What-If 0119: "Laser Umbrella"

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What-If 0119: "Laser Umbrella"

Postby mtavs » Fri Nov 14, 2014 7:15 am UTC

Laser Umbrella

What if you tried to stop rain with a laser that targeted and vaporized each incoming droplet before it could come within ten feet of the ground?

My first thought was that you would need waterproof equipment for the lasers and targeting device, but that would only be the case the umbrella it didn't work. Also, there's no number three [3] footnote thing. It skips from 2 to 4.

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Re: What-If 0119: "Laser Umbrella"

Postby peregrine_crow » Fri Nov 14, 2014 7:54 am UTC

Doesn't it matter at which altitude we're trying to zap the rain? I imagine it would be harder to aim at rain droplets that are further away, but it would also mean that the deadly steam cloud you are creating is further away (and hopefully less deadly).
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Re: What-If 0119: "Laser Umbrella"

Postby bachaddict » Fri Nov 14, 2014 8:06 am UTC

peregrine_crow wrote:Doesn't it matter at which altitude we're trying to zap the rain? I imagine it would be harder to aim at rain droplets that are further away, but it would also mean that the deadly steam cloud you are creating is further away (and hopefully less deadly).

The cloud would also absorb the laser, requiring more energy to get through.

Would a jet engine pointing up work?
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Re: What-If 0119: "Laser Umbrella"

Postby Dr What » Fri Nov 14, 2014 8:16 am UTC

"you just multiply the 2.6 megajoules per liter by the rainfall rate and you get laser umbrella power requirement"
Yes, that result will be close if the rain consists of black body water drops.
For regular water drops, according to this, a 70nm laser would be the best. If UV laser is hard to make, we can try a 3μm IR laser.

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Re: What-If 0119: "Laser Umbrella"

Postby solune » Fri Nov 14, 2014 8:50 am UTC

There were several rockets that exploded because engineers mixed US and metric (or inches and liters (i.e decimeter cube)). For the sake of humanity (and for my eyes :D ), please don't do that.

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Re: What-If 0119: "Laser Umbrella"

Postby Djehutynakht » Fri Nov 14, 2014 9:02 am UTC

BHG in footnote 4 pretty much won this.

As well as the Craigslist ad... which I may or may not be answering...

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Re: What-If 0119: "Laser Umbrella"

Postby Tulkaslan » Fri Nov 14, 2014 9:30 am UTC

Another potential issue with the laser umbrella would be the scattering of high energy laser pulses upon contact with the water droplet. Given that you're likely to be using a class 4 laser, the odds of randomly injuring someone would be fairly high as well.

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Re: What-If 0119: "Laser Umbrella"

Postby peregrine_crow » Fri Nov 14, 2014 12:26 pm UTC

bachaddict wrote:The cloud would also absorb the laser, requiring more energy to get through.

But that is also a problem if you create the steam cloud closer to the ground, so that shouldn't matter, should it?
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Re: What-If 0119: "Laser Umbrella"

Postby LtPowers » Fri Nov 14, 2014 1:24 pm UTC

What happened to footnote 3?


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Re: What-If 0119: "Laser Umbrella"

Postby Neil_Boekend » Fri Nov 14, 2014 1:55 pm UTC

*cringes at the formula*
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Re: What-If 0119: "Laser Umbrella"

Postby Klear » Fri Nov 14, 2014 2:12 pm UTC

That last picture is missing the guy's car:

Image

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Re: What-If 0119: "Laser Umbrella"

Postby rmsgrey » Fri Nov 14, 2014 2:15 pm UTC

If you had a suitable laser and a good enough aim, could you get somewhere by deflecting raindrops rather than vaporising them? Or is the path of an individual raindrop too unpredictable?

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Re: What-If 0119: "Laser Umbrella"

Postby mattg0110 » Fri Nov 14, 2014 2:24 pm UTC

Zach Wheeler, like, the Mets pitcher?

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Re: What-If 0119: "Laser Umbrella"

Postby Whizbang » Fri Nov 14, 2014 2:29 pm UTC

Arrows.jpg

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Re: What-If 0119: "Laser Umbrella"

Postby Moose Anus » Fri Nov 14, 2014 3:27 pm UTC

Reminds me of the laser mosquito net.
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Re: What-If 0119: "Laser Umbrella"

Postby azule » Fri Nov 14, 2014 4:01 pm UTC

A pretty good one, especially of those of recent. Yeah and with Black Hat as a footnote. So sneaky.

What's wrong with the math, N_B? I couldn't see it at the time, nor would I understand it at the time, most likely.

Footnote 3 is showing up for me. It's a bunch of quotes from the reference papers.
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Re: What-If 0119: "Laser Umbrella"

Postby Neil_Boekend » Fri Nov 14, 2014 4:10 pm UTC

azule wrote:What's wrong with the math, N_B? I couldn't see it at the time, nor would I understand it at the time, most likely.

Not wrong as in wrong math, but he combines inches and SI units in one formula. That is a legendary method of inducing off by factor 25,4 errors (such as crashing mars probes).
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Re: What-If 0119: "Laser Umbrella"

Postby ps.02 » Fri Nov 14, 2014 4:22 pm UTC

Neil_Boekend wrote:Not wrong as in wrong math, but he combines inches and SI units in one formula. That is a legendary method of inducing off by factor 25,4 errors (such as crashing mars probes).

Eh ... that seems like it'd only be a problem if you're in the habit of ignoring units and just blindly hoping they cancel the way you imagine. Don't do that! Even purely in SI, you'll be introducing orders-of-magnitude errors all the time, e.g., as you fail to notice that you have meters in the numerator and cm in the denominator.

Of course, with pure SI, there's a small chance that these errors themselves will happen to cancel. But don't rely on that!

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Re: What-If 0119: "Laser Umbrella"

Postby mathmannix » Fri Nov 14, 2014 5:28 pm UTC

equation.png
equation.png (6.88 KiB) Viewed 15224 times

Is that better for everyone's sensibilities?

EDIT: Oh, and Neil B., I feel like that guy in the last comic was claiming your avatar was to blame for the fire.
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Re: What-If 0119: "Laser Umbrella"

Postby jonbly » Fri Nov 14, 2014 8:28 pm UTC

rmsgrey wrote:If you had a suitable laser and a good enough aim, could you get somewhere by deflecting raindrops rather than vaporising them? Or is the path of an individual raindrop too unpredictable?

That's what I was thinking - the ink drop video seemed to suggest a decent deflection effect. Of course, now we're not just targeting moving raindrops, we're trying to hit specific points on them...

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Re: What-If 0119: "Laser Umbrella"

Postby KittenKaboodle » Fri Nov 14, 2014 10:58 pm UTC

solune wrote:There were several rockets that exploded because engineers mixed US and metric (or inches and liters (i.e decimeter cube)). For the sake of humanity (and for my eyes :D ), please don't do that.


Mixing units seems to be a "What if" thing, maybe part of the joke ?

"What if we randomly mixed "US" and SI units in a major engineering project?"

Seems to me the initial requirements were a bit ambitious, I'd think a warm mist would be an acceptable outcome and that would be much easier to accomplish that totally vaporizing every raindrop.
Also, if you are trying to stop rain you don't totally randomly point your laser, you'd at least keep it above the horizon, probably a radian or so above the horizon (Unless you are black hat guy).

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Re: What-If 0119: "Laser Umbrella"

Postby Eshru » Sat Nov 15, 2014 1:44 am UTC

What about a constantly on laser that spins very quickly?

Edit: multiple lasers if necessary.

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Re: What-If 0119: "Laser Umbrella"

Postby Copper Bezel » Sat Nov 15, 2014 2:10 am UTC

Roughly the same as the random paths. Luckily, since it's raining and your laser umbrella doesn't work, nothing should stay on fire for very long.
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Re: What-If 0119: "Laser Umbrella"

Postby Pfhorrest » Sat Nov 15, 2014 2:27 am UTC

Wouldn't shooting a single wide-beam laser of enough power straight up into the sky have the effect of vaporizing all the water somewhere up around the visibilility-through-rain distance, where it (the superheated water vapor and the lasers) won't hurt anything (that's not flying)? The cloud of water vapor would then block and absorb all the laser energy, but that would just keep it (the vapor cloud) hot enough to vaporize any new droplets falling into it from above.
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Re: What-If 0119: "Laser Umbrella"

Postby Mikeski » Sat Nov 15, 2014 5:31 am UTC

Pfhorrest wrote:Wouldn't shooting a single wide-beam laser of enough power straight up into the sky

So, a wider one of these as an umbrella?

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Re: What-If 0119: "Laser Umbrella"

Postby Neil_Boekend » Sat Nov 15, 2014 8:04 am UTC

ps.02 wrote:
Neil_Boekend wrote:Not wrong as in wrong math, but he combines inches and SI units in one formula. That is a legendary method of inducing off by factor 25,4 errors (such as crashing mars probes).

Eh ... that seems like it'd only be a problem if you're in the habit of ignoring units and just blindly hoping they cancel the way you imagine. Don't do that! Even purely in SI, you'll be introducing orders-of-magnitude errors all the time, e.g., as you fail to notice that you have meters in the numerator and cm in the denominator.

Of course, with pure SI, there's a small chance that these errors themselves will happen to cancel. But don't rely on that!

Off by 1k fail the sanity check far more easily than off by 25.4.
mathmannix wrote:EDIT: Oh, and Neil B., I feel like that guy in the last comic was claiming your avatar was to blame for the fire.

Yeah, eh about that. Ehm, you see?
Luckily some idiot was basting away at the neighborhood with a high powered laser.
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Re: What-If 0119: "Laser Umbrella"

Postby Mikemk » Sat Nov 15, 2014 10:23 am UTC

What if you strap a whole bunch of lasers to one of these?
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Re: What-If 0119: "Laser Umbrella"

Postby Heimhenge » Sat Nov 15, 2014 5:18 pm UTC

Aren't raindrops negatively charged by their passage through air? I found "air" near the positive end of several triboelectric series online, but not "water." Still, I'm almost certain raindrops pick up some amount of charge as they fall through air, and participate in the charge separation that eventually generates lightning.

If this is the case, then instead of a laser umbrella, maybe you could just position a powerful Van de Graaff generator at the top of your umbrella and deflect the falling drops to the sides? Shields up!

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Re: What-If 0119: "Laser Umbrella"

Postby keithl » Sat Nov 15, 2014 7:09 pm UTC

Water vapor is lighter than air. Incandescently hot water vapor is much lighter than air. Keep firing an ultrahigh power infrared laser at all the water droplets, and microdroplets, etc. until it is all vapor and you have a large column of superheated air and vapor rising rapidly into the air above you, with surface air pulled horizontally into the column. Probably with raindrops entrained, so the airflow must be tailored to send those upwards into the blast zone. Rain drops fall at 5 to 30 meters per second, the smallest ones are slowest, so if the airstream rises faster than that, the drops won't fall.

You don't need an agile mirror, just a couple of spinning prisms like the inside of a laser printer, scanning the 100 square meter sky patch a few times per second and firing the laser when a drop appears in the crosshairs. Raster, not calligraphy.

USE MORE POWER(tm) and the air where the drop was will be hot enough to vaporize the remaining droplets. All that incandescent air above your head will heat you rapidly, so you will still need an umbrella, this time with an infrared mirror surface on the top, which is much less boring than a rain umbrella. After all, the purpose of this exercise is to avoid boring tents and umbrellas and use powerful lasers. Mission f***ing accomplished.

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Re: What-If 0119: "Laser Umbrella"

Postby Klear » Sat Nov 15, 2014 10:49 pm UTC

keithl wrote:Water vapor is lighter than air. Incandescently hot water vapor is much lighter than air. Keep firing an ultrahigh power infrared laser at all the water droplets, and microdroplets, etc. until it is all vapor and you have a large column of superheated air and vapor rising rapidly into the air above you, with surface air pulled horizontally into the column. Probably with raindrops entrained, so the airflow must be tailored to send those upwards into the blast zone. Rain drops fall at 5 to 30 meters per second, the smallest ones are slowest, so if the airstream rises faster than that, the drops won't fall.

You don't need an agile mirror, just a couple of spinning prisms like the inside of a laser printer, scanning the 100 square meter sky patch a few times per second and firing the laser when a drop appears in the crosshairs. Raster, not calligraphy.

USE MORE POWER(tm) and the air where the drop was will be hot enough to vaporize the remaining droplets. All that incandescent air above your head will heat you rapidly, so you will still need an umbrella, this time with an infrared mirror surface on the top, which is much less boring than a rain umbrella. After all, the purpose of this exercise is to avoid boring tents and umbrellas and use powerful lasers. Mission f***ing accomplished.


Wouldn't that be causing tornadoes or something?

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Re: What-If 0119: "Laser Umbrella"

Postby PinkShinyRose » Sat Nov 15, 2014 11:25 pm UTC

Is there any particular reason why our light source needs to be a laser?

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Re: What-If 0119: "Laser Umbrella"

Postby The Moomin » Sat Nov 15, 2014 11:52 pm UTC

Is all this about making a real life version of the game Pang?
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Re: What-If 0119: "Laser Umbrella"

Postby Various Varieties » Sun Nov 16, 2014 5:13 pm UTC

Is this the first What-If to have alt-text for a picture within a footnote? No earlier examples come to mind for me...

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Re: What-If 0119: "Laser Umbrella"

Postby andy01q » Sun Nov 16, 2014 9:37 pm UTC

I believe this is actually way easier because some of the problems cancel each other out!
The Laser will more likely scatter the rain than outright vaporize it, I agree with that, But: THIS IS A GOOD THING!
Let's target an imaginary 1m² plain 10m above us, assign 1.000.000 points on that plain and shoot all these points randomly with 100 shots per second.
Let's say we can see 200m far in rain. Let's also say this means that by 200m distance 95% of the laser beams will have hit. This means that by 10m only 4,75% of the laser beams would hit; bad quota!
But they do hit! So what happens when they hit? They make the rain-droplets scatter.
What happens when the rain-droplets start to scatter?
They will start to become way easier to hit and with enough scattered droplets soon enough close to 100% instead of 4,75% of our laser will hit something inside of 10m.
This makes the energy we deal to the rain droplets in about 10m distance increase by a factor of 21 and since we mainly hit heated shreds of already scattered rain we will actually heat the water-stuff above 100°C to actually vaporize it, yay.
We're not done yet, what happens to the water-vapor?
First off since it's way warmer and thanks to the rain-scatter also less dense than its surroundings it will decelerate in downwards speed and possible start to thrust upwards.
But since water vapor can cool laser-rays it will still heat up even though already vaporized and thus make our laser less efficient.
But that again is not as bad as it sounds because the heated water vapor will move up even quicker and there it will pre-heat the rain above our vapor-cloud making it easier for us to vaporize it.
Well... when the heat-energy from the water vapor gets transmitted to the rain, that means that the water vapor will condense back to water and thus create a massive wall of water-vapor-water. O_O
At least this will make it yet easier for our laser to hit it's targets. :D
What happens next? Given infinite energy the water condenses farther and farther until we reach a black hole? Just kidding. With more energy the water-vapor-water wall will become bigger than that square meter we target and at the sides of it the water avoids the laser thus it will cool back down and finally fall down - next to us.
I assume that a Laser with twice up to 4 times as much energy as needed to vaporize the rain water within an area 1m² wide and 3m high used with my described targeting algorithm should keep you free of water as you walk along.
Note that it would also shower your surroundings with 99° hot steamy water!
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Re: What-If 0119: "Laser Umbrella"

Postby algorerhythms » Mon Nov 17, 2014 2:03 pm UTC

Seems to me a better idea might be to try to trap the water droplets in optical tweezers, slow them down and deflect them somewhere else, rather than trying to vaporize them. Of course you'd need a lot of very intense, focused laser beams for this, so you'd still set your neighbor's house on fire, but he's probably used to it by now.

Also, I haven't worked out the math for it, so the laser power you'd need might be (probably is) ridiculous.
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Re: What-If 0119: "Laser Umbrella"

Postby jgh » Tue Nov 18, 2014 5:28 pm UTC

People have been trained by dodgy SciFi that it is possible to shoot some form of energy at something and the target simply ceases to exist, instead of all the component atoms dispersing into the surrounding environment as much more energetic particles. I think only Larry Niven got this right when a character uses a device to remove the binding energy from a target, and the target (a city) explodes into lots of subatomic particles shooting out into space.

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Re: What-If 0119: "Laser Umbrella"

Postby dalcde » Tue Nov 18, 2014 8:40 pm UTC

Apparently 3 people made entries for "autoclave you can live in" after the what-if came out. The list was initially empty.

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Re: What-If 0119: "Laser Umbrella"

Postby david.windsor » Wed Nov 19, 2014 1:38 am UTC

dalcde wrote:Apparently 3 people made entries for "autoclave you can live in" after the what-if came out. The list was initially empty.


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Re: What-If 0119: "Laser Umbrella"

Postby cellocgw » Wed Nov 19, 2014 1:02 pm UTC

ps.02 wrote:
Neil_Boekend wrote:Not wrong as in wrong math, but he combines inches and SI units in one formula. That is a legendary method of inducing off by factor 25,4 errors (such as crashing mars probes).

Eh ... that seems like it'd only be a problem if you're in the habit of ignoring units and just blindly hoping they cancel the way you imagine. Don't do that! Even purely in SI, you'll be introducing orders-of-magnitude errors all the time, e.g., as you fail to notice that you have meters in the numerator and cm in the denominator.

Of course, with pure SI, there's a small chance that these errors themselves will happen to cancel. But don't rely on that!


That's why the nuclear physics guys came up with this set of units where c=1 (speed of light), dimensionless. Makes all sorts of things have the same dimensions and units.
And, no, I'm not joking. They kind of make sure to tack on the dimensions at the end of the calculation
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Re: What-If 0119: "Laser Umbrella"

Postby Nicias » Wed Nov 19, 2014 1:28 pm UTC

cellocgw wrote:
That's why the nuclear physics guys came up with this set of units where c=1 (speed of light), dimensionless. Makes all sorts of things have the same dimensions and units.
And, no, I'm not joking. They kind of make sure to tack on the dimensions at the end of the calculation

Its call Geometrized units. Mathematicians use it too. In some sense they are more natural in terms of being truer to the laws of space-time and less dependent on the happenstance around us.

However, they take some getting used to. Our customary units of time, mass, and length are way off. Take the sun for instance. Its mass is 1.5km, a reasonable length. In units of time that is 5 microsecond, and in units of mass it is 10^30 grams.

Of course in GR solutions all scale, so you will often just have a scale parameter M or r_0 or whatever. So for instance the Schwarzschild solution can be described by its Mass from infinity M or the value of the radial coordinate for the event horizon (r_s = 2M) or whatever.


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