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Nitpick, I know, but: it's not powered by gravity it's powered by human muscles, so in the end - by food.
by ajuc 11y ago
Nitpick, I know, but: it's not powered by gravity it's powered by human muscles, so in the end - by food.
- romaniv 11y agoI don't think it's a nitpick. I think this is a very important point. Is the hypothetical fuel more expensive than food?
- ajuc 11y agoLED is much more efficient than kerosene lamp, so I guess yes.
- daveloyall 11y agoI think your muscles are pretty energy efficient. The kinetic potential battery (the weight lifted against gravity) is also very efficient. Food is very energy dense. https://what-if.xkcd.com/128/ https://what-if.xkcd.com/128/ (sandwich vs battery) Uh, change of topic: isn't the caloric value of a food how much heat can be released by burning it (probably when it is dehydrated)? Plenty of energy stored in a sandwich, sure. But a car battery could power a small spaceheater-like thing for... a while. There's a lot of potential for heat production in a car battery (even without BURNING it). Did Mr. Munroe get this one wrong?
- gus_massa 11y agoFrom Wikipedia: https://en.wikipedia.org/wiki/Muscle#Efficiency https://en.wikipedia.org/wiki/Muscle#Efficiency > The efficiency of human muscle has been measured (in the context of rowing and cycling) at 18% to 26%. The efficiency is defined as the ratio of mechanical work output to the total metabolic cost, as can be calculated from oxygen consumption.
- haxiomic 11y agoI take your point, but the food ultimately gets its energy from the sun, which gets its energy from collapsing under gravity. I don't think there's a clear place to stop, but I'd say the name Gravity Light gets the idea across anyhow
- avian 11y agoSun is powered by fusion. Gravity just holds it together.
- Confusion 11y agoThe fusion persists because gravity forces hydrogen sufficiently close together.
- avian 11y agoBy that logic my car is powered by steel, because that's what keeps the fuel-air mixture sufficiently close together.
- scott_s 11y agoClose, but not quite. The steel of your engine did not cause your engine to exist. But gravitational collapse of interstellar gas is the process that formed the sun. So, in that sense, gravity comes before the fusion.
- Confusion 11y agoIt's common to say a car is powered by fuel. It's also common to say a car is powered by its engine. The latter is indeed is a device that brings fuel and air close together (and does some more important things). It's less common, but certainly not wrong, to say a car is powered by combustion. Similarly, I think saying both 'the sun is powered by gravity', 'the sun is powered by hydrogen' and 'the sun is powered by fusion' is reasonable.
- ajuc 11y agoSo gravity is a catalyst, not the energy source.
- jheriko 11y agoits a symbiotic relationship. stars exist generally at an equilibrium point where gravity is trying to make it collapse, but the pressure produced from the constituents pushing outwards balances it out. this is because if something is collapsing from gravity it will stop once some other force is pushing out harder. this explains a lot of phenomena and is a key piece of understanding the modern theories of stellar phenomena, e.g. via the pauli exclusion principle, why neutron stars can form during a supernova (from the extra inward pressure combining with the gravitational collapse exceeding the degeneracy pressure of electrons in the plasma)... its also why beyond a critical mass the collapse doesn't stop at all and produces a black hole. in any case energy is always conserved... but the ultimate place it is being extracted from to power a star is the gravitational field. everything else, e.g. the nuclear fusion, the light pressure from temperature etc. is a result of that energy being transferred into the star as it forms and continues to evolve.
- mdturnerphys 11y agoIf we're playing that game: - Human muscles get energy from food. - Food is or gets its energy from plants. - Plants get energy from the Sun. - The Sun's energy comes from gravity-forced fusion. So gravity :)
- ajuc 11y agoGood point, but it's not the end. Fusion uses up energy put there by the Big Bang :)
- brianpan 11y agoThe Big Bang came from....gravity?
- ajuc 11y agoIs this the science consensus? I thought it's one of competing unconfirmed theories?
- jheriko 11y agoi believe it is very much the consensus. the FLRW model of cosmology, which has made numerous confirmed predictions teaches us that the big bang is a geometric property of space-time i.e. a consequence gravity, and particularly Einstein's field equation from general relativity.
- dear 11y agoSo by your logic fossil fuel is solar energy.
- andrewstuart2 11y agoBy causality, it kinda is.
- mikepurvis 11y agoIt is, it's just massively time-shifted. It's interesting to put that in perspective: the pyramids were built out of solar energy by way of muscle. Skyscrapers are built out of solar energy mostly by way of diesel fuel.