Workshop on tests of gravity at Case Western – day 1
ASTRO, COND-MAT, HEP-TH/PH — By Dmitry Podolsky on May 27, 2009 at 3:16 pm
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Dear friends,
I am sorry for being rather quiet for a while. As many of you may already know, my laptop has decided to enter coma during the trip, as a result, I was left without appropriate internet access (sporadic use of Pascal Vaudrevange’s computer is not counted, thanks, Pascal!). Although the laptop tried to revive after we came back home, demonstrating some kind of “brain” activity, in a couple of days I understood that the growth of entropy is as inevitable as a finally victory of string theory over loop quantum gravity.
Apart from those continuous attempts to revive the bugger, the workshop proved to be fun. I learned tons of staff and had a chance to talk to many people. Here I’ll try to present an informal report from the workshop, while its official webpage can be found on the Case website.
The morning part of the Day 1 program featured several talks (by Blayne Heckel, Tom Bay, Neil Ashby, Ricardo Decca and Casey Wagoner) on tests of gravity at small distances and deviations from equivalence principles.
I guess, the main conclusion I took home from this part of the workshop is that inverse square law clearly holds up to distances as small as 
m. Although the opposite case would most probably mean existence of large extra dimensions and all of us spent years studying and constructing different models of modified gravity 
, I guess, this experimental conclusion is of no surprise for me. It is so unfortunate that gravitational physics at intermediate length scales (that is, between Planckian length and the size of cosmological horizon) is boring, but that’s Life.
Or is it really? Note that the energy density of dark matter is about 
, which corresponds to a length scale of the order 
m – exactly, to the scale that experiment was able to achieve so far. Can DM really mean some kind of modification of gravity at relatively short distances (say, in the sense of MOND)? If yes, future experiments might be able to find it.
Second part of the day was devoted to black holes, and Avi Loeb gave the first talk after lunch. The talk was based on their paper with Broderick and Narayan, which we have already discussed on NEQNET, so I am not going to repeat the conclusion. There were two pieces of information new for me though: one is that Sgr A* is no longer the only candidate which we can detect the angular size of, the BH in the center of M87 will also do (it is 700 times more massive than Sgr A* and 2000 further from us, i.e., its angular resolution is quite small).
The second piece of information was received during one-to-one conversation with Avi. I did not understand why the source would emit thermal radiation if it is not a BH (they strongly used this suggestion in their analysis with Broderick and Narayan). I thought that it has something to do with kinetics of relativistic plasma in a deep gravitational well, but the physics was much simpler of course (and as usual I missed the simplest explanation) – photons in the vicinity of strongly gravitating body often return back to the surface of the body and only small amount of them has a possibility to overcome the gravitational attraction and leave the well (those photons are the only ones we detect of course). Therefore, we essentially deal with a pinhole experiment familiar from the course of general physics for 1 year grades – the one with a sphere filled with black body radiation
In the second afternoon talk Frans Pretorius has explained how black holes collide with each other and showed some rather amazing videos of computer simulations of GR. Imagine that you have two boson stars – that is, gravitationally collapsed objects made of scalar field 
. Both objects are of pre-threshold mass (that is, they are not BHs). Now, what conditions do you need to satisfy if you want to form a black hole after collision of these two objects? As it turns out, they should have relatively large Lorentz factors w.r.t. each other (
around 3). It was very interesting to see how much radiation is emitted in the process of BH formation…
Next talk was by Samir Mathur, and not surprisingly Samir was discussing fuzzball conjecture. His presentation was almost formulae-less (that’s because he knew that you are in the room – as Tanmay Vachaspati explained to me ) I guess, you already know very well the physics behind the conjecture, thereofore, allow me to proceed to my discussions with Samir before and after his talk.
Before the talk I blatantly tried to push my own agenda in the conversation with Samir – the agenda was explanation how coherence pattern (in terms of correlation functions of phase of the modes) of Hawking radiation should change in space and time, and Samir Mathur very quickly explained to me that everything I am talking about is known to string theoretic community for years He was certainly right, Though there was though also some exchange which forced me to get alert for a moment:
- So you are saying that everything is entangled with everything else, myself, Sun, distant galaxy and CMB radiation?, – I asked. – Everything is quantum?
- Sure, – he replied, warmly smiling.
- Wait a moment, we know where quantum mechanical effects are important and where they are not – you need to compare the classical action on a given trajectory with 
! For distances like this QM effects should be negligible.
- (Not so warmly) Yes, but we know what we are talking about – we are talking about decoherence. Of course, everything is decohered at macroscopic scales!
So, in the end of the day I seem to miss the fundamental difference between entanglement and quantum coherence, but I guess I am just nagging and really agree with him deep;y in my heart – everything is entangled. I guess, he wanted to say the following: in full Theory of Everything we are dealing with pure states (a quantum state of the Universe is a pure state), and string theory is a full theory. When we reduce it to GR, we introduce coarsegraining at scale of the order 
and neglect any transplanckian physics. Of course, coarsegraining in turn introduces entropy, and that’s the entropy we have to deal with in our sub-planckian world. Please correct me if I am wrong (Lubos?). With that only difference that he was trying to convince us that in BHs coarsegraining scale is not really 
, but BH radius instead.
(This strongly reminds me an old Russian anecdote stating that “during war times the value of cosine might reach 4″.)
Our second conversation, after Samir’s talk, was devoted to discussion of dS. In some sense, de Sitter is surprisingly similar to BH: it has a horizon (albeit specific for any observer) and thermal radiation coming from it, the essential difference is that we are now living inside the horizon So, how to apply Mathur’s machinery to dS – for example, what would be a counterpart of KK instantons, which are the microstates he would like to count inside BHs? The answer to that question remains unknown.
The last talk afternoon, about 5d BHs, was given by Dejan Stojkovic. Together with Frolov, Dejan has completely classified 5d BHs – I think it is quite a piece of work. Another thing he discussed is how 5d BHs interact with branes and evaporate.
10 Comments
Dorogoi Dmitry, welcome back. First, a few language corrections.
The victory of string theory is not going to be “finally” but rather “final”. What you learned were tons of “stuff”, not tons of “staff” which would be gravitationally equivalent to dozens of employees.
The accuracy of “56 microns” in the upper bound on the distance scale where new gravity-like effects appear looks more accurate than what one would rationally expect, and it is thus probably convention-dependent.
The pinhole explanation of the thermal spectrum in a gravity well is amusing but one must be careful not to confuse the full-fledged thermal curve with the grey body factors which have a similar “gravitational well” origin.
Quite generally, I believe it must be possible to intuitively explain why the BH radiation is thermal by an analogy with the pinholes, too.
The animations are cool. Equally cool are “ad hoc panels of tools” that I accidentally discovered in Windows by moving a zipped directory to the boundary of the screen.
Sumir (rather than Samir haha) and you: There are surely features of the phases in various BH-related events that became calculable in string theory – but you would have to define your agenda more precisely. Everything is quantum: what else could a sane physicist say.
I am not sure whether I understand your query about the relationship between coherence and decoherence
(they are the opposites of each other), so let me try to say what I think you may have wanted to ask, and what the answer is, and why it’s still important that the world is quantum.
Whenever we legitimately use classical physics in the real world, it’s because the classical limit is applicable. Fundamentally, the world is always fully quantum but the (usually) simpler classical description becomes OK under certain conditions.
The classical intuition – with “classical probabilities” only – is valid whenever decoherence is fast enough, which occurs for sufficiently large, sufficiently strongly interacting (with the environment) physical systems after sufficiently long times (usually supershort are already enough). This interaction with the environment destroys the purity of states of the studied quantum subsystem, destroys the quantum coherence, and is therefore called decoherence. It’s the only mechanism by which classical physics becomes legitimate.
You may have wanted to ask whether decoherence doesn’t imply that QM may always be ignored for all questions about large objects such as large black holes. The answer is No.
Why? Because on the other hand, when we study the systems exactly, like with the microstates of black holes, we never encounter any decoherence because there is no separation to the system and the environment. The fate of microstates is exactly localized, coherence and interference is always possible, even with large objects, and information can get out of a black hole because all the classical intuition (including exact causality that would follow from classical or even semiclassical GR) fails in this precise description.
So yes, I kind of agree with your following answer. The decoherence results from an approximate description, analogous to coarse-graining – the separation to the environment and the main system is not the same thing as coarse-graining but both are approximate descriptions of a system.
Whether you’re entangled with a star depends on the state of both of you, and historically such an entanglement is likely if you share a common origin.
Sumir’s fuzzball philosophy really brings the unusual conclusion that one can’t really coarse-grain over Planckian distances only, in the presence of black holes. In reality, one doesn’t really need fuzzballs for this conclusion. The latter holds because the degrees of freedom get rapidly thermalized – and smeared over the event horizon. You can’t really “localize” qubits to small – or even Planckian – regions because the qubits quickly encode themselves to properties of the whole horizon. This inability to localize qubits is pretty much equivalent to the detailed microscopic nonlocality of the black hole microstate dynamics that is needed – and true – to remove the black hole information loss.
Even the membrane paradigm prevents you from localizing things – like charge – on the horizon. The localization and locality of dynamics reappears in the classical limit only, and you are not allowed to follow individual microstates if you want to be satisfied with this classical description.
I have always agreed with the de Sitter – black hole analogy, where the interior and exterior are flipped. If we live inside, the Hawking (now dS thermal) radiation always gets reabsorbed by the horizon, making the dS space effectively constant in size, unlike an unstable BH, but otherwise most other things are analogous.
I thought that the 5D BH classification had been completed by people like Reall, Elvang, Emparan, and others years ago. Maybe not.
Dear Lubos,
About your last sentence, the existence of the hidden symmetry of the rotating 5D black hole characterized by the Killing tensor, separability of the equations of motion, non-existence of the stable circular orbits, properties of the principal null congruences, the existence of superradiance, classification to the Petrov type D class and many other things were actually shown by Frolov and Stojkovic.
I apologize for this sensitive personal inaccuracy, biased by my old Boston-area sources. But I am right that it has been 5 years ago or so?
Dear Lubos,
No need to apologize, really
. And you are right, it was 5-6 years ago. But the talk was much more than just properties of 5D black holes.
Dear 5dbh,
sorry, I realize that I did not cover Dejan’s talk well – just lost momentum closer to the end of the post. Would you be interested to write a guest post about this work?
Cheers,
Dmitry.
I take my misspelling of SM back. Sorry, Samir.
Dear Lubos,
Everything is corrected (apart of SM), love your language corrections (and even more – physics discussion). Thanks for your explanations of locality vs. non-locality in BHs, it is so much more clearer to me now after you presented it that way.
Yes, and now I wonder how would one be able to estimate gray body factors using this analogy and whether there should also be gray body factors for the situation Loeb discusses (that is, strongly gravitating body but not quite yet a BH).
As I understood, his microstates are KK instantons, so if the analogy between dS and BH works literally (it does not have to, I guess, since dS horizon is observer dependent), the question is where are they in dS case?
I also have a related cosmology question to you. How do KK instantons look like for a 4-dim observer, is it just a gravitating matter with some equation of state? If yes, what is the equation of state?
Cheers,
Dmitry.
Hi Dmitry, unfortunately I don’t know anything else about the grey body factors in the context above, and it may be harder for you to ask the right person after the meeting is gone.
Concerning the microstates, first, I guess that you wanted to use the term “KK monopole” rather than “KK instanton”. Second, the microstates are not “just” KK monopoles. A KK monopole is a terribly simple, featureless object. The KK monopoles can appear as geometric parts – or fibers – in a microstate, but they must still be multiplied by (or fibered over) a nontrivial manifold, at least a circle, that stores the information about the microstate.
Well, obviously, if the dS/BH analogy is good for the fuzzball intuition, the fuzzballs are outside your horizon volume or patch. They may be forced to converge to something even further, so that the counting gives you a finite entropy. It should be so because the entropy should be determined by the area of the emergent horizon only, and it is finite both for dS and BH.
Concerning the KK instanton cosmological question, let me assume that you mean KK monopoles. Well, I won’t say anything terribly interesting. KK monopoles are dual – and thus qualitatively equivalent – to ordinary electrically charged objects. They are just magnetically charged but the difference between electricity and magnetism is just a matter of S-duality, which can become very rigorous in N=2 or N=4 theories.
So for large enough observers who don’t care about the internal structure of the particle, KK monopoles are just like charged objects. Charged objects have a bizarre equation of state because you can’t really squeeze too many into a volume – they repel. You want to neutralize them. If you do so sensibly enough, you get the same dust as you do for normal electrons and protons. Not sure whether you can get something else.
It seems that my answer will be disappointing for you – but you tried to combine cosmology with KK monopoles which is a particle physics question, and I attempted to separate these two different disciplines again because they have almost nothing to do with each other in this case.
Sorry, going to watch Hercule Poirot.
Hi Lubos,
How was Poirot?
I did mean KK monopoles, I guess. What is a typical mass scale for them – inverse radius of compactification? If they are heavy, then the EOS is
as you say – and I wonder whether it is possible for them to play a role of dark matter, if they are present in the bulk.
I guess the next question would clarify whether it is possible – what group are they charged with respect to? Is it just ordinary EM U(1) or some “hidden symmetry” U(1)?
Cheers,
Dmitry.
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