# How can black holes be used for daredevil gravity slingshot sport?

This is set in the same universe as my Andromeda galactic war question (same tech level). In this question, I had somewhat purposely labelled the galactic core as "off-limits" (black holes and other nasty things).

However, young fighter-jocks use this space for training and recreation - so much so that it's a broadcast sport.

Black holes can be used as gravity slingshots, and this has turned into an event in itself where pilots attempt to out-do each other in terms of exit speeds from the slingshot before manually shooting at a target.

In order for this to be credible in the story, I'll need some help:

• How much acceleration could you get from using a black hole as a gravity slingshot?
• What might happen if you got slightly too close during the attempt?

Assume that pilots have tech that helps them withstand g-forces, but it has limits - manually shooting at a target on exit demonstrates the ability to continue fighting after extreme-G manoeuvres.

Black holes being used here are Intermediate in size (lets pick one of 5,000 solar masses)

As before, there's no time dilation effects in this fiction - it's lots of gravity and lots of speed.

• It's not a full answer, and I'm not up to writing a full answer at the moment, but keep well in mind that black holes aren't magic. You can treat them as point gravity sources, which makes some things easier (no need to mess around with an actual radius, for example, beyond knowing the event horizon radius) but they have no special gravitational effects or properties beyond those of a different object of a similar mass. They are just a lot more compact than your run-of-the-mill gravity source.
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Commented Mar 13, 2017 at 20:54
• @MichaelKjörling you can orbit them at the speed of light though, so in practice they will be quite different Commented Mar 13, 2017 at 23:45

If the black hole is stationary the net change to your speed is 0. That is to say, once you are as far away from the black hole as you started, you'll be moving at the same speed you started at. We use planets that are already travelling in the general direction we want to go. See Gravity_assist. During a close pass you could certainly pick up some extraordinary speed before shedding it as you move away from the center of gravity.

EDIT: Loren Pechtel pointed out the Oberth Effect, which I don't believe is usually what is meant by a "gravity slingshot", but may certainly be relevant to the story idea. Note, this just means your have a greater exit speed from the gravity well than by using the same thrust in empty space. Your maximum speed will still occur around the closest point of your pass by the black hole.

As for what happens when you get close, well first there's frequently a lot of matter in the accretion disk that is getting sucked in pretty fast. If you get close the gravity on the side of you that is closer to the black hole is actually greater than the gravity on the opposite side, resulting in a stretching force. This force is enough to tear anything apart before you even reach the event horizon. In other words it's never the event horizon that really kills you. See Spaghettification

• Can you get a gravity assist of a stationary thing if you activate your thrusters while near the black hole? Commented Mar 13, 2017 at 23:47
• @PyRulez No, but you do get the benefit of the Oberth effect if you do your burn deep in a gravity well. Commented Mar 14, 2017 at 2:13

If the black hole is spinning, you can still get a speed boost.

I see two spots of trouble.

One is the tidal stress if you get too close (also known as spaghettification).

The other problem is when will you come out. Time travels slower the closer to the black hole you get. You could come out decades or centuries after you went in. Imagine coming out and bragging about your skill at piloting 400 year old tech.

• Huh? Time slows for you but that doesn't mean you fly any slower, you just perceive the pass by the black hole taking less time than it actually did. Commented Mar 14, 2017 at 2:19

There's no reason for the guns.

Put some targets in orbit about the black hole. Closer in targets are worth more, targets that require a greater deflection are worth more. The pilots may manually maneuver at will on the way in, they are not allowed to use their engines after periapsis. Their score is the point value of the targets divided by a factor based on how close to the target they got.

I will propose: the object they are playing with is a pair of black holes, about ready to make one of those merger events people measure at LIGO - think GW190521. The distance and period are up to you - as short, of course, as you can picture your competitors braving. Now black holes have just mass, charge, and spin ... but that means that your contestants are passing through two possibly non-aligned ergospheres dragging along space itself with them, while confronting differences in charge and powerful magnetic effects, all while the constant wobbling spins of the holes around each other on a millisecond scale are threatening to put them into zones of intolerable tidal stress. Then there are the accretion disks and the stray Higgs mines in actively maintained "orbit", left over from the Acronesian War! The need to plot a course that avoids this real debris makes this a much less mathematical exercise (I.e. requires much more math). A skilled robotic gunner capable of outthinking a mine on a nanosecond to nanosecond scale when you can't approach within a fifth of a light millisecond (or else) might make all the difference. But the navigator had better be able to roll with that input and plot a course that doesn't ride an ergosphere straight into the throat of the other hole.

I don't think this would work because you're black holes would be too far apart and provide too small a change in delta-V to be interesting in a sport.

Black holes are generally formed by collapsed stars, and as such they tend to be as far apart as solar systems, ie very far apart. Even if you have technology making black holes closer then would naturally form they would still have to be very far apart or they would suck in each other and collapse into one larger black hole. this means you will have, at most, one black hole close enough to you to have any appreciable gravitational affect on you.

Before anyone says it yes in theory you could on rare occasion have two black holes in stable orbit of each other, but even then they have a minimum distance far enough apart that likely only one will be applying significant gravitational pull at a time.

However, this black hole will not have an interesting affect at a rate worth calling someone a daredevil, though why depends on your FTL travel.

• If you are staying at sublight speeds then you're craft is moving so slow that it would take days to approach the black hole, any maneuver that takes days to complete probably isn't going to make a very fascinating sport.
• Most viable FTL options involve not moving in 'normal' space and thus would not be likely to be affected by gravity.
• If you have FTL travel fast enough to make regular trips between star systems a regular occurrence that doesn't take a decade to complete then you are moving so fast that the you zip by the black hole too quickly to notice it's gravity. The only daredevil sports you could pull off is "how close will that idiot get to the black hole without running into it", but this is really just a game of chicken, no different then sticking a giant pole in the middle of a racetrack and seeing how close your racecar drivers can pass it in a lap without hitting it.
• You could, in theory, create a third type of FTL travel, too slow to make regular trips between systems viable but fast enough to make it possible to approach a black hole fast enough to make it a viable sport. However the delta-V provided by gravity would still be too small relative to the speed your craft was going to have a significant effect, your engines would be able to push you forward at a rate dozens of times faster then the blackhole altered your speed. Furthermore to get close enough to the black hole that it can impart any remotely interesting amount of delta-v for the short length of time you would be near it you would be so close the gravitational shear affects would likely tear you're craft apart.
• A gravity well, like a black hole, will not add speed in the long run, it could in theory help you change direction, but total speed will not be increased by approaching near one and then departing it.

Basically I see no means that black hole dare-deviling makes sense in realistic physics. Luckily as a writer you get quite a bit of suspension of disbelief which allows you to make up your own rules. I'd suggest you create an intra-system handwave FTL that happens to behave in a manner similar to what you want to happen because insert quantum tachyon technobable. First you would need to have two FTL systems, one for traveling between solar systems that was faster and one for intra-sytem travel which is fast enough to allow rapid approach to planets but not too rapid of an approach. Since you are making up a handwave FTL system anyways simply say that this one is explicitly affected by local gravity in a manner more drastic then would happen in if you were traveling in normal space at sublight speeds. In other words your not Just being accelerated by the gravity of a black hole, which doesn't provide enough delta-V to be interesting at those speeds. Instead just getting closer to a massive gravity well allows your intra-system FTL to accelerate your craft faster because quantum magic. Similarly you can have practically any affect you want happen if they get too close to the black hole, since your pretty much making up the physics of your FTL drive anyways.

Of course even then there is another problem, black holes really aren't that different then any other gravitational body. Other then being super-massive the way the gravity work's doesn't change. If you replaced our sun with a black hole of the same mass all planets would orbit in just the same way. Given that fact any massive body could provide the same gravitational pull. It makes more sense to have your dare devils zooming past planets then black holes, they will be closer together and if you just approach them a little closer then the black hole you would still get the same overall gravitational pull on you. That is unless you do more handwave magic about how the density of the gravitational object affects how your FTL system works.