Theoretically if a Birch planet is built around the event horizon of an ultra massive blackhole, if there was a planet or structure outside of range to experience time dilation, self maintained computers could calculate at a speeds of many orders of magnitudes faster (possibly infinitely? if time dilation becomes infinite) than those at the event horizon and they could keep updating the event horizon computers with calculation results at a tremendous rate, theoretically? but could those computers at range be sent information from the Birch planet?

As the escape velocity at the event horizon exceeds the speed of light due to space-time curvature could a message ever be sent out or could information even be received due to the path light takes towards the event horizon?

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    $\begingroup$ Time slows down the closer you get to a mass, relative to a far away observer. $\endgroup$ Commented May 9, 2022 at 12:57
  • $\begingroup$ You might want to look into the ergosphere of a rotating black hole. There are extra things you can do if the ergosphere is bigger than the event horizon. $\endgroup$
    – Daron
    Commented May 9, 2022 at 13:05
  • $\begingroup$ @Daron Yep, I know about Penrose process and theoretical uses for ergospheres (has been a while though, will need to refresh my knowledge) I thought there was a limit of how close to the event horizon a ship could get and still escape. At a safe distance I thought time dilation is not that much $\endgroup$ Commented May 9, 2022 at 17:03
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    $\begingroup$ You might want to edit the question to explain what a Birch planet is. Having looked it up, it seems it means a hollow shell built around the black hole, such that the surface gravity is about 1g. In that case the surface of the sphere is very far from the event horizon and the time dilation at the surface will be negligible - it will be about the same as the time dilation due to Earth's gravity well. Your question would make a lot more sense for an ordinary planet or space station orbiting the black hole just outside the event horizon, where the time dilation is much more extreme. $\endgroup$
    – N. Virgo
    Commented May 10, 2022 at 1:39
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    $\begingroup$ It will be slowed down by a factor of 300 at first, but c/300 is still very fast. The red shift won't degrade the signal at all, it will just change its frequency. So, e.g. you would have a microwave transmitter/receiver on the surface, but the computers would receive and transmit radio waves, at 1/300th the frequency of the microwaves. There would be no practical problems with that if the equipment is designed for it. $\endgroup$
    – N. Virgo
    Commented May 10, 2022 at 13:14

3 Answers 3


If the planet it's outside the event horizon of the black hole, it can send communication outside of it.

However whoever receives it will see it extremely red shifted, due to the distortion caused by the proximity to the black hole.

If the transmission starts at the event horizon or inside it, it won't be received outside of the black hole.



It is impossible to send information from inside (the event horizon of) a black hole to outside. So build your shell planet outside the event horizon to allow messaging other planets.

If the shell is close to the event horizon the other planets will recieve your data in a highly scrambled form due to interference by Hawking radiation and other quantum gravitational mumbo jumbo.

Unfortunately, according to entropy physics and the conservation of information, this scrambling is a necessary feature to prevent paradox. The distortion increases in proportion to the time dilation. Make the shell twice as close to the horizon? Then there's twice as much information being sent out. But it's also twice as scrambled so the information density is the same.

The upshot is this method cannot be used to perform a superfast computation since when you transmit the answer it will be distorted. For example you have a large yes/no question that it would take 1 year to compute normally. You set the computer to emit a prolonged signal of strength 1 if the answer is yes or 0 (no signal) if the answer is no.

With no black hole you just wait a year and then measure the signal. With the black hole you can start measuring almost immediately. Only instead you are measuring the deviation from the average signal emission from the black hole. This will be so slight that you have to wait a year before you can reliably judge if the average is above the normal background.

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    $\begingroup$ i would like to see a citation for this scrambling. $\endgroup$
    – ths
    Commented May 9, 2022 at 22:23
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    $\begingroup$ @ths I figure if Lenny Susskind is allowed to make up black hole stuff then I should give it a go as well. Who knows -- I might be good at it. $\endgroup$
    – Daron
    Commented May 9, 2022 at 22:29
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    $\begingroup$ Lenny's "make up stuff" is at a pretty high level, and backed by enough theory that it seems plausible - in particular since his "stuff" is an untested-yet hypothesis. None of this applies to you making up stuff, in particular since a black hole does NOT scramble anything; you can go down that route, but it's just technobabble and you don't even need a black hole. $\endgroup$
    – toolforger
    Commented May 10, 2022 at 8:29
  • $\begingroup$ @toolforger Black holes absorb stuff and then emit Hawking Radiation until they disappear. So whatever goes in gets scrambled into Hawking radiation. $\endgroup$
    – Daron
    Commented May 10, 2022 at 10:52
  • $\begingroup$ @toolforger Of course I am talking about stuff near the hole and not inside. Edited the answer to mention Hawking radiation interference. Though I already think Hawking was covered under quantum gravitational mumbo jumbo. $\endgroup$
    – Daron
    Commented May 10, 2022 at 10:54

A Birch planet cannot be natural, it is a megastructure that compares to a giant Dyson Sphere. Suppose your civilization would be able to create a Dyson Sphere the size of Neptune's orbit, it needs galactic amounts of iron to construct it. Hundreds (thousands?) of stars and gas giants would have to be dismantled to build it. But.. there are more issues.. I have to shoot down the whole idea, a Birch planet is imho an implausible construct.

Frame challenge

No sunshine and a lot of dangerous impacts

In the usual Dyson sphere, you live on the inside. That works, because there is a sun inside the construct and a Dyson sphere will be rotating to generate "gravity" directed outward. A Birch planet, or Birch Dyson sphere can't possibly rotate fast enough to compensate for the inward gravitational force. As a result, you'll have to live on the outside, where you got your 1G gravity (Neptune orbit). Your construct will receive no sunshine - or too much sunshine - and it will be easily perforated with accretion disk material falling into the black hole, at near light speed.

Time dilation

An object approaching the event horizon would appear to be slowing down as it approaches the black hole instead of speeding up as we would expect. This is because of time dilation, which slows the passing of time for an object moving close to the speed of light with respect to an outside observer. The object eventually would slow down until it stops moving when it reaches the event horizon


Yes, your computers will run faster, but that only counts for the outside observer. The inhabitants of your Birch planet won't notice that. Also, you'll have a problem with communication. For an outside observer, perceived time on your Birch planet will be slowed down. As a result, your radio waves will have a frequency well beyond the receiver capabilities of the observer. The closer your Dyson sphere will be to the event horizon, the worse that issue will become.

Escape will be difficult

Your civilization has built a mega structure around a black hole, they will want to do space travel. With 1G gravity, you'd assume that could be arranged. But you'll have to take into account this 1G gravity will not diminish outward, like it happens when you leave planet Earth. The 1G gravity will extend outward for many AE's, your space vessel would need to maintain escape velocity for days on end, to get away. You'll need a special drive or zillions of liters of fuel, else you can't possibly escape the place. Time dilation will spoil your space travel as well: the nearer your Birch surface is to the event horizon, the larger the time difference your space ship will perceive when it looks back. And the longer it will take your ship to return. Space travel will take forever, for anyone on your Birch planet.

  • $\begingroup$ Of course a birch planet is not natural, how could it be lol. Although time dilation is only relative to the observer, the computers at range will still have tens of thousands or more of years pass by for one year at the black hole relative to the at range computers. $\endgroup$ Commented May 9, 2022 at 16:47
  • $\begingroup$ an accretion disk is far closer to the bh than the shell. and there won't be any anyway, because the builders will have cleaned any matter that could fall in. for any civ able to assemble multiple galaxies' worth of material into a shell world, we can safely assume that space flight is a solved problem. and where would they want to go anyway? there's nothing left for millions of light years around. $\endgroup$
    – ths
    Commented May 9, 2022 at 16:55
  • $\begingroup$ Why would you make the Dyson sphere so big? $\endgroup$
    – Daron
    Commented May 9, 2022 at 22:54
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    $\begingroup$ @Daron this thing is built around a super massive black hole, a galactic core like one Sagittarius-A. Its event horizon is beyond the orbit of Mercury. You need 1G, you don't want to be too near. $\endgroup$
    – Goodies
    Commented May 9, 2022 at 23:05
  • $\begingroup$ @Goodies You write that the outside of the Birchworld would receive no sunlight like it was a bad thing. If people want to see, they will turn on tthe artificial lights. Where will they get energy from? Presumably from fusion power gnerators. $\endgroup$ Commented May 10, 2022 at 0:40

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