# Knight in shiny armor vs the laser

## Situation:

• A knight wants to kill me.
• I have a night to prepare.
• It will be a duel.

## Me:

• I'm a time traveler (and I ended up in the worst possible timelines of the multiverse)
• I have a laser with a 14 kW power output concentrated into a 1 cm diameter circle.
• I also have its power source in the form of the fruit of life a 5 nm traversable wormhole that is used to transfer light from a Dyson sphere orbiting stars to me. This light is harnessed inside the core (NGE is where I get(read: steal) my ideas from), hence one of its Japanese names is Koukyuu, wich means "a sphere of light or radiance"
• I only have my ordinary clothing as "armor"
• I have some training in using laser rifles, but nothing in CQC.

## The Knight:

• Knows what he/she? is fighting against (lasers, that can be reflected)
• Has a nice little reflective plate armor, shield, and sword
• His/her? helmet is protected against lasers, completely (a wizard did it)

Considering the fact, that I don't want to die at the hands of a primitive barbarian (LOL) who can't even use forks while eating, I've decided to make a substance that is a:

• Good heat conductor
• Has a high melting point
• Can stick onto steel armor

...You can figure out, what I'm going to use it for.

## Question:

A: How long it takes until the knight fries collapses from heat exhaustion, thanks to keeping him/her under continuous laser fire?
B: Is there a real-life substance, that meets the criteria of my secret weapon?

• I wouldn't say this knight can't use a fork, I'd just say he didn't. Why use a fork anyways? My 1 year old daughter doesn't. Mar 23, 2017 at 16:47
• Quite relevant answer on Physics.SE: physics.stackexchange.com/a/70701/4083 Mar 23, 2017 at 17:13
• @cobaltduck Thx, but I've already got around this problem. (and any other, involving mirrors against lasers) Mar 23, 2017 at 17:17
• You have the concentrated power of a Dyson sphere and you're messing around fighting knights? You don't have a laser, you have an unstoppable siege weapon! Tell everyone you're a wizard, punch some holes in fortress walls, raise an army, and go take over the kingdom. Mar 23, 2017 at 19:00
• @Schwern If I'd have something bigger than a 5nm wormhole (those things (more specifically, the negative energy density) get ludicrously costly at bigger sizes), maybe a micro fusion reactor, or a nuclear power plant. Mar 23, 2017 at 19:04

I have a laser with a 14 kW power output concentrated into a 1 cm diameter circle.

# Power per unit area

An industrial laser cutter for steel operates in the 500 - 1.5kW range. You have a lot more power. OTOH you have to go through more material. OT3H medieval steel stinks compared to modern steel.

14 kW puts it in the power range of lasers tested to shoot down missiles, clearly a class 4 safety hazard that will at minimum burn skin.

However, industrial lasers are usually focused to less than a millimeter, in part because the target is also millimeters away. The real question isn't about raw power, it's about intensity: power per unit area.

Lasers lose coherence as they travel through the atmosphere. The further away the target is, the larger the area it falls on. You don't say how far away your laser has a 1 cm diameter circle, but I'll guess it's within the few dozen meters of a dueling ring.

Doesn't matter, it's enough to calculate the intensity of the laser, but know that intensity changes inversely with the range.

A circle has a surface area of $\pi d^2/4$. So your 1 cm circle has a surface area of $0.79 cm^2$ or $7.9 x 10^{-5} m^2$. That works out to an intensity of $18 kW/cm^2$. That's roughly the power output of a your typical car falling on an area smaller than your fingernail.

# Wavelength

The missing piece here is the laser's wavelength. Different wavelengths will transfer energy to different materials at different rates. Military and cutting lasers are in the micrometer (μm) range, so go with that.

Has a nice little reflective plate armor, shield, and sword

Medieval steel will not reflect a laser of this power, I don't care how well polished it is. Furthermore, something which is polished to reflect visible light probably won't reflect light in the micrometer range, so it's irrelevant.

[Their] helmet is protected against lasers, completely (a wizard did it)

# Holding it on target

This is the real problem. Developing lasers to shoot down missiles isn't just the laser power, but also holding it on target. That's done with computerized stabilization. You've just got your hand trying to hold on a target the size of your fingernail while that target is trying to kill you.

Your hand will shake, the knight will move, and the power of the laser will be spread out through a much, much larger area over a much, much larger period of time greatly defusing its effect.

At your power levels it might still cause burns to exposed skin, or even ignite exposed clothing on fire, but a knight will be used to pain and will close the distance and cut you down.

# Test the laser beforehand

You have a night to prepare for the knight. Get some armor plate, wave the laser around on it to simulate the knight moving around, and see what happens. If that cuts through, you're good. Confidently stride into the ring and slice him up.

If it doesn't, go find a cow and do the same thing. If it burns the cow, you're in good shape. There's enough exposed skin, clothing, and leather (ie. cowhide) on your standard knight's armor that you can wound him badly.

# Fair fights are for suckers

...but you should probably get rid of that armor before the battle even begins. Armor is custom fitted and cannot easily be replaced at a moment's notice.

Before the battle, while their armor and equipment is hanging on a rack and an easy target (armor is heavy and the less time spent wearing it before a duel the better), use the laser from a distance to covertly ruin the armor. You can steady your aim on a surface, and the armor isn't moving. Burn through the leather straps which hold the armor together and the whole thing will fall apart.

The knight, now lacking armor, will have to face you without that advantage. A much easier target for your laser. If they try to delay to get the armor prepared, boast that you were ready to face a fully armored knight with nothing but the clothes on your back.

• If this is a prearranged duel with t witnesses, damaging the knight's armor before the duel may be considered cheating and the time traveler might be lynched by a mob too big for him to laser them all. Jan 6, 2018 at 7:13

As @Schwern points out in their answer, the power output of your laser is high enough to pierce just about any armor, no matter how shiny it is. But, for the sake of fiction and with a healthy dose of suspension of disbelief...

Black paint. Throw it on the knight's shiny armor, shiny armor is no longer so shiny, armor absorbs all the energy from the laser, laser either cuts through armor and knight instantly (the realistic option) or cooks knight in their armor within a second (the comedic option).

A 100kW laser will destroy a plane. Your 14kW laser should be enough to fry a human. To put things in perspective 3kW is the high end for industrial cutting lasers.

If that doesn't work just shine it in their face. 5mW is enough to blind someone instantly.

• Note that the 100kW laser must punch through kilometers of atmosphere before reaching its target, must hold on a supersonic target, and can only fire for very brief pulses before overheating. If the plane were a few feet away like the knight there would be far, far less power involved. Mar 23, 2017 at 19:04