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So you can burn a piece of paper with magnifying glass and sunlight.

Light from stars are similar to that emitted by the sun, just much, much weaker.

Theoretically, is it possible to cook a thinly sliced piece of meat using powerful magnifying instruments such as gigantic converging lens or dish or combination of those to converge light (and thus, energy) from stars?

This does not have to be on earth, it could be carried out in space for example so there is less interference from external factors such as clouds etc.

If it can not be done, what is the limiting factor?

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  • $\begingroup$ Well, if you go far enough into space, and close enough to another star, you can just do the same thing as you do with the sun :) $\endgroup$ – milo Dec 23 '16 at 10:20
  • $\begingroup$ May be you can, its just a matter of size of your lenses, but the time its takes will be crucial and meat may not be eatable $\endgroup$ – Digvijay Yadav Dec 23 '16 at 10:21
  • $\begingroup$ Also, take a look at this: what-if.xkcd.com/145 . It's a different question because unlike the moon, the stars ARE hot, but still interesting. $\endgroup$ – milo Dec 23 '16 at 10:22
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Purely in theory, yes you could.

You don't really need the light to be of a specific wavelength to generate heat. You mostly just need a high enough concentration of photons.

The problem is that the number of photons falling on earth from a star is many times lower than that from the sun, so you'd have to concentrate the energy from an immense area to get the same intensity.

Ignoring, for the moment, the filtering from the earth's atmosphere (and such), we receive about 1021 photons per square meter per second from the sun. The brightest star (other than the sun) is Sirius. We receive about 109 photons per square meter per second from Sirius, so we'd need to magnify by a factor of about 1012 just to match the intensity we reach from the sun without any magnification1.

Doing a bit of quick math, that works out to a single round magnifier about 10 kilometers in diameter, or equal area in other forms.

Then figure that cooking meat with sunlight does require at least some sort of magnifier, so we can probably multiply that by a factor of at least 3, and probably more like 7-10 or so.

Of course, for now I'm ignoring a such minor details as how you even design (not to mention building) a system even close to that size. There would be decidedly non-trivial challenges involved. For example, the largest mirrors we've built yet have diameters around 10 meters (or non-round mirrors of roughly equivalent area). These are already built in sections, with a computer to control movement of the individual pieces to maintain the illusion of a single mirror acting as a unit. Trying to multiply that out to multiple kilometers--well, I don't think anybody's even contemplated what that would take yet.


1. side note: both these numbers are really for the number of photons hitting the top of the atmosphere, not what we see on earth after being filtered by the atmosphere. We don't really care a whole lot about that though--the intent is just to get at least some idea of what it takes to concentrate starlight to approximately match sunlight.

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Back in the days of Kepler, the theory was, that if sum of the combined light from all visible to the naked eye stars, approximately 5-10,000 max, was equal to or greater than, that received from the Sun. Then we would live in permanent daylight. This theory is still true today!

It would be possible to cook some meat in sunlight. If you live in a desert, just pop a black skillet in the sun and wait couple of hours, this would store enough energy from the sun to fry a egg, albeit slowly, because you need about 160°C to start frying anything, although 55°C is about enough for a rare steak. Now try the same thing under a full moon, it won't work, why? Because the moon only reflects the shorter wavelengths of light and not the infra red wavelengths, which it absorbs, and it is the infra red energy that you need to cook. By the time starlight actually reaches us, all the infra red radiation has been absorbed by interstellar dust or hydrogen molecules.

Therefore, the answer to you question would a be definite NO! No matter how many or how big your optical lenses were!

Another point is, that if infra red radiation from stars were actually reaching us on Earth, there would be a lot of amateur and professional astronomers with burnt retinas.

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  • $\begingroup$ I am sure you are right about how the moon filters sunlight, but starlight is not subject to the same filtration and contains plenty of IR, ionising radiation. To restate, the energy of each photon is the same in starlight, there are just many less of them. Do you need IR to cook, or is it just the most efficient absorption frequency for organic matter? $\endgroup$ – JMLCarter Dec 23 '16 at 12:46
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    $\begingroup$ I would have thought shorter wavelength part of the light carries more energy. What are you basing the statement: 'it is the infra red energy that you need to cook' on? $\endgroup$ – TelKitty Dec 23 '16 at 12:51
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In fact weaker light does not contain photons with less energy, it contains less photons. Which means that the heating and the ionising effect (aka burning) will be the same, just slower. So you could just leave your meat in space for a really long time and eventually it would be burnt. Or if you are not very hungry you can have a really really tiny "piece of meat", like a single atom, it might not take so long.

Cosmic ray ionisation is a real problem/effect both in space and on the surface of the earth, especially for semi-conductor electronics in which it can cause data corruption, a bit-flip. Satellites have to be built with measures to protect against such corruption, either through shielding or detecting it.

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  • $\begingroup$ So this is why Tyrannosaurus Rex never throve in space, no? Your answer is correct in many ways, but I'm not sure about the "just slower" bit: cooking of meat involves change of proteins that in turn requires higher temperature, so I don't think the meat won't get there unless the flux of light is high enough to be equal to $\sigma\,T^4$, where $T$ is high enough to cause this change and $\sigma\,T^4$ is the rate of radiation emission per unit surface area that balances the incoming radiation at steady state. You would need to arrange your magnifying instruments for this to be so. $\endgroup$ – WetSavannaAnimal Dec 23 '16 at 11:54
  • $\begingroup$ I was thinking it might slowly burn but not cook, since ionisation can still occur. $\endgroup$ – JMLCarter Dec 23 '16 at 12:04
  • $\begingroup$ I'm no protein chemist, so I don't really know, but I have a hunch that meat does need to get to a specific temperature to "cook" as we'd normally understand that word. But I think in principle you could gather enough light to do this - there's no principle barring this and in particular it won't worry the second law as there's no possibility of the meat's rising to a higher temperature than the source starlight - it's just going to be really hard to do it with starlight! $\endgroup$ – WetSavannaAnimal Dec 23 '16 at 12:09

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