Scanning for Cosmic Speedbumps –The Search for Quantum Gravity

Blackhole

A race across existence has helped in the search for an explanation of everything.  Light from an event over two billion light years ago allowed scientists to scan space-time itself for speedbumps, signs that quantum mechanics could crumple one of the fundamental rules of the universe.


The idea of quantum gravity is an attempt to find a Grand Unified Theory of Everything (GUT), and the Grand Unified Theory isn't the Holy Grail of physics – the Holy Grail is the Grand Unified Theory of Christianity (the former runs existence itself while the latter is just a fancy cup). 

Quantum gravity aims to merge general relativity, the science of the very large, with quantum mechanics, the math of the very small.  The problem is that while both have had huge successes in their home stadiums, when you try to put them together you end up with infinities – and it doesn't matter what's infinite, that means your math is broken.

Some strive to solve this problem by literally drawing little loops around the infinities and saying "Don't go in there!" (that's superstring), but other attempts to twist the titanic theories together predict grainy spacetime: a "quantum of space", not quite starring Daniel Craig, a smallest possible unit of reality.  This is so stupendously small it has no effect on most things, just as your being made of molecules doesn't come up when you catch a football, but extremely short wavelength light could be slowed by the itty-bitty bumpiness compared to the usual "speed of light is constant" thing (think of a toy car jiggling and slowed on a tarmac road while real cars aren't.)

The universe thoughtfully provided the perfect experiment, planning ahead by two thousand million years to set off an incredible gamma-ray burst.  Gamma rays are extremely high energy photons – according to quantum mechanics high energy means high frequency, and the (presumably) fixed speed of light means small wavelengths.  Detected by the Fermi Gamma-Ray Space Telescope (so we know that's working) the very smallest wavelength photons were delayed by 0.829 seconds relative to the first signal.  Over a two billion year trip.  That's on par with measuring Earth's oceans and finding one sink full missing.

These vital results were examined by a global team from over sixty institutions, with a combination of "incredible new test for fascinating theories" and "seriously, if this works it's a Nobel Prize."  The conclusion?  Lots of quantum-gravity theories don't work – but because this is science, that's a great result.  The data defines a new set of parameters, enabling universologists to narrow down the possible theories and close in on the goal of a GUT.

Image top of page: The black hole in galaxy NGC 4261 pulls in surrounding dust. The challenge is to describe a black hole according to the laws of quantum mechanics.

The Daily Galaxy via nature.com

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