String Theory vs Loop Quantum Gravity

[luckyme]

Apparently they'll be launching the Glast satellite in 2007. It'll have the sensors necessary to detect differences in the time it takes gamma rays from billions of light years away to get here. If there is a difference, it'll be evidence in favor of Loop Quantum Gravity.

There is also a potential to test ( in future experiments) for evidence of time before the big bang , which may turn out to be 'the Big Bounce'.

Not bad for a science that was 'over' a hundred years ago or so.

If Loop Quantum Gravity does pan out, it means that spacetime is not continous but comes in little pieces. I read a comment by Einstein from just before he died where he suspected that (field theory?) may not hold up.. does anyone have that reference?

luckyme

[yukoncpa]

Hi luckyme,
I just finished reading about loop gravity in Wikipedia. But, unless I missed something, how is it that they'll be able to test for evidence of time before the big bang?

[luckyme]

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I just finished reading about loop gravity in Wikipedia. But, unless I missed something, how is it that they'll be able to test for evidence of time before the big bang?

[/ QUOTE ]

The physics is over my head, and the comment was about the "potential to test", it sounded like some of the early math had been done but an actual test is yet to be devised but seemed feasible. It's interesting what this concept does to the 'flow of time' way we experience events.
There's an excellent abstract here (http://relativity.livingreviews.org/open?pubNo=lrr-2005-11&page=title.html)

luckyme

[Borodog]

The problem with explaining these and similar theories is that the number of people in the world capable of truly understanding them is extraordinarily small. I have a Ph.D. in physics, but because my specialty was different (experimental astrophysics via computational fluid dynamics), I have absolutely no idea how these theories work. It would probably take me a really long time to understand these theories myself. What chance does a layman stand? /images/graemlins/frown.gif

[luckyme]

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What chance does a layman stand?

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At any meaningful technical level, None really. That's why peer review and open exchange is so critical in science. If we can rely on that process then a layman can a least grasp the concepts involved and some of their implications, along the lines of how we absorb Relativity and quantum mechanics.

luckyme

[BluffTHIS!]

Scientific American has a special issue on the newstands now regarding these type of physics matters. It is enough to give anyone with at least some prior knowledge of particle physics and cosmology a decent overview, without a need to understand the math involved. Similarly, Brian Greene's oft mention book The Fabric of the Cosmos, along with periodic articles in SA and another book or two is enough to keep up on the subject. However loop quantum gravity is a topic with a highly developed and complicated mathematical foundation which is certainly beyond me.

2007 will also see CERN's Large Hadron Collider come online, with the hope of actually observing the Higgs-Bosun particle, although such an observation still won't convey much information about the underlying Higgs Field.

[Metric]

[ QUOTE ]
Apparently they'll be launching the Glast satellite in 2007. It'll have the sensors necessary to detect differences in the time it takes gamma rays from billions of light years away to get here. If there is a difference, it'll be evidence in favor of Loop Quantum Gravity.

[/ QUOTE ]
Interestingly, this prospect seems to very seriously irritate some superstring theorists -- I have seen the feeling expressed that the LQG people are "desperate" to make a testable prediction before they are truly ready with a solid calculation. However, an observation of a frequency dependent propagation velocity through free space would indeed be a blow to superstring theory, which essentially forbids such an effect from the start.

Personally, I'm rooting for a frequency dependent delay -- there has been a very notable lack of "surprises" in particle physics for several decades. Such an observation might not really establish LQG, but it would probably go a long way to shake up the strange contentment with the "untestable" status of superstring theory.

[yukoncpa]

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However, an observation of a frequency dependent propagation velocity through free space

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Hi Metric,
Is there any way you could explain the above in layman's english? I do love physics, but unfortunately, In CPA school, we only learned how to add and subtract ( multiplication was extra credit).

[Metric]

[ QUOTE ]
[ QUOTE ]
However, an observation of a frequency dependent propagation velocity through free space

[/ QUOTE ]

Hi Metric,
Is there any way you could explain the above in layman's english? I do love physics, but unfortunately, In CPA school, we only learned how to add and subtract ( multiplication was extra credit).

[/ QUOTE ]
In standard relativity, all massless particles (like photons, e.g. gamma rays) travel through free space at the same velocity, "c," according to all inertial observers (locally, anyway). A frequency-dependent velocity would indicate some kind of breakdown of this "Lorentz symmetry" of spacetime. But superstring theory takes this symmetry to be absolutely fundamental -- it's essentially a postulate of the theory. Hence, if it were found that some massless particles were moving ever-so-much-faster than others, it would be bad news for the foundational assumptions on which superstring theory is built.

On the other hand, LQG does not make this assumption. In LQG, spacetime is built-up from "spacetime quanta" instead of postulated to take a specific form at all length scales. In particular, the standard Lorentz symmetry is something that is expected to hold only in an extremely good "large scale" approximation, similar to the way substances (e.g. water) can be thought of as continuous at the large scale, but we understand that they are not really continuous -- it's really a bunch of individual atoms/molecules on the small scale. Since the light that it going to be observed in this experiment has travelled across the universe, tiny imperfections in the Lorentz symmetry may effectively "add up", causing photons of different frequency to move at very slightly different speeds (i.e. a certain frequency would arrive at the telescope first).

Although LQG makes this seem very possible (while superstring theory rules it out), the problem is that LQG calculations have not really even reproduced standard relativity yet -- the problem is that calculations become exponentially difficult as soon as you make "space" bigger than a few quanta across. So it's very hard to get to the "large scale limit" and concretely check that things are behaving as expected. The LQG predictions make heavy use of "reasonable assumptions" that can't be fully justified at this time -- hence the irritation that some string theorists feel at the prospect of the LQG people "claiming victory" if a frequency dependent velocity shift is actually observed...