Wednesday, October 28, 2009

In response to: Gamma-ray photon race ends in dead heat...

In response to:

ScientificBlogging Gamma-ray photon race ends in dead heat; Einstein wins this round. We'll get you yet, though, Al.

Racing across the universe for the last 7.3 billion years, two gamma-ray photons arrived at NASA's orbiting Fermi Gamma-ray Space Telescope within nine-tenths of a second of one another. The dead-heat finish may stoke the fires of debate among physicists…
How do you know they started at the exact same time and not 9/10th a second apart?
Here are my collected comments:

The article itself says that the difference in arrival times is "… likely due to the detailed processes of the gamma-ray burst…", i.e. in the source of the γ rays.

Addressing Kevin’s question in more detail:
The telescope was looking at the same part of the sky, presumably small enough to contain only one source of γ s. A Gamma ray burst was observed (bunch of γ s with energies between the two extremes mentioned in the article.). Gamma ray bursts last on the order of a couple of seconds (See above article.). On a macroscopic scale, the photons traverse the same spatial trajectory and intervening objects, but one is a few seconds behind the other. Nothing changes cosmologically on a scale of two seconds - except of course for gamma bursts and supernovae, which weren't observed. Macroscopic dispersive effects (change in refractive index due to frequency) in large intervening nebulae etc. cannot in principle be ruled out, but what do I know about astrophysics? Presumably they carefully chose a very empty part of the sky.

So, in order to reject the null hypothesis (that there are no differences in the speeds of the photons of different energies) any observed difference has to be >~ 2 secs. If there are hypothesized physical processes that do predict larger differences in arrival times, the observed 0.9 sec difference will put very tight constraints on the viability of those theories.

Now, watch the string theorists squeeze themselves through those very tight constraints! They have had 30 + years of practise!
Bo Asciu wrote:
Kevin good point, keen mind.

Space/Time can't be that "frothy" since the laws of the universe are so precise. Too much looseness and the whole thing falls apart.
Briefly addressing Bo’s comments:

There is classical, deterministic chaos, there are classical uncertainties due to stochastic processes, and we haven't even got to talking about QM! -in terms of causing frothiness in spacetime.

The point is that there are viable and very precise theories - "laws of the universe" - that separately either hypothesize or predict a "frothy" universe on a scale somewhere between 10-18m and the Planck scale (10-35m). The observations discussed put presumably very tight constraints on the free parameters in the theories that distinguish them from Einsteinian General Relativity.
If say the two photons had arrived more than 8 secs apart, the observations would tell us precisely how "frothy" spacetime actually is – how big are the hyopothesized microstructures, on what time scales do they change, how do they interact with different photons.

Even Einstein's very beautiful, very precise etc etc General Theory of Relativity CANNOT rule out frothiness of the universe on some heretofore unobserved, untested scale, though very good arguments can be made that the GTR will break down at some scale > Planck. If frothiness is observed at some larger scale, it will put a lower bound on the domain of applicability of the GTR (Since it hypothesizes a smooth (or at least twice differentiable) geometry for spacetime, GTR is valid above but not below the scale at which those “violations” are observed.).

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