Hawking Radiation - how could it shrink a Black Hole?

Discussion in 'Physics & Math' started by MyBrainHurts, Sep 29, 2008.

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  1. NMSquirrel OCD ADHD THC IMO UR12 Valued Senior Member

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    i do believe you have answered my question..sorry if i wasn't clear..
    the root of this question came from wondering if it was possible,which you say it just may be,(dependent on the math) for a planet to be formed from a black hole..our planet has an iron core.(not steel)..so IF it were to be a possibilty that a small black hole was at the center of a planetary body..
    (and i suppose my ignorance is showing at this point.) would we be able to explain gravity better?( i have read that gravity is the only thing keeping us from a theory of everything..)
     
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  3. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    No, I think a planet formed from a mass cloud near a black hole, by the BH, is impossible. Read about quasars to understand why.

    Even if the "mass cloud" and initial BH masses are comparable, the BH will not cause the mass cloud to compress into a planet. The gravity gradient will help disperse the mass cloud, and as part of it gets close to the EH, heat it white hot, at least. Again read about how quasars make their radiation.
     
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  5. Gravage Registered Senior Member

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    No problem, personally I think I also crossed the line a bit as well. So, take my sincere apologies. Generally, despite my sentences, I do highly respect physicists. I usually contact them by e-mail or listen their lectures whenever I can. The only problem I have is that I need more than math, experimental and observable proofs for mathematical calculations physicists usually make, otherwise all calculations, until experimentally/observably proven are pretty much useless. Physicists can throw them out of the window. This is why I think job to be theoretical physicist is so hard, and that's why all of you (physicists) have my honest respect.

    Cheers.
     
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  7. Gravage Registered Senior Member

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    It is about validity, especially the part where it explains that one anti-particle (particle with negative energy) enters the black hole and the other which is in the vacuum just outside the event horizon runs always. Black hole does not give off any kind of radiation, since the particle that ran away from the black hole is always at all times outside the black hole's event horizon.
    However, my hypothesis for Hawking's radiation is a bit different:
    1) Anti-particle (particle with negative energy) falls in the black hole, and since black hole's event horizon and the black hole itself are made purely of particles with positive energy, the anti-particle that enters black hole will be annihilated with an particle of event horizon's positive energy. In that way, black hole shrinks. Basically, in the same way black hole's surface will evaporate since (matter and anti-matter collision).

    2)The other thing possible is through quantum tunneling where, perhaps particles, travel,from time to time for a very short period of time, travel faster than speed of light.

    That's all I found it in the books.

    Cheers.
     
  8. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Your ignorance plus a dislike for things that are only mathematical has your POV here very silly but amusing.

    First there are no particles at the event horizon, waiting to annihilate some anti-particle that happens to fall in. I know this because the event horizon is just a mathematically computed surface. To make it simple enough for you to understand let me tell you about the Earth's "half G" surface:

    The half G surface, like the event horizon, is discovered or defined by a mathematical calculation. I.e. for both the half G and event horizon we compute a surface, which has an arbitrarily specified gravitational strength.

    In the case of the Earth's half G surface that condition on the gravity strength is such that if you were there, your weight would be 50% of what it is on the Earth's surface.

    In the case of the event horizon surface that condition on the gravity strength is that if there then nothing (light included) can overcome that strong gravity and move farther from the mathematically computed surface we call the "event surface" or the "event horizon."

    We call it that because it is a surface or horizon you cannot see (or in any way measure or determine) what is happening inside of that surface. I.e. no event, not even explosion of a hydrogen bomb, inside that surface can be sensed anywhere outside of that surface by any means - ALL events inside that surface are beyond our observational horizion.

    Both surfaces are ONLY the results of a mathematical calculation designed to find a surface where the strength of gravity has a specified value. There is no real surface. There is nothing there, unless some particle by chance happens to be passing thru the surface.

    You are too ignorant to know that your alternate theory is built 100% on the result of a mathematical calculation, and you do not even believe in mathematical results! Nothing exists there. If you could survive the gravity gradient and fall thru the event horizon from outside to inside, you would not even know when you passed thru that surface - just as you would not know when you fell thru the "half G" surface of the Earth. Again: There is nothing there both are ONLY mathematically defined surfaces. Learn the basic facts before building a theory on what does not exist, except in your despised mathematics. That way your ignorance is not so obvious.
     
    Last edited by a moderator: Dec 6, 2009
  9. Gravage Registered Senior Member

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    I don't ignore, trust me, it's just sometimes I'm lost in the ocean of informations when I read about science (not just physics, but every other scientific discipline so far).
    You're right, I was wrong ACTUALLY EVERY SINGLE ONE OF YOU WAS RIGHT, and I was wrong. Actually, in the book "A Brief history of time", Hawking specifically says how did he get to this solution in 1973., by using Heisenberg's uncertainty principle. Heisenberg's uncertainty principle enables particles for a short period of time and in short distances to travel at speeds faster than light, just long enough to allow sub-atomic particles to get out from the black hole's event horizon. On pages 119-121 of the same book Hawking specifically describes the process of Hawking radiation. It's very interesting how at the end of detailed explanation, Hawking mentions that when he specifically said that energy of the outgoing radiation is positive from the black hole, and it is equalled by the entrance of negative energy particle (because E=mc2), and there is no loss of energy, there is no violation of the conservation energy law. Also, he additionally mentions that entrance of particle with negative energy decreases black hole's mass, so the event horizon is getting smaller and smaller, while this is completely compensated with entropy of outgoing radiation from the black hole (which is is enabled thanks to Heisenberg's uncertainty principle) so the second law of thermodynamics is completely retained.
    What do you think, is this description right or wrong?
     
  10. Gravage Registered Senior Member

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    Public apology to all physicists and mathematicians...

    After all, I've read and re-read and e-mailed other physicists about Hawking's radiation, I decided I crossed the line.
    Prometheus, AlphaNumeric, Billy. T and others, I owe you honest apology. You were right, I was wrong. This is very interesting, when 2 years ago, and physicist explained me why and how Hawking's radiation hypothesis works and he showed me, as well as what and how Hawking came to that conclusion.
    For some reason after that, I started to have doubts about it. Maybe because I stopped reading books for a while. However, everything is clear, now at least they way Hawking described it.
    I also apologize to call some of you "math geeks", I thought this would be a compliment, not verbal offense. Because, honestly speaking if someone called me math geek, I'd know that I possess the knowledge, and I'd proud of it. But if it's offense for you, I apologize I won't bring this term ever again.
    I just hope all of you are not too angry on me, because what I did.
    Maybe I wanted to draw you intention in explaining some things that I obviously forgot, but in totally wrong way, this way totally wrong approach.
    The reason may lay down in the fact none entered this topic for some time, and I was afraid none will answer it at all, or that it will be totally ignored, not answered, and even not to read it at all.
    I learned the lesson, there is no doubt about it.
    Again, take my sincere apologies.

    Gravage

    Cheers.
     
  11. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    To Gravaga:

    I am not angry at you and you owe me no apology. I don’t understand Hawking Radiation, HR, and think few actually do. I got my PhD in physics more than 40 years ago and back then few physicists achieved the level of math required to understand it. Since then, two mutually contradictory popular “explanations” have been advanced for people who want to think they understand HR. One is the one you are obviously familiar with, but I think still do not have it completely understood, so I will say a few words (soon) about it. The other explanation has to do with the fact that the math and the association between entropy and temperature allow you to define a temperature T for the Event Horizon, EH surface. (Perhaps “determine” is better than “define” – I am not sure exactly how “real” this T is.)

    For BHs significantly smaller than the moon in mass, this T is very great and BHs are “perfectly black” (the only thing that is). By that I mean any radiation falling on their EH is exactly 100% absorbed for all wavelengths. It is a general law of physics (which if not true would allow one to make energy from nothing) that the coefficient of absorption, a = e the coefficient of emission WAVELENGTH by WAVELENGTH. (I made that all caps as usually people just say a = e or “a good absorber is a good emitter” and do not realize that at different wavelengths a = 0.8 and e = 0.2 is certainly possible, so in general a is not equal to e, but it is for a BH as both are unity for all wavelengths.)

    Thus a surface which is very hot and has e = 1.0 is losing radiant energy by “black body radiation” BBR. In the case of a tiny BH, the peak of that BBR is in the gamma rays spectrum or at least harsh X-rays. This radiated energy must come from somewhere. The same complex math, which shows that HR exist shows that the mass of the BH is decreasing to keep the total energy of the universe constant.

    In the false popularized explanation you already know, the loss of BH mass / energy is exactly equal to the mass / energy of the PARTICLES which are escaping into our part of the universe. In the other false popularized explanation (the hot EH surface with BBR) the loss of BH mass / energy is exactly equal to the mass / energy of the RADIATION (X-ray and gamma rays, mainly) which are escaping into our part of the universe. I.e. one popularized account has PARTICLES and the other has X-ray and gamma rays, mainly escaping into our ob servable part opf the universe. I.e. the two popular explanations of HR are mutually contradictory – they are both just attempts to give people who cannot follow the math a “warm fuzzy felling” that they understand HR.

    -----------------
    Now for the few promised words on the “particles escaping” popular version of HR:

    It is a fact (even a measurable one) that spontaneously “vacuum polarization pairs” briefly “pop into existence” out of the nothingness of the vacuum. The pair is always the particle and its anti-particle, most commonly an electron and positron. This new “from nothing” energy in our universe can only remain real for the short time the uncertainty principle allows then they must mutually annihilate to return the universe to the same energy it had prior to their existence.

    They are called “virtual particles” by many, but I tend to reserve that term for the particles that appear in Feynman diagrams. For me, they typically are just ordinary electrons and positrons with positive mass and each with +0.511 Mev of rest mass. (Some more recently educated PhD may have reason for say one is somehow with negative mass etc. but how would it always be the one which falls into the BH?)

    I think simple geometry determines which falls into the BH. At the EH, the gravity gradient is unimaginable strong. If the pair pops into existence very near the EH, then whichever one happens to be closer to the EH will be more strongly accelerated towards the EH than the other. I.e. the gravity gradient will increase their separation.

    In some cases both will be eaten by the BH. Usually neither will be as they mutually annihilate again before the speed of light limit on their movement towards the BH allows either to get to the EH. In some rare cases, one and only one will fall in that strong gravity field into the EH of the BH. That leaves the one which did not without a partner to mutually annihilate with. I.e. it can be a permanent addition in the usual pair production case of 0.511 Mev added to our observable universe; however the Hawking math shows that the BH’s mass energy decreases with this event by 0.0511Mev also so total energy of the universe is conserved.

    ---------------------
    I took the time and effort reflected above for two reasons: I like to teach, and correct errors when I can. Also you seem willing to learn. I hope I have shown that the popular accounts you read are just that – to really understand you need to learn the math which accurately describes the physics. I suggest you start with areas where that math is relative easy. For example Special Relativity, then perhaps Quantum Mechanics. (There is a lot of nonsense in the popular accounts of QM, especially about what the uncertainity principle states.) Ideally take some courses, but probably in either of these fields you can teach your self (especially if you come here with well defined questions when confused.) The math of HR is well beyond me, but I think some here can follow it OK. Once I could follow tensor equations, but they are filled with conventions that just reading a book may not make clear – at that level you will almost certainly need a teacher. Good luck.
     
  12. Gravage Registered Senior Member

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    My criticism was about about proving Hawking's radiation in repeatable experiments as well observational proofs. Equations don't prove it until it's proven in LHC or if we have observational proofs. (The only equation that I'm aware of that has been proven is E=mc2.)
    This is why Hawking's radiation is still merely a hypothesis, it's not theory.
    There is a difference between hypothesis and theory: Theory is something that is already proven, hypothesis merely an assumption, no proof of it.
    You can't really say, based on equations, this is a scientific fact, because it isn't. It can go either way. It's still a hypothesis, this is why I truly hope LHC will be able to create micro-black holes long enough to see/detect if there is Hawking's radiation in the process.

    Cheers.
     
  13. prometheus viva voce! Registered Senior Member

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    Hawking radiation is based on the two most successful and tested theories of physics ever (general relativity and quantum field theory). Hawking used these theories together a particular way and I can't see a logical reason why it should be invalid. I've not seen a convincing argument why they should be incompatible.


    Firstly, theories are never proven, they can only be disproven. Secondly, every theory that is used in physics today and for the last 300 years or so have been based on equations - One example of a 300 year old one is Newtons gravitational law, \(F = \frac{G m_1 m_2}{r^2}\), which is enough to land people on the moon. In any case, \(E = mc^2\) is only a special case of \(E^2 = (pc)^2 + (mc^2)^2\), it's not really a very important equation, except in the public consciousness.

    Hawking radiation is a much stronger result than you are giving it credit for. It seems like you want to say that a hypothesis is when you come up with something off the top of your head like "the sun is made of a conglomeration of farting elves." Hawking radiation has been computed in at least three different ways, and you get the same result each time. Also, there isn't (IMO) an argument to say that you shouldn't be able to combine QFT and GR in the way Hawking did it.

    It's true that calling Hawking radiation a theory is not right, because it isn't a theory - it's a prediction of QFT and GR, both of which we have extensively tested and believe to be "right."

    There have been plenty of things that have been theoretically predicted before they were experimentally observed. The one that I can think of off the top of my head is the \(\Omega^-\) baryon. I'm not saying that Hawking radiation is scientific fact, but it's far more robust than you're painting it to be.

    Take it from me - there will be no micro black holes at the LHC.
     
  14. Nasor Valued Senior Member

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    This is exactly what I learned about virtual particles in my university classes about 7-8 years ago.

    Then temur said...

    Really? I would be interested to hear about that. Was this refutation very recent? Was I learning something old and refuted in my class?
     
  15. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I am almost certain temur was referring to the idea that positrons might be electrons traveling from the future into the past. Not commenting on what I learned about virtual pair production ~45 years ago, which they were still teaching you 7 or 8 years ago.
     
  16. Nasor Valued Senior Member

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    Ahhhh yes, upon a second reading that seems pretty obvious.
     
  17. Gravage Registered Senior Member

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    I see the difference.
    Honestly, thank you for the answer. I really did think that you will never respond, since the first time I was behaving like a warlock (I had to make joke on myself, although my jokes are not very good despite my effort).
    I thought I offended you and other posters here too much who know the knowledge.
    And thank you for those equations, I can't believe I forgot them, after all they are fundamental.

    I don't know if this is the right place, should I create new separate post, because my question is what principles have been proven in quantum physics?
    I'm more familiar with general and special theory of relativity, than with quantum physics, where I'm a total rookie.
    Hawking asked in one of his books which theory is right, since theory of relativity nicely describes macrocosmic sizes (from few meters to cosmic proportions if I remember correctly), while quantum physics describes sub-sub-atomic sizes (atoms and sub-atomic particles and etc.

    The question is why is it so impossible to combine both relativity and quantum physics?
    How many scientists tried to combine them?

    Thanks.
     
  18. temur man of no words Registered Senior Member

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    It was referring to the idea that all the electrons are the same. About the idea being one of Wheeler's, I found this in Feynman's Nobel lecture:

    As a by-product of this same view, I received a telephone call one day at the graduate college at Princeton from Professor Wheeler, in which he said, "Feynman, I know why all electrons have the same charge and the same mass" "Why?" "Because, they are all the same electron!" And, then he explained on the telephone, "suppose that the world lines which we were ordinarily considering before in time and space - instead of only going up in time were a tremendous knot, and then, when we cut through the knot, by the plane corresponding to a fixed time, we would see many, many world lines and that would represent many electrons, except for one thing. If in one section this is an ordinary electron world line, in the section in which it reversed itself and is coming back from the future we have the wrong sign to the proper time - to the proper four velocities - and that's equivalent to changing the sign of the charge, and, therefore, that part of a path would act like a positron." "But, Professor", I said, "there aren't as many positrons as electrons." "Well, maybe they are hidden in the protons or something", he said. I did not take the idea that all the electrons were the same one from him as seriously as I took the observation that positrons could simply be represented as electrons going from the future to the past in a back section of their world lines. That, I stole!
     
  19. prometheus viva voce! Registered Senior Member

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    There are too many more important equations to mention - suffice it to say that we are very good at creating mathematical models of different phenomena.

    I'll reply here and if Ben wants to move it he can.

    Basically, we know that nature is quantum mechanical because quantum theories work incredibly well. For example (and it's the most often quoted example) quantum electrodynamics predicts the anomalous magnetic moment of the electron so accurately that it agrees with experiment to one part in a billion.

    Unfortunately, gravity has turned out to be a bit of a head scratcher - there's no reason why gravity should not be fundamentally quantum and there are objects in the universe that are both very small and very massive, for example black hole singularities, that require a quantum theory of gravity to describe them properly.

    Quantum theories are slightly tricky beasts because they need to be renormalised to extract useful information. This is because the definitions we use for mass and charge turn out to be unphysical and cause divergences in the theory that need to be subtracted away. If you can do this, the theory is renormalisable - you only need to make a finite number of subtractions to make get rid of the divergences. It turns out that the naive way of quantising general relativity leads to a theory that is non renormalisable - that is, you need to make an infinite number of subtractions to get rid of the divergences.

    An approach to quantum gravity that does work mathematically is string theory, but it has yet to produce any predictions that differ from the standard model and general relativity so we can't test is right now. There are yet other approaches like loop quantum gravity which are even less well understood.
     
  20. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    I have an original, alternative explanation for the great numerical dominance of electrons. Not well thought out or demonstrated, but more of an analogy than any scientific argument:

    We and all our observational instruments are traveling "forward" in time as are all the electrons. So it is relatively easy for us to observe a “fellow traveling" electron. Sort of like one jet flying alongside another can easily notice it.

    The positrons are traveling "backwards" in time. The rate of time's absolute flow may be very high, so even a positron with small spatial velocity in our space/time frame may be moving very fast from a POV which is traveling forward in time. Sort of like two jets flying in opposite directions on essentially the same trajectory do not see each other. Neither can observe its own or the other’s “absolute speed” nor is there any way an absolute reference can be defined or observed. (That does not necessarily make its existence impossible. – just that it might as well not exist for us.)

    I.e. we do not notice the rapid absolute flow of time, but only it “relative flow.” I.e. we see how objects move wrt each other or chemical process rates, (iron rusting, etc) but cannot sense any absolute speed of time passing as all our clocks of all types are carried forward with time. Likewise we really cannot tell the absolute speed of the difference between “plus time” time and “minus time” or even if it stopped and started up again “later” (whatever that could possible mean).

    A couple of years ago a small private Embraer jet collided large Boeing plane. All six passengers in the private jet survived, including one NYT author who wrote several articles about it, but all 300 or so in the Boeing died as it broke apart in mid air. (Embraer had to hire extra personnel to respond to the order / information requests in the months that followed and even with the recession, there is a multi- year wait to get a new one. Used ones for immediate delivery sell for more than the new price, or at least did a year ago.)

    Not one person saw the Boeing pass by and at least two of the passenger were looking out the window of the side that had the end of the wing sheared off and neither pilot even saw the approaching Boeing! They initially thought their plane had hit a bird.

    Perhaps there are an equal number of positions existing. Only those few traveling at near the speed of light (in “God’s time frame”) appear to be moving at more modest speed in our space time frame. – I.e. slow enough for us to observer them (That “slow enough” for our instruments to record could, and often does, have an observable positron traveling at near speed of light in our space time frame.)

    As I said – This is just a wild ill-stated idea – definitely “thinking outside of the box.” Perhaps you want to help clarify this vaguely stated idea, or just “shoot it down.” However, until someone explains why all electrons are identical, I am attracted to the idea they are all the same one so I try to help make that idea agree with the observed dominance of electrons.
     
    Last edited by a moderator: Dec 12, 2009
  21. Gravage Registered Senior Member

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    Thank you for these valuable answers.


    About Hawking’s radiation for you, but I also hope other will join the club:
    I have several more questions regarding Hawking’s radiation.
    I’ll try to analyze how Hawking’s radiation really work:

    When pairs of virtual particles (such as an electron and positron) are created in the vicinity of the event horizon, the random spatial and chronal distribution of these particles may permit one of them to appear on the exterior; this process is called quantum tunneling. The gravitational potential of the black hole can then supply the energy that transforms this virtual particle into a real particle, allowing it to radiate away into space.
    In exchange, the other member of the pair is given negative energy, which results in a net loss of mass-energy by the black hole. The rate of Hawking’s radiation increases with decreasing mass, eventually causing the black hole to evaporate away until, finally, it explodes.

    Ok, here is what I don’t understand:
    When an particle falls inside the event horizon of the black hole, shouldn’t this particle simply keep falling until it reaches the centre of the black hole-singularity?
    So, what makes one of many, many particles stop falling towards the singularity of the black hole and than this same particle through quantum tunneling process get out from the black hole’s event horizon?

    And how do we know that photons, electrons, quarks, neutrinos are not destroyed/transformed into energy by the black hole’s extremely intense gravitational field?

    Is the answer photons?
    Since photons are travelling exactly at the speed of light are they one of the particles that are placed exactly at the border of the event horizon, but still can’t get out from the event horizon, and that’s why black hole is still invisible!?

    Quantum tunneling:
    You have mentioned quantum tunneling which is the key process for Hawking’s radiation. So, basically particle (only one of many, many particles) gets enough energy to get into quantum tunneling process for short distances, just enough to get out from the black hole’s event horizon and run away into deep space?
    And since the number of these particles is very small, it is impossible to detect such low level radiation through telescopes, since their radiation is mixed with cosmic background radiation as well the intense radiation created by accretion discs.
    True or false?

    One thing I don’t understand: How come both electron and positron don’t fall into a black hole and they annihilate each other inside the event horizon?
    That would make Hawking’s radiation process impossible to occur.
    Is there a solution for this problem?

    Thank you for your answers, time and patience.
     
  22. Pandaemoni Valued Senior Member

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    In thinking about a matter particle "falling into" a black hole you have to bear the relativistic effects in mind. From our perspective, outside a black hole, Things fall toward the black hole and time slows for them. When they reach the event horizon, time stops. From our perspective they can't move after that, They shift outside our visual spectrum, but they are trapped and smeared along the event horizon, technically always falling toward it but their temporal frame of reference always approaching (but never actually reaching) zero.
     
  23. BobG Registered Senior Member

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    No that's not true, the particle crosses the event horizon in a finite proper time. From the point of view of the particle nothing unusual happens as it crosses the event horizon. A stationary observer would not see it cross however.
     
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