# Spinfoam 4D unification hazards a prediction

1. Apr 28, 2006

### marcus

To be science a theory must bet its life on the outcome of some future observation. That is what it means to predict something new and to be falsifiable. If the observation goes against the theory's prediction then the theory is wrong.

so the theories of space time and matter that are currently under development are in a kind of informal rivalry to see which can first make a falsifiable prediction.

The question that one expects a theorist to answer is "What experimental result would make you give up your theory and explore something else?

Einstein 1915 Gen Rel survived that kind of test in the solar eclipse of 1919. It could have gone against, and GR would be dead. It is the honor of the theorist to make a theory which predicts something new, not observed yet, and bets its life on it---so that if that thing is not observed the theory is refuted.

A theory which can accept any possible outcome of any future experiment is mush---it may be suggestive but it has no predictive value since it can accomodate anything that happens.

=================

OK one of the theories being developed by the Loop QG community is
described in http://arxiv.org/hep-th/0512113 and http://arxiv.org/gr-qc/0604016 which has references to the extension to 4D, work in progress. The 4D version should be out in a few months, certainly by the time LHC starts. It is a theory of space time and matter using the spinfoam formalism---let's call it SPINFOAM 4D UNIFICATION THEORY. Freidel is one of several spinfoam theorists developing it.

What could LHC show that would make Freidel give up on this approach of 4D spinfoam unification?

If there is something like that, then Freidel's theory makes a falsifiable prediction that it will NOT be seen. Making a falsifiable prediction scores points for the theory.

Well, what about little Black Holes? They would be evidence of extra dimension, because in plain 4D there is not enough energy in LHC to produce black holes.

any reaction?

I have to go, back later.

Last edited: Apr 29, 2006
2. Apr 28, 2006

### marcus

what I am using here is an article by string theorist Steve Giddings.
http://www.sciam.com/print_version.cfm?articleID=000CCC72-2AED-1264-980683414B7F0000

As I read it, it says that string/M thinking DOES NOT PREDICT black holes at LHC, in the sense that no string thinker will feel forced to give up his theories if holes arent seen

and also it does not predict NOT SEEING black holes at LHC.

If you dont see little black holes, well hey, maybe the energy is just not high enough!

Either way it's cool, dudes, we can adjust to it either way. In other words the theory is mush as far as black holes at LHC. It has no predictive value on that question in the sense of required commitment to one or the other. It can accomodate any future outcome about BH at LHC.

The relevance to LQG-and-allied QG----why this is actually a test of NON-STRING qg, is this:
Steve Giddings article says LHC ENERGY IS NOT HIGH ENOUGH TO MAKE BLACK HOLES unless there are EXTRA DIMENSIONS.
SO SEEING BLACK HOLES WOULD INDICATE EXTRA DIMENSIONS.

So seeing black holes would kill Freidel's program of 4D unification in spinfoam.
=================

The Giddings link is just a SciAm article and not a journal paper
I hope that is OK with readers: it has the fact we need and the argument's straightforward.

Last edited: Apr 29, 2006
3. Apr 29, 2006

### marcus

Not as a criticism of string, but to illustrate the point about falsifiable prediction, there is this famous quote of Feynman where he is supposed to have said

"String theorists don't make predictions. They make excuses."

Obviously the kind of predictions he was talking about was not the "wouldn't it be nice if" or the "hey, it could be this way, or again it wouldnt have to" sort, because string has always mades plenty of those hopeful suggestions

Like "you might see SUSY at that energy but then again, if you don't then maybe the energy wasnt high enough, so whatever happens is OK. You will probably find it at SOME energy." That is not a real empirical science type prediction, because it does not commit the theory to a specific outcome of a specific measurement---consequently there is no specific real-life situation where it actually makes a hard definite prediction.

Of course string thinking may have CHANGED since when Feynman said that. This example is just to illustrate the ideas. For all I know maybe last week a string theorist made a definite prediction. "If such and such is seen at LHC at a certain energy, then string is refuted and I will get out of string." Or equally well "If such and such other thing is NOT seen at LHC at a certain energy then string is wrong and I will get out of string."

I didn't hear of anybody saying that, but if you know of someone please tell me! I would be delighted to hear if some clear thinking researcher has recently made such a statement. Feynman's quip was made some years back and perhaps things have changed and maybe nowadays they make something besides hopeful suggestions, and excuses.

Anyway the Feynman quip illustrates that in empirical science, where you test theories, a prediction of a theory is something like "my theory requires that you see this or not see that and if you see something else then my theory is WRONG"

It is a prediction that allows one to TEST the theory, which implies that one possibility is that it turns out not to be correct.
=======================
A propos of falsifiability, and tests:

Freidel faces another serious test in 2007-2008, if GLAST flies.
there is widespread agreement that it will be sensitive enough to detect the energydependence of the speed of gamma photons that the spinfoam 4D unification model predicts.

so the theory is exposed to the risk of falsification there too.

and by the way, the 4D version isn't ready yet! They are still working on it. they say they have the results but we are still short one or two papers (that I, anyway, have not seen). It would be nice if all the tees were crossed and the eyes dotted before actual experiment time.

Last edited: Apr 29, 2006
4. Apr 29, 2006

### wolram

Lets see now, how many predictions, (that are testable) are there,and when it will be possible to falsify them, (if ever), what exeperiments are needed,
and when will a theory be finally destroyed.
Glast may falsify one, but what of the others.

Last edited: Apr 29, 2006
5. Apr 29, 2006

### marcus

you I know will not be satisfied with the progress being made, Wolram. You rarely are content, but eagerly look forward to everything being settled.

It is hard for me to sketch the whole picture of testing non-string QG. I may not know enough to do it adequately. This is just a rough sketch.

1. Lisa pathfinder could falsify (Bekenstein TeVeS) MOND
2. Any evidence of extra dimensions (like LHC BHs) would destroy most non-string QG being worked on today. It would be "back to the drawing board" for a large part of the non-string QG community.
3. SOME but not all non-string QG theory requires a form of doubly-special-relativity (dsr) with energy-dependence in the speed photons travel.

As a rough estimate, about half of current LQG research would be blown away if GLAST soes not see some energy-dependent speed. But the community could recover---they would know better then which lines of research to pursue and which not to.

===============
Why points 1-3 interest me, and might interest you as well.

1. Bekenstein TeVeS MOND is technically superior to all the other MOND, and being more mature makes it more TESTABLE. (As a general fact, the more mature and worked out a theory is in its details the more it precisely it predicts and the the more exposed to falsification.) LISA mission is near-term.

If Lisa, or some similar probe, does not find the MOND effect in the solar system which Bekenstein/Magueijo calculate should be there then, for me (and I guess them too) MOND is dead.

If MOND were to survive it would have extremely interesting consequences for non-string QG research. Smolin gave a series of talks about this in February 2004. the connection of MOND with QG is one reason that Magueijo is at Perimeter. It is an exciting research area and largely untouched.

Eventually non-string QG has to explain dark matter and dark energy. Right now the effort is on explaining ordinary matter (i.e. in spinfoam research) but they have to eventually get to dark matter. So any experimental test of MOND is relevant.

2. what people TALK about as testing string theory by looking for extra dimension is actually a test of LQG-and-allied QG. Because the string thinkers DONT SAY AT WHAT ENERGY YOU FIND EVIDENCE. So they commit to no firm predictions about specific experiments.

I dont think extra dimensions are very likely. Quite possibly braneworlds and leaking gravity and curled up dimensions are just fantasy that will go eventually go away. One reason I don't find string-think very interesting is the artificially contrived nature.

But if you think of it as really a test of NON-STRING QG, then testing for extra dimension is actually very exciting. Because non-string QG theories are typically 4D-----or they are toy models in 3D that people are using to ramp up to the realworld 4D case.

so NON-STRING makes a hard explicit prediction that you SEE NO EXTRA DIM AT any energy. So every time, at LHC, that they see no black holes or other evidence of extra dimension then non-string has PASSED A RIGOROUS TEST that could have falsified it.

(on the other hand not seeing evidence of extra dimensions does nothing to the truth-status of string-think, because the researchers have not said at what energy level we should expect it----the thinkers can accomodate any experimental outcome that comes along. It is a striking contrast.)

3. To discuss this, I should focus on Freidel's work because it is the farthest along. And even in this case Freidel et al ARE NOT FINISHED. To have GLAST be a good test, they have to make an airtight case that their theories require energy-dependent speed. And they have to do this BEFORE GLAST FLIES. In the scientific tradition, what counts is PREdict, not POSTdict. To be tested a theory has to tell you to look for something that you dont already know is there. There can be any number of theories explaining what we already know is there. To distinguish itself a theory must make a definite prediction about something new.

In the case of Freidel's spinfoam 4D unification of spacetime with matter, the final papers (which should nail down the DSR connection) are listed as "in preparation". In the past Freidel has made good on promises. I believe they will be appearing, but I have not seen the papers.
Preliminary results were discussed at the October 2005 Loops conference.

Some DSR requirement seems to be GENERIC to non-string QG. Smolin has argued this in several papers. But I am not satisfied with this. I want to see an unequivocal prediction from a mature theory. Freidel's is the closest to being fully worked out.

It is ironical that, with scientific theories, if one likes a theory--admires its ingenuity and scope--then one WANTS it to be at risk of falsification.
There is no predictive value without danger.
I wonder how Freidel feels now, as it becomes clear that his project is unequivocally predictive and will undergo testing.

Last edited: Apr 29, 2006
6. Apr 29, 2006

Staff Emeritus
Lisa Randall's specific AdS/CFT model does make explicit predictions: new particles with stated momenta, spin, etc. LHC is going to look for them. If found they will at least confirm a five dimensional hyperbolic geometry bulk. Whether you regard that as killing any QG theories I don't know.

7. Apr 29, 2006

### marcus

Great! AFAICS this is a big change!
what would you say to starting a thread on "Lisa Randall's New Particles"

I give her a lot of honor for developing a theory that makes definite predictions of particles appearing (I assume) at some definite level of energy. So if LHC looks, and does not find, then Randall's theory is shot.

This deserves a lot of respect---if she is doing this, as it sounds like she does. I would appreciate our having a thread, listing the particulars (if not too much trouble). Is there a link to some article that lists these new, unfound-as-yet, particles?

===================

as far as the side-effect on QG theories. Yes definitely, evidence for a higher dimension bulk would have an enormous effect on non-string QG----would in effect redirect the entire effort, I suspect. Might stimulate a lot of new non-string QG growth---make people more creative. Hard to tell.
But it seems like our first job should be to focus on Randall's theory and say what it is that will be tested.

If you don't want to start a Randall's predictions thread, maybe you could just give a link here to a list of her new particles that LHC might see.

Last edited: Apr 29, 2006
8. Apr 29, 2006

Staff Emeritus
I'm starting to get the first post for that together. It'll just be out of her book.

9. Apr 29, 2006

### marcus

I'll try to help by doing an arxiv search. I was just now having trouble with their search engine timing out---which is unusual.

Cornell's engine must be down. I switched to LANL and got links to her two most recent, but neither seems quite the ticket

http://lanl.arxiv.org/abs/hep-th/0512247

http://lanl.arxiv.org/abs/hep-th/0507102

Last edited: Apr 29, 2006
10. Apr 29, 2006

Staff Emeritus
What I'm planning is a first post (hopefully today) that just lays out her predictions and assumes the model. Then subsequent posts a la your thread on LQG to introduce each of the papers where the built up the model (e.g. at one point she and a coworker treated the "leaking gravity" of the brane/bulk model by Einsteinian math and derived the hyperbolic geometry; this is the origin of the word "warped" in her title). Hence if the bulk was constant negative curvature hyperbolic then it was Anti-deSitter, and so on. So a post on that paper and then one on each paper on each step from there on.

11. Apr 29, 2006

### marcus

sizable project. more power to you!

12. Apr 29, 2006

### wolram

Trying to see through the fog is so difficult to me, but this sounds interesting, it seems to have some mechanics to it.

By Marcus.

If MOND were to survive it would have extremely interesting consequences for non-string QG research. Smolin gave a series of talks about this in February 2004. the connection of MOND with QG is one reason that Magueijo is at Perimeter. It is an exciting research area and largely untouched.

13. Apr 29, 2006

### wolram

I spent most of the night looking at, testing gravity theories by the slowing
dowm of gamma rays, it seem an extremely difficult task.

14. Apr 30, 2006

### marcus

I sympathize. I know it's difficult, but how difficult depends on how you go about it. a short cut is to check out who Shahn Majid is and if you decide you can trust him, that makes part of it easier. You can take part on faith, or expert testimony.

You may have heard of Majid. He is at Queen Mary London----and sometimes at Oxford or Cambridge on leave. He gets to do what and go where pretty much as he pleases, seems to me.

I shouldn't minimize the difficulty. It is very hard to follow every detail, for sure.

but Majid does one step of it in a rough back-of-envelope calculation here:

look at page 19 of
http://arxiv.org/hep-th/0604130

. In that case one may argue[1] that the speed for such waves can be computed as $$e^{\lambda p}$$ in units where 1 is the usual speed of light. So the prediction is that the speed of light depends on energy. What is remarkable is that even if$$\lambda \approx 10^{-44} s$$ (the Planck time scale), this prediction could in principle be tested, for example using gamma-ray bursts. These are known in some cases to travel cosmological distances before arriving here, and have a spread of energies from 0.1-100 MeV. According to the above, the relative time delay$$\Delta T$$ on travelling distance L for energies p, $$p +\Delta p$$ is
$$\Delta T \approx \lambda \Delta p L/ c \approx 10^{-44}s \times 100 MeV \times 10^{10} years \approx 1 millisecond$$ which is in principle observable by statistical analysis of a large number of bursts correlated with distance (determined for example by using the Hubble telescope to lock in on the host galaxy of each burst).

I am sleepy, will have to check his calculation in the morning. The gist is that if a burst travels for 10 billion years and part of the burst is photons which are 100 MeV more energetic than the others, then a detectable part of the burst will arrive 1 millisecond earlier than the rest. So there will be some dispersion that you can see. this is even if his coefficient lambda is planck scale i.e. very very small.
So if his theory is right, then even in the extreme case you should be able to see some dispersion. The conclusion is, if you dont see any (if you dont see one part of the burst coming in before the rest) then his theory is wrong.

I will see about checking his calculation in the morning when I'm more awake.

Last edited: Apr 30, 2006
15. Apr 30, 2006

### Chronos

Tiny black holes produced by the LHC? No way. High energy cosmic ray collisions in earth's upper atmosphere generate energies far beyond what the LHC is capable of achieving. Albeit that leads to an interesting question . . . are GRB's evaporating black holes?

16. Apr 30, 2006

### marcus

I myself would be astonished if LHC reported production of small black holes.
However Steve Giddings is a prominent string thinker at Santa Barbara, and he has just written in the Scientific American contemplating the possibility that LHC MIGHT produce little black holes.

(energetically this would only be possible if extra dimensions of string theory actually exist, instead of being pure fantasy, as I interpret Giddings)

this offers a test of Loop-and-allied QG-----like the 4D spinfoam unification theory that we hear a bit about these days----because those theories swear by their beards that no black holes shall be seen.

as long as the world-famous expert Giddings brings it up, I believe I will not dismiss the opportunity of a test of QG.

YOU Chronos are welcome to pooh-pooh and say "No Way!" but you must, I think, admit it is a test.

17. Apr 30, 2006

### francesca

I look forward for next January! Bojowald and Horowitz are organizing in Santa Barbara a mini-program lasting 3 weeks in Spacetime Singularities :tongue2:
We will see...

18. Apr 30, 2006

### marcus

Wolram, I checked Majid's calculation and it came out right.
As one would expect since he is a worldclass expert in noncommutative geometry (more or less same caliber as Alain Connes I guess----one of the originators of the field)

Majid's not likely to make a mistake. but anyway I checked.

In case anyone is interested, here is the check: it is an ORDER OF MAGNITUDE (nereid says "oom") calculation of by how much one bunch of photons lags behind another if they both travel for 10 billion years.

Majid assumes that in Planck unit terms the DIFFERENCE IN SPEED IS PROPORTIONAL TO THE DIFFERENCE IN ENERGY

going to the NIST website, one sees that the planck energy is E19 Gev

and 10 billion years is E61 placktimes.

Now one can OBSERVE that in Gammaray Bursts some photons are 100 Mev more energetic than others. that is 0.1 Gev.

so the difference in energy between the two groups is E-20 planck.

SO ONE GROUP IS E-20 FASTER THAN THE OTHER.

and say they both travel a time E61 (not unusual for GRB light to have traveled 10 billion years to reach us)

and so one group goes E41 plancklengths farther and arrives E41 plancktimes sooner.

BUT E41 plancktimes IS 5.4 MILLISECONDS.

Majid is conservative, by his assumptions one group arrives 5.4 millisecond sooner. But he says, because he is doing a rough OOM calculation that it arrives ON THE ORDER OF ONE MILLISECOND sooner.
Right. I shouldnt have said 5.4 because the calculation is really rough, but the spread is on the order of a millisecond.

the only arithmetic, besides having stuff in planck units, was to multiply E61 times E-20 and get E41

===============

this is regardless of which way the effect goes. I dont think we can be sure at this point WHICH WAY the effect is. If I hear anything about that I will let you know.

this business of a dispersion----this energydependence of speed---is extremely unintuitive

it is like the business of 1905 Special where things seem to gain inertia as they approach c. And their clocks slow down. Why the hell should their clocks slow down?

What has appeared is that Freidel's theory will not work without this tiny dispersion-------a difference of a millisecond that shows up after 10 billion years. And Freidel's theory is very PROMISING in several ways----it brings spacetime geometry and matter closer together, it has nice classical and zerogravity limits. So this promising theory can be SHOT DOWN if we can observe that the dispersion does not happen. And this makes it interesting, to me at least.

I do not say you or anyone should find it intuitive, or that anyone should BELIEVE (I'm not sure that science theories are about belief.) But it is emerging as a serious high-stakes game.

we are coming to the part of the movie where Robin Hood and the Sheriff of Nottingham fight on the staircase. and the swords are predictions.

Last edited: Apr 30, 2006
19. Apr 30, 2006

### marcus

WOW!!!!!!!
------quote-----
The Quantum Nature of Spacetime Singularities

(Martin Bojowald, Robert H. Brandenberger, Gary T. Horowitz, Hong Liu)

Jan 8, 2007 - Jan 26, 2007

One of the main goals of quantum gravity is to provide a better understanding of physics near the big bang and inside black holes where general relativity breaks down. Recently, there has been considerable progress toward this goal in both of the main approaches to quantum gravity. In string theory, this includes string gas cosmology, tachyon condensation, and holographic descriptions using the AdS/CFT correspondence. In loop quantum gravity, space is fundamentally discrete, and evolution in symmetric cosmological models is nonsingular. This three week miniprogram will combine experts in both areas and on the classical approach to singularities to make further progress on this fundamental problem.

------endquote-------

Francesca this is a gold information nugget. I hadnt seen the announcement. thanks so much!

Since you have not, I will make a thread about it, so that more readers get the news.

20. Apr 30, 2006

### Sauron

In these article http://arxiv.org/gr–qc/0602100 Bojowald reviews current staate of black hole theory in the frame of (canónical) loop auantum gravity.

If i dón´t understand bad it points that in an aproximation,consisting in obtianing an effective classical model wich incorporates some aspects of the full theory you find that you can´t have black holes smaller than a certain size iirrespctively of how much energy you apply.

That would certainly rule out LHC black holes in 4d LQG.

Anyway, i dón´t see a definitive reason why LQG couldn´t be formulated in 5, or upper dimensions.

On the other side the earlier papers of Lisa Randal on non warped extra dimensions (i haven´t seen the recent ones) didn´t depend in a definitive way in string theory because they used effective QFT´s mixed with classical GR.

So i wonder if LQG couldn´t equally face the posibility of remain valid in extra dimensions. and if so if the bojowald result of small black holes production remains valid in extra dimensional LQG´s.

Of course i am talking alltime about canonical LQG, the only part of the theory i have a reasonable knowlege. I am still begining with spin foam models (with the aid of the papers here recomended) and don´t know how well or bad they do with 5and upper dimensionality.