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Laser point defense is a known concept.

A powerful laser beam, either continuous or pulsed is used to shoot down or deflect either a projectile or object out of the sky. The concept has been speculated to be very beneficial to a multitude of systems. Vehicles, artillery, base protection etc. etc.

One idea I've seen floating around is the idea of miniaturizing such a concept down to where an infantry soldier can have on mounted onto their person to intercept bullets and shrapnel.

While the idea is interesting, and suggested in place of your typical sci-fi energy barrier-type shielding. It has always sounded a bit...off to me.

First off, would the sensors be able to detect and respond in time? Computers today certainly have the speed and power, but would the detectors be able to sense the projectile in the heat of combat?

And how far out does this system have to be to actually work? Too close and the laser may not be able to do its job.

Additionally, the chances of the laser emitter (which is presumably a form of orb with a lens on the front and mounted on a swivel) missing are uncomfortably high.

These and many more issues brings me to ask:

Assuming that the technology and engineering is available, is the concept of a infantry mounted point defense laser for intercepting shrapnel and bullets viable?

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    $\begingroup$ Are you asking about sensor response time, time needed to neutralize incoming projectiles, or how to mount the laser aperatus? If you say the tech exists you can just as easily your system has the response time power, and other design elements necessary to be a viable system in your fictional world. $\endgroup$
    – sphennings
    Oct 10, 2023 at 7:17
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    $\begingroup$ You're missing a lot of important details. The power required to melt a bullet between when it is fired and when it hits a person would weigh many times what the person themselves weighs. Even with powered armor, the power output would be insane. Even if you manage that, you now have a chunk of melted slag heading towards you. Vaporizing it would require an order of magnitude MORE power. As phrased, this question is a brainstorming ask. Stack Exchange isn't suited for "I have this great idea, can you make it work for me" kind of questions. $\endgroup$ Oct 10, 2023 at 17:34
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    $\begingroup$ @RobertRapplean What WBSE isn't suited for is dictating plot. "Why does my character like/do [X]" or "why does [Y] empire invade another empire" does not fit, but "is it possible for my idea to work" is. $\endgroup$
    – KEY_ABRADE
    Oct 10, 2023 at 18:07
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    $\begingroup$ Seraphim, you have both the Science-based and Internal-consistency flag. I know that Internal-consistency used to be considered synonymous with reality-check, and was often paired with Science-Based, but the standards around this have changed a bit. Internal-consistency is only used for things like magic and pseudoscience where you only care if the logic of a thing contradicts itself or the setting. So, if you want Science based lasers, remove Internal-consistency, if you want Clark Tech or magic lasers, remove science-based $\endgroup$
    – Nosajimiki
    Oct 10, 2023 at 21:02
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    $\begingroup$ I can't vote to reopen. Let me explain why. "Viable" is as bad a word as "possible," "plausible," and "realistic." This Stack's goal is to help you build a fictional world. It is NOT to help you realize as fact a fictional idea. I know several reasons why this isn't "viable" in the world today - but why should that matter in your fictional world? Consequently, you're not asking a question that makes sense. In your world it's obviously "viable." So what's stopping you from moving forward? Do you have an actual worldbuilding question, or are you simply seeking permission to use the idea? $\endgroup$
    – JBH
    Oct 11, 2023 at 15:35

4 Answers 4

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Potentially, but a minor frame challenge

Other answers may bring up power-density or computation issues. I have a much simpler one.

Tanks can get away with active protection systems which blow apart incoming shells/missiles/penetrator darts/etc. because the chunks of the incoming projectile simply bounce off the tank's rather durable armor. However, a destroyed bullet still flying at a soldier is much more dangerous to that soldier, proportionately speaking, than a destroyed anti-tank missile still flying at a tank is to that tank.

Dressing the infantry in a suit of powered armor as proportionately durable to bullet fragments as a tank is to missile fragments may solve this. However, this runs into another thing: a laser capable of tracking and destroying an object as small, fast, and durable as a bullet is likely capable of doing the same to the infantry that bullet is aimed at.

In other words: if lasers capable of this exist in-setting, they're probably going to replace bullets. Even if the lasers are designed to simply vaporize part of the surface of the bullet to push it off-course, that still implies very high power (i.e. laser output in joules/second) and energy (i.e. joules per kilogram or liter of battery) densities, which means that each soldier can likely carry around weapons far, far more destructive — and harder to stop — than bullets. One of these active protection lasers can assuredly do some unpleasant stuff to a humanoid if it can disrupt a bullet enough to save a humanoid. Now imagine ten strapped together and converging on the same point.

Here is a hard-science death ray calculator you may enjoy. You will find that it takes either (a) a lot of power or (b) a lot of time for a laser to do damage.

Real life active protection systems are, to simplify things, tiny guns that shoot down incoming missiles whereas their much bigger cousins — tank guns — fire super-fast kinetic energy penetrators active protection systems cannot defend against. The same will likely happen with your lasers (presuming power density/computation issues are solved): a little version for shooting down the occasional chunk of shrapnel, a big version for turning portions of people into rapidly-expanding clouds of hot gas, and a complete inability for the little version to deflect the big version.

This will likely end up resulting in just as much infantry mortality as if they were getting shot with bullets and had no APS.

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Not even close in the simplest case

I worked in the ballistic missile defense (BMD) field for a while. Lasers are a bad idea for intercepting missiles (despite the US DoD trying to use lasers to do just that, because it sounds really cool), and they would be a bad idea for bullet and shrapnel interception for some of the same reasons.

The biggest problems are a combination of sensor accuracy and just plain physics.

  • A) The footprint of a laser on its target is a tiny dot, a few millimeters in diameter at most.
  • B) Lasers do not instantaneously blow holes in things Star Wars style- they have to dwell on the same point on the target for a time to compromise it.
  • Therefore, C) the sensor needs to be accurate enough to "see" where the laser dot is on the target (which is moving faster than the speed of sound), and the actuators physically pointing the laser must be precise enough to keep that dot in that exact spot long enough. This is further complicated because missiles and bullets spin around one axis to stabilize their flight; that means you lose line of sight on your dot. Shrapnel spins randomly around all 3 axes.

Radar is the sensor of choice in BMD applications, and it's a good one. But the best that a radar can do, even when it already knows where to look for its target, is tell you that it is somewhere inside the width of the beam (for azimuth and elevation), and that it is at some distance between two pulses (range). That gives you a box that the target can be in, not the XYZ points down to the millimeter where a laser dot should go now. Oh, and this target will be a kilometer away in less than a second. Any sensor system will suffer the same or similar issues.

BMD radars are also fairly stationary. Your infantryman is sprinting, ducking, sliding all over the place, which the physical actuators have to account for.

But assume you have an perfect sensor and a perfect actuation system. If a hostile is firing at you with a (slow) Mach 1 bullet from 50 meters away, you have 0.146 second (50m / 343 m/s) to detect a shot, get a target lock on the bullet, decide if it's a threat, and disintegrate it. It needs to do it about three times as fast for Mach 2.9 NATO 5.56 round.

While you're calculating the number of gigajoules it would take to disintegrate a jacketed lead round and praying I don't switch to steel ammo, I'm going to be pulling out my semi-auto shotgun loaded with tungsten double-ought buckshot.

...but if you can disintegrate a bullet at 50 meters, you should be aiming that laser at the guy shooting you

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    $\begingroup$ It is possible to disintegrate a bullet into a cloud of hot plasma, but that requires phenomenal power and energy densities which, as you and I have pointed out, mean you're likely to be getting shot with lasers rather than bullets. $\endgroup$
    – KEY_ABRADE
    Oct 10, 2023 at 18:15
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    $\begingroup$ The last line really is the definitive answer - a system that powerful would be better utilized as your primary weapons system. especially if it can track and intercept multiple targets in fractions of a second. $\endgroup$ Oct 10, 2023 at 19:56
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Use predictive AI to aim your ADS

Other answers are correct. There is no way a laser can wait for someone to pull a trigger and have time to respond by intercepting the bullet, but a much more reasonable expectation would be a system that uses cameras and other sensors positioned around your body that feed into an image recognition AI to predict incoming threats and aim your lasers BEFORE the bullet it fired. If the AI sees someone turning their gun towards you, or a grenade land nearby, it can respond by aiming the lasers to intercept the predicted angle of attack.

This makes all of the concerns regarding how quickly your laser needs to aim or time required to power it up a non-issue because as long as your ADS can out-draw an attacker by reading body language, you have eliminated the time it takes to arm and aim vs when you only start to respond once the shot is fired. So your response time is then reduced to how long it takes a computer system mounted to a high speed camera to respond to the mussel flash which could easily be less than a thousandth of a second. The bullet could still be within inches of the mussel by the time the ADS fires in response. So depending on just how handwavy your laser themselves are, this system could potentially be effective at ranges of just a few meters.

As for accuracy issues...

There is really no rule saying a future tech society can't make a mini orb laser accurate enough to directly hit the bullet, again, using predictive AI, you should be able to map not just where the bullet is, but where it will be so you can hold your laser on target, but what if you don't want this level of auto-aiming technology in your setting?

As it turns out, you don't actually need to hit a projectile for a laser to be an effective ADS. If you base your lasers on the Boeing Plasma Shield concept, you just need to shoot somewhere in the path of the attack, you don't need actually target the projectile itself.

Instead of firing a single laser to try to burn away the bullet, you could fire a set of lasers that converge in the air between you and the enemy just before he fires creating a mid-air plasma burst. This uses less power than is needed to make a viable laser rifle (which solves internal consistency issues), and less accuracy than is needed to directly intercept a supersonic projectile. The plasma burst can do a wide range of things including blocking enemy lasers, overwhelming enemy sensors, visually blinding the enemy, and/or creating a shockwave that can detonate missiles and deflect shrapnel.

The big drawback here is that it will not be able to stop bullets fired at close quarters, but long-ranged sniper shots (the kind where wind speed needs to be taken into account) can be redirected by enough to miss you if it has to fly through a plasma burst somewhere far enough away from you that the slight change in angle would compound into a major offset.

In this way you can use lasers that may not necessarily make for a good primary weapon, but still able to protect you in battle.

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As automaton predicted:

Other answers may bring up power-density or computation issues (...)

Let's assume a bullet made of lead and weighting 10 grams. You need to heat it up to 1749 degrees Celsius to vaporize it. Let's assume a starting temperature of 25 C for the bullet. You need to give it 1.3 joules per degree Celsius, so about 2.2 kJ. But you also need 22 J/g for fusion and 860 J/g for vaporisation.

All in all, 11 kJ to handle a bullet. Let's assume the laser is 85% efficient in terms of energy, a performance similar to a typical laser pointer. You'll be spending 13 kJ on a bullet, of which 2 kJ will be dissipated as heat in your equipment. That's enough energy to heat a gram of water by 478 C, so be careful that the wiring does not touch your skin.

Supposing you need the bullet vaporized in 0.1 seconds or it hits you, you're cycling 130 kilowatts in the equipment's bus. And you know, you can convert watts to horsepower. 130 kw is nearly the same as 174 hp.

I have a mid-size crossover SUV. Its curb weight is ~1900 kg and it reaches 100 km/h in a few seconds with a 173 hp engine. I think my car's engine alone weights around 175 kg.

You need a reactor just slightly more powerful than my car's engine to power your laser. It will probably be just as massive as well. And then you will also need to add fuel or batteries, piping and wiring, casing etc.

More realistically, your point defense would be a set of really heavy turrets. This could protect individuals in a closed space they have to defend, but I doubt it would be very mobile.

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