Physics Question



  • Thank you.

    You are right that the part of the chain that enters the event horizon would then require infinite energy to get out.  So you would have to let go of the chain (otherwise you would get pulled in also 😮  ).


  • Moderator

    @Mary:

    Nicely put. I had this weird idea of dropping a chain into a black hole and slowly pulling it out 🙂 I guess the part that enters the hole would require infinite energy to get back out 😞

    Looks like you’re thinking along these lines:

    At the centre of the black hole, well inside the event horizon, general relativity predicts a singularity, a place where the curvature of spacetime becomes infinite and gravitational forces become infinitely strong. Spacetime inside the event horizon is peculiar in that the singularity is in every observer’s future, so all particles within the event horizon move inexorably towards it (Penrose and Hawking). This means that there is a conceptual inaccuracy in the nonrelativistic concept of a black hole as originally proposed by John Michell in 1783. In Michell’s theory, the escape velocity equals the speed of light, but it would still, for example, be theoretically possible to hoist an object out of a black hole using a rope. General relativity eliminates such loopholes, because once an object is inside the event horizon, its time-line contains an end-point to time itself, and no possible world-lines come back out through the event horizon.



  • no possible world-lines come back out through the event horizon.

    With the exceptions being the strange, unproven Hawking radiation, and whatever happens once this radiation causes the black hole to evaporate.  I wonder if it wouldn’t someday be possible to use this radiation to “see” beyond the event horizon.


  • Moderator

    Yeah, that was just an exerpt from Wiki on Black Holes.

    Thought Mary might be interested in John Michell, since I guess his theory had to do with the rope-chain thing.

    I’m not familiar with it but still thought it was cool.

    You should submit your own Wiki def, but instead call it “Baker Radiation”  😄



  • OK, just correcting what I think is an initial misunderstanding…

    Escape Velocity means that, with a single initial burst of energy, that energy must be sufficient to accelerate the object to 17,000 mph to clear Earth’s gravity well.  Anything less, and the object falls back to Earth at some point.

    Thus in a way, the initial concept is valid.  The Saturn V’s did not need to hit 17,000 mph to clear the Earth’s gravity well.  They just needed an amount of thrust (force) spread over time that, had it been released immediately, would have been able to accelerate the mass to 17,000 mph immediately.

    Now, just outside the Event Horizon of a Black Hole, the escape velocity is a fraction under 186,000 mps (per SECOND).

    Thus, to escape from JUST OUTSIDE the Event Horizon, you would need sufficent force applied over time to be equivalent to the force necessary that, if released immediately, would accelerate the object to a fraction under 186,000 mps.

    Once you cross the Event Horizon, you need to be able to have enough thrust force to be equivalent to a one-time burst of the energy required to accelerat to greater than 186,000 mps.

    The calculation for that force is infinite, since the amount of energy required to instantly translate an object to greater than 186,000 mps is infinite, based on Einstein.

    An infinite amount of energy realease over time is an infinite amount of energy at ALL times.

    Did that work for ya?



  • DM, i doubt that a non-relativistic “concept” of a black hole makes a lot of sense. Using this 1783 thing as an early reference is not very useful. Even though Newtonian mechanics and the speed of light were known at that time, the problem would have been the “mass” of the light-particle, and i wonder why the idea was remembered after Maxwells equations postulated that light is a wave and not a particle.

    Baker Street, there was a pretty famous bet, about whether information can escape the black hole / whether the Hawking Radiation carries information about what “fell into”  the hole when it evaporates again.
    http://en.wikipedia.org/wiki/Black_hole_information_paradox 
    My own -personal- gut feeling is that information is physical and thus can be destroyed by decoherence. As some states are “immune” to several decoherence processes, information expressed in these states is more stable. Yet, i doubt that passing something through a singularity is a unitary process: We have no idea how the physics looks inside a black hole, so i wouldn’t have given in in the bet.



  • @DarthMaximus:

    From Wiki:

    Not sure what is says, I didn’t have time to read the whole thing but it does have the equations and mentions energy.

    http://en.wikipedia.org/wiki/Escape_velocity

    I am not sure where Mary got the 17,000 mph.

    Wiki gave a range of 7.1 to 11.2 km/s depending on your altitude.

    7.1 km/s * 60 s/min * 60 min/hr * 3280 ft/km * mi/5280 ft = 15878.2 mph
    11.2 km/s * 60 s/min * 60 min/hr * 3280 ft/km * mi/5280 ft = 25047.3 mph

    The whole idea is at every speed, you will achieve a certain orbital distance away from the center of the earth.  Lose some of that orbital speed, and your orbit drops you closer.  Gain some speed and you can achieve a higher orbit.  Gain enough speed and you will break orbit altogether.

    You can run some calcs based on the formulas on that site and then punch in an orbit distance = a few miles above the atmosphere to get the speed you need.



  • @F_alk:

    DM, i doubt that a non-relativistic “concept” of a black hole makes a lot of sense. Using this 1783 thing as an early reference is not very useful. Even though Newtonian mechanics and the speed of light were known at that time, the problem would have been the “mass” of the light-particle, and i wonder why the idea was remembered after Maxwells equations postulated that light is a wave and not a particle.

    Baker Street, there was a pretty famous bet, about whether information can escape the black hole / whether the Hawking Radiation carries information about what "fell into"  the hole when it evaporates again.
    http://en.wikipedia.org/wiki/Black_hole_information_paradox 
    My own -personal- gut feeling is that information is physical and thus can be destroyed by decoherence. As some states are “immune” to several decoherence processes, information expressed in these states is more stable. Yet, i doubt that passing something through a singularity is a unitary process: We have no idea how the physics looks inside a black hole, so i wouldn’t have given in in the bet.

    Interesting stuff.  As the article says, Hawkings paper isn’t yet published (I don’t think it has been published yet) … but since he paid the debt, he believe the information isn’t lost.  This is stuff that is way over my head, but my gut feeling on the matter is that the information isn’t lost.  It will be interesting to see Hawkings paper once it is completed.



  • The link at the bottom with the “hep” in there could be to a pre-print version of the paper (the address is the common pre-pront address). It is freely available if you are interested. I haven’t had a look into it though, so i can’t tell wether it is the preprint of the mentioned publication.


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    My post also points out the following:

    The Earth’s escape velocity is 11.2 kilometers per second (about 25,000 m.p.h.), while the Moon’s is only 2.4 kilometers per second (about 5300 m.p.h.).I dont think the other figure is correct ( 17,000)



  • Thanks for noticing the pre-print version of the paper.  Its only 5 pages and surprisingly not too terribly difficult to follow.  From the article:

    How does information get out of a black hole? My work with Hartle[8] showed the radiation could be thought of as
    tunnelling out from inside the black hole. It was therefore not unreasonable to suppose that it could carry information
    out of the black hole. This explains how a black hole can form and then give out the information about what is inside
    it while remaining topologically trivial. There is no baby universe branching off, as I once thought. The information
    remains firmly in our universe. I’m sorry to disappoint science fiction fans, but if information is preserved, there is
    no possibility of using black holes to travel to other universes. If you jump into a black hole, your mass energy will
    be returned to our universe but in a mangled form which contains the information about what you were like but in a
    state where it can not be easily recognized. It is like burning an encyclopedia. Information is not lost, if one keeps the
    smoke and the ashes. But it is difficult to read. In practice, it would be too difficult to re-build a macroscopic object
    like an encyclopedia that fell inside a black hole from information in the radiation, but the information preserving
    result is important for microscopic processes involving virtual black holes. If these had not been unitary, there would
    have been observable effects, like the decay of baryons.

    I gave John an encyclopedia of baseball, but maybe I should just have given him the ashes.

    Falk wrote:

    i doubt that passing something through a singularity is a unitary process

    Hawking basically states this is a unitary process by what isn’t observed in other quantum effects…i.e. the decay of baryons.  I’m not sure if that is really proof or just a strong indication however.


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