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Escape Velocity of virtual particles

  1. Apr 6, 2003 #1
    I know that the escape velocity for virtual particles being emitted from black holes is c2+ (in the black hole entropy formula, there is an escape velocity of c3). I also know that Hawking radiation and for that matter*(I don't mean matter in physics terms), any radiation being emitted from a black hole supposedly escapes the gravitational pull of the black hole. But how does this happen? Does this imply superluminal travel?
  2. jcsd
  3. Apr 7, 2003 #2
    the way i understand hawking radiation it is not radiation that is emitted by blackholes but the result of a pair of virtual particles being created near the black hole, ie outside the event horizon, with one of the particles entering the blackhole and the other flying off into space
  4. Apr 7, 2003 #3


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    It is only at the event horizon itself that the escape velocity is c. Get a little above the horizon and the escape velocity is some value less than c. And it is a little above the horizon that the Hawking radiation arises, so the escaping particle does not have to move faster than light.
  5. Apr 7, 2003 #4


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    Greetings MajinVegeta !

    To make it clearer, gravity is not really the
    point here. The point is a VERY strong field -
    it could be an electric field (for particles
    that have electric charge) too for example.
    If it is strong enough then it can "split"
    the virtual pairs of particles created as
    a result of zero-point-field quantum fluctuations.
    But, like I said, it has to be VERY strong.
    So, when it comes to a gravity field it is only
    strong enough near(OUTSIDE !) a BH's EH to be able
    to split the ZPF particles.

    Live long and prosper.
  6. Apr 12, 2003 #5
    What's a EH and a ZPF?
  7. Apr 12, 2003 #6
    Re: Re: Escape Velocity of virtual particles

    Why do virtual particles do behave like so?
    Can you give me an approximate escape velocity of a particle NEAR the event horizon?
  8. Apr 12, 2003 #7
    Virtual particles behave more-or-less like real particles, which would do the same thing.

    As for escape velocity near the black hole... hmmm. That's a GR question, and I'm not sure how to do it -- or even how to frame it coherently right now. The problem is that around the event horizon (drag's 'EH') which frame you measure the energy in is tremendously important.

    One thing to remember is that the whole virtual-particles thing is just kind of a heuristic approximation: it almost always vaguely works, but the actualy theory is a field theory, of interacting fields, not particles. The actual derivation of Hawking radiation comes from looking at the ground state of the field near the black hole from two different reference frames (moving relative to each other), not from invoking virtual particles at all.
  9. Apr 12, 2003 #8
    aren't interacting fields composed of particles?
  10. Apr 12, 2003 #9


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    Shorts for Event Horizon and Zero Point Field(which
    I wrote in the body of the message too :wink:).
    ZPF quantum fluctuations are the virtual particles
    that are momentarily created in a relativly (for
    you as an observer) "empty" part of space with no
    strong fields passing through it - not fluctuations
    that are directly due to a "real" energy source.

    Live long and prosper.
  11. Apr 12, 2003 #10


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    According to QM - yes. What happens is that due to
    the HUP the value of these fields (or virtual
    particles if you wish) also fluctuates, which means
    that it could "jump" high enough in some places
    to allow other virtual particles to be created

    Live long and prosper.
  12. Apr 12, 2003 #11
    I am unsure about what a ZPF is (this is, its definition, not what it stands for).
  13. Apr 12, 2003 #12
    "Ground state", "zero point energy", and "vacuum state/energy" are more common terms AFAIK. In a quantum system, the lowest energy state -- this is the unexcited state in quantum mecahnics, or the empty state (no particles) in a field theory -- actually has nonzero energy. This lowest state is the vacuum/ground/zero point state.
  14. Apr 12, 2003 #13
    Whats the difference between zero point fluctuations and zero point quantum fluctuations?

    are virtual particles, from the way I understand them, particles, although they themselves cannot be detected exactly, but are detected to their indirect (virtual=indirect) influence in space?
  15. Apr 13, 2003 #14
    >>Whats the difference between zero point fluctuations and zero point quantum fluctuations?

    Same thing.

    Technically, virtual particles actually arise when you consider the perturbation-series approximation to a quantum field interaction.

    They are sort of "temporary" or short-lived particles... kinda. Real particles the long-time or long-distance limits of virtual particles.
  16. Apr 13, 2003 #15
    so they're not real particles? (i'm getting a bit confused here...)
  17. Apr 13, 2003 #16


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    Not really, they're the result of mathematical
    abstractions. In QM you can't have any
    continuos interaction/field/whatever because
    everything is made of qauntums - individual

    Live long and prosper.
  18. Apr 13, 2003 #17
    What do you mean by mathematical abstractions? What is their relation to their surroundings?

    I'm getting a notion this has to do with dark matter?
  19. Apr 13, 2003 #18
    Don't worry, you're not the only one confused about this! :)

    Nothing to do with dark matter -- dark matter is just anything not currently undergoing nuclear fusion and glowing like the sun. You are dark matter, I am dark matter, the Earth is all dark matter...

    No, virtual particles are not real particles. Hmmm... okay, they arise in the context of quantum field theory; in QFT everything consists of interacting fields. However, symmetries and conservation laws make it so that non-interacting fields always are of a form that can be described by some number of particles. That's why the idea of a particle is so useful: on a large enough scale, we can ignore the field-like nature of what's going on and talk about discrete particles.

    Now, when you get to interacting fields -- take two particles and shoot them really close together for instance -- the picture of these discrete particles breaks down, and you have to look at the underlying fields. However, much of the time, it turns out you can approximate the field picture very closely and in a very intuitive (if you know some QFT) way by introducing virtual or 'temporary' particles. They appear like regular particles, but can have any mass, velocity, etc (they need not "be on their mass shell") and only exist for short periods of time/distance.
  20. Apr 14, 2003 #19
    why do they exist for short periods of time?

    Dark matter is involved in the brane theory. Dark matter accounts for the wierd velocity of stars for instance. Their calculated velocities don't correspond with their current velocity. Their velocity relative to their placement in their galaxy (in the outskirts of a galaxy, this is a common occurance) doesn't make sense. For example, a star in the outskirts of a galaxy wouldn't be traveling at c+, right? That sort of thing happens. That's when dark matter comes in. Oh dear..I have to go to bed, but I'm pretty sure you know what I'm talking about, damgo.

    BTW, I'm only 13, in pre-algebra so if my questions involve some seriously complex math, can you try and simpliyfy it for me or just tell me what I need to know or something?
  21. Apr 14, 2003 #20
    maybe since the singularity is a source of untapped energy,outside the event horizon or in it spacetimes interaction with energy is what makes particles or matter just like in the begining.the amount of energy in spacetime somehow converts this excess energy into particles.so next to the black hole spacetime is highly charged with energy from the gravity,so what if every now and then spacetime steals energy from the blackholes gravity and creates particles from it!then when the two virtual particles come into existence,the normal one is then attracted by the blackholes gravity and is pulled back,then the anti particle then leaves travelling at light speed because of it's repulsion to spacetime,like photons just pure anti energy repelling against the positive energy background of spacetime.and all thats leaving the blackhole is anti matter.
    Last edited: Apr 14, 2003
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