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Speed of light enough to escape black holes ?

  1. Oct 20, 2012 #1
    I wad thinking since black holes are so dense ... lights speed would get slow significantly , so isn't it that if you are at light speed ( 3 x 10^8 m/s ) then you might come out of a black hole ?
    Its the same concept we learn in 10 grade !!
    Am i right ?
     
  2. jcsd
  3. Oct 20, 2012 #2

    Chronos

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    No, the escape velocity of a black hole is c - by definition.
     
  4. Oct 20, 2012 #3
    But light doesnt escape because it slows down ? :p
     
  5. Oct 20, 2012 #4

    NWH

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    I believe this is more of a General Relativity question?
     
  6. Oct 20, 2012 #5

    Chronos

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    Yes, it cannot escape the event horizon.
     
  7. Oct 20, 2012 #6

    Drakkith

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    That is an easy way to remember it, though technically it's that there are simply no paths through spacetime that lead out from beyond the event horizon. However if you don't have a basic understanding of General Relativity just remember that light is "pulled" back in.
     
  8. Oct 20, 2012 #7

    mfb

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    In other words, once you are inside, all directions are "towards" the center. There is no "outwards" direction for your flashlight.
     
  9. Oct 20, 2012 #8

    RUTA

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    The "escape velocity" analogy isn't a good one. When an object is ejected/launched radially at escape velocity its speed is reduced as measured by local observers along its path until at infinity it has slowed to zero speed. However, light always moves a c as measured by observers locally. And, light does not leave the Schwarzschild radius even though according to the escape velocity analogy it should get to infinity.
     
  10. Oct 22, 2012 #9
    Light, climbing out of a gravitational well, doesn't slow down, it loses energy. This increases the wavelength (reduces the frequency). At the horizon it's frequency is zero.
     
  11. Oct 22, 2012 #10

    pervect

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    Not really. Light emitted outward exactly at the time someone crosses the event horizon will simply "hang" at the event horizon, not loosing or gaining energy.

    Someone else falling into the black hole on the same trajectory can see the light left there, an image of the previous traveller.

    No physical observer can hover at the event horizon. Any physical observer passing through the event horizon , using their own local clocks and rulers ,will measure the speed of any trapped light there to be equal to "c", just as they would measure the speed of any other light to be "c" (with the same conditions, the measurement must be a local one).

    The above requires exact timing. If you consider a bunch of photons emitted over a period of time from an infalling object, (more realistic), as time advances a smaller and smaller number of the photons will be close enough to the exact time to be close to the event horizon. Those that are emitted "too late" will fall into the central singularity. Those emitted "too early" will escape to infinity.

    Reference: see for example http://casa.colorado.edu/~ajsh/singularity.html#r=1, Hamilton's website on black hole's. Hamilton is a physics professor with several published papers on black holes.

     
  12. Oct 23, 2012 #11
    That is a bit difficult for me to picture. What I do understand is how Einstein calculated light bending in a gravitational field with the Huygens construction; such gravitational lensing is a true physicists approach. However, considering that method, it is not clear to me why perfectly "outwards" should not be possible. What exactly prevents this possibility in terms of that approach?
     
  13. Oct 23, 2012 #12

    PAllen

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    Actually (let's pretend supermassive black hole with minimal tidal forces after crossing horizon; as usual, ideal SC geometry), after you cross the horizon, up until you crunch, you can point your flashlight any direction, and locally see its light move away from you at c in any direction (assuming, further, you are inertial). So, locally (as required by definition of semi-riemannian manifold), everything still looks Minkowski sufficiently locally.

    However, if you define radial position in terms of circumference of a circle about the singularity / 2 pi, what happens is: your radial coordinate is decreasing much faster than the outgoing light (which is also moving - slowly - in the decreasing r direction). Note, that if someone falls in shortly after you, you can continue sending them light signals until you reach the horizon. To you, they are outgoing light signals, meeting this later infaller who is futher from the singularity than you. In terms of r coordinate, everything is ingoing, but at different rates.

    A key point is that a line of constant r (as defined above) is a spacelike curve inside the horizon. Thus, a light like path must decrease in r with increase in its affine parameter.

    [Upshot: I would qualify mfb's statement: all timelike or light like directions inside the horizon point in a decreasing r coordinate direction; outgoing r directions exist, but they are spacelike.]
     
    Last edited: Oct 23, 2012
  14. Oct 24, 2012 #13
    OK... that I understand. Now, the Huygens method is non-local, as pictured from a distant frame far in space. So, I guess that my question boils down to asking how to transform that description into a description based on such a non-local frame.
    Thanks, but that isn't a Huygens construction - not even a "non-local" description. If someone can translate the above into a non-local description, that would be very helpful for me and no doubt many others. I guess that I put my finger on the Schwartzschild singularity issue. :tongue2:

    Aren't clocks supposed to stop at the Schwartzschild radius and should thus also the frequency of light emitted from that point be zero? How can frequency be anything less than zero? :confused:

    When searching a little about this question I found this:

    http://casa.colorado.edu/~ajsh/schwp.html

    (I only read the first half)

    as well as this:

    http://blogs.discovermagazine.com/badastronomy/2007/06/19/news-do-black-holes-really-exist/

    That makes sense to me.
    There is also an interesting discussion included which I did not yet fully read; post 10 provides a slight correction in phrasing by the author.
     
    Last edited: Oct 24, 2012
  15. Oct 24, 2012 #14

    PAllen

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    I don't know anything about this Huygen's method. It is not used in any texts I have or papers I've read on gravitational lensing (at least by that name). I've studied methods that simply follow null paths in SC coordinates.
    .
    The r coordinate I've described is simply the Schwarzschild coordinate r coordinate - I've given its physical definition. In contrasting the local inertial frame observation that you can point a flashlight in any direction, of flash a bulb and get a spherical wave front, with the global statement that all timelike or null paths (inside the EH) progress toward the singularity, you must define some such global coordinate. I don't know of any simpler coordinate for this purpose than SC r coordinate.
    This false belief has been refuted numerous times on these forums. It's all about relativity. A distant or external hovering observer sees infalling clocks slow and their emitted light redshift, as they approach the horizon. The infalling observer sees no such thing. Their clock proceeds normally right up to the singularity, and they see external clocks also proceeding at a normal rate (slower or faster depending on the exact infall trajectory, but with a strictly finite Doppler factor).

    One way of explaining this asymmetry is simply noting that ingoing light has no trouble decreasing r coordinate to the singularity; while outgoing light has increasing 'difficulty' escaping as the EH is approached, up until not escapting at all if emitted at the EH (or inside). Personally, I do see this [freezing of clocks as viewed external to EH] as purely an gravitational optical effect [on outgoing light], somewhat analogous to Lene Hau's freezing light in a BEC.
     
    Last edited: Oct 24, 2012
  16. Oct 24, 2012 #15



    What do you mean by this? It appears you may be thinking of a black hole, inside the event horizon, as dense matter??.....so 'light would slow' as, for example, in glass or fiber optic cable??

    That is NOT what is believed to be inside a BH event horizon....all the mass that caused the original BH to form is crushed from existence and resides at the singularity.

    There is a good discussion about spacetime geometry inside a black hole here:

    http://www.jimhaldenwang.com/black_hole.htm


    In summary here is what you get inside a black hole horizon....all the way to the singularity at the center of the BH:

    [r = 2M is the Schwarschild radius, the location of the BH horizon]

     
  17. Oct 24, 2012 #16
    :uhh: It was the method that Einstein famously used to calculate the light bending by the Sun. On this forum I elaborated on that several times, with a link to the paper. Here once more: https://en.wikisource.org/wiki/The_...erihelion-motion_of_the_paths_of_the_Planets..

    That would mean that the only references that I found elsewhere are wrong according to refutations on this forum. In case you or someone else remembers any of such refutations that give a correct "distant" perspective instead, a link to it would be great! :smile:
    Yes, that is obvious (well, to me it is) and not in question! :smile:
    According to the second link that I gave, it is an unrealistic assumption that there would be anything to emit light "at or inside" the EH; seeing the there provided arguments, that made perfect sense to me.
     
    Last edited by a moderator: May 6, 2017
  18. Oct 24, 2012 #17

    PAllen

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    In reference to the quote in #15, while this description is often given, it is not quite accurate IMO. The switching places of r and t in SC coordinates is primarily a coordinate effect that disappears in a number of well behaved coordinate systems for this geometry. The change of role for t is exclusively the result of defining it so that fixing r,theta,phi and varying t labels points on spheres of fixed surface area (each distinguished by a different t). Since a path inside the EH that does not progress toward the singularity is spacelike, t labels points on the spacelike path, so it becomes spacelike.

    Instead, consider Lemaitre coordinates. Here, you have a radial coordinate that remains spacelike all the way to the singularity, and a time coordinate that remains timelike all the way to the singularity. This is achieved by allowing the radial coordinate to be non-static (effectively describing a collapsing space). Specifically, fixing radial and angular coordinates and varying Lemaitre time coordinate produces a path connecting spheres of decreasing surface area.

    The key physical statement about interior SC geometry is just that all timelike and null paths reach the singularity.
     
  19. Oct 24, 2012 #18

    PAllen

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    Well then I've just not heard it called Huygen's, and don't see much difference between it and other methods.
    The statement I was responding to was:

    "Aren't clocks supposed to stop at the Schwartzschild radius and should thus also the frequency of light emitted from that point be zero? How can frequency be anything less than zero?"

    The key point being that the understanding: "Only from the point of view of an external observer"
    is missing. The reputable sources have this understanding.

    You mean the arguments described in Discover magazine? The classical portion of these have been raised and refuted many times. External observers are seeng a frozen image - that is all. They can deduce that the object has reached the singularity. Specifically, they can compute when to send a signal such that the infaller will receive it an instant before reaching the singularity. The argument about Hawking radiation is more speculative because there is no complete theory of quantum gravity. Most other researcher's analyzing the same situation come to opposite conclusions (that horizon [or quantum analog thereof] forms in finite external time when quantum corrections are taken into account). However, this is an active, disputed area. For the purposes of this forum and thread, I am only discussing classical GR, classical EM.
     
  20. Oct 24, 2012 #19
    This is matter of context; the understanding that you say to be missing was specified in my post:

    "the Huygens method is non-local, as pictured from a distant frame far in space. So, I guess that my question boils down to asking how to transform [your local] description into a description based on such a non-local frame. [..] If someone can translate the above into a non-local description, that would be very helpful for me and no doubt many others. I guess that I put my finger on the Schwartzschild singularity issue.
    Aren't clocks supposed to stop at the Schwartzschild radius and should thus also the frequency of light emitted from that point be zero? How can frequency be anything less than zero?"

    So, once more: In case you or someone else remembers any post that gives a reasonable answer from that same perspective, a link to it would be very helpful. :tongue2:

    That is quite what I understood from it (without contemplating what distant observers actually "see"; surely distant observers will not see anything after a while).
    That's fine to me; this is also not the forum for quantum gravity!
     
    Last edited: Oct 24, 2012
  21. Oct 24, 2012 #20

    PAllen

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    Ok, then how much of what you're looking for is addressed or not addressed by the following observation I made:

    "They can deduce that the object has reached the singularity. Specifically, they can compute when to send a signal such that the infaller will receive it an instant before reaching the singularity. "

    Even though they won't get a reply, the distant observer knows when to send a signal that will be received by a given infaller:

    - at moment of crossing horizon
    - at an any point between horizon and singularity

    None of these signals involve waiting for infinite time to pass [before sending them]. Further, the infaller can flash a radio burst on receiving each of these signals. It just so happens that the bursts (that propagate away from the infaller in all directions, in their frame) never escape the horizon. The whole assemblage (infaller, out going signal sphere) is moving towards the singularity [in terms of SC r coordinate, and in terms of termination world lines and null paths on the singularity].
     
    Last edited: Oct 24, 2012
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