If a body is too dense, I've read it can form a star or collapse into a black hole. Is there a physical limit on density (like the speed of light being a speed limit)?

If no, what are the consequences of magically introducing a super-dense (but not too big) object into the universe? Can it cause the universe to collapse into itself? Will time behave significantly differently near the body?


closed as off-topic by Mołot, Josh King, James, Frostfyre, Twelfth Apr 4 '17 at 19:45


  • "This question does not appear to be about worldbuilding, within the scope defined in the help center." – Mołot, Josh King, James, Frostfyre, Twelfth

  • $\begingroup$ I'm not 100% sure here, but I think that absolute mass is more important than density. Consider that most black holes are presumably comprised mainly of helium and hydrogen. $\endgroup$ – Matt Bowyer Apr 4 '17 at 15:52
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    $\begingroup$ You are probably better much better off at physics.SE (with a slight rewording of the second part) Astronomy.SE might be able to answer the very closely related "is the galactic center black hole the same density as other black holes?" $\endgroup$ – user25818 Apr 4 '17 at 16:10
  • $\begingroup$ I think the physical limit on density can be taken from the Big Bang...all matter existing at a single point. IE, no limit. $\endgroup$ – Twelfth Apr 4 '17 at 16:55
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    $\begingroup$ I rarely agree with closure to another site on WB science questions but this may be one of the few. $\endgroup$ – James Apr 4 '17 at 17:50
  • $\begingroup$ There is a theoretical physical quantity called the Planck density which is far more dense than degenerate matter, ASAIK. $\endgroup$ – catsteevens Apr 6 '17 at 20:16

Yes, if something is too dense, it can become a black hole. The Schwarzschild-radius of an object is 2*M*G/c**2, where c is lightspeed, M is the mass of it, and G is the gravitational constant. If the given mass is inside a sphere having Schwarzschild radius as r, it is a black hole. (it's about 8.8 mm for Earth.)

There is no absolute limit, nothing like: 'no area can have density over 10100 kg/m3!', but compressing things too much is impractical for this reason.

A black hole won't cause the collapse of the universe. In fact, there are a lot of black holes out there, and we are still alive.

Yes, black holes (and any masses by some degree) cause gravitational time dilatation.

But your superdense object can have other problems. If it is not dense enough for a black hole, it may explodes from the internal pressure, or, if you can magically keep it together, it has chance to become some form of degenerate matter:


Supporting a small but very heavy object to prevent it from falling into the planetary core is also a difficult task.

