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Imagine a constellation of satellites equipped with sensors capable of identifying any object posing a risk to humans and targeting it with lasers to neutralize it. The constellation covers the entire globe 24 hours a day.

For example a bullet is fired in the direction of a human, the satellite sees the object, based on its speed and visual characteristics it determines its a bullet and then calculates based on its location, direction it poses a risk to a human. It then shoots it out of the sky with laser. It could do the same for larger projetiles such as shells, grenades, rockets or missles.

Would such a system render modern warfare which is based on projectile weapons obsolete? What would replace it?

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – Monty Wild
    Dec 3, 2023 at 13:30
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    $\begingroup$ A satellite at 160km orbit directly above the bullet, that has instantaneous sensors and targetting (grossly implausible) of a bullet traveling at 3000kph can only destroy the bullet after it has traveled 0.4m because of the limitation of the speed of light (in a vacuum). Start adding in the physical complexities around the real world problem of gun related deaths and that distance will soon be too far away to be simply be handwaved away to a casual but physics aware reader. $\endgroup$ Dec 3, 2023 at 19:53
  • $\begingroup$ Aside from the political consequences of having a lethal satellite network watching everybody at all times, the social consequences of having sensor technology capable of doing that, and the small matter of presumably destroying the universe by breaking speed-of-light limits— Presumably, if such laser technology existed, guns would just use miniaturized versions of that instead of projectiles. $\endgroup$
    – Will Chen
    Dec 4, 2023 at 4:02
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    $\begingroup$ People will start shooting each others indoor ? $\endgroup$
    – armand
    Dec 5, 2023 at 4:29

7 Answers 7

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Such a system, assuming that for a moment it could actually work, would make war terribly easy!

Having a laser capable of vaporizing a bullet from a distance of at least 80 km in the few milliseconds it takes for the bullet to reach its target would require an humongous amount of power$^*$, such that any stray light resulting from the laser itself would as a minimum blind whoever happens to be around the trajectory of the bullet, not mentioning what would happen with the bright flash of light caused by the sudden expansion of the cloud of lead plasma produced as a consequence or with the unavoidable diffraction induced spread of the beam on its path, or what would happen to the unlucky target who would inhale abundant lead vapors produced by the condensing plasma.

A soldier would be trained in just firing in the general direction of its target, and let the satellites taking care of harming them.

$^*$ for a reference, the energy needed to convert to liquid a .44 magnum bullet from room temperature is 1.35 kJ, which assuming the time available is 1 ms, converts to a whopping 1.35 MW. Then vaporizing the lead takes additional 19 kJ, that is 19 MW. Without counting the energy needed to bring the molten lead to the boiling point, we are already talking of 20 MW on the bullet. You don't want to be anywhere close to the bullet when it pops!

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    $\begingroup$ And just imagine the havoc caused by firing a bullet at someone on the ground floor of a crowded skyscraper! $\endgroup$
    – Jedediah
    Dec 3, 2023 at 3:42
  • $\begingroup$ The soldier firing would also be harmed, however. You need more caution. $\endgroup$
    – Therac
    Dec 3, 2023 at 10:17
  • $\begingroup$ Depends on how soon the bullet gets blasted. If trully any projectile with line of sight gets destroyed, no matter the range, it has to be very quick, making the attacker almost certain to get scorched, though the target propably won't be safe either. $\endgroup$
    – Kugelblitz
    Dec 3, 2023 at 12:47
  • $\begingroup$ You don't need to reduce everything to gas in order to stop it; that's a common mistake when talking about defensive lasers. You need to ablate enough of it in a short enough period of time that you can either slow it to harmless velocities or safely deflect it. $\endgroup$ Dec 4, 2023 at 16:17
  • $\begingroup$ @StarfishPrime how can I calculate how much energy would be required to ablate it? $\endgroup$ Jan 2 at 7:59
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The first effect? There'd be a lot more anti-satellite weapons developed. The second effect would be a lot more light based weapons in general use.

Weapons and armour are an age long ever evolving cycle. From the pointy stick and basic clothing to nuclear weapons and their matching bunkers. Sometimes one is dominant, sometimes the other.

You're introducing active defences and defining them as fully and totally effective. The next step is always going to be a weapon that neutralises them, just as once upon a time having a thick hide was sufficient against the pointy stick of the primitive man, but then he tied a sharpened flint to it, and having a thick skin was no longer enough. The tank followed the machine gun and soon enough you got anti tank missiles and the anti-materiel rifle. It's a perpetual cycle.

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    $\begingroup$ If there are satellites capable of literally shooting bullets out of the air, and doing this simultaneously for every bullet shot anywhere on Earth, there would be no anti-satellite weapons. It would be futile to try. Whatever entity put the satellites there has total control of the surface of Earth. $\endgroup$
    – causative
    Dec 3, 2023 at 0:46
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    $\begingroup$ @causative you are just being unimaginative. The controllers aren't omniscient, and tracking takes time/calculations. Somebody could develop something insanely fast in secret to blow them up, or even a laser based counter weapon $\endgroup$
    – Marie
    Dec 3, 2023 at 17:59
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    $\begingroup$ @Marie To shoot bullets out of the air everywhere on Earth, the controllers must be taking snapshots of the entire surface of the Earth to about 5mm spatial resolution and 1 millisecond temporal resolution. (Because, a rifle bullet can travel 1 meter in 1 millisecond, so they have to be as fast as that, and even then there's speed of light delays.) The system that could do this would have billions to trillions of satellites. Even if you could develop a ground based laser in secret, five milliseconds after you turn it on and maybe shoot down one of their billion sats, they vaporize yours. $\endgroup$
    – causative
    Dec 3, 2023 at 18:07
  • $\begingroup$ The question mentions any required technology exists. I could just create a nanite based rocket and warp space time to teleport to orbit, then have them consume the satellites from within and self-replicate. Easy peasy $\endgroup$
    – Marie
    Dec 3, 2023 at 18:13
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No, this would not render modern projectile weapons obsolete. It would make them more difficult to use, and it would change the tactical environment.

Consider that Adam wants to kill Bob. Adam has a gun:

  1. Adam ambushes Bob in the hallway of an apartment building and shoots at Bob. Because this is taking place under cover without line-of-sight to the sky, there is no way that the lasers from the gods can see this and intervene. Bob gets shot...

  2. Adam ambushes Bob in the glassed-in lobby of an apartment building and shoots at Bob, from the doorway of an elevator as Bob is about to walk out of the building. Can the Sky Laser Satellites (henceforth SLS) see this through the glass? Will they shoot through the glass? Let's suppose that they do. The SLS shoots through the glass. It's regular green architectural glass, not optical glass. The laser which is capable of vaporising a bullet through a heavy pane of glass is fired through the glass and hits the bullet. The bullet vaporises, and the glass - which absorbs some of the laser's energy - shatters from the shock of the glass vaporising, filling the air with glass shrapnel. Now what happens?

  • Let's say that the laser has shot down the bullet, but does nothing more. Bob is in the path of flying glass from a violently exploding window, while Adam ducks back into the cover of the elevator. Bob may be seriously injured or killed, even if the bullet might not have caused such a severe injury.

  • Alternatively, lets say that the SLS shoots down the glass fragments too. The glass fragments are rapidly turned into smaller and smaller fragments of glass and then glass vapour. Bob is killed by the massive explosion of the building's glass, and possibly Adam too.

  • If the lasers don't shoot through the glass, Bob gets shot...

  1. Adam ambushes Bob outdoors, in clear view of the SLSs. Adam presses the gun against Bob's body and pulls the trigger. The SLS never see the bullet. Bob gets shot...

  2. Adam and Bob are outdoors in a high-rise city. Adam shoots at Bob from some distance away... but do the SLSs have line-of-sight to the bullet in the artificial canyons? They can't cover the whole sky, can they? If not, Bob might get shot...

  3. Adam shoots at Bob from a carefully-chosen position out of doors, in sight of the SLSs, a metre or so away from Bob. The best-placed SLS shoots the bullet... and also Bob, given the massive over-kill necessary to destroy the bullet. Bob has now been shot by a freakingly powerful laser, and not just a .22...

Now let us suppose that diplomatic relations between two nations, Ayastan and Beeland have broken down and they have gone to war. Both of these countries have modern state of the art kinetic weapon systems. They are fighting outdoors for the most part.

The firing starts. The SLSs begin to shoot down the bullets they can see... but as more and more soldiers and vehicles begin to shoot at each-other, there are more and more projectiles in the air. The SLSs cannot be infinite in number, otherwise Earth would be surrounded by an opaque shell that would block out the sun and condemn Earth to becoming a frozen ice-ball world. So, the limited number of SLSs must track and destroy each projectile, or track and prioritise each projectile according to its potential for harm. As the quantities of projectiles increase, it becomes harder and harder for the SLSs to keep up with the target load. As the SLSs reach target saturation, projectiles start getting through to hit their targets.

  • What happens if the forces of Ayastan and Beeland introduce technology to synchronise the firing of all of their projectiles only on the tick of each half second? Suddenly, there aren't a bunch of asynchronously-fired projectiles, but a bunch of synchronously fired projectiles. The workload of the SLSs suddenly goes way up, making it easier for the armed forces of Ayastan and Beeland to saturate the SLSs' capabilities, and more projectiles get through.

  • What if the SLSs still shoot down all the projectiles, but just take a bit longer? Well, some projectiles are rather large, and being destroyed - i.e. being turned into plasma by lasers and exploding - is dangerous on its own, potentially more dangerous than the projectile was in the first place. What if the projectiles contain toxic substances such as depleted uranium or even just lead? People are going to get poisoned...

Now, let's consider rockets. Are these lasers going to prevent harmless satellite launches, or only potentially harmful weapon launches? What about rocketry hobbyists' launches? Kids with bottle rockets with an improvised bomb duct-taped on top? How are the SLSs going to tell the difference, or are they just going to shoot down everything?

Then what's going to happen when Ayastan and Beeland get frustrated at the huge amounts of ammunition that they must expend to kill even one of the other side's soldiers with their guns, and build lasers to shoot down the SLSs? If the SLSs allow satellite launches, what's to stop geosynchronous SLS-killer lasers from being emplaced, that shoot down the SLSs while they're busy elsewhere?

All that also assumes that the SLSs have the ability to shoot through atmosphere (which is non-trivial at the energy levels required to vaporise a bullet or missile), and have the sensor capability and processing power to detect and prioritise the destruction of anything threatening, which are both also non-trivial problems.

TL:DR: It's not going to work as well as you might hope.

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  • $\begingroup$ I love questions with "absolutes". (No, actually, they depress me.) $\endgroup$
    – RonJohn
    Dec 3, 2023 at 9:23
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  • The sensors for this system would change everything.
    The ability to detect and recognize a bullet in flight would mean detecting and classifying bullet-sized objects in the environment, worldwide, 24/7. Outdoors, privacy is gone. The system can peep into outdoor swimming pools, it can track serial killers burying their victims, and all things in between. Disputed speeding ticket? Replay the 'bullet-finder' camera and count how many moles the driver had. Then fast-forward to the present day to ascertain that the driver is the defendant.
  • The police would be disarmed.
    There are scenarios where the police will shoot people who do not have a firearm themselves. For instance, when there is a threatening person with a knife, the the officer might decide not to get into knife range. Can the system tell legitimate from illegitimate use of lethal force?
  • Not every threat is tiny.
    In addition to stopping bullets, you expect it to stop grenade launchers, tank-guns, howitzers, etc. What happens if rescue services use a line thrower? The system would either have to calculate that the projectile poses no threat to people, and let it fly, or neutralize it. Would it know better than the people on site if that risk is acceptable in the situation?
    And not all threats fly. Cars can be lethal, too. So would the system decide that a car is going to hit a pedestrian, and 'neutralize' it? Vaporizing the car would be an enormous energy release in a populated area, merely damaging it removes any ability of the driver to drive. (Every day I face dozens, if not hundreds of situations where a car would kill me if both I and the motorist did continue at a constant speed and direction. Normal motorists don't, and I also respect red traffic lights ...)
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  • $\begingroup$ The last point hints at something between the Trolley Problem and a Three Laws issue: A super-laser can't simply stop a car in its tracks - even vaporising the engine wouldn't help as the car has momentum. Maybe it could vaporise just enough for the explosion to stop it - but the rest of the explosion has to go somewhere, harming those who were about to be hit by the car. Is the robo-laser's action or inaction going to lead to a worse outcome? $\endgroup$
    – Chris H
    Dec 4, 2023 at 11:27
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As described so far, the laser would destroy any lethal projectile to which it has line of sight. This wouldn't make projectine weapons obsolete, indoors or with any cover from the laser they will work just fine, but it would make some alternatives more popular.
I'd say everyone will just use lasers, given how both effective and numerous the space ones are, but if that is impossible for whathever reason I think that tactics to conceal the fight would become commonplace. Attacks could be planned for the worst weather possible instead othe best, and on clear days entire battlefields could be covered in smokescreen. It might also disperse any laser fired there, but if the space lasers can pierce the atmosphere from top down and still hit their target with pin-point focus I wouldn't be so sure about it, simply hiding the fight is propably the only way.

But as was already said, the vaporization of the projectiles won't delete them, it just gives their mass enough energy to disperse itself in a violent burst of plasma. This might actually be an upside, for example the system wouldn't have to be perfectly reliable since even 10% shoot-down chance now means a 10% chance of the shooters skin beying burned off their flesh, which might not be worth the 90% hit rate, but if what you had in mind were bullets disapearing in thin air this is a big issue and you might have to come up with a different way to make shooting hard.

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Couldn't be done

The biggest threat would be the satellite system itself. Would birds swooping get targeted and fried? Baseballs? Spitwads? Snowballs? Random pieces of gravel thrown by a passing truck?

There is far too much to try and stop. Laser blasts would be raining down like a thunderstorm which would be a greater threat to life.

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Part of the difficulty of laser anti-ballistic missile systems (part of Reagan's Strategic Defense Initiative) was the thermal bloom from high powered lasers. The idea was that since ballistic missiles tend to have very thin skin (for weight reasons), that puncturing them and igniting the propellant would destroy the missile's ability to reach their target.

High powered lasers introduce so much energy that they turn the atmosphere into plasma. Plasma is generally opaque enough that it stops the laser. So the laser has to burn through the plasma that it just created. So now the laser has to be more powerful just to burn the plasma away, which makes more plasma, resulting in a viscous cycle.

The October 1984 issue of Scientific American listed a number of inexpensive methods that could defeat laser based anti-ballistic missile technologies. One example would be spinning the missile so that the surface has time to cool off before the laser can burn through the hull. Another would be venting a liquid out the nose-cone to cool off the hull during the boost phase.

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