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seminars

I was so busy I forgot to include an entry about last week's seminar by Joe Lykken on searching for extra dimensions. It was quite good, and I was surprised by how close we are to being able to detect consequences of various extra-dimensional models, as well as determine how many extra dimensions there are. Some of it is just barely out of the range of current experimentally testable limits (but not out of the range of LHC which will be operational in 2007). This was the title of the lecture:

String theory predicts extra dimensions of space, and suggests a number of physical mechanisms to hide them from our everyday experience. Some of these mechanisms are so efficient that even extra dimensions of macroscopic size could have eluded detection. Experiments are already underway, using a variety of techniques, aimed at the direct or indirect discovery of extra dimensions.


One interesting slide was his ranking of how certain we should be of the existence of extra dimensions in comparison to other theories which are very likely but not yet experimentally confirmed. Lykken said that on a scale of 1 to 10... where 10 is perfect ontological certainty, 9 is our certainty that Higgs particles should exist, 8 is our certainty that supersymmetry exists at some energy scale, ... and so on and so forth until you reach some joke about a lack certainty in our ability to elect good political leaders in this country (I think that was the joke, something of that nature) around 1 or 2. He ranks extra dimenions as an 8. In other words, up there with supersymmetry. I thought that was surprisingly high.

This week there's a seminar on detecting quantum gravity, and next week one about measuring how the cerebral cortex in rat brains transmits and stores information (using particle-detectors, somehow) to gain an understanding of the human cerebral cortex and its role in higher reasoning. Good stuff, I just need to make sure I don't spend all my time attending seminars and forget to get my work done! (Just got my midterm back--yet again, the highest grade in the class. I guess I can justify turning in incomplete homeworks a bit more now. For some reason, it just never seems like I'm getting this stuff, but I must be doing something right.)

Comments

( 15 comments — Leave a comment )
onhava
Feb. 24th, 2004 03:10 pm (UTC)
(Problems posting, so apologies if this eventually shows up multiple times.)

Lykken must be one of the coolest physicists ever. For proof, see the Halloween party videos here.
spoonless
Feb. 24th, 2004 10:57 pm (UTC)
Christ. You Chicago folks really know how to party! Is that Timothy Leery he's dressed up as???
onhava
Feb. 25th, 2004 07:56 am (UTC)
Seems to be. I don't think many of the people in that video are Chicago people; it looks like mostly Fermilab people. Although I only recognize a few.
spoonless
Feb. 25th, 2004 10:35 am (UTC)
I meant the city as much as the school (perhaps I was being intentionally ambiguous)

Before I forget... congratulations on all your acceptances. And good luck with deciding, visiting and choosing a school is a wonderfully exciting time.
onhava
Feb. 25th, 2004 11:12 am (UTC)
I was also being ambiguous -- Fermilab isn't really in Chicago, so most of those people probably live in Batavia or Geneva or some other outlying town. But, I shouldn't be nitpicking.

Thanks for the congratulations. It is pretty exciting to be in the process of deciding on a school.
pbrane
Feb. 24th, 2004 05:55 pm (UTC)
He ranks extra dimenions as an 8.

Damn. That's balls. He's pretty much saying "String theory is it" by saying this (although not in so many words). I'd certainly not rank it that highly, but who am I?

I'd put GUT at 8, SUSY somewhere at maybe 6, string theory of some kind (including extra dimensions) at 5.
spoonless
Feb. 24th, 2004 08:32 pm (UTC)

Damn. That's balls. He's pretty much saying "String theory is it" by saying this (although not in so many words). I'd certainly not rank it that highly, but who am I?

From the way he described it, it sounded to me like he considers extra dimensional models (which he abbreviated "ED's" for most of the talk) to be a superset rather than a subset of string theory. I was a little surprised by this, unless he's just using a narrow definition of string theory which doesn't include brane world models and such. I suppose Kaluza-Klein was around before all that stuff, but I didn't think it had much relevance any more outside of string theory.

I'd put GUT at 8, SUSY somewhere at maybe 6, string theory of some kind (including extra dimensions) at 5.

Yeah, I was surprised he didn't have GUT up there for comparison. Curious, how would you rank Higgs compared to GUT?

Whenever I hear a physicist lecture on their own field and compare it to others, I always knock off a bit from whatever measure of importance they give it, assuming that everyone automatically has some degree of professional myopia.
pbrane
Feb. 24th, 2004 09:31 pm (UTC)
From the way he described it, it sounded to me like he considers extra dimensional models (which he abbreviated "ED's" for most of the talk) to be a superset rather than a subset of string theory.

Yeah, I've heard of that kind of talk from phenomenologically minded people before, but I think that's kindof a load of hooey: the only known consistent QFTs in higher that 4d are those which are string derived, so people who talk about 6-d "large extra dimensions" theories seriously without talking about embedding them in a string theory are really kinda just saying "I have no idea what happens in the UV", and don't have much of a theory, per se.

Basically, once you get beyond 4-d, QFT behaves about as badly as quantum GR, which is to say - fairly badly, so it shouldn't be considered any sort of final theory.

Curious, how would you rank Higgs compared to GUT?

These questions should almost be on a logarithmic scale, actually. I'd put the probability of "some sort of Higgs like resonance" (ie. spin zero state which is part of a SU(2) doublet, singlet under everything else, which couples linearly to the chiral fermions and weak bosons) pretty damn near close to 1, probably only marginally less certain I was that we'd see the top quark back around '94. No Higgs-like state == > Standard Model is in big trouble.

GUTs are really aesthetically pleasing, and the way the SU(3), SU(2) and U(1) couplings run really hint (especially with SUSY) toward it, but there's nothing *flawed* in the theory if it's not there. But it requires the least leap of faith to have it there - electroweak symmetry already has exactly that kind of symmetry breaking/unification pattern, so it would be continuing an established trend in the same way. So I'd bet there is unification, given the minimal circumstantial evidence, together with the aesthetic reasons and precedent.

SUSY and ED models require pretty big leaps of faith (with I'd say the latter requiring more leaps of faith, as well as a larger fudge factor in getting the whole program consistent), but both aim to fix large problems. SUSY has the advantage in that with minimal SUSY, GUTs also seem to suddenly work out better as well, and low energy SUSY makes sense in string theory, while ED models *can* work in string theory, but have to be really crammed in there in most cases, from what I've seen.

But then again, you should follow your last sentence's advice re: me - I was "raised" a SUSY field theorist, and heard Savas Dimopoulous and Nima Arkani-Hamed *joke* about their "wacky numerology" they'd concocted (that later became the first "large extra dimensions" theories), so I probably have a similar degree of "professional myopia". :)
onhava
Feb. 24th, 2004 10:40 pm (UTC)
people who talk about 6-d "large extra dimensions" theories seriously without talking about embedding them in a string theory are really kinda just saying "I have no idea what happens in the UV", and don't have much of a theory, per se.

But all of these people are aware of that. I think that's part of the reason so many people are working on "warped geometries" so they can appeal to AdS/CFT, hoping that string theory will provide a good description of the 5D AdS end of things. Even if the universe isn't "really" five-dimensional, they give us another way of understanding strongly coupled gauge theories. I don't necessarily see anything wrong with just trying to develop effective field theories we can compare with experiment. That these look 5D doesn't necessarily mean the underlying geometry -- if that means anything -- really is 5D. Isn't that a lesson we can take from Arkani-Hamed's deconstruction stuff? I'm sure that at some level geometry is an emergent property (probably noncommutative or god knows what, and possibly without any obvious "dimensionality"), so the fact that these theories look 5D really is just a manifestation of the Wilsonian viewpoint that effective field theories need all allowed terms, even the nonrenormalizable ones.

Also, ED models make GUTs work much better in some cases, just as SUSY does. And they address the hierarchy problem.
pbrane
Feb. 25th, 2004 09:34 am (UTC)
But all of these people are aware of that.

I know the researchers themselves are aware of this - I was just trying to make more public (to sr. spoonless, in particular) what I think the "party line" is - that these models with extra dimensions or warped geometries or brane worlds, they all could be good effective theories, but if you're still looking for a UV theory, they pretty much all need to be embedded in string theory to really make sense (and even there, moduli stabilization and brane world stabilization are *far* from understood to work).

And I'm not saying working on these theories as effective theories is a bad idea either, but looking a little farther ahead to their completion is important - it'd be like writing down the Standard model in 1980 or so, but only going up to the bottom quark, because that's the only one that fit the phenomenology, even though we *knew* there had to be a top quark around the corner eventually. Similarly, writing down a 5-d theory without trying to embed it in some theory which can make sense of the UV behaviour of the field theory is similarly incomplete.

Heh... I must sound like I hate phenomenology, I don't! I just like to make sure to clarify that this is what is going on...

Maybe it's just that I'm comfortable with what's going on in a field theory, but not so much in some nebulous deconstructed brane world... so for example - I'm not so keen on Randall-Sundrum in its original form, because it "looks like" a full fledged theory, but it's really pretty phenomenological. OTOH, Strassler's mechanism for taking orbifolds of AdS that yeild nonsusy field theories which have naturally light scalars (due to weird strongly coupled dynamics), is just the field theory dual to the same basic idea, but yeild an actual *construction* of the Randall-Sundrum *idea*.

I guess that's my issue - I want actual constructions, not just neat ideas, before I feel like I want an experimenter to go looking for evidence of it.
onhava
Feb. 25th, 2004 11:08 am (UTC)
moduli stabilization and brane world stabilization are *far* from understood to work

OK, point taken. I don't know nearly enough about string theory to even know where I would start reading about such things. I guess I should finish Polchinski first....

it'd be like writing down the Standard model in 1980 or so, but only going up to the bottom quark, because that's the only one that fit the phenomenology, even though we *knew* there had to be a top quark around the corner eventually.

This comparison seems like a bit of a stretch.... Adding a top quark is not difficult. Finding a UV complete theory reducing to these LED models is much harder. In principle I'm sure everyone working on them would like to do this, but work on them shouldn't grind to a halt just because no one knows how to do this.

OTOH, Strassler's mechanism for taking orbifolds of AdS that yeild nonsusy field theories which have naturally light scalars

Awesome. I had never noticed that paper before. I'll study it tonight.
romanarce
Feb. 25th, 2004 04:39 am (UTC)
"These questions should almost be on a logarithmic scale, actually. I'd put the probability"
I'll make a totally idiotic comment about what a crazy mathematician would say about that: probability goes always from 0 to 1, now in a logarithmic scale every step you go the right you multiply by a number so if you have a probability between 0 and 1 then you can reach 1 after some steps, but if you go to the left you're just dividing by a number and you'll never get to 0, so a conventional scale could go from 0 to 10 but a logarithmic one would need to go from minus infinity to 10 to cover all posibilities. Anyway I'm just joking with all this. And I'm just realizing that for example an earthquake of magnitud 0 on the Ritcher scale does not have intensity 0, it would have (I think) an intensity 30^(-6) of an earthquake of magnitud 6, an intensity 0 earthquake would be minus infinity on the Ritcher scale (I only heard that it was a logarithmic scale with a factor of 30). And the stars with that annoying absolute and relative magnitud, what's wrong with watts and distance? Ok, going too off topic now. And why can't Winamp use a logarithmic scale for the sound volume, at the left of the scale a tiny step changes the volume a lot but at the right of it you need to move a lot to change the (human perception of) volume in the same amount, the reason is the same as with probability. A lame logarithmic scale could be ((n^x)-1)/((n^10)-1) for a scale from 0 to 10 with a factor n, it would be 0 for x=0 and 1 for x=10. Ok, shutting up now.
romanarce
Feb. 24th, 2004 06:06 pm (UTC)
A quote from that webpage:
"The simplest way that you can see evidence for an extra dimension is to try to produce a high-energy graviton, the quantum of gravity, which could then move off into the extra dimensions. In a particle accelerator we try to collide very high-energy particles and hope occasionally to produce a high-energy graviton that moves off into extra dimensions and disappears.
This is something we don't see directly -- we don't see the graviton that disappears -- but we notice that energy and momentum were carried off by some invisible particle and from that we deduce that something strange happened. In this case a high-energy particle, a graviton, moved off into extra dimensions. So this is the simplest kind of experiment you can do, and if you can eliminate other kinds of possibilities for things that carry off energy invisibly, you would then be able to claim that you've seen evidence for extra dimensions of space.
"
I think I'm gonna need a bit more than that, a photon always has a probability of avoiding detection and it's energy/momentum rate would be the same as with gravitons so they miss a photon at the LHC and therefore extra dimensions exist? I like physics in which theory with little arbitrarity has a lot of complex consequences in experiments like Schodinger's equation with the exclusion principle and spin explaining the entire periodic table, now I know that I still need to learn details of all that so I shouldn't be jumping to conclusions but extra dimensions because you seem to have lost some energy and momentum is going too far I think.
And about supersymmetry, again I need to learn more about it but the laws of quantum mechanics were always the same for all particles without any distinction between elementary and non-elementary, but supersymmetry is like drawing a line between them, unless they expect a fermion partner for phoNons, a boson partner for electrons/holes in a semiconductor and a fermion partner for electron pairs in a superconductor. Anyway experiments dictate.
spoonless
Feb. 24th, 2004 08:52 pm (UTC)

I think I'm gonna need a bit more than that, a photon always has a probability of avoiding detection and it's energy/momentum rate would be the same as with gravitons so they miss a photon at the LHC and therefore extra dimensions exist?

It would be up to the experimenter to make sure that the disappearance exceeded any statistical fluxuations. And as with anything, there is a danger of a false positive the first few times you see something--it would have to be confirmed in multiple experiments.

I like physics in which theory with little arbitrarity has a lot of complex consequences in experiments like Schodinger's equation with the exclusion principle and spin explaining the entire periodic table, now I know that I still need to learn details of all that so I shouldn't be jumping to conclusions but extra dimensions because you seem to have lost some energy and momentum is going too far I think.

I agree, predicting the entire periodic table, or the equivalent, is much much better. What we really need eventually is something that predicts all the masses of the particles correctly. That would be the ultimate confirmation (of any unified field theory), in my mind. Of course, it may not be possible. But it's nice to dream of.

One of the things he talked about is that some of these ED models predict that extra copies of the current particles will be seen at higher energies, [because of some kind of wrapping around of the dimensions?]. If those extra copies are seen, then it's hard to deny. He mentioned that if those are seen, it might be difficult distinguishing those from SUSY partners... but he outlined what would be the distinguishing factors to look for.
pbrane
Feb. 25th, 2004 09:19 am (UTC)
The rate of energy loss via gravitons in ED scenarios is a power law in energy (with power determined by the number of extra dimensions), so this is easily distinguishable from photon or neutrino energy loss.
( 15 comments — Leave a comment )

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