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

Black Hole Information Loss Question

  1. Nov 23, 2011 #1
    If I understand correctly, Hawking Radiation is created when the anti-particle of a virtual particle pair falls into a black hole while the particle of the pair escapes. The anti-particle goes on to annihilate with a particle in the black hole. Is not the annihilated particle identical to the real particle that escaped the black hole?
  2. jcsd
  3. Nov 24, 2011 #2


    User Avatar
    Science Advisor

    Either the anti-particle or the particle can escape. It doesn't matter.

    As for what happens inside the black hole, we can't say. All that we do know is that the particle that escapes the black hole carries with it some of the mass of the black hole.
  4. Nov 25, 2011 #3
    How does the escape of am anti-particle reduce the mass of a black hole? Such an event would have a particle falling into a black hole, thereby increasing the mass of the black hole and decreasing the mass of the rest of the universe.
  5. Nov 25, 2011 #4
    That's one way of looking at Hawking radiation, there are others.

    That's not quite what happens. What happens is that the energy from the gravity creates a particle/antiparticle pair near the event horizon. The particle falls in. The anti-particle escapes. Because the pair gets created from the energy of the gravitational field, some of that mass/energy of the black hole leaves via the anti-particle.
  6. Nov 26, 2011 #5


    User Avatar
    Science Advisor

    Annihilation isn't the cause of the reduction of mass of the black hole. It can't be, because if you annihilate two particles within the event horizon, the photons that are produced in the annihilation will have the same energy, and those photons will just remain in the black hole with no change in the behavior.

    Instead, the particle that ends up falling into the black hole turns out to have negative mass (this isn't a problem for short-lived particles).
  7. Nov 27, 2011 #6


    User Avatar
    Science Advisor
    Gold Member

    It really does not matter which particle falls into the black hole, all that matters is the one that escapes has positive energy. Call it a quantum entanglement thing if that makes it more palatable.
  8. Nov 28, 2011 #7
    So why do they talk about quantum fluctuations springing into existence and then annihilating each other? Are you saying that photons spring up out of nothing every time there is a quantum fluctuation?

    Also wouldn't you think that a black hole would be accumulating far far more stuff than it could ever evaporate? It should continue to grow until there was no more space/universe to feed it.
  9. Nov 29, 2011 #8


    User Avatar
    Science Advisor

    The actual picture of the Hawking effect in terms of virtual particles is actually rather murky. Hawking's original derivation made no mention of virtual particles slipping over event horizons; he instead examined the asymptotic transformation of a field fluctuation brought in from far away, past a black hole, and back out to very far away. If the initial field fluctuation far away was in its vacuum state, the final fluctuation far away generally is not (as a result of its gravitational interaction with the black hole). John Baez has a nice write up about this: http://www.weburbia.com/physics/hawking.html

    The idea of the Hawking effect being the result of virtual particles popping into existence (and, yes, this could be e+/e- pairs popping out of the vacuum) near the event horizon, one falling in and one flying off, is more an illustration for the sake of popular science than an accurate description of the process, which admittedly, is hard to understand in terms of particles. After all, if there's any lesson we learn from playing with quantum fields in general relativity, the very concept of a particle has questionable significance in curved spacetime.

    If the black hole is in a particularly crowded part of the universe (like hungry black holes at the centers of some galaxies) they will gorge themselves on surrounding matter and increase in mass. Only when there is little left to eat will they begin to incur a net loss in mass due to Hawking evaporation.
  10. Nov 29, 2011 #9
    Bill I have seen a few of your posts and just want to clarify something in case you were unsure.

    At a distance beyond the Event Horizon of the Black Hole the gravitational effects are the same as any other body of mass, Black Holes are not hoovers that suck everything in. As a thought experiment: if the Sun were to turn into a black hole of 1 solar mass then the Earth would continue to orbit in EXACTLY the same way (except maybe a little colder!)

    You may already know this but it seems like you may have this common misconception and I am just trying to help :smile:
  11. Nov 29, 2011 #10


    User Avatar
    Science Advisor

    Yeah, pretty much. Though they can't stay around for long or else they'll violate conservation of energy.

    If a black hole absorbs a negative-mass particle, it reduces in mass, which means it drops in size.
  12. Nov 29, 2011 #11
    Thank you, actually I did realize that. It would be the same as if a mass suddenly turned into a star, the gravity would not change as long as the mass stayed the same. What I was saying had to do with Hawking's evaporation theory. I believe through the effect of gravity that all massive bodies are fed continuously. Even after all the visible surrounding matter has been pushed into the black hole, space is still filled with particles and energy. As I said before there would be far more going into the black hole than could ever evaporate. As long as there was any SPACE/UNIVERSE left. Of course this is my opinion and I'm just looking for feedback.
    Thanks again
  13. Nov 29, 2011 #12


    User Avatar
    Science Advisor

    The black hole will only continue to feed as long as the temperature of its surroundings are higher than the black hole's temperature. If we imagine a situation where all of the normal matter is gone, and we only have black holes, then the temperature in question is the temperature of the CMB. So once the CMB temperature drops below the black hole temperature, it will start to evaporate in net.
  14. Nov 29, 2011 #13
    I agree and always have that black hole may evaporate and I can understand how this could happen. All I have been suggesting is that a black hole will continuosly be fed as long as there is space, given it hardly a chance to ever evaporate.
  15. Nov 30, 2011 #14


    User Avatar
    Science Advisor

    Again, it has to do with temperature. If the temperature of the space is lower than that of the black hole, then the black hole will emit radiation more rapidly than it feeds.
  16. Nov 30, 2011 #15


    User Avatar
    Science Advisor

    As already said the explanation with particle antiparticle is only an analogy. In the original paper Hawking writes:

    "One might picture this negative energy flux in the following way. Just
    outside the event horizon there will be virtual pairs of particles, one with negative
    energy and one with positive energy. The negative particle is in a region which
    is classically forbidden but it can tunnel through the event horizon to the region
    inside the black hole where the Killing vector which represents time translations
    is spacelike. In this region the particle can exist as a real particle with a timelike
    momentum vector even though its energy relative to infinity as measured by the
    time translation Killing vector is negative. The other particle of the pair, having
    a positive energy, can escape to infinity where it constitutes a part of the thermal
    emission described above. The probability of the negative energy particle tunnelling
    through the horizon is governed by the surface gravity K since this quantity
    measures the gradient of the magnitude of the Killing vector or, in other words,
    how fast the Killing vector is becoming spacelike. Instead of thinking of negative
    energy particles tunnelling through the horizon in the positive sense of time one
    could regard them as positive energy particles crossing the horizon on pastdirected
    world-lines and then being scattered on to future-directed world-lines by
    the gravitational field. It should be emphasized that these pictures of the mechanism
    responsible for the thermal emission and area decrease are heuristic only
    and should not be taken too literally.
  17. Dec 1, 2011 #16
    So Hawking's argument is that a black hole has a non-zero temperature and so radiates as a thermal black body. His justification is based in QM but with that caveat, "It should be emphasized that these pictures of the mechanism responsible for the thermal emission and area decrease are heuristic only and should not be taken too literally," as posted by martinbn.

    John Baez took up the argument, as linked by bapowell, beginning with, "In 1975 Hawking published a shocking result: if one takes quantum theory into account, it seems that black holes are not quite black! Instead, they should glow slightly with "Hawking radiation", consisting of photons, neutrinos, and to a lesser extent all sorts of massive particles."

    And ending with, "Now in fact when you do a Bogoliubov transformation to the vacuum you get a state in which there are pairs of particles and antiparticles, so this is possibly the link between the math and the heuristic explanation."

    This leads me back to the original question, which was (admittedly) understated. If Hawking radiation is more than simply thermal (i.e. comprised of "photons, neutrinos, and to a lesser extent all sorts of massive particles") then should not an observer at infinity be able to (at least in principle) measure the properties of all of the particles emitted by the black hole?
  18. Dec 2, 2011 #17
    Yes sorry if this is just asking the same question again in a different way :uhh:...

    I keep hearing that one of the big issues that is had with Hawking Radiation (or is it only a problem in relation to the Information Loss Paradox maybe??..) is that there is no mechanism for information to be transferred from the incoming B.H. food to the outgoing Hawking Radiated photons? I've heard it could be said that the two pass by one-another, but there's nothing suggestive of an interaction between them. Am I anywhere near on the right track so far? :smile:

    And I guess if that's truly the question, anyone around here who might be capable of recounting the popular possible solutions would be great. I've seen that holographic theories may allow the intricacies of a B.H.'s event horizon's shape to preserve the information? Something like the shape of the (entire) horizon changes simultaneously with any incoming matter, and therefore exactly how the B.H. contacts the adjoining space around it (creating the vacuum gradient or whatever [you can probably tell how over my head I am here by now heh] that generates the radiation) can be effected by something seems unconnected. (And the key being the singular, integral nature of the B.H. I take it?)

    Are there other ideas for a mechanism? Or just giving up on the problematic quantum information tenant?

  19. Dec 2, 2011 #18


    User Avatar
    Science Advisor

    Well, no, the information of what enters a black hole is necessarily encoded in the Hawking radiation that leaves it. It's just that that information is so exceedingly garbled that it is in practice impossible to determine from the outgoing radiation what fell into the black hole.
  20. Dec 4, 2011 #19
    :blushing: Ugh...So the paradox was that, before the idea of Hawking Radiation, it seemed like the quantum information of the ingoing matter was disappearing forever, but Hawking's discovery (if true) shows how it can reemerge, thereby satisfying the universal bookkeepers?

    I've actually read (e.g.) the wikipedia entry on this, etc., but it's still confusing to me.
  21. Dec 4, 2011 #20


    User Avatar
    Science Advisor

    Well, Hawking radiation is on very firm footing, and can be derived in a large number of different ways. Hawking radiation has also been observed in analog systems where we produce sound horizons (as opposed to the light horizon of a black hole). So basically, if black holes exist at all (which is nearly certain), they produce Hawking radiation.

    With that aside, the picture you paint is largely accurate. The main issue, however, is that it was a long time between the discovery of Hawking radiation and the solution of the black hole information paradox. Early on, it was largely believed that Hawking radiation could not contain any information about what went into the black hole. This has since been proven false.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Black Hole Information Loss Question
  1. Black hole question (Replies: 2)

  2. Black hole question (Replies: 11)

  3. Black Hole Question (Replies: 6)