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Minimum safe distance to black hole

  1. Dec 5, 2011 #1
    I wonder what the minimum safe distance would be for a black hole with mass of a couple of billion solar masses. With that I mean at what distance would the gravitational pull be negligible, I guess. I am sorry if this is the wrong kind of question to ask in this forum.
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  3. Dec 5, 2011 #2


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    If the black hole were the only thing in the universe and you were not moving with respect to it, there would be no safe distance. The reason why the black hole at the center of our galaxy doesn't pose a threat is because we are orbiting it.
  4. Dec 6, 2011 #3

    The most massive black hole yet found is eighteen billion times the mass of the sun. So how close could you get? In one sense the gravity doesn't matter: you are falling, so you don't feel anything as long as you don't bump into anything. The gravity could be a million times that of earth and you wouldn't even be able to tell. The limit comes when the difference in gravity between different parts of your body becomes large enough that it is like being stretched on a rack. If you hang from your arms with your feet off the floor, that is called 1G of stretch. I figure about 1G per meter would be the most that anyone would care to tolerate for very long. Special compression body suits would help. You would have to be careful to perpendicular to the black hole, so that the stretching is along the small distance across the body instead of the large distance from head to toe. So with that universal-record-holding black hole, that would occur at a distance of seven million kilometers. That would be about as close as you could go.
  5. Dec 6, 2011 #4


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    Note that that is way inside the horizon. The horizon of this black hole completely dwarfs the solar system.

    Or do you mean 7 million kilometers from the horizon? It is not clear from your description.
  6. Dec 6, 2011 #5

    OK, what's the radius of the horizon?
  7. Dec 6, 2011 #6


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  8. Dec 6, 2011 #7
  9. Dec 6, 2011 #8
    As you have probably figured out from the very nice replies already posted, these are two very different questions...."minimum safe distance" and "negligible"....and neither is well defined. For example, the gravitational strength is proportional to 1/r2 so it gets asymptotically small, but never reaches zero in our universe.
  10. Dec 6, 2011 #9


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    A better way to define "safe distance" would probably be, "how close can you get before tidal forces (different gravitational forces on different parts of your body) tear you apart?"

    See for example Larry Niven's short story "Neutron Star" which has a nice description of traveling close to a very small but massive object, not a black hole, but the principles are the same.

  11. Dec 6, 2011 #10


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    Right, but for a massive black hole, you get way inside the event horizon before you feel tidal forces. Are you safe before this? Nope - you are long past the point of no return; nothing you do will even significantly slow down your arrival at the singularity.

    Meanwhile, near the horizon, you feel nothing locally, but it would still take enormous fuel consumption to escape.

    Here's a thought: there are two aspects to safety:

    - ability to survive tidal forces
    - ability to survive escape

    The former has been discussed (e.g. force per meter of your body). The second might be based on the idea that you wouldn't want to experience more than e.g. 3 G acceleration for an extended period of time. So, it is not safe to get closer to the horizon than would require greater acceleration to escape.
  12. Dec 6, 2011 #11
    It is actually difficult to fall into a black hole.

    Even a little angular momentum could either swing you right past it, or have you enter into an orbit or even let you make a 360 turn around the black hole (even multiple times) and then travel away from it.

    If you are on a radial collision course all you need to do is to calculate how much you have to accelerate laterally to avoid crashing into the black hole.
  13. Dec 6, 2011 #12


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    Good points! So you could safely pass close to a horizon and escape; only hovering close to a horizon would require excessive g force.

    But you could never consider yourself safe once you've crossed.
  14. Dec 6, 2011 #13
    Thanks for all answers, I feel like I should explain my worries maybe hehe. The reason I asked was that I read a huge black hole is 300 million lightyears away. So lets say it takes 100 generations until its possible to evacuate earth, we wouldnt need to worry then until it is a couple of hundred "plutos" away ? :-)
    Edit: ignore the generations thing, obviously it depends on the trajectory.
    Last edited: Dec 6, 2011
  15. Dec 6, 2011 #14


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    A huge black hole 300 million light years away will take at least 300 million years (probably much longer) to get here (if it's even heading here at all). There's a large black hole much closer to us at the center of our galaxy (~50,000 Light years away), but we are orbiting that one and there's no danger posed by that black hole.
  16. Dec 9, 2011 #15
    Or less fuel than required to reach escape velocity.
  17. Dec 9, 2011 #16

    George Jones

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    How close can one get to a Schwarzschild black hole while staying freely falling? I think I know the answer.
  18. Dec 10, 2011 #17
    The escape velocity at a distance of "two hundred plutos" from an 18 billion solar mass black hole is close to a tenth of the speed of light. Any escape plans should have been exercised long before the black hole gets that close to the solar system. If you haven't taken a fast ship out of town by the time the black hole is "two hundred plutos" away, you will be subjected to an enormous and fatal amount of radiation from the acretion region activity long before you encounter the Event Horizon.
  19. Dec 10, 2011 #18
    Does not make any sense to me, the chances are already very slim you would ever get trapped and if you happen to approach the black hole radially (if we take the simple view of a non rotating black hole) then all you need is some very small and short acceleration in the theta and phi direction.
  20. Dec 10, 2011 #19
    I was responding to post #13 in which a black hole was approaching the Earth. The question in that post seemed to be how close can the black hole get to the Earth before
    its presence become a serious concern to the Earth. The escape velocity at a distance of 200 times the average radius of Pluto's orbit from the center (not the Event horizon) of an 18 billion solar mass black hole is more than 0.22c. There is no way the earth can be accelerated to miss the approaching black hole. A ship might still be able to accelerate radially from the black hole to reach escape velocity. Anything less than 0.22c would at best only put the ship in orbit around the black hole. Even if the ship could achieve an orbit around the black hole, the radiation from the acretion area of the black hole would be lethal.
  21. Dec 10, 2011 #20
    That is not correct, why do you think that is the case? It depends on the approach, some will 'smash' into the black hole, others might orbit, and yet others might be slung back after circumnavigating the lack hole one or more times. We can calculate this based on the so-called impact parameter.

    There seems to be a myth that a black hole is something like pulling the plug in a bathtub full of water sucking everything in, but that is not the case.
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