For 130 million light years the difference between gravitational wave signal and gamma burst is 2s. That constrains the speed difference to 1.6e-18 m/s. This is fascinating number especially given that speed of light is 3e8.
> For 130 million light years the difference between gravitational wave signal and gamma burst is 2s. That constrains the speed difference to 1.6e-18 m/s.
This assumes that the production of gravitational waves and the gamma rays occurred at the same time.
Newbie question: While this is a very small fractional difference, could it be theoretically significant? What is the explanation of this difference? My best guess is that this difference must also be there in when the waves _started_.
It reminds me of a point from non-Euclidean geometry. In Euclidean geometry a triangle has 180 degrees, but in hyperbolic or elliptic geometry it has more or less (I forget which way). So you could start measuring triangles to find out whether our universe is Euclidean or not. But since every measurement has an error interval, e.g. 180 +/- 0.000001 degrees, you may some day prove the universe is non-Euclidean, but you could never prove it is Euclidean! It may be "skewed", but just less skewed than your instruments can measure.
As neutron stars are small, like 20 kilometers, while the light travels 600 000 km within 2 seconds and one can roughly assume that speed of processes that generates the gamma burst during the stars collisions matches the speed of light the delay may need some explanations. It could be just scattering of gamma rays by intergalactic medium that does not affect the gravitational waves, but what ever it is, I suppose it matches models or this delay will be in the news.
Simplest explanation I can think of: there's no reason to model this event as if all of its energy is emitted from a point source. As the NYT article put it, the size of the explosion is comparable to the orbit of Neptune. Meanwhile, two seconds at the speed of light is less than the orbit of Earth's moon.
So, if the gravitational effects originate from the center of mass of the explosion, and the gamma rays originate from some kind of Big Bang-like recombination phenomenon happening a few hundred million km away from the center of the expanding shell, that would easily account for the difference.
I'm not a physicist, but I did do the 101 course a long time ago. I read "That constrains the speed difference to 1.6e-18 m/s" as "we measured as accurately as we could, and if they do differ, they must differ by this amount or less, which could just be the tolerances of our experiment"
In other words, there's always a confidence interval, the trick is to measure it and minimise it: The speeds could be identical numbers, but what was measured was an either no or a very small difference. Larger differences have been ruled out. Identical numbers are _suggested_.
What is observed is the delay between the two signals. Fractional speed difference is calculated from it. As I understand, there is little chance that the 2 second delta isn't real.
OK, but there are other possible explanations besides fundamental physics: either the order of events at the source, or the effects of the interstellar medium along the way.
Basically, yes: The speed of both gravity waves and light waves is limited by c, the speed of light in vacuum.
But actually, no, because the light is not in a vacuum - it's ricocheting out from the center of a star, and bouncing through the interstellar medium (which is incredibly sparse; very nearly a vacuum: but there's an awful lot of it between these events and us).
This is also the mechanism behind neutrino detectors. When a star's core goes supernova, it releases a cataclysm of neutrinos which pass through the upper layers of the star almost unaffected. The light and radio waves are still bouncing around trying to get to the surface of the star, and the blast wave is still physically propagating much slower than the speed of light, but the neutrinos are long gone. That gives scientists a short time period to point their telescopes in the right direction!
Yes and this is different in as much as LIGO / gravitational waves gave us the 'tip off' that the light was incoming, we fortunately had a satellite that captured that.
It's a whole other information stream we are just learning how to use, basically with using gravitational waves we are at the stage Galileo was at with light. Very early, very low res but enough to give us massive new discoveries.
