Probably some kind of force field, possibly electro-magnetic. Throw random matter, possibly replaced and transported in front of the beam.

@cybernard Give me specifics on that force field, convince me it's realistic and that answer may win

my knowledge of this is quite low. If I understand correctly (in lamen's terms) antimatter is basically electrons with the mass of protons and protons with the mass of electrons?

@TheGreatDuck Possibly true, but the main property it's exploited for is the opposite charge - positive electrons and negative protons - which means that when it collides with matter, both are annihilated

@TheGreatDuck the mass of an anti-electron is the same as the mass of an electron, but it has the opposite electrical charge, and as Zxyrra says, if an anti-electron meets an electron, both are annihilated, and are converted (usually) to a pair of gamma rays.

I guess you meant spaceships? Because my first reaction was to answer "air", as hitting an ocean-going ship with antimatter would require you to be really close, otherwise the antimatter would react with and dissipate in the atmosphere.

@vsz I mean spaceships. Edited for clarity

Chaff made of microsingularities might diffuse the beam.

Antimatter has the same mass as the matter it is intended to annihilate, so whatever antimatter beam you can construct, it should be even easier to construct a matter beam of the same mass and speed. So we're not trying to block this fast beam with slow missiles, we just need to block it with an equally fast beam. Timely detection of the incoming beam would be the main problem to solve for an active defense.

@JamesK That's what I said. A positron is a proton with the mass of an electron.

@TheGreatDuck -- There's no such thing as a proton with the mass of an electron. The proton and the positron are two of several distinct particles with +1 charge. Very different things.

@MackTuesday It isn't just a smaller proton? Cause I thought there were proton sized particles with -1 charges. I always thought antimatter was just proof that electrons and protons can vary in size all the way from mazzive planet sized versions to smaller than electron sized versions. I thought the electron-proton particles are just the most common sizes because they sit in a nice balance of magnetism.

@TheGreatDuck A positron and a proton are extremely different things. A positron is (to current physics knowledge) a fundamental (anti-)particle. A proton is a particle that consists of fundamental particles (quarks) held together by what's known as the strong force (i.e. certain quarks interact via the strong force to 'form' a proton). While neither the proton or positron interact via the strong force, the (relatively) large difference in mass is due to the strong force. Apart from mass, there's also a difference in things like the magnetic moment, size etc.

@TheGreatDuck If you're interested, you might want to try reading en.wikipedia.org/wiki/Subatomic_particle and en.wikipedia.org/wiki/Standard_Model to get some sort of an idea of what's going on - it seems that you're thinking in terms of classical physics, where everything is continuous, as opposed to quantum physics, where things are discrete ('quantised'). Classical physics is great for 'everyday life', but quantum physics is needed at the atomic level

@Mithrandir24601 I didn't realize that a positron was a fundamental particle. I always assumed that protons and electrons were just masses of negativity and positivity that can shear and break apart. That sounds very complicated. I'll read about that sometime (maybe).

@RudolfL.Jelínek Obviously the antimatter containment field. That's... that's what it's there for!!

Surround the enemy with a cloud of matter.

I mis-read the title as "Can spacecraft defend against antimatter dragons?" (there was a question next to this one about dragons).

@DavidK You'll want to fire a lot more matter than there is antimatter in the beam. Matter is much cheaper, though, this shouldn't be a problem. The hard part is targeting.

@LorenPechtel How much matter you need depends on how well you can direct it to where the antimatter will be. But if you're suggesting that it's better to throw a lot of matter to many places where the antimatter might or might not be aimed, rather than try to throw less matter with pinpoint accuracy, I agree.

@DavidK No, I'm saying you want to be sure to annihilate it all, so send extra as you won't get a 100% collision rate.

If a particle beam hits you with almost the speed of light, it’s not really important whether it’s matter or anti-matter. (it doesn’t really matter, scnr)

@RudolfL.Jelínek The best I can come up with is that these antimatter weaponizers are themselves antimatter beings with antimatter ships, which just makes the whole thing awesome.