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Message: [QUOTE="Steven D'Aprano, post: 5165023"] I'm not making any value judgements ("better" or "worse") about cars based on their speed. I'm just pointing out that the speed limits on our roads have very little to do with the speeds cars are capable of reaching, and *nothing* to do with ultimate limits due to the laws of physics. Chris made the argument that *the laws of physics* put limits on what we can attain, which is fair enough, but then made the poor example of speed limits on roads falling short of the speed of light. Yes, speed limits on roads fall considerably short of the speed of light, but not because of laws of physics. The speed limit in my street is 50 kilometres per hour, not because that limit is a law of physics, or because cars are incapable of exceeding 50kph, but because the government where I live has decided that 50kph is the maximum safe speed for a car to travel in my street, rounded to the nearest multiple of 10kph. In other words, Chris' example is a poor one to relate to the energy efficiency of computing. A more directly relevant example would have been the efficiency of heat engines, where there is a fundamental physical limit of 100% efficiency. Perhaps Chris didn't mention that one because our technology can build heat engines with 60% efficiency, which is probably coming close to the practical upper limit of attainable efficiency -- we might, by virtue of clever engineering and exotic materials, reach 70% or 80% efficiency, but probably not 99.9% efficiency. That's a good example. Bringing it back to computing technology, the analogy is that our current computing technology is like a heat engine with an efficiency of 0.000001%. Even an efficiency of 1% would be a marvelous improvement. In this analogy, there's an ultimate limit of 100% imposed by physics (Landauer's Law), and a practical limit of (let's say) 80%, but current computing technology is so far from those limits that those limits might as well not exist. "Better" is your word, not mine. I don't actually care about fast cars, but if I did, and if I valued speed above everything else (cost, safety, fuel efficiency, noise, environmental impact, comfort, etc) then yes, I would say 250 mph is "better" than 150 mph, because 250 mph is larger. And yet you're going to disagree with it, even though you agree it is correct? This makes no sense at all. Your two statements about speeds are logically and mathematically equivalent. You cannot have one without the other. Take three numbers, speeds in this case, s1, s2 and c, with c a strict upper-bound. We can take: s1 < s2 < c without loss of generality. So in this case, we say that s2 is greater than s1: s2 > s1 Adding the constant c to both sides does not change the inequality: c + s2 > c + s1 Subtracting s1 + s2 from each side: c + s2 - (s1 + s2) > c + s1 - (s1 + s2) c - s1 > c - s2 In other words, if 250mph is larger than 150mph (a fact, as you accept), then it is equally a fact that 250mph is closer to the speed of light than 150mph. You cannot possibly have one and not the other. So why do you believe that the first form is acceptable, but the second form is nonsense? I do not understand what confusion you think you see here. If we agree on the value judgement "greater top speeds are always better", and the law of physics "c is the upper-limit to speeds", then the following two statements are logically equivalent: "Car HV is better than car FT because the HV has the greater top speed." "Car HV is better than car FT because the HV's top speed is closer to c than the FT's top speed." These sorts of value judgments are independent of the *cause* of the upper limit. Sticking to Chris' example of speed, if we agree that faster travel is better than slower travel, then in the state of Victoria, Australia, the ultimate upper-limit on (legal) speed is 110kph. If we decide to value faster speeds, then the Hume Freeway with its 100kph speed limit is better than my suburban back street with a 50kph speed limit, even though the limit is a social restriction, not an engineering limit or physics limit. If I were arguing that there are no engineering limits prohibiting CPUs reaching Landauer's limit, then you could criticise me for that, but I'm not making that argument. I'm saying that, whatever the practical engineering limits turn out to be, we're unlikely to be close to them, and therefore there are very likely to be many and massive efficiency gains to be made in computing. [/QUOTE]

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