What-If 0013: "Laser Pointer"

[Quilbert]

One interesting question is, what would the Earth look like from the dark side of the moon at stage 2 (green 1W lase pointers)? Half of it (Asia, that is) would obviously be lit by the sun and have its usual blue-green appearance, but on the dark side of the Earth, you would see Africa and Europe glowing in green! Actually, Asia would also be a little greener than usual. Anyone up for an artist’s impression?

Apologies if I'm mistaking a slip of the tongue (fingers?) for uninformedness of astronomic terms, but the dark side of the moon is the one not facing earth - that's why is dark, it does get sunshine - we just don't see it; a remnant of a geocentric vocabulary.
From the side of the moon that's not iluminated by the sun the earth should appear pretty solid green - we're giving off half a lux with the lasers, and full moon on earth is about 0.27 lux. Agreed that it's going to be an interesting sight but I'm not the right person to that - my drawing skills are limited to a Mohammed stick figure (in an effort to inspire one person in each country to produce something crude to offend him, ridiculing the fundamentalists and distributing the damage done to embassies, reducing it to a manageable level).

Wow, that would be a pretty strange naming convention … Actually, Wikipedia says:

[cite]The dark side of the Moon can refer to:

• the lunar hemisphere that is not lit by the sun at any particular moment in time, or
• the far side of the Moon that is permanently turned away from the Earth (although it receives approximately the same amount of light as the near side of the moon)

[/cite]

and:

[cite]The far side of the Moon, sometimes called the "dark side of the Moon" in the sense that it is in a radio blackout in respect to transmitters on Earth,[1][2] is the lunar hemisphere that is permanently turned away, and not visible from the surface of the Earth.[/cite]

But I don’t really care … I take it that your 0.27 lux are probably more correct than the 1 lux in the what-if. But we’d have to take into account that the Earth is bigger than the Moon, has a different albedo, and is only half lit in this scenario. Also, the Earth’s population is not nearly evenly distributed …

[hroobarb]

My first thought was the question of lightspeed--for example, aiming at the sun would not work too well, as the light takes 16 minutes for a round trip, so the rotation of the earth puts your aim off by 4°, which is a big factor when your target is only about 0.5° wide. However, since the moon is only about 1.25 light seconds away, it takes only 2.5 seconds round trip, so the aim is not a big factor-the moon just doesn't move fast enough to escape.

The rotation of earth isn't a factor, is it? It's not like the laser beam is a solid rod that rotates with the earth. The light keeps heading toward the sun no matter what, as long as the entire solar system doesn't travel around the galaxy too quickly.
Somebody please correct me if I'm wrong.

[conorjh]

There was an obviously doomed public arts project organized around this in 2001

http://www.afineline.org/projects/paint.html

[peewee_RotA]

I'm quite disappointed that this was not one of the projected outcomes.

[Vroomfundel]

Wow, that would be a pretty strange naming convention … Actually, Wikipedia says:

[cite]The dark side of the Moon can refer to:

• the lunar hemisphere that is not lit by the sun at any particular moment in time, or
• the far side of the Moon that is permanently turned away from the Earth (although it receives approximately the same amount of light as the near side of the moon)

[/cite]

and:

It's indeed not ideal - the far side of the moon would be more intuitive; but then again, electricity flowing in the direction in which electrons travel would also be - some confusing conventions once coined just stuck (at least until someone fixes it - http://xkcd.com/567/)

I don't think that the citation claims otherwise, it just states that "the dark side of the moon", if taken literally, means the side that is, well, dark. However most people (e.g. the guy I ninja'd, hehe) would assume that you are refering to the far side when you use the phrase, I think it's pretty widespread. Maybe if English is not your first language it will be even less intuitive. It's a second language for me but I get to read about celestial bodies mostly in English so I've been indoctrinated early on.

[Max™]

I'm quite disappointed that this was not one of the projected outcomes.

As am I, chum.

[Kaden]

Did anyone else notice that there's an automatic chat window that opens up on the wickedlasers.com link Randall included in the article? Here's how my conversation went:

Joe N.: Hi, thanks for visiting, please let me know if you have any questions.
me: uh... hello?
Joe N.: how can I help you?
me: do you guys really chat with every person who visits this page?
Joe N.: If we are on duty and they want to chat about our products
me: you must be getting a lot of traffic from xkcd
Joe N.: what is that?
people are mentioning it today
me: http://what-if.xkcd.com/
scroll down to "You can pick one up for $300."
it's a direct link to this page
and that's one of the most popular webcomics on the internet, so... yeah
Joe N.: what is the site name?
me: expect to be bombarded lol
xkcd.com
it's pretty famous
http://en.wikipedia.org/wiki/Xkcd
has its own wikipedia page and everything lol
Joe N.: we have beengetting traffic from there
me: it was just posted today, the thing that links here
Joe N.: our screen shows that is where you came here from
me: neat stuff
hope it helps your sales!
please don't sue him or anything haha
he's a nice guy
Joe N.: we welcome the traffic, so long as they don't put us down
me: great
well I'm going to mosey along now...
nice talking to you!
Joe N.: you too, have a good day!

Wonder how many additional visitors they'll have. Probably won't be too too many since it'll only be people that visit the what-ifs, and then only the people that read that far and clicked on the link.

What a nice guy! I'm glad he seemed to be taking it in stride. That's good customer service, IMO.

I'm quite disappointed that this was not one of the projected outcomes.

Same here.

[commodorejohn]

The moral of the What-if series seems to be that the more hypothetical questions you ask, the closer you bring the world to (un)natural disaster or outright apocalypse. It's kinda fascinating, really.

[bmonk]

The moral of the What-if series seems to be that the more hypothetical questions you ask, the closer you bring the world to (un)natural disaster or outright apocalypse. It's kinda fascinating, really.

I believe Mythbusters has this tendency too. Or they would if they could command enough explosive power.

[johnfrye3]

I am very surprised that no one, not Randall or anyone else, suggested doing this during a total lunar eclipse. Personally, what I would have liked to see is Randall finding the next total lunar eclipse that is visible from all of Eurasia, extrapolate population trends and laser pointer numbers/technology to that year, and then show the result.

[Moose Anus]

Did anyone else notice that there's an automatic chat window that opens up on the wickedlasers.com link Randall included in the article?

I got that too. I thought it was like one of those ones where women come up and want to chat, and then get all pissy when you don't reply, when you go to po... some websites.

[rmsgrey]

I am very surprised that no one, not Randall or anyone else, suggested doing this during a total lunar eclipse. Personally, what I would have liked to see is Randall finding the next total lunar eclipse that is visible from all of Eurasia, extrapolate population trends and laser pointer numbers/technology to that year, and then show the result.

Total solar eclipse might be more interesting - though aiming could be awkward for people not currently in the umbra

[Max™]

I want a shirt of this.

I just found my new avatar

Also, I love black hat guy, we need more people that will blow up the earth... FOR SCIENCE!

*EDIT*
Found this on reddit, the neighbours will complain

Spoiler:

Being Australian myself, I lol'd pretty hard.

For you:

Spoiler:

[niky]

A gleefully glib what-if. I found myself reasing each "More power!" line in Jamie Hyneman's voice.

Why did people have to start talking about Xeelee? Now I'm going to have to go back and read all one trillion pages of the Xeelee Sequence... again.

[johnfrye3]

Total solar eclipse might be more interesting - though aiming could be awkward for people not currently in the umbra

With a total solar eclipse the effect would be even harder to see than in Randall's scenario. His considerations were: "We can try to illuminate either a new moon or a full moon. The new moon is darker, making it easier to see our lasers. But the new moon is a trickier target, because it’s mostly visible during the day—washing out the effect." The solution seems obvious to me: full moons are even darker than new moons during a total lunar eclipse, thanks to the lack of reflected earthshine, and are mostly visible during the night, ensuring that the effect is maximum and the majority of persons with lasers can see it. Plus, it would be interesting to see how the brightness changes from the penumbra to the umbra, and vice versa. Granted this is not as visually obvious as the half moon depicted by Randall, but it would be nice to see it addressed rather than overlooked.

[ChrisTilley]

I like the video on the 1W Laser site but it got me wondering back the Larry Niven's Ringworld. Would the balloons pop if they were the same color as the beam? How close in color would they have to be?

[Fieari]

It'd have to be a VERY shiny green. Most balloons are matte, which means there'd be plenty of energy absorption.

[Jamaican Castle]

In the final scenario, where Randall finally finds enuff dakka for his task, would the Earth really be hit by the emerging plasma? I mean, I know there's a lot of plasma happening here, most of it quite energetic, but it's no piddling distance back to Earth.

(This, incidentally, is how the Death Star was unaffected; they weren't dumb enough to get up close and personal with an exploding planet.)

[Max™]

A gleefully glib what-if. I found myself reasing each "More power!" line in Jamie Hyneman's voice.

Why did people have to start talking about Xeelee? Now I'm going to have to go back and read all one trillion pages of the Xeelee Sequence... again.

Sorry, that's me, was seeing if anyone else was a Baxter fan... or maybe I just want a starbreaker and nightdark wings a 100 miles wide... though oddly I'm in a Xeelee-ish mood due to recently finishing Blame! and NOiSE, check em out.

[niky]

I only read Baxter whenever I want to feel inadequate about myself.

Of course, these What-Ifs are more Niven-ish. Though Niven did once try to blow up Pluto with frozen hydrogen and oxygen that turned out, in the end, to not be there.

[Max™]

I only read Baxter whenever I want to feel inadequate about myself.

Of course, these What-Ifs are more Niven-ish. Though Niven did once try to blow up Pluto with frozen hydrogen and oxygen that turned out, in the end, to not be there.

Ah, I'm not as into the whole... human experience thing as much as I am the stuff that fills my head with images of galaxies sailing past me and ridiculously gorgeous vistas out of hubble deep field shots (deep field since most everywhere redder except near the ring, of course) though I want to see one of those really talented space artists do a rendition of the last scene in Manifold: Space.

But yeah, it was actually keithl's post where he basically described a Type-4 civilization that decided it really didn't like the moon... or anything else within several light-seconds of it which got my head going "STARBREAKERS!", more power indeed.

[Quicksilver]

I want a shirt of this.

I just found my new avatar

Also, I love black hat guy, we need more people that will blow up the earth... FOR SCIENCE!

*EDIT*
Found this on reddit, the neighbours will complain

Spoiler:

Being Australian myself, I lol'd pretty hard.

For you:

Spoiler:

Despite most people not knowing most of the criminal lines from those ships died out with the gold rush in the 1850's, I still lol'd at the second stereotype.

[cantab314]

full moons are even darker than new moons during a total lunar eclipse, thanks to the lack of reflected earthshine

I thought the Earthshine-lit moon was fainter than the eclipsed moon. Certainly I had no trouble seeing the latter the last time I saw a total lunar eclipse, while Earthshine is hard to see without magnification.

Also, the original question was never answered.

"If every person on Earth aimed a laser pointer at the Moon at the same time, would it change color?"

Going from very dark grey to lighter grey to white is not changing colour!

[sam_i_am]

Didn't Goldfinger have a laser that could project a spot on the moon?

[rmsgrey]

full moons are even darker than new moons during a total lunar eclipse, thanks to the lack of reflected earthshine

I thought the Earthshine-lit moon was fainter than the eclipsed moon. Certainly I had no trouble seeing the latter the last time I saw a total lunar eclipse, while Earthshine is hard to see without magnification.

I'll thank you to keep your attribution straight!

And, yeah, the red glow of sunlight gently scattered through the atmosphere to hit the moon while it's eclipsed appears brighter than the pale glow of reflected light for a new moon, but since the latter tends to be against a brighter sky, I'm not prepared to conclude that the latter is dimmer.

[Oktalist]

Why does Randall use EST when talking about the time of day in the Indian subcontinent? I thought the E for Eastern meant East Coast, not Far East.

[Max™]

Why does Randall use EST when talking about the time of day in the Indian subcontinent? I thought the E for Eastern meant East Coast, not Far East.

2 PM in the EST timezone means it's daytime there and nighttime in India?

[Oktalist]

What does the local time on the US east coast have to do with anything? The moon wouldn't even be above the horizon there. In India it would already be the next day.

Through the rest of the What If he uses GMT like a sensible person. (I'll forgive him not using UTC instead... for now.)

[Max™]

What does the local time on the US east coast have to do with anything? The moon wouldn't even be above the horizon there. In India it would already be the next day.

Through the rest of the What If he uses GMT like a sensible person. (I'll forgive him not using UTC instead... for now.)

I assume he used it as a reference for US readers, but yeah, UTC is preferable.

[sirkha]

I want
http://what-if.xkcd.com/imgs/a/13/laser_pointer_more_power.png
on a tee shirt. Anyone else?

[gmalivuk]

My first thought was the question of lightspeed--for example, aiming at the sun would not work too well, as the light takes 16 minutes for a round trip, so the rotation of the earth puts your aim off by 4°, which is a big factor when your target is only about 0.5° wide. However, since the moon is only about 1.25 light seconds away, it takes only 2.5 seconds round trip, so the aim is not a big factor-the moon just doesn't move fast enough to escape.

The rotation of earth isn't a factor, is it? It's not like the laser beam is a solid rod that rotates with the earth. The light keeps heading toward the sun no matter what, as long as the entire solar system doesn't travel around the galaxy too quickly.
Somebody please correct me if I'm wrong.

You're not wrong.

[bmonk]

My first thought was the question of lightspeed--for example, aiming at the sun would not work too well, as the light takes 16 minutes for a round trip, so the rotation of the earth puts your aim off by 4°, which is a big factor when your target is only about 0.5° wide. However, since the moon is only about 1.25 light seconds away, it takes only 2.5 seconds round trip, so the aim is not a big factor-the moon just doesn't move fast enough to escape.

The rotation of earth isn't a factor, is it? It's not like the laser beam is a solid rod that rotates with the earth. The light keeps heading toward the sun no matter what, as long as the entire solar system doesn't travel around the galaxy too quickly.
Somebody please correct me if I'm wrong.

You're not wrong.

True. But I was more thinking of the moon being slower moving...

[Draco18s]

In the Lensman series, the next step up from throwing planet-sized negative masses or catching an enemy world between a couple of spare planets with equal and opposite velocities is to use the sun's entire energy output focused into a single beam (strictly as a defensive measure since the power-source was non-portable - at least at that stage of the arms race...)

That level of output would actually accelerate the solar system out of its orbit around galactic central point.
It'd be a Class C Stellar Engine.

It wouldn't be much, but it would add up over time (reflecting half of our sun's output for a million years would impart a stellar velocity of 20 m/s).

[jboy55]

My biggest problem with this is that Randal starts with the apparent size of a dot projected from a laser pointer on the moon in what seems all subsequent calculations.

My question would be, what degree of collimation and therefore aiming accuracy would people need in order for a spot to be visible say from the 1W green pointer?

And for aiming accuracy, instead of each person having a 1 TW laser, we could first just assume they all have a motorized telescope mount, calibrated to keep a point on the moon.

Some quick Excel calcs shows. If only 2000 people, aimed their 1W laser (producing 150 lumens) to the same 100KM spot on a new moon. This would produce a 'star' with a -3.15 magnitude (or above the new moon, and visible during the day). It would require collimation < .2mrad (not out of the question).

* Edit: I realize this is with the moon being a perfect reflector.

[PinkShinyRose]

Everytime someone mentions increasing laserpower in a microscopy practical I think of blackhat guy, I just can't get it out of my head.

Wow, that would be a pretty strange naming convention … Actually, Wikipedia says:

[cite]The dark side of the Moon can refer to:

• the lunar hemisphere that is not lit by the sun at any particular moment in time, or
• the far side of the Moon that is permanently turned away from the Earth (although it receives approximately the same amount of light as the near side of the moon)

[/cite]

and:

It's indeed not ideal - the far side of the moon would be more intuitive; but then again, electricity flowing in the direction in which electrons travel would also be - some confusing conventions once coined just stuck (at least until someone fixes it - http://xkcd.com/567/)

I used to think that too. But in the lifesciences, where really any flow of charged particles is considered "electricity" (I have no idea of whether this is a correct use of the word from a physics perspective) and the major particle flows considered consist of potassium, sodium or calcium ions it makes perfect sense to consider the electricity to flow from the positively charged side to the negatively charged side (if that still applies in those cases, I'm not sure).

Why does Randall use EST when talking about the time of day in the Indian subcontinent? I thought the E for Eastern meant East Coast, not Far East.

And it's even the east coast of the small continent, so the term makes no sense at all to begin with. The only reasonable argument for it I can think of is that the east coast of Afro-Eurasia varies more in longitude.

[rmsgrey]

I used to think that too. But in the lifesciences, where really any flow of charged particles is considered "electricity" (I have no idea of whether this is a correct use of the word from a physics perspective) and the major particle flows considered consist of potassium, sodium or calcium ions it makes perfect sense to consider the electricity to flow from the positively charged side to the negatively charged side (if that still applies in those cases, I'm not sure).

Also, when dealing with semi-conductors, about half the time the charge carriers are the (positively charged) holes in the electron sea. On the other hand, it is still all about the movements and arrangements of electrons, which would make more sense to call positive if someone with a time machine and a working knowledge of Classical Greek were in a position to do something about it...

[hooiberg]

The price has gone down from 300 to 200 dollar.
No Kinder Eggs for my Murican fellow human beings, but killer lasers are fine...