“Space-Time is a Mirage” – Will the Physics of Supermassive Black Holes Overthrow Einstein?

Massive_star_explosion Space-time that plastic fabric whose geometry can be changed by the gravity of stars, planets and matter – was born may be no more than a mirage, according to Peter Horava. Horava, who is at the University of California, Berkeley, wants to rip this fabric apart and set time and space free from one another in order to come up with a unified theory that reconciles the disparate worlds of quantum mechanics and gravity.

The world's physics community has started using Horava heresy to explain away the twin cosmological mysteries of dark matter and dark energy. Others are finding that black holes might not behave as we thought. If Horava's idea is right, it could forever change our conception of space and time and lead us to a "theory of everything", applicable to all matter and the forces that act on it.

For decades physicists have been stymied in their efforts to reconcile Einstein's general theory of relativity, which describes gravity, with quantum mechanics, which describes the sub-atomic world of particles and forces on the smallest scales. 

Space and time according to quantum theory are a static backdrop against which particles move. In Einstein's theories, by contrast, not only are space and time inextricably linked, but the resulting space-time is shaped by the matter within it.

Much of the motivation behind the quest to square relativity and quantum theory – to produce a theory of quantum gravity reflects the need understand what happened immediately after the big bang or what's going on near the event horizon of black holes, where the gravitational fields are massive.

Horava found his solution in the the physics of condensed matter, specifically in a carbon atom one atom thick, called graphene, whose electrons ping around the surface like balls in a pinball machine and can be described using quantum mechanics. Because the graphene atoms are moving at only a fraction of the speed of light there is no need to take relativity into account.

But cool this graphene down to near absolute zero and something extraordinary happens: the electrons speed up dramatically. Now relativistic theories are needed to describe them correctly. It was this change that sparked Horava's imagination. One of the central ideas of relativity is that space-time must have a property called 

What struck Horava about graphene is that Lorentz symmetry isn't always apparent in it. Could the same thing be true of our universe, he wondered. What we see around us today is a cool cosmos, where space and time appear linked by Lorentz symmetry – a fact that experiments have established to astounding precision. But things were very different in the earliest moments. What if the symmetry that is apparent today is not fundamental to nature, but something that emerged as the universe cooled from the big bang fireball, just as it emerges in graphene when it is cooled?

Horava tweaked Einstein's equations in a way that removed Lorentz symmetry: a property that keeps the speed of light constant for all observers, no matter how fast they move, time slows and distances contract to exactly the same degree. This led Horava to a set of equations that describe gravity in the same quantum framework as the other fundamental forces of nature: gravity emerges as the attractive force due to quantum particles called gravitons, in much the same way that the electromagnetic force is carried by photons. He also amended general relativity to include a preferred direction for time, from the past to the future -the way the universe as we observe it appears to evolve. 

"All of a sudden, you have new ingredients for modifying the behaviour of gravity at very short distances," Horava said in an interview with New Scientist.

By breaking asunder the symmetry between space and time, Horava's theory alters the physics of black holes – especially microscopic black holes, which may form at the very highest energies, which means for the formation of these black holes, and whether they are what they seem to be in general relativity "is a very big question."

Horava gravity might also help solve one of the great unsolved mysteries of modern cosmology: the puzzle of dark matter if the equations of motion derived from general relativity are slightly off this could explain the observed speeds of the stars and galaxies without dark matter playing a role.

"It is possible that some fraction of the dark matter picture of the universe could be coming from corrections to Einstein's equations," Horava says.

Ditto for dark energy: theories of particle physics predict the strength of dark energy to be about 120 orders of magnitude larger than what is observed, and general relativity cannot explain this enormous discrepancy. But Horava's theory contains a parameter that can be fine-tuned so that the vacuum energy predicted by particle physics is reduced to the small positive value that is in line with the observed motions of stars and galaxies.

The ultimate answers of course will come with Improved observations of supermassive black holes, which contain regions of intense gravity, which could reveal the necessary corrections to general relativity and prove Horava's theory of quantum gravity, in much the same way that unexplained measurements of Mercury's orbit showed that Newton's laws were incomplete, opening the door for Einstein.

Casey Kazan via newscientist.com

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