View Full Version : Time and Space Dimension and the Big Bounce

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

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
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
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."

2010-Mar-21 Sun, 17:43
Hi Michelle,

I hope it's OK to go through old topics like this one without invoking a 'newbie-is-digging-from-the-graveyard' phenomenon that is so prevalent with forums. I just wanted to share a tip with you.

In your opening paragraph you mention wanting to print and store these articles. Well, aside from the regular Copy & Paste, most (if not all) web browsers allow you to save the web page you're viewing. Usually this is available from the 'File' menu item and sometimes also from the context menu (right-click on a PC, Option-click on a Mac). You can create a folder on your hard drive or removable media such as a USB stick and save or relocate the files there for archiving. Usually the web pages are stored as a HTML file along with a folder containing linked files such as images, style information and (page interactivity) scripts.

Alternatively there are also tools which allow you to automate these things. You can also download entire websites with those tools. (Just don't attempt to download something like youtube.com... :D.)