4 ms·
I'm a bit skeptical of the conclusion and the reasoning process used to arrive there. For example, the article states "In other words, all a lens system can do
by robertelder 8y ago
I'm a bit skeptical of the conclusion and the reasoning process used to arrive there.
For example, the article states "In other words, all a lens system can do is make every line of sight end on the surface of a light source, which is equivalent to making the light source surround the target."
If you forget about optics for a second, imagine that the outer surface of the sun were wrapped around a point (think of the image shown in the article). If you consider conservation of energy for the energy flux from the surface of the sun being entirely directed to a single body of matter that absorbs this heat (assume it's a penny), the steady-state blackbody emission of the penny would have to equal the energy flux from the entire surface of the sun. I think this situation would end up making the 'temperature' of the penny much higher than the surface of the sun for the same reason that the center of the sun is hotter than the surface: There is energy expended, and it comes from the fusion of light elements inside the sun, so there is no violation of entropy as stated in the article: "you'd be making heat flow from a colder place to a hotter place without expending energy."
- aqme28 8y agoYou cannot use a magnifying glass to make something hotter than the source of the light. The thermodynamics principle is unyielding. This is a tough tricky thermodynamics question because it really seems like you can use a really big magnifying glass to make say an object that is hotter than the sun. I remember working through it in a physics class.
- JimboOmega 8y agoIf you directed all the input to the penny, then you've created a close system that can't lose energy, and doesn't the whole system - including the sun - keep rising in temperature anyway? Ultimately the outside of this inverted sun system needs to vent the whole of the energy, the energy can't just fall inwards. What you're describing is a reversal though; it requires the penny to be much hotter than the sun, only as a result of the sun's energy flowing into it.
- landryraccoon 8y agoThat doesn't work because when the penny is hotter than the sun it radiates an equal amount of energy back to the sun. The sun is a blackbody, it radiates light because it's hot. Once the penny is the same temperature it will radiate back at the sun until the two are in equilibrium.
- robertelder 8y agoIf the size of the penny was reduced to just a few atoms, wouldn't changing the number of atoms in the penny affect the rate at which the penny itself could emit radiation through blackbody emission? If so, I would expect there would be a size at which the penny could be small enough that it's temperature could grow arbitrarily large once the heat flux in = heat flux out from the penny.
- schoen 8y agoI don't know very much thermodynamics, but I'm not sure that thermodynamic temperature is well-defined for a system consisting of "just a few atoms".
- jarfil 8y agoA penny at a temperature high enough to radiate the same amount of energy as the entire surface of the sun, would be at a far higher temperature than the sun.
- Dylan16807 8y agoIf you surround the penny with sun with no gaps, all the light hits the penny. If you add more sun, you add gaps, and the amount of light hitting the penny stays constant. It never has more than one penny-surface-area of light hitting it, so it never has to get hotter than the sun.
- nomel 8y agoThanks, this is what allowed it to click for me.
- shkkmo 8y agoI think the answer here is that you can't do this. I don't think you can build such a system to redirect all the light from the sun to a surface with a smaller area than the sun because of "conservation of étendue".