## Intergalactic Race Shows That Einstein Still Rules 227

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."*
## Re:i'm confused (Score:4, Informative)

## Re:i'm confused (Score:5, Informative)

The spread in travel time of 0.9 second between the highest- and lowest-energy gamma rays, if attributed to quantum effects rather than the dynamics of the explosion itself, suggested that any quantum effects in which the slowing of light is proportional to its energy do not show up until you get down to sizes about

eight-tenths of the Planck length, according to the Nature paper, whose lead author was Sylvain Guiriec of the University of Alabama.Granted they say it would have to be proven much smaller than a planck length for most people to accept this as empirical proof, it is empirical data backing Einstein. The 9/10s could be due to the explosion or a physical effect but the latter is now more unlikely given the many light year distance.

## Re:How do they know (Score:5, Informative)

The event was approximately 2.2 seconds long. Thus it is plausible that these two photons left .9 seconds apart.

## Re:How do they know (Score:5, Informative)

We don't, and it doesn't matter. .9 seconds is much smaller than would be predicted by the theories in question.

## A decade long project (Score:5, Informative)

This was proposed by G. Amelino-Camelia et al. back in 1998 [nature.com]; here [arxiv.org] 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 [nature.com].

It should be noted that this does not rule out quantum gravity - it seems pretty clear that General Relativity and Quantum Mechanics cannot

bothapply 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.## Re:i'm confused (Score:5, Informative)

It doesn't prove that the speed of light is constant, but it does reasonably prove that the speed of light is independent of wavelength, since they left from the same source at the same time.

## more information (Score:5, Informative)

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 [arxiv.org] 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 [wikipedia.org] (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 [arxiv.org] 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 [wikipedia.org] 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.

## Re:How do they know (Score:4, Informative)

We're talking about a distance of 7.3 billion light years. Even if the expected difference in speed is 1m/s (absolutely miniscule difference against the speed of light), we're talking a difference significantly greater than 3.1s in travel time.

## Planck length (Score:5, Informative)

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.

## Re:i'm confused (Score:3, Informative)

Exactly.

Competing theories stated the difference would be more than 3.1 seconds (The possible variance given a 2.2 seconds event and 0.9 seconds difference). Therefore they are invalid.

Einstein stated the total variance would be equal or less than the total duration of the event. Total duration was 2.2, difference was 0.9, this fits. Einstein's theory is not proven invalid.

Any other conculsions are logical falacies.

## Re:Mass, gravity, and the speed of light (Score:5, Informative)

Anything that moves at the speed of light does not experience a passage of time and has no "internal clock" to speak of.

This is how people figured out that neutrinos had a rest mass when at first the Standard Model assumed that they didn't. It was discovered that neutrinos could oscillate between different lepton flavors. But for that to happen they would have to experience the passage of time. And since particles without a rest mass always travel at the speed of light, they had to have mass.

## Re:Mass, gravity, and the speed of light (Score:1, Informative)

Photons do not have mass. "Relativistic mass" is a terrible term and I hate it. You have to look at the relativistic equations to understand where it comes from. You can express the total relativistic energy, in short, as E=sqrt((pc)^2+(mc^2)^2). For a particle without mass, such as a proton, this reduces to E=pc. However someone figured out that for a particle with mass, you can represent the energy as E=gamma*mc^2, which is Einstein's famous equation with a factor dependent on the speed of the particle.

So someone figured that if you find the energy using the original equation, then equate it to the second way of writing it, you can kind of take that second "m" to be a "relativistic mass" of sorts, and apply it to any kind of particle, even massless ones. But it isn't actually "mass" in the true sense of the word. In reality saying something has "relativistic mass" is more like saying that total energy of the particle, while in motion, is the same as if the particle was not moving at all but just got more massive.

While that might be a fun mental exercise to undergo, it isn't a very useful thing from a theoretical standpoint.

So no, photons do not have gravity wells, as they do not have mass.

By the way, the reason photons are affected by gravity at all, according to Einstein, is that things with mass curves spacetime itself. The photons have to move through spacetime, so they curve with it.

## Re:string theory (Score:3, Informative)

Actually, the big contender here was quantum gravity, which is more or less a competitor to string theory.

## Re:string theory (Score:3, Informative)

thanks

http://en.wikipedia.org/wiki/Quantum_gravity [wikipedia.org]