Further to Gravitational waves reliably detected – Updated IV, from Sean Carroll at the Atlantic:
Einstein’s gravitational waves rest on a genuinely radical idea.
Einstein’s general relativity is a theory of gravity. It says that spacetime can be curved, and we feel the effects of that curvature as the gravitational force. According to relativity, the speed of light puts an absolute limit on how fast influences can travel through space. The Andromeda galaxy is two and a half million light years away, so it would take at the very least five million years to send a signal there and get a response back.
We’ve all heard about this speed-of-light barrier, which applies to gravitational waves just as much as everything else in the universe. But let’s think a bit more deeply about why there is such a limit at all. That’s where locality comes in.
Drop a pebble into a still pond. Small waves ripple outward in a circular pattern. We naturally think of those waves as “things” that “travel” through the water, but at the same time we recognize that there is a deeper description. The water is made of molecules, and those molecules keep moving around and pushing on other molecules. At this microscopic level of description, molecules in the pebble pushed on water molecules at the location where the pebble entered the water. Those molecules in turn pushed on other water molecules nearby, and those pushed on ones a bit farther out, and so on. It’s the collective action of all those molecules together that gives us the impression of “waves traveling through water.”
That’s the origin of gravitational waves: disturbances in the gravitational field, rippling through spacetime at the speed of light, just like the ripples we get from throwing a pebble into a pond.
The molecules themselves only interact when they are very close by—when they literally bump into each other. The pebble doesn’t instantly affect all the water in the pond. It affects water molecules in one location, which then affect those nearby, in a chain that ripples to the edges of the pond. That’s locality in action: Even though the waves might travel impressive distances, at a deeper level it’s just each molecule talking to others right next door. There is no action at a distance, spooky or otherwise. More.
Well, it’ll certainly be action at a distance when a tsunami arrives. It’s unclear how the fact that the gravity travels locally changes that.
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