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Quantum mechanics and black holes

  1. Nov 5, 2009 #1
    Does it make sense to try to combine quantum mechanics with relativity when trying to understand black holes when the mass of the black hole is so huge?

    In nuclear physics people use something called the liquid drop theory to model a heavy nucleus using classical physics not quantum mechanics because the nucleus is big enough that this approximation is reasonable.

    Michael Fothergill
    Last edited: Nov 5, 2009
  2. jcsd
  3. Nov 5, 2009 #2
    as well, one can perfectly use newtonian mechanics by considering the BH as a big massive sphere. It just depends what you want to do...
    However to gain knowledge on the BH itself and its physics one has to go beyond such simplifications.. just like for nuclei (one has to do particle physics).
  4. Nov 5, 2009 #3

    George Jones

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    Most physicists think think that quantum mechanics and general relativity must be combined deep inside a black hole, but no one knows how to do this in usable way.
  5. Nov 5, 2009 #4
    It depends what part of the black hole you want to understand. Some things you can model without quantum mechanics, some things you can even model without much general relativity. Other things you can't.

    "Liquid drop" is semi-classical which means that you sort of try to sweep the quantum parts under the rug.

    And for things outside the black hole there is something called the membrane paradigm which lets you think about the situation using more or less classical physics.
  6. Nov 6, 2009 #5
    I'm no physicist, but I had always guessed that material in the black hole was normal material but the atoms are completely squashed to the point electrons/protons etc are physically touching.

    (I really don’t understand how mathematicians calculate the black hole to have these infinite values like infinite gravity at the centre.)
  7. Nov 7, 2009 #6
    Yes because time stops at the point of the singularity the mass of a black hole does not exist at where the star collapsed. Therefore there is a worm hole that leads to the singularity. Quontum machanics predicts that the worm hole will be of an infinitly small size 1/infinity and matter will be squeezed to a size of 1/intinity which means that even elcotrons will be crushed
    This leads to other questions. Since the singularity has no time or space where does it exist? Is it everywhere in the universe at the same time? (bad word time as time does not exist for a singularity)! Do all worm holes lead to the same singularity? If so will all the matter in the universe end up in the sigularity and would this lead to another big bang?
  8. Nov 7, 2009 #7
    Electrons are point particles and protons are made of point particles so it doesn't make sense to talk about them "physically touching". There is something in quantum mechanics called the Pauli exclusion principle which says that some types of particles can't have the same energy state at the same time. Electrons and protons obey the exclusion principle. Photons do not. The Pauli exclusion principle is why you can build stuff out of protons and electrons but not light. If you have a floor made of protons and electrons, the Pauli exclusion principle will keep you from falling through it. Whereas if you have a floor made of light, it won't.

    The trouble with the Pauli exclusion principle is that if you have enough gravity, it will overwhelm it. It's a consequence of special relativity. To put it crudely, if you have a chair and then you try to compress it, the atoms will try to vibrate faster. Once the atoms in a block of matter start to vibrate at close to the speed of light, they can't vibrate and faster, and so if you press on something hard enough, the atoms will be unable to vibrate fast enough to resist the pressure and resist the pressure that you are putting on it.

    The problem is that once you have enough matter in a black hole, nothing can stop the collapse to infinite density. In order to resist the collapse, you need energy. Energy is equivalent to mass (E=mc^2) and mass has gravity. What happens in a black hole is that the energy that you need to stop things from getting crushed creates enough gravity to crush things even more.

    So what happens is that things get crushed to infinity because there is nothing to stop it using known theories. There may be some unknown theory that will stop it, but it's hard to guess what that might be since we don't have observations to create theories.
  9. Nov 7, 2009 #8
    This is wrong. Sometimes you just have to say "I don't know."

    Once you get to black hole densities and you put in numbers into our current theories to ask what happens, the theories spit out the answer "I don't know."
  10. Nov 8, 2009 #9


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    It is likely that any theory predicting an infinite anything is incomplete. The universe abhors infinities.
  11. Nov 8, 2009 #10
    If time slows and stops from the reference point of the object being sucked in, then wouldn't nothing happen?
  12. Nov 9, 2009 #11
    I will admit it may not be right but its better than an i don't know. Before a theory can be conceived it has to be thought and evaluated and re-though. So let an idea run until it can be disproved.
  13. Nov 9, 2009 #12
    As the universe is still expanding and time stops for the singularity then the distance between them will be increaseing. Time does not stop at the event horizon.
  14. Nov 9, 2009 #13
    Theorist abhor infinities as they can't at the moment accept them. The universe has no knoledge of infinity, probably does not remember Einstine.
  15. Nov 9, 2009 #14
    Matter = energy. I believe that a singularity is made of just energy compressed infinitly.
  16. Nov 18, 2009 #15
    I got a few questions .

    Since the time breaks down inside the bh , that could mean that at the moment of the creation of the same bh inside of it have already merged with all the other bhs and eated the whole universe. Right?

    If soo then inside all the bhs of this universe resides a copy of this universe in mass?.

    If soo then the bhs would be a likely face of a duplicating mechanism?. in terms of mass.

    It makes some sense as the universe is slowly accelerating on the expansion.
    At least in my mind .
    Last edited: Nov 18, 2009
  17. Nov 28, 2009 #16
    The whole thing bows down to what you are describing within the blackhole. If you are discussing mass on the large scale of the blackhole, classical gravity is a good hook. Here physicists are describing the forces of nature which deals with force carrying particles with zero mass or mass with range with just about a few eV/c^2, which other branch of physics you think deals exclusively with such small scales? That's why they try to combine quantum Physics with General relativity. The standard has combined the three forces with the exception of gravity. We know that gravity is brought on by mass. To go back to before Planck's time (which is where is supposed gravity combined with the other three forces), one has to go beyond the standard model. Quantum Gravity is the best candidate to date.
  18. Nov 28, 2009 #17
    "Time stops in the BH" is one of the popular misconceptions.
    No, it does not stop.
    It stops in a coordinate system of a distant observer.
    Freely falling observer reaches the singularity in finite time
  19. Nov 30, 2009 #18
    Hey, we will have to be careful how we present things here. you will have to understand there are people on this forum who take thing as we write them. Things dont get crushed to infinity. Saying this is illogically irrelevant. infinity has two subsets, large and small and we will have to point that out. How you put it is "MATTER GET COMPRESSED TO INFINITE DENSITY". This is simply saying since the mass is large, we see from our secondary school formula for density (D=M/V) that if the volume shrinks toward zero, the density becomes infinite. However, the volume here depends on the radius of the BH(which we assume is a sphere). Knowing the volume of a sphere is given by V=4/3pi r^3, playing with our equations a little we get the density D=3M/4pi r^3. Here now its clear the we are actually saying as the radius goes to 0, the density becomes infinite.
  20. Nov 30, 2009 #19
    Inside a black hole, it's not. I don't think a black hole has a well defined volume.
  21. Nov 30, 2009 #20
    No, as said before, matter crosses the horizon in a finite proper time. Yes, the volume of the BH is not well defined (you cant use a formula for a volume of a sphere because it is applicable to the euclidean space only). But even if we ignore this fact, the position of the event horizon is irrelevant: matter is compressed to extremely high densities DEEP INSIDE THE BLACK HOLE.
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