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Mass and space

  1. Sep 21, 2003 #1
    A question has been bothering me for some time and I haven't been able to find an anwser to it yet, so I was wondering if you might know the anwser.
    Why is it that for example a white dwarf hasn't become a black hole, while other stars which have a lot less mass do become black holes?

    I find this very weird, since white dwarfs have a lot of mass in one "spot".
  2. jcsd
  3. Sep 21, 2003 #2


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    What gave you this idea? Black holes (that come from collapsed stars) have more mass than neutron stars, which have more mass than white dwarfs. There is a theoretical concept of mini black holes, which were born at the time of the big bang, but none have been detected so far.
  4. Sep 22, 2003 #3
    Black holes have more mass than Neutron stars? How can this be? A black hole's mass would be almost completely negative, wouldn't it, since it is a deep indentation in the fabric of spacetime?

    Please bare with me if this is a stupid question, I am no expert in the field of astrophysics (obsiously ).
  5. Sep 22, 2003 #4


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    Er... no? By GR, indentations are assoiciated with positive masses....

    It's important to clairfy this point. What makes a BH a BH is not so much it's mass, than it's density.
  6. Sep 22, 2003 #5


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    BH (in 'theory'): yep, could have 'any' mass.

    BH (observed/inferred): >~1.44 M sol, consistent with the stellar evolution part of astrophysics.

    Nothing, in theory, that says a BH can't be less massive than a neutron star, and that in turn less massive than a white dwarf. However, how would any such form?

    Of course, if you keep adding mass to a white dwarf, at some point it will become a neutron star; similarly a neutron star will become a BH once its mass exceeds the Chandrasekhar limit.

    Expect some fireworks along the way.
  7. Sep 23, 2003 #6
    Talking of mini black holes - they're the primordial black holes which were created in abundance during the initial moments after the big bang. They are nowhere to be found today - why? Did they die out because of the famous Hawking radiation? We don't know yet - but there's new research around which suggests they were not frittered away by nature - they formed stable bound states - which interact only gravitationally! These stable bound states have been named holeums. The research papers can be seen online at:

    http://xxx.arxiv.cornell.edu/ftp/gr-qc/papers/0309/0309044.pdf [Broken]
    Last edited by a moderator: May 1, 2017
  8. Sep 23, 2003 #7


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    IIRC, the smaller the BH, the faster it evaporates. So, the smallest primordial BHs would be gone by now. But some of the slightly larger ones (yet still much smaller than stellar-formed BHs) should still be around.
  9. Sep 23, 2003 #8
    I thought General Relativity was what showed that gravity was to be considered as "Negative Energy" (which is what leads to people concluding that the net energy of the Universe is zero - since the positive energy of the bodies is cancelled out by the negative energy of the gravitational field).

    A little more help please.
  10. Sep 23, 2003 #9
    Only point out that the limit that separate a neutron star to form a black hole is not the Chandrasekhar limit, but the Oppenheimer-Volkoff limit, that is equal to 3 solar masses
  11. Sep 24, 2003 #10
    Hmmm...agreed - the smaller the BH, the faster it evaporates. But - there's one very important thing to consider: this will happen only if the BH is an isolated one! Primordial BHs whould not evaporate if they were densely packed together - as in the immediate aftermath of the Big Bang. This is analogous to the behavior of neutrons - they are unstable in the free state and disintegrate, but are eminently stable inside a bound state - an atom (except for the heavier atoms like uranium and co). The primodial BHs would then be free to form stable gravitational bound states of their own - which can't be detected as yet as we still don't have gravitational wave detectors.
    What this means is: the primordial BHs did not die out - they hung around in the form of stable bound states...and are out there, waiting to be detected. Smells like dark matter? :)
  12. Sep 26, 2003 #11


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    What would their (present day) space density be?
    Last edited: Sep 26, 2003
  13. Sep 26, 2003 #12
    It can't be too high. Otherwise we would have observed a lot more gravitational microlensing.

    And I have qualms about this since it was formulated using nonrelativistic quantum mechanics. I'm curious if relativistic quantum mechanics forbids such states to exist. After all, the black holes that exist in bound states would eventually coalesce if one puts relativity in the picture (gravity wave bleeding)
  14. Sep 26, 2003 #13


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    There would seem to be rather a lot of 'otherwise's (with much dependency on their mass distribution); e.g.
    -> wakes in the ISM
    -> different initial stellar mass distribution
    -> peculiar objects, esp in young globular and open clusters
    -> strange transients

    If we're talking about holeums (which are approx the size of a nucleus or an atom), none of the above apply (assuming no significant coalescence). But different 'otherwise's could be listed...
  15. Sep 26, 2003 #14


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    You don't have to measure the energy of the universe as zero -- you can simply define it as such. Energy is a conservative quantity, so you can simply call any energy level you'd like zero, and measure other energies relative to it. If you want the energy of the universe to be zero -- you got it. It's free.

    And besides -- gravity is not energy!

    - Warren
  16. Sep 29, 2003 #15
    Gravity is not energy? I thought Einstein had postulated that non-uniform motion and gravity are precisely the same, since they are both warpings of spacetime, and that gravitational pulls were taken as "negavite energy" since they produce a "friction" of sorts on any object that tries to leave them. Thus, more positive energy is needed to escape stronger gravity wells, since they negate that positive energy.

    I suppose it suffices to say that, if gravity is indistinguishable from accelerated motion, then all objects that attempt to escape a gravitational pull are trying to accelerate away from previous acceleration in the opposite direction (otherwise the "friction" wouldn't exist, would it?).

    A little more help on this topic please.
  17. Sep 29, 2003 #16
    No. Gravity is the curvature of spacetime. Spacetime curves because of energy density. In other words, a certain amount of energy density will warp spacetime a certain amount and thus produce what we call gravity. Saying gravity is energy is equivalent to saying a wave is energy. Waves carry energy, yes, but waves and energy are two different, yet related concepts.
  18. Sep 29, 2003 #17
    I didn't think Gravity was a "real" force though, I thought it was an inertial force. It seems we're going to drift to antimatter. Antimatter certainly has positive *energy* and positive *inertial mass*; people want to verify experimentally that it also has positive *gravitational mass* - i.e., that the equivalence principle holds for antimatter. Nobody in their right mind thinks antimatter could possibly have negative gravitational mass, but it's nice to check things experimentally, even when you feel sure they're true!

    We say SPACE is positively curved if the angles of a triangle add up to more than 180 degrees, and we say it's negatively curved if they add up to less than 180. For a 2-dimensional example, compare the surface of an egg to the surface of a saddle.

    The geometry of space in the universe as a whole is rather lumpy,
    thanks to the lumps of matter (e.g. galaxies) that bend it.
    However, people are very interested to know whether, if you ignore
    the lumps, space is on average positively or negatively curved -
    or flat. It's pretty close to flat, that's all we can say for
    sure... though some indications suggest it might be a bit negatively

    And this is why GR is so hideously non-linear and hard to solve. The geometry dictates the configuration of the mass in the universe, but the mass configuration changes the geometry.

    Oh one more thing. A white dwarf keeps a constant size as it evolves. It radiates away its heat and grows cooler and dimmer simultaneously. After billions of years, a white dwarf becomes so dim it is difficult to detect, wouldn't it? A 2 solar mass core of a star contracts after using its nuclear fuels. Explain why we can be sure that the star will not become a white dwarf? Hmm?
  19. Sep 29, 2003 #18


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    I'm not sure I understand what you're asking, but here goes:
    a) because the gravitational force making it contract would be greater than the electron degeneracy pressure resisting contraction
    b) because gravitational contraction cannot 'ignite' the nuclear ashes which remain in the core
    c) because a 2 solar mass collapsing core bounces off the incipient neutron star
    d) because the pressure from the neutrinos is great enough to slow down the collapse so that a neutron star can be formed
  20. Sep 29, 2003 #19
    But we have checked trillions of times that antimatter has positive gravitational mass. Indeed, everytime we create antimatter, we have to be careful to confine it lest it have a slow tendancy to drift down...especially when we create antihydrogen.

    As far as we can tell space is more or less flat, however indications point towards some scalar field operating on cosmic distances driving forward an accelerative expansion, that would require a very observable deviation from flatness if it was purely the curvature of spacetime at fault.

    And the math of GR is difficult to solve indeed, but not solely because it is nonlinear.

    And while in the right forum, the last part about the white dwarf is slightly off topic. We can be sure because above a certain limit, 1.4 solar masses more or less, the degeneracy pressure from electrons obeying Fermi-Dirac statistics is no longer enough to counteract the weight of the core itself, hence the star "fuses" into a neutron star.
  21. Sep 29, 2003 #20
    Thanks Nereid and of course Brad.

    But one more thing:

    I just read an article about Quantum gravity and it says "Gamma ray bursts -- those terrific and mysterious flashes of high-energy light now considered to be probes to the farthest reaches of the Universe and earliest moments of time -- may have yet another secret to reveal: quantum gravity." What would gravity be described in there? Energy? No?
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