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I imagine such a creature would be composed of thousands, possibly millions of planets, gas clouds, asteroids and stars. Almost like a giant living engine powered entirely by gravity.

Obviously the time scale this creature would live and perceive on would be vastly slower than ours. Taking information over such incredible distances would take millions of years.

Would we even be able to detect such a creature? I imagine we would simply see it as planets and stars following their normal path. I guess humanity would blink in and out of existence in the time it took the creature to think a single thought. We wouldn't even know we were living inside it like a form of bacteria. Our satellites, radio signals and space stations might even be harmful to the overall working of such a being.

It's biology would be like nothing we have ever seen. I'm happy to leave that up to the readers to figure out. Whether it passes information / signals around it's "body" via gravitational fields or asteroids etc.

Are there any killer physics principles that would absolutely rule out the existance of such a creature?

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  • $\begingroup$ I think it's mostly philosophical, not something that can be answered. "humanity would blink in and out of existence" - exactly. So we cannot observe it behaving as a creature. No chance. $\endgroup$ – Mołot Oct 26 '16 at 13:52
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    $\begingroup$ This question is not too broad v.v; The answer is no you can't rule it out, but you should look at the arguments for and against why a groups of people could be considered one creature... and then the argument applies across a galaxy. The transmitters, actions, and reactions might be slower than we can comprehend as "living" but one can make solid arguments for why groups of higher level entities, galaxies, and even the univers are indeed "sentient" or "living" $\endgroup$ – Durakken Oct 26 '16 at 14:42
  • $\begingroup$ Classic SF on this very theme: Olaf Stapledon's Star Maker and Nebula Maker, and on a somewhat smaller scale, Fred Hoyle's The Black Cloud. $\endgroup$ – John Dallman Oct 26 '16 at 19:24
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Smallest known bacterium is perhaps Pelagibacter ubique, with the size about $0.2 \mu{}m$. Assuming its density is comparable to water, it weights $1000 kg m^{-3} \cdot (0.2\cdot10^{-6}m)^3 \approx 10^{-17} kg$. One oxygen atom weights $16u \approx 2.6\cdot 10^{-26}kg$, i.e. the bacterium contains about $1.5\cdot 10^9$ atoms. I deliberately left out viruses, because their classification as "alive" is controversal. So, assuming a stellar (or perhaps a Jovian) system takes the role of an atom, you need more than $10^9$ stars to get a being of similar complexity as the Pelagibacter. This on the same order of magnitude as a dwarf galaxy. Scale the size appropriately for bigger creatures.

However, consider the fact that interaction between atoms is quite complex (forming molecules etc.), while you are limited basically to gravity in your "star creature", so it does not look like you would be able to achieve the same stability and complexity as a typical bacterium, and gravitationally bound systems have the unpleasant tendency to either coalesce or relax, and there is still the black hole formation threat if you get too close (which you have to, if you want to have interactions between the atoms-stars).

Edit: numbers fixed, I left out a factor of $10^3$.

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  • $\begingroup$ What about civilisations acting as those other interactions. You could perhaps argue that over the course of billions of years civilisations may tend to act in a predictable manner. A tendency to transfer some resources and not others could act as a vague sort of attraction / repulsion between systems. $\endgroup$ – user3161729 Oct 26 '16 at 15:14
  • $\begingroup$ @user3161729 One problem is that when stars are interacting (gravitationally), even planets are insignificant, so you won't get anything like atom-like interactions. Small stars+big superjovians are probably the only way. So unless your civilization(s) indulge in astroengineering projects that move superjovians (at least over billions of years), they will have about zero effect. Thinking about it, such projects might be exactly what is missing to make the system atom-like - the civilization wants to live together, so they move the stellar systems closer and guard against black hole formation. $\endgroup$ – Radovan Garabík Oct 26 '16 at 15:23
  • $\begingroup$ @HDE this was only first approximate. Answer's author questions this assumption in the answer. Second paragraph. $\endgroup$ – Mołot Oct 26 '16 at 15:50
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I can't think of a feasible way to make it work with an organism being made of planets and solar systems, however if you're looking for a galactic scale intelligence then perhaps a vast densely packed gas cloud surrounding millions of planets and stars might work?

If the gas naturally arranged itself into some kind of a lattice formation and each of the gas particles contained a charge, then signals could be transferred across the lattice as differences in potential (though I'm not sure if that's plausible around highly electromagnetic things like suns)

The regular movement of planets through each solar system could indicate a standard signal and any deviation of that such as some outside planet entering the solar system would cause a change in the distribution and density of the gas in the regions around the new object.

Of course you'd still need some regions of the gas to do the processing of the signals and act as the brain, but perhaps you could have the brain distributed throughout the gas cloud or perhaps have the molten cores of planets and suns act as the information processors through some kind of quantum entanglement with the gas cloud ?

If you're dead set on it being planets and Suns making up the organism then perhaps you could invoke dark matter as some component of the organism's makeup. We know so little about it that it would be hard for someone to pick holes if you chose to have your organism transmit signals through dark matter excitations or vibrations?

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A few things make the possibility of such a creature extremely unlikely. These are related to biology though, but the intrinsic principles definitely fall in the realm of physics. Here they are:

1- Respiration is a key feature of living organisms. Respiration simply means to release energy. Living things either release energy (which they took as food) by reacting it with oxygen (aerobic respiration) or without oxygen (anaerobic respiration). In order for your hypothetical creature to be called alive, it must eat food and release its energy into the universe. Do you have any realistic details about what does this creature eat and how the stored energy is released?

2- Another key feature of a living organisms is that they must have been born. Who/what gave birth to your creature? The implication is that your creature cannot be alone and only one of its kind. There have to be others like it.

3- Yet another key feature of living organisms is reproduction. Will your creature ever give birth to other creatures like it? If yes, how?

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  • $\begingroup$ I was thinking it might simply absorb planets and stars into itself as they are drawn to it via gravity. Not sure if it would have a way of releasing that energy. Though I guess consuming too much mass being stuck because a massive black hole has formed could be a kind of death for such a creature. I'm not really concerned about the creature being born, I don't need an entire species, just an individual and I feel that kind of detail is best left unexplained anyway. $\endgroup$ – user3161729 Oct 27 '16 at 6:50
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At that scale, I don't think it could think. The electrical signals that would need to cross whatever functioned for neurons in its brain would have to be so powerful, they'd destroy the planets and stars which were the neurons. It is very hard for electricity to cross the vacuum of space, so it would need to be astronomically powerful to cross to other solar systems.

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  • $\begingroup$ There is no proof thinking is only electric. $\endgroup$ – Mołot Oct 26 '16 at 13:50
  • $\begingroup$ I was thinking it might use the movements of planets as it's thinking. Perhaps asteroids if it needed a signal to take a more direct route. $\endgroup$ – user3161729 Oct 26 '16 at 14:06
  • $\begingroup$ @user3161729 Well, you could make a primitive computer by moving objects about. I don't think you could make a brain work with asteroids and planets. If you can, that is far beyond my level. $\endgroup$ – J. Doe Oct 26 '16 at 14:09
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    $\begingroup$ JDoe: pure mechanical computers are viable, and they can be Turing-complete. What that means is that if AI is possible in a computer (still an open question, with some theories saying sentience requires quantum interactions) then it can be built out of physical movements with any sort of signaling method: gravity shifts, light emitted/reflected, physical delivery of an asteroid, etc. $\endgroup$ – SRM Oct 26 '16 at 15:12
  • $\begingroup$ @SRM Yes, mechanical computers would be fully possible. $\endgroup$ – J. Doe Oct 26 '16 at 21:19
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I don't think so, considering the distances involved, and the kind of density you'd need, unless a lot of it is "below the surface" embedded in the dark matter or in subspace.

Maybe if the whole galaxy was the organism, and the central black hole was its brain? That would put the impending collision with Andromeda in a new light... Could be they are either going to fight or mate?

How about something like a nebula or stellar nursery, where the gaseous clouds and stars make for a much denser volume of space.

Really, it's your story, so if having a living nebula is important then I don't see why you couldn't come up with some kind of reason for it to work, but if it works on cosmic time scales then I don't know how it being alive will matter for the story at all...

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In a steady-state universe this might (but probably isn't) be an option. We don't live in a steady-state universe, we live in one whose meaningful life is measured in at most 14 digits of years.

As you say, such a creature would operate on a time scale vastly slower than ours--which means it's evolution would also operate on such a time scale. Your creature simply doesn't have time to evolve even if somehow such life could exist.

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