I don’t know if this should be here or great debates, I need some factual (according to our current understanding of physics) ways for a sufficiently advanced civilization (say Type II or III, maybe even Type I) to generate energy while inside the event horizon of a black hole, for a writing project of mine (fiction).
ASSUMING that what the Pop Physicist Michio Kaku says is true: If a black hole is large enough, tidal effects of the singularity wouldn’t be very large even inside the Event Horizon.
So now once trapped inside, how to generate energy? One way would be to use rotation and take advantage of the tidal forces with an artificially created “Io” with an Iron core and Mantel specifically designed to generate and exploit heat from the tidal forces, this would be more of a “Type I” civilization move.
Another way would be to use insanely long tethers with weights on the end to tap into the gravitational field for energy itself (Nasa has done experiments with this, wish I could find a link).
Another way would be to build a ring around the singularity far enough away to be “safe” but still close enough to tap into tidal forces, perhaps with movable joints in the ring and a slightly jittered orbit. Or some other way, a Type II civ move.
Now in my mind at least, a Type III civ should be able to tie into the gravity field ITSELF for energy, any Ideas how?
Any Ideas?
Also Mods if this is in the wrong place, please move it.
As I’ve always understood it, you can get energy from a black hole, but not from inside the EH. All effects are outside it.
If the BH is rotating, you throw your garbage down to it, and gather energy from the acceleration of the matter as it approaches the EH. (Sort of like the slingshot orbit where we tap just a little of Jupiter’s orbital energy to send spaceships outward at higher velocities.)
There’s also the idea of harnessing Hawking radiation from micro black holes.
To be clear, you want your characters themselves to be inside the black hole, and still maintaining their high-energy lifestyle for as long as they can? It’s probably simplest if they just keep on using whatever it is they used on the outside. I can’t actually think of any particular way to take advantage of being inside the hole.
First, please note: Once you’re inside the event horizon of a black hole, you have a limited time before you crash into the singularity. (At least, under our current understanding of physics. Nobody has ever checked, and even if they did, they wouldn’t be able to report back [under our current understanding of physics.]) Roughly speaking, a black hole curves space & time so much that “inwards” and “the future” become the same directions. The reason you can’t escape turn around and fly out of a black hole is because there’s no way for us to start travelling backwards in time.
This means that for anything that falls into a black hole, the deep unpleasantness of the singularity is inexorably in its future; the radial distance from the event horizon to the center has become a certain amount of time, and once that time has elapsed, it will crash into the singularity. Even for the most massive black holes we know of, with billions times the mass of the sun, this period of time doesn’t exceed a few hours. A civilization inside a black hole is a contradiction in terms.
If you want a way for a Type II or Type III civilization to mine black holes for energy, there is a way: the Penrose process. Very roughly speaking, this is not unlike a controlled sort of Hawking region: you can, in principle, send an object very close to the event horizon of a spinning black hole, and have it split in two in such a way that one of the pieces has negative mass (and falls into the black hole) while the other object escapes the black hole with more energy than it started out with. This process also necessarily causes the black hole to spin a bit less; and once its spin has gone to zero, you can’t extract any more energy from it. But we’re still talking about the energy equivalent of several solar masses’ worth of mass, so that should be plenty to power pretty much anything you’d care to name.
I think you’re remembering the Tethered Satellite System that flew in the '90s. However, that was a system that used the Earth’s magnetic field to generate energy, not the gravitational field.
And actually, a satellite tether just robs you of your own orbital energy. Which you had to pay dearly for, using highly inefficient rockets. You’d be much better off skipping the tether, saving that rocket fuel, and carrying batteries or fuel cells.
Once you’re inside the black hole the maximum time to the singularity (and the insanely large tidal forces that occur just before you reach the singularity) depends on the mass of the black hole and even in the largest known supermassive black holes the maximum time from the horizon to the singularity would be, almost exactly, one week.
Ok I may be missing something but couldn’t the advanced civilization be living on a massive satellite thats orbiting the singularity at some significant percentage of c? If it’s orbiting the super massive black hole singularity at 0.9c then sure eventually its going to still hit the singularity but its going to take a lot longer than a week isn’t it?
BTW there is a very good short story “The Planck Dive” by Greg Egan about uploaded minds launching clones of themselves into a black hole and their experiences. Full text is available online (legally): http://gregegan.customer.netspace.net.au/PLANCK/Planck.html
For a Schwarzschild black hole at 1.5 times the Schwarzschild radius (i.e. the photon sphere - the point where photons can orbit a black hole), a free-falling object with purely tangential velocity needs a velocity (relative to a Schwarzschild static observer) of 1c not to fall into the black hole, however a free-falling object with purely outwardly-directed radial velocity needs only a velocity of >0.82c (approx.) not to eventually fall into the black hole. So for free-falling objects starting at 1.5 times the SR, an object with initially a 0.9c purely tangential velocity meets the singularity at some finite proper time, whereas the proper time for the object with 0.9c purely outwardly-directed radial velocity to meet the singularity is infinite (i.e. it escapes).
The moral of the above paragraph is that to increase the amount of time you survive around a black hole it is best to direct all the velocity you have in the outwards radial direction.
Now in general relativity a key feature (tied into the equivalence principle) is that the most time experienced between two events is experienced by a free-falling observer. If we view the event horizon and the singularity both as two sets of events, then the maximum time experienced by any observer between the two sets of events must be experienced by a free-falling observer (i.e. an observer experiencing acceleration cannot be the one who experiences the most time).
Taking these two observations together, the upper limit of the time experienced between event horizon and singularity must be the time experienced by a free-falling observer who is dropped from rest at the event horizon as the maximum possible outwardly directed radial velocity at the event horizon is zero.
For a 40 billion solar mass black hole this gives a maximum time of one week, give or take a few minutes.
Ok, its makes my head hurt but I think I get it. However you could still have an advanced civilization inside a black hole if we assume they are AI or uploaded minds which are experiencing time millions or billions of times slower than we do. In the Greg Egan story they send six minds inside the event horizon in a computer built of counter rotating light pulses (I don’t understand it but Greg Egan is pretty thorough in making his ideas scientifically plausible). They are attempting to see if there is a way to use various properties of the inside of the black hole to achieve infinite computing time and thus escape the inevitable big crunch the rest of the universe will succumb to.
Jumping into a black hole to escape the inevitable Big Crunch is the very definition of counterproductive. The singularity at the center of a black hole and the end state of a Big Crunch universe are two examples of exactly the same thing, just on different scales. Note that I did not say “very similar” there: That was intentional. If you jump into a black hole, there is a Big Crunch in your future, and a lot sooner in your future than it would be otherwise.
Egan was probably referring to Frank Tipler’s ideas about the Big Crunch; Tipler suggested that the Crunch would get hotter and hotter, allowing computation to proceed faster and faster until it reached effectively infinite speed.
My explanation is a bit hand-waving, but it basically comes down to the fact that the most efficient way to get away from a black hole is to move directly away from it. Inside the event horizon the singularity is inescapable, but you can maximize your time still by heading in a certain direction as fast as you can.
I could only bring myself to read a little bit of that Greg Egan story, but I would not place too much value in what a science fiction story says.
As I recall, the characters’ motivation had nothing to do with evading the Big Crunch - they just wanted to study the laws of physics under extreme circumstances and were willing to sacrifice their lives (or rather, one version of their lives) to learn something (even though they wouldn’t be able to communicate the results out of the black hole). They did think there was a tiny chance that what they learned would allow them to escape the black hole’s singularity, but that wasn’t really a part of their motivation.
So this is sorta interesting, even if I do have the expression of a glazed donut on my face for parts of it.
Why would the fall have to be completely unpowered? If unpowered how do you accelerate to the right free-falling frame soon enough or even at all? (remember, for some of the explanations, I turn into a glazed donut)
If 40 billion solar masses gives you about a week inside the horizon, does that ratio of mass to time remain constant at different scales i.e. 160 billion masses equals about 4 weeks? (I know that “about” makes it fuzzy and that “4 weeks” may be 3 or 5 or whatever) for simplicity lets say that the ratio is exactly 40B to 1W
The best frame is a free-falling one. If you’re not already in that frame, then you will want to turn on your rockets long enough to get into it ASAP. How quick that is, or whether you can even get to it at all, depends on how powerful your rockets are and how much acceleration you can tolerate.
Are you aware of the twin paradox of special relativity? In the twin paradox one twin heads off into space whilst one remains on Earth, after a while the space-travelling twin returns and they compare clocks. To their surprise (well probably not surprise as you would think that if they had the technology to travel into space they would be aware of special relativity) less time has passed for the space-travelling twin than the Earthbound twin. The resolution of this apparent paradox is that the spacebound twin experienced some form of acceleration (or at least a change of inertial reference frame) which breaks what otherwise seems to be a symmetric situation between the two.
In general relativity, that a non-accelerating free-falling observer starting at event a and finishing at event b experiences more time between the two events than an accelerating observer who also starts at event a and finishes at event b ,carries over from special relativity (but with some caveats - caveats though that are not relevant here!). Indeed this is one of the most important insights that allowed Einstein to conceive general relativity and it is related to the equivalence principle (i.e. locally, a free-falling observer is equivalent to an inertial observer).
In our analysis of the black hole we can view the event horizon as a set of spacetime events A, with each event being a spatial location on the event horizon at a particular time. The (Schwarzschild) singularity is a bit trickier as it isn’t really an event, its more like a hole in spacetime where it seems like an event or set of events should be, but if we put such an event or events in the curvature diverges (goes to to infinity) making it impossible describe. However it can be shown (by using a bit of trickery) that in this analysis we should treat the singularity as a set of events B (rather than a single event).
Taking some very straightforward arguments it can be shown that for every accelerating observer who starts at some event a in A (the set of all events at the event horizon) and finishing at some event b in B (the set of all events at the singularity), there is also a free-falling observer who starts at event a and finishes at event b. From this we can see that as free-falling observers always maximize the time experienced between two events, then the observer who experiences the most time must be a free-falling observer (or observers).
However the time taken for all free-falling observers from the event horizon to the singularity is not the same and if we imagine a free-falling observer whose initial state of motion is not such that they will maximize the time from the event horizon to the singularity their best option is to accelerate (i.e. use powered spaceflight) as quickly as possible until their state of motion matches that of the free-falling observer(s) who does maximize the time. This fact is not that well-known due to the potentially misleading way this (standard) problem is sometimes presented in textbooks.
Yes, the maximum time experienced between the event horizon and singularity is directly proportional to the mass of the black hole. In fact to calculate the time for a 40 Bn solar mass BH, I simply took the value that someone had already calculated for a 1 solar mass BH and multiplied it by 40 Bn.
Edited to add: tl;dr - read Chronos’s post for a more concise answer.