# Black hole ramblings **URL:** \ **Category:** In My Humble Opinion\ **Created:** [October 5, 2020, 7:48pm UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282 "2020-10-05T19:48:35Z")\ **Posts on this page:** 14\ **Page:** 3
**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:** [October 23, 2020, 5:57pm UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/41 "2020-10-23T17:57:06Z")
I shoudl add though that at the point where the black hole does evaporate quantum effects are going to be significant so the “by hand” model cannot be expected to hold. The point I would make is for an evaporagting balck hole observed by a faraway observer, the infalling object approaches the event horizon as t goes to the point of evaproation, rather than as t goes to infinity. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 23, 2020, 11:45pm UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/42 "2020-10-23T23:45:45Z")
I thought black holes are always evaporating. They just do it really, really slowly. I mean, in order to completely evaporate sometime super far in the future they either need to be evaporating all the time or there has to be some arbitrary time when they start and the latter would be weird. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [October 24, 2020, 12:42am UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/43 "2020-10-24T00:42:56Z")
Black holes lose mass through Hawking radiation, but they gain mass though all the shit they vacuum up (beyond all else, there is a ridiculous amount of neutrino flux in the universe for them to suck in). The current thinking is that a black hole larger than a few microns will absorb more cosmic background radiation than it loses from it Hawking radiation, so any naturally formed black hole will get larger. Of course, the region inside the event horizon is a physical unknown. It may somehow be possible that there is, therein, some dynamic that would eventually lead to the bubble bursting. The only solid data we have to go on is conjecture. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2020, 1:01am UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/44 "2020-10-24T01:01:31Z")
Does that mean a black hole will never evaporate but rather they will all grow in size forever? (understanding that we are talking about ridiculously distant future events) I was under the impression that they will all, eventually, poof out of existence too but not till loooong after all the stars are gone and pretty much everything else too. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2020, 1:42am UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/45 "2020-10-24T01:42:34Z")
> [@k9bfriender](#): > > Space itself is being pulled into the black hole at the speed of light at the event horizon. What does this mean? I know we have seen frame dragging where a massive object (like a planet) sorta twists space around it a bit from its movement…kinda like twisting your foot on an area rug. But what does it mean for a BH to hoover up space at light speed? Is new space being made behind what is being drawn in? It sounds like pulling a rug. The earth is eight(ish) light minutes from the sun…if the sun became a BH does that mean in eight minutes the earth would be drawn to the BH? I am pretty sure no. If the sun poofed into a black hole the earth would still merrily orbit it as usual (ignoring other ill-effects for those on the planet). So what does it mean to have space falling in at light speed to a black hole? ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [October 24, 2020, 2:00am UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/46 "2020-10-24T02:00:49Z")
> [@Whack-a-Mole](#): > > I am pretty sure no. If the sun poofed into a black hole the earth would still merrily orbit it as usual (ignoring other ill-effects for those on the planet). Of course. If the sun suddenly turned into a black hole, the orbital dynamics around it would remain exactly the same as long as its mass remained the same. What @k9bfriender is referring to is a valid (or at least, plausible) interpretation of the field equations at the limit of the event horizon (not far away from it). ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2020, 2:07am UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/47 "2020-10-24T02:07:39Z")
But if I am dragging space in at light speed…even only the space very near me…is new space spontaneously forming behind it? Put another way…what is being dragged in, where does it come from and is it an infinite resource? The [Big Rip](https://en.wikipedia.org/wiki/Big_Rip) is one theory of what happens to the universe where spacetime itself is ripped apart as space expands. Is this happening at the event horizon of a black hole? ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [October 24, 2020, 2:37am UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/48 "2020-10-24T02:37:06Z")
> [@wolfpup](#): > > Of course. If the sun suddenly turned into a black hole, the orbital dynamics around it would remain exactly the same as long as its mass remained the same. I think that is not correct. Nothing orbits the sun, each planet orbits a barycenter, based on it gravitational pull upon the sun. The barycenter of Jupiter’s orbit lies above the “surface” of the sun by about 30K miles. Here is a [graphic depicting the motion of the sun](https://skydogs.files.wordpress.com/2011/04/motion-of-barycenter-of-the-solar-system-relative-to-the-sun.jpg) within the solar system. Now, take that motion and apply the gradient of a black hole the mass of the sun (a few miles in radius) to that motion. Suddenly, the gradient experienced by the inner planets is changing drastically. Over a few centuries, Mercury’s orbit would become increasingly elliptical, leading to it either being flung out of the solar system or pulled into the black hole. The other inner planets would be less affected, but their orbits would become increasingly erratic. Jupiter and the outer planets would eventually be perturbed by the instability of the inner planets. The solar system would most decidedly not remain “exactly the same”. ---
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [October 24, 2020, 6:05am UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/49 "2020-10-24T06:05:49Z")
> [@eschereal](#): > > I think that is not correct. Nothing orbits the sun, each planet orbits a barycenter, based on it gravitational pull upon the sun. The barycenter of Jupiter’s orbit lies above the “surface” of the sun by about 30K miles. > > Here is a [graphic depicting the motion of the sun](https://skydogs.files.wordpress.com/2011/04/motion-of-barycenter-of-the-solar-system-relative-to-the-sun.jpg) within the solar system. Now, take that motion and apply the gradient of a black hole the mass of the sun (a few miles in radius) to that motion. Suddenly, the gradient experienced by the inner planets is changing drastically. I don’t see why any of that is relevant, but I could be wrong and if so, perhaps someone can explain why. If the sun were to collapse into a black hole of identical mass, its center of mass would remain exactly where it is now. You’ll note that in **[the simple two-body problem](https://en.wikipedia.org/wiki/Barycenter#Two-body_problem)**, the equation giving the location of the barycenter is determined only by the masses of the two bodies and the distance between them. The radii of the bodies is irrelevant. In the case of two bodies like the sun and Jupiter, the radius of the sun is irrelevant except insofar as it tells us whether the barycenter is inside or outside the sun’s present radius. It’s certainly true that a black-hole sun would be quite small, and there would be an increasingly great gravitational gradient as one approached it from where its outer layers currently reside, with an immense gravitational gradient at the event horizon. But the planets all orbit well outside this region, where as far as I can see the gravitational gradient should remain the same as now. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [October 24, 2020, 3:15pm UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/50 "2020-10-24T15:15:02Z")
> [@Whack-a-Mole](#): > > Does that mean a black hole will never evaporate but rather they will all grow in size forever? (understanding that we are talking about ridiculously distant future events) Forever, no. For a long time, yes. The cosmic microwave background is cooling off as the universe expands, as well as any neutrinos or other particles or radiation that it is currently absorbing. Space is a few degrees above absolute zero, and any non-primordial black hole is going to be trillionths of a degree, and probably a whole lot less. A supermassive black hole is going to need some negative exponents to describe its temperature. Give it a couple few tens or hundreds of trillion years (or more), and the CMB will have cooled off enough that these black holes will start to be warmer than their surroundings, and then they will actually start to shrink. Black holes are going to be around for a very, very long time. They won’t start really going poof until an age of the universe that requires exponential notation to express. > [@Whack-a-Mole](#): > > But what does it mean for a BH to hoover up space at light speed? Is new space being made behind what is being drawn in? Space _is_ being created all the time, that’s what dark energy is. But, space is also infinite in all ways that matter. It’s not a physical quantity, not a particle or a distinct _something_ that you can point at. Anything embedded in space is being dragged along with it. A rather imperfect analogy is an infinite shallow lake with a hole in a spot on the bottom. This analogy is close enough that some useful experiments are done with this set up, where the water flowing out through the plug at greater than the speed of sound in water acts as an event horizon. It doesn’t really explain what is going on behind the scenes(which to be honest, we don’t actually fully understand anyway), but does demonstrate the effect that it has on objects floating (embedded) in it. A similar horizon exists a few billion light years away, where the expansion of space is carrying things away from us at greater than light speed. If you observe an object cross _that_ horizon, it would act pretty much the same as an object falling into a black hole. > [@Whack-a-Mole](#): > > It sounds like pulling a rug. Not really, as a rug is a discrete object, and space is infinitely flexible and stretchable. > [@Whack-a-Mole](#): > > if the sun became a BH does that mean in eight minutes the earth would be drawn to the BH? It _is_ being drawn toward the sun, and if the sun _bamfed_ into a black hole, then it would be drawn towards that black hole in exactly the same manner. The fact that we are in orbit keeps us from falling in. > [@Whack-a-Mole](#): > > So what does it mean to have space falling in at light speed to a black hole? It means that anything embedded in space (which is everything we know or can conceive of) is also falling through the event horizon at the speed of light. > [@Whack-a-Mole](#): > > Put another way…what is being dragged in, where does it come from and is it an infinite resource? Spacetime, the inherent properties of spacetime, and yes. > [@Whack-a-Mole](#): > > The [Big Rip](https://en.wikipedia.org/wiki/Big_Rip) is one theory of what happens to the universe where spacetime itself is ripped apart as space expands. Is this happening at the event horizon of a black hole? No, that is a hypothesis that dark energy is not a constant, and is instead increasing. If this is the case, then eventually, the expansion of space will overcome gravitationally bound systems like galaxies, then solar systems, then electromagnetically bound objects like people and molecules, then eventually strong force bound objects like nuclei and the protons and neutrons themselves. It is _not_ a favored hypothesis, fortunately, but we don’t know enough to conclusively rule it out. In some ways, it is a bit similar to the spaghettification that you would experience as you approached the “singularity”, but only superficially, and for different reasons. Spaghettification can happen outside of a black hole as well, and in fact will for all but the most massive of ones. > [@eschereal](#): > > I think that is not correct. Nothing orbits the sun, each planet orbits a barycenter, based on it gravitational pull upon the sun. The barycenter of Jupiter’s orbit lies above the “surface” of the sun by about 30K miles. It has nothing to do with the size of the sun, only its mass and the location of that center of mass. The barycenters would still be the same, they would trace the same ellipses, but _all_ of them would now be outside the event horizon, as the event horizon of the sun would only be a few miles wide. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2020, 3:37pm UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/51 "2020-10-24T15:37:19Z")
Thanks for the detailed response but you have not answered what it means for a black hole to hoover up space at light speed. What does that mean? If we look at a quiescent black hole we can see nothing much is happening. But you would have it that it is sucking in spacetime itself at an amazing rate. BUT…despite this hoovering in of space it has no discernable effect except, maybe, at the edge of the event horizon and even then we don’t see it. What am I missing? (And please don’t nitpick my use of “hoovering”…I get that a BH is not a vacuum) ---
**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:** [October 24, 2020, 4:25pm UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/52 "2020-10-24T16:25:30Z")
The size of the Sun will have higher order relativistic gravitational effects. For example, gravitational radiation produced by the solar system will be affected by the internal properties of the Sun, which in turn will have a back-reaction on the solar system. However it is safe to say these would be negligible. ---
**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:** [October 24, 2020, 4:54pm UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/53 "2020-10-24T16:54:42Z")
In all honesty it doesn’t have a defintive physical meaning to say that _space_ is being hoovered up by the black hole at lightspeed. However it can be useful to think in these terms. Light moving outwards from a black hole is ‘frozen’ on the event horizon and you can think of this as being due the space at the event horizon being ‘pulled’ in to the black hole at lightspeed. There’s a little bit more to it, but the idea of the “apparent horizon” frames this idea in more formal and physically-meaningful terms. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [October 24, 2020, 5:32pm UTC](https://boards.straightdope.com/t/black-hole-ramblings/922282/54 "2020-10-24T17:32:43Z")
> [@Whack-a-Mole](#): > > If we look at a quiescent black hole we can see nothing much is happening. But you would have it that it is sucking in spacetime itself at an amazing rate. There is no such thing as a truly quiescent black hole, as the universe is full of particles and radiation. Those are being dragged in, or towards, the event horizon. If it were truly a black hole in an otherwise empty universe, then we wouldn’t see much happening, just as it wouldn’t seem as though there is much happening on a steadily flowing river with nothing floating in it or anything to disrupt its laminar flow. Space itself is not a thing, like a particle or quantum of radiation, it is just where things happen, it is where things are. However, the properties of space, as they are influenced by the things that are in it, do influence the movement of the things in it. OTOH, space kinda _is_ a thing. It is a sea of virtual particles. In many ways, all a real particle is is a fluctuation in that virtual particle field that has the right properties to be stable. Those virtual particles are being drawn in to the black hole, and are taking anything that relies on them for their interactions (which is everything) along with them. And I know you asked not to nitpick, but “hoovering” does give a somewhat useful impression. You _could_ think of it as every individual object that has mass (including radiation) does its own “hoovering”. Every particle and quantum is drawing space into itself, distorting space as space rushes in to fill the “vacuum” left by the space that is being absorbed by the things that are embedded in it. The more stuff you have in an area, the more that area pulls in space. It’s not exactly right, and I may get myself in trouble with this (sorry to those who know better, but I don’t mind being corrected), but you can maybe think of it as real particles absorbing the virtual particles, leaving a lower density of virtual particles near a real particle, which causes the virtual particles to be drawn towards it, and since the virtual particles are how particles interact, that causes it to draw in everything else. That’s about as “behind the scenes” as I think I can get away with without invoking the “Just do the math and forget about understanding it intuitively” concept that usually comes about when we get into this level. On the surface of the Earth, space is being “hoovered” by the Earth at 11.19 km/s. In a single thrust, if you are not capable of reaching that velocity, then you will inevitably fall back to the surface. It’s no different than a black hole, other than the “surface” of a black hole has an escape velocity at the speed of light. There is nothing really special about the event horizon of a black hole other than the fact that at that point, there is enough stuff below it that it pulls in enough space that what’s rushing to fill that vacuum exceeds the speed of light. [Previous page](https://boards.straightdope.com/t/black-hole-ramblings/922282.md?page=2)