Edit: while asking this question I had mixed up 'one way function' and (just in general) 'one way nature' (since, a black hole has this 'kind of nature', stuff can go in but not out) so, my apologies.

Just to add a example of 'why' black holes has this nature is, because all possible 'directions' a 'thing' can take, points into the black hole. This is probably where I got confused. A one way function is, something that takes a input, and produces a output, which by definition is hard to 'reverse' given only that output.

Where's a black hole takes in but does not output anything (or, not anything of 'value' that is) (maybe hawking radiation, but not any 'thing').

Even if this doesn't make the question re-openable I decided I would clarify it anyway.

Note I am studying Physics just for fun, I am in no university yet. (As my other questions on this site point out.)

! Note: when asking this question I forgot the detail about a one-way function, that it (of course) outputs a value and that value is difficult to compute to the input (broadly speaking) sorry. ! I totally forgot that part.

I have quite a (unique, in a sense) question that I have to admit I am not sure this is Physics, or Computer Science.)

A black hole, is of course Physics - while a one-way function (as far as I know) is more cs.

What am I basing this on? Both the Wikipedia site - One-way _function. But also, on just my own thinking (not opinion), to me at least, the name itself says it all:

one-way function

What I have tried searching for to find this

  • "one-way function physics"
  • physics one-way function black hole
  • gravitational singularity one-way function


on both google, Wikipedia and this site.

my main question:

Is a black-hole really - an ideal one way function, in practice?

Note to be clear: I mean a static , not spinning, (no charge) - so a Schwarszchild Black Hole.

I thought of this for some time now, and is still unsure about where to ask this. It seems it is both Physics and cs.

I did find a tag but it is not really what I am after.. It at least doesn't have any description anyway [one-way-speed-of-light].

note I am not stating there is no one-way function or functions in physics. Just wondering about this.


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    $\begingroup$ NOTE - if I can improve this , please point it out! And, I did search for the tag computer-science, or cs, (...) But didn't find any. So I didn't really know what more tags to add. Wishes from Sweden! $\endgroup$ Jan 8, 2023 at 17:34
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    $\begingroup$ The definition of a one-way function is a mathematical operation that is easy to compute, but difficult to invert. ("easy" and "difficult" can be given more precise definitions -- as in the wikipedia article you link to). A black hole is a region of spacetime where no object can escape once it enters, due to the existence of an event horizon. (Here I'm ignoring Hawking radiation, which anyway has never been observed for astrophysical black holes). Can you elaborate on why you think there is a connection between these two concepts? Based on their definitions, they seem quite different to me. $\endgroup$
    – Andrew
    Jan 8, 2023 at 17:47
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    $\begingroup$ I would guess that this question will probably be closed for lacking sufficient clarity to allow for a reasonable answer (other than a "not-even-wrong"-type answer). I'm not sure if you can delete your own question once it has a posted answer. See, for example: meta.stackexchange.com/q/5221 $\endgroup$
    – hft
    Jan 8, 2023 at 18:24
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    $\begingroup$ Anyways, if you want to delete it there may still be a link at the bottom that says "delete" (but I don't see one--although you might) $\endgroup$
    – hft
    Jan 8, 2023 at 18:26
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    $\begingroup$ I think maybe a better analogy would be that a black hole is somewhat like a function that maps any input to $0$. It is impossible to recover any of the inputs once the function has been applied / once the material has fallen into the black hole. (Again we're ignoring Hawking radiation). To me, a better physical analogy to a "one-way function" would be the second law of thermodynamics -- systems will naturally evolve so that the entropy increases, and it is "difficult" to force a system to reduce its entropy to recover the initial state. (for example: it is hard to unscramble an egg). $\endgroup$
    – Andrew
    Jan 8, 2023 at 18:27

1 Answer 1


A black hole is not a function of any kind in the mathematical sense that a one-way function is. There's no connection. A one way function takes input in a way that's very difficult if not impossible to reverse so that the output cannot easily be used to compute the input. However the output must be consistent in that it must correlate to the input.

If a black hole could be said to be any kind of function it would simply be a random one, as at best what you can get out (via Hawking radiation or during a black hole merger) would be random with no correlation to anything that went in.


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