If the center of our galaxy is tens thousands of light years away and hypothetically collapsed on itself, we wouldn’t know for tens of thousands of years. So then in the meantime, what exactly is our solar system orbiting, if it no longer exists?


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

1. Orbiting what used to be there, because its gravity is still propagating toward us at the speed of light.
2. Even after it collapses, it will still have the same total mass, hence the same total gravity.

The supermassive black hole is relatively inconsequential on a galactic scale. We (and by and large the rest of the galaxy) are orbiting the *center of mass* of the galaxy itself. That is to say, not an object at all, but a point in space as a result of the interactions of all the stuff in the galaxy as a whole. We definitely do not orbit the black hole, and while it is near this center of mass, it is not the center of mass nor the specific cause of it.

We expect a change that substantially altered the location of the center of mass would propagate outwards at the speed of light. There’s no known mechanism by which it could travel faster. So if something “deleted” the other half of the galaxy, the gravitational effects (flinging the galaxy apart?) would work their way outwards at lightspeed.

>Does that star’s supernova explosion light up in the sky, or does that impact from this supernova hit us and cause catastrophic damage?

I would question here, “impact” of what? In any (remotely) likely scenario where a supernova damages us, the impact itself is going to be of *light*, which can still be quite damaging. If you mean light versus any physical matter of the supernova, the light would reach us well in advance.

The speed of light is the speed limit of the universe. Nothing can propagate throughout space faster than the speed of light.

So, changes in gravity propagate throughout the universe at the speed of light.

As for a supernova, you have to realize that there is no matter in space for a shock-wave to propagate through like here on Earth. In order for us to feel an impact from a supernova, we would have to be hit with ejected material and light *from* that supernova. Since the ejected material does *not* move faster than light, we will see the light first before being hit by anything else.

It’s also important to note that the speed of light is not the limit *because* light moves that fast, but rather the reverse. Light in a vacuum just cannot move faster than that speed limit. It is a universal constant.

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

>If the center of our galaxy […] hypothetically collapsed on itself

Ain’t nothing hypothetical about it, the center of our galaxy has collapsed on itself. We are very confident [there is a supermassive black hole](https://en.wikipedia.org/wiki/Supermassive_black_hole#In_the_Milky_Way) at the center of our galaxy. In that article is a black and orange image; that’s an image of a supermassive black hole in another galaxy – I believe ours is still too hard to image since we’re looking at it on end with a spiral galaxy (our own) in the way. The black hole in the middle of that image is just larger than our solar system.

>…we wouldn’t know for tens of thousands of years. So then in the meantime, what exactly is our solar system orbiting, if it no longer exists?

Nothing moves faster through space than causality, which is why light moves at that speed. So do gravitational waves.

Let’s reduce this scope to something more manageable. If our sun collapsed into a black hole, right now. we wouldn’t see the effects for 8 minutes. Then, suddenly, the lights would go out. In the mean time, while we’re waiting for the light of the event to reach us in 8 minutes, we would continue to orbit as normal.

And after? Ostensibly, we would continue to orbit as normal! Gravity is the effect of mass over distance. The further away you get from something massive, the less the significance of gravity. If you parked a space ship at the same distance from the black hole as the “surface” of the sun once was, you would feel the same amount of gravity act on your body as though you were standing on the surface of the sun. It’s only when you start getting closer to that infinite well of gravity does the effects of a black hole we talk so much about actually come into play. If the sun collapsed into a black hole, it would be no more than 6 kilometers across – the entire mass of the sun in that little space. Don’t wander too close…

In contrast, as you go underground and get closer to the center of the Earth, you get lighter. Scales with 3 or 4 places of precision need to be calibrated depending on where they are on Earth, because their distance from the center, and the amount and composition of rock underneath them will influence the precision of the scale, including going down into the deepest mine shafts.

Now if all these things just *vanished*, then in 8 minutes, we would go from orbiting the sun to being cast off at some trajectory, into the darkness.

There isn’t any physical process that could make actual mass suddenly vanish from the universe. So, asking what the laws of physics say would happen when a physical impossibility occurs isn’t especially useful.

1. The center of our galaxy is already collapsed, its a supermassive blackhole. There is no “collapsing” beyond a singularity, and even if there was, it wouldn’t affect the mass that’s already in it. If you’re saying “remove the mass” then that’s a different story, we’d probably just find our solar system keeping its current trajectory forever, falling further and further away from the rest of the milky way.
2. The gamma radiation/xrays from that supernova would probably scour the atmosphere off the planet and eradicate all organic life far before anything would push us out of orbit. The physical pressure wave would be not near speed of light, and would lose energy much more rapidly.

EDIT: Spelling and clarity

Wow, I can’t thank you all enough for taking the time to comment on this. It was extremely humbling, and I have learned more by reading your comments and subsequently researching your points, than I have my entire life until now! I did not know that the center of our galaxy was not indeed a super massive black hole, but a center point of mass where the black hole is in proximity of.

You guys are awesome, thank you so much

First, to clear up a couple of misconceptions, our sun and everything else in the solar system does NOT orbit a supermassive black hole at the center of the galaxy. Everything in the galaxy orbits the center of mass of the galaxy. A supermassive blackhole is completely insignificant in terms of the total mass of the galaxy. Just for comparison, the supermassive black hole at the center of our galaxy is estimated to be roughly 4.3 million solar masses, whereas the total mass of the Milky Way is estimated to be no less than 1.5 *trillion* solar masses, of which 90% is dark matter. That means that black hole is about 0.00028% of the mass of our galaxy. Second, the galactic center is not an object, but an area of space, and there’s no method by which it can just “collapsed on itself”.

That being said, you’ve answered your own question simply by knowing that changes in gravity propagate at the speed of light. Earth is roughly 8 light minutes from the sun. If the sun were to magically disappear, we would not notice for 8 minutes. We would receive sunlight for 8 more minutes, and the Earth would continue to orbit the spot where the sun was for 8 minutes. That doesn’t violate the laws of physics because it literally *is* a law of physics.

The same applies to your supernova scenario. The star goes supernova, but we don’t see it for 100 years. The rest of your scenario is impossible though. There’s no such event that can cause gravitational disturbances strong enough to toss planets from orbit 100 light years away, and supernovae don’t create debris that can fly through space and smash into other planets. There’s nothing to impact or cause damage. What you get is radiation, gas, dust, and charged particles. The radiation travels at the speed of light, but everything else goes slower. The charged particles can travel very close to the speed of light, but the gas and dust might only be expanding outward at a few km/s. That’s roughly the same speed as spacecraft in Earth orbit. It would take millions of years for that dust to reach earth.