Is there anyone who could provide a more extensive explanation of Horava Gravity? Poking Holes in Einstein with Horava Gravity Thank you, Michael
Prefix - I've been to a few seminars about Horava-Lifshitz gravity, but that's my limit. Firstly, that is not the best article in the world. In fact, I'm being polite - it sucks. The stuff in pencils is graphite, not graphene. Graphene is a 2 dimensional sheet of carbon atoms and it seems to have interesting electrical properties. See the wiki for an overview. I'm not completely sure whether he's talking about graphene or graphite. Also, and more importantly, he claims that quantum mechanics breaks Lorentz symmetry. While that's true for the Schrodinger equation, it's emphatically not true for modern quantum mechanical theories like QED and the standard model (if you're feeling brave, have a look at the Coleman Mandula theorem for more information). It's commonly known that in relativity space and time are treated equally and form spacetime, while in quantum theories space is treated differently from time. Specifically, in quantum mechanics any observable is represented by a Hermitian operator - one can write an operator for position in space \(\hat{\underline{x}}\) but you cannot write one for time, ie, time is not an observable in quantum theories, and that is a problem when trying to write a quantum theory of gravity. Put simply Horava's idea was to say "what the hell, in quantum theories space and time are not the same so let's write a theory of gravity that does exactly that." The way he did that was to say that Lorentz symmetry is broken, but what seems quite miraculous is that when you turn the handle, you find that Lorentz symmetry emerges at long distances / low energies. That's the "perfect result" that your man refers to in his article, but it's not as good as it seems - Horava gravity makes some weird predictions that I don't know too much about. It contains a scalar mode of the graviton that grows uncontrollably. Normally that means your spacetime is unstable and that it is trying to flatten out, but this seems to happen in flat space as well. It seems like the theory will need some tweaking before it's ready to take the mantle of "correct theory of gravity." What would be really nice is if you could take a theory of everything (not getting into a debate about it, but string theory is the only possible example of this AFAIK) and show it reduces to Horava gravity in the appropriate limit.
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Thanks, prometheus. Does the Coleman-Mandula theorem basically put a restriction on the groups that can serve as physical symmetry groups? How is the word trivial used? What are they referring to? Thanks, Michael
Given a few assumptions (as outlined in the theorem) it follows that if a theory has a symmetry group which is a 'big group' containing both gauge symmetries and Lorentz symmetries then that symmetry group is the 'trivial' tensor product of the gauge symmetry and the Lorentz symmetry, there's nothing more complex you can do. And the reason it's a 'theorem' is that its an unavoidable mathematical consequence of the assumptions. The theorem isn't a theorem about the universe, ie it doesn't say "This is how the universe is", but instead it says "If the universe obeys A, B and C then the following must be true also....". A way to avoid (the typical terminology is 'evade' in the theoretical physics community) the conclusion is to use fermionic generators, which go not generate a Lie algebra.
Wow! Thanks, AlphaNumeric. That’s very impressive. I hate to push it because my understanding is very limited but you seem to know a great deal about it. So, this Horava Gravity is supposed to be regarded as a complete ultraviolet candidate for general relativity, right? It says it describes non-relativistic gravitons. Is there any chance that you could elaborate on some of the LMP black hole papers? Can you give an explanation that would be easier for me to understand? http://arxiv.org/PS_cache/arxiv/pdf/0905/0905.0179v3.pdf Thanks, Michael
HG is, I think, supposed to be UV complete but it does this by chucking the fundamental tenant of all current high level physics, ie special relativity, in the bin. If you don't require space-time to have Lorentz invariance down to the Planck scale then there's lots of problems which get altered but typically also a great deal more are made. Symmetries are extremely useful and powerful in theoretical physics so binning one needs a hell of a lot of motivation. As for the paper you've linked to you're going to have to be a bit more specific. Like Prom I have passing familiarity with the notions of entropic gravity models but not detailed familiarity so I can't summarise that paper off the top of my head and likely it'd be pointless to do so unless the answer is tailored to you in a more specific way. Is there any single bit which you have a question about? Are things like Schwarzchild-AdS R-N black holes something you are least grasp the definition of in terms of what them mean in regards to space-times and field equation solutions? I was still doing theoretical physics when the first papers on entropic gravity came out and overall I must admit I felt that they seemed to be a new way of reformulating known stuff, in a way which felt a lot like "I know what the answer should be, can I motivate such a formula?". It developed beyond that but interest tailed off pretty quick. Though I don't entirely agree with everything he says Lubos Motl's blog is a good place to look for detailed explanations of why various theoretical physics concepts might be flawed. For instance he gives a very good explanation as to why Penrose's recent 'pre big bang' cosmology might not be as ground breaking as Penrose hoped. Motl's pro-string theory so don't expect any compelling arguments against it from him, you might have to look elsewhere for that if that's what you're after Please Register or Log in to view the hidden image!
No, that's perfect. Thanks! You've been extremely helpful. Lubos Motl's blog, I'll check it out. Have a good one. Happy Holidays, AlphaNumeric! Please Register or Log in to view the hidden image!
Thanks! His blog was very informative, especially on the Lorentz violation. He did a nice job at pointing out why the interest in the paper has declined. A few papers indicate that Horava gravity ignores the Stueckelberg field. Does anyone know if low scale Lorentz violations means you have to have large Stueckelberg charges? The Stueckelberg Trick? What is that? Is that just about an electron moving backwards in time? Where it would appear to be moving forward in time because the sign difference is placed on the charge instead of the mass, e.g. anti-electron, or is there way more to it? Please Register or Log in to view the hidden image!
