# (Almost) Black Hole = Fast Space Ship?

1. Jul 4, 2013

### GurSybngvatOen

Hello,
I am new to this forum! I have only just completed my first physics class, but I like to learn about
some of the more advanced topics in physics on my own time. I was watching a video about black holes and I noticed a part of the video that stated "any matter that enters this region of space spins so fast that centripetal force tends to fling it out" (22 minutes). I was wondering, how fast is that? Could you fling a space ship that fast? If not, if the object were near the density and mass of a black hole, but was not at the density and mass of a black hole, then would it be possible? If not, with the said object if you had a part of the space ship inside the warped region of space effected by the almost black hole's gravity and a engine outside of said region it pushing the craft, relative to the part of the ship inside the gravitational region of the black hole, would it be moving faster than the engine actually is pushing it (given that the part of the craft holding the engine on was not warped)?

(Image of what I am describing attached)

Sorry if this sounds silly, I just like to theorize even though I do not have large amount of knowledge about physics.

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2. Jul 5, 2013

### Simon Bridge

Welcome to PF;
the gravitational slingshot is the usual approach for using gravitation to accelerate a spacecraft by passing close to a large mass.

The video you show is being somewhat glib and you shouldn't base any theories on what they say.

3. Jul 5, 2013

### Staff: Mentor

I'm not up to speed on relativity, but when they say "space itself falls in at the speed of light", it sounds like nonsense to me. I'm sure that there are effects like frame dragging in effect, but "space itself" falling in at the speed of light doesn't make any sense. Space is not a "thing".

No, the video is talking about what goes on BEYOND the event horizon of a black hole, near the singularity. So you have to have a black hole, not something close to it.

To my understanding what the video is saying is that near the singularity of a rotating black hole the effects of frame dragging (I think it's frame dragging at least) are so extreme that it adds energy to the infalling matter, kicking it back out before it can fall in. BUT, this is all within the event horizon.

Also, I just want you to understand that everything they say is based off pure math and since it's beyond the event horizon it can never be verified, so take it with a grain of salt.

I assume that at the conditions at that location inside the black hole you cannot have a spacecraft. It would have been ripped apart and turned into atoms or plasma long ago.

4. Jul 5, 2013

### Simon Bridge

That's entirely poetic, yeah, like most of the narrative in the video. They are trying to describe space inside the ergosphere ... it's a frame-dragging feature.

All rotating masses drag space-time around like that though - just not to the extent described.
The forced co-rotation is a feature of Kerr/Kerr-Newman holes though - you do have to have one of those.

Rotating black holes have two singularity-ish surfaces - the top of the ergosphere and the event horizon inside it.

I'm not the first person to speculate that frame dragging may be expoited as a kind of gravity assist - borrowing some of the angular momentum from the massive object. But now I'm thinking... wouldn't the effect cancel out? https://www.physicsforums.com/showthread.php?t=677865

That's pretty much what I got too.

They ignored quite a bit ... which makes room for the speculation here.
The video makes it look like matter could escape the event horizon by gaining energy from the hole ... this is misleading: the whole process takes place outside the event horizon, inside the ergosphere.

It's called the "Penrose process"... the effect is that Kerr holes (would) radiate energetic particles.
However, I understand the Penrose process involves the infalling particle splitting in half?

Depends on the black hole - supermassive Schwarzschilde holes can have quite gentle gravitational gradients so a trip to the inside is feasible. Only - you won't be able to tell anyone back home about it.

References:
... I don't really like wikipedia refs but this has an OK synopsis.
http://www.jimhaldenwang.com/black_hole.htm
... the usual Jim Haldenwang: space-time geometry inside a black hole, beginners should skim past the math at the start.

Last edited: Jul 5, 2013
5. Jul 5, 2013

### pervect

Staff Emeritus
There is some basis in the literature for the notion of "space falling into a black hole". In particular, http://arxiv.org/abs/gr-qc/0411060

published in the AJP as http://dx.doi.org/10.1119/1.2830526

It's not really personally my favorite picture. In particular, there is no instrument that can measure the "velocity of space".

However, it's probably less misleading than the popular notion that "time stops at the event horizon", and when correctly interpreted can be valuable.

I haven't watched the website in question, but use of the river model isn't a definitive "red flag". That said, it's wise to be very cautious about things that haven't been peer-reviewed that one finds on the web with regards to relativity.

6. Jul 7, 2013

### Staff: Mentor

I'm pretty sure they were talking about the region inside the event horizon nearer to the singularity during the time frame of the video the OP was talking about. I remember they specifically talked about the "firestorm" or something, but that it was cut off from us by the event horizon.

7. Jul 7, 2013

### Simon Bridge

Well, there's a number of interpretations that could be made on what they showed and said: your guess is as good as mine - I just tried to fit what they showed to the physics of a Kerr hole.
They probably tried out a number of different visuals and narratives and picked the one that looked and sounded the most entertaining.
This is why my standard response is as per last line, post #2.

8. Jul 8, 2013

### bahamagreen

Technical and pedagogical aspects aside, isn't the important feature of the concept that a tremendous acceleration is provided over a short amount of time without any inertial stress to the craft, equipment, or passengers... because they are in free fall for the maneuver?

9. Jul 8, 2013

### Staff: Mentor

Why wouldn't there be any stress? Wouldn't the parts of the ship closer to the black hole or singularity or whatever be affected more than those further away, leading to stress?

10. Jul 8, 2013

### nitsuj

Nah it's just a fictitious force, through and through. just the spacetime is "stressed", mechanical physics stays the same. Bahamagreen mentions the ship is inertial, (freefall)

11. Jul 8, 2013

### Staff: Mentor

Drakkith is right - he's describing tidal forces which are experienced even in freefall. Whether the resulting stresses are significant depends not on the strength of the gravitational forces, but how much that strength varies from point to point.

12. Jul 8, 2013

### nitsuj

oh yea, that's one of the discerning differences for local weak equivalence, tidal force. I should stay away from GR, thanks for the correction so like a big black hole (weak tidal )to a small black hole (strong tidal)

13. Jul 8, 2013

### Staff: Mentor

By the way, what exactly is "inertial stress"?

14. Jul 8, 2013

### soothsayer

If the video is talking about a gravitational slingshot, it is hardly unique to black holes...I think maybe the "outer horizon" they are referring to is the ergosphere, and the "inner horizon" is the actual event horizon, which would make the idea of matter being "flung out" physically viable, since the matter has not yet crossed the event horizon, but I don't think it is exactly likely for matter to escape from the ergosphere. Maybe radiation that is scattering off of it could escape, but I think conventional, in-falling matter is realistically doomed, no matter how much energy it picks up, because it's total energy is still lower than the escape potential.

(There's also Superradiance, which is a really cool phenomenon, but is so minute, it wouldn't contribute to this discussion.)

The idea of space "falling into the black hole" is not really canonical, but if space were treated of as a fluid that flows toward massive objects, the mathematics would be essentially unchanged. That's basically the basis of Superfluid Vacuum Theory.

15. Jul 8, 2013

### Simon Bridge

It isn't.
That was my take on things too - others got something different ... the whole thing is too poetic to be sure what they mean or even if the writers know themselves.

16. Jul 9, 2013

### pervect

Staff Emeritus
It sounds like the ergosphere to me.

17. Jul 9, 2013

### Staff: Mentor

Go watch the video from 20:45 and on. They specifically talk about an "inner horizon" around the singularity, not the event horizon and not the ergosphere.

18. Jul 9, 2013

### pervect

Staff Emeritus
Ah - that makes sense. I'd watch the video if I could find the link to it.

Kerr black holes do have an inner horizon. But the Kerr solution isn't a stable one, so it's a mistake to take it too seriously. Current thinking about the interior structure of rotating black holes is more along the lines described in http://arxiv.org/abs/1010.1269.

19. Jul 9, 2013

### Staff: Mentor

Here you go.

Last edited by a moderator: Sep 25, 2014
20. Jul 11, 2013

### stevebd1

While it would be considered incorrect to refer to the ergosphere as the outer horizon and the event horizon as the inner horizon, wikipedia seem to make the same assumption-

http://en.wikipedia.org/wiki/Kerr_metric#Important_surfaces

It seems that some sources don't recognise the Cauchy horizon (or the 'inner' inner horizon) as some people think this is nothing other than a quirk of the metric.

The video shows two spheres which would also be incorrect as the ergopshere makes contact with the inner horizon at the poles but this wouldn't be the first time TV science graphics haven't been 100% correct.