Eli5 –> Speed of Gravity and GW170817 – Why was it a big deal anyway and won the Breakthrough of the Year award for 2017
In: 70
The GW170817 observations were very good evidence for gravitational waves, and that they travelled at approximately the speed of light.
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In modern physics there are four fundamental interactions; electromagnetism, strong nuclear, weak nuclear, and gravity. Our understanding of gravity is fundamentally different to the others (which have all been combined into a single Standard Model). One of the big questions of modern physics is how to fit gravity with the others.
As part of that there is the question of how gravity is transmitted; whether it is immediate or travels at a certain speed, and how it does so.
The main theory (as of 2017) was that gravity travelled as waves, at the speed of light. Essentially if something massive accelerates a lot, the change in its gravitational field would ripple out away from it at the speed of light, and those ripples would have local effects on space and time (due to General Relativity stuff). But gravity is a really weak interaction (compared with the others) so it would take a really huge mass, accelerating an awful lot, to have even a chance of making measurable ripples.
On 17 August 2017 three different gravitational wave detectors picked up a 100 second or so signal. Gravitational waves rippling through them. Understandably this caused a lot of excitement and a mad rush to confirm the others had picked up on it, and try to triangulate where it came from. Once that had been done, there was a second frantic effort to point any available telescope at that patch of sky to see what was going on there, and if it was anything visible (previous suspected gravitational waves had probably come from black holes, which are black, so hard to spot or get secondary confirmation of).
Over the next few days, weeks and months 70 different observations were made of this area of the sky (using different telescopes and detectors, covering a whole spread of the electro-magnetic spectrum), and they saw two neutron stars collapsing together; exactly the sort of high-mass, high-acceleration interaction that would produce significant gravitational waves.
This was a big deal. It was the first time that detectors had picked up both electro-magnetic signals from an event *and* gravitational waves – pretty much confirming that gravitational waves are real, and behave as predicted. It also put a limit on the difference between the speed of gravity and light, at a factor of about 10^(-15).
It is always exciting when experimental observations back up long-standing hypotheses, and this did so [although arguably it is more exciting when they don’t!].
Grumblingduke has given a fantastic response relating to the speed. I will add a bit more about why GW170817 is a big deal and won the Breakthrough award.
As well as allowing us to measure the speed of gravitational waves, it allowed us to measure the distance to the galaxy that the collision occurred in (for more info on method, look into “standard sirens”) and by then looking at how fast the galaxy is moving away from us we can figure out the expansion rate of the universe. While we already had more accurate methods of measuring this, there are concerns about the other distance measures we use, and so having this method that is independent may be quite important.
Comparing the expected amplitude at that distance to the actual amplitude also allowed a test to a number of general relativity alternatives with extra dimensions, and ruled out the gravitational wave “leaking” into extra dimensions. Going back to the speed measurement again, the fact that the speed was the same ruled out several GR alternatives.
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