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Intergalactic Race Shows That Einstein Still Rules
Ponca City, We love you writes "The NY Times reports that after a journey of 7.3 billion light-years, a race between gamma rays ranging from 31 billion electron volts to 10,000 electron volts, a factor of more than a million, in a burst from an exploding star, have arrived within nine-tenths of a second of each other. A detector on NASA’s Fermi Gamma-Ray Space Telescope confirmed Einstein’s proclamation in his 1905 theory of relativity that the speed of light is constant and independent of its color, energy, direction or how you yourself are moving. Some theorists had suggested that space on very small scales has a granular structure that would speed some light waves faster than others — in short, that relativity could break down on the smallest scales. Until now such quantum gravity theories have been untestable because ordinarily you would have to see details as small as the so-called Planck length, which is vastly smaller than an atom — to test these theories in order to discern the bumpiness of space."
they arrived within 9/10th of a second of each other
which indicates the opposite of the story's summary
intellectual property law is philosophically incoherent. it is your moral duty to ignore it or sabotage it
that the photons all left at the same time?
that these rays are all from a star which exploded 7.3 billion light years away? what was monitored to predict arrival?
What a time we live in where a "slow new day" consists of a 7.8 million year race being recorded (regardless of the results), a fusion reactor is being developed, and a real time speech translator was released.
Some people just like to get offended. It validates their existence.
I'm curious, why is the Plank-length "So-called"? Hasn't it been firmly established as a unit of measurement?
If our elected representatives no longer represent us, do we still live in a Democracy?
This was proposed by G. Amelino-Camelia et al. back in 1998; here is a review from 2004. Even though the wavelengths of even the most energetic gamma rays are much, much, longer than the Planck length, roughness in space time at the Planck length adds up over cosmological distances, and could be in principle detectable. (The Planck length can be thought of heuristically as the length at which the gravitational effects of virtual particles should be strong enough to create virtual black holes; general relativity cannot be ignored in quantum mechanics at that scale, and vice versa.) What this current test is ruling out is a particular violation of Lorentz invariance - a variation of photon speed with energy. There were similarly negative results using radiation from the Crab nebula in 2003.
It should be noted that this does not rule out quantum gravity - it seems pretty clear that General Relativity and Quantum Mechanics cannot both apply at the Planck scale. What this work is doing is beginning to constrain models of quantum gravity (there is as yet no general theory that makes precise predictions). What would be really cool is to detect some effects, which would maybe help nudge the theorists along.
So the $500 high-energy gamma-output cables I bought actually DON'T improve my ping?
FUCK YOU MONSTER CABLE!
Aaarrggghh!!!
Genesis 1:32 And God typed
This Einstein guy is starting to really piss me off. Someone gotta take him down a notch or two.
Fuck systemd. Fuck Redhat. Fuck Soylent, too. Wait, scratch the last one.
OK, here's one for the physicists in the audience (and pardon the simplification of terms here, but...)
1) Being deeper in a gravity well slows time relative to being further out.
2) All things which have mass have gravity wells.
3) Photons have mass (NOTE TO THE CLUELESS: "mass" and "rest mass" are two different things - photons have no rest mass, but they most certainly have relativistic mass).
4) By 2 and 3 photons should have a (small) gravity well. More massive photons (higher energy and thus shorter wavelength) have deeper wells.
Thus, wouldn't 1 and 4 lead to higher energy photons "clocks running slower" (since they are deeper in a gravity well) and thus propagating as a lower speed as viewed by an observer outside their gravity well - and that effect would be negligible for all but the most massive photons.
(for the physicists: feel free to expand and clarify on the oversimplifications I've made here. This is, after all, targeting a Slashdot audience which has rather a wide spread of backgrounds).
www.eFax.com are spammers
This is actually just the latest in a series of measurements of this type. Since the Nature paper isn't free online, people may want to look at this similar paper from earlier this year that is available.
The article talks about testing "some theories" of quantum gravity. AFAIK the only theory of quantum gravity that makes anything like a prediction that could be tested in this way is loop quantum gravity (LQG). The two leading contenders for a theory of quantum gravity are LQG and string theory. String theory essentially assumes a background of flat spacetime (plus an xtra 6 rolled-up dimensions), so I don't think it's capable of addressing the issue of whether spacetime is frothy at the Planck scale. LQG doesn't assume a background of flat spacetime, and in fact one of the main research programs in LQG is focused on showing that flat spacetime can emerge as a solution to LQG in the appropriate limit. LQG unambiguously predicts that the vacuum is dispersive, i.e., that the speed of light depends on the energy of the photon. However, LQG does not unambiguously predict the exact form of the energy-dependence. The possible form that is usually assumed in order to evaluate observational tests is |v/c-1|~(E/E_P)^n, where v is the speed of the photon, c is the speed of cause and effect in relativity (often referred to as the speed of light), E is the energy of the photon, E_P is the Planck energy, and n=1 or 2. Previous observations, such as the one in the arxiv paper I linked to above, have pretty much ruled out n=1, so if LQG is right, we'd presumably have to have n=2. Some people have been saying that LQG is ruled out by these measurements, but I don't think that's really correct, it's just constrained by them. Here is a paper by LQG researchers discussing the empirical tests, and they don't seem to be saying "OK, we give up." It's actually very exciting for people in quantum gravity to have observations that even have some chance of disproving a theory (or some version of a theory); the whole field is a dead end if it can never be tested by experiment.
In a broader sense, the holographic principle gives strong, model-independent reasons for believing that spacetime is probably discrete, not continuous, at the Planck scale. Otherwise it's hard to imagine how there could be an upper bound on the information content of a given region of space. And any theory in which spacetime is discrete at the Planck scale will naturally give a dispersive vacuum. Therefore I'd say that either (a) we should eventually observe dispersion of the vacuum once the observations get sensitive enough, or (b) the holographic principle is telling us something that we don't yet understand.
Two good popular-level books that get into this kind of thing are Three Roads to Quantum Gravity by Smolen, and The Black Hole War by Susskind. Because Smolen and Susskind represent very different points of view on quantum gravity, anything that both books agree on is probably correct.
Find free books.
Yah, but think of the poor string theorists. Here they spend 30 years working on it, and they stack the department hiring processes so they will not be criticized for never have created a testable conjecture. Now some data comes in and half their theories crash.
I thought the speed of light does depend on the medium through which light travels.
* http://en.wikipedia.org/wiki/Dispersion_(optics)
* http://en.wikipedia.org/wiki/Prism_(optics)
What they measured is a bit surprising that way.
Stephan
http://stephan.sugarmotor.org
I heard a good analogy once explaining just how small the Planck length really is--and why it's so out of reach of any conceivable measurement we can even dream of:
If the nucleus of a single atom were expanded to the size of the known universe (15 billion light years across--itself an almost unimaginable distance), the Planck length would be about as long as a tall cedar tree.
SJW: Someone who has run out of real oppression, and has to fake it.
One interesting thing that stood out is they used the assumption that the effect of quantum gravity would be proportional to the energy of the light; is this what the theories suggested or is this another case of science getting lost in the translation to newspapers?
As far as I understand from my colleagues who worked on this analysis (now departed for other groups/institutions), the theories of quantum gravity which predict a linear relationship between photon energy and propagation speed are the simplest to test. There are other theories, and they are worth testing too, and some of them would no doubt also be falsified by the Fermi data, but the analysis to do so is more difficult and more complicated, so nobody has done it yet.
SIGSEGV caught, terminating
wait... not that kind of sig.
I thought electromagnetic radiation of different frequencies traveled at different speeds through a medium (as opposed to a vacuum). In this case, the medium would be intergalactic gas, very thin, but there's 7 billion light years of it. How come that didn't spread things out? Is it because the frequencies involved are so high?
In theory, theory and practice are the same; in practice they're different. (Yogi Berra & A. Einstein)
Photons with different wavelengths carry more or less energy depending upon frequency. High-Frequency is Higher-Engergy. E=MC^2. So, Higher Energy Photons are affected greater by gravity. Are they not?
Over-the-top Response Guy! Giving "Over-the-Top Responses" since 1970.
Funny... I sent the link to a friend who does GRB-related research, and she said "thanks, I already know about it though, I'm one of the authors" :). Apparently there are 210 authors on that paper though. Imagine coordinating that.
Anyway, I don't know a thing about astrophysics so that's about all I can contribute to this discussion.