighed down by the effect of quantum gravity.

A gamma-ray burst represents the greatest outpouring of energy the Universe has ever seen aside from the Big Bang. Each burst is as powerful as a billion trillion suns, and satellites detect a burst or two a day. As common as the bursts are, though, no one is certain about what causes them. They are seen only in the gamma-ray waveband, although their afterglows fade away slowly in the X-ray and optical realms.

Gamma-ray bursts were discovered in the late 1960s, decades after the concepts of General Relativity and Quantum Mechanics first spiced the physics lexicon.

General Relativity accounts for gravity, the force that acts across large scales. Quantum Mechanics, part of the Standard Model, describes the behavior of the other three fundamental forces: electromagnetism, weak forces (seen in radioactive decay), and strong forces (holding subatomic particles together). These three forces act over small scales, and each has a corresponding particle that transmits that force: namely, photons (for electromagnetism), gluons (for strong forces) and Z and W particles (for strong forces).

The hypothesized particle that would account for the force of gravity is the graviton. Now, a graviton is not something you can look for in a giant particle accelerator, unlike a Higgs Boson or other exotic particles. Scientists instead look for the effects of the graviton, such as in gravitational waves rocking objects in space or, in the case of the gamma-ray burst, gravitons slowing a passing photon.

In quantum mechanics, the vacuum of space is not a vacuum; rather, it is a field with virtual particles, such as the graviton. Light passing through this field of virtual particles is refracted, just as it is when passing through water or any medium.

The graviton, being the essence of gravitational force, would interact with (or slow down) those particles with greater gravitational potential. With mass directly proportional to energy, as expressed in e=mc2, photons of higher energy have greater gravitational potential than lower-energy photons - as if they "weigh" more.

The highest-energy photons would therefore travel through space more slowly than lower-energy photons. (This does not violate the constancy of the speed of light, for light travels at the same speed only in an absolute vacuum.) To detect the very slight difference in photon speed, one needs an extremely distant source emitting extremely high-energy photons: that is, the gamma-ray burst.

One of the gamma-ray bursts that Fermi has now seen, known as GRB 090510, fulfilled scientists' needs by having a redshift of z=0.9. The scientists did not detect high-energy photons moving at different speeds from low-energy photos.

Studies with Fermi are ongoing, and more observations can help to confirm or disprove the idea of the graviton.

Several groups of scientists are working on the topic of quantum gravity and how to detect it. A team led by Dr. John Ellis of CERN is looking at low-energy gamma-ray bursts; Dr. Karl Mannheim of Universitäts Sternwarte and his group are looking at previously detected high-energy burst photons; and a group under the guidance of Dr. T.C. Weekes of the Whipple Observatory in Arizona is pouring through the data of the highest-detected gamma-ray photons, from relatively nearby active galaxies with black holes.

Clear-cut evidence for quantum gravity would ultimately open new pathways for physicists' prime goal of uniting all four fundamental forces under a Grand Unified Theory - a theory that explains the behavior of all matter and energy in all situations.

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Author: Christopher Wanjek; updated by Kelly Whitt

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