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Why doesn't our galaxy fall into the black hole at its centre?

Gordon Ansell Disquisition, Science 656 words.

I was asked this question the other day.

It can see why people might think that would happen. The supermassive black hole at the centre of our galaxy is a big unit. It has a mass that’s about 4.3 million times the mass of our Sun and that mass is squeezed into a diameter that’s only 20 times bigger than the Sun’s and a volume that’s only 5,500 times bigger. I say only 5,500 times bigger because that’s piffling in relation to 4.3 million times bigger the black hole’s mass is.

The thing is, the galaxy as a whole is much more massive than the black hole at its centre. The black hole represents less than 0.5% of the mass of the galaxy. And even though the black hole has a diameter of about 23 million km, that’s nothing compared to the diameter of the galaxy, which is over 100,000 light years wide (and each light year is about 9.5 trillion km).

Sagittarius A - where they think our galaxy's black hole is.
Sagittarius A - where they think our galaxy's black hole is.
Credit: NASA

As a singular object the black hole is a mighty beast but its mass and sphere of influence isn’t enough to have too much effect on the galaxy as a whole. For the most part, the structure and motion of the galaxy is driven by other forces.

That’s not much consolation to aliens living close to the black hole, though. They’ll certainly be concerned about its influence. They might, however, be pleased to know they won’t suffer from spaghettification. This is a term cosmologists use to describe how objects are stretched out and eventually torn apart as they cross a black hole’s event horizon.

But this effect is more noticeable with smaller black holes and tends not to happen with the big ones (or at least not so soon). It’s still no fun day out being close to a supermassive black hole either, though. You won’t die the instant you cross the event horizon — in fact you will notice nothing at all at first — but your ultimate fate can now be nothing other than an appointment with the singularity at the centre of the black hole.

There’ll be time for some laughs before that happens, though. Supermassive black holes rotate and they drag spacetime around with them (this is called frame-dragging and it’s another one of Einstein’s gems) and odd things happen in such situations. Let’s say you had an ice-rink in your spaceship — and what self-respecting spaceship wouldn’t? When a skater starts spinning with their arms out and then they bring them in, they speed up, but the opposite could happen in a frame-dragging situation. It’s the laws of physics, Jim, but not as we know them.

Closer still it would be even more weird because cause would no longer precede effect. You’d get up before you went to bed and the council would collect the bins before you’ve put them out. Aliens would be raising many complaints with their local MPs about the latter.

You might think your best bet would be to start your engines up and try to accelerate away from the singularity, thus prolonging your demise. But, whichever way you think is ‘opposite’ will in fact not be opposite at all. All directions lead to the singularity once you’re across the event horizon. So accelerating in any direction at all just speeds up your demise. It’s like standing precisely on the North Pole. Whichever way you step is going to be south.

What’s interesting is that as far as someone outside is concerned, you never cross the event horizon at all. According to them, you’d sit there motionless, gradually getting redder and redder as the gravity redshifts the light. You’d appear to flatten out across the surface somewhat and eventually it would look as if you’ve been consumed by radiation at the black hole’s event horizon, but never crossed it.

So, in summary, black holes are completely bananas but they don’t have as big an influence on the rest of the galaxy as you might think.