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Thread: How does spaghettification work?

  1. #1
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    How does spaghettification work?

    Hi, I'm trying to understand the spaghettification effect as something or someone approaches a black hole event horizon. I guess you could say there's an acceleratory force that acts stronger on your feet than on your head, hence you get stretched? However I'd rather understand it in terms of spacetime geometry. However, here I'm thinking there'd be more length contraction (and also time dilation) at your feet than your head? Hence I'm struggling to visualise why you get spaghettified rather than squashed. Thanks, Steve

  2. #2
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    Length contraction does not actually contract your length, just your observed length from an outside reference frame.

    Otherwise, you're correct, tidal forces are differences in acceleration; your head and feet are trying to orbit separately.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  3. #3
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    I think you are right the first time. The gravity gradient is huge so going in feet first, your feet see a much greater pull than your head. There are also near speed of light particles in orbit not quite ready to go through the horizon, so approaching radially encounters another hazard. If you are approaching with angular momentum, that will accelerate your rotation too, with more on your near side (feet).
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

  4. #4
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    Hmm OK, but I'm trying to get the John A Wheeler perspective on this, where gravity isn't a force it's just geometry. You say length contraction is just an issue for the observer, but if I'm observing my feet from my head?

  5. #5
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    Geometrically, your feet and your head are in non-inertial frames, because neither is following a spacetime geodesic. If they were not subject to some sort of restraining force, they'd quickly move apart. So in order to keep your feet and head the same distance apart, your body must generate a tension force, continuously pulling your head away from one geodesic trajectory and your feet away from another. As you get closer to the black hole, the geodesic trajectories for your head and feet steadily become more divergent, and the force necessary to prevent them flying off divergently steadily increases, until it overwhelms your body's ability to restrain them.
    Special relativity, and its associated time dilation and length contraction are not really relevant here, firstly because the forces generated are a consequence of general relativity, and secondly because the relative velocity of your head and feet is very small.

    Grant Hutchison

  6. #6
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    That's helpful, thanks Grant.

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