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Large Intellect Collider - a discussion with Brian Greene
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The string theorist Brian Greene sat in discussion with Timothy Ferris in a packed hall at the San Francisco Jewish Community Center last night (Wednesday, 9/10).
Brian Greene
Although I usually enjoy Brian Greene's talks, I was frustrated by the conversational format. Turn Brian loose and let him hold forth on his own with his visuals behind him and he can take an audience on a stunning journey, leaving them a lot smarter about string theory, the birth of the cosmos, and other such topics, than they ever thought they could be. Force him to give 2-3 minute answers to a scattershot spectrum of questions, and he can never really get rolling.
He could have given a one-hour talk about any of the topics brought up by his interviewer, but 2-3 minutes on each didn't give him enough room to build up a head of steam and really cut loose. Clearly, the evening left some of the audience more than a little exasperated.
Having said that, there were a few showers of information that escaped from the collision of these two minds that were eagerly detected and absorbed by the audience.
Extra dimensions.
String theory predicts that space must be composed of 10 or 11 dimensions, most of which are hypothesized to be curled up so tightly that we don't notice them. In order to verify this prediction, string theorists have to be able to provide experimental evidence of the existence of these extra dimensions.
Image showing 2 extra dimensions curled up tightly
Question: How do you demonstrate the existence of extra dimensions?
Answer: You use the Large Hadron Collider (LHC) to smash particles together with so much energy that some of the debris from the collisions gets crammed into the extra dimensions. This would show up as a violation of energy conservation, as you would not detect the energy carried away from the collision into the extra dimensions, and would therefore come up with an energy deficit compared with the energy that went into the collision. Greene admits that there could be lots of other reasons why you might come up with an apparent violation of energy conservation. However, an energy deficit in the appropriate LHC experiments would give renewed hope to many string theorists, and lead them to attempt to eliminate experimentally the other possible explanations.
Greene thinks that the LHC might be just barely capable of generating collisions with sufficient energy to cram some of the debris into extra dimensions. If the scale of the curled up dimensions is on the order of 10 to the minus 19th meters, you would be able to cram enough debris into them to come up with a measurable energy deficit.
However, 10 to the minus 19th meters is a pretty large size for a string theorist, and if the curled up dimensions take up much less space than this, then the LHC would not be capable of cramming collisions debris into them. Thus, not detecting an energy deficit would put an upper limit on the size of the curled up dimensions. All this assumes that you can cram collision debris into other dimensions - something which nobody is really sure of. So, there is a small chance that the LHC could provide positive support for string theory, but it is not a very big chance.
Gravity and Dark Matter
Another topic where Greene was able to cast off a few sparks was on the subject of gravity. String theory is an attempt to reconcile Einstein's general relativity - a theory about the very large - with quantum mechanics - a theory about the very small. Both bodies of theory have been validated to very high-precision within their respective domains of application, but they cannot be reconciled mathematically in those areas where their domains overlap.
We believe that most of the forces that govern physical processes in our universe - the electromagnetic force, and the strong and weak nuclear forces - are nicely constrained to play out their role on our traditional three dimensional stage. Gravity, as Einstein pointed out, requires us to think in terms of 4-dimensional spacetime. If there are higher dimensions, it may be that gravity freely extends into them as well, unlike the other forces.
If so, then that would explain a couple of riddles that have perplexed scientists about gravity. First, it could explain why gravity is such a surprisingly weak force compared with the other forces: it is spreading out across many more dimensions than they are (albeit tightly curled dimensions). Second, it might explain dark matter as being the gravitational influence of matter in "nearby" dimensions spreading out to affect matter in our familiar 3 dimensions.
Image of dark matter clumping (blue) in collision of two galaxy clusters.
Because it is in another parallel universe, we cannot detect this "nearby" matter via our normal means of detection, which rely on forces constrained to our 3 dimensions of space, and hence this matter is "dark" to us. However, if gravity can seep across higher dimensions, then matter in our universe could be affected by matter in other universes - and visa/versa. Thus, the matter in parallel universes lying "close" to one another in a higher, roughly 10-dimensional, spacetime may become gravitationally coupled, to some extent.
For more information about string theory:
Three excellent video resources:
Brian Greene's String theory talk at TED
Nova - The Elegant Universe - with Brian Greene
Discover Magazine - string theory in two minutes or less
Other sites:
A surprisingly good Wikipedia page on string theory
Seed Magazine's string theory crib sheet
