Posted on 10/02/2004 4:43:49 PM PDT by PatrickHenry
It seems like the most obvious physical fact: The universe has four dimensions--three spanning space and one ticking away time. But the ultimate theory of gravity should explain why the universe is four-dimensional and how those dimensions arose, say researchers trying to unify the theories of quantum mechanics and relativity. Now, calculations in the 24 September PRL [Phys. Rev. Lett.] show that when all possible microscopic contortions of spacetime are added together, a large-scale four-dimensional universe can emerge.
For nearly 80 years physicists have struggled to reconcile the prevailing theory of gravity with quantum mechanics. According to Einstein's general theory of relativity, mass and energy warp spacetime. The undulations then affect the trajectories of passing objects, producing the effects we call gravity. In Einstein's theory, spacetime is a stretchy, dynamical entity.
However, the precise state of any dynamical thing remains uncertain, according to quantum mechanics. So at lengths of about 10-35 meters, spacetime can no longer be smooth, but must be roiling and frothy. That "quantum foam" bedevils researchers trying to concoct a quantum theory of gravity because in it concepts such as "ahead" and "behind" or "sooner" and "later" can loose their meaning. And no one has explained how the four-dimensional spacetime we take for granted emerges from the fantastical foam. Researchers have attempted to generate familiar four-dimensional spacetime by adding up all the possible configurations of the foam, thus borrowing a page from quantum mechanics, in which theorists assume that a particle travels between two points by taking every conceivable path at once. But these calculations have produced spacetimes that have either just two dimensions, or infinitely many.
The approach can produce a four-dimensional universe, however, if each particular version of the foam preserves a certain notion of cause and effect, report Renate Loll of the University of Utrecht in the Netherlands and colleagues. As in previous calculations, the researchers used a computer to generate approximate versions of all possible curved and frothy spacetimes by gluing together little triangular bits of flat four-dimensional spacetime, much as an artist might fashion abstract sculptures curving every which way by randomly gluing flat triangular tiles edge to edge. However, this time the researchers required that in each bit of spacetime nothing could travel faster than the speed of light, which implies that causes could never precede effects. In previous work, researchers imposed no such "causality." The team also took care to connect the triangles together in a way that preserves the cause-before-effect ordering.
The researchers added up all the possible spacetimes to see if something like a large-scale four-dimensional spacetime would emerge from the sum. That was not guaranteed, even though the tiny bits of spacetime were four-dimensional. On larger scales the spacetime could curve in ways that would effectively change its dimension, just as a two-dimensional sheet of paper can be wadded into a three-dimensional ball or rolled into a nearly one-dimensional tube. This time the researchers found that they could achieve something that appeared to have one time dimension and three space dimensions--like the universe we know and love.
"It's exceedingly important" work, says Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada. "Now at least we know one way to do this." Des Johnston of Heriot-Watt University in Edinburgh, Scotland, agrees the work is "very exciting" and says it underlines the importance of causality. "The other neat thing about this work is that you're essentially reducing general relativity to a counting problem," Johnston says. "It's a very minimalist approach to looking at gravity."
ok, NOW my head hurts.
Hmm what would Doctor Who think about all this???
ping for later
I had a hunch that this was the case.
I suppose that in light of a 4 dimensional world I don't feel so bad about higher tax rates.
"concepts such as "ahead" and "behind" or "sooner" and "later" can loose their meaning."
I had this epiphany after drinking a few Guinness in Temple Bar, Dublin late one evening.
Where would we be without FreeRepublic?
However, this time the researchers required that in each bit of spacetime nothing could travel faster than the speed of light, which implies that causes could never precede effects.
Do you think they meant effects could never precede causes, or is it just the usual handwaving?
My question is, if the universe is expanding, why aren't my pants?
"Des Johnston of Heriot-Watt University in Edinburgh, Scotland, agrees the work is "very exciting" and says it underlines the importance of causality."
Now if only the democrats could be persuaded to admit the importance of causality.
. . . which implies that causes could never precede effects
It is probably just sloppy writing (the writer got it backwards) but these two statements contradict each other. A cause precedes an effect by definition.
Or maybe it is just QuantumSpeak. Meaning it has no discernable meaning.
|
|
|
"My question is, if the universe is expanding, why aren't my pants?"
They are but your eyes are expanding too, so everything seems the same relatively. (And you can just keep my relatives out of this discussions - they never really existed. It's all been a dream.)
Looks like sloppy journalism. Unless this article bled through from some flip-flop Kerry-style universe.
Look behind you.(humor)
Too much cheese moose perhaps, could be series, ask your sister.
I feel so Homer Simpson, trying to grasp a 3D world.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.
