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Thread: Would you see billions of years happening in seconds behind you going in a black hole

  1. #1
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    Question Would you see billions of years happening in seconds behind you going in a black hole

    Since the stronger the gravity gets the closer you get to a black hole as you get really close or start to pass the event horizon if you were looking behind you or going in backwards would you see stars going supernova and being born in seconds as you went in?

    How much time would you see passing by away from the black hole as you went through the event horizon? Thousands, millions, billions, trillions of years in seconds?

    My guess is that once you pass the event horizon since this is the region where space is falling faster than light you'll no longer be able to see anything behind you and time doesn't exist anyway as Michio Kaku explains in this video: Fast forward to 4:15 https://www.youtube.com/watch?v=t2mjoI7bgXM

  2. #2
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    I addressed pretty much all of those questions less than a
    month ago in this thread:

    http://cosmoquest.org/forum/showthre...-from-the-edge

    Although I didn't give any quantitative answers.

    An observer free-falling into a black hole would see everything
    happening outside the black hole apparently slowed down and
    squished into a small circle directly overhead. Light from
    outside would be redshifted. The observer would reach the
    center of a supermassive black hole in a few minutes or hours
    of his own proper time, depending on the size of the black hole.
    The observer would be spaghettified before reaching the center.

    An observer hovering on a fantastically powerful rocket just
    above the event horizon would also see everything outside
    squished into a circle overhead, but light from outside would
    be blueshifted, and everything would appear speeded up.
    How much time the observer would see pass in the outside
    Universe depends on how long the rocket can support him.
    If the rocket is close enough to the event horizon to speed
    up the apparent passage of time in the outside Universe
    significantly, then the rate of fuel useage would be vastly
    greater than it would be for the same rocket supporting the
    same mass near Earth's surface, for example.

    Instead of hovering on a rocket, the observer could sit on a
    platform built around the black hole. The time dilation would
    be exactly the same as it would be for the hovering rocket at
    the same distance from the black hole. If the platform were
    built so that the acceleration of gravity on the platform is 1 g,
    the same as it is at Earth's surface, then the observer would
    see time in the outside Universe speeded up by exactly the
    same amount we see it speeded up because we are sitting on
    Earth's surface. A very, very tiny amount. But measureable
    and important in a few things such as GPS. If the platform
    were built close enough to the black hole to give an obvious
    time dilation, the material it was built from would have to be
    magic. No real material could support the weight.

    -- Jeff, in Minneapolis

  3. #3
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    Quote Originally Posted by Jeff Root View Post
    If the platform were
    built so that the acceleration of gravity on the platform is 1 g,
    the same as it is at Earth's surface, then the observer would
    see time in the outside Universe speeded up by exactly the
    same amount we see it speeded up because we are sitting on
    Earth's surface.
    Actually, the time dilation depends on the gravitational potential, not the gravitational acceleration. To go from the first to the second, multiply by a characteristic length, which is much smaller for a stellar-mass black hole, and much larger for a supermassive black hole. This means the effect (hypothetically) would be much more apparent when the acceleration is 1 g around a supermassive black hole.

  4. #4
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    Yes. That was careless of me. If the platform were built so
    that the gravitational potential on the platform is the same as
    it is at Earth's surface, then the observer would see time in the
    outside Universe speeded up by exactly the same amount we
    see it speeded up because we are sitting on Earth's surface.

    -- Jeff, in Minneapolis

  5. #5
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    If Earth were compressed to a black hole, the event horizon
    would be about a centimeter in diameter. (Between the size
    of a large pea and a small marble.) If a platform were then
    built around this mini black hole with the same diameter as
    the uncompressed Earth, both the gravitational acceleration
    and the gravitational potential on the platform would be the
    same as on the surface of the uncompressed Earth.

    I think that such a platform could theoretically hold up under
    its own weight if built of real materials. Theoretically...

    -- Jeff, in Minneapolis

  6. #6
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    It might be tough to make an arch that could support against 1 g with no support over such huge distances. Judging from things like bridges, we seem to have trouble supporting large structures against 1 g unless we can support them every few hundred meters. A single arch with a huge radius of curvature might be a considerable challenge.

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