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jacob_cannell comments on [Link] Study: no big filter, we're just too early - Less Wrong Discussion
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Unrelated. There is baryonic and non-baryonic dark matter. Most of the total dark matter is currently believed to be non-baryonic, but even leaving that aside the amount of baryonic dark matter is still significant - perhaps on par or greater than the baryonic visible matter. Most important of all is the light/dark ratio of heavier element baryonic matter and smaller planets/planetoids. There are some interesting new results suggesting most planets/planetoids are free floating rather than bound to stars (see links in my earlier article - "nomads of the galaxy" etc).
There is a limit to how big a giant computing device can get before gravitational heating makes the core unusable - the ideal archilect civ may be small, too small to detect directly. But perhaps they hitch rides orbiting larger objects.
Also, we don't know enough about non-baryonic dark matter/energy to rule it out as having uses or a relation to elder civs (although it seems unlikely, but still - there are a number of oddities concerning the whole dark energy inflation model).
Well we are talking about hypothetical post-singularity civs . . ..
There doesn't appear to be any intrinsic limit to computational energy efficiency with reversible computing, and practicality of advanced quantum computing appears to be proportional to how close one can get to absolute zero and how long one can maintain that for coherence.
So at the limits, computational civs approach CMB temperature and use negligible energy for computation. At some point it becomes worthwhile to spend some energy to move away from stars.
Any model makes some assumptions based on what aspects of engineering/physics we believe will still hold into the future. The article you linked makes rather huge assumptions - aliens civs need to travel around in ships, ships can only move by producing thrust, etc. Even then from what I understand detecting thrust is only possible at in-system distances, not light year distances.
The cold dark alien model i favor simply assumes advanced civs will approach physical limits.
The CMB temperature (2.7 K) is still very warm in relative terms and it's hard to see how effective large-scale quantum computing could be done at that temperature (current crude quantum computers operate at millikelvin temperatures and still have only very miniscule levels of coherence). The only way to get around this is to either use refrigeration to cool down the system (leading to a very hot fusion reactor and refrigeration equipment) or make do with 2.7 K, which would probably lead to a lot of heat dissipation.
You would absorb a large amount of entropy from the CMB at this temperature (about 1000 terabytes per second per square meter); you'd need to compensate for this entropy to keep your reversible computer working.
The CMB is just microwave radiation right? So reflective shielding can block most of that. What are the late engineering limits for microwave reflective coatings? With superconducting surfaces, metamaterials, etc?
Some current telescopes cool down subcomponents to very low temperatures without requiring large fusion reactors.
If the physical limits of passive shielding are non-generous, this just changes the ideal designs to use more active cooling than they otherwise would and limit the ratio of quantum computing stuff to other stuff - presumably there is always some need for active cooling and that is part of the energy budget, but that budget can still be very small and the final device temperature could even be less than CMB.
I'm afraid it can't. The 'shielding' itself would soon reach equilibrium with the CMB and begin emitting at 2.7 K. It makes no difference what it's made of. You can't keep an object cooler than the background temperature indefinitely without expending energy. If you could, you would violate conservation of energy.
And, again, the process of generating that energy would produce a lot of heat and preclude stealth.
But the gross mass of the telescope is never lower than (or even equal to) the background temperature. JWST, for instance, is designed for 50 K operating temperature (which emits radiation at about 100,000 times the background level according to the Stefan-Boltzmann law).
Again, this would just make the problem worse, as a decrease in entropy in one part of the system must be balanced by a larger increase in entropy elsewhere. I'm talking about the possibility of stealth here (while maintaining large-scale computation).
This is a non-obvious statement to me. It seems that a computation on the level you're describing (much larger in scale than the combined brainpower of current human civilization by orders of magnitude) would require a large amount of mass and/or energy and would thus create a very visible heat signature. It would be great if you could offer some calculations to back up your claim.
Years ago I had the idea that advanced civilizations can radiate waste heat into black holes instead of interstellar space, which would efficiently achieve much lower temperatures and also avoid creating detectable radiation signatures. See http://www.weidai.com/black-holes.txt and my related LW post.
It's an interesting idea.
Stable black holes seem difficult to create though - requires alot of mass. Could there be a shortcut?
EDIT: After updating through this long thread, I am now reasonably confident that the above statement is incorrect. Passive shielding in the form of ice can cool the earth against's the sun's irradiance to a temp lower than the black body temp, and there is nothing special about the CMB irradiance. See the math here at the end of the thread.
Sure - if it wasn't actively cooled, but of course we are assuming active cooling. The less incoming radiation the system absorbs, the less excess heat it has to deal with.
Sure you need to expend energy, but obviously the albedo/reflectivity matters a great deal. Do you know what the physical limits for reflectivity are? For example - if the object's surface can reflect all but 10^-10 of the incoming radiation, then the active cooling demands are reduced in proportion, correct?
I'm thinking just in terms of optimal computers, which seems to lead to systems that are decoupled from the external environment (except perhaps gravitationally), and thus become dark matter.
The limits of reversible computing have been discussed in the lit, don't have time to review it here, but physics doesn't appear to impose any hard limit on reversible efficiency. Information requires mass to represent it and energy to manipulate it, but that energy doesn't necessarily need to be dissipated into heat. Only erasure requires dissipation. Erasure can be algorithmically avoided by recycling erased bits as noise fed into RNGs for sampling algorithms. The bitrate of incoming sensor observations must be matched by an outgoing dump, but that can be proportionally very small.
I think you're still not 'getting it', so to speak. You've acknowledged that active cooling is required to keep your computronium brain working. This is another way of saying you expend energy to remove entropy from some part of the system (at the expense of a very large increase in entropy in another part of the system). Which is what I said in my previous reply. However you still seem to think that, given this consideration, stealth is possible.
By the way, the detection ranges given in that article are for current technology! Future technology will probably be much, much better. It's physically possible, for instance, to build a radio telescope consisting of a flat square panel array of antennas one hundred thousand kilometers on a side. Such a telescope could detect things we can't even imagine with current technology. It could resolve an ant crawing on the surface of pluto or provide very detailed surface maps of exoplanets. Unlike stealth, there is no physical limit that I can think of to how large you can build a telescope.
Not theoretically, no. However, at any temperature higher than 0 K, purely reversible computing is impossible. Unfortunately there is nowhere in the universe that is that cold, and again, maintaining this cold temperature requires a constant feed of energy. These considerations impose hard, nonzero limits on power consumption. Performing meaningful computations with arbitrarily small power consumption is impossible in our universe.
You're repeatedly getting very basic facts about physics and computation wrong. I love talking about physics but I don't have the time or energy to keep debating these very basic concepts, so this will probably be my last reply.
No - because you didn't actually answer my question, and you are conflating the reversible computing issue with the stealth issue.
I asked:
The energy expended and entropy produced for cooling is proportional to the incoming radiation absorbed, correct? And this can be lowered arbitrarily with reflective shielding - or is that incorrect? Nothing whatsoever to do with stealth, the context of this discussion concerns only optimal computers.
Don't understand this - the theory on rev computing says that energy expenditure is proportional to bit erasure, plus whatever implementation efficiency. The bit erasure cost varies with temperature sure, but you could still theoretically have a rev computing working at 100K.
You seem to be thinking that approaching zero energy production requires zero temperature - no. Low temperature reduces the cost of bit erasure, but bit erasure itself can also be reduced to arbitrarily low levels with algorithmic level recycling.
Which are?
Such as? Am I incorrect in the assumption that the cost of active cooling is proportional to the temperature or entropy to remove and thus the incoming radiation absorbed - and thus can be reduced arbitrarily with shielding?
Limit: External surface area of computer times σT^4.
As for active cooling, I think the burden of proof here is up to you to present a viable system and the associated calculations. How much energy does it take to keep a e.g. sphere of certain radius cold?
The thermal power you quoted is the perfect black body approximation. For a grey body, the thermal power is:
P = eoAT^4
where e is the material specific emissivity coefficient , and the same rule holds for absorption.
You seem to be implying that for any materials, there is a fundamental physical law which requires that absorption and emission efficiency is the same - so that a reflector which absorbs only e% of the incoming radiation is also only e% efficient at cooling itself through thermal emission.
Fine - even assuming that is the case, there doesn't seem to be any hard limit to reflective efficiency. A hypothetical perfect whitebody which reflects all radiation perfectly would have no need of cooling by thermal emission - you construct the object (somewhere in deep space away from stars) and cool it to epsilon above absolute zero, and then it will remain that cold for the duration of the universe.
There is also current ongoing research into zero-index materials that may exhibit 'super-reflection'. 1
If we can build super-conductors, then super-reflectors should be possible for advanced civs - a super conductor achieves a state of perfect thermal decoupling for electron interactions, suggesting that exotic material states could achieve perfect thermal decoupling for photon interactions.
So the true physical limit is for a perfect white body with reflectivity 1. The thermal power and entropy absorbed is zero, no active cooling required.
Furthermore, it is not clear at all that reflection efficiency must always equal emission efficiency.
Wikipedia's article on the Stefan-Boltzmann Law hints at this:
What do you make of that?
Also - I can think of a large number of apparent counter-examples to the rule that reflection and emission efficiency must be tied.
How do we explain greenhouse warming of the earth, snowball earth, etc? The temperature of the earth appears to mainly depend on it's albedo, and the fraction of incoming light reflected doesn't appear to be intrinsically related to the fraction of outgoing light, with separate mechanisms affecting each.
Or just consider a one-way mirror: it reflects light in one direction, but is transparent in the other. If you surround an object in a one-way mirror (at CMB infrared/microwave wavelengths) - wouldn't it stay very cold as it can emit infrared but is protected from absorbing infrared? Or is this destined to fail for some reason?
I find nothing in the physics you have brought up to rule out devices with long term temperatures much lower than 2.7K - even without active cooling. Systems can be out of equilibrium for extremely long periods of time.
Again, you're getting the fundamental and basic physics wrong. You've also evaded my question.
There is no such thing as a perfect whitebody. It is impossible. All those examples you mention are for narrow-band applications. Thermal radiation is wideband and occurs over the entire electromagnetic spectrum.
The piece in the wikipedia article links to papers such as http://arxiv.org/pdf/1109.5444.pdf in which thermal radiation (and absorption) are increased, not decreased!
Greenhouse warming of the Earth is an entirely different issue and I don't see how it's related. The Earth's surface is fairly cold in comparison to the Sun's.
One-way mirrors do not exist. http://web.archive.org/web/20050313084618/http://cu.imt.net/~jimloy/physics/mirror0.htm What are typically called 'one-way mirrors' are really just ordinary two-way partially-reflective mirrors connecting two rooms where one room is significantly dimmed compared to the other.
Well, firstly, you have to cool it down to below 2.7K in the first place. That most certainly requires 'active cooling'. Then you can either let it slowly equilibrate or keep it actively cold. But then you have to consider the carnot efficiency of the cooling system (which dictates energy consumption goes up as e/Tc, where Tc is the temperature of the computer and e is the energy dissipated by the computer). So you have to consider precisely how much energy the computer is going to use at a certain temperature and how much energy it will take to maintain it at that temperature.
EDIT: You've also mentioned in that thread you linked that "Assuming large scale quantum computing is possible, then the ultimate computer is thus a reversible massively entangled quantum device operating at absolute zero." Well, such a computer would not only be fragile, as you said, but it would also be impossible in the strong sense. It is impossible to reach absolute zero because doing so would require an infinite amount of energy: http://io9.com/5889074/why-cant-we-get-down-to-absolute-zero . For the exact same reason, it is impossible to construct a computer with full control over all the atoms. Every computer is going to have some level of noise and eventual decay.