Gravity around a super massive black hole

[Derek Smith]

The maths is beyond me, but:

We were discussing spaghettification.

Two questions came up.

1/ Why don't astronauts try going in lying down, and

2/ Would it also apply in the situation of a super massive blank hole. Whilst a small black hole would increase spaghettification, the bigger the SMBH, the lower the effect, or at least that's what was suggested.

It's beyond me. Anyone help?

[Derek Smith]

MarvGTI said:

Hi Derek,

Here's my 2 cents, but please be gentle as I am in no way qualified or trained to be answering these questions just have a healthy interest in all things spacey!

As per your first point, would an approaching astronaut's relative position to the black hole really make a difference ? I thought that once you go past the event horizon, your position, weight (mass), etc don't matter anymore as you can't escape the gravitational pull no matter how ? Also, if let's say you're 'coming in' in a horizontal position, wouldn't that make spaghettification last infinitely longer and thus make it infinitely more painful compared to entering the BH in a vertical position ?

Concerning the second point, with a SMBH the gravitational radius is going to be completely different from a small black hole's. The event horizon now being reached much further away from the black hole's centre, spaghettification would be less intense due to the lower gravitational pull (that will amplify in strength the closer you get to the centre). I took this to mean that while spaghettification will be less intense and abrupt, you get spaghettified over an exponentially longer distance and time, accelerating as you get closer to the centre.

Correct me if I'm wrong of course =)

Marvin

Thanks for that. With regards the second point, that's what I suggested but all the literature has no one getting past the event horizon. What started this was a How the Universe Works programme (repeat) early this week.

Further, something else that came up, was that as a 'person' gets closer to an even horizon it will speed up, approaching the speed of light, sow from my understanding, they would, from an outsider's point of view, slow. I remember a science fiction book, Stargate?, about just this feature. So would a body ever reach the speed of light?

Now according to some recentish stuff I read, black holes lose mass, slowly but then there's a long time to go. As the universe becomes less cluttered - one chap said that the universe is made up of black holes and some debris - then the rate of accretion would be slower than the loss of material. So could we travel to the end of the universe?

Edited to add: The book is Gateway, by F. Pohl. The premise of the book is that an observer will never see an object cross the even horizon.

Edited by Derek Smith on Friday 21st February 12:38

[Derek Smith]

MarvGTI said:

I read that once you cross the event horizon, people outside of it would perceive you as though you were infinitely slowing down until you'd appear to be standing still in space and time, even though you'd be near light speed.

Observers would see this stand-still image in a red&greenish colour, distorted residual light that can be picked up optically as far as I understand. The aftermath itself won't be visible, as the light emitted from you getting sucked in would not be able to escape the black hole's gravity.

The 'ghostly' image of the astronaut standing still in the void would be visible for millions of years, even though they'd be long gone.



The universe being ever-expanding, the distance between planetary objects and stars, and indeed the galaxies themselves, will increase as they are constantly being pushed away from each other (even our moon gets further and further away from Earth at a rate of a few cm per year).

In so far, and again this is based on what my limited capacity brain was able to pick up, there is no clearly defined end to the universe... if you want to travel to a point that's, let's say, a billion lightyears away, you'd have to bend space/time - i.e. literally bending the universe so that the starting point from which you depart and the arrival point at the destination would almost 'overlap', and you can go from one point in the universe to the other with merely a footstep.

So methinks that intergalactic travel, should we achieve it, will not be us travelling for prolonged periods of time over increasingly vast distances; we just have to drag the destination to our doorstep, but then we're already getting into transdimensional crap



I find this accretion rate / loss of material idea very interesting! As a black hole loses more and more mass, there will have to be a point where it can't sustain itself anymore, where it will 'die'. Do you think the black hole's 'contents', the stuff it swallowed over its lifetime, will be released or lost? Like, do you think they will behave similarly to a supernova and release elements (metals, gases, precious metals through fusion etc) into space once they explode ?

Perhaps I should read the Brief History again but surely the important thing is time dilation from the point of view of the person approaching, and reaching at the event horizon?, the speed of light. Therefore, if the ultimate speed is reached then that person will see the end of the universe, or at least far enough in the future for the black hole to disintegrate.

Tau Zero, Anderson's classic, suggests that this is possible. I'm not sure this has been verified by independent research.

[Derek Smith]

don4l said:

All this talk of accretion disks leaves me a bit cold.

If the sun were to collapse into a black hole, then everything would carry on orbiting around it as it does at the moment.

All that would change is that it would stop shining.

Stuff disappears into the sun now and again. If it became a black hole then less stuff would end up "accreting" than does at present. This is because the event horizon would be much smaller than the sun's current circumference. And stuff that currently gets between the two gets gobbled up.

Doesn't that presuppose a stable solar system? Which it ain't.

[Derek Smith]

Toltec said:

Besides, the radiation will get you first.

Would there be radiation around a black hole that had, say, been catapulted from its galaxy during a collision and is all on its own, away from matter so drawing nothing in? Would not the black hole's gravitation stop any escape of radiation?