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In the future, defensive technology has advanced so much that weapons strong enough to kill an enemy soldier tend to cause unacceptable collateral damage. Sure, we can make a weapon that generates the force required to punch through a personal forcefield, but the blast tends to also destroy everything else within hundreds of meters making war a very messy and environmentally unfriendly business.

Then one day, a well intentioned inventor figures out how to cause oscillations in an object's effective mass causing gravitational waves to radiate from the object. These waves have the same average pull as the original object, but the oscillations cause an alternating plus and minus gravitational effect. Any effects that gravitational waves have on space-time are assumed to still apply.

The inventor quickly finds ways to manipulate these gravitational waves in pretty much all of the same ways that we can currently manipulate radio waves such as adjusting frequency, amplitude (measurable as plus or minus m/s²), directional transmission, etc.

The government funding his research quickly realizes that gravity is not affected by any known defensive technologies; so, they decide to take over the inventor's research and try to find a way to use gravitational waves to kill a person through their armor, shields, etc. without causing lots of collateral damage.

Given the nature of the human body, what is the ideal amplitude, frequency, and methodology required to kill a person with gravitational waves while minimizing collateral damage.

As per comments: While this would require a ridiculous, down-right stupid amount of power as per our current understanding of physics, assume his method relies on a handwavium mechanic that can be achieved with a power source that would not itself cause collateral damage.

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  • $\begingroup$ Not going to work without either utilizing energies many orders of magnitude higher than would be needed to vaporize your target, or finding a way to collimate gravitational waves like light is collimated in a laser. $\endgroup$ – Alexander Jun 6 at 19:26
  • $\begingroup$ @Alexander The energy requirement did occur to me, but for purposes of the question, assume civilization has enough power to do what they need to do (or they can do this in a way that cheats the matter energy equations). The weapon does not need to expend less power than the doom weapons they already have, but it does need to solve the collateral damage issues that are making warfare too atrocious to justify. $\endgroup$ – Nosajimiki Jun 6 at 19:46
  • $\begingroup$ I don't know if there's any way to create gravitational waves in a laser-like "beam" as opposed to spreading spherically...as an alternative would you consider creating a microscopic black hole, which also means using huge amounts of energy to create a localized region of strongly curved spacetime? If it's not charged then no non-gravitational forces should affect its path, and at the right choice of size it could probably create localized spaghettification for anyone directly in its path but non-fatal tidal forces for those off its path. $\endgroup$ – Hypnosifl Jun 7 at 19:29
  • $\begingroup$ I'm open to frame challenges as long as they could deliver the defence breaking ability of a nuke without making a big crater or causing major environmental damage. Micro-black holes may be a good idea along those lines as long as you can answer what properties they'd need to be able to not slowly suck up the whole planet into one big black hole over time. $\endgroup$ – Nosajimiki Jun 7 at 20:23
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Balance

A small, constant wave could be used to knock someone off balance. If they're crossing a bridge or atop a ladder or the roof of their house at the time, that could kill them.

Cardiac Arrhythmia

Pass a wave through a human's heart and see what happens! Cardiac arrhythmia is when the heart starts beating in the wrong way, and that leads to death, too.

Unconsciousness, concussion, or coma

Pass that wave through the brain and watch it shut down! give it a good yank to the left and see if they can ever draw a straight line again!

Why are these suggestions useful?

Weaponizing isn't always about killing. A friend of mine in Finland, where they have mandatory military service, once explained that killing people wasn't at all what he was trained to do. If you kill someone, you've removed but a single person from the battlefield. But if you wound someone, it takes 1-3 more people to get that person off the field. Now you've removed up to 4 people from the battlefield.

And that assumes that you're on a battlefield. Just because the gun shoots bean bags, rubber bullets, blasts of water or air, doesn't mean it's any less of a weapon. Gravity waves would be great for crowd control. Yup! Make them all unbalanced (think about the "sick stick" from the movie Minority Report. That's what you'd have, people hurling just like they would on roller coasters.)

You also have the stunning applications, like tasers. Small gravity waves knocking people unconscious or senseless have great applications in the field of kidnapping!

And you wouldn't need the gravity of a planet to pull some (or, I suspect, most) of this stuff off. What you need is precision. You need to hit just the heart, or just the right part of the brain or inner ear.

Would it be practical? Heck if I know. That's Nosajimiki's problem. But there are ways to weaponize the use of gravity waves, IMO.

As for ideal amplitude, frequency, etc. I'm voting for the 10-30 Hertz range as that's where audio tends to move people in a more physical, less emotional way. I haven't the foggiest what the amplitude would be, but if the amplitude is measured in "standard gravity" (i.e., if Earth's g=9.807 m/s2 then let's call that "unity" and name the unit of measure the "Nosajimiki.") So, you'd likely start really screwing up the human body with 1.5-5 Nosajimikis.

The one problem you do have is that gravity goes through things in a way light and slugs don't. The collateral damage might be ugly. This could, perhaps, be mitigated by using a stereoscopic transmitter that had to be focused such that the maximum amplitude hit just where you wanted it, but every where else the two beams were smaller. Just a thought.

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If you wish to cause a human to be thrown in any direction with this technology, you will need to summon up the mass of an Earth. That is not sanitary, as the collateral damage will be the planet as a whole.

Rather, you could cause someone to vibrate very fast by changing their mass. Increase and decrease in short intervals and see they become jelly.

However, that still takes a lot of energy. To increase a person's mass by even 1 gram takes close to 9 x 1015 joules. If you just pointed a laser in their direction, that laser could probably pierce through meters of concrete. For comparison, the energy input the Earth receives from the Sun on its day side is about 1.73 x 1014 watts. You read it right - you would need to collect all the sunlight that hits the Earth for almost a a minute and a half in order to have enough energy to make someone marginally lighter or heavier.

The amount of heat you're going to generate from the power source alone can probably destroy a few ecossystems, so you should probably collect power and fire your weapon from space.

Much cheaper and cleaner would be to just vaporize people, which takes in the vicinity of 3 x 109 joules per adult.

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Massive amounts of unobtanium, handwavium, and pure old magic.

Gravity is a force with a very small local effect, but very large effective range. That is, if you grab your typical magnet and lift a paperclip with it, you are using the force of magnetism to counteract the force of gravity of the entire world.

That is, a 25 gram piece of magnetized metal is lifting a 1 gram paperclip, despite 6 septillion kilograms of mass (gravity) working to pull it the other way.

If you have a weapon that can harm a human using gravity waves, it also does the same amount of damage to the entire planet.

There is no known mechanism is science or fiction that would contain a gravity wave, so your only option is to just make something up. Reverse the triangularity of the flux proton sequencer.

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Think outside the metric. Directly hitting the opposition with gravity waves might be less useful than other easy possibilities. What you want to do is distort things in a way that causes problems.

Brute force example: The novel Protector by Larry Niven had the idea of using gravity lenses for an interesting telescope. His telescope had an enormous input aperture so that he could observe very faint objects. Imagine then, a gravitational lens system about 100 km across that focuses the light from the sun down into a nice narrow beam only 1 km across. It's then 10,000 times as intense as sunlight. Instead of 1 kW per square meter, it's 10 MW per sq meter. Probably that is a city buster.

Subtle-then-brute-force example: Fairly small pushes could re-direct a comet. So a comet that is out in the Oort cloud, nosing along in an orbit that nobody will notice, gets nudged into an orbit that will intersect the target. Maybe nobody notices you doing the pushing, especially if you are careful to keep the unusual stuff pointed away from the opposition. After orbital mechanics takes it's toll, a 20 km comet hits your target. Yes, it has a few year delay for the time required for the comet to get from the Oort cloud to the planet you are attacking. How patient are you?

Subtle example: If the opposition is doing space travel, little pushes can make their space vehicles drift off course. Presumably they have a finite ability to correct. Combined with the gravity telescope of the first part, maybe incoming potential attackers can all get flung to oblivion before they can fight back.

Subtle example: Bending light might be used to produce mirages, hidden objects, etc. When the opposition looks at your base maybe they see a duplicate of some other part of the sky. Maybe when that comet is heading toward them they see also a duplicate of some other bit of empty space. So they don't get warning and so can't do anything about it.

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The key way to cause damage with gravity will be tidal forces. These are strongest in regions where the gravity gradient is steepest. This doesn't necessarily mean where gravity is strongest; larger black holes actually cause weaker tides than smaller black holes.

So consider your gravitational waves like sound waves. It is possible to mix multiple frequencies of sound waves so they cancel out everywhere except for one small point where they add up. So they're silent in most places, but in that one spot they're extremely loud. If your equipment is very precise, this point can be arbitrarily small, meaning the sound starts and stops very abruptly. I suggest you do something similar with gravitational waves. Generate multiple frequencies synced up just right so they cancel out in most places, but add up abruptly inside your opponent's ship. One half will be pulled in one direction, the other half in the opposite direction.

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