Can spacecraft defend against antimatter weapons? How?

Question

Here is what we know:

• We have the means to create stable antimatter
• We can direct it in a beam
• It can travel nearly as fast as light with ease
• Antimatter and matter combine in dangerous ways: one microgram of antimatter colliding with one microgram of matter can create $1.8\cdot 10^8$ Joules, equivalent to over $40$ kilograms of TNT exploding

Theoretically, future weapons could produce antimatter, shoot it as a beam at speeds too fast to react to with missiles, projectiles etc, and annihilate basically anything made of normal matter.

In a future where antimatter weapons are common and easy to manufacture, how can spacecraft defend against antimatter beams effectively?

Even if this is optomistic and speculative, humor me. Pretend these weapons are relatively cheap (and possible) and that the technology exists. What's a correspondingly feasible basis for the tech to defend against them?

Answer 1

Don't be there when the antimatter beam arrives.

There is no shortage of ways to destroy a spaceship: high-powered lasers, antimatter particle beams, near-lightspeed projectiles, and so on. There's no reliable way to shield a ship against energies of this level. Instead, the basic defense is maneuver.

Nothing travels faster than light. The Moon is a light-second away from Earth: this means that if you're trying to shoot something on the Moon with a laser, you don't aim at its current location, you aim where it will be two seconds from now (one second to deal with light reaching Earth, and one second for your shot to reach the Moon). Extending this to space combat, your basic defense is to maneuver randomly, and to stay far enough away from the enemy that you can move the full width of your ship in less than the round-trip time between the two ships. The enemy is reduced to firing randomly and hoping they get lucky.

There's historic precedent for this in wet-navy combat: destroyers are fragile, especially in comparison to battleship armament. But destroyers are fast enough, and battleship shells slow enough, that a destroyer at extreme range can move several times its own length in the flight time of a salvo, letting it get out of the target zone.

Answer 2

The mass-energy in a gram of anti-matter is $0.001 c^2 = 9 × 10^{13}$ joules or about 20 kilotonnes of TNT, that is substantial! (and you get another $9 × 10^{13}$ from the annihilation of the equivalent matter

On the other hand, if you accelerate 1g of anything (matter or antimatter) to 0.99 c, it has a kinetic energy of $0.001c^2/\sqrt(1-0.99^2) - 0.001 c^2= 5.5 × 10^{14}$ joules, over 3 times more!

Once you have some antimatter, then accelerating it to 0.99c takes exactly the same amount of energy as accelerating the same mass of matter. However antimatter has to be made first, and the process of making antimatter is inefficient: it takes much more energy to make 1g of of antimatter moving at 0.99c than it takes to accelerate 1g of matter to the same speed.

The conclusion is that if you can accelerate something to "nearly the speed of light", it doesn't matter whether it is matter or antimatter since the kinetic energy is substantially more than mass-energy. If you get hit by something like this your ship won't survive. The only defence is to not be hit. Tactically this means keeping hidden, and ultimately it means either diplomacy, or shooting first.

There would be no benefit in using an antimatter beam, since at those speeds, just using matter would pack almost the same punch. Antimatter could be used as a miniaturized nuke. A gram of antimatter, fired by railgun, would still release 20 kilotonnes on impact, but be a lot smaller than an equivalent thermonuclear weapon.

All this is, of course, way beyond current science. Our current best antimatter manufacturing site (CERN) is the size of a city and can make 0.000000001g per year.

Answer 3

Assume, for starters, that the antimatter weapons produce and fire the simplest kind of antimatter beam. Namely, a beam of positrons. Once your vessel detects an incoming beam of positrons, it aims its own antimatter beam weapons in the direction of the attacking vessel.

Both positron beams will be positively charged. This will cause them both deflect and diverge each of the positron beams. Essentially backscattering the attack positron beam and allowing a large volume of positive space charge to build up between the two vessels. Effectively using a positron beam to attack a positron beam blocks it from reaching your vessel. This defence tactic will work best when your positron beam is travelling along the same path of the attacking positron beam.

A proton beam beam also be used in the same manner to block positron beam weapon attacks. This will have an added advantage that protons have more mass than positrons, so the proton beam will 'penetrate' further along the positron beam path.

The attacked spacecraft can use electrostatic technology to induce a large positive charge on its hull to further deflect the positrons in enemy antimatter beam.

If the spacecraft under attack has escort drones or auxiliary vessels in formation with it, these craft can fire electron beams to pass near or adjacent to an attacking positron beam. Since electrons and positrons have opposite charges they will attract each other and cause the beams to diverge from their intended paths. If this is maximally effective the attacking positron beam will miss its target.

This effect can be increased if the support craft fire a positron beam that passes on the opposite side of the attacking antimatter beam to that where the electron beam is passing it. The combination of external positive and negative charges will increase the deflection of the enemy's antimatter beam.

Another defence against a positron beam would be firing an x-ray laser beam weapon along the beam-path of the antimatter. The x-ray photons in the defensive laser beam will deflect the positrons with Compton scattering.

A spacecraft suitably armed with its own positron beam weapons and x-ray laser weapons, supported by escort drones equipped with positron and electron beam weapons, will be able to defend itself against attacks using antimatter beam weapons.

Answer 4

Since we don't know the density of the beam, we don't know if there's any hope of protecting the ship from a hit. I will assume that we're not talking lightsabers here. That is, a ship could take several shots assuming it maneuvers to prevent the damage from piling up in one area.

Cover your ship in an ablative armor made to soak up the anti-matter particles and burn off. As the Ablat annihilates, the surrounding areas not directly struck but caught in the heat, burn and produce smoke/soot particles. These then also absorb anti-matter particles. The Ablat could be manufactured so the burning parts spew soot/smoke outwards, away from the ship. Now the secondary annihilations happen further from the ship. The inverse cube law is your friend.

The entire ship does not need to be covered in the thickest Ablat. WWII-era battleships were designed with a 'citadel', a part of the ship deemed critical and up-armored significantly.

An ablative armor has advantages:

• It is passive armor and works on its own
• Burning Ablat carries heat away from the ship
• It's cheap compared to the cost of the weapon it is intended to defeat
• It's easy to replace

EDIT - There were some good questions in the comments.

How does Ablat handle the radiation generated by mass annihilation?

I hadn't thought of that, but it could be made from a material good for soaking up specific types of radiation. Or not - being in space, without warfare, is extremely hazardous due to radiation. Solving that problem might help with this problem.

How does it burn in space

I was using "burning" loosely, but I'd expect the Ablat to include some oxygen in its structure. Basically, we need a chemical reaction that causes particles to be spewn forth. Given there's going to be a flash of heat, some manner of "burning" seems reasonable.

The hit of antimatter results in a horrific explosion. Thermonuclear warfare is nowhere near in energy/volume ratio.

You don't know how damaging the beam is since the OP has not quantified the strength of the beam. So while what you say is true to my layman's ears, it does not follow that the OP's weapon is necessarily this powerful.

If an antimatter ray hits a vessel, there's no way any ablation is going to save its surface. Emitted stream of fast particles just doesn't work like that.

I disagree. If the beam hits, you have a choice - it hits the hull of the ship, or it hits the Ablat. It seems to be Ablat is preferred. There will surely be a hole in the Ablat due to annihilation, maybe down to the hull, who knows. The 2nd beam that hits a little later has to pass through the cloud of "smoke" and this will surely be helpful.

I agree that if the beam is dense enough to cause an energy release equal to the power of megaton (or even kiloton) nuclear weapons, there is no chance of the ship surviving unless magic is used.

This Ablative armor is not instead of other defenses such as stealth, agility, or spewing clouds of ablative dust. It is in addition to these other defenses.

EDIT 2 -

Let's talk about "beams" for a second. It is very difficult to make a beam that's not actually a conic. That is, instead of being a cylinder, the "beam" will be wider that the far end. Machinery is not perfect. Also consider the ranges we're talking about fighting. The anti-matter weapon might start out producing a beam, say, 5mm in diameter. 300,000 kilometers later, however, it might be 50m in diameter. The beam is no longer a saw, it's a shotgun. Ablative armor would be very effective against such a weapon. At close range, everyone is going to had a bad day, I think.

EDIT 3 -

The Hiroshima bomb involved about 700 milligrams of matter being converted to energy. That's about a 15KT equivalent. To get to 1.5 MT, we need 70 grams of matter converted to energy. Safety tip - if that gets loose, the firing ship is erased from the inside. And it may take a long time to generate that much anti-matter increasing the chance of a mishap. Current technology allows us to create hundreds of particles on-the-fly, and it takes the LHC to do it. Between the safety issues and practicality issues, it could be that anti-matter beams are not primary weapons but instead are dandy for point defense. Perhaps they are equivalent to the Vulcan cannons on modern ships, or the 8" guns on WWII battleships. (There is reasoned conjecture that an 8" shell from BISMARCK destroyed HMS HOOD. 8" guns' projectiles had a plunging trajectory. A different weapon for a different purpose...)

Answer 5

Your question cannot be answered because it depends on incorrect assumptions.

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First, your claims:

We have the means to create stable antimatter

At already obscene cost, we can create on the order of a few hundred anti-atoms that sometimes last for several minutes. You need more like a billion trillion (10^21) atoms, and indefinite storage.

We can direct it in a beam

This is minor residual kinetic energy from the generating beams, which themselves come from an enormous particle accelerator. To maintain these parent and daughter beams, extremely powerful superconducting magnetic fields are required along with extremely high vacuum--neither of which are reasonable assumptions for the free space between two ships. The beam also spreads anyway.

It can travel nearly as fast as light with ease

No, these are the generating beams, which are pushed to this speed by literally multi-hundred-megawatt powerplants. This, despite containing very little mass. (Initial estimates put power consumption of Cern at 200 megawatts, just to save up enough energy to send tiny packets of protons through the tubes. This isn't easy, and scaling this to a weapon, operating continuously, is ridiculous.)

Antimatter and matter combine in dangerous ways

This is correct, although the energy is typically released as gamma radiation, which is more dangerous to living tissue than e.g. spaceship hulls. (Different reactions are possible. For a proton/anti-proton annihilation, such as you'd get by throwing antihydrogen ions at hull plating, you get a couple loose quarks and some utterly-harmless muon neutrinos.)

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So, the problem here is that to produce an "antimatter beam", inasmuch as that concept even makes sense, you basically need several hundred-megawatt power stations, with all the heat radiators that requires, powering a particle accelerator which is maybe 25,000-30,000 meters in diameter. Then to make it actually dangerous, you need about 100 of these assemblies, and the target needs to be at point-blank range and essentially unarmored.

The generating proton beams themselves are much more dangerous, since they have relativistic mass, and you can actually direct them places.

Obviously, if you can afford a power-source that can convert energy into weapons-grade quantities of antimatter in real-time, you might as well instead dump that energy into a whole fleet of death star lasers.

If you really must have antimatter, confine it in bombs. Strategy and tactics for using/defending-against these are identical to nuclear weapons.

Answer 6

Today in science we can generate anti-matter, and contain it in electro magentic fields.

http://singularityhub.com/2011/06/11/scientists-trap-antimatter-for-more-than-16-minutes-video/

Quotation from above

Matter and antimatter make for bad company. When they come in contact with one another they annihilate as, in a flash, their masses are converted to energy. The challenge for the CERN scientists then was to find a way to trap the anti-matter without allowing it to come in contact with matter. Instead of regular matter, they used magnetic fields to contain the antihydrogen. But a magnetic field only works if the particle is charged. At extremely low temperatures–near absolute zero–antiparticles becomes charged and the magnetic field becomes an effective barrier.

The scientists collaborating on the so-called ALPHA (Antihydrogen Laser Physics Apparatus) experiment, were able to trap the antihydrogen atoms for as long as 1,000 seconds–or just over 16 minutes–“which is forever,” says CERN

This is from 2011.

Now advance into the future with space ships. The energy and tech to make these fields are probably super easy. Most of space is near absolute 0 anyway except for the radiation left from the big bang.

The ability to create 2 fields to form a barrier and drop the temp to near absolute 0 is probably easy. In the future the whole absolute 0 thing is probably made obsolete by advances in tech.

The reason being that you can not build a container out of pure electrons, or pure protons, etc… Therefore, magnetic and electric fields must be used to trap the antimatter.

additonal reading material. http://www.forbes.com/sites/brucedorminey/2016/02/24/antimatter-space-propulsion-possible-within-a-decade-say-physicists/#11f05dfb65f0

Answer 7

This weapon would have interesting ramifications. Open spaces become dangerous. You'd hide in dust and gasses. You'd get matter between you and any potential threat pretty quickly.

If you could get matter within close proximity to the attacker, the antimatter weapon turns back on itself.

Dust, gas, charged particles.

Maybe send out a bunch of drones, small and difficult to detect. These drones are able to emit strong magnetic fields (energy is cheap and portable because antimatter) They then feed small and amounts of charged particles into the fields. The charged particles travel at very high speed and can be redirected quickly by shifting the field. Alone these fields are completely harmless, but in the presence of antimatter they become something else entirely. Your enemy charges up his anti-matter beam, your drone flicks on and starts spinning matter around right near his ship. The antimatter beam licks out, immediately interacts with tiny amounts of matter and destroys itself and the ship and anything nearby.

Of course if you can get a drone close without being detected you might as well deliver a tiny antimatter bomb with it instead. But as a non-offensive deterrent it might work.

Answer 8

Erm . . . just chuck normal stuff at it?

In a scene from Star Wars Episode II: Attack of the Clones, Obi-wan Kenobi is followed by Jango and Boba Fett. They fly through an unrealistically dense asteroid field, shooting lasers that go "pew-pew" in the vacuum of space and essentially defy laws of physics.

Jango Fett sends several missiles at Obi-wan's ship. Master Kenobi's first thought? Chuck the spare parts at them:

(Two seconds later. . .)

The same principle can be used here. Matter and antimatter will annihilate one another. All you need is enough junk that can be destroyed as a shield while you quickly mosey on out of the way.

Now, Obi-wan's advantage is that he has a small ship (one Jedi and an astromech droid), a lot of things to block the shrapnel (asteroids), and only a couple of missiles to avoid. In a full-scale battle, with larger ships, this becomes a nearly impossible defense - assuming the beam could be large enough, which would seem to take enough energy such that it could only be fired very few times.

Then, you just have to shoot the other guy first.

It turns out that certain types of flares are used for the same purpose by aircraft. I wonder if spacecraft could manage to do something similar here, for antimatter beams. The difficulty, of, course, is that flares are designed to confuse the heat-seeking missiles, but you could still likely confuse the beam gunners by adding more lights and explosions.

Answer 9

Antimatter is not a power source, it's an energy transport mechanism. To generate a beam of antimatter with a given energy content requires generating substantially more power in the ship's powerplant (due to thermodynamic losses), or tapping a Penning trap which can be likened to a battery.

How does a ship avoid being destroyed by an antimatter beam? Exactly the same way it avoids being destroyed by a particle beam, railgun, laser, or any other weapon of comparable energy, all relying on converting electrical power to a form that can be delivered to the target.

My point is, whatever defenses work for these other weapon types- mostly 'not getting hit' as Mark decribes- will work equally well against antimatter. It's not uniquely destructive weapon so much as yet another method of transporting energy from your ship's power source to the hull of an enemy ship.

Answer 10

Using current physics, the only way to defend against weapons we can build (antimatter, relativistic beams, nuclear bombs) is to be missed.

We are able to concentrate energy at the point where both the electron bonds within molecules, and the electron bonds between electrons and protons, fail. In effect, our weapons are capable of disassembling matter.

More advanced weapons will start breaking nuclei.

If you aren't at the location the weapon hits, you aren't killed. We are not capable of making weapons that can saturate a solar system (or larger) with deadly amounts of energy; ie, nova-scale or higher. Generally, your only defensive friend is the inverse square law.

If they beam their attacks, even better, it becomes easier to dodge.

Antimatter is really dross. The KE of relativistic weapons surpasses their mass-energy already; extra effort to send antimatter is pointless. If a relativistic weapons hits, whatever is there is gone, and a large explosion occurs. It is like designing a high-caliber machine gun to fire bullets, but we also lace the bullets with cyanide.

In theory, if you could contain antimatter, you could use it as a mine or bomb or energy storage source. If you could efficiently produce antimatter (ie, more less 2 grams of energy per gram converted from matter), you'd have an insane power source; that insane power source would have far larger tactical uses than "throw a beam of antimatter at your target", or even "fire a high-speed missile armed with antimatter".

For example, missiles fueled with antimatter could reach higher velocities, because a hard part of getting matter up to high speeds is doing it quickly with a small barrel. We probably don't want to devolve matter down to quark-gluon plasma (or even less structured!) in the barrel of the gun.

For any weapon hit to be survivable, you'd need something science-fantasy like force fields. Even shells of defensive matter don't help much, as the collision just turns the defensive matter into more relativistic shrapnel.

Your only real bet is to not be where they think you are; random movement. There is no stealth in space, so you have to not be where they think you will be after their light-speed delay.

They counter this with smart relativistic missiles, who have large ridiculously powerful thrusters to redirect themselves to match your random movement. As they finish the approach, they turn themselves into shrapnel to cover as large of an area as they can against your final dash.

Answer 11

vent your waste at it, as soon as the beam hits any normal matter you get an explosion that will disrupt the beam. wipple shields would work too, remember it is still a stream of matter. It would also be worse than useless in an atmosphere, it would actually damage the ship firing it.

Answer 12

Mobile, ablative armour.

Many of the techniques used for stopping RPGs would work, at a larger scale. Oddly little difference between antimatter and a shaped charge, with space(d) armour and reactive armour working here.

So, light spaced out armour, capable of moving pre-emptively out to cover holes in protection, far enough to withstand the effects of matter/antimatter annilation. You don't need solid mass and a lightweight, mostly empty material would mean its easier to move it around as needed.

So.. Large plates of something like aerogel propelled by small motors with enough freedom as mobile ablative armour, forming a controlled defensive cloud. As units are destroyed they movie to cover holes in the armour, moving inwards. (another model would be brilliant pebbles with satillites creating a dumb cloud of lightweight chaff)

Alternately they close in on the enemy so firing the gun would result in the ship firing the round annihilating itself.

Chaff in this case would be a possibility too. While covering the entire arc of fire in chaff in space combat is impractical at best you could have your interceptors either break into chaff or have local interceptors release chaff after the first hit

Answer 13

If the beam is charged, then it can be deflected with a sufficiently strong magnetic field, so if the enemies are smart, they will neutralize the beam (this also prevents the beam from repelling itself). So the beam will be antihydrogen.

When the beam hits the target (assuming it does), The positrons annihilate, creating gamma rays. The second reaction is between the antiprotons and protons, creating neutral pions (which decay almost instantly into gamma rays), and charged pions.

Charged pions have a half-life of 26 nanoseconds, moving at about the speed of light, half of them will have decayed in about 7.8 meters (possibly inside the ship), into neutrinos and charged muons, which then decay into electrons, positrons, and more neutrinos. The positrons then annihilate into gamma rays.

The biggest threat seems to be the gamma rays. Heavy metals with high atomic numbers are very good at defending against gamma rays, and they may affect the charged pions (I guess they would be affected by the electrons in the atoms?)

If antimatter is cheap, then really none of this matters. If the beam deposits about a kilogram of antimatter on the target, the resulting explosion will be equivalent to about 43 megatons of TNT, and no ship can survive anywhere close to that, even if only 1 gram of antimatter was deposited, the explosion would be 43 kilotons. Practically nothing can survive a hit by an antimatter particle beam.

As stated in another answer, the good thing is, you don't have to be hit. If you are 1 light-second away, the enemy will only know where you were 1 second ago.

The weapon technology scales with engine technology. If you can have practical antimatter particle beams, you can have practical antimatter engines, capable of multi-gee accelerations at immense efficiencies. So the effect of light-speed lag is increased. If the ship can dodge 1 ship length in 1/10 of a second (VERY high accelerations, crew NOT recommended), the lower limit for combat range is 1/10 of a light second. Any less and it's a bloodbath. And as the capability for dodging increases, so does the capability to saturate the area of possible maneuvers with shots, keeping the probability of hit at a given range stable as technology gets better.

Another option for defense would be to operate like a bussard ramjet (great if the ship IS a bussard ramjet). If the beam is neutral, then it cannot be deflected by magnetic fields. But what if you ionize it? Bussard ramjets face the same problem when collecting interstellar hydrogen, and if antihydrogen has the same spectrum as hydrogen (I think it would), a laser designed to ionize hydrogen would be just as efficient at ionizing antihydrogen.

Of course, if the beam is moving at near the speed of light, you won't see it until it's right on top of you. So the laser would have to be constantly directed at the enemy, to defend against a sudden antihydrogen beam (or as an offensive weapon), and the deflector magnets would also have to be constantly running.

More:

http://www.projectrho.com/public_html/rocket/slowerlight.php#id--Go_Fast--Bussard_Ramjet

http://www.projectrho.com/public_html/rocket/spacegunexotic.php#id--Antimatter

http: (remove this and spaces) //www.projectrho.com/public_html/rocket/spacegunconvent.php#id--Particle_Beams

http: (remove this and spaces) //www.projectrho.com/public_html/rocket/spacewardefense.php#id--Evasive_Maneuvers

Answer 14

The method you use for directing the antimatter as a weapon and the method you use for defending against it are going to be closely linked. The simple answer being:

What works for the weapon will work for defence.

Somehow you have to manipulate the antimatter into a beam, quickly directed or redirected to use as a weapon, without it colliding with the inside of the weapon itself. Whatever method you use for that will equally work to defend another vessel against the beam itself.

At the technological point where antimatter can be used as a weapon, it can equally be defended against. The trick is going to be that nobody would believe you'd use a weapon with such a high risk to your own ship and hence wouldn't have defences against it. This only works once.

Answer 15

It depends on whether some new technology/physics is invented or not. I present some examples from sci fi.

If there is no such thing as super luminar communication, then the detection of an incoming projectile / beam (anti-matter or not) moving at near light speed is going to be useless, as you can't signal to a target to take evasive action fast enough. So, your defenses are either: armour, speed, cover or stealth.

1. Armour: In some books this takes the form of large amounts of ice layered around the outside of a hull
2. Speed: simply moving fast and unpredictably is going to make scoring a hit pretty hard. This will require the ability to sustain high G forces. This naturally leads to the idea of multiple robotic fighter craft/munitions that can sustain very high accelerations
3. Cover: Just stay behind that moon, planet/ asteroid belt
4. Stealth: Maintain a super cooled ship exterior and emit no radiation signature (or angle it away from the likely attacker). If you can't accurately pinpoint your target's location, its not going to be easy to hit it. This leads to a problem of the 2nd law of thermodynamics (that heat has to go somewhere eventually...). You also have to worry about your ship occluding the stuff behind it. If you transit a near by sun, you might get seen.

For invented technology/physics:

Super luminar detection of incoming projectiles is possible:

1. Just move out of the way (maybe teleport your self!)
2. Teleport the matter somewhere safe, or maybe back at your attacker
3. construct cover using nearby mass by teleporting it into the way
4. detect teh attackers targetting and actively shut down your attacker's systems before shots are even fired
5. Deflect incoming beams using force fields
6. Ablate / absorb incoming matter using force fields traps. For example, a ship with exceptional field management could use matter contained in a field structure to interact with the incoming matter, trap the resultant annihilation photons and construct lasing chambers to create laser beams to shoot at an attacker.
7. Bend space time to curve sufficiently that an attacker's beams end up looping back at the attacker.

The list goes on and on...

One thing that is certain about all of the above approaches is that they all require some serious computational ability and quick reactions. Hence all will require some sort of AI or battle computer.

See Iain Bank's "culture" books for some high tech space battles.

Answer 16

The thing is, there's a lot we still don't know about anti-matter, because it is so rare and short-lived. We don't even know how gravity works with it. So if you are doing this for story purposes, honestly there's a lot you could probably more-or-less make up.

For example, its possible that matter and anti-matter repel each other. So if your target is massive enough, or you can create some kind of mass-distortion field in front of it, that might divert the particles. I'm not sure how you'd create a mass-distortion field. But I'm not sure how you're creating enough anti-matter to use in a weapon either, rather than just using the ridiculous amount of energy required to make the antimatter directly against the enemy. Perhaps there's some planet-sized facility, and this is a way to store up that huge amount of energy only available to such facilities into a shipboard weapon. So warships fly around with a set stock of anti-matter ammunition.

Since the point of using anti-matter is presumably the matter-destructive properties is has, another defense may simply to be much more massive (regardless of gravitational concerns). To use extreme examples, a planet will shrug off a hit that would obliterate a house.

Answer 17

If the bad guys can accelerate their antimatter to relativistic speeds, their beam is very likely to consist of charged particles. Positrons or possibly antiprotons, as others have suggested. As cybernard pointed out, physicists routinely control beams of positron or antiprotons with magnetic fields (at places like CERN, FERMIlab, DESY et cetera), making them go around in circles.

Which brings me to the suggestion (a variant of cybernard's idea).

Shield your ownship with a suitable magnetic field. With more advanced tech you could steer the incoming beam into a loop around your ship, and then aim it back at where it came from.

A more serious problem might be, if the baddies come from a different part of the universe, where there has been a surplus of antimatter (as opposed to matter as in these parts). Then their entire spaceship would be made of antimatter, including their regular gun shells. Of course, their journey through a part of the universe, where usual matter is the norm, would be perilous :-/

Answer 18

The main problem for this weapon is firing the antimatter at the kinds of speeds that the question is proposing.

In order to get even small quantities of particles shooting toward your enemy at near-light speeds, your weapon would need to be a pretty serious particle accelerator. The final shot would be at very high speeds, but it would take some time and a lot of energy to get the particles accelerated up to that speed, so the enemy ship would have plenty of warning if they were capable of monitoring power usage, and plenty of chance to disrupt the shot by taking out the accelerator before it is ready to fire.