I am genuinely curious about this. I *kind of* understand that gravity, like light, moves at the speed of light (right?). So then would our solar system, and millions of other star systems, just continue orbiting what USED to be the super massive black hole in the center of our galaxy, if said black hole just suddenly (hypothetically) collapsed/vanished? How does that not violate the laws of physics?
Furthermore – let’s say a star… a hundred light years away went supernova. We’re still receiving it’s light for a hundred years right? It would just look totally normal to us in the sky, for the next hundred years. Well let’s say that supernova was so awesomely powerful as to truly push our planet out of orbit from our sun. What happens first? Does that star’s supernova explosion light up in the sky, or does that impact from this supernova hit us and cause catastrophic damage? What’s faster – the impact or the supernova?
I really want to understand distance/time better as it relates to astrophysics, I just can’t comprehend the insane distance and the deltas between distance + time = what we experience on earth. It’s truly humbling.
In: Physics
You’ve touched upon part of the extreme weirdness which is causality at long distances.
I’ll start with your supernova question because it’s far more straightforward – the visual change of the supernova is first, as it’s purely light-based which is of course as fast as anything can be. The impact of a supernova is a shockwave of other particles which are all traveling very *very* fast… but much slower than light. Measured supernova shockwaves travel at somewhere around 0.01% of the speed of light.
As for the sudden change in the black hole at the center of the galaxy – what is important to understand is that the concept of “now” is not a real one. There is no objective “right now,” only a “right now” from a given frame of reference. From the point of view of an observer on earth, any visible change in a distant object is happening “right now” even if it is millions of light years away.
One can talk about the change happening 100 million years ago, but it’s meaningless because
* There is no objective time scale; from the “point of view” of a photon leaving a star, the moment it leaves the sun and the moment it hits the earth are simultaneous
* There is no causal relationship between you and events further away in space than in time; and without a causal relationship the idea of a “length of time” is not a very meaningful one
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