What-If 0141: "Sunbeam"

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Quaaraaq
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What-If 0141: "Sunbeam"

Postby Quaaraaq » Tue Jan 12, 2016 3:33 pm UTC

Sunbeam
What if all of the sun's output of visible light were bundled up into a laser-like beam that had a diameter of around 1m once it reaches Earth?
—Max Schäfer

Here's the situation Max is describing:
Image


Something tells me that this may have been inspired by the new Star Wars movie.

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Re: What-If 0141: "Sunbeam"

Postby Beavertails » Tue Jan 12, 2016 3:59 pm UTC

A Dyson Sphere Death Star?

Awesome.

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Re: What-If 0141: "Sunbeam"

Postby yakkoTDI » Tue Jan 12, 2016 4:15 pm UTC

Beavertails wrote:A Dyson Sphere Death Star?


Buy their vacuums or else the Earth gets it!!!!

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Re: What-If 0141: "Sunbeam"

Postby Jeff_UK » Tue Jan 12, 2016 4:40 pm UTC

This is quite possibly the best line in a What-If, if not all of science fiction, so far:
You wouldn't really die of anything, in the traditional sense. You would just stop being biology and start being physics.
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Re: What-If 0141: "Sunbeam"

Postby sevenperforce » Tue Jan 12, 2016 8:10 pm UTC

Jeff_UK wrote:This is quite possibly the best line in a What-If, if not all of science fiction, so far:
You wouldn't really die of anything, in the traditional sense. You would just stop being biology and start being physics.

Yeah, the scenario itself didn't get quite as complex or interesting as a lot of other What-Ifs, but his writing is definitely on point.

I particularly loved this:
Randall wrote:Normal light interacts with the atmosphere through Rayleigh scattering. You may have heard of Rayleigh scattering as the answer to "why is the sky blue." This is sort of true, but honestly, a better answer to this question might be "because air is blue." Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons.

Exactly, exactly, exactly! And then when people pause, puzzled, and ask "Well why doesn't air look blue?" you just answer "Because it's usually not thick enough for you to notice." Which is basically true.

A few follow-up questions I have...
  • If the device DID have tracking ability, how long would it take to bore completely through the planet? Or would the continued collapse of the planet make this impossible? Would the Earth ever absorb enough energy to blow up/change shape/etc, or would it eventually reach a radiative equilibrium?
  • Can we do this? Say with a Dyson swarm....
  • If we did this with mirrors alone, how narrow could we make the beam?
  • How much longer would we survive if the moon was in the way when it happened?
  • If you wanted to build a bunker to protect yourself, how long could you hope to survive?

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Re: What-If 0141: "Sunbeam"

Postby edzieba » Tue Jan 12, 2016 10:48 pm UTC

Sunbeams: good for all your Boskonian problems!

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Re: What-If 0141: "Sunbeam"

Postby a_cat » Wed Jan 13, 2016 12:22 am UTC

sevenperforce wrote:I particularly loved this:
Randall wrote:Normal light interacts with the atmosphere through Rayleigh scattering. You may have heard of Rayleigh scattering as the answer to "why is the sky blue." This is sort of true, but honestly, a better answer to this question might be "because air is blue." Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons.

Exactly, exactly, exactly! And then when people pause, puzzled, and ask "Well why doesn't air look blue?" you just answer "Because it's usually not thick enough for you to notice." Which is basically true.


Following this idea wouldn't the answer to 'Why are sunsets orange?' be 'because that air over there is orange'? Doesn't seem all that convincing...

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Re: What-If 0141: "Sunbeam"

Postby HES » Wed Jan 13, 2016 12:29 am UTC

Jeff_UK wrote:This is quite possibly the best line in a What-If, if not all of science fiction, so far:

Who said anything about fiction?
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Re: What-If 0141: "Sunbeam"

Postby beverson » Wed Jan 13, 2016 4:17 am UTC

I initially missed this "What If?" Is the feed not working correctly? I tried deleting and re-loading it in my feed reader to no avail. I'm not sure who to contact about this.

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Re: What-If 0141: "Sunbeam"

Postby Eternal Density » Wed Jan 13, 2016 5:12 am UTC

Randall just wanted an excuse to draw stick-figure Twilight covers.
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Re: What-If 0141: "Sunbeam"

Postby BlueSloth » Wed Jan 13, 2016 10:09 am UTC

Collimating incoherent light, reaching millions of degrees when the photosphere is only thousands... I think thermodynamics would have something to say about this. (The corona is over a million degrees, but it's too sparse and transparent to do much.)

It does, however, seem like something like this could at least produce hotter temperatures than the current photosphere temperature because more light would reflect back to the sun, heating it up. It's essentially insulating the sun.

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Re: What-If 0141: "Sunbeam"

Postby eviloatmeal » Wed Jan 13, 2016 10:22 am UTC

a_cat wrote:Following this idea wouldn't the answer to 'Why are sunsets orange?' be 'because that air over there is orange'? Doesn't seem all that convincing...

"Why are rainbows pink?"

"Because rain is pink."

"Why are rainbows blue?"
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Re: What-If 0141: "Sunbeam"

Postby sevenperforce » Wed Jan 13, 2016 1:17 pm UTC

BlueSloth wrote:Collimating incoherent light, reaching millions of degrees when the photosphere is only thousands... I think thermodynamics would have something to say about this. (The corona is over a million degrees, but it's too sparse and transparent to do much.)

It does, however, seem like something like this could at least produce hotter temperatures than the current photosphere temperature because more light would reflect back to the sun, heating it up. It's essentially insulating the sun.

The question specified "bundled up into a laser-like beam [with] a diameter of around 1m" so ostensibly there is some sort of energy transformation involved. Less like a lens; more like a solar panel connected to a laser diode.

a_cat wrote:
sevenperforce wrote:I particularly loved this:
Randall wrote:Normal light interacts with the atmosphere through Rayleigh scattering. You may have heard of Rayleigh scattering as the answer to "why is the sky blue." This is sort of true, but honestly, a better answer to this question might be "because air is blue." Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons.

Exactly, exactly, exactly! And then when people pause, puzzled, and ask "Well why doesn't air look blue?" you just answer "Because it's usually not thick enough for you to notice." Which is basically true.

Following this idea wouldn't the answer to 'Why are sunsets orange?' be 'because that air over there is orange'? Doesn't seem all that convincing...

The answer to that is "because sunlight turns orange at sunset". If they press, you can explain that it's because the air between you and the horizon is even thicker, making it REALLY blue, so the sunlight looks even more orange.

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Re: What-If 0141: "Sunbeam"

Postby zombie_monkey » Wed Jan 13, 2016 2:23 pm UTC

Beavertails wrote:A Dyson Sphere Death Star?


A Nicoll-Dyson Laser

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Re: What-If 0141: "Sunbeam"

Postby Zinho » Wed Jan 13, 2016 7:28 pm UTC

a_cat wrote:Following this idea wouldn't the answer to 'Why are sunsets orange?' be 'because that air over there is orange'? Doesn't seem all that convincing...

Does it seem strange to you that a thing can look different colors when viewed from different angles? There are several metal coatings with that property, it's called opalescence.

Spoiler:
Example image:
Image
I teach my kids that they can't see stars during the day because the air in the sky glows when sunlight hits it. We see it glowing blue when the Sun is overhead, and we see it glowing yellow/orange/red in the evening. The difference in color is due to the angle we see it from, because over the horizon someone is looking up at the same air and seeing blue. They wrap their heads around it pretty well, and the discussion of Tyndall effect/Rayleigh scattering can start from there with a firm base and continue for as long as they have patience/unexploded brain cells left.

Spoiler:
Different angle, different color:
Image

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Re: What-If 0141: "Sunbeam"

Postby Boilerplate » Wed Jan 13, 2016 8:29 pm UTC

"If the beam was restricted to aiming at a fixed point in the sky, it would only take the Earth about three minutes to move out of the way."

This must presume that the beam started at the midpoint on the earth, because my rough calculations tell me the earth moves by its own diameter in about 7 minutes.

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Re: What-If 0141: "Sunbeam"

Postby rmsgrey » Wed Jan 13, 2016 8:55 pm UTC

edzieba wrote:Sunbeams: good for all your Boskonian problems!


Almost - hyper-velocity planets can still get through, and it's purely a defensive installation, so you still have to figure out a way of dealing with those pesky enemy bases.

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Re: What-If 0141: "Sunbeam"

Postby BFGaloot » Wed Jan 13, 2016 10:05 pm UTC

Someone asked if we could physically do this with mirrors. The answer is "no", since etendue (or radiance or luminance or in non-qualitative terms as used by laser jocks, brightness) is a conserved quantity. These quantities have units of power per steradian, and since the sun emits over 4 pi steradians, you can't just stuff it all into a collimated beam.

This is often missed by even experienced engineers, they often think that you can just put in a lens or something to increase the luminance of a source. As an example, if you tried to focus the sun's energy on a 1-meter diameter optical fiber, the vast majority of the light would not continue down the fiber but would leak out the sides since it is not confined within the numerical aperture of the fiber.

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Re: What-If 0141: "Sunbeam"

Postby Thrawn » Wed Jan 13, 2016 10:58 pm UTC

Quaaraaq wrote:Something tells me that this may have been inspired by the new Star Wars movie.

Actually it's much older (en.wikipedia.org/wiki/Second_Stage_Lensmen#Plot_synopsis).

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Re: What-If 0141: "Sunbeam"

Postby Beavertails » Thu Jan 14, 2016 1:24 am UTC

Thankfully, we have learned that a Dyson Sphere Death Star can destroy a Proton Earth and Electron Moon.
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Re: What-If 0141: "Sunbeam"

Postby buddy431 » Thu Jan 14, 2016 6:55 am UTC

How long would it take earth to be destroyed? I'd take as a first approximation the gravitational binding energy of the planet, and see how long it takes for that much energy to be dumped in. In reality it's going to be a lot more than that, because the earth is going to heat up and be radiating energy like crazy.

Earth requires 2.2*1032 J to blow apart. Sun puts out 3.8*1026 J per second, which I get to be about 160 hours. That's a lot longer than I expected.

And yeah, I think thermodynamics is going to have something to say about this scenario, however you try to rig it.

sevenperforce wrote:I particularly loved this:
Randall wrote:Normal light interacts with the atmosphere through Rayleigh scattering. You may have heard of Rayleigh scattering as the answer to "why is the sky blue." This is sort of true, but honestly, a better answer to this question might be "because air is blue." Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons.

Exactly, exactly, exactly! And then when people pause, puzzled, and ask "Well why doesn't air look blue?" you just answer "Because it's usually not thick enough for you to notice." Which is basically true.



But the color of air is different than what we traditionally think of color. Usually when we say "color", we're talking about reflected or occasionally transmitted light, rather than scattered. It's accurate in the traditional sense to say that ozone is blue, or chlorine is yellow, or nitrogen dioxide is brown. I would argue that it's not really appropriate to assign a material a color based on how it scatters light. Any of those gasses would also scatter blue light preferentially, but we wouldn't call any of them blue.
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Re: What-If 0141: "Sunbeam"

Postby markfiend » Thu Jan 14, 2016 10:10 am UTC

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Re: What-If 0141: "Sunbeam"

Postby aegidius » Thu Jan 14, 2016 12:04 pm UTC

OK, I think the fixation with the atmospheric plasma etc. is irrelevant. The atmosphere will be totally blown away.
The sun gives out 10^26 W, which is to be put in one place on the Earth (1m^2). The Tsar Bomba is the biggest H-bomb evah, at 10^17 J. (wikipedia figures) So that one m^2 patch is going to get 10^9 Tsar Bomba's exploding on it every second. This will vaporise a hole deep into the Earth's mantle. The ejected gases could propel the Earth forcefully away from the beam like some hellish rocket engine, but I think it more likely that they would blow the Earth to pieces.

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Re: What-If 0141: "Sunbeam"

Postby notthepope » Thu Jan 14, 2016 1:21 pm UTC

What's the "Breaking Dawn" joke in footnote/popup [3]?

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Re: What-If 0141: "Sunbeam"

Postby rmsgrey » Thu Jan 14, 2016 1:28 pm UTC

notthepope wrote:What's the "Breaking Dawn" joke in footnote/popup [3]?


The image layout is very similar to the stylised title of the TV series Breaking Bad - except that both Br and Ba are symbols for chemical elements; Da isn't. Also, there's unlikely to be much crossover between the two fandoms, so a lot of people are going to be inclined to take offense at the implied comparison.

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Re: What-If 0141: "Sunbeam"

Postby sevenperforce » Thu Jan 14, 2016 2:28 pm UTC

buddy431 wrote:
sevenperforce wrote:I particularly loved this:
Randall wrote:Normal light interacts with the atmosphere through Rayleigh scattering. You may have heard of Rayleigh scattering as the answer to "why is the sky blue." This is sort of true, but honestly, a better answer to this question might be "because air is blue." Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons.

Exactly, exactly, exactly! And then when people pause, puzzled, and ask "Well why doesn't air look blue?" you just answer "Because it's usually not thick enough for you to notice." Which is basically true.

But the color of air is different than what we traditionally think of color. Usually when we say "color", we're talking about reflected or occasionally transmitted light, rather than scattered. It's accurate in the traditional sense to say that ozone is blue, or chlorine is yellow, or nitrogen dioxide is brown. I would argue that it's not really appropriate to assign a material a color based on how it scatters light. Any of those gasses would also scatter blue light preferentially, but we wouldn't call any of them blue.

I think Randall's point was that reflection and transmission don't constitute "color" any more than scattering does; they're just phenomena in physics.

Perhaps a better way of stating it would be "the sky is blue because air turns blue when you stack it that high directly under a bright light".

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Re: What-If 0141: "Sunbeam"

Postby Pfhorrest » Thu Jan 14, 2016 4:24 pm UTC

buddy431 wrote:It's accurate in the traditional sense to say that ozone is blue, or chlorine is yellow, or nitrogen dioxide is brown. I would argue that it's not really appropriate to assign a material a color based on how it scatters light. Any of those gasses would also scatter blue light preferentially, but we wouldn't call any of them blue.

Except ozone, which you just called blue.
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Re: What-If 0141: "Sunbeam"

Postby chenille » Thu Jan 14, 2016 5:04 pm UTC

buddy431 wrote:But the color of air is different than what we traditionally think of color. Usually when we say "color", we're talking about reflected or occasionally transmitted light, rather than scattered.

Very much agreed. Saying the atmosphere is blue might let you skip a physics lesson for the moment, but it is going to come back double when you notice that the thicker atmosphere makes the sunset look red. Fine, it turns out that's from it being more than just the pure air; then let me point out that you could certainly have a planet where scattering continues until the sky look white, even with the same gases we have.

Structural colors are not like pigment colors. "Blue-sky research" into them has given us so much, it amazes me how many science fans would rather just say air is blue and skip all those differences.

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Re: What-If 0141: "Sunbeam"

Postby sevenperforce » Thu Jan 14, 2016 5:48 pm UTC

chenille wrote:
buddy431 wrote:But the color of air is different than what we traditionally think of color. Usually when we say "color", we're talking about reflected or occasionally transmitted light, rather than scattered.

Very much agreed. Saying the atmosphere is blue might let you skip a physics lesson for the moment, but it is going to come back double when you notice that the thicker atmosphere makes the sunset look red. Fine, it turns out that's from it being more than just the pure air; then let me point out that you could certainly have a planet where scattering continues until the sky look white, even with the same gases we have.

Structural colors are not like pigment colors. "Blue-sky research" into them has given us so much, it amazes me how many science fans would rather just say air is blue and skip all those differences.

But "pigment colors" do not constitute color in any way that is more scientifically concrete or valid than the way "structural colors" constitute color. In fact, one could argue the reverse; pigmentation is actually the absorption of other wavelengths, so red pigment is actually a "orange-yellow-green-blue-indigo-absorbing color".

Zinho's explanation that "the air in the sky glows when sunlight hits it" is a pretty good approximation.

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Re: What-If 0141: "Sunbeam"

Postby SDK » Thu Jan 14, 2016 6:34 pm UTC

It's not a question of being scientifically concrete. When I say something is a certain colour, 99% of the time I'm talking about the light being reflected off that something. Common usage defines that. The sky operating by a different mechanism than common usage is a relevant distinction.
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Re: What-If 0141: "Sunbeam"

Postby sevenperforce » Thu Jan 14, 2016 7:25 pm UTC

SDK wrote:It's not a question of being scientifically concrete. When I say something is a certain colour, 99% of the time I'm talking about the light being reflected off that something. Common usage defines that.

What color is your skin when you go outside?

What color is your hair?

What color are your eyes? What about your significant other's eyes?

What color is this dress?

What color is that car?

What color is your cellphone background image?

What color is your computer desktop?

What color was the traffic light?

What color are your car's headlights?

What color is that necklace you bought?

What color is that blinking indicator over there?

What color is a firefly?

What color is that candle?

What color is lava?

What color is that flame?

What color is bluegrass?

What color is that window?

What color is lightning?

I daresay that 99% figure is a little bit overestimated.

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Re: What-If 0141: "Sunbeam"

Postby chenille » Thu Jan 14, 2016 9:43 pm UTC

sevenperforce wrote:But "pigment colors" do not constitute color in any way that is more scientifically concrete or valid than the way "structural colors" constitute color.

It's not a question of the mechanism so much as its consistency. When you say something is blue, it normally means you can expect it to look blue in most contexts where the light allows for it. If that isn't the case – if for instance the color depends on the angle like Zinho's example, or on fluorescence excited by certain wavelengths, or in the case of substances if they're dichromatic – it makes for a very poor description.

Here on Earth the atmosphere is in fact slightly blue thanks to things like ozone, and I understand that for instance influences its color from space. But like I said you have cases where the same atmosphere makes the sun look red; and elsewhere you could have the same gases scatter enough to appear white, or blue skies from gases that are not bluish from above. Saying the sky is blue because the air is blue is true in the special case but misleading in general.

To me that's an explanation of the worst kind, a glib answer that hides all the ways sky color doesn't work the way you might expect. Meanwhile talking about scattering doesn't just explain it but is a first step into a larger world, where you learn how birds and trees and blood vessels can be blue without any blue substances in them, how color depends on structure and context, and so on. I'm amazed anyone rejects it in favor of pretending it's just more of the same, physics like everything else, nothing of special interest.

Not anyone who likes science, anyway. I get why people who don't care would be happy to stop there, but there's a reason blue-sky research is a term.

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Re: What-If 0141: "Sunbeam"

Postby SDK » Thu Jan 14, 2016 9:57 pm UTC

sevenperforce wrote:I daresay that 99% figure is a little bit overestimated.
Lava is pretty awesome.

How are my skin or hair colours not reflection?

You're right, though. I forgot about light. Let's add simple light scenarios to selective absorption (covering reflection and transparent materials) as being intuitively obvious. That'll hit your 99%. Describing most other cases of colouring in the same simple terms is doing a disservice to the complexity of the universe! Rainbows, man! Wow!
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Re: What-If 0141: "Sunbeam"

Postby sevenperforce » Thu Jan 14, 2016 10:18 pm UTC

chenille wrote:It's not a question of the mechanism so much as its consistency. When you say something is blue, it normally means you can expect it to look blue in most contexts where the light allows for it. If that isn't the case – if for instance the color depends on the angle like Zinho's example, or on fluorescence excited by certain wavelengths, or in the case of substances if they're dichromatic – it makes for a very poor description.

Here on Earth the atmosphere is in fact slightly blue thanks to things like ozone, and I understand that for instance influences its color from space. But like I said you have cases where the same atmosphere makes the sun look red; and elsewhere you could have the same gases scatter enough to appear white, or blue skies from gases that are not bluish from above. Saying the sky is blue because the air is blue is true in the special case but misleading in general.

To me that's an explanation of the worst kind, a glib answer that hides all the ways sky color doesn't work the way you might expect. Meanwhile talking about scattering doesn't just explain it but is a first step into a larger world, where you learn how birds and trees and blood vessels can be blue without any blue substances in them, how color depends on structure and context, and so on. I'm amazed anyone rejects it in favor of pretending it's just more of the same, physics like everything else, nothing of special interest.

I'm surprised, because I see it the opposite way. I remember my dad mentioning that the Bohr model wasn't exactly right when I was about 12, and I asked him what atoms were really shaped like, and he said I wouldn't be able to understand it unless I understood probability distributions and electron degeneracy. I wish he had just said "they are shaped differently based on how much energy their electrons have!" because I could have wrapped my head around that enough to ask more questions later on.

Being able to offer a deceptively simple answer like "air is blue" can, I think, help someone to realize that it's not hopelessly complicated and a sensible explanation is entirely within reach.

SDK wrote:
sevenperforce wrote:I daresay that 99% figure is a little bit overestimated.
How are my skin or hair colours not reflection?

When you're outside in sunlight, a good bit of the color of your skin is actually fluorescence. And depending on the pigmentation, thickness, and volume of your hair, it can appear partially transparent or otherwise interact with light in ways other than pure reflection.

Let's add simple light scenarios to selective absorption (covering reflection and transparent materials) as being intuitively obvious. That'll hit your 99%.

Eye color is partially the result of scattered light in the same way that makes the sky blue. Rainbows, prisms, and opalescent materials are colored due to refraction. Light sources are sometimes the result of excitation emission, sometimes the result of fluorescence, and sometimes the result of thermal blackbody radiation.

Sure, selective absorption is one major source of perceived color, but thermal emission, excitation, fluorescence, refraction, and scattering are far more common than just 1%.

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Re: What-If 0141: "Sunbeam"

Postby sevenperforce » Thu Jan 14, 2016 10:44 pm UTC

(Apologies for the double post but editing posts from mobile is hell on this crapphone.)

Perhaps to clarify....

I don't see "air is blue" as a dismissal of the fantastic physics of Rayeigh scattering at all, and I don't think Randall does either. Rather, I see it as an invitation to explore...an subtle implication that even something so "mundane" as color is anything but. Selective absorption may be closer to intuitive experience but it's actually REALLY complicated. Reflection alone requires pretty substantial quantum mechanisms. Try explaining EXACTLY why red paint is red, and you'll long for the days of explaining refraction and scattering.

EDIT: Besides, isn't a substantial part of why sunsets appear red due to atmospheric dust rather than the comparatively meager enhanced scattering?

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Re: What-If 0141: "Sunbeam"

Postby chenille » Thu Jan 14, 2016 11:01 pm UTC

sevenperforce wrote:Besides, isn't a substantial part of why sunsets appear red due to atmospheric dust rather than the comparatively meager enhanced scattering?

Yes, I gather that enhanced scattering from a thick layer of air would instead make things more white. You're still left with an atmosphere of slightly-dusty air that colors the sun yellowish when it's high, red when it's near the horizon, yet at noon manages to appear blue both high and low, which is an interesting difference.

sevenperforce wrote:Being able to offer a deceptively simple answer like "air is blue" can, I think, help someone to realize that it's not hopelessly complicated and a sensible explanation is entirely within reach.

I really don't see how "blue light is scattered more by the air", or some rephrasing like Zinho's, doesn't do the same. And yet I'd argue it's less misleading, doesn't encourage you to think it works the same way as blue paint, and is the sort of concept you are ultimately going to need anyway if you ever want to explain blue eyes or bluebirds.

For the record, blue paint tends to be blue in most contexts. But if a child asked me why, I'd explain that light comes in packets with a certain energy for each color. When they reach the paint molecules, some of those energies fit into them and so get absorbed, but for this substance blue doesn't fit and is left behind. It's very far from the whole story, but I think it at least points you in the right direction, and maybe hints a little about how the world works in general. That's what an explanation is supposed to do.

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Re: What-If 0141: "Sunbeam"

Postby netsplit » Fri Jan 15, 2016 4:29 am UTC

How about this very simple definition of color?

Code: Select all

Color: the wave length of photons leaving an object by means of radiation or reflection as perceived by human eyes


Perceived is a key word, it neatly avoids the violet/purple issue where red and blue photons look like violet due to how people see them. violet is a shorter wave length than either red or blue of course.

I don't find air is blue very satisfying. If that were true, pictures of the ground from space would have a blue tint, and Mars would look purple through ground telescopes. The answer I find most satisfying is that the air is iridescent like a hummingbird feather and translucent, color depends on the angle relative to the light source.
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Re: What-If 0141: "Sunbeam"

Postby Pfhorrest » Fri Jan 15, 2016 5:38 am UTC

netsplit wrote:The answer I find most satisfying is that the air is iridescent like a hummingbird feather and translucent, color depends on the angle relative to the light source.

That's not really accurate though, because it's not as though the sky near the sun is red and the sky far from the sun is blue. Two people on different parts of the Earth looking at (very nearly on a solar scale) the same angle toward the sun, but through different amounts of air, will see different colors, e.g. if it's noon where you are and you look at where Venus should be (not that you can see it in broad daylight), and it's dusk or dawn somewhere else and someone there is also looking at where Venus should be, you'll see blue at that spot while they see red-orange.
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Re: What-If 0141: "Sunbeam"

Postby netsplit » Fri Jan 15, 2016 6:34 am UTC

Pfhorrest wrote:
netsplit wrote:The answer I find most satisfying is that the air is iridescent like a hummingbird feather and translucent, color depends on the angle relative to the light source.

That's not really accurate though, because it's not as though the sky near the sun is red and the sky far from the sun is blue. Two people on different parts of the Earth looking at (very nearly on a solar scale) the same angle toward the sun, but through different amounts of air, will see different colors, e.g. if it's noon where you are and you look at where Venus should be (not that you can see it in broad daylight), and it's dusk or dawn somewhere else and someone there is also looking at where Venus should be, you'll see blue at that spot while they see red-orange.


That's a good point about thickness of air affecting the color, so maybe iridescent isn't the right word. I can't think of a good single word or simple name that describes the coloring of the atmosphere. At short distances it scatters blue, at longer distances only red remains to be scattered. Its really the only thing we deal with of it's size in every day life, and particular color schemes happen at specific times so that's what langauge, at least English, went with.

What's even odder about it is it's only blue because our eyes suck at purple compared to blue, it should be scattering violet light better than blue.

Edit: I forgot, there's even an XKCD about this https://xkcd.com/1145/
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Re: What-If 0141: "Sunbeam"

Postby GulliNL » Fri Jan 15, 2016 9:11 am UTC

sevenperforce wrote:
Jeff_UK wrote:This is quite possibly the best line in a What-If, if not all of science fiction, so far:
You wouldn't really die of anything, in the traditional sense. You would just stop being biology and start being physics.

Yeah, the scenario itself didn't get quite as complex or interesting as a lot of other What-Ifs, but his writing is definitely on point.

I particularly loved this:
Randall wrote:Normal light interacts with the atmosphere through Rayleigh scattering. You may have heard of Rayleigh scattering as the answer to "why is the sky blue." This is sort of true, but honestly, a better answer to this question might be "because air is blue." Sure, it appears blue for a bunch of physics reasons, but everything appears the color it is for a bunch of physics reasons.

Exactly, exactly, exactly! And then when people pause, puzzled, and ask "Well why doesn't air look blue?" you just answer "Because it's usually not thick enough for you to notice." Which is basically true.

A few follow-up questions I have...
  • If the device DID have tracking ability, how long would it take to bore completely through the planet? Or would the continued collapse of the planet make this impossible? Would the Earth ever absorb enough energy to blow up/change shape/etc, or would it eventually reach a radiative equilibrium?
  • Can we do this? Say with a Dyson swarm....
  • If we did this with mirrors alone, how narrow could we make the beam?
  • How much longer would we survive if the moon was in the way when it happened?
  • If you wanted to build a bunker to protect yourself, how long could you hope to survive?


And another question, when building a Dyson Sphere/Swarm with mirrors, aren't we already focussing the light from the star in the building progress? Every unit (mirror) that gets put in it's position blocks the light from going straight outwards to back to the star or another direction. I'm no physicist but my guess would be that when the building progress is at -say- 50% that would mean that the light that is still able to escape the star has to be 200% of its original intensity. This intensity would increase the further you get in the process (since the net amount of light still has to remain the same, only the surface over which it can dissipate would decrease, just like a garden hose where you place your thumb over the spout) which should make it harder to finish the Sphere with every unit you put into orbit. Eventually you would be left with the final unit, the final piece of the puzzle and this hole is the only exit for the light to leave the Sphere... I think I just came up with the biggest Double Slit experiment evarr!

Another way of building could be prefabricating two hemispheres and trying to quickly slam them together trapping the star in their midst. But in this case too, the light (and all the other forms of energy a sun emits) would be forced to leave the star in an ever decreasing space making it harder and harder to place the two hemispheres together. Like trying to screw a lid on a broken fire hydrant.

One final question (really, I don't have plans of building a Dyson Laser. Trust me. That Powerball money will be invested in bonds and better health care for puppies), how would the star itself react to being trapped in a Sphere? It is now bombarding itself with insane amounts of heat, light, x-ray, matter and probably building up pressure inside the Sphere. Will the star collapse under this stress? Will it become stronger? Will the entire Sphere start acting as a new star?

Really, how does one build a Dyson Sphere?

**edit** I realise that a "normal" Dyson Sphere wouldn't completely enclose a star but in this 'What-If' it would, for the sake of harnessing all the power of the sun, so my questions regard this kind of Sphere.
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