Laser recoil in zero gravity?

indianroads · Jun 30, 2018

I'm back from my motorcycle road trip from Colorado to the Outer Banks Islands in NC. It was a good time and I feel refreshed and ready to hit my next novel hard.

A question though:
This novel starts off in Earth orbit with the MC firing a laser rifle at a bunch of people swarming up the space-elevator hoping to catch a ride on ships that are escaping a solar catastrophe. My question is, would his laser have a noticeable recoil that would be enough to push him away from the escaping ships?

Shoeless · Jun 30, 2018

Assuming you're talking about a "true" laser, that is simply a more powerful version of the cat toys and grocery scanners that we have today, and not a Star Wars laser that is a "bolt" which is apparently more like emitted plasma, then no, there shouldn't be any recoil. Kid's don't have to brace their shoulders and dig their feet in for recoil when they hold a magnifying glass over ants to burn them. Same thing here, there shouldn't be enough recoil from emitted light to actually push your MC around.

Guns with bullets on the other hand... yeah. You'll need to compensate for that.

Brightdreamer · Jun 30, 2018

If you want your gun to have a recoil, as Shoeless mentioned, maybe make it a plasma bolt shooter or technowhizbang weapon. A laser is just light, and wouldn't have a kick like something propelling a solid projectile at lethal speeds. (That said, you could probably come up with a malfunction or other issues with a hypothetical weapon designed to shoot a deadly beam of light over presumably long distances; things can go all melty or kabangy, still, if you want them to.)

indianroads · Jun 30, 2018

Thanks!

I have two different weapons in use in my future scifi world - the first is a laser rifle, the other is a mini-railgun. In this case, I'll have the character use a railgun.
Thanks again.

themindstream · Jun 30, 2018

I don't think it would have recoil in the way we think of for projectile weapons but I do wonder if it would have an effect like a solar sail. In this zero-G scenario, the laser wielder would drift back slightly over the course of the action and possibly need to readjust every few minutes.

Someone who knows more about radiation particle physics would have to confirm/deny. I don't think it would be a lot of movement, if at all.

Albedo · Jun 30, 2018

I think a bigger problem with laser weapons in a vacuum would be heat dissipation, particularly for a weapon small enough to be hand-wielded. You need some way to deal with waste heat, and the only way to do that without magic technology is radiators, the more powerful the weapon the bigger. You might describe the extended muzzle of the gun glowing white hot after being fired too many times in short succession. Or just hand wave the problem away with hyperefficient future laser tech.

jjdebenedictis · Jul 1, 2018

Woo-hoo, this is my field! The phrase you want to google to learn about this is "radiation pressure".

EDIT: Updating because I screwed up my definition of a peta-watt.

The quick version is: If you have one of present-day Earth's most powerful lasers, it emits about 2,000,000,000,000,000 Watts of power. This translates to about 6,700,000 Newtons of force spread out over the cross-section of the laser beam. That's your recoil.

A 100 kg (220 lbs) person firing such a laser will accelerate by about 67,000 metres per second-squared. In other words, if they fire the laser continuously for one-tenth of a second, while floating in space, they will be moving at a speed of 6,700 metres per second, or about 15,000 miles an hour, at the end of it!

So for a ferociously powerful laser, yes, that's a ferocious recoil speed. Presumably, however, your laser rifle wouldn't be that brutal. A 50,000 Watt laser can shoot down a drone two kilometres away and is probably fine for your purposes.

But that one won't give you bupkiss for a recoil. Running through the math for a 50,000 Watt laser gives, for the same constraints (100kg person, firing for 1/10 of a second), a recoil speed of 166 NANOMETRES per second--which is less than the length of a single wavelength of visible light.

Shoeless · Jul 1, 2018

jjdebenedictis said:
Woo-hoo, this is my field! The phrase you want to google to learn about this is "radiation pressure".

Damn. Today I learned the Death Star had recoil. Holy crap.

Shoeless · Jul 2, 2018

In related news, it looks like the Chinese have developed a working laser rifle.

Kind of surprised they beat the Americans to it. Or maybe the Americans still have theirs classified, and the Chinese just don't care.

Isilya · Jul 3, 2018

jjdebenedictis said:
Woo-hoo, this is my field! The phrase you want to google to learn about this is "radiation pressure".

EDIT: Updating because I screwed up my definition of a peta-watt.

The quick version is: If you have one of present-day Earth's most powerful lasers, it emits about 2,000,000,000,000,000 Watts of power. This translates to about 6,700,000 Newtons of force spread out over the cross-section of the laser beam. That's your recoil.

A 100 kg (220 lbs) person firing such a laser will accelerate by about 67,000 metres per second-squared. In other words, if they fire the laser continuously for one-tenth of a second, while floating in space, they will be moving at a speed of 6,700 metres per second, or about 15,000 miles an hour, at the end of it!

So for a ferociously powerful laser, yes, that's a ferocious recoil speed. Presumably, however, your laser rifle wouldn't be that brutal. A 50,000 Watt laser can shoot down a drone two kilometres away and is probably fine for your purposes.

But that one won't give you bupkiss for a recoil. Running through the math for a 50,000 Watt laser gives, for the same constraints (100kg person, firing for 1/10 of a second), a recoil speed of 166 NANOMETRES per second--which is less than the length of a single wavelength of visible light.

This is why I love this place. Awesome knowledge is awesome.

Vhb_Rocketman · Jul 4, 2018

Shoeless said:
In related news, it looks like the Chinese have developed a working laser rifle.

Kind of surprised they beat the Americans to it. Or maybe the Americans still have theirs classified, and the Chinese just don't care.

"can pass through windows and cause the “instant carbonisation” of human skin and tissues."

That is both awsome and terrifying.

Brightdreamer · Jul 4, 2018

Vhb_Rocketman said:
"can pass through windows and cause the “instant carbonisation” of human skin and tissues."

That is both awsome and terrifying.

I love how they also say it's "non-lethal", yet it can light a person's clothing on fire...

Shoeless · Jul 5, 2018

Brightdreamer said:
I love how they also say it's "non-lethal", yet it can light a person's clothing on fire...

To be fair, if a weapon is designed to maim, injure, and/or permanently cripple/disfigure, and put people in the hospital while inspiring fear in others, that's not actually lethal.

Like Batman.

indianroads · Jul 5, 2018

Shoeless said:
To be fair, if a weapon is designed to maim, injure, and/or permanently cripple/disfigure, and put people in the hospital while inspiring fear in others, that's not actually lethal.

Like Batman.

I've trained in martial arts for most of my life - anything, whether you call it a weapon or not, can be lethal in the right hands.

Dave.C.Robinson · Jul 15, 2018

This reminds me of Larry Niven's Man-Kzin Wars. Upon first contact, the human ships were completely unarmed because Earth had totally demilitarized; unless you count the 1-G photon drives which were basically planet-killing lasers.

Laser recoil in zero gravity?

indianroads

Wherever I go, there I am.

Shoeless

Brightdreamer

Just Another Lazy Perfectionist

indianroads

Wherever I go, there I am.

themindstream

Albedo

Alex

jjdebenedictis

is watching you via her avatar

Shoeless

Shoeless

Isilya

Rogues, thieves, and knaves abound

Vhb_Rocketman

Brightdreamer

Just Another Lazy Perfectionist

Shoeless

indianroads

Wherever I go, there I am.

Dave.C.Robinson

... with the High Command