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Science

Have Scientists Finally Made Sense of Stephen Hawking's Famous Black Hole Formula? (science.org) 26

Slashdot reader sciencehabit shares this report from Science magazine: Fifty years ago, famed physicist Stephen Hawking wrote down an equation that predicts that a black hole has entropy, an attribute typically associated with the disordered jumbling of atoms and molecules in materials.

The arguments for black hole entropy were indirect, however, and no one had derived the famous equation from the fundamental definition of entropy — at least not for realistic black holes. Now, one team of theorists claims to have done so, although some experts are skeptical.

Reported in a paper in press at Physical Review Letters, the work would solve a homework problem that some theorists have labored over for decades. "It's good to have it done," says Don Marolf, a gravitational theorist at the University of California, Santa Barbara who was not involved in the research. It "shows us how to move forward, that's great."

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Have Scientists Finally Made Sense of Stephen Hawking's Famous Black Hole Formula?

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  • It won't know the current result, because that came after it's training cut-off, but that's all the better: It can't copy the result, so it has to derive it from the fundamental equations. Gotta be sure.

  • by Press2ToContinue ( 2424598 ) on Sunday April 07, 2024 @10:16AM (#64376498)
    this whole idea of "quantum hair" that's pretty key in figuring out how black holes might actually hold onto information about their past, despite gobbling up everything in sight. It's a big deal because it challenges the notion that all that data just gets erased from the universe, tying back into the whole entropy and black holes debate. This insight could be a game-changer in meshing together the worlds of quantum mechanics and relativity.
    • by Dorianny ( 1847922 ) on Sunday April 07, 2024 @01:43PM (#64376726) Journal
      There is no issue with stuff (quantum information) falling in a black hole and forever being inaccessible to the rest of the Universe. The paradox comes about as a direct result of an analyses of Hawking's original paper predicting that Hawking's radiation is purely thermal and has no knowledge of the black-hole's quantum state (Often referred to as the no hair theorem.) Recent work on correlation's between Hawking's radiation and black-hole entanglement entropy pokes holes in the validity of the theorem but more work remains before it can conclusively be overturned.
    • by Tablizer ( 95088 )

      "The quantum hair theory is a bald lie!"

  • I call it a Hawking Hole.

  • by Okian Warrior ( 537106 ) on Sunday April 07, 2024 @12:30PM (#64376652) Homepage Journal

    Thought problem for the physics mavens here.

    The event horizon is usually described as requiring an escape velocity faster than the speed of light, and anything that falls in can't get out.

    Suppose an object came in on a parabolic or hyperbolic course, in the manner of a meteor or comet going around the sun. Ignore tidal and time dilation effects for the moment because that's something the object will experience and I want to view this from a reference frame outside the black hole.

    Suppose the orbit of the object goes inside the event horizon at an angle, so that the object wouldn't intersect the singularity at the middle.

    Would it come out again?

    In Newtonian terms the object would speed up as it approached the black hole and crossed the horizon, and it could never exceed or attain the speed of light, but would get kinetic energy in excess of it's actual speed. Things appear heavier as they are accelerated, and more and more of the energy is put into mass while the velocity only approaches the speed of light.

    Coming around the object the same process happens in reverse, so the object isn't travelling at escape velocity but the pull from the singularity takes mass energy instead of slowing the object down. Without slowing down appreciably, the object should pop back out of the black hole and continue on it's original course.

    Is there a good reference that points out the fallacy in this argument? I'm just a little surprised that there's this area in space that will grab anything that flies by and suck it in permanently. Especially since the black hole has roughly the same mass as a regular star, so flying around in the vicinity should be no more difficult than flying around in the vicinity of a typical star.

    (I've been looking into whether the universe is computable, and the existence of boundary discontinuities 'kinda throws a wrench into those theories.)

    Is there a good reference online that explains this?

    • Due to that Einstein dude, it doesn't have enough time to get out again.

    • by PLAST ( 416196 )
      I'm not a physicist, but I do know that the event horizon is the surface below which even light cannot escape the gravitational pull.
      So you just need to substitute "photon" for "object" in your question and take into account that it moves at light speed, and then you have the answer.
    • My thought experiment is, what if two black holes were approaching each other very rapidly on a not-quite-collision course, so that the sides of their event horizons briefly overlapped as they passed. Would they stick together?

      ISTM that if anything was inside the overlapping area they'd have to stick, since otherwise that thing would be escaping from one of them. But is there anything there? Maybe something that just now fell in and hasn't had time to fall to the center? Or, is there quantum foam inside

      • These two black holes wouldn't stick to each other, but start swirling around each other and eventually merge together. Further outside of the event horizon particles that move slower than the speed of light will also not be able to escape. Their trajectories will be altered by the mass in the center and only when the speed is high enough they will (partially) go around and fly off. Anything that goes slow enough to be captured into an orbit will eventually spiral inwards.
        • These two black holes wouldn't stick to each other, but start swirling around each other and eventually merge together.

          This is partly because of friction with and among other stuff in orbit around the black holes in their "accretion disks". (Black holes experience friction by eating the stuff in the other hole's disk of debris, with the momentum of the black-hole-plus-dinner thus being different from the black-hole-before-dinner.)

          It's also partly because the rapid acceleration of things passing near a blac

        • Anything that goes slow enough to be captured into an orbit will eventually spiral inwards.

          Well, most of it (when we're talking matter not already in another black hole). Ordinary stuff orbiting near a black hole gets torn apart by the enormous tides and forms a disk-like structure similar to a gas giant's rings. Interactions among it and with the black hole's magnetic and gravitic fields can eject a bit of it in a pair of jets out along the axis of the disk, powered apparently by the rest of the stuff fa

      • Oh fuck. I just spent an hour flogging through this, trying to get your thought experiment to work, and not really getting there. Then Slashdot's Cloudflare comment-destruction mechanism ate my comment, and I hadn't saved a copy.

        Short version : I don't think your idea works, but it was worth an hour of thinking and typing. The details have vanished into Cloudflare's bit-bucket. Sorry. Blame Slashdot.

        I should remember to copy-before-submitting. But the blame is still on Slashdot.

    • by Sique ( 173459 ) on Sunday April 07, 2024 @02:31PM (#64376798) Homepage
      As the time slows down when approaching the Black Hole, you have to wait for infinity until the object comes out at the other side. Hence for all you know, the object disappeared into the Back Hole forever.
      • by Sique ( 173459 ) on Sunday April 07, 2024 @02:39PM (#64376800) Homepage
        PS: Alternatively, you can imagine to unroll the time-space-curve around the Black Hole into a flat surface, and if you then plot the hyperbolic curve of your object onto that flat surface, you will notice that it winds infinitely often around the singularity before leaving the Event Horizon.
        • PS: Alternatively, you can imagine to unroll the time-space-curve around the Black Hole into a flat surface, and if you then plot the hyperbolic curve of your object onto that flat surface, you will notice that it winds infinitely often around the singularity before leaving the Event Horizon.

          Ah, a -funroll-loops solution to a Slashdot problem.

      • In your external frame of reference. In the impinging particle's FoR ... less so. After a few microseconds - including time dilation - it reaches whatever is happening at the centre. How much it interacts with whatever is there ... we don't know.
    • I

      n Newtonian terms the object would speed up as it approached the black hole and crossed the horizon, and it could never exceed or attain the speed of light, but would get kinetic energy in excess of it's actual speed. Things appear heavier as they are accelerated, and more and more of the energy is put into mass while the velocity only approaches the speed of light.

      Coming around the object the same process happens in reverse, so the object isn't travelling at escape velocity but the pull from the singular

      • by HiThere ( 15173 )

        No, the part he's missing is the idea that one can't model this while ignoring time dilation. Not even approximately accurately.

        • What do you think is the velocity of a particle (say, falling from Sedna's aphelion to the surface of a Sun-size BH) when it crosses the event horizon. And consequently, what is it's time dilation factor?

          It might break the speed limit. For tarmac roads. Small-integer fractions of c ? I don't think so.

    • Beyond the event horizon of an axisymmetric black hole the time and radial coordinates exchange roles - which implies that in order to exit the event horizon one would have to travel back to the past. Thus, any object crossing the event horizon, at whatever angle, will necessarily end up trapped there. This is described in any decent textbook on general relativity.
    • by ceoyoyo ( 59147 )

      Your scenario is fallacious. You can’t ignore things like time dilation. The situation is not Newtonian. Time dilation is not something the object experiences, it’s something you would see as a faraway observer.

      As a viewer far away you would see the object slow down as it approaches the event horizon, and freeze there forever, never crossing it.

    • "but would get kinetic energy in excess of it's actual speed" as I recall from my physics days, at the speed of light, the mass of the particle would become infinite, hence no real particle can be accelerated to the speed of light. The particle will never 'pop back out' because it could never get faster than lightspeed,

      If light, the fastest moving thing in the Universe, can't get out, then neither can a slower moving thing.

      The best reference for this is to start with a good modern physics class and the

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