# A few questions about black holes **URL:** \ **Category:** Factual Questions\ **Created:** [March 13, 2013, 6:52pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841 "2013-03-13T18:52:57Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![Gnarlswood](https://avatars.discourse-cdn.com/v4/letter/g/e9a140/32.png) [@Gnarlswood](https://boards.straightdope.com/u/Gnarlswood)\ **Post date:** [March 14, 2013, 10:53am UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/21 "2013-03-14T10:53:12Z")
If you take me to the biggest galaxy core available in universe, and if i had some uncurable cancer maybe i would try it! Maybe a white hole spits me out and i can tell you? I know the einstein equations look not so complicated in tensor form, but outwritten they baffle me absoluteley! 10 coupled nonlinear partial differential equations? Not withe me. Also not the linearized ones. I have just a very basic understanding of differential equations and tensor calculus, so i can follow some of the simple examples, but i could not calculate them by myself. So i will not even try to make sense out of the Einstein equations! But i will gladly try the books you mentioned, if they provide a good solid base to get me further. Are there some mathematical explanations in it? That helps me a lot. If it goes not too far… By the way, i love mathematics, but everything more than basic algebra i have learned by myself. I know how the techniques work, but i have no training in using them. At least most of the time i know what happens why in a calculation. Thanks to all ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [March 14, 2013, 3:47pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/22 "2013-03-14T15:47:43Z")
> [@Gnarlswood](#): > > If you take me to the biggest galaxy core available in universe, and if i had some uncurable cancer maybe i would try it! Maybe a white hole spits me out and i can tell you? Hey, this [bloody and long piece of spaghetti](https://en.wikipedia.org/wiki/Spaghettification) is trying to tell me something! ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [March 14, 2013, 4:34pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/23 "2013-03-14T16:34:01Z")
You are right that there may or may not be a singularity at the center of a black hole. As moriah says the math starts to break down. It may be that there is a whole new set of physical laws that start taking effect under those circumstances, and which are an expansion on relativity in the same way relativity is an expansion on Newtons laws. Unfortunately we will probably never know because there are no naked singularities, and so all of the physics of what goes on inside is shrouded by an impenetrable event horizon. From the outside view treating it as a singularity appears to be correct. Which as I am writing this brings up a question I have for physicists. Suppose that instead of matter being crunched down to a singularity in a black hole, other unknown forces take over and it is crunched down to a small but finite volume (one that is still small enough to form an event horizon). Clearly with accurate enough measurements of the gravitational field this difference would be detectable, but given that our current observations of black holes are indirect at best, is there any way to rule this out, or at least put an upper bound as the the size of the core relative to the size of the event horizon, with current technology? ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [March 14, 2013, 5:34pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/24 "2013-03-14T17:34:07Z")
> [@Buck\_Godot](#): > > Which as I am writing this brings up a question I have for physicists. Suppose that instead of matter being crunched down to a singularity in a black hole, other unknown forces take over and it is crunched down to a small but finite volume (one that is still small enough to form an event horizon). Clearly with accurate enough measurements of the gravitational field this difference would be detectable, but given that our current observations of black holes are indirect at best, is there any way to rule this out, or at least put an upper bound as the the size of the core relative to the size of the event horizon, with current technology? We would not expect to see any difference in the curvature of spacetime around the mass whether it is a singularity or just a very compactified, as we would expect any theory of quantum gravity to reduce to the classical limit for macroscale behavior. The primary difference would be thermodynamic properties and behavior. Stranger ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [March 14, 2013, 6:21pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/25 "2013-03-14T18:21:22Z")
> [@Stranger\_On\_A\_Train](#): > > We would not expect to see any difference in the curvature of spacetime around the mass whether it is a singularity or just a very compactified, as we would expect any theory of quantum gravity to reduce to the classical limit for macroscale behavior. The primary difference would be thermodynamic properties and behavior. > > Stranger I thought we were currently having difficulty in getting quantum physics to reduce to classical behavior at the macroscale already, let alone trying to work gravity in. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [March 15, 2013, 12:18am UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/26 "2013-03-15T00:18:42Z")
> [@Lumpy](#): > > I thought we were currently having difficulty in getting quantum physics to reduce to classical behavior at the macroscale already, let alone trying to work gravity in. In general, quantum behavior reduces nicely to classical mechanics in fields of optics, electrodynamics, molecular chemistry and reaction kinetics, et cetera. Many everyday phenomena such as the photoelectric effect or chemistry are fundamentally dependant upon quantum mechanics, but given the classical (and often empirical) models that proceeded them you don’t really have to apply quantum mechanics to solve a problem any more than you need to understand the nuances of spherical cartography to find coordinates on a map. Quantum phenomona mush into essentially classical mechanics on a macroscale in a process physicists call “decoherence” for all but the most unusual or extreme conditions. As an example, we got by with classical optics including the electromagnetic from the time of Euclid all the way up to Planck without ever realizing that light is not actually a continuum. (To be fair, Isaac Newton postulated light in individual particles called “corpuscles” which held sway for a while but was unable to cope with the wave-like behavior of electromagnetic fields.) Maxwell’s eqautions, the classical unifed model for electromagntics, serves even today for nearly every practical application on scales larger than an atom. Stranger ---
**Author:** ![Gnarlswood](https://avatars.discourse-cdn.com/v4/letter/g/e9a140/32.png) [@Gnarlswood](https://boards.straightdope.com/u/Gnarlswood)\ **Post date:** [March 15, 2013, 1:23pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/27 "2013-03-15T13:23:26Z")
> [@Asympotically\_fat](#): > > Here’s an interesting factoid: > > When studying a black holes, unless your particularly interested in the collapse process, the collapse is usually ignored. This is for several good reasons: firstly it has been shown that black holes quickly settle down in to the highly symmetrical objects we all know and love (this is the famous no hairs theorem), so they are pretty much the same objects governed by a small set of parameters regardless of how they formed; secondly it vastly simplifies the spacetime under study to a highly-symmetric and stationary one. When you ignore collapse you study a black hole as an object that has existed for all time i.e. an eternal black hole. > > Now what happens when you trace the path of test particles, such as light rays, travelling radially outwards from the event horizion an eternal black hole back in time (assuming they have always been travelling radially outwards)? To an observer in the exterior region of the black hole they seem to have been travelling outwards for all time, but this does not actually give you the maximal extension of their paths back in time. If you take the maximal extension, you actually find that they originate from inside the event horizon of a **white hole** and (if applicable) they take finite amount of proper time (i.e.time from their own perspective) to travel from the white hole interior region to the black hole exterior region. That´s quite interesting. I know you can´t tell the difference between a frozen star as i call them and an actual black hole, so treating them as eternal black holes is just for convenience? And regarding to my question in the OP, how can anyone say black holes will decay in (insert some random long time) ? Does Hawking radiation also work with black holes as they are from our view, including the collapse process? Thanks for all your input! ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [March 15, 2013, 6:18pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/28 "2013-03-15T18:18:30Z")
> [@Gnarlswood](#): > > That´s quite interesting. > > I know you can´t tell the difference between a frozen star as i call them and an actual black hole, so treating them as eternal black holes is just for convenience? Yes treating them as an eternal black hole is a matter of convenience, but as I say the existence in general relativity of a 2-D spacelike surface where light is ‘frozen’\*, under reasonable energy conditions\*\* and also where causality is obeyed, necessarily implies that the spacetime is singular. This is the null version of the Penrose-Hawking singularity theorem. So a frozen star IS a black hole and there IS a singularity\*\*\* as long as we disallow weirdness such as negative energy and time travel. > [@](#): > > And regarding to my question in the OP, how can anyone say black holes will decay in (insert some random long time) ? > Does Hawking radiation also work with black holes as they are from our view, including the collapse process? > > Thanks for all your input! The physics behind the decay of black holes is slightly cobbled together, basically it relies on a few fairly sensible assumptions because the actual solutions that describe the dynamics of the evaporation process aren’t know (AFAIK). The idea is that Hawking radiation can be shown to cause a negative energy flux across the event horizon (simply speaking black holes radiate energy outwards) and, as long as we assume that energy is conserved in the same way as it is in the fully classical descriptions of black holes, this means that the mass must reduce to reflect the loss of energy. We can then estimate the flux of energy and get an order of magnitude estimate for the rate of mass loss of a black hole. As the rate of mass loss is inversely proportional to the mass squared, the smaller the black hole gets, the more mass it loses, so it will eventually evaporate if the mass is not replaced. Hawking radiation specifically comes from the theory of black holes which have formed by gravitational collapse at some finite point in the past. The existence of the white hole in the solution for eternal black holes actually throws a massive spanner in the works when trying to calculate whether they emit Hawking radiation. \*the technical term is a “trapped null surface” \*\*I think Hawking radiation might not obey these energy conditions, so to be fair there is a bit of latitude \*\*\*The exact nature of this singularity though may be more elusive, as singularity theorems merely prove the existence of singularities but don’t usually say much about how they ‘appear’ in the spacetime. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [March 15, 2013, 7:57pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/29 "2013-03-15T19:57:05Z")
> [@Asympotically\_fat](#): > > Hawking radiation specifically comes from the theory of black holes which have formed by gravitational collapse at some finite point in the past. The existence of the white hole in the solution for eternal black holes actually throws a massive spanner in the works when trying to calculate whether they emit Hawking radiation.. Well if you presume that black holes radiate, then there’s no such thing as an eternal black hole; and if there are eternal black holes than they can’t radiate. Or am I missing something? ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [March 15, 2013, 10:30pm UTC](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841/30 "2013-03-15T22:30:21Z")
> [@Lumpy](#): > > Well if you presume that black holes radiate, then there’s no such thing as an eternal black hole; and if there are eternal black holes than they can’t radiate. Or am I missing something? Because the calculations are done in quantum field theory in curved spacetime, the back reaction of Hawking radiation on the black hole isn’t factored in at this stage. [Previous page](https://boards.straightdope.com/t/a-few-questions-about-black-holes/652841.md?page=1)