# What is the maximal gravity assist boost achievable in the Milky Way galaxy using black holes?

This question on the world building site sparked my curiosity on whether we could see signs of an intergalactic civilization. The question itself effectively asks whether this is possible for our own galaxy. Since the Milky Way is 100 lightyears across, and assuming there is an alien self replicating machine capable of traveling at 1% of c, then it would be able to colonize the whole galaxy in about 10mn years give or take. This is a tiny amount of time in the grand history of the universe so it's at least plausible.

However when you extend this thought to intergalactic distances it quickly becomes unfeasible. The distance to the Andromeda galaxy is 2mn light years and the width of the local supercluster is 110mn. Travelling these distances at 1% c would take the entire age of the universe...

So on to my question: is it possible to achieve anything close to 80% of the speed of light using gravitational assistance from the black holes orbiting the center of our galaxy?

I don't know in detail how gravity assists work so I have no idea what the limiting factors are, but let's assume we have millions of years to time it out and a delta v budget of 1% the speed of light.

Since the boost depends on the proximity to the boosting body I assume the biggest hurdle would be the acceleration you would have to endure as you get closer to the blackhole boosters. Is there an easy way to estimate what it would take?

Some napkin math tells me it's somewhat plausible:

• The diameter of the black hole at the center of our galaxy is on the order of a light minute.
• It takes on the order of 10 minutes accelerating at 50000g to get to the speed of light.
• Accelerations on the order of 50000g were achieved by the rail gun prototypes. Since there are some proposals to add electronic chips to rail gun projectiles, I'll assume our autonomous alien friends have the tech to withstand this.
• Gravity assists are as concerned with the mass of the body as they with its velocity. Even if the body being used in the gravity assist is as massive as a black hole, it may lack any meaningful velocity and thus boost potential, at least when compared to 80% c. Apr 20, 2021 at 1:24

This is an interesting question in that it highlights what gravitational assists can and can not do.

To think about this, first choose a frame, then find something moving fast in that frame, then come at it and let it gravitationally accelerate you for a few hours roughly in the direction of its motion.

For example if the frame is our solar system and the fast-moving thing is Jupiter, then come up on Jupiter and let it pull you along in somewhat the same direction that it's moving in the frame of the solar system.

Now, how would this translate to using black holes in the galaxy?

If you can find a pair of black holes orbiting each other quickly (fraction of the speed of light), i.e. frighteningly close to each other, perhaps you could think about doing this.

But the supermassive black hole at the center of the Milky way pretty much just sits there and occasionally munches on dust and gas spiraling into it, giving off an occasional flare.

• Would this make the maximum boost speed roughly double the speed of the orbiting black hole? I'm thinking that you come at this system with a galactic speed of about 0, which from the black hole's frame of reference is the negative of it's speed, you get picked up on a ride and exit the system at the positive of the black hole speed (by symmetry) and therefore 2x it's speed in the galactic reference frame.
– csiz
Apr 20, 2021 at 1:26
• @csiz I've labeled this as a partial answer because that may turn out to be a more difficult calculation than I can do. I'm not sure how far you'd have to drop down into the black hole's gravitational well, or what the general relativity implications are. Your question is new and I have a hunch one or more math-based answers may be forthcoming. Let's see how this goes.
– uhoh
Apr 20, 2021 at 3:19
• After some digging, black holes reach about half the speed of light during mergers, but the event only takes a few seconds which seems awfully difficult to time. The speed of binary black holes would be 0.1c or lower at stable orbits (decaying slowly). It seems like multiple bounces are required which should also make the conditions for a single slingshot more tolerable. As long as the distance between slingshots is large enough the spacecraft could also refine the timing with a small amount of delta v.
– csiz
Apr 20, 2021 at 6:04
• If you were willing to throw some mass into the BH and could find a suitably spinning one, you could use the Penrose process to get some extra boost -- really this is extracting angular momentum from it. Otherwise I think you're likely correct.
– user21103
Apr 20, 2021 at 13:55