Saturday, September 24, 2011

Theory Carnival: Phenomenological Quantum Gravity

[Geek Mommyprof from Academic Jungle is hosting a carnival on real people's work in theoretical or computational sciences, and what that work entails. She asked me to contribute some lines about what I do for a living, so here we go.]

I am a theoretical physicist and I work on the phenomenology of quantum gravity. Phenomenology is the part of theory that makes contact with experiment. (For more read my earlier post On the Importance of Phenomenology). Quantum gravity is the attempt to resolve our problems in formulating a common treatment for the quantum field theories of the standard model and Einstein's general relativity. Quantum gravity has for a long time been dominated by theory, and it's only been during the last decade or so that more effort has been invested into phenomenology.

I like working in this area because it offers interesting and still unexplored topics, and if there will ever be an experimentally confirmed theory of quantum gravity there's no way around phenomenology. My work requires keeping track of what the theorists are doing and what the experimentalists are planning and trying to find a way to connect both. Since gravity is a very weak interaction, finding evidence for its quantum effects is difficult to do, and so far there has been no signature. In fact, it can be quite frustrating if one puts in the numbers and finds the effect one considered is 40 orders of magnitude too small to be measurable, which is the normal state of affairs. I've joked on occasion I should write a paper "50 ways you can't measure quantum gravitational effects," just so all my estimates will finally be good for something. But there are areas, early universe and high energy densities, high energies and large distances, where it doesn't look completely hopeless.

Lacking a fully established theory of quantum gravity, phenomenology in this area requires developing a model that tests for some specific features, may that be extra dimensions, violations of Lorentz Invariance, antigravitation or faster-than-light travel. Model building is like having a baby. While you work on it, you have an idea of how it will be and what you can do with it. Yet, once it's come into life, it starts crying and kicking and doesn't care at all what you wanted it to do. Mathematical consistency is a very powerful constraint that is difficult to appreciate if one hasn't made the experience: You can't just go and, for example, introduce antigravitating masses into general relativity. It sounds easy enough to just put in stuff that falls up, but once you look into the details the easy ways are just not compatible with the theory, and it turns out to be so easy not. (I should know, since I spent several years on that question and out came a paper that I doubt anybody read.)

You might ask now, well, what has antigravitation got to do with phenomenological quantum gravity? Nothing actually. It's just that people always ask me what I work on and I used to say: A little bit of particle physics and a little bit of cosmology and my recent paper was about this-and-that and I'm also interested in the foundations of quantum mechanics and organizational design, and then I wrote this paper on the utility function in economics and so on. But I figured that what they actually wanted was a three word answer, so that's why I work on phenomenological quantum gravity. On the institute's website it says I work on "high energy and nuclear physics," which isn't too far off, still, 5 is larger than 3.

But no matter what the headline, what my work looks like is like this: I start with an idea and try to build a model that incorporates it while maintaining mathematical consistency, after all that's what I sat through all these classes for. In addition, the model should be compatible with available data and ideally predict something new. The failure rate is high. But there's the occasional idea that turns out not to be a failure. It gets written up and submitted to a journal and, if all goes well, gets published. I usually publish in Classical and Quantum Gravity, Physics Letters B or Physical Review D.

In the process of working on a paper, I almost always have an ongoing exchange with some people who work on related topics. If the finances allow it, I might visit them or invite them to come here. I might also attend a workshop or conference, or organize one myself. In addition, my work brings the usual overhead like writing or reviewing grant proposals, attending or giving seminars, coming up with a thesis topic, reading applications, reviewing papers, attending faculty meetings and so on. I presently work at a pure research institute, the Nordic Institute for Theoretical Physics in Stockholm, and have no teaching duties, which has advantages and disadvantages. And if you are following this blog you know that I'm only just back from parental leave.

For more on what my work is like, see also What I am is what I am and One day. You can also follow me on Twitter, or Google+.

27 comments:

Juan F. said...

If you are referring on the paper on the antigravitation you wrote and you think nobody read, you can safely say you are wrong. Some people, like me, read it! :D
I am happy to follow your blog. After all, there are no many people who have tried to write about antigravitation. By the way, Schërk tried to show antigravity was not a crazy idea. Only weird.

I would remark three areas where now everyone is pushing ahead:

-Neutrinos ( interesting for a QG paper like Neutrinos and Quantum Gravity)

-TeVatron/LHC and Higgs (desperately, very desperately seeking the Higgs signal)

-Entropic gravity ( you did a great work writing the review; somone asked yesterday in the OPERA results about the temperature dependence on the result, something wrong directed, I guess, to argue against a thermodynamical/entropic origin of gravity).

Plato said...

Hi Bee,

The Theory of Carnival of course set off neurons firing:)Pirsig called them,"the traveling tent-show Chautauquas." Just adding a little ole fashion flavor to an institution.:)Oh Dear!... How Technology has Changed Things.


The "Talking Pictures" Projection Wagon-

In the 1920's about the only entertainment that came to the rural community of Leakey, Texas was the traveling tent shows. This form of family entertainment would come to the canyon about once a year to the delight of all. Everyone looked forward to the horse drawn wagons that brought the much anticipated entertainment to town. In later years the horses were replaced by the Model T Fords but this form of transportation did not deter the excitement.

See:"Leakey's Last Picture Show" by Linda Kirkpatrick
Vintage photos courtesy Lloyd & Jackie Shultz

Best,

Plato said...

One millionth of a second after the Big Bang, the universe was a hot, dense sea of freely roaming particles called quarks and gluons. As the universe rapidly cooled, the particles joined together to form protons and neutrons, and the unique state of matter known as quark-gluon plasma disappearedString theory may hold answers about quark-gluon plasma

So where did gravity start?:)

Best,

Uncle Al said...

http://arxiv.org/abs/1109.4897
http://press.web.cern.ch/press/pressreleases/Releases2011/PR19.11E.html
Superluminal neutrinos

Observation falsifies defective theory. Pure theorists whine non-Euclidean path (gravity well), Sagnac effect, inverse Shapiro delay... systematic error. Dark Star, "teach it phenomenology."

Valid measurement, valid GR. Propagation through a dispersive medium chirped pulse shape. Optical "early" signal detection happens at will (phase velocity, group velocity). Relativistic neutrinos are chiral, the vacuum is selectively chiral toward mass, neutrino pulses' centroids shifted forward. Chiral vacuum background active only toward mass horrifies theory because it is quantifiable in existing apparatus. Opposite shoes on a vacuum left foot vacuum free fall non-identically,

http://www.mazepath.com/uncleal/erotor1.jpg
Two geometric parity Eotvos experiments

Theory makes more theory - an unemployment bureau firing half its staff to become a closed system with 100% reimbursement. Ya gotta look! Fix the problem, mourn the dead, get on with the job.

Bee said...

Hi Juan,

Oh, I am maximally flattered you read my equation cluttered masterwork ;-) More seriously, I'm somewhat cynical because if you'd plot the effort that went into my paper vs the attention they got, you'd probably find they are anti-correlated, which is somewhat depressing.

And who is Schërk?

In any case, one very important area where everyone is pushing ahead too is all the early universe stuff, go back in time as far as you can.

I've also been thinking about the entropic gravity recently, but I still haven't made up my mind about it. It could just be true, but then again, it seems too cheap. Best,

B.

Bee said...

Hi Plato,

Here's some more neurons firing upon hearing "theory carnival" ;-)

"Have I been wrong
Have I been wise
To shut my eyes
And play along
Hypnotized
Paralyzed
By what my eyes have found
By what my eyes have seen
What they have seen?

Have I been blind
Have I been lost
Have I been wrong
Have I been wise
Have I been strong
Have I been
Hypnotized
Mesmerized
By what my eyes have found
In that great [blog] carnival
In that carnival?

~ Natalie Merchant, Carnival

Best,

B.

Giotis said...

This means you are doing String phenomenology too?

GMP said...

Bee, thanks for writing this post -- it was a pleasure to read! --and for participating in the carnival!

Bee said...

Hi Giotis,
Not presently. Best,

B.

Plato said...

"Observations always involve theory."Edwin Hubble


Bee,

Analyses of these light flashes are already providing tantalizing informationMiniBooNE

Could have been a "Three Ringed Circus".....yet, you've kept it real enough.:)

Researchers also have found that the class of ancient exploding stars, or supernovae, used to measure the expansion of space today look remarkably similar to those that exploded nine billion years ago and are just now being seen by HubbleHubble Finds Evidence for Dark Energy in the Young Universe

Best,

Mud said...

Cool Stuff!
I understand some of the ideas although I'm pretty ignorant of the procedural details involved with these models, which renders my opinion useless. That's a lot of ground to cover! Your right though, trial and error will eventually weed out what is considered true and what is considered absurd. Your practical interpretation of it is surely appreciated.

Plato said...

A back yard party and Chirping crickets?

Technology most certainly provides for better observations and helps us break up the misconceptions we can have. As well, as to hone in on what that information and data did provide.

Best,

Plato said...

Bee,

Maybe yo already have some answers for?:)

Best,

Juan F. said...

Dear Bee. I am happy to read you. I hope I can finish my MSch on schedule...So I am not posting as much as I could and I am thinking and writing a lot for work.

Joel Schërk wrote some nice papers on SUSY-SUGRA and "antigravity". His notion of antigrativation was something similar but not the same than the one you wrote in your paper. Any N>1 SUSY theory contains a graviphoton that can acts as a vector field whose propagator provides a term opposited to the usual newtonian potential. Schërk understood by antigravity the phenomenon of cancellation between couplings that could mark the point in which antigravitational (graviphoton mediated) repulsion dominated the usual attractive force.
Do not you know the paper?

Juan F. said...

And respect entropic gravity, well...It works to quantitave and somelike qualitative stuff. But the important thing that worries me is that, again, it is something circular ( in spite of the nice and straightforward derivations it allows) and, it is a wibbly wobbly mixing of principles or formulae( holography plus equipartition plus Unruh) with no, yet, a general background to understand why it works. It just works, like QM. LOL...Astonishing, but remarkable, it was the entropic gravity strikes back recent paper in reply to Kobhalikdhe. Cheers. PS: I hope my next post will be done with my Master thesis finished.

Bee said...

Hi Juan,

No, I don't know Schërk's papers, otherwise I'd have cited them. If it's SUGRA, it sounds like something I'd have missed, so thanks for mentioning. Reg entropic gravity, Matt Visser had a nice paper recently pointing out some problems with Verlinde's approach. Best,

B.

Bee said...

Hi Plato,

Sorry, I haven't solved the neutrino puzzle ;o) I just pointed out somewhere (Chad's?) that if the effect is real it might not actually be the neutrinos being superluminal, but some intermediate state which then decays into a neutrino (+something). That would be more plausible because of the conflicting supernova data. In any case, if I were to look for an error, I'd start with Δt_BCT. Best,

B.

Plato said...

Bee:.....but some intermediate state which then decays into a neutrino (+something). That would be more plausible because of the conflicting supernova data. In any case, if I were to look for an error, I'd start with Δt_BCT.

Yes, most indicative of the approach to phenomenology? Would you use "past experiments" in order to lead others through to this approach? How would Quantum Gravity be related? Maybe another post perhaps?:)

So basically you wold start with this article referenced below?

Measurement of the neutrino velocity with the OPERA detector in the CNGS beam

Best,

Plato said...

The analysis of internal neutral current and charged current events, and external νμ CC interactions from the 2009, 2010 and 2011 CNGS data was carried out to measure the neutrino velocity. The sensitivity of the measurement of (v-c)/c is about one order of magnitude better
than previous accelerator neutrino experiments


Are you satisfied with this Bee?

Best,

Evil String Theorist said...

Dear Sabine,

It really is a shock that you don't know who Scherk was:

http://en.wikipedia.org/wiki/Jo%C3%ABl_Scherk

As you can see, he did some very important work on both string theory and supergravity before his untimely death.

Robert L. Oldershaw said...

No, not so.

Scherk just transformed himself into graviphotons and left for an 11-dimensional anti-verse in the multiverse.

He's just doing his laundry at the moment.

Not to worry.

Phil Warnell said...

Hi Bee,

A very nice synopsis of what it is you do professionally which boils down to being a researcher looking for phenomena which begs not simply explanation yet which also deserves further attention. In this regard although you might not find the connection I have long thought you to be a physicist of the J.S. Bell genre and wish there were more.

“Theoretical physicists live in a classical world, looking out into a quantum-mechanical world. The latter we describe only subjectively, in terms of procedures and results in our classical domain.”

-John Stewart Bell, " Introduction to the hidden-variable question " (1971), included in Speakable and Unspeakable in Quantum Mechanics (1987), p. 29

Best,

Phil

Steven Colyer said...

Very nice Bee, but isn't Dark Energy "like" anti-gravity? Anyone have a link to Bee's paper?

bee, I know you're busy, but a book by you (takes about 2 years to write/edit, I know) on PQG would be great. Sort of a popular history of starts, false starts and dead ends, successes, etc. Hey look at me! Mr. Taskmaster, sheesh.

The important point is that Bee bases theory based on that which we KNOW to be true, i.e., experimental results. that means you have both feet on Terra Firma, Bee.

Now .... let's talk about the crazy NON-phenomenological stuff the popular scientific press writes about ... no, better yet, not.

Bee said...

Hi Steven,

Try this summary. No, antigravitation is *not* like dark energy. I know people keep saying that but it's bullshit. It's quite easy to see why: If you have matter that anti-gravitates in the sense that it's just like normal matter (i.e. attractive to itself) but repulsive to our stuff, then it has a normal equation of state. But for something to have the effect of dark matter you need a funny equation of state. It is not sufficient if you just switch signs for density and pressure because the density also comes into the first Friedman equation, where it can't make too much of a contribution, otherwise you'd have a square of a real number become negative. See, a cosmological constant does well as dark matter. But it's constant. It's not like matter that dilutes, which is what antigravitating stuff would also do. (Unless you make it not-matter-like, which defeats the purpose.) Best,

B.

Steven Colyer said...

Thanks bee, you're right, I see that all the time, that it's "like" anti-gravity. The new word on the street is that it's "like", or actually "is": "negative energy." Care to comment on that? Here's the link:

The Accelerating Universe and Dark Energy Might Be Illusions

That's based on work by Christos Tsagas, a cosmologist at Aristotle University of Thessaloniki in Greece.

I myself am somewhat skeptical of Space.com preenting the full story and often feel something is lost in translation. After all, it was there that I learned about Randall Mills and his nutty "Hydrino" theory, presented as fact there, and well deconstructed since.

But I just don't know. Gravity is its own beast, apart from the other 3 forces, or 2 if one considers E-M and Weak = Electroweak. Like E-M (and unlike Weak and Strong) it seems to work over infinite distances, but unlike E-M I see no proof of repulsion anywhere. It's so vexing. I don't think it's "entropy" either as Verlinde promotes, but possibly a function of Geometry. Shrug.

Bee said...

Hi Steven,

The article you link to is terrible, to begin with it's bluntly wrong that "gravity pulls stuff inward." To see this, replace t with -t. Or, more to the point here, look at the 1st Friedman equation: You get expansion by virtue of totally normal gravity on totally normal matter. What you don't get is *accellerated* expansion.

In any case, I have a great sympathy for attempts to explain dark energy by "local" effects and at some point in time when my life was not cluttered with two crying babies and a snoring husband I'd have gotten very excited about that. Also because it would fit nicely with the antigravity which doesn't give you a dark energy, but it should, on very large scales, have segregated which is why we don't have much of it, but it has to be elsewhere. That is, not having read the papers they talk about all I can say is I wouldn't just dismiss it. On the other hand, I have to caution you that ΛCDM is very successful and fitting cosmological data is difficult to say the least. Best,

B.

Juan F. said...

Hello Bee! As I promised, I post by the first time since I passed my Master thesis defense.
On the whole thread. "Antigravity" by Scherk is not you antigravitation and also it is not what cosmic repulsion by the cosmological constant says. I agree on that.
I am expecting the results on free fall experiments with antiprotons by the CERN.
By the way, I will write you an e-mail on the Ph.D positions at Nordita. Just curious about it.
Cheers!