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The black hole and the milk

  1. Aug 23, 2004 #1
    Nothing can escape from a black hole; so no experiment results can be report out from a BH.
    If I say that a BH is full of milk how can be proven I'm wrong?

  2. jcsd
  3. Aug 23, 2004 #2


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    I'm not sure if it can be proven wrong. An interesting characteristic of the structure of a black hole is its relative simplicity compared with most objects on that scale. A black hole is, in a sense, simple because it can be described using very small number of fundamental properties. In this way it is more like a fundamental particle, which can be described fully using certain attributes such as charge, mass, spin, etc. I think this is the basis of the so called "no hair theorem". Furthermore, I believe that this lack of complexity (once you know a handful of properties of the black hole, you know everything), is an aspect of the "loss of information" problem: My astronomy prof noted that simply observing a black hole tells us nothing about the nature of the original matter that "collapsed" to form it. She even said "it could have been peanut butter for all we know", but she was merely making a point. The loss of info problem is further compounded by the fact that nothing that goes in ever goes out. I've read that this has surprisingly drastic implications (something Hawking was talking about). That's all I know for sure. I'm speaking based on what I learned in a first-year astronomy course, I certainly haven't studied black holes in detail, or GR in general.

    Can anyone clarify whether one can say that a black hole is "full of" anything, because it is, after all, a singularity. What is this "inside" that people refer to when they talk about matter entering a black hole?
    Last edited: Aug 23, 2004
  4. Aug 23, 2004 #3


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    Perhaps by looking at the processes that create a black hole one could say it is not likely full of milk. But I'm not sure it could be proven.
  5. Aug 23, 2004 #4


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    You can't be proven wrong. A giant ball of milk, if compressed adequately, would form a black hole. It might seem reasonable to say that a black hole formed from a ball of milk "contains" that milk.

    On the other hand, you could never see the milk inside it ever again, and, in physics, it's considered a faux pas to speak of what you cannot observe. While no one can disprove that your black hole has milk inside, you also lack the ability to prove it.

    There is a famous theorem called the "no hair" theorem, which states that the only physically observable characteristics of a black hole are its mass, charge, and angular momentum. Black holes literally have no other properties besides those three quantities, no matter what kind of matter formed them.

    You can think of the formation of a black hole -- the actual collapse -- as an event that destroys some of the characteristics of the collapsing matter. Analogously, if you were to heat any two substances up sufficiently -- milk and orange juice, say -- they would break down into protons and neutrons and electrons. You could not tell the two substances apart anymore except perhaps by counting the number of neutrons vs. protons, and even that knowledge is not sufficient to conclusively indentify one substance as milk and the other as orange juice.

    In collapsing into a black hole, matter loses all its characteristics except mass, charge, and angular momentum. There is no way to tell a "milk" black hole from an "orange juice" black hole, even in principle.

    - Warren
  6. Aug 23, 2004 #5
    By going into the black hole yourself. But that may hurt. :surprise:

    If you want to stay on the outside, and live, then it is impossible to say what is inside the event horizon. All you can tell is how much mass is inside and what the angular momentum is. You can't tell what form the matter takes.

  7. Aug 24, 2004 #6
    Obviously my question was a joke.
    But, more seriously, the BH internal status it the only - as far as I know - theory of the physics that can't be verified through experiment. I'm not sure if it is still "physics" or "philosophy".

  8. Aug 25, 2004 #7


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    Physics. We have mathematical models that describe BHs and that can give us testable hypotheses about what properties such an object would have. Then you couple that with the observation evidence of objects that display such properties.

    A strong description of a BH singularity (and perhaps the workings of spacetime within the event horizon?) is still lacking though.
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