Dave

imagine two cars traveling at 50 versus 50.5 kph: after a few seconds you’ll hardly see
any difference, but over an hour they’re separated by half a kilometer. So the longer the
trip, the easier it is to measure.

Does the tiny-scaled foaminess and bumpiness imply that photons travel the entire distance at different but constant speeds?
It seems to me if tiny-scaled foaminess is affecting speed, than the effect on the speeds of two photons should average out over the entire distance, just as the Fermi results show.

?

There’s also some misconception going on here about how this relates to special relativity. Special relativity doesn’t predict that space is smooth at all scales. It postulates that the speed of light is the same within every inertial frame of reference. This is a local theory, which means that it only holds strictly on an infinitesimal scale. If the geometry of space meanders about in a complex way at some finite scale, then naturally light should behave differently in practice than in the idealized system of special relativity. If the two gamma photons, of differing wavelengths, had arrived at significantly different times, it wouldn’t have been a categorical violation of relativity.

“In other words, as an effective field theory, gravity is no worse, nor better, than any other of the effective field theories we know and love. The trouble is that all hell breaks loose for ε ~ 1. Then all of these infinite number of coupling become equally important, and we lose control, both computationally and conceptually.”

Moreover, under certain implausible but possible conditions (i.e. we would know all particles up to Planck scale) and augmented with some technicalities “that theory would be perfectly predictive within its realm of validity.”

Btw, I note that Loop Quantum Gravity (the usual name for LQT) is discussed as having a fundamental length scale because of difficulties of fine-tuning, in Distler’s word because some people “give up on taking the continuum limt [sic]“. Well then, it’s not the impression I had gotten, but if so formally there is a way out.

None of those made much sense anyway, at least to me.

Spacetime is emergent like classical objects independently of gravity being an effective theory, so wouldn’t be susceptible to quantum uncertainty wholesale as an effective quantum field would. (Think ice/spacetime vs water/fields.)

And Lorentz invariance may be broken as any other invariance. But never destroyed along the simpleminded axiomatic path of some theoreticians “fundamental length scale”.

More interestingly, this is to my knowledge the first observation beyond Planck scale! In principle “beyond Big Bang”. (IIRC at least Planck length/1.3 or so, also a hint of possible observation to Pl/100 or so – but not this time, alas.) I wasn’t sure we could ever make one, it’s a huge thing in my view.

@ NBwaW, Moonshark:

The foaminess was supposed to be spacetime itself being subjected to uncertainty. In these hypotheses its geometry would go from large scale flat spacetime to a small scale dynamic froth of curvature going this way and that, with spacetime itself popping in and out of existence.

This is quite different from uncertainty of particle fields that regular quantum field theories AFAIU predict and which I believe NBwaW describes. For one, spacetime uncertainty would make the concept of fields and particles “uncertain” too.

There has been a minor subfield trade of proposing more or less unphysical theories of “quantum gravity” above and beyond simply quantizing the effective field theory of gravity (which is simple at low energies) and get gravitons.

LQT mentioned above is the prime example. Besides now failing its main prediction of a fundamental length scale big time, it wasn’t much of a physics theory to begin with. [Think mathematicians with delusions of physics grandeur. Never mind that some of those actually contributed to physics elsewhere too.] Famously LQT could neither establish a lowest energy state nor a simple harmonic oscillator to populate it with, so there was no dynamics to be had.

Interaction with “virtual” particles would obviously be greater for short wavelengths than long. So why didn’t they use gamma vs visible? Probably because visible wavelengths interact very strongly with “real” particles and over a seven billion year trip, that would be too much interference to provide good data.

Great job, Fermi. My congrats to the research crew.

GAry 7

Kuhnigget – Oh…um…of course I knew

Is there a more detailed yet still accessible paper about what Fermi observed and how it relates to those other observations?

I’m googling ‘GRB090510 quantum gravity’, and it looks like articles in the Nov 19 issue of Nature are most relevant. An ArsTechnica article also looks helpful. Off to read
http://arstechnica.com/science/news/2009/10/quantum-gravity-theories-meet-a-gamma-ray-burst.ars

Sometimes irony /sarcasm / Poe-etry is all too easy for folks to confuse & mistake as the “real” thing, alas.

Oh & Darwin & Einstein being “consorts” as you delicately put it – nice touch! That’d really have the RTC’s* gasping. LOL.

—————————————————

* RTC = “Real True Christian” – y’know the small-minded sort that are intolerant of homosexuals & unmarried mothers, hate the poor, hate the reality of evolution and the need to teach it in science class, hate .. well most people that aren’t *them*, and think Jeebus’es message was all about abortion, teh Gayzz & teh Apocalypse / Rapture. That acronym there (no idea how widely its known) comes from this blog here :

http://slacktivist.typepad.com/slacktivist/left_behind/

which is another of my faves.

Yes, there was a large implied there. If not an even larger

Guess I’m really asking the same thing as others. What amount of delay was expected due to foaminess? I’m sure it depends on which exact QM theory you use, but still, what was the median?

That’s still an assumption. Yes, it is FAR more likely that you are correct, but where’s the proof?

some quantum mechanics theories propose that space is irregular, foamy, and bumpy on incredibly small scales

Just some? I have not come across any that did not propose that.

and this means the speed at which photons travel may change very slightly if they are more or less energetic

I only have a lay knowledge of quantum mechanics, and I have never come across any predictions that the speed of light would vary by frequency due to spacetime foaminess, so it doesn’t seem like a consensus idea.

Also, how does this article square with this article? I think the popular science press is falling down again.

Gib – Well, within 0.9 seconds. It’s possible they arrived at the exact same time, but we would have no way of knowing – it’s a matter of the sensitivity of our instruments. And anyway, 0.9 milliseconds of wiggle room is nada compared to a few billion years of travel time!

Kee – So they were emitted at different times and from different distances, and somehow arrived at nearly the exact same time? That’s too big a coincidence for scientists to accept

(Sorry couldn’t resist.)

@ 1 Kuhnigget : I presume you forgot the emoticon again there?

But still, I suppose that a difference of 0.9 seconds after a 7 billion year interval does say something about the stability of lightspeed regardless of energy.

Or are they assuming that they arrived 0.9 second apart because the two different frequencies were generated at slightly different times/places in the original explosion ? And hence it would have been 0.9 second apart anywhere along the path between us and the explosion ?

Sheesh! Don’t they teach this stuff in Sunday school anymore?