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B Can a "thing" go through a black hole?

  1. Jul 13, 2018 #1
    Can a "thing" go through the event horizon of a black hole? I mean, everything inside a black hole can travel only in one direction: inward. That destroys the "thing" itself, since independent parts of the "thing" are no longer connected to one each others. Perhaps it's impossible for a "thing" to trespass the event horizon of a black hole. In that scenario the light could trespass the event horizon because the light it's not a thing, but a phenomenon. Or maybe I misunderstood some concepts and a "thing" could remain like a "thing" locally, even though it's impossible for it to escape from the black hole.

    Thanks and sorry for my weird English.
  2. jcsd
  3. Jul 13, 2018 #2


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    Composite objects can pass through the event horizon of a black hole without any problem. Locally, it is as if the horizon is an imaginary plane moving at the speed of light across objects in the neighborhood.

    Locally, the "one way" nature of the horizon is enforced by its speed. You cannot send anything across the horizon because you cannot send anything faster than light. Right this minute, an imaginary surface is passing through your body at the speed of light (just imagine it) and you are not falling into pieces. It's the same in a black hole. No local effect.
  4. Jul 13, 2018 #3


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    Why not? As long as the black hole is super massive so that the tidal forces are small, then there is nothing to pull the pieces apart at the horizon
  5. Jul 13, 2018 #4
    I think what you want to say is, if have two parts of the object inside the event horizon, then one part (A-part) can send a signal to the other (B-part, that is situated closer to the singularity, a.k.a. more "inward"), because a signal can go inwards, but it can't receive the signal from it, because the signal can't go back (it can only go in one direction)? This is incorrect, of course, because the A-part doesn't just stand there and wait for a signal from B, it flies inward itself and meets the signal sent from B (by catching up to it). Locally it looks like nothing out of the ordinary is happening and two parts are exchanging signals in their usual manner (that is, until the tidal forces, caused by the non-uniformity of the gravitational field, become too noticeable).
  6. Jul 13, 2018 #5


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    A "thing" can fall through the event horizon of a large black hole without a problem. As others have mentioned, it's important for the hole to be large or the "thing" will expereince large tidal forces that will likely tear it apart. From the "things" viewpoint, the "thing" is stationary, and the event horizon passes through it at the speed of light.

    What's impossible is for a "thing" to have a velocity other than "c" relative to the event horizon when the "thing" is at the horizon. The event horizon is a lightlike (or null) surface, so thinking of it as an ordinary place is not quite right and leads to confusion.
  7. Jul 13, 2018 #6


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    If a large free-falling physics lab is crossing an event horizon, will the occupants notice any discontinuities between the portion of the lab that is across the horizon and the portion that is still free falling through?

    Large being large enough that it takes an hour (or some human scale noticable time) for the entire lab to free fall through, from leading edge to trailing edge. That's pretty large - maybe its an enormous scale particle accelerator.
  8. Jul 13, 2018 #7
    Tidal force makes everything like spageti, I think.
  9. Jul 13, 2018 #8


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    That would be about a billion kilometers, or about 7 times the radius of earth’s orbit. I’m not sure there is a supermassive BH large enough for that.
  10. Jul 13, 2018 #9


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    As long as the lab is small compared to the size of the hole (so that tidal gravity is negligible, or at least manageable, within the lab), then no.

    An hour according to a clock at the center of the lab, I assume?

    Roughly speaking, that means the lab has to be about 1 light-hour in size (since, from the lab's point of view, the horizon is moving outward across it at the speed of light), which is, as @PAllen said, about a billion kilometers.

    If we want the size of the lab to be small compared to the size of the hole, then the hole's mass would have to be, say, a trillion kilometers in geometric units; that works out to about a trillion solar masses, since a solar mass is about 1 kilometer in geometric units. According to Wikipedia[1], the most massive black hole currently known seems to be about 200 billion solar masses, which is still large enough for a light-hour sized lab to be reasonably intact falling through the horizon.

  11. Jul 13, 2018 #10


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    Of course - hence its not possible to observe any discontinuity - I might have thought that through for myself. Thanks, Peter.

    Edit - and post #2 had the answer, I just didn't get it.
    Last edited: Jul 13, 2018
  12. Jul 13, 2018 #11


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    As Peter has pointed out, that is not true for supermassive black holes. You don't notice anything as you cross the EH.
  13. Jul 21, 2018 #12
    Thanks to everyone for replying my question. I think that my error was thinking in the "space" in the traditional way, instead of something that can be distorted. The black hole is, indeed, swallowing the space itself. In that case, a black hole is quite similar to a waterfall.

    Again, sorry for my English :)
  14. Jul 21, 2018 #13


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    None of that is correct. The "distortion" is not a distortion of something material like water and black holes don't "swallow" space, they just swallow material around them that is IN space.

    "Distortion of space" means that objects IN space travel on paths that are very curved when looked at from the point of view of Euclidean Geometry.
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