What-If 0145: "Fire From Moonlight"

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Re: What-If 0145: "Fire From Moonlight"

Postby BullCityFats » Mon Feb 15, 2016 6:17 am UTC

There's another way this can be wrong which wasn't touched on at all. The magnifying glass doesn't have to be between the Moon
and the Earth. Instead, use a large magnifying glass to heat the Moon up to ~4000C. This will nearly double the amount of light the
Earth receives, which will eventually almost certainly set something (probably lots of somethings) on fire.

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Re: What-If 0145: "Fire From Moonlight"

Postby JohnCD3141 » Mon Feb 15, 2016 5:15 pm UTC

There's another way this can be wrong which wasn't touched on at all. The magnifying glass doesn't have to be between the Moon
and the Earth. Instead, use a large magnifying glass to heat the Moon up to ~4000C. This will nearly double the amount of light the
Earth receives, which will eventually almost certainly set something (probably lots of somethings) on fire.


Oh, that's brilliant!

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Re: What-If 0145: "Fire From Moonlight"

Postby twerpod » Mon Feb 15, 2016 5:50 pm UTC

bill@xoom.org wrote:
twerpod wrote:This Winston paper "The thermodynamic limits of light concentrators" from 1990 is full of math,
Spoiler:
and includes discussion of how thermodynamics support concentrations greater than the surface of the sun through combination of non-imaging and a high dielectric material like sapphire. It also mentions that a rough surface can absorb 4x more energy, independent of concentration.


Thank you for introducing non-imaging optics into this. ...
Spoiler:
The way I understand the conservation of etendue is similar to how Randall describes it. I can believe that there is some purely geometric argument that means if you squeeze down more photons per unit area passing through some plane, the angles of the photons coming out will be more "spread out" in some way that I don't even care about making more precise. All I care about is that it seems as though that light is less capable of heating things up. However you multiply together the numbers of photons and angles and whatnot to derive values like intensity or illuminance or radiant intensity or whatever, I don't really care.
The interesting part to me is that this light that "wants to spread out" (before it even has a chance to do any spreading) does a lousy job of heating things up. It's not just the number of photons passing through the plane that matters (and this is a point I think Randall doesn't make clear, and many explanations here hide behind other terms) but their directions, somehow. Since some of the terms in play are directional, that makes sense.


I think the etendue concept is much less important in the case of the Winston Cone concentrator, since it has an arbitrary output aperture (the example was ~1mm), you can press this directly against the black rough paper for ignition. There is no "focus" for this non-imaging concentrator, but it can act like the focal length is +/- 1 mm.

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Re: What-If 0145: "Fire From Moonlight"

Postby buddy431 » Tue Feb 16, 2016 7:47 am UTC

He explained why the reflected sunlight wouldn't be enough to start a fire with his comment at the end: Buzz Aldrin is still alive. We have put lots of stuff on the moon. Much of it, it is true, is pretty reflective, but some of it is not. These things on the moon are surrounded by roughly a hemisphere of moonlight, and yet they don't get hot enough to catch on fire. At the end of the day, the moon just isn't that reflective.
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Re: What-If 0145: "Fire From Moonlight"

Postby wayne » Tue Feb 16, 2016 9:59 am UTC

Gan_HOPE326 wrote:
paha arkkitehti wrote: the Sun couldn't get rid of all its energy any more, and its nuclear fusion reaction would begin to heat it up. Which would lead to it expanding or probably ending up even hotter.


Except that as a star expands, fusion slows and it starts to lose more energy than it produces. If it heats up from its current state, it will expand, and then cool. Which will make it contract again, which will make it heat up again. Which...

So maybe putting a Dyson sphere around a star would make it act like a much larger periodic star?

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Re: What-If 0145: "Fire From Moonlight"

Postby x7eggert » Tue Feb 16, 2016 10:08 am UTC

If, like sb said, the black body temperature of the light would determine the maximum heat of a target, wouldn't it be possible to filter out the low frequencies to make moonlight resemble sunlight?

Filtering would use energy, so "Lenses and mirrors work for free; they don't take any energy to operate." does not apply.

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Re: What-If 0145: "Fire From Moonlight"

Postby tdjewell » Tue Feb 16, 2016 1:59 pm UTC

Could one use "angle of incidence" to get more than one sun's worth of energy?

I tried to draw ASCII art, but failed utterly.

Imagine the regular direct path passes through an angled pane of glass, and does not reflect.
Imagine a set of mirrors set up to reflect light from elsewhere and bounce it off the far-side glass from an angle, so it reflects instead of passing through.

If I am breaking the laws of physics, what have I forgotten?

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Re: What-If 0145: "Fire From Moonlight"

Postby stevendesu » Tue Feb 16, 2016 2:03 pm UTC

I understand the basic explanation for why multiple mirrors reflecting light from different angles or a very large lens cannot make an image brighter than the original... it's because optics are two-way. If the fire were brighter than the sun, more light would travel from fire to sun than from sun to fire, causing energy to flow away from the higher energy source.

However I don't understand the explanation for why you can't heat something up to hotter than the surface of the moon using moonlight. If I use a mirror to redirect sunlight towards a magnifying glass then I can light a fire with this light, can't I? However the mirror is not super-heated to the temperature of the sun.

So if we think of the moon as a mirror, why is the surface temperature of the MOON a limiting factor in how hot we can make something?

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Re: What-If 0145: "Fire From Moonlight"

Postby jgh » Tue Feb 16, 2016 4:30 pm UTC

I was so sure the discussion was introducing conservation of énnuie.

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Re: What-If 0145: "Fire From Moonlight"

Postby mcdigman » Tue Feb 16, 2016 5:41 pm UTC

x7eggert wrote:If, like sb said, the black body temperature of the light would determine the maximum heat of a target, wouldn't it be possible to filter out the low frequencies to make moonlight resemble sunlight?

Filtering would use energy, so "Lenses and mirrors work for free; they don't take any energy to operate." does not apply.


Nope! That would only work if the lower frequencies were somehow actually cooling down the target, which they are not; adding more power at any wavelength will always let you heat up the target more.

Or, thinking in terms of the blackbody spectrum, even though the peak wavelength is smaller for sunlight, a hotter blackbody actually radiates more total power at every wavelength than a cooler one, so removing power at longer wavelengths actually makes the moon look less like the sun, not more.

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Re: What-If 0145: "Fire From Moonlight"

Postby SuperSteve » Wed Feb 17, 2016 5:33 am UTC

Positron wrote:I'm still confused a bit here as to why you wouldn't be able to take, say, 5 ideal convex lenses refracting different points of the moon's reflection and overlap their outputs to get a theoretical max of 500 degrees. It seems to violate the basic thermodynamics argument presented (the cumulative total is reversible, and you're boosting 100 degrees theoretical max up to 500 ("something for free")). But I don't see anything in the rules about optics mentioned that says you can't since there wouldn't be a single source point exceeding it's origin or a single lens with multiple "choices" of reverse paths...


Even if you completely surround it with lens, you still won't heat it enough.

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Re: What-If 0145: "Fire From Moonlight"

Postby Tyndmyr » Wed Feb 17, 2016 3:24 pm UTC

You should be able to, with enough effort concentrate enough light to start a fire. Single lens optics do indeed have limitations, but that doesn't necessarily apply to every solution.

Now, it's extreme in that moonlight is pretty weak because the moon is an awful reflector, but that's just a numbers game. Temperature of the moon is not particularly important. I mean, the moon averages what, -50? And yet it still provides some warmth. Sure, any single lens can't do it, unless you're essential positing extremely unusual manufacturing processes that are effectively a giant dome of seperate lenses, but that has little to do with temperature.

A big mirrored funnel should suffice.

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Re: What-If 0145: "Fire From Moonlight"

Postby Echo244 » Wed Feb 17, 2016 4:38 pm UTC

Tyndmyr wrote:A big mirrored funnel should suffice.


A thought along similar lines flitted through my head, but... what about the angles of reflection? How much of the light will be correctly channelled down the "funnel", how much will be bounced back out of the "top" end, as every reflection of a ray of light increases its angle from the path of the funnel? I'm not convinced it effectively concentrates light like that onto the narrow end, which seems to match the conservation of étendue thing.
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Re: What-If 0145: "Fire From Moonlight"

Postby Tyndmyr » Wed Feb 17, 2016 9:21 pm UTC

Echo244 wrote:
Tyndmyr wrote:A big mirrored funnel should suffice.


A thought along similar lines flitted through my head, but... what about the angles of reflection? How much of the light will be correctly channelled down the "funnel", how much will be bounced back out of the "top" end, as every reflection of a ray of light increases its angle from the path of the funnel? I'm not convinced it effectively concentrates light like that onto the narrow end, which seems to match the conservation of étendue thing.



A parabolic mirror, with the flammable bit in the focus point would probably be more efficient, yeah. You'd just need a horribly large one, because the moon is a crappy reflector. Pretty much any solar stove design scaled up to ridiculous degrees should suffice.

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Re: What-If 0145: "Fire From Moonlight"

Postby mfb » Wed Feb 17, 2016 11:45 pm UTC

Tyndmyr wrote:A parabolic mirror, with the flammable bit in the focus point would probably be more efficient, yeah. You'd just need a horribly large one, because the moon is a crappy reflector. Pretty much any solar stove design scaled up to ridiculous degrees should suffice.
It does not, for reasons explained both in the what-if and at least ten times here in this thread.
Unless you do something clever with the spectrum, as also explained in the thread already.

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Re: What-If 0145: "Fire From Moonlight"

Postby Soupspoon » Thu Feb 18, 2016 12:23 am UTC

How about a rapidly approaching mirror? At a significant fraction of the speed of light, will the blue-shifted reflections of the not-energetic-enough moonbeam quanta now have enough energy to them to act as if sunbeams. At least once the mirror of any given size attains the appropriate proximity to also have sufficient apparent brightness of reflection to ensure the result we want.

And then, shortly afterwards, you/the planet gets hit by the disintegrating mirror! Free silvered glass!


(The problems with the above scenario is left as an exercise to some other reader. A reader who was expecting the boring small-print of an insurance policy, probably.)

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Re: What-If 0145: "Fire From Moonlight"

Postby mcwaffles » Thu Feb 18, 2016 8:46 am UTC

i dont think the thermodynamics argument Randall puts forward is really all that sound. the better your aberration of the lens and how well matched up with the focal point you are with the same energy being deposited, i see no reason why you shouldn't be able to make an extremely small spot 10000 degrees. sure the black body profile would show a higher temperature than the of the sun but the surface area, being so small, would still emit a very limited amount of energy.

light is energy. i dont think i need to point that out here but moving on. if a 1000m wide (i like exaggerating a scenario) perfect lens were to focus light on a piece of black paper already at the same temp or higher than the moons surface hes saying it would do nothing? then what happened to all that energy?

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Re: What-If 0145: "Fire From Moonlight"

Postby Tyndmyr » Thu Feb 18, 2016 9:14 pm UTC

mfb wrote:
Tyndmyr wrote:A parabolic mirror, with the flammable bit in the focus point would probably be more efficient, yeah. You'd just need a horribly large one, because the moon is a crappy reflector. Pretty much any solar stove design scaled up to ridiculous degrees should suffice.
It does not, for reasons explained both in the what-if and at least ten times here in this thread.
Unless you do something clever with the spectrum, as also explained in the thread already.


Look, from earth's perspective, all photons coming from the moon are essentially on a parallel track. Yes, yes, lots of scattering happened, so the moon's a really shitty mirror, but...you're still getting what, 1/400th of the light? Plenty. We just have to go big.

We don't care about imaging it. This is basically an exercise where non imaging optics are perfectly acceptable since we're after heat, not light, so we're not actually restricted by the behavior of a single lens*, and can gleefully concentrate light ridiculously**.

Sure, such a device pointed at the moon is going to be vastly less efficient, but we're still, using current-record earth magnification, effectively pulling 140x concentration of sunlight, which should be more than adequate to light fires.

*This may be slightly stretching the definition of magnifying glass, but if we consider this as among the "what if went faster/bigger/etc" options, this seems pretty reasonable.
** http://www.nature.com/nature/journal/v3 ... 198a0.html

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Re: What-If 0145: "Fire From Moonlight"

Postby Neil_Boekend » Fri Feb 19, 2016 9:26 am UTC

If all photons were parallel then the moon would look like a point. The eye sees objects as having size because the photons enter the eye from different angles, which causes them to strike the retina at different places.
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Re: What-If 0145: "Fire From Moonlight"

Postby Hafting » Fri Feb 19, 2016 2:27 pm UTC

I think we can make fire from moonlight - I believe there is a subtle flaw in Randalls argument. We may need a material with a very high index of refraction though - which might be hard to get.

You only need so much energy to set a piece of paper on fire. The moon is huge, and radiates much more energy than that. Collect it all, or some sufficient fraction, and you can set paper on fire.

A lens with a given focal length will make an image of the moon with a given size. Make the lens cover more area while keeping the focal length the same is a way to capture more energy. The aperture increases, and a higher index of refraction is needed to make the lens. In the beginning, doubling the area (nearly) doubles the energy collected - still concentrated on the same small spot. There is diminishing returns here though - an infinitely large plane-filling lens will only capture half the moon's radiated energy. Still, a lens with a diameter proportional to the earth-moon distance, will collect a nice fraction of the moonlight. And if we can make that with a few centimeters focal length (using some exotic material with a very high index of refraction), then we'll have plenty of energy to start a fire. If high index of refraction is unrealistic, make a gargantuan curved mirror instead. A very large mirror is tricky too, but won't need any weird materials. Build it in orbit, if weight is a concern.

I believe the flaw in the argument is the assumption that the lens system must be passive and reversible. A lens won't change the temperature of the radiation, because radiation frequency does not change. But concentrate enough light onto a target, and you get non-linear optic effects. You get events where a single atom/electron is hit simultaneously by several photons - because you have such a high photon concentration.This is a bit similiar to "frequency multiplication" in green lasers, or increasing the temperature of radiation by trading two (or more photons) for a a single photon with a higher frequency. Nonlinear optics are not trivially reversible. Temperature is increased in a small spot, and we get our fire. The lens system may have fully reversible optics - but not the paper itself.

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Re: What-If 0145: "Fire From Moonlight"

Postby mcdigman » Fri Feb 19, 2016 3:32 pm UTC

Hafting wrote:I think we can make fire from moonlight - I believe there is a subtle flaw in Randalls argument. We may need a material with a very high index of refraction though - which might be hard to get.


My back of the envelope calculation is that to get a factor of 400,000 concentration for an object that takes up 0.52 degrees in the sky is that the index of refraction would have to be at least 2.87. There are materials with indices of refraction higher than that (certainly at least metamaterials), but I don't think they work in the visible spectrum, or at least enough of it to concentrate that much power. But given that the index of refraction of diamond (highest index material most people might have access to) is 2.419, I don't think its inconceivable that something could have an index of refraction that high.

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Re: What-If 0145: "Fire From Moonlight"

Postby ijuin » Fri Feb 19, 2016 7:57 pm UTC

That only applies to a single-lens system however. A multiple-lens-in-series setup could allow for more refraction.

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Re: What-If 0145: "Fire From Moonlight"

Postby Eternal Density » Sat Feb 20, 2016 12:40 am UTC

Does capturing and storing the energy from moonlight using photovoltaics, to release it later in a concentrated burst that would start a fire, count? Yes I know most solar panels would be losing power under light that low, but if we specially designed one to get the most out of moonlight and had a really good battery...

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Re: What-If 0145: "Fire From Moonlight"

Postby Flumble » Sat Feb 20, 2016 9:44 am UTC

Eternal Density wrote:Does capturing and storing the energy from moonlight using photovoltaics, to release it later in a concentrated burst that would start a fire, count?

For some reason[1] I have my doubts.


[1]
Rogier Spoor wrote:Can you use a magnifying glass and moonlight to light a fire?

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Re: What-If 0145: "Fire From Moonlight"

Postby dawolf » Sat Feb 20, 2016 9:37 pm UTC

Ok, my physics degree is over a decade old, but this just doesn't feel right to me.

If the moon was emitting as a blackbody at 100°C, we wouldn't see it apart from as a black shape.

So the light which is reaching us from the moon ISN'T from a body at 100°C, it is from the sun.

Now forget temperature for a moment - temperature controls the emitter (as black bodies), but what we're actually taking about are photons coming from the sun.

And if Randall's numbers are correct, that the lunar irradiance is 1/400,000 that of the sun, then yes indeed a lens 400,000 times the size required by a solar one would work as far as I'm concerned.

We can check this a bit further.

https://spie.org/samples/FG11.pdf

Full Moon: 0.2 Lux
Quarter moon: 0.01 lux

So let's go, using the stefan boltmann law.

If someone brighter than me can work out what lux we'd actually get from a 100°C moon shape in the sky, you could plug it into the stefan boltzmann law. For example, if it was contributing 0.01 lux we see that e is proportional to t^4

So, using Randall's number of 100°C (moon) we use the following equation

T(moon) = ((373.15^4)*20)^0.25 = 789°K = 526°C even if we assume that a dark moon at 100°C is 1/20th as bright as a full moon.

I think the lux number is lower than that, and that the implied temperature of the moon is therefore higher, and that it CAN set fire to things. I wouldn't be surprised if the moons implied temperature was somewhere above 500°C

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Re: What-If 0145: "Fire From Moonlight"

Postby ijuin » Sat Feb 20, 2016 9:53 pm UTC

Are we talking bolometric luminosity or human-visible-wavelength luminosity?

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Re: What-If 0145: "Fire From Moonlight"

Postby mfb » Sun Feb 21, 2016 12:32 am UTC

dawolf wrote:And if Randall's numbers are correct, that the lunar irradiance is 1/400,000 that of the sun, then yes indeed a lens 400,000 times the size required by a solar one would work as far as I'm concerned.
It does not, as the what-if describes, and has been pointed out numerous times in this thread now.
How is it possible to overlook this? Serious question, I just don't understand it.

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Re: What-If 0145: "Fire From Moonlight"

Postby DanD » Wed Feb 24, 2016 10:19 pm UTC

With a mirror, it is absolutely impossible, I can state that definitely. Let's assume, for a minute, that the moon is powered rather than reflective, allowing us to use optics to capture 100% of it's energy without blocking the input. If you surround it with a dyson ellipsoid with the moon at one focus and your object at the other, your object sees 100% of it's "sky" filled with the moon, and reaches the black body temperature dictated by the energy.

However, and this is the critical bit, most of the energy from moon misses your object. That elliptical mirror forms an image of the moon that is the same size as the moon, and most of the energy misses your (less than moon sized) target and ends up reflecting back to the moon's surface. You can narrow the image of the moon the mirror creates, but only by designing the mirror such that only a portion of the moonlight from any given point on it's surface reaches the target. At no point can you concentrate more of the total energy of the moon on your object than the ratio of it's radius to the moon's.

Now, I'm less certain that this holds true for lenses. I believe it does (optics enforces thermodynamics), but feel free to continue the discussion there.

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Re: What-If 0145: "Fire From Moonlight"

Postby dawolf » Wed Feb 24, 2016 11:34 pm UTC

mfb wrote:
dawolf wrote:And if Randall's numbers are correct, that the lunar irradiance is 1/400,000 that of the sun, then yes indeed a lens 400,000 times the size required by a solar one would work as far as I'm concerned.
It does not, as the what-if describes, and has been pointed out numerous times in this thread now.
How is it possible to overlook this? Serious question, I just don't understand it.


I think the what-if is wrong, simply put.

Thought experiment. You're a long way away from the sun, such that the irradiance from the sun is 1/400,000 that of the sun.

Could a lens focus this light to burn something? Of course, the source is ~5000 degrees, so why not?

What is magically different about the two situations? One is reflected light, one is being further away.

It is the same light, at the same intensity, from the same source.

For me, the arguments about whether such a lens system is actually feasible are stronger.

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Re: What-If 0145: "Fire From Moonlight"

Postby KittenKaboodle » Thu Feb 25, 2016 3:35 am UTC

dawolf wrote:For me, the arguments about whether such a lens system is actually feasible are stronger.


Since the original question involved a "magnifying glass", that would seem to be a reasonable stance. However, we are talking about "What if" here, where shooing a BB gun at a train escalates into the locomotive moving backwards at nearly the speed of sound. Where pointing a laser pointer at the moon ends up setting the earth on fire without even using a magnifying glass!

Granted, the moon is a really bad mirror, but that is still reflected sunlight, and the sun is pretty darn hot.

What if we took the Fresnel lens from an old style light house and put in on top of a solar power tower (at night, with a full moon) and then put a cotton ball soaked in carbon disulfide at the focus? Sure a box of matches would be cheaper and more portable, but then a set of locomotive air brakes is cheaper than arming 40,0000 people with AK47s,and even that is much more practical than: "Ok, let’s mount a megawatt laser on every square meter of the surface of Asia. Powering this array of 50 trillion lasers would use up Earth’s oil reserves in approximately two minutes". So, "impractical" is not really a valid objection in the "What if" world. However, in the latter case, one could then start a fire with "moon light" and a magnifying glass.

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Re: What-If 0145: "Fire From Moonlight"

Postby DanD » Thu Feb 25, 2016 2:46 pm UTC

It is the same light, at the same intensity, from the same source.//


No, it really isn't. If the moon were a perfect, spherical, specular reflector, then you would get a small spot of full sunlight intensity, and the rest of it would appear dark. This is evident if you look at a mirrored sphere with a single light source, only the portion of the sphere that is reflecting light directly at you appears fully illuminated. Since the sun has a non-zero angle at the moon's surface, the spot you would get would be a function of the apparent size of the sun and the radius of the moon, but it would be significantly smaller than the apparent size of the sun from earth. If you were able to focus all of this light sufficiently (which you would be able to, but the spot size would be minuscule) then you could use it to light a fire.

However, if the moon were a perfect spherical diffuse reflector, you would get that same amount of light, coming from the entire moon (the average moonlight reaching any particular spot remains constant, the apparent source increases to the full surface of the moon with diffusion). Since the light is coming from a larger angle, it is more difficult to focus to a tight spot. The maximum concentration possible is C=n^2 sin^2 b/sin^2 a where n is the index of refraction, b is the angle between the edges of the optic and the edges of the spot size, and a is the angle of the light arriving (angle from parallel for both, 2a and 2b for the total angle)

The moon is actually a mix of these two, but, given as mcdigam points out on page two the moon absorbs more light than it reflects, which means the majority of energy is coming back out as perfectly diffuse black body radiation, and significant portion of the remainder is diffused by the nature of the regolith. Therefore, the net result is much closer to a diffuse reflector than a specular one, the light from the moon is significantly different and more diffuse than sunlight, and there is a finite limit on concentration.

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Re: What-If 0145: "Fire From Moonlight"

Postby trpmb6 » Thu Feb 25, 2016 3:38 pm UTC

I think what we should really be concerning ourselves with here is that some day in the future we are going to need to utilize parabolic mirrors to change how much energy from the sun reaches Earth. When the Sun becomes a red giant (assuming we aren't engulfed by the corona at this point) we would need a lens that reduces the amount of energy reaching Earth. When the sun shrinks to a white dwarf we would need a lens that increases the amount of energy reaching the Earth.

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Re: What-If 0145: "Fire From Moonlight"

Postby HES » Thu Feb 25, 2016 3:54 pm UTC

At that point it's probably just easier to move the Earth to a better orbit.
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Re: What-If 0145: "Fire From Moonlight"

Postby Soupspoon » Thu Feb 25, 2016 4:03 pm UTC

trpmb6 wrote:When the Sun becomes a red giant (assuming we aren't engulfed by the corona at this point) we would need a lens that reduces the amount of energy reaching Earth.
Sunglasses don't need to have prescription lenses. (Neither do slatted window-shades.)

Feel free to design the giant lens to grab more of the sun's energy, in it's later life, perhaps1, but develop first a sunshade or other more exotic solution that would protect the Earth from not just the light from the sun but also the expanded volume. Assuming we think that's even worthwhile (and doable!) by that point.


1 Assuming we aren't above such concerns, having a totally different energy-economy at that point, including whatever measures we implement to maintain the deliberately frozen-in-time environments of all the less self-determining creatures on Earth that we wish to keep safe and 'static' in the face of the effects of such a deep-future scenario.

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Re: What-If 0145: "Fire From Moonlight"

Postby dawolf » Thu Feb 25, 2016 5:50 pm UTC

DanD wrote:
The moon is actually a mix of these two, but, given as mcdigam points out on page two the moon absorbs more light than it reflects, which means the majority of energy is coming back out as perfectly diffuse black body radiation, and significant portion of the remainder is diffused by the nature of the regolith. Therefore, the net result is much closer to a diffuse reflector than a specular one, the light from the moon is significantly different and more diffuse than sunlight, and there is a finite limit on concentration.


The difference is to do with power, and where it comes from.

A sealed system WITHOUT the sun, I agree, you can't get hotter than the surface of the moon as it'll reach thermal equilibrium.

But this is not a sealed system, you've got a huge amount of energy being constantly generated and reflecting off the surface of the moon and I have so far not seen a good reason why this energy is constrained in the same manner.

DanD
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Re: What-If 0145: "Fire From Moonlight"

Postby DanD » Thu Feb 25, 2016 8:50 pm UTC

Re: What-If 0145: "Fire From Moonlight"

Postby dawolf » Thu Feb 25, 2016 5:50 pm UTC
DanD wrote:

The moon is actually a mix of these two, but, given as mcdigam points out on page two the moon absorbs more light than it reflects, which means the majority of energy is coming back out as perfectly diffuse black body radiation, and significant portion of the remainder is diffused by the nature of the regolith. Therefore, the net result is much closer to a diffuse reflector than a specular one, the light from the moon is significantly different and more diffuse than sunlight, and there is a finite limit on concentration.



The difference is to do with power, and where it comes from.

A sealed system WITHOUT the sun, I agree, you can't get hotter than the surface of the moon as it'll reach thermal equilibrium.

But this is not a sealed system, you've got a huge amount of energy being constantly generated and reflecting off the surface of the moon and I have so far not seen a good reason why this energy is constrained in the same manner.


Imagine, for a moment, that the moon is a black body radiator that has a total energy output equal to the current reflected light+black body radiation output. If you focus as much of that radiation as you can on your target, the best you can do is reach the same temperature and produce the same black body output. (This is ignoring, admittedly, the second order effects of your target radiating some of it's energy back to the moon, and vice versa, which does raise the temperature slightly. This effect is negligible as long as the size of your target is small relative to the size of the moon.) This would occur when you construct optics that make 100% of the "sky" of your target occupied by the moon (a dyson ellipsoid being the easiest approach). However, unless your target is also the size of the moon, much of the light at the target focus would miss the target and end up returning to the moon. You cannot optically concentrate light any more tightly than this. In order for the "sky" of your target to be brighter than the object it is receiving the energy from you would need to be able to bring two light rays that came off the source object from a different position and/or angle into the target at the same position and angle. (That would be the definition of concentration greater than 1 relative to the source). There is simply no way to do that with optics.

So whether the moon is the source of the energy, or merely an imperfect diffuser of it, the net result is the same.

Tyndmyr
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Re: What-If 0145: "Fire From Moonlight"

Postby Tyndmyr » Thu Feb 25, 2016 11:04 pm UTC

KittenKaboodle wrote:Where pointing a laser pointer at the moon ends up setting the earth on fire without even using a magnifying glass!


....that's an excellent way to get a "yes" answer. Technically moonlight, yes? And you can throw a magnifying glass in the air to technically satisfy the question, even if it's utterly unnecessary.

DanD wrote:. In order for the "sky" of your target to be brighter than the object it is receiving the energy from you would need to be able to bring two light rays that came off the source object from a different position and/or angle into the target at the same position and angle. (That would be the definition of concentration greater than 1 relative to the source). There is simply no way to do that with optics.

So whether the moon is the source of the energy, or merely an imperfect diffuser of it, the net result is the same.


Again, who cares? We're not imaging it. We don't need to restrict ourselves to a single image of the moon.

Aright, take your black body. Place a perfectly mirrored surface around the whole thing, save for a tiny hole aimed for your target. Works, doesn't it? In this case, because of abusing those second order effects, but it's not the only possible solution, merely an easy example(that happens to be theoretically challenging).

The total energy is there. You merely have to focus enough of it. The fact that it is scattered means that the focusing cannot both be of fire lighting power and be a good single image of the moon, but that isn't actually a requirement here.

jseah
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Re: What-If 0145: "Fire From Moonlight"

Postby jseah » Fri Feb 26, 2016 10:53 am UTC

I believe it may be possible to perform an energy calculation to show you can indeed set things on fire.

Moonlight power: 0.00146 watts per square meter
Ignition Temperature: 500C
Assumption: Moonlight as viewed from Earth is parallel rays (we will assume the pesky Earth doesn't get in the way of the construction)
Assumption 2: We have a lens material capable of generating a 1cm wide image of the moon from a 3 square meter lens at the focus distance of 5 m (aka. the image of the moon at focus is 1cm across)
Assumption 3: Our target is a blackbody
Assumption 4: Earth's surroundings do not contribute heat to the target (or exist at all), the target's blackbody emissions are not reflected by the setup onto the target (aka. not reabsorbed)
Assumption 5: The target absorbs incident light perfectly

Therefore, if we can concentrate sufficient power onto a 1cm size target to equal the blackbody radiation energy of a 1cm diameter sphere at 500C, we can achieve ignition.

Stefan–Boltzmann law: Emitted power per area of the object (W m-2) = 5.67×10−8 W m−2 K−4 * temperature in K ^ 4
500C = 773K
Emitted power = 629.7 W m-2
1cm sphere = 3.142 cm2 = 0.0003142 m2

Sphere emitted power @ 500C = 0.1979 W
= 135.5 m2 of moonlight

A single 3 sq m lens will not do this, you need just over 45 of them. Obviously you can't angle the lenses because you need to catch the full 3 square meters or you mostly defeat the point.

But what you can do is put a lens somewhere else, then bounce the moonlight off a flat mirror (or refractive material to bend the light) before the focal point such that it still focuses onto the target. That might allow you to fit more in. The total distance of lens to mirror to object just has to be 5 m.
http://i253.photobucket.com/albums/hh47 ... cture1.png
(the original lens on top isn't drawn to the same height because I am too lazy to fix the picture)

At first, you can get away with surrounding the initial lens with a few other lenses a little bit lower than the 5 m height and reflecting off a mirror below the object to focus onto the target. You might be able to cram a few more in with creative mirror sizes and sending light beams in between the lens and mirrors for those which are positioned on either side of the object.

In fact, what you need is longer focal lengths so you can put the mirrors further out. Longer focal length lenses will then let you put mirrors further away and still focus onto the 1cm target. Get long enough focal lengths and they basically turn into telescopes, and with the right system of lenses for each spot, you can put them quite far away. (Hubble's focal length is 50+ meters, so you have at least that much to play with)
You can also change the diameters of the lenses to play with the packing. Smaller lenses will waste less space and allow better mirror packing at the bottom too (the mirrors have less of a problem since they're all smaller than the lenses which must be as big as the light area they collect.

In the limit (assuming infinitely tiny lenses and mirrors), you are limited by the full incoming angle of the target since the mirrors cannot occlude each other. The bottom half of the sphere around the target, in other words. Assuming a spherical series of microreflectors, your limit of incoming power is thus the area of the circle around the edge of the sphere. Which leads back to needing a circle with an area of 135.5 sq meters = 13.1 m diameter. Which gives quite a large leeway in long focal lengths.
For lens/mirror systems that bounce light off the bottom onto the target, the longest focal lengths occur nearest the target because they bounce the light off the bottom then all the way back up to the target again. But of course, you can also just go for a bigger collection area then change the shape.

This ends up looking like a shallow bowl with the target suspended in the center.
http://i253.photobucket.com/albums/hh47 ... cture2.png
(Not all lens systems drawn in)

And yes, you can light it on fire.
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pileofrogs
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Re: What-If 0145: "Fire From Moonlight"

Postby pileofrogs » Fri Feb 26, 2016 5:09 pm UTC

Hi all. I almost understood why the moon can't light a fire on the earth. I only have one question: Could I light a fire on earth with two moons?

Could I make a spot on earth as hot as two suns if I used two lenses and two suns? More realistically, could I focus light from two lenses at two different parts of the same sun and get a spot on earth that was hotter than the surface of the sun? If I can take two lenses and make two spots that are each as hot as the sun, could I combine them into one spot and make it twice as hot as the sun? What If I shone the two spots at both sides of a black piece of never-burning paper? Etc.. etc... ???

Thanks all!

-Dylan

ijuin
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Re: What-If 0145: "Fire From Moonlight"

Postby ijuin » Sun Feb 28, 2016 12:40 am UTC

jseah wrote:Assumption: Moonlight as viewed from Earth is parallel rays (we will assume the pesky Earth doesn't get in the way of the construction)
Assumption 2: We have a lens material capable of generating a 1cm wide image of the moon from a 3 square meter lens at the focus distance of 5 m (aka. the image of the moon at focus is 1cm across)


In regards to Assumption 1, the rays are not in fact exactly parallel, but are distributed across a half-degre-wide arc.

In regards to Assumption 2, can a lens with the described properties actually exist, or are the numbers arbitrary? (IDK what the precise limits on image width are).


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