Can you use a magnifying glass and moonlight to light a fire? (2016) • Hacker News Board

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#231

Post by Retric » Sat, Dec 22, 2018, 4:05 PM UTC

nshepperd wrote: post #168

skolos wrote: post #145

Interesting article. Right in many places. Wrong (possibly) in main conclusion. Entropy argument - correct in the sense that using radiation from black body we cannot use lenses to heat another body to the temperature higher than original. Easy to understand why - the first body has a temperature, radiation has the same temperature, if we apply the radiation to another object it will not heat up more than the radiati…

The conclusion is correct. It doesn't rely on the moon being a black body radiator, but on etendue, which shows that the most you can do with a system of lenses and mirrors is to create an environment (on earth) as bright as the environment on the moon. The fact that rocks on the moon's surface only reach 100°C shows that an environment like that is not bright enough to light a fire.

First some light is reflected from the moon without increasing the temperature of moon rocks. Second, the moon landings avoided peak moon temperatures making the fact astronots survived irrelevant. Just look at how long their shadows where, it was a long way from noon and 127c surface temperatures.

Finally paper burns at ~233c but plenty of things ignite at lower temperatures. Hay’s ignition temperature is ~130c which considering the moon is 127c and you will concentrate extra reflected sunlight seems very viable.

However, the earth’s atmosphere blocks a lot of moonlight, so it’s more likely to work in a pressurized cabin at high altitude or spacecraft. Even your lenses are going to be an issue.

PS: A lunar day is almost a month long. They are casting long shadows which means they are a long way from noon. https://en.m.wikipedia.org/wiki/Apollo_11#/media/File%3AAldr...

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#232

Post by ufmace » Sat, Dec 22, 2018, 4:14 PM UTC

skolos wrote: post #145

Interesting article. Right in many places. Wrong (possibly) in main conclusion. Entropy argument - correct in the sense that using radiation from black body we cannot use lenses to heat another body to the temperature higher than original. Easy to understand why - the first body has a temperature, radiation has the same temperature, if we apply the radiation to another object it will not heat up more than the radiati…

Trying to think about this some.

If the Moon was actually a giant mirror-lens thing that was the size of the Moon, but perfectly shaped to reflect and concentrate sunlight shining on it into as small an area as possible on the Earth's surface, you'd start a fire with that very easily. Probably turn a county-sized area of the crust into lava or something actually.

On the other hand, if the Moon was a perfect reflector, but a big sphere/hemisphere pointed at the sun, then it would receive a lot of high-energy radiation, but reflect it back out in every direction all over the universe. I'm not sure of the math for that offhand, but it sounds difficult to concentrate it back to something even as concentrated as direct sunlight. I suppose it would involve considering the fraction of the total solid angle the reflecting area of the Moon would shine at. Even the entire Earth's surface would be a really tiny fraction of that area, so sounds impossible to concentrate things back to fire-starting intensity.

Assuming these scenarios are basically right, then the actual Moon is a lot closer to the second than the first. So starting a fire isn't practical with any lens system you could build on Earth. Maybe you could build some massive mega-lens thing in space near the moon that concentrated enough of it back on the Earth to be near-sunlight brightness. Or just set up a more manageable sized reflector on the Moon itself, or in space, to reflect sunlight in a more concentrated way to a small area on the Earth.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#233

Post by nkurz » Sat, Dec 22, 2018, 4:16 PM UTC

dekhn wrote: post #227

nkurz wrote: post #139

Earlier quoted context omitted.

It is a truism: no matter how many times this is explained What exactly is the "this" that you refer to? I don't think the issue is that the "engineers" disagree with the physical principles, rather they tend to disagree that the physical principles apply in quite the way that the author claims. Many of the engineers probably believe is that Munroe is correct in claiming that on cannot start a fire with a low tempera…

The this I refer to is "the principle of etendue": https://en.wikipedia.org/wiki/Etendue and I agree it's counterintuitive and it's OK for people to come up with ideas, but when you hit the point of "hey, the thermo people have a nice collection of proofs demonstrating this, and it fits very well with the underlying theory, oh, and if you do manage to violate etendue, you could probably build a perpetual motion machi…

The this I refer to is "the principle of etendue"

Great principle. I agree with it, and think that most of the people offering objections here do as well. Our question is whether it's it's being applied correctly in this case. For me, the sticking point is whether surface temperature can be used as a proxy for the brightness we care about.

BTW, what's your obsession with Munroe?

I have none. He's a great explainer, probably occasionally gets details wrong, and (so far as I can tell) is a positive force for spreading scientific understanding. My "obsession" seems to be that I have poor tolerance for overly broad smackdowns of critics. If you are going to tell someone else that they are wrong (as opposed to Munroe who is trying to explain what he believes is true) I think you have a higher obligation to get all the details right. I presume I'm sensitive to it because I'm often on the receiving end.

How about you? Why does it bother you so much that some people say that Munroe's argument is logically flawed?

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#234

Post by skolos » Sat, Dec 22, 2018, 4:22 PM UTC

robbiep wrote: post #209

skolos wrote: post #197

Earlier quoted context omitted.

Looking through comments. There is still lots misunderstanding. So let's get deeper into the topic. In particular - how stuff gets heated by light and what light "temperature" is. Thermodynamics arguments are always good but you really need to understand that most of them describe closed systems under equilibrium. Real world is messier and if your understanding of thermodynamics is shaky its easy to get to wrong conc…

I am not an expert but on your oversimplification on the ‘High energy = higher wavelength’ Whilst this is true, it is also a bit irrelevant. Gamma rays are higher energy than visible light but you don’t need to focus gamma radiation (should that be easily possible) to start a fire, you can do it with visible light; or infrared light (which is lower wavelength again) at a high enough qanta

Higher energy = lower wavelength is not oversimplification. The formula is: photon energy is plank constan times speed of light divided by wavelength. You right - you don’t need gamma rays, but you cannot do it with low frequency radio waves. You need photons that energetic enough to break chemical bonds.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#235

Post by mmmBacon » Sat, Dec 22, 2018, 4:42 PM UTC

skolos wrote: post #234

robbiep wrote: post #209

Earlier quoted context omitted.

I am not an expert but on your oversimplification on the ‘High energy = higher wavelength’ Whilst this is true, it is also a bit irrelevant. Gamma rays are higher energy than visible light but you don’t need to focus gamma radiation (should that be easily possible) to start a fire, you can do it with visible light; or infrared light (which is lower wavelength again) at a high enough qanta

Higher energy = lower wavelength is not oversimplification. The formula is: photon energy is plank constan times speed of light divided by wavelength. You right - you don’t need gamma rays, but you cannot do it with low frequency radio waves. You need photons that energetic enough to break chemical bonds.

Depends on what you consider low frequency. Radio waves can be used to excite vibrational modes. The resultant thermal energy can be used to break chemical bonds.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#236

Post by neltnerb » Sat, Dec 22, 2018, 4:52 PM UTC

tbabb wrote: post #155

neltnerb wrote: post #149

Earlier quoted context omitted.

Yeah, this was my immediate thought as well. Yes, you can't concentrate solar energy to heat something hotter than the surface of the sun. But the emission of a 100C blackbody is not visible either, so it's clearly an incorrect hypothesis on it's face that this temperature causes moonlight. It's a bad mirror, not a radiator.

And it's a mirror that subtends a tiny fraction of the sky, and a tiny fraction of the sun's blurred virtual image.

For sure, the light is super dim. I'm not sure it's possible to focus the energy enough to light a fire. I'm just sure the reason is not because moonlight is entirely composed of radiation from a 100C blackbody. This is just immediately obvious because we can see it, so this whole article is a little weird.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#237

Post by alanh » Sat, Dec 22, 2018, 5:06 PM UTC

starbeast wrote: post #92

baddox wrote: post #35

Earlier quoted context omitted.

> But you're right, the light from the moon is reflected sun light, plenty hot to start a fire. That's my impression as well. A blackbody has albedo 0. The moon has an albedo of around 0.12. While I suspect you can't start a fire from moonlight in practice, I don't think the arguments in this article are correct.

>While I suspect you can't start a fire from moonlight in practice Ivanpah may be able to, I wonder if we can borrow it at night for an experiment - https://en.wikipedia.org/wiki/Ivanpah_Solar_Power_Facility

now this is an idea i like.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#238

Post by [deleted] » Sat, Dec 22, 2018, 5:13 PM UTC

nshepperd wrote: post #168

skolos wrote: post #145

Interesting article. Right in many places. Wrong (possibly) in main conclusion. Entropy argument - correct in the sense that using radiation from black body we cannot use lenses to heat another body to the temperature higher than original. Easy to understand why - the first body has a temperature, radiation has the same temperature, if we apply the radiation to another object it will not heat up more than the radiati…

The conclusion is correct. It doesn't rely on the moon being a black body radiator, but on etendue, which shows that the most you can do with a system of lenses and mirrors is to create an environment (on earth) as bright as the environment on the moon. The fact that rocks on the moon's surface only reach 100°C shows that an environment like that is not bright enough to light a fire.

[deleted]

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#239

Post by nshepperd » Sat, Dec 22, 2018, 5:33 PM UTC

nkurz wrote: post #180

nshepperd wrote: post #161

Earlier quoted context omitted.

The thermodynamic argument applies to black bodies, and says that you can't make an object hotter than the surface of the emitter. That's pretty uncontroversial. The more general argument based on etendue, which applies to the moon, is: you can't make the incoming light any brighter than it is on the surface of the source (which doesn't have to be the original emitter, but can be any point along the path of the light…

you won't be able to make your object much hotter than that. I'm bothered by the modifer "much". If you are indeed talking about a physical principle, shouldn't this be an absolute limit rather than a suggestion? How much hotter does physics allow you to go? Are you sure it's not enough to allow ignition? Along those lines, I'd assume that the surface temperature of the depends on the moon's shape and thermal conduct…

> I'm bothered by the modifer "much". If you are indeed talking about a physical principle, shouldn't this be an absolute limit rather than a suggestion?

It's an absolute limit on the amount of incoming irradiance you can create to your object. The actual equilibrium temperature it reaches will depend on additional factors like how well your object loses heat (eg. by conduction) compared to a moon rock.

In this case, the temperature of moon rocks is probably a reasonable upper bound of the achievable temperature of an object on the earth:

- Moon rocks are in vacuum, while something on earth is in contact with air and dissipating heat by convection.

- Moon rocks are in contact with the surface of the moon (~100°C), whereas an earth object is in contact with the ground, or your hand, or whatever (~37°C, assuming your hand). So heat loss by conduction will be greater on the ground.

If you rigged up something to suspend your object in vacuum without touching anything so that conductive heat losses ~0, maybe you could get something slightly hotter than the average surface temperature of the moon. But not hotter than a well placed moon rock that already happens to be making near 0 contact with the moon's surface (due to standing on a point or something).

> If I were to change the moon to be an ultra-thin and highly heat conductive hemispherical shell rather than a solid sphere, I'd assume the surface temperature would drop.

In that case much more heat would escape around to the unlit side, and the moon's surface temperature would reach somewhere between the "day" (~100°C) and "night" (~-200°C) temperatures. Say around -50°C. In that case the surface temperature will be less representative of that achievable for an object on earth. A moon rock touching the ground would be in contact with -50°C, which is colder than the 37°C for an object held in your hand.

Re: Can you use a magnifying glass and moonlight to light a fire? (2016)

#240

Post by Dylan16807 » Sat, Dec 22, 2018, 5:50 PM UTC

AlexCoventry wrote: post #230

Dylan16807 wrote: post #208

Earlier quoted context omitted.

Take your target and trace a ray in every direction away from every point on its surface. The more of these that hit the energy source, the more energy you're getting. And 100% is obviously the best you can do.

From an optical perspective, how does the argument dismissed at the start of the OP break down? Is there some reason the moon's light, gathered from a lens covering hundreds of acres, carries insufficient energy to light a fire in concentrated form?

The optical argument goes like this: Measure the brightness (in energy per square meter) right at the moon's surface. No matter what you do with lenses, you can't concentrate moonlight beyond this level.

You can make an enormous lens where all the energy coming off a particular acre of moon goes through it. But you can't focus all of it onto a spot smaller than an acre, no matter what you do.