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This is based on: Black Hole as a Storage Device

What goes into a black hole doesn't come out (except as random electrons or positrons).

However, is it possible to store something within the event horizon of a black hole and then use hawking radiation to shrink the black hole to the point that the event horizon retreats past the stored object?

This scenario has some built in assumptions:

  1. The object is strong enough to withstand the tidal forces that close to the black hole.
  2. You have the energy to enter into a decaying orbit that will slow the fall to a reasonable rate.

For assumption 1, we can just propose some unobtanium gotten from the same place that will give us enough energy to have a measurable effect on the radius of the event horizon.

Assumption 2 might not be too difficult to achieve since the rate of fall slows as you approach the black hole (because time slows). So, you just have to help that process along by making the spiral as shallow as possible.

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    $\begingroup$ @kleer001, the unobtanium is not involved in the answer, just the premise. The question is can the Event Horizon move past an object that somehow has remained intact. $\endgroup$ – ShadoCat Oct 24 at 21:30
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    $\begingroup$ @kleer001 they arent mutually exclusive. If I have unobtainium powering my railgun and ask for what a specific energy output from the unobtanium can do for the railgun, then the unobtainium isnt really a factor but some fluff to get the question going. This question has unobtanium to make the object survive the tidal forces, but it is really about how a BH can "store" something. Science-based you can answer this multiple ways, such as that the event horizon will shrink slower than the object will fall no matter what happens so it will always hit the singularity before the horizon reveals it. $\endgroup$ – Demigan Oct 24 at 21:40
  • $\begingroup$ @Demigan, RE: falling faster than shrinking. Do you know that or are you just giving that as an example. That's the kind of answer that I'm looking for. $\endgroup$ – ShadoCat Oct 24 at 22:10
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    $\begingroup$ @Demigan thanks for the clarification, I'll delete my comment. $\endgroup$ – kleer001 Oct 24 at 22:48
  • $\begingroup$ How is this not a dupe of the linked question..? $\endgroup$ – dot_Sp0T Oct 25 at 6:08
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I'll try to keep it simple.

The gravitational pull on earth is +/- 9.8m/s^2. To float without going closer to the surface you need to push yourself away with exactly that amount of counter acceleration force every second to cancel the gravitational acceleration. This is what something like the ISS orbiting the earth does. It moves so fast sideways (7660m/s) that it falls as much towards earth as it moves around it.

No matter the size of the BH, at the event horizon the gravitational pull is 299792458m/s^2 (reading Mark Olson's answer and looking into it there seems to be no consensus yet for the "surface" gravity at the event horizon so I'll just stick with this for now as it does illustrate the problem well I think). To reach that kind of acceleration with any kind of mass you need infinite energy. This is why it is impossible to escape once you meet the event horizon. The example above to cancel out 9.8m/s you needed 7660m/s to not fall into earth. To cancel out an acceleration the speed of light you would need to go faster than light. I hope your unobtainium has some infinite power reactors and doesn't mind time traveling because that's what's going to be needed to do this.

You try to find a trick around this by launching the object in a trajectory that will cause it to "orbit" the BH and take longer to reach the center, then let the event horizon shrink fast enough that it catches up on the object. Now we know that light cannot escape if it enters the event horizon of a BH so even when you do go the speed of light and try to go directly against the pull of a BH the event horizon will never shrink fast enough to reveal the object, because the object cannot reach the speed of light itself.

There is also another problem: Evaporation rate. I'm using this calculator because this kind of math isn't something I'm going to take time learning any time soon: https://space.geometrian.com/calcs/black-hole-params.php

A BH with a 1m event horizon radius is 10 septillion KG, or 6.733176548*10^26 (as I understand it E+26 means 10^26, correct me if I'm wrong). That's a "mere" 112 earths that fit into that BH or 3x our sun. This BH already takes 8.135956940*10^44 TRILLION years to evaporate. In years thats 8.135956940*10^56 or 80 septen-decillion years. When you have to look up the names of the numbers you know you are already pretty far gone. Again, that is 10^44 Trillion years to evaporate a 1m radius, so your object would have to be small! A BH that evaporates in a day has an event horizon radius of 1*10^-20. An atom is the size of 1*10^-10, so your item would need to be 10 magnitudes smaller than an atom to fit in a short-lived BH. I'm ignoring the explosion this BH causes as it evaporates it's last energy as your object can survive in a BH. But even then it would never be able to have a total velocity greater than the speed of light and would never get out of the BH event horizon.

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Short answer: No.

Long answer: Not as far as was can tell.

Black Holes (BHs) are solutions to the equations of General Relativity (GR) and GR is a phenomenally accurate and mathematically elegant theory of gravity. It shows that gravity is not a force at all, but a side-effect of the curvature of space-time. GR has been tested repeatedly and passed every test with flying colors. No other theory of gravity is as elegant or matches experiment better than GR. For those reasons, we think that GR gives a good picture of what's really going on with space, time and gravity.

A Black Hole is a region of space-time where mass-energy has been so concentrated and the curvature become so extreme that space-time inside the surface of the BH is fundamentally different than outside. Specifically, (and inevitably approximately since I'm using English and not math) space and time twist around so that inside the BH the radial coordinate is timelike. And that means that, just like outside the BH we can't stop ourselves from moving forward in time, inside the BH, we can't stop ourselves from falling towards the center.

Anything put inside the BH (inside its Event Horizon) inevitably, no matter what velocity it is moving at and no matter what kinds of acceleration is undergoes, falls to the center and is obliterated by the singularity that lurks there. In particular, unobtanium does not work if it is made of either matter or energy or a mixture of the two.

Assuming that Hawking Radiation exists (we have never observed it, but it's on pretty firm theoretical ground), all BHs eventually evaporate, but not before anything that crossed the Event Horizon has gone splat (or whatever) against the singularity and been destroyed.

Now we have some pretty good reasons for thinking that GR isn't the final theory of space-time and gravity, but those reasons also tell us that it is a very good approximation in big BHs and away from the central singularity. (Called the "weak field" region, though it's only weak by mathematical standards.) We also have good reason to think that whatever replaces the singularity in the New and Improved (and so-far Undiscovered) theory will be just as dangerous to matter and energy.

About the only way you can put something into a BH and get it back later is to either (a) ignore General Relativity altogether (in which case what's a Black Hole doing there?) or (b) assume that the super-scientists who discovered unobtanium have an arbitrary new theory which allows the construction of arbitrary new gadgets which do arbitrary things. (Either way you're using something called "magic".)

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  • $\begingroup$ The thing is that the Event Horizon is not a physical thing. It is simply the point at which the pull of gravity equals the speed of light. If that point is moved, what happens to the stuff that that point moved past? $\endgroup$ – ShadoCat Oct 24 at 21:32
  • $\begingroup$ The reason the event horizon shrinks is because there's less gravity. The reason there's less gravity is because there's less stuff. What happened to it? It evaporated. (Sort of.) Hawking radiation isn't a very fast process, though. It will be hard to appreciate the difference on human timescales. $\endgroup$ – Cadence Oct 24 at 21:35
  • $\begingroup$ @Cadence, yes, but if you could speed up the process by running a laser tangentially across the event horizon, causing it to evaporate faster, what happens to the stuff in the black hole that hasn't been fully compressed? $\endgroup$ – ShadoCat Oct 24 at 22:00
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    $\begingroup$ Maybe I'm missing something, but how is shining a laser going to accelerate the black hole's decay? Either it will be trapped and add (somewhat) to its mass, or it will miss and nothing particularly happens. $\endgroup$ – Cadence Oct 25 at 0:46
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    $\begingroup$ @ShadoCat I beat my head against the massive Misner-Thorne-Wheeler Gravitation (en.wikipedia.org/wiki/Gravitation_(book)) (the book won). PBS SpaceTime has some good YouTube videos. $\endgroup$ – Mark Olson Oct 30 at 2:32
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No, because black holes have no hair.

The no-hair theorem states that all black hole solutions of the Einstein–Maxwell equations of gravitation and electromagnetism in general relativity can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum. All other information (for which "hair" is a metaphor) about the matter which formed a black hole or is falling into it, "disappears" behind the black-hole event horizon and is therefore permanently inaccessible to external observers.

This leads to the black hole information paradox

The black hole information paradox is a puzzle resulting from the combination of quantum mechanics and general relativity. Calculations suggest that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state.

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  • $\begingroup$ That's a good point, but it isn't what prohibits things which fall into a BH to later exit it. Two problems: (1) It only applies to equilibrium solutions (e.g., it obviously does not apply during "ring-down") (2) The "no hair" theorem is proof that solutions of the entirely classical Einstein–Maxwell equations are extremely limited and ignores quantum effects. But the change in space-time geometry inside the EH is more fundamental. $\endgroup$ – Mark Olson Oct 25 at 13:54
  • $\begingroup$ Not being able to communicate to the outside world doesn't mean you no longer exist. $\endgroup$ – candied_orange Oct 26 at 16:58
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Storing any objects, as kind guys before me explained, is not possible, but storing some data? Not much but its something.

How to do it?
Its simple, as we know, black holes have given charge, this charge is depended on charge of things BH consumed. So, lets assume you give certain values for certain charge values of BH, even something simple like negative charge is 0 and positive is 1. Later combine arrays of black holes and bam, you can store any data on black holes.

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