Viewing a single comment thread. View all comments

Muvlon t1_je7f7cf wrote

Reply to comment by amitym in Ultramassive black hole over 30 billion times mass of our sun has been spotted. The discovery had been made possible thanks to a phenomenon known as gravitational lensing — the first time a black hole has been spotted in such a way. by Wagamaga

> Anyone in orbit just above the event horizon would move at about 6km / s, roughly comparable to low Earth orbital velocity, and would be subject to only 50 gees

Wait, what? Wouldn't they be subject to 0 gees, seeing as they're in orbit, i.e. experiencing no acceleration?

5

amitym t1_je7mice wrote

Yes, practically speaking, but under a steep enough gravitational gradient you can no longer ignore the difference between, for example, the gravity acting on your head versus your feet. Or one end of a structure versus another. That's what causes "spaghettification" for example.

However in this case the gravitational gradient is still pretty shallow, as far as I can tell.

1

Plan-B-Rip-and-Tear t1_je7p075 wrote

Astronauts in low earth orbit still feel like they are falling the whole time. Orbit means your velocity perpendicular to the action of gravity matches the rate you would be falling otherwise, resulting in you following a circular path/orbit around the object.

Same principle the vomit comet plane uses, except parallel to earths gravity instead of perpendicular. The plane loses altitude at the same rate you would fall due to gravity, so inside the cabin it’s as if you are weightless. But you feel the acceleration the whole time.

1

amitym t1_je89s60 wrote

It doesn't have to do with inertia. Astronauts orbiting Earth feel like they're falling, instead of feeling like they're being extruded into a thin bloody dribble, because the pull of gravity is effectively the same at their feet as it is at their heads.

That's not the case when very close outside the Schwarzchild radius of smaller black holes. But at 30 billion solar masses, the Schwarzchild radius is so far out from the singularity that the gravitational gradient is, as around Earth, negligible.

1

Plan-B-Rip-and-Tear t1_je8cb19 wrote

I really should have responded to the poster you were responding to rather than you as that’s the question I intended to address about feeling 0 g’s, not gravitational gradients on intermediate and stellar sized black holes.

1

Muvlon t1_je90mb6 wrote

Right, but those are tidal forces, and how much of those you experience depends not only on what your orbit looks like but also on how big you are (and how you're oriented). So I'm not sure how you arrived at the 50G number, there must be some hidden assumptions.

1

amitym t1_jea7h3g wrote

It's the gravitational acceleration at that distance from a body of that mass, at least based on the random internet calculator I used. (~500 m/s^(2))

Under that kind of gravity, it doesn't really matter how big you are or what your orientation is. The gradient isn't going to be enough to spaghettify you. It might matter if you want to build a large structure in close orbit around the black hole, but even then, a reasonably sized, properly engineered steel-reinforced structure should be able to handle that level of stress.

1

Muvlon t1_jebecrf wrote

Ah, so it's the gravitation acceleration for a still standing obverser, not an infalling one or one that is in orbit.

1