3
$\begingroup$

As has been pointed out by some scientists, that black holes aren't stable bodies but are eternally collapsing objects. How does one come to such a conclusion? Is hawking radiation related to this in some way?

$\endgroup$
  • 1
    $\begingroup$ Wrt "eternally collapsing", they are probably referrring to the fact that in the reference farme of an external observer, time dilation prohibits matter from ever reaching the event horizon (the "surface") of the black hole. However, this is only for the external observer; an observer falling with the collapsing matter would cross the horizon and reach the center in a finite time. Wrt "not being stable", you're probably right that they are referring to Hawking radiation which presumably makes the black hole slowly "evaporate". $\endgroup$ – pela Jun 2 '15 at 7:42
  • $\begingroup$ As to your first question, it's very similar to this one: physics.stackexchange.com/questions/5031/… As for your 2nd question, that's a no. The eternal collapse which may not be true (except from the point of view of an observer - see Pela's answer) and the Hawking radiation are entirely separate things. $\endgroup$ – userLTK Sep 1 '15 at 10:25
1
$\begingroup$

With respect to "eternally collapsing", they are probably referrring to the fact that in the reference frame of an external observer, gravitational time dilation prohibits matter from ever reaching the event horizon (the "surface") of the black hole.

Denoting the radius of the event horizon $r_\mathrm{S}$ (for "Schwarzschild radius"), time runs slower by a factor of $(1 - r_\mathrm{S}/r)^{-1/2}$ for an observer at a distance of $r$. As $r$ approaches $r_\mathrm{S}$, this factor goes to infinity, i.e. the observer will never reach $r_\mathrm{S}$

However, this is only for the external observer; the falling observer would cross the horizon and reach the center in a finite time.

With respect to "not being stable", you're probably right that they are referring to Hawking radiation which presumably makes the black hole slowly "evaporate". Near $r_\mathrm{S}$, pairs of virtual particles are being created and can be turned into real particles by the gravitational field. If they avoid falling into the black hole, energy is taken away from the hole, reducing its mass.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.