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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" disag
by nkurz 8y ago
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 temperature blackbody radiation source regardless of the size of one's magnifying glass, but disagree that it is reasonable to consider sunlight reflected by the moon as fitting this model.
Presumably, you agree that it's possible to start a fire using a magnifying glass using sunlight on earth. I'd guess you also believe that it's possible reflect the light from small handheld mirror into the magnifying glass, and still start a match, even though the mirror is much lower than the temperature of the sun? While the specular reflection from the mirror is different than the diffuse reflection from the moon, one might note that the words "specular" and "diffuse" don't appear in Munroe's exposition. Would you agree that Munroe's argument would appear to prohibit this behavior?
Now assume that the moon was replaced by an equally sized parabolic mirror aimed to be focused on the earth. Would it be possible to light a match using this light if one's magnifying glass was large enough? Which thermodynamical principle am I violating in thinking that it might be possible? And which part of Munroe's argument do I invalidate by making these modifications? Again, my point isn't that Munroe's conclusion is wrong, just that there might be something flawed about the argument he uses to reach that conclusion. This might be a "brain bug", but from the inside it just feels like an attempt to understand truth.
- dekhn 8y agoThe this I refer to is "the principle of etendue": https://en.wikipedia.org/wiki/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 machine", if you willingly continue to argue and get shot down, it's time to go back and re-read the books. BTW, what's your obsession with Monroe? What he's referring to is a scientific phenomenon, Monroe is just a science popularizer, and if he got the details wrong- well, the point of xkcds like that is more to inspire people with ideas, than get the exact details right.
- nkurz 8y agoThe 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?
- dekhn 8y agoI don't have any problem with people complaing about Monroe's argument. I am fairly certain he wrote what he did in consultation with optics engineers, and then modified it so it was intellectually comprehensible by a nerd-but-not-optical engineer audience.
- nkurz 8y agoThanks for the response. If you are still interested in this, there's an useful link on the second page of this thread to a discussion of the same question: https://physics.stackexchange.com/questions/370446/is-randall-munroes-what-if-xkcd-correct-that-magnified-moonlight-cant-get-th/370600 https://physics.stackexchange.com/questions/370446/is-randal.... If you expand the comments on the answer, the back and forth presents most of the argument I would make, in a clearer form than I could muster.