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What's the difference between a point and singularity?

  1. Sep 2, 2015 #1
    When an neutrino is observed, it becomes a point particle, but it has mass. Is that mass not concentrated into a point? Why doesn't it collapse down to a micro black hole? What's the difference between a neutrino and a micro black hole of the same mass? I know it has 1/2 spin and other properties, but why?

    In a similar thought, what would happen if you tried the double slit experiment with micro black holes? Does relativity and QM agree in the expected result or is it too fuzzy to even lean one way or the other (blotches or interference patterns) in one or both theories? I would expect relativity to predict no interference pattern. Would string theory provide a prediction?
  2. jcsd
  3. Sep 2, 2015 #2


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    An exact answer would really require quantum gravity, but a rule of thumb is that something is not a black hole unless its Schwarzschild radius is larger than its Compton wavelength. The idea is that the object can't be a black hole unless it is naturally localized within the length scale associated with gravity. This puts a lower bound on the mass of a black hole of around the Planck mass, which is around 20 micrograms.

    A micro black hole would evaporate extremely quickly. Although the semiclassical treatment of Hawking radiation isn't really valid for a Planck-scale black hole, it predicts that the entire mass of the black hole is radiated away in a few emission events. If we assume for the sake of discussion that we could do the experiment while the black hole still exists, since our criterion above was that a black hole is quantum mechanically localized, we shouldn't expect to find measurable interference. A Planck mass is around 1/3 the mass of a human eyelash hair, which we don't expect to behave quantum mechanically either. Any theory of quantum gravity that actually agrees with known quantum mechanics would lead to the same result.
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