nuances in the imagery of Nolan's 'intestellar'

[bwanaaa]

Much has been said about the physics used to model the images produced for 'interstellar'. In this forum, this thread, http://forums.anandtech.com/showthread.php?t=2383278&highlight=interstellar&page=12
discusses the movie but I know there are some REAL astrophysicists here, who perhaps out of politeness, have not commented.

An interesting blog here,
http://ikjyotsinghkohli24.wordpress.com/2014/11/07/on-the-science-of-interstellar/
discusses the significance of a rotating black hole having significantly different physics than a non rotating one. The point being made is that it is possible to stably orbit a rotating black hole and experience time dilation without crashing into it. In the movie, I believe the actors were depicted as almost matching the orbit of the water planet (taking a slightly larger orbit around gargantua- the black hole). By simply descending to the planet surface, the gravitational field slowed time sufficiently to allow 1 hour on the planet surface to account for 7 years in the spacecraft. I do not understand though how the gravitational field of gargantua did not crush them into a molecular monolayer of proteins, lipids and minerals on the planet surface. and what role the angular velocity of gargantua plays in protecting them from this fate.
Perhap's one of the astrophysics gurus here could clarify Prof. Kohli's blog on the point.

I can imagine a way possible for humans to safely experience high gravitational fields - imagine chasing a black hole that is accelerating through space (perhaps by the attraction of a super black hole). As your spaceship follows the black hole towards the superb black hole, the occupants will be in its intense gravity. However, since both the black hole and the space ship are accelerating through space, their distance will remain relatively constant. And since they are both 'falling' towards the super black hole, the sensation of weightlessness is preserved.
No crushing of bones or popping of eyeballs occurs. Just progressive time dilation.

Also Kohli's blog mentions that ' the astro community are largely mistaken on this whole tidal force ripping up the planet.' Putting aside the hubris of such a comment, the ensuing lines in the blogger's argument tend to sound 'hand-wavy'. The distinction of the Kerr metric is lost on me. I reach out again to the astros here to if its possible to explain the lack of tidal destruction. Please note, I am not questioning the cause of the large waves on the planet, I would expect them to occur. I am just asking why this water planet was not spread out into water crescents.

Another effect not depicted is the presence of gravitational drag. Does this concept have any validity? In other words, if you approach a spinning black hole directly, will its spinning gravitational field impart a perpendicular velocity component to your trajectory? As an analogy consider a giant magnet (like the earth but much stronger), and i were in an iron spaceship (like a hollow asteroid for example), then the rotational velocity of that planet should have no effect as a accelerate towards the planet. Spinning a magnet on its polar axis does not change its attraction to a ferrous material, nor does it induce an electric current in a nearby wire. I just did that experiment with my dremel to which I had glued a super magnet. It didn't attract nearby paper clips any more or less easily. It did not budge my voltmeter needle. However, I suspect paramagnetic aluminum motion does depend on the angular velocity of the drill. I just didn't have any aluminum massive enough to detect that I suppose. Still, is there a mechanism ( 'eddy currents' or 'gravity waves') being the source of the energy that would alter the trajectory of an approaching body?

Finally, I enjoyed the fact that the light from the accretion disk of the 'far side' of the black hole was bent around, effectively creating a halo as seen from any vantage point. However, I do not see why there were no polar jets. Don't quasars have polar jets of energy? I am not talking about Hawking radiation that derives from the event horizon. I am talking about the massive amount of energy released by the plasma fusing outside the event horizon. I guess that's another benefit of spinning a black hole very fast that I do not understand.

In the interpretative drawings where a black hole is depicted consuming matter, the accretion disk is usually shown as asymmetric as well, where the matter streaming towards the black hole goes down a tornado like vortex. Perhaps the source of matter in the movie was on the far side of gargantua. Oh well, I dislike knowing I will die someday without knowing these answers.

 

[Biftheunderstudy]

First, though I am an astrophysicist, I don't profess to be an expert on GR, especially the Kerr metric.

That said, as to the reason why the high gravitational force doesn't crush you, I think you are on the right track with your guess.

As for his hand waving about the tidal forces, I can't say for sure, but I think he is slightly off there. There was no need to invoke any strangeness to explain why the tidal force doesn't rip and spaghettify everything. The tidal forces go like M/r^3 while the schwartzchild radius scales like M. This means that if the black hole were "small" (on the order solar masses), the tidal forces would in fact be dangerous close to the event horizon. But the black hole in question is a supermassive one, on the order 100 million solar masses and so the above scaling means that the tidal forces are very weak. They are actually weak enough that a human could easily survive crossing the event horizon of a non rotating SMBH without dying or perhaps even noticing it.

For the "gravitational drag", I'm not sure what you mean. Do you mean the frame-dragging effect of GR? This will definitely be important, but I can't wrap my head around how it would manifest.

For the jets, yes, in principle there probably could/should be jets. It's just that we really don't understand them and the environment in which they form. Empirically, we see jets where ever there is a lot of matter accreting onto something, it doesn't even have to be a black hole. My suspicion is then that this black hole is "quiescent", meaning there isn't much stuff falling on it, much like our own galactic supermassive black hole. The basic theory is a very tightly wrapped up magnetic field from the accretion disk funnels relativistic particles into a beam. The stuff in the accretion disk heats up to VERY high temperatures due to friction or viscous forces and a dynamo mechanism can amplify a very high strength magnetic field. Observed jets have velocities which are comparable to the speed of the innermost orbit of the disk, in a black hole this is very close to the speed of light.

How this all works in the Kerr metric? I have no idea.

 

[bwanaaa]

Thank you bitofunderstudy for your clear statement about tidal forces. I understood that and it did not require pages of integrals. Reminds me of when I derived Einstein's SR theorem for my kids on a napkin while we were waiting for dinner - just a creative application of the Pythagorean theorem.

And yes, I was thinking of frame dragging.

There is so much chaff on the net these days; grains of truth are lost in the maelstrom. I applaud this film for trying to bring the concept of 'time dilation' and the vastness of the universe to human comprehension. It emphasizes how fortunate we are to be on this magical planet Earth.