It would obviously utilize a much more powerful substance than simple electricity. Maybe plasma could constantly be siphoned from the infinitude of stars in the universe to keep the battery charged constantly?

This question was inspired by L. Ron Hubbard’s story “Revolt in the Stars”, which tells of an alien dictator named Xenu. He ended up being condemned for genocide on a cosmic level, and was locked away in a force-field prison. Said force-fields were powered by an eternal battery.

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    $\begingroup$ What's wrong with an array of photovoltaic panels powered by a long-lived star? The Sun has powered life on Earth for billions of years, and will continue to do so for several billion years more. $\endgroup$
    – AlexP
    Commented Oct 20, 2019 at 22:46
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    $\begingroup$ PV cells degrade over time, at a rate of ~1%/year. $\endgroup$
    – Hobbes
    Commented Oct 21, 2019 at 9:18
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    $\begingroup$ Call me an extremist, but I believe the capital punishment to be quite fitting for genocidal dictators on a cosmic level. Why would you ever give someone like that a chance to ever get free again? And if they're hard to kill, toss 'm in a black hole using a temporary prison. Or any star/supernova, really. $\endgroup$
    – Gloweye
    Commented Oct 21, 2019 at 11:34
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    $\begingroup$ It would obviously utilize a much more powerful substance than simple electricity There seems to be a fundamental misunderstanding here. Batteries produce electricity. They store energy, usually in the form of a chemical reaction that proceeds when the terminals care connected. $\endgroup$
    – nzaman
    Commented Oct 21, 2019 at 12:22
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    $\begingroup$ I have to start a close vote as "Unclear". You shouldn't mix reality-check and science-fiction together unless you are very careful with the wording and scope of your question. as @o.m. stated, there's no answer. $\endgroup$ Commented Oct 21, 2019 at 16:22

4 Answers 4


Yes. It's called a "betavoltaic" battery (there are also theoretical alpha- and gamma-voltaics devices, but betavoltaic batteries - or rectius, unreachargeable betavoltaic cells - are known to already exist).

To build an "eternal" battery, you'd use a very large quantity of 128Te (or any other "barely radioactive" element) surrounded by substances that react to its double beta decay by generating photons, and suitably doped semiconductor layers (essentially a photovoltaic cell). The tellurium will decay very slowly, supplying energy.

Since 128Te is unbelievably long-lived, the energy generated is minuscule. So, you will need a lot of tellurium.

On the other end of the spectrum, plutonium 238 decays in less than 90 years, and its activity (more than 600 GBq/g) and energy output is high enough to keep a nugget of PuO red-hot.

A large enough deposit of uranium ore should be enough to keep Xenu locked for the foreseeable life of most current generation's stars.

The quantity of material required is probably pretty much constant with the power requirement over the same period of time; that is, if you double the decay rate you do double the power, and require half the quantity, but then you need to double that again since otherwise it would burn in half the time.

Raw calculations for 235U

From Wikipedia I get uranium 235's half life, about 700 million years. So if I have a kilogram of 235U, in 700 million years I'll have half a kilo, after 1.4 billion years 250g, after 2.1 billion years 125g, and so on.

Activity: 2.12 microCurie per gram. What does this mean: since one Curie is 3.7 x 1010 atoms per second, and one gram contains 2.12 microCurie, so 2.12 x 3.7 x 1010 x 10-6 = 78440 atoms per gram disintegrate each seconds. Each of those gives out 4.39 MeV, or 1.60217733 x 10-13 J.

78440 x 1.6E-13 J each second is 1.25 x 10-8 W, which is very little (this is spontaneous radioactivity; of course were we to enclose the metal into a neutron reflectant material, such as beryllium or tungsten carbide (you might want to google "Demon Core"), the radioactivity would increase sharply, and the half life go down accordingly.

Assuming an advanced enough technology we should be able to recover a good 50% of that energy, so we can extract 6E-9W from each gram, 6E-6W from each kilogram, 6 milliwatt from each ton, and 6 W from a block of one thousand tons. A million tons of U235 (divided into subcritical masses separated by neutron absorbers, to avoid uncomfortable nuclear explosions) would supply initially six kilowatts, down to 3 kW after 700 million years; and so on.

Given a density of around 19, one cubic meter of uranium weighs 19 tons, and one million tons of uranium are one cube with a side of 37 m (given the need of subdividing it into subcritical masses and gather energy, I imagine it would be more like 50 or 60 m).

If the field generator requires one hundred kilowatts (100 / 6 kW = about 17 million tons) and we want it to be running for twenty billion years (20,000 / 700 = 28 half-lives), we need (100/6)*228 = 4.5 billion millions tons, or about 220 thousand cubic kilometers; a sizeable asteroid of about 75 km diameter.

Such a mass would significantly distort the planet's local gravitational field, so if that's how the eternal battery works, I feel some confidence in telling you that good ol' Xenu might currently be doing time somewhere in the vicinity of Manaus, Brazil.

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The battery is not the problem

As user @Hobbes noted, the problem is not a battery running for billions of years - the problem is maintenance. The field generator, and the battery itself will degrade over time (all the more so since radioactivity plays hell with electronics and semiconductors). Even with self-recycling and self-repair, chances are that after a paltry few million years, Xenu is going to get free.

  • $\begingroup$ Out of Tellurium, Plutonium, and Uranium, which element would be the most efficient on keeping Xenu rotting in prison? You made it sound like uranium would be, but I’m also inclined to believe tellurium would be the best if enough of it could be obtained. $\endgroup$ Commented Oct 20, 2019 at 23:44
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    $\begingroup$ Tellurium is probably overkill, as it would outlive practically anything. Most isotopes of uranium have a half life sufficient for several billion years. I'd worry much more about malfunctions in the field generators. Redundancy and self repair can go a long way, but with the passing of strange aeons, even they may die :-) $\endgroup$
    – LSerni
    Commented Oct 21, 2019 at 0:02
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    $\begingroup$ Current nuclear batteries (RTGs) degrade from the radiation hitting the thermocouples. This would also apply to PV cells in a beta decay battery. $\endgroup$
    – Hobbes
    Commented Oct 21, 2019 at 9:21
  • $\begingroup$ "going to go rabbit" Is this a regional(?) idiomatic phrase or a typo? I googled and couldn't find any references to such an expression. $\endgroup$
    – jmbpiano
    Commented Oct 21, 2019 at 18:16
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    $\begingroup$ @Niobium_Sage I do not think that I can give an unbiased opinion. Starfish Prime's answer is scientifically sound: essentially you build a Seebeck effect matrioska (plus a black hole levitation system, which is the real problem if you are not in space) and reradiate the hole's heat, extracting energy. What can I say, I upvoted his answer 😁 $\endgroup$
    – LSerni
    Commented Oct 21, 2019 at 23:27

With the tag, there is no such thing. The first law of thermodynamics says that there are no perpetual motion (or perpetual power) machines. You tried to get around this by tapping stars, but even stars are not eternal.

  • $\begingroup$ LSerni provides a pretty good answer for an “Eternal” battery. One that figuratively lasts an eternity anyways lmao. $\endgroup$ Commented Oct 21, 2019 at 5:37
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    $\begingroup$ @Niobium_Sage, if you think a billion years is an eternity, maybe. But the best stories happen when "eternal" prisons finally run out. $\endgroup$
    – o.m.
    Commented Oct 21, 2019 at 5:50
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    $\begingroup$ You’ve got me there, action is what all works of fiction should strive for. $\endgroup$ Commented Oct 21, 2019 at 5:53
  • $\begingroup$ If you’re wondering where at least one downvote came from, I accidentally clicked it when reading your post, and now can’t remove it. Sorry about that. $\endgroup$
    – DonyorM
    Commented Oct 29, 2019 at 12:01

I'd suggest a substellar mass black hole. Given that we're dealing with science fantasy silliness, the safe anchoring of the black hole to the planet is left as an exercise to the reader, but it'll probably help if your black hole is charged. Good luck!

Black holes evaporate over time due to Hawking radiation. This can be captured by various means such as photovoltaics or even a heat engine, given that you can use the rest of the earth as a heat sink.

Lets be less optimistic than LSerni, and assume the prison needs about a megawatt to run. The Hawking radiation power of a black hole with mass $M_0$ is $P \approx 3.56345\times10^{32} / M_0^2$, so if we need a megawatt we'll need a black hole with mass of 1.9x1013kg, or about 20 billion tonnes. This compares favourably with LSerni's millions of billions of tonnes, and provides more power and by way of a bonus black holes develop more power as they shrink... no half-life issues here.

A black hole with initial mass $M_0$ will evaporate over this timescale: $t_\mathrm{ev} \approx 8.41092 \times 10^{-17} \;M_0^3$. For our initial megawatt power source, we get a lifetime of 5.6x1023 seconds, or about 18 quadrillion years, vastly outperforming LSerni's and Gloweye's suggestions and when the black hole runs out of mass it goes bang spectacularly rather than just turning off and letting the occupant of your prison escape. Not that it will matter because the universe isn't guaranteed to last that long.

Aha! I suspect people will say. There's no way your prison equipment could last that long! They're probably right of course.

I'd just throw Xenu into the event horizon of a larger black hole (such as the regular, natural stellar-mass ones). If they're genuinely immortal (and they'd have to be, if imprisoning them for eternity is a worthwhile punishment) then I'm sure a little thing like spaghettification won't bother them too much and not much gets out of event horizons. Problem solved.

edit: to counter the "the black hole will eat the earth!" argument, consider than the event horizon of a black hole is found at the Schwarzchild radius which for this miniscule object will be significantly smaller than a single atom. It cannot eat the Earth quickly enough to make any difference to the lifetime of either the black hole or the Earth itself (which will of course be eaten by the Sun in the future). Have a read of this related answer for a bit more on the matter.

Obviously, the most sensible thing to do is no not drop the black hole, hence the usefulness of it being charged. You might need a lighter, more energetic black hole if you wanted to get enough energy to electromagnetically levitate it against Earth's gravity, but maybe you should just keep the whole assembly sensibly in space instead. The OP, of course, did not mention planets or gravity at all, merely eternity...

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    $\begingroup$ Similar answer I would have given but I would have placed the BH and prison in space, because why not? And I would also add a way for mass to keep falling into that BH to make it last for a few seconds of eternity more because F Xenu. Then should the prison be about to fail Xenu's force field stops and Xenu is exposed to the remaining moments of the BH as it explodes. $\endgroup$
    – Demigan
    Commented Oct 21, 2019 at 15:00
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    $\begingroup$ @Demigan I think the story has him being trapped in a mountain or something, which implied on a planet. The OP didn't specify that, of course, so firing it into deep space at galactic escape velocity would be a reasonable thing to do. $\endgroup$ Commented Oct 21, 2019 at 15:13
  • $\begingroup$ @StarfishPrime In regards to the story, he is indeed imprisoned in a mountain peak somewhere on Earth. Starfish Prime’s answer is probably the best one, but I don’t think any science Mumbai jumbo could make a loophole for the black hole to have its energy drained to Earth and not destroying it in the process. $\endgroup$ Commented Oct 21, 2019 at 16:31
  • $\begingroup$ @Niobium_Sage what do you mean by "energy drained to earth"? That makes no sense. A black hole of this mass has an event horizon smaller than an atomic nucleus and is actively radiating a lot of energy that will deflect stuff falling into it. It will not destroy the earth within the lifetime of the solar system. $\endgroup$ Commented Oct 21, 2019 at 16:40
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    $\begingroup$ @Niobium_Sage as I mentioned in my answer, you can capture the energy radiating out of the black hole using a heat engine (treating the black hole "simply" as a source of intense heat) or via photovoltaic means (which is harder, for reasons of wavelengths, but not impossible). I'm not sure what "Transporting the energy to earth" is supposed to mean. The black hole assembly is the battery, not a charger. $\endgroup$ Commented Oct 21, 2019 at 16:47

We already have them.

Since it's cosmic-scale stuff we're talking about, a star can last a pretty long time, and 10 billion years is very possible. The vast majority of that time (like 80% or something), it'll have a pretty constant output, much like our own sun.

And if you have FTL travel, then you can even abandon one star and head to the next if you're close to running out. It shouldn't be that much of a burden to do that once every X billion years.


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