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In the short story Neutron Star by Larry Niven, the main character Beowulf Shaeffer's face is sunburned somehow when his ship - built from a General Products starship hull - passes very fast and very close to a neutron star. These hulls were said to only allow visible light (and gravity) to penetrate (edit: at least that's how I remember it). I think there is something about blueshifting needed to explain the sunburn, but if so, is the light shifted after it passes through the window? How is the physics behind the sunburn described?

edit: I should clarify that I'm interested primarily in the explanation given in the story itself. If there is any further elaboration in other stories within the series, that would be also interesting.

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    There are two other ways to penetrate a GP hull, as explained by later stories. Antimatter is one, also a Beowulf Shaeffer story. The other one is explained in the Fleet of Worlds series and it's a major retcon/spoiler so I won't mention it here. – user45485 May 18 '16 at 11:16
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    This stack could use a lot more questions about General Products hulls, IMHO. +1 – Todd Wilcox May 18 '16 at 12:31
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    @Hans thanks - I'll get right on them! – uhoh May 18 '16 at 12:59
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    @uhoh Keep in mind that the Fleet of Worlds series is "written" by Larry Niven. The books are clearly by Edward Lerner and Niven's name is on them for what is most likely copyright ("licensing") reasons. This is not a negative; I quite liked FoW, more so than The Ringworld Throne and Ringworld's Children. The only thing I missed in FoW was the tying up of one loose end. (Which I can't elaborate on since it would be a major spoiler.) – user45485 May 18 '16 at 13:34
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The first mention of light being able to penetrate a GP hull says this.

Nothing, but nothing, can get through a General Products Hull. No kind of electromagnetic energy except visible light.

A little later there is a small, but significant, expansion.

No such force could penetrate a General Products hull. Neither could heat, except in special bands of radiated light, bands visible to at least one of the puppeteers' alien customers.

It's not stated explicitly, but this would mean that if one of the puppeteers' clients can see beyond violet that ultraviolet would be allowed to go through a GP hull.

Then at several points in the story mention is made of starlight turning to blue.

Were the stars turning blue?

The stars were fiercely blue, [...]

Then at the end Beowulf states he "got a bad sunburn from exposure to starlight". Normally for a bad sunburn you need high-intensity UV but Beowulf is an albino.

To be honest, I think Niven made a mistake because Beowulf, as a spaceship pilot, takes "tannin pills" to counteract his albinism.

(Edit: I used the paperback version of 'Crashlander' as the source.)

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    Your answer is most appreciated! This is exactly what I was looking for. First "...except visible light." then later on we realize that "visible" isn't necessarily what our anthropocentrism has assumed, without realizing it. Further, I now see that the sunburn is from blue-shifted starlight and not the neutron star itself. (I've never seen one up close myself, but) They aren't supposed to be particularly luminescent, so this explains it. Didn't know about the albinism either. Wow - Thanks! – uhoh May 18 '16 at 10:44
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    Whenever the colour of the neutron star itself is referenced it's as (dull) red. – user45485 May 18 '16 at 10:56
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    Because of the steep gravity gradient central to the story, radiation from the neutron star outwards (including to Shaeffer's ship) would be red shifted, and radiation from outside the gravity well coming inwards would be blue shifted. Another thing that could be seen as a mistake is I should think UV and IR filtering would be applied during ship construction to hulls that are intended for use by humans, who can't see those bands and therefore don't need them. – Todd Wilcox May 18 '16 at 12:31
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    @ToddWilcox Such filtering might not be perfect, like other GP hull properties. – Yakk May 18 '16 at 13:01
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    The story specifically mentions Beowulf Shaeffer asking the puppeteer to not coat the inside of the hull so he has an unrestricted view in all directions. Having said that, you'd think he would ask for shielding for dangerous radiation (high UV, gamma), certainly given that he's an albino. – user45485 May 18 '16 at 13:25
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According to the actual story itself, as it originally appeared in If, October 1966, it was just visible light. You can read the whole story at the Internet Archive. The following quotation is from pages 11-12:

"Two humans named Peter Laskin and Sonya Laskin wished to use the ship. They intended to come within one mile of the surface in a hyperbolic orbit. At some point during their trip, an unknown force apparently reached through the hull to do this to the landing shocks. The unknown force also seems to have killed the pilots."

"But that's impossible. Isn't it?"

"You see the point. Come with me." The puppeteer trotted toward the bow.

I saw the point, all right. Nothing, but nothing can get through a General Products hull. No kind of electromagnetic energy except visible light. No kind of matter, from the smallest subatomic particle to the fastest meteor. That's what the company's advertisements claim, and the guarantee backs them up. I've never doubted it, and I've never heard of a General Products hull being damaged by a weapon or by anything else.

P.S. The clarification pointed out in Hans's answer, that "visible" does not necessarily mean "visible to Earthmen", can be found on p. 16, column 2 of the original magazine publication.

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    Thank you for this - and for the the link to the Internet Archive! This is exactly the sentence that seems to have made an impression on me long ago: " No kind of electromagnetic energy except visible light ." And I seem to have missed the "... visible to Earthmen ..." all these years. – uhoh May 18 '16 at 10:57
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According to larryniven.net, General Products hulls are transparent to visible light and UV rays.

http://www.larryniven.net/puppeteer/puptech.shtml

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    Thanks - I'll take a look, but I'm most interested in what Larry Niven wrote in the actual story itself. Somehow I remember that it was stated somewhere in the story before the swing past the neutron star that it was only visible light. Am I wrong? UV is after all dangerous radiation, some wavelengths are even ionizing. Also, isn't blue shifting mentioned there? If other stories in the series elaborated, that would also be interesting. – uhoh May 18 '16 at 9:29
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    @uhoh See my long answer. Concerning other stories elaborating: Niven has forever been retconning as (his knowledge of) science progressed. You would have to take all of his Known Space novels into account and Ringworld and Fleet of Worlds already make up 3300+ pages. For an in-story explanation Neutron Star should be enough. – user45485 May 18 '16 at 10:31
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Although I haven't read the book, I'll bring my solid state physics training to bear on this question. When the intensity of the EM field -- even in the visible spectrum -- is high enough, there will be some nonlinear interactions between the field and the light is passing through. One consequence of this nonlinearity is that it becomes possible for two lower energy photons to be converted into one higher energy photon, with the crystal acting as a medium to help conserve linear momentum and total energy. The early versions of green laser pointers worked on this very principle where two IR photons interacted with a nonlinear "frequency-doubling" crystal to get green photons out.

  • Yep yep! All my green laser pointers are DPSS 532nm - aren't they still cheaper than green diode lasers simply because they've been around so long? They have an IR diode laser, a IR solid state laser, and a frequency doubling crystal and all the associated optics and electronics in that little package - beautiful! – uhoh May 19 '16 at 0:52
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    As far as the General Products hull technology though, it's supposed to be extremely advanced - so they should be supposed to stand up to (I would speculate) even an infrared Directed Energy Laser weapon, which would frequency multiply like crazy in most "normal" window materials. – uhoh May 19 '16 at 0:55
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If you came within -one mile- of a neutron star (and survived), I would imagine that gravitational lensing of the ambient starlight would produce a very high intensity light beam in your immediate vicinity. Sunburn? Oh, yeah!

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    Lensing of a single star can make flashes, but you can't lens many stars in different positions to a single point. Go outside on a clear day, with a magnifying glass and a piece of paper. You can focus the suns light just fine, but try to focus "blue sky" and nothing happens. Concentration of extended sources requires really funky optics like those used in solar concentrators for higher efficiency photovoltaic operation. – uhoh May 19 '16 at 1:02

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