Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What's inside the event horizon

  1. Sep 22, 2010 #1
    Don't know where I picked it up, but something
    indicated to me that inside R_eh = 2GM/c^2 lies
    a black hole whose R_bh = GM/c^2. And that at
    R_bh lies the energy singularity. And that at R_eh
    there is not an energy singularity, but only an end
    to communication with the world outside. I know
    that outside observers cannot detect what is inside
    the event horizon sphere, but that should not stop
    the mathematics from predicting what is going on
    in there. Especially since authors(wiki) say that
    nothing in particular is experienced by an observer
    passing through the event horizon. The "mathematical
    breakdown" seems only to be a "communications
    breakdown" in other words. Mathematics predicts
    stuff we cannot see...like the "dark properties"...
    What's the problem here? Is quantum mechanics
    going on in there?
  2. jcsd
  3. Sep 22, 2010 #2
    Not entirely sure what you mean here, but the event horizon is the boundary within which nothing can escape from the black holes gravity, including light. Which is why it appears black.
    There wouldn't be anything special in the event horizon, it is just the boundary for the above mentioned limit.
    Anything within the gravity of a black hole gets pulled into it unless you can provide enough energy to escape, however, once past the event horizon you would need an infinite amount of energy to get away.

    (This all ignores the fact you wouldn't survive the immense gravitational forces long enough to get to that point.)

    A bit simplistic, but I believe it covers it.

    I'm not sure what these 'dark properties' you refer to are. Dark Matter? Dark Energy?
  4. Sep 22, 2010 #3


    User Avatar
    Science Advisor
    Gold Member

    The event horizon marks the point where observational evidence, as we know it, ends. It can only be probed using math. As history has taught us time and again, math without observational evidence is unreliable.
  5. Sep 23, 2010 #4
    Unreliable it may be, but then if it shows nothing can escape it, what is there to 'observe'? Just black, as we see (or don't on the backdrop of space). You'd have to go in, but then you'd never be able to tell anyone what you observe. Bit of a pickle you'd find yourself in (assuming you survive).

    I guess we'll never know...
  6. Sep 23, 2010 #5
    ...Especially when the math in question breaks down at that specific point. Lets be honest, at this point John Earman's hypothesis is as good as any: "lost socks and green slime."

    JarednJames: We may never know, but if there is a way to explore the Planck scale someday, and an effective theory of quantum gravity or something else emerges "never" may just mean "never within the foreseeable future".
  7. Sep 23, 2010 #6
    Yes, when I "probe" the event horizon with Newton's equation for orbital

    V = square root[GM/R] and plug in R = 2.95 Kilometers and M = our sun,
    I get 212000 km/s, not 300000 km/s as I expected. I'm not telling you
    my values for G and M because I am now thinking that I've got them
    wrong...do you get 300000 km/s for the orbital velocity near the event
    horizon? Does Newton's equation need more terms when relativity
    is accounted for?
  8. Sep 23, 2010 #7
    Never for me means up to the point I expire. After that, can't say I'll be that bothered if they do or don't get to explore it. :approve: Everyone does keep calling me a pessimist so saying 'never' sounds about right from that point of view.
  9. Sep 23, 2010 #8
    I didn't think our sun was big enough to become a black hole, I thought it had to be quite a bit larger, which would increase your mass figure somewhat. (According to another site, it's around 10 times the mass of the sun). By factoring this in, it would add a 0 to your required orbital velocity increasing it by a factor of 10.)

    Also, why orbital velocity? Surely you mean escape velocity? You could calculate the gravity using:

    F = ( G M1 M2 ) / r^2 and that gives you the acceleration due to gravity. From that you can work out the orbital velocity. (I don't do this stuff much so I'm not the best person here).
    Last edited: Sep 23, 2010
  10. Sep 23, 2010 #9
    Well, don't get discouraged...

    Just build a very durable black hole probe, which trails a very long antenna which transmits in a clever way via Hawking radiation or something like that for the few milliseconds while the probe is inside and the antenna is still outside of the event horizon.

    An interesting question for bar room conversation is: What sensors to put on the probe?? A camera? is there anything to see in there, like another universe or something? What would you look for?

  11. Sep 23, 2010 #10
    Anyone desire to point me to a wiki or such?...this is not a homework
    problem...so fear not. I don't know how to get to orbital velocity from
  12. Sep 23, 2010 #11
    I've just checked and your equation for orbital velocity is correct.

    As I pointed out though, a typical black hole (apparantely) is about 10x the mass of our sun. If you factor this into your calculation, it increases the orbital velocity to 2,120,000 km/s (well over c).
  13. Sep 23, 2010 #12
    And R_eh becomes 29.5 km...it's still 212000 km/s, or did I add insult to
    injury again. Hate it when the answer is not what I get.
  14. Sep 23, 2010 #13
    Well, I don't believe in an afterlife so I'm not inclined to characterize your view as pessimistic, just realistic; I doubt we'll have these answers within our lifetimes, so yeah... never works. :tongue: It's a novel use of the word, but I understand your position.
  15. Sep 23, 2010 #14
    The answer I want is 299792 km/s...the answer you want is DEATH AND TAXES.
  16. Sep 23, 2010 #15


    User Avatar
    Science Advisor

    Er, the math doesn't break down at that specific point. The Schwarzschild metric breaks down, but that's just because of the particular coordinate system used. There are other coordinate systems for describing the same black hole that have no unusual behavior at all at the horizon.

    The math doesn't break down until you get to the singularity at the center, but I strongly suspect that General Relativity gives the wrong answer before that.
  17. Sep 23, 2010 #16
    This is the post I give 5 stars because it got me thnking about
    escape velocity and its relation to orbital velocity. Yup, it applies
    here...Newton found that V_escape = sqrt(2) times V_orbital.
    Same thing seems to hold classically as well as relativistically. So,
    when V_escape becomes c, V_orbital = c/sqrt(2) = 212000 km/s.
    Hats off to jarednjames for finding a way to make me think.
  18. Sep 23, 2010 #17
    Anyway, the hypothetical probes don't have any wiki's.
    so put them low on the list of things to be ruled-out. After
    reading wiki's on black holes, worm holes, and white
    holes, the scenario I put on the top of the list to rule
    out is the freefall into a non-rotating BH experiment.
    I think it makes sense that, for the entire experience,
    the subject would be weighess and unharmed. The subject
    would of course enter the event horizon going very fast
    but nonetheless weightless. But suddenly, a star filled sky
    would appear where none was an instant before. IE, from
    BH horizon, through the wormhole, to the WH horizon in an
    instant (with no possibility of return). The wiki's don't seem
    to rule this out. If no human wanted to volunteer for this
    experiment, a huge and powerful radio transmitter would
    suffice. The experiment would continue by searching the
    heavens for the WH by searching for the radio source that
    was introduced into the BH earlier. If the radio source could
    not be found, it was either worm holed too far away, or got
    ground to dust. If the radio source was eventually found, its
    WH would be nearby and it would get written up in journals.
  19. Sep 23, 2010 #18


    User Avatar
    Science Advisor

    No, according to GR this isn't what happens. Basically, as a macroscopic object falls inward, the gravitational force difference between the bits closer to the black hole and the bits further away gets greater and greater. As the object draws towards the singularity, this difference gets so great that the object basically gets drawn into a long thin strand. I've heard it referred to as "spaghettification".

    For smaller black holes, this process occurs outside the event horizon. For more massive ones, objects pass through the event horizon more or less unharmed (though they apparently encounter an infinite blast of radiation as they pass the horizon), and don't spaghettify until they get closer to the singularity.
  20. Sep 23, 2010 #19
    Where is the wormhole and WH in your theory? May we limit our discussions
    to situations where we assume wormholes and WHs exist...instead of that
    pesky singularity?
  21. Sep 23, 2010 #20
    Blue sheet?
    This is applicable only to rotating BH, and it happens when thje probe crosses the second horizon.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook