unification theory- Petr Horava

bogwalrus

I thoroughly enjoyed this article. I hope you do too.

And can someone explain to me whats going on in his theory then:)

http://www.newscientist.com/article/mg20727721.200-rethinking-einstein-the-end-of-spacetime.html

Anonymo

bogwalrus wrote: »

I thoroughly enjoyed this article. I hope you do too.

And can someone explain to me whats going on in his theory then:)

http://www.newscientist.com/article/mg20727721.200-rethinking-einstein-the-end-of-spacetime.html

Ok I'll give a stab at this one, but I'll need to explain one or two concepts before getting into the nitty gritty.

Lorentz invariance says (roughly) that no matter your frame of reference the result of a single experiment at the one location doesn't change. There have been loads of tests to check this as it is a foundation stone for special and (locally) in general relativity. Maxwell's equations follow exact Lorentz invariance for instance. It's really this invariance that links space and time into 'spacetime'. Redlights should start flashing when you see a theory that breaks Lorentz invariance. One thing essential for any theory is that it satisfies the experimental constraints on Lorentz invariance.

Ok the next thing to talk about is quantum mechanics against general relativity. There are scenarios such as just after the big bang and inside black holes where you need both theories to describe what's going on. However in this regime the results are nonsense - all you get are infinities. Theorists have been working for years on getting a 'Theory of Everything' to join these theories together. There's a book by Lee Smolin called Three Roads to Quantum gravity that talks about this. One road is to assume something is wrong with GR and QM is ok. Another is to assume something is wrong with QM and GR is ok. The last is to assume something is wrong with both of them.
Since QM has given a theory that describes the standard model forces (Strong, Weak and Electromagnetic) most people take the first route. String theory is an example of a theory that takes this road. Another is this Horava-Lif****z gravity. The basic idea of such theories is to mess around with the geometrical assumptions in GR e.g. GR assumes you can go to zero distance but in string theory the string length is the smallest distance that can be probed). In Horava-Lif****z they make space and time independent with the spacetime notion an approximate (but accurate) description at large distances.

The theory gives some nice predictions. I don't think the dark energy problem is really one of them since it really replaces one fine-tuning problem with another (but I don't know so much about this). The main problem as written in the article is that it's not clear whether it's a consistent theory. One of the problems with it, as I understand, is that the theory predicts a strong coupling at low energies and has some instabilities inherent. These problems are mainly to do with initial separation of space and time in the context of the theory which introduces a scalar mode of gravity which does the damage. There is an extension to Horava-Lif****z which gets around the problem by making the relation between space and time dynamical rather than the fixed relation in the original theory. This gets rid of the dangerous mode but the problem now is that there may still be strong coupling at low energies. Essentially the strong coupling changes the experimental predictions from what we observe (and which obey GR very well). So that is a pretty serious problem. People are working on getting around these problems but the key message is to take the theory with a grain of salt!

PS I tried as best I could to avoid over technical language. If anything isn't clear I'll give it another shot later.

Anonymo

I can't believe the second guys name was scrubbed out. It's Lifschitz without the 'c' by the way!!

bogwalrus

thanks again for that explanation. i followed what you were saying using some reference points.

It seems to be such a straight forward theory in some ways compared with string theory and its complexity with many alternate dimensions.

occums razor seems to have popped its head up. I wonder how this theory will develop.