Could you make an umbrella out of lasers?

Stopping rain from falling on something with an umbrella is boring. 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?

Henry Reich is the creator of MinutePhysics and executive producer of MinuteEarth and MinuteFood and founder of Neptune Studios (the parent company for all three youtube channels).

Credits
Narrated by and based on "What If?" by Randall Munroe
Written & Directed by Henry Reich
Illustration and Video Editing by Lizah van der Aart
Illustration and Animation by Ever Salazar
Music & Sound Effects by Know Art Studios

Laser droplet animation based on footage by:
Physics of Fluids - University of Twente

What If? The Video Series is the official adaptation of the What If? books by Randall Munroe and is produced by Neptune Studios LLC.

©2023 xkcd, inc.

[jesseisstuckinside]

I love how most of his answers can be boiled down to "please don't"

[veuriam]

Out of sheer coincidence, I recently did the exact same research as a joke with some of my friends. We concluded that the whole system (with generators, cooling, targeting, etc.) would cost $8, 000, 000, weigh roughly 30, 000 pounds, chug an entire gallon of gasoline every few seconds, and "protect" 4 m^2 in hurricane weather from the horrors of *liquid* rain. It would also blind everyone.

[laalaastl]

I kind of love Randall doing the objecting lady voices.

[svenvandenberghe]

Actual laser engineer over here o/
My job isn't exactly "zapping water droplets with lasers"... But I do zap other "stuff" with lasers, which is then used to squirt droplets... which are then zapped by lasers!
I'm not 100% clear on what my NDA says, but that's probably as close as a description of my job, that I can share on YouTube😅

So about your laser-umbrella-concepts: I would actually do this differently, and more safely for your neighborhood - although the same could probably not be said for the airline pilots flying above you. Instead of aiming the beam parallel to the ground, I'd aim it up, straight into the direction the rain is coming from. And I would also fit a huge defocusing lens (the official jargon would be: "beam expander") on the laser, so the parallel beam, which is normally quite narrow, can expand to have a cross section of about a m².
This would essentially "cook" the droplets slowly as they are falling down to you, since they will all only receive a small portion of the power emitted, but for quite a bit longer than a single "zap". This means they likely won't fragment like you suggest in your video, but boil slowly. Even if they do fragment, your laser beam would be omnipresent to the perspective of the droplet, those fragments will still get blasted until they make it out of the m² thick beam.
As you mentioned in your video, the beam would be going up for a few 100 meters, until pretty much all of the light has collided with a rain drop, perhaps reaching the clouds producing the rain to begin with.
This brings to mind another tantalizing option: Why not evaporate the piece of the cloud that's raining on you? You could feel like God and create your own ray of "monochromatic sunshine" to blast a hole through the cloud.

Looking at the absorption curves for liquid water, I would also suggest using a Near-Infra-Red (NIR) laser, such as a Neodymium YAG laser, which has a wavelength of 1064 nm, giving it an absorption length of about 1mm. perfect for absorbing the power throughout the whole droplet, and not immediately at the surface, which would indeed make it explode.

If you have doubts about my strategy because the raindrops don't fall in a perfect straight line towards you, then I'd counter by suggesting that you could perhaps defocus your laser beam slightly (again with lenses) so you are shining the light in a cone upward, hitting more droplets the further away the beam gets, and therefore creating a larger (no)safe-zone further upward. You may need to increase your power to compensate for spreading out the beam even more though, but I seem to have read more outlandish solutions than that in your books... 😁

[harrisonirving]

After hearing the lady's voice I can't wait to see him doing Hat Guy saying, "What if we tried more power", during the laser pointers at the moon one.

[abbiearcher]

"Why is my house on fire again" is a killer line.

[danwylie-sears]

An umbrella diverts water sideways. Splattering the droplets away, instead of evaporating them, is exactly what you would want to do.

[jaromdl]

2:31 "...how far will it go before it hits a drop? This is a pretty easy question to answer; it's the same as asking how far you can see in the rain."

The simplicity of this brought me joy.

[darksamrai]

Finally, an xkcd video that doesn't end ion a world-ending apocalypse

[hellomark]

I enjoy that the last power setting on the laser is "please don't"

[pariahzero]

Another important question: How loud would it be to instantly vaporize a drop of water with a laser? Sudden conversion of a raindrop to gas has got to create a shockwave that would be *loud*. Repeat that with as many raindrops in close proximity over the user's head and it's got to be... unpleasant.

[camsy]

The character voices are absolutely delightful

[LightsOnTrees]

I like how it's "why is my house on fire AGAIN?", because it has happened more than once

[johnredford]

Swiveling lasers to target raindrops is about what they did at CMU to make rain-removing headlights. They replaced the single bulb of the headlight with a projector with a million pixels. It would turn off the pixels that would hit raindrops, letting the remaining light go much, much farther. They had high-speed cameras to find the raindrops, and then calculated where they would be when the pixel was ready to fire a few ms later. This apparently really worked, but needed massive computer power. That's a mere matter of hardware design...

[SuperFromND]

can we talk about that very well-done rotoscoped water droplet animation at 1:34, it looks very nice

[zachw]

OMG it's so awesome to see my question animated 😁 This was a lot of fun to watch and it made my day the same way it did way back when you took the time to answer it on WhatIf. Thanks so much

[jdotoz]

I have to think there's another layer of problem here. Let's say you vaporize the falling water above you and remove the kinetic energy of the drop in the process. You now have a tiny amount of steam, which I would expect to rise. But it's rising into a shower of relatively cold water, which will cause it to condense and join the rain falling on you. So now you have more liquid water falling on you than before, increasing your power requirement. Yes, the condensed steam shouldn't take as much energy to re-evaporate, but you still have to hit it quickly.

[CSarchitecture]

i find xkcd's "voice of reason" character quite charming lol. It's voiced like a pleasant version of Charlie Brown's teacher - the trombone "wah-wah" voice :)

[QuantumHistorian]

The lasers splattering droplets rather than evaporating them might help for the umbrella actually. You don't need to turn the droplets to mist to stay dry, you can simply deflect them so they land somewhere else than on you. That probably takes a lot less energy. Deflection rather than evaporation is how a boring umbrella works after all.

[JesseFeld]

So what you're saying is we should use microwaves as an umbrella instead?