Once upon a time, I was talking to a young engineer about some new wrinkle in solar concentrators.  He was enthusiastic: he thought that with a little effort, you could focus sunlight enough to generate a temperature higher than that of the Sun itself.

I said ” Nope. ”

Theory is your friend. Correct theory, that is.

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61 Responses to Theory

  1. dearieme says:

    he thought that with a little effort, you could focus sunlight enough to generate a temperature higher than that of the Sun itself.”

    Did he design perpetual motion machines in his spare time?

  2. Smithie says:

    I wonder how possible it would be to build a giant shade to block some of the sun’s light from falling on Venus.

    I don’t see how it could be kept in place.

  3. jbbigf says:

    It depends upon what you mean by “a little effort”. Obviously, electrical energy produced by a system of solar collectors could be used to produce a temperature hotter than that of the apparent surface of the sun. I suspect they do it fairly regularly at Livermore.

    • gcochran9 says:

      You can’t get heat to spontaneously flow from a cooler to a hotter object.

      Trust me on this.

      • jbbigf says:

        “spontaneous” is a rather tendentious term, under the circumstances. Read what I said. There are heat pumps. Are their actions “spontaneous”? They are certainly possible.

      • morris39 says:

        Certainly not possible to circumvent the 2nd Law in the usual definition ie within the system. But if work is done by bringing in energy from outside it should be possible. Maybe this is too pedantic but maybe you are a little too hard on the young guy.

      • dearieme says:

        It’s even been set to music.

      • brokenyogi says:

        And you would be wrong on this. Radiant light energy is very different than conductive heat energy.

        A large but simple set of mirrors could gather enough sunlight energy and focus it in a very concentrated manner to produce temperatures even higher than the surface of the sun.

        I trust me. The math checks out.

        • gcochran9 says:

          How much would you like to bet?

          • brokenyogi says:

            I’d love to bet quite a lot, but I really don’t have the money to pay off if I lost, so it wouldn’t be fair to you. Plus, you’re an easy mark, and it just wouldn’t be fair.

            Again, you don’t understand the second law of thermodynamics. In a passive system, you of course can’t heat something hotter than the original heat source. But we’re talking about an active system. Work changes everything.

            For example, thunderstorms concentrate solar energy in massive clouds, and are able to produce lightning sparks exceeding 30,000K in temperature, far higher than the sun’s 5800K.

            The Sun’s corona is heated by the Sun’s 5800 K photosphere to a few million K. How is that possible under the second law of thermodynamics? Well, again, work does the trick.

            The LHC operates on solar energy, and produces temperatures to near Big-Bang awesomeness. It’s all a question of how you manipulate the energy through design and work.

            Likewise, creating a gigantic array of solar mirrors ground to focus solar energy from several square kilometers of incoming sunlight into a tiny area could produce fantastic temperatures. Easier to do if floating in outer space. Not sure what the usefulness of that would bem other than proving you wrong, but it’s still quite possible. Again, work added changes everything. That’s what engineers do. They add work to natural phenomena to produce unnatural results.

            Re-think what you think you know.

            • gcochran9 says:

              I’m an easy mark with a Ph.D. in physics.

              ” gigantic array of solar mirrors ground to focus solar energy from several square kilometers of incoming sunlight into a tiny area could produce fantastic temperaturesZ”

              No hotter than the surface of the Sun. But, oddly enough, if you had a vast array of mirrors in space concentrating light from Vega – vastly weaker than sunlight – , you could heat something to a temperature greater than the Sun.

              • brokenyogi says:

                You’re not thinking it through properly. People who think they know the answers already stop thinking.

              • gcochran9 says:

                Sure I am. If you’re cash-short, How about betting a kidney?

              • brokenyogi says:

                Trying to see where you’re coming from, and I guess it’s because you imagine that we’re talking about shining the light on an open black body sitting at the focal point of the concentrated sunlight. In that case, you might be right, insofar as the object being heated would also be radiating away its own heat, and the break even point wouldn’t get as high as the surface of the sun.

                But that’s not how engineers would build such a device. For example, imagine a thermal container capable of severely preventing the heat from escaping. The light enters the thermos through a small glass hole. The light energy is converted into heat, and the heat can’t radiate away. So it just keeps building up higher and higher. The thermos could even be a magnetic one so that it can’t be melted away, surrounded by reflective media directing all light back inside. There are perhaps practical limits to such a device, but probably very high ones. The Tokamak for example, is able to use magnetism to contain temperatures in the millions of degrees. So it’s definitely possible.

              • brokenyogi says:

                What am I going to do with one of your kidneys?

                Anyway, your link is exactly what’s wrong with your thinking here. Again, you’re looking at this as a textbook physics problem, rather than an engineering problem. As I point out below, the basic problem is one of thermal equlibrium. Your answer assumes a free point in space that can’t focus light energy to a higher temperature than that of the source. That’s true in the abstract. But that presumes nothing else is done to change the equilibrium between incoming and outgoing radiation. A thermal device which contains the outgoing radiation while allowing the incoming radiation to continue to heat the objects (gases, past a certain point) inside the thermos, allows much higher temperatures to be acheived. Because adding energy will always increase temperature until thermal-radiative equilibrium is again achieved, but this time at a higher level. Otherwise, you are breaking the law of the conservation of energy. And that’s a real no-no.

                So, given that I have no use for your kidney, I ask that you donate it to the kidney banks that serve those who really do need a healthy kidney. Would yours qualify?

              • gcochran9 says:

                You’d bet your kidney: I’d bet money. And I’d win, of course.

                It is a textbook physics problem: and you would not ace the course.

              • brokenyogi says:

                Oh, I did fine on textbook stuff. All you have to do is give the answers the textbooks are looking for. But creative solutions to engineering problems are a different story. That requires imagination. Often sorely lacking among physicists.

            • gcochran is correct here. Relevant xkcd: This was definitely one of the more surprising things I’ve ever learned about physics.

              Light does have entropy, and it can have a definite temperature too, if you put it in a situation where it can reach thermal equilibrium. The relevant degrees of freedom are things like the distribution of directions of photons, and the distribution of their wavelengths.

              Your example with the laser in your comment below is an interesting one. Blackbodies like the sun produce light with a definite temperature, but lasers do not. (Neither do fluorescent light bulbs or LEDs. Incandescent light bulbs work via black-body radiation, and so they do produce light with a definite temperature.) As it turns out, the laser light contains a lot of energy, and almost no entropy. This means that focusing it is a good way to create high temperatures. Sunlight has quite a bit more entropy, which puts an upper limit on how high of a temperature you can create with it.

              Q: “But what about solar panels? You can create high temperatures by powering a laser with a solar panel, right?”

              A: This is a good question, and I think what jbbigf was getting at originally. Solar panels work by taking in hot sunlight, converting most of the energy into heat, and a bit of it into electricity. All the entropy that was originally in the sunlight is carried away in that heat. The physical body of the panel is at much lower temperatures than the solar surface, so this heat can carry more entropy per given amount of energy than the sunlight could. That’s why some of the energy is left over as useful electricity. It’s basically the same principle as a heat engine: Dump some of your energy into a colder system to get rid of the entropy, keep the remaining energy as useful work. Typical optical components like mirrors and lenses arranged in some static configuration can’t really act as a heat engine. They just transmit and reflect all the incoming light. In real mirrors and lenses, there would be a little bit of energy loss, but they wouldn’t be able to extract extra entropy from the transmitted light.

              • brokenyogi says:

                “Light does have entropy, and it can have a definite temperature too, if you put it in a situation where it can reach thermal equilibrium. ”

                Yes, that’s precisely my point. Change the thermal equilibrium, and the equation changes accordingly. Our stubborn host clings to the idea that all we are doing is focusing light on a black body object floating in space. That’s how people who learn physics from textbooks think. But engineers think differently. A black body object receiving sunlight will radiate away that energy as it receives it, acheiving an equilibrium that won’t exceed the sun’s original temperature, because of wavelength issues. So the solution comes in changing the outgoing radiation, while keeping the incoming radiation constant. As I mentioned elsewhere, you can do this by designing a thermos of various kinds, probably partially magnetic, partially reflective, to contain the heat and prevent it from radiating away faster than the incoming radiation from the sun. By changing that equilibrium, you can raise the temperature to much higher levels than its original radiant source.

                The “temperature” of light isn’t an actual temperature, it’s range of light frequencies corresponding to the radiative curve of the original heat source’s temperature. When the incoming light heats the object to the same temperature as the original source, their radiative signature matches. Outgoing equals incoming. But if you prevent the outgoing from going out, that changes everything.

                And that’s basically what solar furnaces do. Not to this extreme extent, because they don’t need to reach such high temperatures. They contain the incoming heat and re-direct it towards a useful purpose, such as driving a steam-engine dynamo to generate electricity. They try to minimize radiative losses. They are not trying to heat black body objects suspended in mid-air. Apply that same principle to achieve extreme temperatures, preventing heat loss by ,pre esoteric means, and you get very high remperatures as a result. Above that of the sun’s surface even.

              • brokenyogi says:

                Also, none of you guys are explaining how the surface of the sun, at 5800k, can somehow heat the sun’s photosphere to several million k. Hint: It doesn’t do that by violating the second law of thermodynamics.

            • jb says:

              Hint: Nobody (except you) believes that the Sun’s corona is heated by radiation from the photosphere. It’s an unresolved problem, but the general feeling is that it has something to do with solar magnetic fields.

              Also, you have this strange idea the engineers are somehow able to get around the laws of physics because they “think differently”. Can you explain where you got that idea?

              • jb says:

                brokenyogi is employing an extremely popular style of argumentation that I will dub The Trollcratic Method. It goes like this:

                You must either prove — to my satisfaction! — that I am wrong, or else you must acknowledge that I am right. There are no other options.

              • brokenyogi says:

                I don’t think the sun’s corona is heated by the radiant light from the surface either. But it’s an example of how heat can be concentrated by thermal containers, in this case likely magnetic, to heat far above the temperature of the heat source. And that’s basically what my model is. Energy from the sun heating a thermal container that allows it to attain much higher temperatures than the source temperatures.

              • brokenyogi says:

                JB, I’m not asking you to prove or disprove anything. I’m just trying to have an interesting conversation that challenges textbook limits. Some people aren’t up to that, I understand that quite well. I guess this probably isn’t the place for that anyway.

          • brokenyogi says:

            Simple question: Do you think light has a temperature?

          • brokenyogi says:

            As you can see in this article, X-ray lasers here on earth can achieve fantastic temperatures, because “the machine can focus the laser pulses to a point three times smaller than the width of a single blood cell.” The source of the laser light is not 3.6 million degrees, but it can achieve that temperature by focusing the light energy into a tiny point.

            Same principle applies to concentrating sunlight.

            So, pay up dude. Oh, crap, I forget to name an amount.


            • Frau Katze says:

              All these examples create very hot spots but they don’t last, It’s like particles in accelerators. You can get the particles energetic enough, surely at least as hot as the sun, but for an extremely brief period of time, Doesn’t defy thermodynamics. You have to apply energy to get them going.

              But what good is it? In the outside world people are fretting about global warming, they want fusion power. I’m very far from knowing what techniques have been tried but I’m starting to think it might not be possible. You need a sustained release of energy. None of the examples given here are sustainable. And practical.

              I hope I’m wrong.

              • brokenyogi says:

                You’re not wrong that much of this is impractical, and besides the point. We’re arguing if it’s at all possible, even for a brief period of time. Using a lower temperature light source to create a higher temperature end product is certainly possible. It’s been done before. Would anyone want to build a solar array like this in space with a thermal receptor to get higher temps? No, they wouldn’t. Could they? I think so. I could be wrong. Everyone here seems to think I’m wrong, and a troll to be arguing otherwise.

                I’ve never suggested any of this would defy the laws of thermodynamics. That’s the argument being made against this. I don’t think it applies to a system to which work and design is applied, which is what engineers do.

                As mentioned, the sun’s surface heat of 5800K is somehow able to heat the coronosphere to several million degrees. Undoubtedly by some mechanism which concentrates that micro-flare heat energy through magnetic fields. So there are natural examples of something like this. How practical it would be to repeat on our own is besides the point.

              • gcochran9 says:

                You are wrong. At this point it’s boring. You know, I spend a certain of time slamming various kinds of idiots. Not so often engineers – they’re a positive force in the world – but yes, they have their own characteristic idiocies, and this is one of them.

                Imagine for a moment what you could do if you could get energy to spontaneously flow from a cooler to a hotter body. Think of the _consequences… hint hint.

                On the other hand, you guys look damn good compared to epidemiologists.

        • Frau Katze says:

          If this is true, why haven’t scientists been able to create fusion power? But I think the fusion is not on the surface of the sun but deep inside at much higher temperatures, that’s likely the answer.

          Do the readers here think fusion power on earth will ever be a possible and controlled way to generate energy?

          • brokenyogi says:

            Because the energy problem of containing and sustaining that energy is very difficult. Not because the laws of thermodynamics prevent it.

            • Frau Katze says:

              I know that but I am blatantly trying to change the topic. The other day, on their weekly podcast, the biologists Bret & Heather Weinstein were musing on global warming.

              They don’t like nuclear fission plants, because of the unresolved problem of disposing of the waste and assume it wouldn’t be a problem with fusion power. Look, I’m not an engineer, but it seems to me that disposing of the waste would be much easier that going for fusion power. There was this proposal to use an old and very geologically stable salt mine but it was rejected by the general public (nimby stuff).

              I also suspect that fusion power would irradiate a lot of nearby matter, so you’d still have waste. But that’s sheer guesswork.

              It was like the failure of the breeder reactor. I read about and how the required coolant was liquid sodium. As soon as I saw “liquid sodium” I thought, that’s going to be very hard. And all attempts have failed. I get that feeling about fusion power too.

              • brokenyogi says:

                With current reactors, waste generation and disposal really is a big problem. The problem is that the waste has to be contained for tens of thousands of years, and we can’t rely on the human race to stay alive, much less on top of things like this, for that long.

                The LFTR (Liquid Flouride Thorium Reactor) idea is being intensively studied by a joint Chinese-American effort. It’s not only theoretically possible, it’s also extremely safe and potentially plentiful, since Thorium is far more common and cheap than Uranium. A huge benefit would be that it not only produces very little waste, it also can be fed the waste from our current Uranium plants and gobble up 99% of it, mostly solving the overall nuclear waste problem. But there are indeed many engineering issues to resolve. Progress is not expected to be fast. They expect to have a small pilot reactor operational within a year or two that can generate electricy, and big commercial ones operational by 2030.

            • Frau Katze says:

              @brokenyogi I’ve heard of the thorium reactor, but few details. I didn’t know a pilot project was in the works. Yes, I know about the long time scale for uranium fission reactors.

    • William Newman says:

      “electrical energy” cleverness “could be used to create a temperature hotter”

      It could indeed, but a process like that is stretching the term “focus” awfully far. One can make an apparatus that does what you describe, but such an apparatus is not focusing the sun’s energy so much as separating the stream of solar energy into at least two fractions, putting one of those fractions (necessarily less than 100%) onto the target at high temperature, and dumping the other fraction into some low-temperature sink (the heatsinkiness of which is necessarily used up, at least in the case of any finite sink). Such a process, requiring diversion of energy and also requiring using up a heat sink, is not what people usually mean by “focus”.

      If you want to describe it informally, with the usual implicit assumption that you are operating on Earth, and the understanding in this situation that you are dealing with energy flow only as concentrated as solar radiation, you could get away with casually omitting the heat sink as in common usage. However, I don’t see how to get “focus” in there idiomatically without leaving people feeling misled once they understand what’s actually going on. I would expect something more like “you can build an apparatus driven by solar energy which uses part of the incoming energy to heat something hotter than the surface of the sun.” Perhaps you could even say “concentrates” instead of “uses”, but either getting “focus” in there or omitting “part of” seems like a stretch.

    • Coagulopath says:

      Lenses work for “free”. They don’t take energy to operate.

      This means you can’t ever use lenses to make something hotter than the energy source the lens is reflecting. Doing so violates the second law of thermodynamics.

  4. jbbigf says:

    I had recent occasion to visit Livermore. In the large, glass-lined outer entryway to a fairly low-security (by Livermore standards) office and conference building, there was a splendid bronze bust of John Von Neumann. My hosts seemed rather puzzled by the interest I displayed in what I suppose was to them a rather dull and unconsidered part of the working environment.

  5. pyrrhus says:

    There have been many media articles in the past claiming that an automobile covered with solar panels could be powered by the sunlight hitting it……Physics is a foreign land to most…

    • jbbigf says:

      There is nothing contrary to physics in the claim that an automobile could be powered by the sunlight hitting it. The thermodynamic consideration is that it has a reservoir — the parts of it that are NOT exposed to the sun — into which it can dump heat. Since it can sit in the middle of a heat-flow, it can do work.The issue is what kind of performance could be obtained. It is difficult to imagine circumstances under which such a system could achieve anything like the acceleration an ICE achieves. But I suppose that on level ground, a great deal can be accomplished by eliminating friction.

      Nah. I was kidding myself. Maybe with some kind of transmitted-power technology. I’m not seeing it.

    • jb says:

      What’s wrong with you? It is totally possible to power a car with solar panels! (As long as you have plenty of time to get where you are going…).

  6. James Thompson says:

    I remember the second law by saying that energy runs downhill.

  7. jb says:

    Years ago I read a science fiction short story involving a room full of people being mysteriously incinerated, and no one could figure out how. The resolution was that a skyscraper miles away was equipped with computer controlled windows, each of which, like a fleet of signal mirrors, had been simultaneously commanded to tilt in such a way as to reflect the sun towards that particular room. The idea was that the skyscraper had briefly been transformed into a huge curved mirror, and that the entirety of the solar energy falling onto that mirror had been focused onto the room, instantly destroying everything.

    It wasn’t a particularly plausible story, and really the only point was to showcase the idea of turning a skyscraper into a death mirror. The idea just seemed wrong though, but it took me a few minutes to figure out why. It isn’t that a skyscraper can’t also be a huge destructive mirror — that can actually happen! But the solid angle of a skyscraper seen from miles away is comparable than that of the sun, so to do that much damage that fast the skyscraper would need to appear to the occupants of the room to be much hotter than the sun. Which, as Greg has pointed out, is impossible.

    • Coagulopath says:

      You used to hear about Archimedes’ “death rays”: hexagonal mirrors that he used to set fire to Roman ships at Syracuse. They make an appearance in the game Age of Empires – they kept zapping my ships, which was annoying.

      It doesn’t seem practical, though, even if it worked in principle. There’s no way you could hold a focused beam of sunlight on a flapping sail on a pitching ship accurately enough for it to catch fire.

      I’ve heard speculation that his death rays were actually Greek fire.

    • jb says:

      I just want to add a bit to this. If it were possible to focus every last bit of solar energy falling on a skyscraper onto a single room miles away then it would indeed be a death ray, because that is a lot of energy. But since the sun isn’t a point source it’s impossible to get the focus that tight, and so the Second Law lives.

      • brokenyogi says:

        Yes, you could focus that light very tightly, not to an actual point, but to a very tiny area. Enough to create a temperature in that small area hotter than the surface of the sun, if the area of the original mirror system or lens was large enough.

          • brokenyogi says:

            Yep. Light is just energy. It has no temperature. It doesn’t remember where it came from. Gather enough of that energy, focus into small enough space, and the temperature will reach whatever level you can manage, regardless of the source of the light.

            You can do this with mirrors, with lenses, with magnetic fields, with hand pumps. The second law goes out the window when work is added. You can take an air compressor, and increase temperatures beyond the original temp just by increasing the pressure. That’s another way of concentrating energy. That’s what engineers learn how to do. When it comes to practical matters, they are much smarter than physicists.

  8. epoch says:

    Lockdowns will select for ever more contagious strains, whereas forced quarantine of those tested positive will select for ever less severe strains.

  9. Cat Rationalist says:

    What if you design refractive medium with index, say, in 10^5 range? (and tolerant to 10^5 K temperature)
    Doing so allows you to make collecting lens with f-ratio of 0.5*10^-5.

  10. moscanarius says:

    On a small enough volume, though? I doubt the guy was meaning to have a sun-sized object reaching a higher temperature than the sun using only solar energy.

    (of course nothing of this would be simple, but then, people mean a lot of different things with “simple”)

  11. quickly burn the boats says:

    OK, but what if the arrangement of lenses is hurtling towards the sun so that the spectrum is blue-shifted?

  12. quickly burn the boats says:

    I see that brokenyogi doesn’t know anything about nuclear reactors either. Fast breeder reactors can fast-fission long-lived actinide waste because they have a surplus of fast neutrons. This allows them to fission materials that are fissionable but not fissile. The same is not true of LFTRs, which would be graphite-moderated. If it’s moderated, it don’t have fast neutrons. By definition. In fact, a large attraction of U-233 as a fuel is that it has favorable eta curves for breeding even when the neutrons are thermal.

    I, like all rational people, think that nuclear reactors are a no-brainer, but I’m tired of knuckle-dragging LFTR fans who don’t know which way is up. This is some basic-ass shit right here, and if you can’t keep that straight, then please don’t open your mouth about nukes in public. It’s an uphill battle even without your “help.”

  13. Casper says:

    When a flashlight battery heats a filament to incandescence, that is a spontaneous transfer of energy from cooler to hotter, no?

    Energy is still flowing downhill, but through a mechanism that isn’t driven by temperature differences.

    [Minor quibble with some of the more strongly-worded Second Law claims here. Not resurrecting the lens idea.]

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