Okay. So, I will try to break this down into sections. We have what I believe are agreements, questions and possible disagreements.
To set the stage, let me start with agreements (most of the times followed by a “however”):
Agreements
The standard cheeky description: I 100 % agree. However, I think speaking of structural degradation may serve as a guide for finding ways of using up structure as a means of converting a fraction of heat used in the breaking of structure into useful energy.
Big-O: I 100 % agree, and I find your intuition “One of the things that I think you’ll find approaches zero more quickly than most is the rate at which it is possible to extract work from the system. This is a general property of systems designed to be highly efficient in terms of entropy generation.” Entirely valid, and plausible. However, this system is complex, and I lack many of the skills you have.
Model switching: Yes, this is something I do. However, I am interested in what happens at different levels of the setup. If one model can be used in one instance (to best explain certain features), and another model seems better suited for analysing a different section (for certain features), I will do so. I am interested in things like: “Will any atoms evaporate?”, “How will this affect the atmosphere?”, “Will we get a density accumulation close to the shell?”.
Not adding unnecessary complexity: Yes, that is good advice. However, when I do this, I may be criticized for doing so (e.g. for not adding details about keeping the shell in place).
What you call the correct premises.
I agree that the apparatus being used for energy extraction will necessarily have a degradation scaling with the energy extraction. However, this would be equally true for e.g. solar cells. Are you saying solar cell structural degradation are bigger than energy gains? Basically, that energy converted by the solar cell, would not be sufficient to fix solar cell structural degradation (if structure from the outside couldn’t be brought in). If so, I agree that this is an intriguing possibility (from my current knowledge base and vantage point).
Questions
Would you prefer for me to tighten my language, as best as I can, or shall we deal with specific questions in a back and forth right now? I am not questioning your main point here.
Follow up: Would you like for there to be a Google Doc, where I try to express things like you suggest, like “What object has what interaction with what other object?” and so on?
Have I missed something obvious in this reply? Like mischaracterized you, or seemingly missed something important you said?
Possible disagreements
Perhaps not necessary, but the shell could be seen as “mostly outside the gravity well”, say 31 radii away, making gravity 0.1 % of gravity close to the asteroid. The shell can be attached with wires or pillars, to ensure stability. A symmetrical arrangement of four carbon nanowires ought to be enough in an extremely stable environment with very low level of unbalance and temperature (5 K).
If I have a container in an environment of comparatively high air density, I could just close a lid on a jar in order to trap some air. No information about individual objects needed. If I sat in the International Space Station, I could easily entrap some air. And if the ISS hang above Earth, I could definitely lower my jar towards earth, get some potential energy before earth density became a thing to worry about, open the jar, and release the air, and pull my jar back up again.
I will try to tackle this “lateral motion” question as well as I can. You say: “There’s never any kind of pattern of inelastic interactions that slows the objects down to the right orbital velocity to stay near the shell.” Let us be careful here. I will list a few premises and statements, and you can tell me were I am wrong:
A) Premise: Temperature and gravity well are aligned such that only a tiny minority of atoms will be able to overcome the gravity well.
B) Premise: There are no rotations to take into account (as opposed to here on Earth)
C) Statement: A majority of molecules leaving the atmosphere will move almost radially towards the shell. Leaving radially requires the least amount of speed. Given the speed distribution of molecules at a certain temperature a vas majority of the helium atoms will travel towards the shell along a very radial path.
D) Statement: The speed of atoms hitting the shell are very low for most atoms.
E) Premise: The shell is not microscopically smooth. It is a standard surface, and atoms “bouncing” against it are very unlikely to go out along the same line they came in along.
F) Statement: Temperature in solids can be thought of as random vibrations. As an atom comes in, it will get some kind of push redirecting it. This will cool the shell (since the incoming atom is so slow). Some incoming atoms will get a slow speed (compared to the reference mean speed associated with a temperature of 5 K), some will get a fast speed. Some will go back radially towards the asteroid, most will not.
G) Statement: Most of the atoms traveling out will be pulled back to the asteroid eventually, due to gravity.
H) Statement: There will be an increase in density close to the shell, as opposed to in the space between the shell and the asteroid. I suspect you may have objections here, but I am not entirely sure why.
3. Model switching: Having multiple models at different levels of precision and abstraction is useful and switching between them is useful. But, you need to make sure that when you switch, you really make all necessary changes and understand which points you can carry over and which need what kind of reassessment or adjustment. Otherwise you’re introducing new and unnoticed errors every time you switch. Doing this well enough to form a useful thought experiment means writing down, as an equation or very precise verbal description, every boundary condition, every initial condition, and every force or law governing the evolution of the system.
4. Complexity: The point is, it is a mistake to consider such details unimportant. You mention keeping the shell in place—in place relative to what? Those “details” mean either some sort of active thrusters that consume work, or some sort of extremely long tethers or pillars that change the set of reference frames with respect to which you’re defining the velocity of the particles moving around. They mean your shell and asteroid are not at rest with respect to the reference frame of the CMB, which creates some Doppler shift so the flux is not spatially uniform (turning momentum into a temperature differential, among other things), and also the speeds and frequencies at which the atoms hit the shell and return to the asteroid are not spatially or temporally uniform.
6. It’s not about degradation, it’s about being able to define such a mechanism at all. You’ve essentially define a balloon around a thin gas gravitationally bound to an asteroid, such that it has a scale height. If you know where every atom is, then sure, you can intercept the ones falling down and ignore the rest and thereby do work. But then you can’t talk about T=5K, because you actually have and are relying on your knowledge of the specific microstate. For you, who somehow has such knowledge, T=0, or at least, T<5K. Otherwise, if you don’t have such knowledge, then whatever your try to set up will have to also deal with the atoms moving upwards balancing out the atoms moving downwards, and produce no net work. Solar panels do not have this problem. They have a net flux of high-T sunlight with a known thermal distribution of photon energies coming in to a lower-T environment, and this creates a very predictable theoretically efficiency limit based on the panels’ composition. What gets ‘degraded’ is the sunlight’s photon distribution, not the panel’s structure.
Questions:
I think tightening up your language as described will require a lot of tightening up of your thinking, and make it clearer what is going on. So yes, you should try to do that first and then see where that leaves you. But feel free to ask what you want to ask along the way.
No preference.
I don’t think so, no.
Possible disagreements:
It really can’t. This does not work or help. See above. You’re still trying to call things “small” without comparing their effect size to the other effects you’re using them to balance or dismiss. To put it another way: Is there some finite limit to shell radius beyond which you think your model doesn’t hold up? What happens as you increase the shell radius without bound, or decrease it to be much small? Which effects scale in which ways, which claims break in which order? If there is no clear upper bound you can reason out in this way, then you could just remove the shell entirely (aka place it at infinity) without changing any implications, which I don’t think you believe.
This works because of the jar, and does not work without the jar. Without the jar, you’re essentially claiming that there is some height above the air at which you can place some apparatus in ‘still’ air which will nevertheless produce net work by extracting energy from falling gas atoms in a way that is not balanced by the forces being applied to the apparatus by rising gas atoms. This is why the thermal-vs-individual-objects model switch matters. You can see the jar, and choose to act on information you have about the jar, and the cost of acquiring such information is small relative to the work you can get from the jar. This is not true for individual atoms.
Ok
Assuming this is about the starting conditions, sure. Not true if you’re saying this stays true over time
Ditto
Almost is not entirely. Over time the ‘almost’ adds up
True, if you set up the initial distribution carefully, but again, drawing conclusions based on this requires a semi-quantitative understanding of what ‘very low’ means.
Ok, so not graphene as described, then :-) I was assuming physisorption on a low-energy atomically-flat surface with very high and uniform emissivity due to it being a zero bandgap semiconductor. It’s been a while but I took a whole class in grad school on low temperature vacuum pumps
Yes, a lot of the thermal energy in solids is phononic. No, your conclusions don’t follow, because the real interactions are mediated by specific mechanisms that meaningfully change the result, especially in a thin atmosphere at very low temperature. Classical approximations like heat and temperature are inadequate to predict outcomes here. The phonon density and wavevector distribution, and their effects on momentum and scattering, are quantized and that matters. Example: I once had a conversation with someone who was building quantum computers. They told me about a time they had a piece of metal sitting on a substrate in a very cold vacuum and it just wouldn’t cool off, instead staying at the same temperature for days. Turns out the metal wasn’t clamped hard enough to the surface it was sitting on. There was essentially a phononic band gap between the metal and the surface that meant the heat just couldn’t get out in their setup by conduction without phononic tunneling through a large energy barrier. And it was already too cold for radiative cooling to help, especially since that also would need to account for the phonon momentum distribution since photons have very little momentum. “Very slow atoms bouncing off a very cold surface” is the kind of scenario where you just can’t rely on classical approximations.
This is also an instance of insufficiently careful model switching. Are we talking about a gas at rest on average, with a shell positioned at many times the atmosphere’s scale height? Or are we talking about atoms which tend to convert radial into orbital motion over time (with large enough mean free path that they can orbit in opposite directions without too many collisions)? In the former case, sure. In the latter case, no. Your original description assumes both, without relative quantification of how large each effect might be.
I think my other response about the tennis ball thought experiment should help clarify this. Again, you’re stating multiple assumptions that are approximate and that push in opposite directions, or that can be operationalized in ways that have many different implications.
I learn some new things here. Mostly I learn about how much I do not know. I appreciate that. However:
Let us back away from what I have written as specific conditions.
Suppose we have a very advanced tech base to play around with. Anything not forbidden, they can build. Suppose we can jump into any time in the universe = any CMB temperature between say 3 to 3000 K. Suppose we can make an asteroid of any shape and any mass. Suppose we can add any type of atmosphere (our starting condition atmosphere).
Suppose we can shape the shell in clever ways, and attach it in clever ways at the most advantageous distance. Suppose we can have shell geometry that funnels incoming atoms, built in order to trap them (in order to build up a small density gradient near the shell kind of analogous to how earth plus atmosphere makes for a geometry that traps heat in a way the moon does not).
Suppose we can have any kind of fancy way of passively entrapping pockets of atoms that exist in a built up density gradient. Maximally clever. Suppose we can have any way we want to send the entrapped atoms through the near vacuum between the shell and the asteroid, where we convert potential energy into work, after which we release the atoms back to the atmosphere again.
Would you say (in your own words) something like:
A) This clearly can’t work, no matter how you tweak it!
B) Hmm… With the right amount of tweaking, perhaps ambient heat would be able to create a temperature gradient, kind of analogous to Planet X. Structural degradation would ensure it couldn’t keep going forever, but it would be an impressive setup in the meantime. In the meantime we probably could get cycles, where each cycle would generate work in excess of the energy cost for the cycle.
C) Fascinating! I know precicely how I would want to tweak the parameters.
D) I don’t know. It just got too complex, with too many unknowable hypotheticals.
I would say that if you find a place in the universe where there exists any kind of free energy gradient—any differential in pressure, temperature, composition, or other ‘structure’ as you’ve been calling it—then with Sufficiently Advanced (TM) technology you can extract work from it. If you’re Sufficiently Smart and patient you may be able to build the equipment in a way that allows you to later reversibly extract the work that went into its construction.
What you can’t do is start from a lack of such gradients, and create a system that causes them to passively form. As described, your system can’t work, because it’s trying to claim it can have passive diffusion that is net in one direction, and work extraction from controlled flow in the other direction that consumes the gradient produced. You’re trying to make this work by taking in thermal radiation from the CMB, but this also doesn’t work, because the CMB is uniform and there’s no process cooling the shell’s outer surface below the CMB temperature and no process that passively could even in principle.
All that adds up to (A). You cannot define the system in a way that is both self-consistent and functional.
Let’s compare with processes that could work:
You find an asteroid with a liquid ocean and no atmosphere. The liquid evaporates, and you run turbines off the escaping gas. This is Planet X again.
You find an asteroid hurtling through space at you. You place some device in its way, and extract work from the energy of collision.
You find a star, and surround it with a Dyson Sphere. The star consumes its own mass to form a hot plasma emitting lots of photons, which you capture and convert to electricity. No problem! You’re working off the stored potential of hydrogen to undergo fusion. You consume some of the work to keep the Dyson sphere in place.
Your Dyson Sphere enclosed star goes supernova and leaves behind a black hole. You hurl damaged pieces of your Dyson Sphere into it, and the (somehow surviving or repaired) remaining pieces extract work from the gamma rays and other radiation given off as matter falls towards the event horizon. No problem—you’re consuming the structure and mass-energy of the sphere’s matter.
When you run out of spare Dyson Sphere parts, you’ve got to use more of that cleverness. The black hole will be colder than the CMB, and will begin gaining mass by absorbing the CMB, and become even colder in the process. I cannot think of a way to use that to do (a very small amount per unit time of) work, but a Sufficiently Advanced Alien might. This works until, eventually, the universe expands and the CMB cools to below the black hole’s temperature, after which the black hole’s evaporation by Hawking radiation outpaces CMB absorption. Even then you might be able to do work by turning your equipment around and harnessing the Hawking radiation and using the CMB as the heat sink, right up until it finishes evaporating.
So overall: Yes, I can imagine there could be a system that couples some astronomical object to the CMB, absorbing its heat and doing work. No, from within the universe, “we” cannot set up such a system except by finding a pre-existing gradient of structure to extract from, or by doing more work to create such a gradient than we can extract by consuming it.
I can’t take much credit, they’re ideas generally in the zeitgeist at the boundary of physics, sci-fi, and speculative engineering.
If you like sci-fi, and haven’t read these already, you may want to check out Asimov’s short story The Last Question, William Olaf Stapledon’s short novel Star Maker, and Clarke’s trilogy A Time Odyssey. All have elements of “What would it take and look like for a civilization to actually survive into the utmost future, long after all the stars have burned out?” They don’t talk about these specific mechanisms (the first two were from before we knew about the CMB!) but I find them really interesting and thought provoking.
I like Asimov, and I love Clarke’s storytelling. Reading his books, it amazes me how he seemingly predicted some of the technology we take for granted today. I can’t help wondering if he may not in part have manefested his predictions, by inspiring the actual inventors. I have never read William Olaf Stapledon. A recomendation, I take it?
I wonder, are you planning to answer these two questions. You have no obligation to do so, obviously. Only if it feels constructive to do so.
Yeah. Stapledon is older—Star Maker was written in 1937, and it builds on the themes of Last and First Men, a book he wrote in 1930. They don’t really have much plot to speak of, they’re more purely exploratory and written as a kind of future history/scifi cosmogony/speculative evolutionary engineering/secular eschatology. But they’re quick reads and I think they’re interesting worldbuilding thought experiments.
I do think there’s some inspiration of that type that goes on, yes. But also, it is often possible for a field to know early on what some of the theoretical limits are for what can be achieved through it, even if it takes decades or more to even start seeing it happen. The great scifi authors are the ones that ask what it will mean when they do.
1) What if you could use the Cosmological Degradation as your entropy sink? What if you could tweak the asteroid sufficiently cleverly to make this work. The law about “entropy always increasing” would not be broken.
2) What if the structure of your setup was your gradient? What if the asteroid and its gravity well, the shell, the atmosphere and the energy conversion equipment would degrade in an edgecase like this in a way that could never be repaired from the energy converted. The law about “entropy always increasing” would not be broken.
Do you see anyting absolutely forbidding those two possibilities? And for 2, I would be interested in your intuition both for the asteroid setup, and in general.
Hey, sorry, I thought I’d responded to this one and apparently hadn’t.
I think my black hole discussion is essentially my answer to (1). I don’t think I could think of a way to make it work with an asteroid or similar setup. I am not entirely sure your discussion of cosmological degradation is well-defined enough to answer more precisely than that.
For (2), my other comments about you can of course do work to create a gradient you then consume, and get some of the work back. But as written, no, that doesn’t mean the setup as described can work.
Okay. So, I will try to break this down into sections. We have what I believe are agreements, questions and possible disagreements.
To set the stage, let me start with agreements (most of the times followed by a “however”):
Agreements
The standard cheeky description: I 100 % agree. However, I think speaking of structural degradation may serve as a guide for finding ways of using up structure as a means of converting a fraction of heat used in the breaking of structure into useful energy.
Big-O: I 100 % agree, and I find your intuition “One of the things that I think you’ll find approaches zero more quickly than most is the rate at which it is possible to extract work from the system. This is a general property of systems designed to be highly efficient in terms of entropy generation.” Entirely valid, and plausible. However, this system is complex, and I lack many of the skills you have.
Model switching: Yes, this is something I do. However, I am interested in what happens at different levels of the setup. If one model can be used in one instance (to best explain certain features), and another model seems better suited for analysing a different section (for certain features), I will do so. I am interested in things like: “Will any atoms evaporate?”, “How will this affect the atmosphere?”, “Will we get a density accumulation close to the shell?”.
Not adding unnecessary complexity: Yes, that is good advice. However, when I do this, I may be criticized for doing so (e.g. for not adding details about keeping the shell in place).
What you call the correct premises.
I agree that the apparatus being used for energy extraction will necessarily have a degradation scaling with the energy extraction. However, this would be equally true for e.g. solar cells. Are you saying solar cell structural degradation are bigger than energy gains? Basically, that energy converted by the solar cell, would not be sufficient to fix solar cell structural degradation (if structure from the outside couldn’t be brought in). If so, I agree that this is an intriguing possibility (from my current knowledge base and vantage point).
Questions
Would you prefer for me to tighten my language, as best as I can, or shall we deal with specific questions in a back and forth right now? I am not questioning your main point here.
Follow up: Would you like for there to be a Google Doc, where I try to express things like you suggest, like “What object has what interaction with what other object?” and so on?
Have I missed something obvious in this reply? Like mischaracterized you, or seemingly missed something important you said?
Possible disagreements
Perhaps not necessary, but the shell could be seen as “mostly outside the gravity well”, say 31 radii away, making gravity 0.1 % of gravity close to the asteroid. The shell can be attached with wires or pillars, to ensure stability. A symmetrical arrangement of four carbon nanowires ought to be enough in an extremely stable environment with very low level of unbalance and temperature (5 K).
If I have a container in an environment of comparatively high air density, I could just close a lid on a jar in order to trap some air. No information about individual objects needed. If I sat in the International Space Station, I could easily entrap some air. And if the ISS hang above Earth, I could definitely lower my jar towards earth, get some potential energy before earth density became a thing to worry about, open the jar, and release the air, and pull my jar back up again.
I will try to tackle this “lateral motion” question as well as I can. You say: “There’s never any kind of pattern of inelastic interactions that slows the objects down to the right orbital velocity to stay near the shell.” Let us be careful here. I will list a few premises and statements, and you can tell me were I am wrong:
A) Premise: Temperature and gravity well are aligned such that only a tiny minority of atoms will be able to overcome the gravity well.
B) Premise: There are no rotations to take into account (as opposed to here on Earth)
C) Statement: A majority of molecules leaving the atmosphere will move almost radially towards the shell. Leaving radially requires the least amount of speed. Given the speed distribution of molecules at a certain temperature a vas majority of the helium atoms will travel towards the shell along a very radial path.
D) Statement: The speed of atoms hitting the shell are very low for most atoms.
E) Premise: The shell is not microscopically smooth. It is a standard surface, and atoms “bouncing” against it are very unlikely to go out along the same line they came in along.
F) Statement: Temperature in solids can be thought of as random vibrations. As an atom comes in, it will get some kind of push redirecting it. This will cool the shell (since the incoming atom is so slow). Some incoming atoms will get a slow speed (compared to the reference mean speed associated with a temperature of 5 K), some will get a fast speed. Some will go back radially towards the asteroid, most will not.
G) Statement: Most of the atoms traveling out will be pulled back to the asteroid eventually, due to gravity.
H) Statement: There will be an increase in density close to the shell, as opposed to in the space between the shell and the asteroid. I suspect you may have objections here, but I am not entirely sure why.
Agreements
3. Model switching: Having multiple models at different levels of precision and abstraction is useful and switching between them is useful. But, you need to make sure that when you switch, you really make all necessary changes and understand which points you can carry over and which need what kind of reassessment or adjustment. Otherwise you’re introducing new and unnoticed errors every time you switch. Doing this well enough to form a useful thought experiment means writing down, as an equation or very precise verbal description, every boundary condition, every initial condition, and every force or law governing the evolution of the system.
4. Complexity: The point is, it is a mistake to consider such details unimportant. You mention keeping the shell in place—in place relative to what? Those “details” mean either some sort of active thrusters that consume work, or some sort of extremely long tethers or pillars that change the set of reference frames with respect to which you’re defining the velocity of the particles moving around. They mean your shell and asteroid are not at rest with respect to the reference frame of the CMB, which creates some Doppler shift so the flux is not spatially uniform (turning momentum into a temperature differential, among other things), and also the speeds and frequencies at which the atoms hit the shell and return to the asteroid are not spatially or temporally uniform.
6. It’s not about degradation, it’s about being able to define such a mechanism at all. You’ve essentially define a balloon around a thin gas gravitationally bound to an asteroid, such that it has a scale height. If you know where every atom is, then sure, you can intercept the ones falling down and ignore the rest and thereby do work. But then you can’t talk about T=5K, because you actually have and are relying on your knowledge of the specific microstate. For you, who somehow has such knowledge, T=0, or at least, T<5K. Otherwise, if you don’t have such knowledge, then whatever your try to set up will have to also deal with the atoms moving upwards balancing out the atoms moving downwards, and produce no net work. Solar panels do not have this problem. They have a net flux of high-T sunlight with a known thermal distribution of photon energies coming in to a lower-T environment, and this creates a very predictable theoretically efficiency limit based on the panels’ composition. What gets ‘degraded’ is the sunlight’s photon distribution, not the panel’s structure.
Questions:
I think tightening up your language as described will require a lot of tightening up of your thinking, and make it clearer what is going on. So yes, you should try to do that first and then see where that leaves you. But feel free to ask what you want to ask along the way.
No preference.
I don’t think so, no.
Possible disagreements:
It really can’t. This does not work or help. See above. You’re still trying to call things “small” without comparing their effect size to the other effects you’re using them to balance or dismiss. To put it another way: Is there some finite limit to shell radius beyond which you think your model doesn’t hold up? What happens as you increase the shell radius without bound, or decrease it to be much small? Which effects scale in which ways, which claims break in which order? If there is no clear upper bound you can reason out in this way, then you could just remove the shell entirely (aka place it at infinity) without changing any implications, which I don’t think you believe.
This works because of the jar, and does not work without the jar. Without the jar, you’re essentially claiming that there is some height above the air at which you can place some apparatus in ‘still’ air which will nevertheless produce net work by extracting energy from falling gas atoms in a way that is not balanced by the forces being applied to the apparatus by rising gas atoms. This is why the thermal-vs-individual-objects model switch matters. You can see the jar, and choose to act on information you have about the jar, and the cost of acquiring such information is small relative to the work you can get from the jar. This is not true for individual atoms.
Ok
Assuming this is about the starting conditions, sure. Not true if you’re saying this stays true over time
Ditto
Almost is not entirely. Over time the ‘almost’ adds up
True, if you set up the initial distribution carefully, but again, drawing conclusions based on this requires a semi-quantitative understanding of what ‘very low’ means.
Ok, so not graphene as described, then :-) I was assuming physisorption on a low-energy atomically-flat surface with very high and uniform emissivity due to it being a zero bandgap semiconductor. It’s been a while but I took a whole class in grad school on low temperature vacuum pumps
Yes, a lot of the thermal energy in solids is phononic. No, your conclusions don’t follow, because the real interactions are mediated by specific mechanisms that meaningfully change the result, especially in a thin atmosphere at very low temperature. Classical approximations like heat and temperature are inadequate to predict outcomes here. The phonon density and wavevector distribution, and their effects on momentum and scattering, are quantized and that matters. Example: I once had a conversation with someone who was building quantum computers. They told me about a time they had a piece of metal sitting on a substrate in a very cold vacuum and it just wouldn’t cool off, instead staying at the same temperature for days. Turns out the metal wasn’t clamped hard enough to the surface it was sitting on. There was essentially a phononic band gap between the metal and the surface that meant the heat just couldn’t get out in their setup by conduction without phononic tunneling through a large energy barrier. And it was already too cold for radiative cooling to help, especially since that also would need to account for the phonon momentum distribution since photons have very little momentum. “Very slow atoms bouncing off a very cold surface” is the kind of scenario where you just can’t rely on classical approximations.
This is also an instance of insufficiently careful model switching. Are we talking about a gas at rest on average, with a shell positioned at many times the atmosphere’s scale height? Or are we talking about atoms which tend to convert radial into orbital motion over time (with large enough mean free path that they can orbit in opposite directions without too many collisions)? In the former case, sure. In the latter case, no. Your original description assumes both, without relative quantification of how large each effect might be.
I think my other response about the tennis ball thought experiment should help clarify this. Again, you’re stating multiple assumptions that are approximate and that push in opposite directions, or that can be operationalized in ways that have many different implications.
Thank you, Anthony!
I learn some new things here. Mostly I learn about how much I do not know. I appreciate that. However:
Let us back away from what I have written as specific conditions.
Suppose we have a very advanced tech base to play around with. Anything not forbidden, they can build. Suppose we can jump into any time in the universe = any CMB temperature between say 3 to 3000 K. Suppose we can make an asteroid of any shape and any mass. Suppose we can add any type of atmosphere (our starting condition atmosphere).
Suppose we can shape the shell in clever ways, and attach it in clever ways at the most advantageous distance. Suppose we can have shell geometry that funnels incoming atoms, built in order to trap them (in order to build up a small density gradient near the shell kind of analogous to how earth plus atmosphere makes for a geometry that traps heat in a way the moon does not).
Suppose we can have any kind of fancy way of passively entrapping pockets of atoms that exist in a built up density gradient. Maximally clever. Suppose we can have any way we want to send the entrapped atoms through the near vacuum between the shell and the asteroid, where we convert potential energy into work, after which we release the atoms back to the atmosphere again.
Would you say (in your own words) something like:
A) This clearly can’t work, no matter how you tweak it!
B) Hmm… With the right amount of tweaking, perhaps ambient heat would be able to create a temperature gradient, kind of analogous to Planet X. Structural degradation would ensure it couldn’t keep going forever, but it would be an impressive setup in the meantime. In the meantime we probably could get cycles, where each cycle would generate work in excess of the energy cost for the cycle.
C) Fascinating! I know precicely how I would want to tweak the parameters.
D) I don’t know. It just got too complex, with too many unknowable hypotheticals.
I would say that if you find a place in the universe where there exists any kind of free energy gradient—any differential in pressure, temperature, composition, or other ‘structure’ as you’ve been calling it—then with Sufficiently Advanced (TM) technology you can extract work from it. If you’re Sufficiently Smart and patient you may be able to build the equipment in a way that allows you to later reversibly extract the work that went into its construction.
What you can’t do is start from a lack of such gradients, and create a system that causes them to passively form. As described, your system can’t work, because it’s trying to claim it can have passive diffusion that is net in one direction, and work extraction from controlled flow in the other direction that consumes the gradient produced. You’re trying to make this work by taking in thermal radiation from the CMB, but this also doesn’t work, because the CMB is uniform and there’s no process cooling the shell’s outer surface below the CMB temperature and no process that passively could even in principle.
All that adds up to (A). You cannot define the system in a way that is both self-consistent and functional.
Let’s compare with processes that could work:
You find an asteroid with a liquid ocean and no atmosphere. The liquid evaporates, and you run turbines off the escaping gas. This is Planet X again.
You find an asteroid hurtling through space at you. You place some device in its way, and extract work from the energy of collision.
You find a star, and surround it with a Dyson Sphere. The star consumes its own mass to form a hot plasma emitting lots of photons, which you capture and convert to electricity. No problem! You’re working off the stored potential of hydrogen to undergo fusion. You consume some of the work to keep the Dyson sphere in place.
Your Dyson Sphere enclosed star goes supernova and leaves behind a black hole. You hurl damaged pieces of your Dyson Sphere into it, and the (somehow surviving or repaired) remaining pieces extract work from the gamma rays and other radiation given off as matter falls towards the event horizon. No problem—you’re consuming the structure and mass-energy of the sphere’s matter.
When you run out of spare Dyson Sphere parts, you’ve got to use more of that cleverness. The black hole will be colder than the CMB, and will begin gaining mass by absorbing the CMB, and become even colder in the process. I cannot think of a way to use that to do (a very small amount per unit time of) work, but a Sufficiently Advanced Alien might. This works until, eventually, the universe expands and the CMB cools to below the black hole’s temperature, after which the black hole’s evaporation by Hawking radiation outpaces CMB absorption. Even then you might be able to do work by turning your equipment around and harnessing the Hawking radiation and using the CMB as the heat sink, right up until it finishes evaporating.
So overall: Yes, I can imagine there could be a system that couples some astronomical object to the CMB, absorbing its heat and doing work. No, from within the universe, “we” cannot set up such a system except by finding a pre-existing gradient of structure to extract from, or by doing more work to create such a gradient than we can extract by consuming it.
I really enjoy imagining your last point, by the way ^^. I do not know if you meant to, but you paint a beautiful picture.
I can’t take much credit, they’re ideas generally in the zeitgeist at the boundary of physics, sci-fi, and speculative engineering.
If you like sci-fi, and haven’t read these already, you may want to check out Asimov’s short story The Last Question, William Olaf Stapledon’s short novel Star Maker, and Clarke’s trilogy A Time Odyssey. All have elements of “What would it take and look like for a civilization to actually survive into the utmost future, long after all the stars have burned out?” They don’t talk about these specific mechanisms (the first two were from before we knew about the CMB!) but I find them really interesting and thought provoking.
I like Asimov, and I love Clarke’s storytelling. Reading his books, it amazes me how he seemingly predicted some of the technology we take for granted today. I can’t help wondering if he may not in part have manefested his predictions, by inspiring the actual inventors. I have never read William Olaf Stapledon. A recomendation, I take it?
I wonder, are you planning to answer these two questions. You have no obligation to do so, obviously. Only if it feels constructive to do so.
Yeah. Stapledon is older—Star Maker was written in 1937, and it builds on the themes of Last and First Men, a book he wrote in 1930. They don’t really have much plot to speak of, they’re more purely exploratory and written as a kind of future history/scifi cosmogony/speculative evolutionary engineering/secular eschatology. But they’re quick reads and I think they’re interesting worldbuilding thought experiments.
I do think there’s some inspiration of that type that goes on, yes. But also, it is often possible for a field to know early on what some of the theoretical limits are for what can be achieved through it, even if it takes decades or more to even start seeing it happen. The great scifi authors are the ones that ask what it will mean when they do.
Ah! I’m glad I asked. So I had two guesses.
1) What if you could use the Cosmological Degradation as your entropy sink? What if you could tweak the asteroid sufficiently cleverly to make this work. The law about “entropy always increasing” would not be broken.
2) What if the structure of your setup was your gradient? What if the asteroid and its gravity well, the shell, the atmosphere and the energy conversion equipment would degrade in an edgecase like this in a way that could never be repaired from the energy converted. The law about “entropy always increasing” would not be broken.
Do you see anyting absolutely forbidding those two possibilities? And for 2, I would be interested in your intuition both for the asteroid setup, and in general.
Hey, sorry, I thought I’d responded to this one and apparently hadn’t.
I think my black hole discussion is essentially my answer to (1). I don’t think I could think of a way to make it work with an asteroid or similar setup. I am not entirely sure your discussion of cosmological degradation is well-defined enough to answer more precisely than that.
For (2), my other comments about you can of course do work to create a gradient you then consume, and get some of the work back. But as written, no, that doesn’t mean the setup as described can work.