In our earlier post we discussed why this bound they put forward was based on a weak argumentation. The essential point was assuming the highest energy photon had been emitted during a particular late peak in the low energy spectrum. Since that peak almost coincided with the arrival of the photon the resulting bound was very strong. There is however no knowing exactly when the photon was emitted. The most plausible assumption is that it wasn't emitted before the onset of the burst in the low energy regime. This assumption however gives a much weaker bound, pretty much exactly at the Planck scale.
The paper got now published in Nature, but it is significantly toned down from the original claim. The "most secure and conservative new limit" is at 1.2 times the Planck scale. The limit of 102 times the Planck scale that arises from associating the 31-GeV photon with the 7th spike is still offered, but explained to be "not very secure." Seems to me the referees did a good job...
The topic even made it into the New York Times. Dennis Overbye writes that 7.3 Billion Light-Years Later, Einstein’s Theory Prevails, and quotes the eternally optimistic Lee Smolin:
The good news, astronomers said, is that more data expected from Fermi could decide the question. As Lee Smolin, a quantum gravity theorist from the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, said, “So a genuine experimental test of a hypothesized quantum gravity effect is in progress.”
New Scientist reports on Giovanni Amelino-Camelina's stomach aches, and SymmetryMagazine explains the why and how in a very recommendable article Gamma-ray burst restricts ways to beat Einstein’s relativity.
For more details, see also our earlier post Constraining Modified Dispersion Relations with Gamma Ray Bursts.
I am confused. I see two papers:
ReplyDelete1) A limit on the variation of the speed of light arising from quantum gravity effects concerning GRB 090510.
2) A gamma-ray burst at a redshift of z approximately 8.2 concerning GRB 090423.
The first has indeed been discussed previously, but the second appears to be a different paper, right?
Thanks, I fixed the link.
ReplyDeleteIf you read the NYT article and Lubos' blog only, you'd think that Quantum Gravity was dead based on these results, in spite of Lee's claim to the otherwise.
ReplyDeleteScience News also has an article up today by Ron Cowan on this :
http://www.sciencenews.org/view/generic/id/48891/title/Gamma-ray_observations_shrink_known_grain_size_of_spacetime
Which seems more reasonable. Science Daily has an article up today as its headline re GRB 090423, but it seems to be on a completely different subject:
http://www.sciencedaily.com/releases/2009/10/091028142231.htm
Dear Steven, if you read some sources that indicate that the Lorentz-violating theories are not dead, maybe you shouldn't read them because they're not telling you the truth.
ReplyDeleteThey're gone - and surprisingly enough, a dozen of the popular articles about this experimental result pretty much agrees with this assertion. Ars Technica perhaps has the most catchy title describing this fact,
Quantum gravity theories wiped out by a gamma ray burst,
but the remaining articles say the same thing. It's still bizarre that the proponents of the junk theories are consulted as the key sources for many of these articles - but at least the truth is out now.
That's the annoying problem with falsifiable theories: they sometimes get falsified.
ReplyDeleteFair's fair, it shows you were right all along to stick with unfalsifiable theories, Lubos.
Dear Andrew, thanks ;-) but the last two words of yours are funny in a subtle way.
ReplyDeleteYou know, predicting that the delay of a photon will be exactly zero is *exactly* as strong a prediction as the prediction that it will be 3 minutes.
And saying that the delay is *zero* is much more specific a prediction than the general statement that the delay will be "something of order one minute" - because it's very accurate, unlike the "something of order one minute" poem.
You might think that the "zero delay" is not a "spectacular" prediction.
But being "spectacular" is a subjective matter, and this definition of a "spectacular" would actually be perverse because if some prediction is claimed to hold *completely exactly*, it is always spectacular once it's confirmed :-) because in a world of chaos and ever-changing dirty laws of physics, as LS imagines, it's infinitely unlikely that something would work exactly. ;-)
So the only "problem" was that the prediction by string theory - of the exact Lorentz invariance at the Planck scale - is not really "new" because Einstein could have expected it (and probably did expect it) back in 1905.
However, Einstein had to assume the postulates. In string theory, they can be derived from a different starting point. So it is a progress. And string theory allows us to answer many related questions whose answers were completely unknown to Einstein.
And again, the problem with falsifiable theories is that they often get falsified, as you correctly wrote - sometimes too quickly (but, fortunately for LS, not quickly enough to annihilate the profit from extremely silly blue books with shoes on the cover).
However, string theory is neither in the category of falsified theories nor in the category of unfalsifiable theories. In this particular case, it predicted the exact "dead heat" in the race of the photons, regardless of their energy, and the prediction was just right.
Guess whether it's right about all other observable predictions that are being disputed in the blogosphere - such as the validity of holography, the equivalence principle, viscosity bounds, preservation of information in black holes, etc.
Thanks, Lubos. I assure you, I was trying to be funny! :)
ReplyDeleteLuboš, a vacuum chiral pseudoscalar background in the massed sector contradicts no prior observation. Photons are inert. Quantized gravitation requires adding an odd-parity Chern-Simons term to even-parity Einstein-Hilbert action. Static atomic mass distribution is an extrinsic property. Chirality is an emergent phenomenon. Physics ignores them with disdain.
ReplyDeleteIt is testable in existing equipment. Given extended calculation, the one parameter fit for emergence of crystal lattice parity divergence is exact. If the math says it's there, what fool fails to look?
Uncle All disdains perturbational string theory's 10^(50,000) acceptable vacua with one bench top experiment. It's a fair fight you will lose.
Bee said: "The "most secure and conservative new limit" is at 1.2 times the Planck scale. The limit of 102 times the Planck scale that arises from associating the 31-GeV photon with the 7th spike is still offered, but explained to be "not very secure."
ReplyDeleteHaha, that's hilarious! Maybe they should have said that the difference between 1.2 and 102 is just a matter of typography!
I don't agree that the referees did such a great job. The paper was still accepted, after all....
I think we can safely conclude that this bizarre claim of using one [sic] photon to demolish all these theories can be dismissed.
Hi Steven,
ReplyDeleteI didn't find the NYT article too bad. My impression was however that the NYT and NewScientist probably sat on an article saying "LQG falsified!," waiting that the Fermi paper got published. When it finally got published, but didn't contain the originally strong claims, they had to rewrite the article, but it still rings through. Best,
B.
Hi Pope,
ReplyDeleteThe Fermi paper reports on a very interesting data set, and I think it deserves to be published in Nature. The problem I had with the preprint was the interpretation of the data they offered, which seemed to me strongly tainted by wishful thinking, instead of based by solid argumentation. The published version is much clearer and much more careful with the conclusions. It is perfectly okay they are offering several estimates with varying amounts of plausibility, from the very conservative to the "unsecure" as long as they say so.
As we discussed in the earlier post though, the only way to get more solid bounds is more statistics. Clinging to one photon is ridiculous, since the effect, if there is one, could have a stochastic component. Best,
B.
Hi Bee,
ReplyDeleteCertainly a lot of fuss over a single photon. It had me to wonder how Michelson mentioned in Stefan's and your last post would have thought about all the goings on. In a related matter I noticed in a Physicsworld article posted yesterday there is a little bit of the green eyed monster overtaking the Fermi Telescope team in relation to outsiders scooping them on their data in terms of its significance. If it demonstrates nothing else, it does once again confirm there is more than the altruistic goal of the expansion of knowledge that is brought into play when it comes to research :-)
Best,
Phil
Hi Phil,
ReplyDeleteYeah, I'm all in favor of a dead-physicists hotline. Dial 1-800-AlbertE and find out what Einstein would think. Best,
B.
This effect occurs in Nature too. It's called Alexander's band.
ReplyDeletehttp://en.wikipedia.org/wiki/Alexander's_band
Chirality violation is off topic here, but it can be observed by "naked eye" by asymmetric jets of black holes.
ReplyDelete/*..that's the annoying problem with falsifiable theories: they sometimes get falsified...*/
ReplyDeleteThis is entertaining point, illustrating the nature of emergence, Gödel's incompleteness theorem, etc. in one sentence.
/*..NewScientist probably sat on an article saying "LQG falsified!"...*/
ReplyDeleteFunny thing is, first NS article interpreted the very same observation in exactly opposite way:
http://www.newscientist.com/article/mg20327210.900-late-light-reveals-what-space-is-made-of.html
The NewScientist article you mention is about the MAGIC result, not the GRB of the Nature paper. The MAGIC observation wasn't even a GRB, but a flare. It is true however that one would like to better understand how come the disagreement in the limits obtained. Best,
ReplyDeleteB.
/*..The NewScientist article you mention is about the MAGIC result..*/
ReplyDeleteTry to use paging feature:
"It is recent results from NASA's Fermi Gamma-ray Space Telescope, launched last year, that provide the most tantalising glimpse yet of something extraordinary going on out there. Last September, it spied a BURST of gamma rays from a source nearly 12 billion light years away..." ..and the more...
Sorry, you're right, I only read the first page.
ReplyDelete"Dear Steven, if you read some sources that indicate that the Lorentz-violating theories are not dead, maybe you shouldn't read them because they're not telling you the truth."
ReplyDeleteDear Lumos, how do I know not to read something until I've read it?
The "truth" is what I'm about, and articles by science journalists who are not themselves Scientists I will always read with great suspicion. If newspapers botch up something as simple as an obituary, it boggles the mind what happens when they tackle cutting-edge Physics.
The "truth" in this case is still controversial. Not to you perhaps, because you're an extremist. My English/Dutch half is open-minded however and insists on more data be collected before jumping to conclusions.
My Slovak half on the other hand is always looking for a fight, however I see no point in arguing with you at this juncture, my Czech cousin, as we all know where you stand. You keep doing your thing, I see you have another article up on the same subject and look forward to reading it.
Bee: "It is perfectly okay they are offering several estimates with varying amounts of plausibility, from the very conservative to the "unsecure" as long as they say so."
ReplyDeleteSorry, I should not have given the impression that I think there is anything wrong with the observational work. I'm sure it deserves to appear in Ap J. But remember that Nature is supposed to publish *important* new work of interest to people outside the community. The final result doesn't come anywhere near that standard.
It's not so much a case of self-deception; it's something much worse, namely an urge to tell certain people what they want to hear.
"As we discussed in the earlier post though, the only way to get more solid bounds is more statistics. Clinging to one photon is ridiculous, since the effect, if there is one, could have a stochastic component"
This is so obviously correct that it is clear that sociological effects have taken over in this case.
Steven: "Dear Lumos, how do I know not to read something until I've read it?"
ReplyDeleteAnswer: well, it's simple, and even Matthew 7:15-20 knew (and even the Christians know) the right method:
"You will recognize them by their fruits. Are grapes gathered from thornbushes, or figs from thistles? So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire."
I just recommended you not to read the same sources that you have quoted again because the "fruits" they previously gave you were rotten.
Mario Livio, an astronomer at the Space Telescope Science Institute in Baltimore, called the Fermi results an interesting effect but not revolutionary by any stretch. “The beauty of the experiment is not as much in what it achieves,” Dr. Livio said, “as in the fact that you can use astronomical observations to place some interesting limits on very fundamental physics.” See: 7.3 Billion Years Later, Einstein’s Theory Prevails
ReplyDeleteI think this is an important point in relation to what cosmologically one sees in events and what one is doing in relation to LHC.
Might I suggest Bee and Stefan that an article on synchrotron radiation and the understanding of M87's Energetic Jet, in that discussion. Why it is important to understand what those Jets are doing.
It is a simple deduction on how much energy is actually projected towards, and how we see the totality of information, as it is released?
Best,
The point is, we still don't know, if Lorentz symmetry was really broken. What we only know is, whole cluster of photons arrived at single moment.
ReplyDeleteBut we still don't know, if this cluster moved in speed of light, the speed of microwaves in particular. For example, whole Earth planet is composed of bosons of different wavelengths, but it still propagates through vacuum as a single body by subluminal speed. So it violates Lorentz symmetry heavilly, although every boson in it travels along complex spiral path, along which Lorentz symmetry is maintained. Both interpretations are correct at the single moment.
Inside of solitons wave packets waves of different wavelengths are propagating like single object - although in isolated state such waves would spread in different speed. The soliton mechanism is quite common in physics, it just requires select proper combinations of wavelengths in distribution, which is related to gamma function. Of course, such combination of photons would be quite rare, but from the very same reason it would be the very last combination, which could travel through the whole universe, so we can detect it right now.
What we are facing by now is sort of natural selection process, just applied to cluster of gamma photons.
Another point is closely related to metamaterial models of vacuum (you know, all these "black holes for light"). In general, metamaterial is basically kind of foam, in which positive and negative curvatures are (nearly) balanced. The trick is, people are choosing such structure of vacuum automatically by observation at large distance, because just this structure remains the least dispersive environment possible.
ReplyDeleteDuring heavy rain water droplets are forming kind of metamaterial foam in atmosphere. When we are observing rainbow, we can often see the dark area between primary and secondary rainbows. It corresponds the angle of observation, in which the dispersion of light by inner and outer surfaces of water droplets becomes balanced. In such direction particle environment slows down the energy spreading - but it doesn't exhibit the dispersion. The point is, inside of such environment light is spreading in slowest speed too, so that this environment appears as huge, as possible. In my opinion, this is exactly the case of vacuum, which we are observing at cosmic distances via gamma ray bursts.
This geometrical model vacuum goes pretty deep, as it explains, why universe exhibits universal symmetry violation, omni-directional space-time expansion, event horizons and other theoretical stuffs. It just waits for you - mathematicians.
Bee said:
ReplyDelete"Amen".
The Pope says: HA!
:-)
Whether the bound is 1, 10 or 100 times the planck scale doesn't really matter much at this point.
ReplyDeleteThe point is Lorentz violation, if it exists, is more or less irrelevant for answering the peculiarities of quantum gravity.
The theories based on VSL, DSR, modified dispersion relations and other similar models that predict strong lorentz violating effects (and its worth pointing out that they don't always do depending on the modeling choices) are pushed into smaller and smaller parameter spaces and consequently loses explanatory power for open questions in cosmology.
Theoretically, they were always long shots to begin with, its not like its a big revelation that one of the most cherished principles of physics is in fact going to be exact (or so nearly exact that its irrelevant).
"Certainly a lot of fuss over a single photon."
ReplyDeleteYes one photon, but some photon. A photon nobody will be able to make for a very long time. Treat it with respect.
I can afford to jump ahead of sound statistics, and make some careless remarks:
The writing is on the wall. Lorentz symmetry is absolute. Fundamental. Quantum may be fundamental for matter, but probably not for space.
Tkk: Look, I really sympathize with what you say, but it's simply incorrect. For one, as I explained several times, these tests check a first order modification only. Second, if the photon wasn't emitted at the 7th peak as was being used for the 102 M_qg limit, then all you need is a factor of sqrt 2 to push the observation out of the current limit. Third, if the effect has a stochastic component, a single photon simply can't do the trick.
ReplyDeleteAs I've mentioned earlier, the model I've worked with doesn't have first order modification, and the second order modification it has doesn't entail an energy dependent speed of light, so I'd be happy to rule this case out. I'm just saying the evidence isn't yet as solid as necessary to actually draw this conclusion. Best,
B.
Hi Tkk.
ReplyDelete“Yes one photon, but some photon. A photon nobody will be able to make for a very long time. Treat it with respect.”
I have no disrespect for the photon or the data collected thus far, it’s just to hang ones hopes or reputation on the significance of this can’t be described as enough evidence to have the matter decided either way and thus prudent or good science. To be honest I would like to see Lorentz invariance to hold since I’m a principle/symmetry guy at heart and would like to think that nature has no choice but to be so constrained. It also shouldn’t be confused that QM is not ruled or affected by such a symmetry, since the development of the Standard Model with Quantum field theory and beyond has this built into it and things as basic as antimatter are dependant and resultant upon it. This is one instance where it is certainly not easy to throw the baby out with the bath water.
Best,
Phil
Hi Bee,
ReplyDelete“Dial 1-800-AlbertE and find out what Einstein would think”
Thanks I dialled it up yet got nothing but a busy signal with not even the opportunity to leave a voice mail:-) Seriously though I do sometimes wonder what those like Michelson or Einstein would make of all this arrogance, confidence and venom expressed by their modern peers hinged and as being felt justifiable on the weight of such evidence.
Phil
They would likely find it very familiar. Human nature can't have changed all that much in only a century.
ReplyDeleteHuman nature can't have changed all that much in only a century.
ReplyDeleteDear Bee,
It is the acceptability/not-acceptability of various behaviors that may have changed, i.e., there may have been a cultural shift.
Best,
-Arun
May have been, yes. But do you really think there was? I'm not much of a historian, but I can relate very well if I read stories from a century ago. So, neglecting the impossibility of that task, why would somebody from a century ago not be able to relate to stories from today? Best,
ReplyDeleteB.
Bee:
ReplyDeleteI kinda knew I would touch a nerve with my 'careless' remarks.
Indeed, this is 1st-order effect from a single powerful photon and one needs to be careful.
While your own model does not require energy dependency on 2nd order, those models that require 1st order energy dependency are probably in serious question.
Since an experiment has successfully been conducted on one very special photon, it is only a matter of time more such photons will be found and analyzed. And credible statistics built up.
I stick with my 'writing on the wall'. 1st-order energy dependent c QG models are probably on shaky grounds. Maybe that's why they have not thus far yielded any credible hints of unification with matter at low energy? (Such as LQG 'twisting braids' going nowhere.) Perhaps there is no quantum at all at Planck scale? That quantum effects is an emerging effect? Why should there be quantization of matter and space at Big Bang 'time'? Shocking!
why would somebody from a century ago not be able to relate to stories from today?
ReplyDeleteThat is different from Einstein, etc., being shocked by arrogance, etc. of some scientists today.
/*...Lorentz symmetry is absolute. Fundamental....*/
ReplyDeleteThis is somewhat religious stance, as in AWT every phenomena should have its deeper explanation. Lorentz symmetry is consequence of rather special insintric (relativistic) perspective at the exact center of gravitational lens, when observer becomes always affected by gravity field in similar way, like space-time around him.
This is difficult to maintain for every heavier objects, which deforms by gravity in more difficult way, then the vacuum itself, being precompacted. Only most lightweight artifacts possible, i.e. the fluctuations of microwave radiation can fulfill LS perfectly. Therefore only microwave radiation can propagate through vacuum in "pure waves" without presence of both tachyons, both heavier particles (photons).
Quantum mechanics perspective doesn't uses LS at all, as in this theory LS remains ALWAYS violated by its definition. The most universal perspective therefore uses both relativity, both quantum mechanics in combination.
The tricky point is, whatever perspective can be made quite universal, depending on your particular stance. You can imagine Universe stitched from whatever local elements, like strings, loops or many geometrodynamic deforms of space-time, which are locally fulfilling the Lorentz symmetry perfectly.
But such construction would always violate Lorentz symmetry at least in infinitesimal way at global level. Universe as a whole is insintrically random, every rule or law in it remains just our anthropocentric projection of reality. This randomness can be understood both like consequent ion of LS, both like extreme violation of it.
Albert Einstein: As far as the laws (of mathematics) refer to reality, they are not certain; and as far as they are certain, they do not refer to reality.
Dear Arun,
ReplyDeleteYeah, I suppose you're right. But then, today physicists look back on a remarkably successful century, so is it a surprise they have gotten more arrogant? Best,
B.
Hi Tkk,
ReplyDeleteI agree with you. I was just trying to distinguish between evidence and opinion. Leaving aside the quantum gravity questions, there is definitely something going on with these gamma ray bursts that requires better understanding. Best,
B.
I must say, Bee, it is fun to contemplate explaining the billion transistors on an Intel quad-core Nehalem processor to an Einstein or Bohr, woken up from the dead.
ReplyDeleteOr showing them Black-Scholes. :)
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ReplyDeleteGood old Albert would probably just throw his hands and say God does not play with dice, that time is but an illusion, and darn it if that young Hubble lad isn't on to something.
ReplyDeleteBut forget Einstein. Has Lee Smolin weighed in on this yet? And besides Lee, what do you think the following real living people (none of which are Michael Green, John Schwarz, Brian Greene, Michio Kaku, Leonard Susskind, Joe Polchinski, David Gross, or Ed Witten) will have to say:
- Roger Penrose
- João Magueijo
- Alain Connes
- Stephen Hawking
Steven: What's great about people who are still alive is that you can ask them what they think, thus I'll restrain from speculating. Lee had something to say about That Photon here.
ReplyDeleteHi Bee,
ReplyDeleteYou are absolutely correct to say that one could never claim to know what someone might think who has passed in regards to a current scientific theory or scientific issue. However, that was not the point I was making in regards to this particular debate. The point I was making is that I find the respect, civility and decorum scientists have amongst themselves is in decline, which I find to be synonymous with the current culture we live in.
I myself am a bit of a science history buff and have read fairly extensively things written by people like Einstein and others which are more of a personal nature and have discovered in general that although they held strong conviction and opinions about their subject, seldom if ever had it spill over to be of a slanderous or defamatory nature in regards to their colleagues.
Personally I think scientists in general should recognise they have an expanded responsibility in a world that is becoming more secular in nature, to act as better examples and role models for people in terms of what should be considered reasonable action and conduct, rather than simply acting as mirrors and echoes of their times.
Best,
Phil
Phil, have mercy; If I see another reference to Einstein I'll shut down my Internet connection:-)
ReplyDeleteIt's not completely a bad thing scientists don't present themselves as god-like infallible creatures. It's kinda funny that when people say they want to communicate science is done by human beings and not by brains with hands to write papers, they usually mean scientists have sex and hobbies, and not scientists burp, curse, and have bad manners. Make your choice ;-p
ReplyDeleteHi Bee,
ReplyDeleteI’m not saying scientists should not have it seen they are as vulnerable to human weakness as others, yet rather how reason can be instrumental in having those weaknesses be diminished to a practical minimum; for whom else could be better able to this then scientists?
Best,
Phil
Hi Phil,
ReplyDeleteI understand what you are saying. I was just asking why do you expect scientists to be any better than the rest of mortals? Just turn on a TV, if you dare to, and find out the world is full of defamation, lies, and personal attacks. I agree that this is particularly damaging for science, and I wish we would hold up the torch of decency and integrity, but how much does my wish count? Best,
B.
Hi Bee,
ReplyDeleteI understand what you are saying. I was just asking why do you expect scientists to be any better than the rest of mortals?
Simply because the focus and intent of the profession itself is to demonstrate how pure reason when tempered by what’s observably confirmed is able to separate what is more likely to be true from what is demonstratably false and not to be ruled by simple conjecture(s) whose mitigated actions formed out of and by personal opinion, feelings and belief.
Best,
Phil
I guess somebody should have told them when they accepted their job?
ReplyDeleteHi Bee,
ReplyDeleteIn as those most learned in your discipline and that of the medical profession are recognized as being doctors I believe you should like them have your own version of the Hippocratic Oath, which for them first and foremost is to pledge “do no harm’. However, in the case of scientists, perhaps it should read “do nothing stupid” which is to suggest their actions in regards to others should be ruled and dictated by sound reason :-)
Best,
Phil
But stupidities, Phil, are what make life interesting.
ReplyDeleteHi Bee,
ReplyDeleteBut stupidities, Phil, are what make life interesting.
I would agree and yet I consider there to be a distinction and thus a fine line between simply being niave and being stupid. The former results out of honestly and unwillingly not knowing better and the latter as intentionally not wanting to;-)
Best,
Phil
Hi Phil,
ReplyDeleteI hope it is clear that I was just teasing you. I basically agree with what you say. However, as far as stupidities are concerned, what I meant to say is simply that with hindsight it is easy to call risk-taking and failure a stupidity, but it's an essential ingredient for progress. So you might say, maybe looking for quantum gravity in gamma ray bursts is a stupidity, but who knows what we'll learn along the way? Best,
B.
This comment has been removed by the author.
ReplyDeleteHi Bee,
ReplyDeleteOf course I realize you were just funning and to some extent also to provoke a response. I actually hold a lot of value for all the serious differences and directions being taken, as this is in essence is a vital part of the methodology of good science. The only issue I have is when those whom are considered by the public to be the keepers and protectors of reason, devile and ridicule each other resultant of not using and acting upon those tools and methods for which they are responsible to have grow and preserve.
No, I quite like the Einstein who sticks out his tongue, as to be seen as being part of, and to identifying with humanity, yet am strongly opposed to those who do the same as directed to their peers, only to have the public’s faith in the salvation found only in reason to be diminished and marginalized. That is to admit if there is anything I would like to think has us to be distinguished from less evolved creatures it is this.
Best,
Phil
Bee,
ReplyDeleteIt's refreshing to hear that Nature only accepted the paper after its wild claims were toned down. Personally, I doubt if any theory of QG could be tested by the 31 GeV photon from that gamma ray burst - 31 GeV is well below the Planck energy (10^19 GeV) and we don't know exactly when the photon was emitted during the GRB event. For that matter, GRBs are still not completely understood.
Lubos of course will insist only string theory is right , but again, where is the evidence for extra dimensions, supersymmetry, and all the other postulates of ST that must be taken on faith? The answer is simple - there is no evidence for any of these things.
String Theory fails the Occam' Razor test badly ( we must add extraneous extra dimensions, etc to make ST work). Someone over at Cosmic Variance once remarked that it's incorrect to say ST makes no predictions - I agree, it's just that all of its predictions are wrong!
LQG may not be the correct description of gravity at the quantum level, but at least it's based on one observed fact - spacetime is four-dimensional.
That's 4 dimensions, not 10,not 11, not 26. Sorry, Lubos.
/*..there is no evidence for any of these things...*/
ReplyDeleteThe more exact comment may be, there's no evidence for understanding of these things - even from side of string theorists.
Just try to think about it: how can we detect, line at 2D list of paper is curved? We should look at it from perspective of at least three dimensions. How can we detect, 3D space is curved? We should look at it from 4D or higher dimensions. But we know already, vacuum is not flat, it contains tiny fluctuations, which manifest itself like photons of microwave background radiation.
Therefore these fluctuations are at least fourdimensional - or we couldn't observe them. And because we are able to detect them, we are high dimensional creatures as well.
In such way, extradimensions are all around us - and not just four, six or nine - but many of them. We just don't realize it. Pure 3D universe would be transparent like glass and we could see anything from it.
how can we detect, line at 2D list of paper is curved? We should look at it from perspective of at least three dimensions. How can we detect, 3D space is curved? We should look at it from 4D or higher dimensions.
ReplyDeleteSee Gauss' Theorema Egregium ("Remarkable Theorem"). One of the most beautiful results in mathematics, inspired the name of my blog.
/* See Gauss' Theorema Egregium ..*/
ReplyDeleteYep, but the problem persist. You cannot measure distances onto surface, until you haven't reference points for it. In empty vacuum as such reference points can serve density fluctuations, i.e. objects, which are higher dimensional, then the space-time itself.
On the other hand, without these minute fluctuations we coudn't talk about things like curvature of space-time at all. It means, curvature of space-time is just a density profile of another minute space-time curvatures.
I'm preaching like Einstein or Newton by now, which may sound funny for someone - but thinking of most people is really quite schematic in this point.
"it's just that all of its predictions are wrong!"
ReplyDeleteThis is not quite true. String theory predicted Gravity. Gravity is true, isn't it?
That's a postdiction, not a prediction.
ReplyDeleteI say it's a prediction. Gravity pops up out of the blue without any previous knowledge of that force.
ReplyDeleteGravity has been known since Newton at least.
ReplyDeleteCan string theory predict value of gravitational constant? Can it explain inverse square law?
ReplyDeleteIf not...
"Gravity has been known since Newton at least"
ReplyDeleteYes, I google it and you are right. Gravity was a known force before String theory. Damn the devil!
I say it's a prediction. Gravity pops up out of the blue without any previous knowledge of that force.
ReplyDeleteHistorically, string theory as a theory of the strong force was going nowhere when S and G thought to identify the massless spin-2 thing in the string spectrum with the graviton. So in that sense it is a post-diction.
I recently posted that the so0called predictions of ST are all wrong. Let me clarify what I meant by example.
ReplyDeleteEd Witten, one of the founders of string theory, once said (perhaps in jest) that string theory "predicts" gravity...
Well, gravity has been known for a long time and Newton first wrote the famous inverse-square law equation.
Perhaps it would be more correct to say ST "postdicts" gravity :)
Anyway, ST doesn't really "predict" gravity - it predicts the existence of a spin-2 particle which string theorists immediately claimed as the graviton. Of course , it isn't the graviton, it's just a spin-2 field arising from the theory. The trouble is ST not only predicts spin-2 but spins 3,4, 5, etc. So what are those "particles"?
Gravitons in fact arise from from perturbations of the metric tensor in Einstein's field equation. One one quantize this field by saying the energy carried by the field E = h f. None of this requires ST.
Warren Siegel argues that QFT predicts supersymmetry: The standard calculational method for particles uses an approximation method ("perturbation") that involves adding correction terms. When the infinite number of terms is summed to find an exact answer, predictive power is lost in the absence of supersymmetry. Thus, particle theory predicts supersymmetry.
ReplyDeleteand Maximal supersymmetry requires (super)gravity, so the existence of the graviton (particle of gravitational force) is predicted by that particle theory.
His score card of QFT and string theory is:
* Particle theory correctly predicts D=4, string theory doesn't.
* Both particle and string theory have difficulty with unification.
* Both theories predict supersymmetry. Unfortunately, it hasn't been observed yet.
* Both strings and maximally supersymmetric particles predict gravity, but neither solves the problem of predictable quantum gravity in D=4.
Maybe Warren just had a good sense of humour?:)
ReplyDeleteAnonymous: I deleted your last 2 comments. I don't tolerate anonymous insults, no matter who they are directed at.
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