String Theory Acquires Rival In Loop Quantum Gravity

UniSci.com ^ | 31 Oct 2001

[sourcery]

String Theory Acquires Rival In Loop Quantum Gravity

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String Theory Acquires Rival In Loop Quantum Gravity Loop quantum gravity (LQG), rival of string theory in the quest to unite quantum mechanics with general relativity, does not suffer from certain mathematical "infinities," a new study shows. (Those infinities correspond to ephemeral but numerous alternatives in the way that interactions take place in spacetime.) This clears up some doubts as to the theory's usefulness. What is LQG, and why has it been so difficult to quantize gravity? To address this question, return to classical (pre-1900) physics, a regime in which space was fixed. Then the relativity and quantum revolutions changed everything utterly. With the advent of general relativity, space was combined with time in an integrated, but deformable, spacetime. Meanwhile, in quantum mechanics, spacetime remains fixed but matter becomes fuzzy; the whereabouts of particles can only be expressed in terms of probability clouds. In a theory that would combine quantum and gravity features, spacetime would then have to be both deformable and fuzzy, and this has been difficult to do. In string theory, the merger is accomplished by imagining that matter ultimately consists of tiny strings. In loop theory, the merger is attempted by imagining that space itself consists of moveable tiny loops. Carlo Rovelli (Center for Theoretical Physics, Marseilles, also University of Pittsburgh) argues that loop theory does not have to import the extra commodities (additional dimensions and particles) needed by string theory. Rovelli argues to loop theory offers, in principle, more testable predictions, such as the idea of quantized surface areas (that is, regions of space would come in discrete chunks and there would be a minimum possible size scale) and the notion that quantized spacetime might manifest itself as a minute difference in the speed of light for different colors. The new version of loop gravity studied by Rovelli and his colleagues pictures spacetime as being foamy: points in space sometimes grow into bubbles. The bubbles are not "in" space; they constitute space itself. The infinities pondered in the present paper represent not difficulties posed by the reality of particles within particles (a necessary complexity dealt with in Richard Feynman's quantum electrodynamics theory) but rather, analogously, to those potentially corresponding to interactions occurring on spacetime loops within loops. (Crane et al., Physical Review Letters, 29 October 2001; text at this URL.) (Editor's Note: This story, with minor editing, is based on PHYSICS NEWS UPDATE, the American Institute of Physics Bulletin of Physics News, Number 562, October 23, 2001, by Phillip F. Schewe, Ben Stein, and James Riordon.) 31-Oct-2001

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[BillF]

Bump to RealScience.

[JRandomFreeper]

So, does this mean I get my gravity generator or not?

/john

[JenB]

Intersting article. Will this have any real-world application or just be a good way to look at things?

[Hillarys Gate Cult]

So, does this mean I get my gravity generator or not?

According to this article, yes and no.

[PatrickHenry]

Sounds loopy. Bump.

[realpatriot71]

Cool science bump!

[monsterbunny]

OK...So help me out here, Your Sourceriness. My vague understanding of string theory is that at string-size scale (micros of micros of micros of micros) either ten or eleven dimensions exist. One source, Your Sourceriness, seems to indicate that with the eleven dimension theory time is both finite and all-encompassing which lends itself to the concept that the past, present, future all exist simultaneously. If my understanding is correct does that imply that time travel is possible if we could just get a handle on manipulation of these elements? Is this also the fundamental source of the Uncertainty Principle? Or am I waaaaay off base?

[ramdalesh]

Bumps-esh: view loop theory...RE

[Stirner]

I knew it! Space-time has to be quantized because that is the only real way to avoid Zeno's paradoxes. If space an time were really continuous, then any motion or change would require an infinite series of distinct points to be occupied. The usual solution to Zeno's problems is to point out that some infinite series (such as 1/2 + 1/4 + 1/8 ...) converge on a finite sum. But others (such as 1/2 + 1/3 + 1/4 ...) do not. If space-time is quantized, then apparently continuous space and time are "gappy" and apparently continuous motion and chage involve jumps.

[r9etb]

Is this also the fundamental source of the Uncertainty Principle?

The source of the uncertainty principle is that the act of measuring the energy of a particle disturbs its whereabouts, and finding the position of a particle disturbs its energy. You can't know both at the same time, because you can't measure them both at the same time.

This is not (to my simple mind) the same as saying that the particles don't have specific position and energy, but just that we can't know them.

[r9etb]

If space-time is quantized, then apparently continuous space and time are "gappy" and apparently continuous motion and chage involve jumps.

1. What's between the quanta?

2. Supposing you're correct -- does this suggest that we can exploit the "gaps" to get around space-time?

[monsterbunny]

It's a DIGITAL world afterall !!!

[PatrickHenry]

Quantum ping.

[VeritatisSplendor]

The usual solution to Zeno's problems is to point out that some infinite series (such as 1/2 + 1/4 + 1/8 ...) converge on a finite sum. But others (such as 1/2 + 1/3 + 1/4 ...) do not.

Yes, this is the usual solution, and yes, some infinite series do converge and others don't. You just don't understand the math well enough to realize why this is true so you insist that the mathematicians who proposed "the usual solution" must be wrong.

Look. It is OBVIOUS that the sequence 0.9, 0.99, 0.999, 0.999 has a finite limit, and that the sequence 1, 2, 3, 4, 5, ... has no finite limit. If you can grasp those concepts you can understand the resolution of Zeno's paradox. (That the sequence 1/2, 1/2 + 1/3, 1/2 + 1/3 + 1/4, 1/2 + 1/3 + 1/4 + 1/5, ... has no finite limit is not as easy to show as that the sequence 1, 2, 3, 4, 5... has no finite limit, but that's not relevant, the point is just that some sequences have limits and some don't.)

What Zeno's argument DID show is that if space is not quantized, then time can't be quantized either. And it is true that LATER scientists showed that matter is quantized, though the jury is still out on whether space and time are. But Zeno did NOT show that either space or time is quantized.

[spycatcher]

Quantum Hall effect gets new dimensions

‘The higher-dimensional generalization of the quantum Hall effect has been sought for a long time, but no one has succeeded before,’ Zhang told PhysicsWeb. ‘The mathematical structure could be very relevant to string theory, but the model is far, far from a realistic model of the universe.’

[freedomcrusader]

But the String Theory folks say that their strings can form loops, so is it really all that different (other than the extra dimensions thing)?

[VeritatisSplendor]

Typo -- where I wrote "0.9, 0.99, 0.999, 0.999" I of course meant to write "0.9, 0.99, 0.999, 0.9999"

[genefromjersey]

Hot dawggies ! This'n splains why the foil in mah trusty deerstalker hat ? Why it gits so dadburn crinkly ? Who'd a thowt it ?