Michele

2009-Dec-13 Sun, 05:39

This is an interesting article. I wanted to post it here so that can be part of the search returns for future references. I wish I had space to print and store all of these articles, but I don't. I'm always afraid the links will break. Anyway, here is the article.

Splitting Time from Space-New Quantum Theory Topples Einstein's Spacetime

Buzz about a quantum gravity theory that sends space and time back to their

Newtonian roots

By Zeeya Merali

http://www.scientificamerican.com/article.cfm?id=splitting-time-from-space

Was Newton right and Einstein wrong? It seems that unzipping the fabric of

spacetime and harking back to 19th-century notions of time could lead to a

theory of quantum gravity.

Physicists have struggled to marry quantum mechanics with gravity for

decades. In contrast, the other forces of nature have obediently fallen into

line. For instance, the electromagnetic force can be described

quantum-mechanically by the motion of photons. Try and work out the

gravitational force between two objects in terms of a quantum graviton,

however, and you quickly run into trouble-the answer to every calculation is

infinity. But now Petr Hořava, a physicist at the University of California,

Berkeley, thinks he understands the problem. It's all, he says, a matter of

time.

More specifically, the problem is the way that time is tied up with space in

Einstein's theory of gravity: general relativity. Einstein famously

overturned the Newtonian notion that time is absolute-steadily ticking away

in the background. Instead he argued that time is another dimension, woven

together with space to form a malleable fabric that is distorted by matter.

The snag is that in quantum mechanics, time retains its Newtonian aloofness,

providing the stage against which matter dances but never being affected by

its presence. These two conceptions of time don't gel.

The solution, Hořava says, is to snip threads that bind time to space at

very high energies, such as those found in the early universe where quantum

gravity rules. "I'm going back to Newton's idea that time and space are not

equivalent," Hořava says. At low energies, general relativity emerges from

this underlying framework, and the fabric of spacetime restitches, he

explains.

Hořava likens this emergence to the way some exotic substances change phase.

For instance, at low temperatures liquid helium's properties change

dramatically, becoming a "superfluid" that can overcome friction. In fact,

he has co-opted the mathematics of exotic phase transitions to build his

theory of gravity. So far it seems to be working: the infinities that plague

other theories of quantum gravity have been tamed, and the theory spits out

a well-behaved graviton. It also seems to match with computer simulations of

quantum gravity.

Hořava's theory has been generating excitement since he proposed it in

January, and physicists met to discuss it at a meeting in November at the

Perimeter Institute for Theoretical Physics in Waterloo, Ontario. In

particular, physicists have been checking if the model correctly describes

the universe we see today. General relativity scored a knockout blow when

Einstein predicted the motion of Mercury with greater accuracy than Newton's

theory of gravity could.

Can Hořřava gravity claim the same success? The first tentative answers

coming in say "yes." Francisco Lobo, now at the University of Lisbon, and

his colleagues have found a good match with the movement of planets.

Others have made even bolder claims for Hořava gravity, especially when it

comes to explaining cosmic conundrums such as the singularity of the big

bang, where the laws of physics break down. If Hořava gravity is true,

argues cosmologist Robert Brandenberger of McGill University in a paper

published in the August Physical Review D, then the universe didn't bang-it

bounced. "A universe filled with matter will contract down to a small-but

finite-size and then bounce out again, giving us the expanding cosmos we see

today," he says. Brandenberger's calculations show that ripples produced by

the bounce match those already detected by satellites measuring the cosmic

microwave background, and he is now looking for signatures that could

distinguish the bounce from the big bang scenario.

Hořava gravity may also create the "illusion of dark matter," says

cosmologist Shinji Mukohyama of Tokyo University. In the September Physical

Review D, he explains that in certain circumstances Hořava's graviton

fluctuates as it interacts with normal matter, making gravity pull a bit

more strongly than expected in general relativity. The effect could make

galaxies appear to contain more matter than can be seen. If that's not

enough, cosmologist Mu-In Park of Chonbuk National University in South Korea

believes that Hořava gravity may also be behind the accelerated expansion of

the universe, currently attributed to a mysterious dark energy. One of the

leading explanations for its origin is that empty space contains some

intrinsic energy that pushes the universe outward. This intrinsic energy

cannot be accounted for by general relativity but pops naturally out of the

equations of Hořava gravity, according to Park.

Hořava's theory, however, is far from perfect. Diego Blas, a quantum gravity

researcher at the Swiss Federal Institute of Technology (EPFL) in Lausanne

has found a "hidden sickness" in the theory when double-checking

calculations for the solar system. Most physicists examined ideal cases,

assuming, for instance, that Earth and the sun are spheres, Blas explains:

"We checked the more realistic case, where the sun is almost a sphere, but

not quite." General relativity pretty much gives the same answer in both the

scenarios. But in Hořava gravity, the realistic case gives a wildly

different result.

Along with Sergei M. Sibiryakov, also at EPFL, and Oriol Pujolas of CERN

near Geneva, Blas has reformulated Hořava gravity to bring it back into line

with general relativity. Sibiryakov presented the group's model in September

at a meeting in Talloires, France.

Hořava welcomes the modifications. "When I proposed this, I didn't claim I

had the final theory," he says. "I want other people to examine it and

improve it."

Gia Dvali, a quantum gravity expert at CERN, remains cautious. A few years

ago he tried a similar trick, breaking apart space and time in an attempt to

explain dark energy. But he abandoned his model because it allowed

information to be communicated faster than the speed of light.

"My intuition is that any such models will have unwanted side effects,"

Dvali thinks. "But if they find a version that doesn't, then that theory

must be taken very seriously."

Note: This article was originally printed with the title, "Splitting Time

from Space."

Splitting Time from Space-New Quantum Theory Topples Einstein's Spacetime

Buzz about a quantum gravity theory that sends space and time back to their

Newtonian roots

By Zeeya Merali

http://www.scientificamerican.com/article.cfm?id=splitting-time-from-space

Was Newton right and Einstein wrong? It seems that unzipping the fabric of

spacetime and harking back to 19th-century notions of time could lead to a

theory of quantum gravity.

Physicists have struggled to marry quantum mechanics with gravity for

decades. In contrast, the other forces of nature have obediently fallen into

line. For instance, the electromagnetic force can be described

quantum-mechanically by the motion of photons. Try and work out the

gravitational force between two objects in terms of a quantum graviton,

however, and you quickly run into trouble-the answer to every calculation is

infinity. But now Petr Hořava, a physicist at the University of California,

Berkeley, thinks he understands the problem. It's all, he says, a matter of

time.

More specifically, the problem is the way that time is tied up with space in

Einstein's theory of gravity: general relativity. Einstein famously

overturned the Newtonian notion that time is absolute-steadily ticking away

in the background. Instead he argued that time is another dimension, woven

together with space to form a malleable fabric that is distorted by matter.

The snag is that in quantum mechanics, time retains its Newtonian aloofness,

providing the stage against which matter dances but never being affected by

its presence. These two conceptions of time don't gel.

The solution, Hořava says, is to snip threads that bind time to space at

very high energies, such as those found in the early universe where quantum

gravity rules. "I'm going back to Newton's idea that time and space are not

equivalent," Hořava says. At low energies, general relativity emerges from

this underlying framework, and the fabric of spacetime restitches, he

explains.

Hořava likens this emergence to the way some exotic substances change phase.

For instance, at low temperatures liquid helium's properties change

dramatically, becoming a "superfluid" that can overcome friction. In fact,

he has co-opted the mathematics of exotic phase transitions to build his

theory of gravity. So far it seems to be working: the infinities that plague

other theories of quantum gravity have been tamed, and the theory spits out

a well-behaved graviton. It also seems to match with computer simulations of

quantum gravity.

Hořava's theory has been generating excitement since he proposed it in

January, and physicists met to discuss it at a meeting in November at the

Perimeter Institute for Theoretical Physics in Waterloo, Ontario. In

particular, physicists have been checking if the model correctly describes

the universe we see today. General relativity scored a knockout blow when

Einstein predicted the motion of Mercury with greater accuracy than Newton's

theory of gravity could.

Can Hořřava gravity claim the same success? The first tentative answers

coming in say "yes." Francisco Lobo, now at the University of Lisbon, and

his colleagues have found a good match with the movement of planets.

Others have made even bolder claims for Hořava gravity, especially when it

comes to explaining cosmic conundrums such as the singularity of the big

bang, where the laws of physics break down. If Hořava gravity is true,

argues cosmologist Robert Brandenberger of McGill University in a paper

published in the August Physical Review D, then the universe didn't bang-it

bounced. "A universe filled with matter will contract down to a small-but

finite-size and then bounce out again, giving us the expanding cosmos we see

today," he says. Brandenberger's calculations show that ripples produced by

the bounce match those already detected by satellites measuring the cosmic

microwave background, and he is now looking for signatures that could

distinguish the bounce from the big bang scenario.

Hořava gravity may also create the "illusion of dark matter," says

cosmologist Shinji Mukohyama of Tokyo University. In the September Physical

Review D, he explains that in certain circumstances Hořava's graviton

fluctuates as it interacts with normal matter, making gravity pull a bit

more strongly than expected in general relativity. The effect could make

galaxies appear to contain more matter than can be seen. If that's not

enough, cosmologist Mu-In Park of Chonbuk National University in South Korea

believes that Hořava gravity may also be behind the accelerated expansion of

the universe, currently attributed to a mysterious dark energy. One of the

leading explanations for its origin is that empty space contains some

intrinsic energy that pushes the universe outward. This intrinsic energy

cannot be accounted for by general relativity but pops naturally out of the

equations of Hořava gravity, according to Park.

Hořava's theory, however, is far from perfect. Diego Blas, a quantum gravity

researcher at the Swiss Federal Institute of Technology (EPFL) in Lausanne

has found a "hidden sickness" in the theory when double-checking

calculations for the solar system. Most physicists examined ideal cases,

assuming, for instance, that Earth and the sun are spheres, Blas explains:

"We checked the more realistic case, where the sun is almost a sphere, but

not quite." General relativity pretty much gives the same answer in both the

scenarios. But in Hořava gravity, the realistic case gives a wildly

different result.

Along with Sergei M. Sibiryakov, also at EPFL, and Oriol Pujolas of CERN

near Geneva, Blas has reformulated Hořava gravity to bring it back into line

with general relativity. Sibiryakov presented the group's model in September

at a meeting in Talloires, France.

Hořava welcomes the modifications. "When I proposed this, I didn't claim I

had the final theory," he says. "I want other people to examine it and

improve it."

Gia Dvali, a quantum gravity expert at CERN, remains cautious. A few years

ago he tried a similar trick, breaking apart space and time in an attempt to

explain dark energy. But he abandoned his model because it allowed

information to be communicated faster than the speed of light.

"My intuition is that any such models will have unwanted side effects,"

Dvali thinks. "But if they find a version that doesn't, then that theory

must be taken very seriously."

Note: This article was originally printed with the title, "Splitting Time

from Space."