I am writing an essay about something semi related. When did the word or concept of antimatter first appear? When was it first used as a weapon system, in film, book or comic?
Several years before Jack Williamson (writing as "Will Stewart") began his Seetee series (seetee = CT = contraterrene = antimatter) with his serial "Opposites—React!" in Astounding Science-Fiction, January 1943 (available at the Internet Archive), the chemist and sci-fi writer John D. Clark, Ph. D. introduced antimatter (as "reverse matter") to science fiction with his novelette "Minus Planet" in Astounding Stories, April 1937 (available at the Internet Archive). Reprinting this story in his anthology Before the Golden Age, Isaac Asimov wrote (p. 909):
By 1937, I had heard of anti-matter, and Clark's story, which was the first ever to deal with it in science fiction, excited me tremendously.
Here is the explanation in the story, with its now outdated theory of the proton:
"You remember, when we first saw this thing, I put you through a quiz on matter? I had a hunch then, and I've proved it. You described the sort of matter with which we are familiar. Look here. You said that matter was made up of neutrons and positrons, in the last analysis, in the nuclei, and of electrons on the outside. Well, there is another sort of matter possible. What is to prevent an electron from combining closely with a neutron, and forming a negative proton? The possibility was mentioned way back in 1934, and I think the old boy even gave his hypothetical particle a name—an 'antron,' I think he called it. Now take some of these antrons, and some extra neutrons, and make a nucleus out of them, and then release enough positrons on the outside to balance the antrons. And one has an atom with a negative atomic number, since the atomic number of an atom, of course, is the number of positive charges on the nucleus.
"And now one makes a whole universe with these minus elements. And one makes oneself out of them, too, and lives in the place, and can't tell the difference between it and a regular universe. All the physical laws will be the same—but just wait until part of your new universe hits part of a regular universe! Figure it out. What do you think will happen?"
"Uh—let's see. First the outer electrons in our matter will neutralize the outer positrons in the reverse matter—and there'll be a hell of a lot of light or other radiation—UV, gamma, cosmic and what not. Then the nuclei will get together. Nothing will happen to either set of neutrons. But the positrons on the protons will neutralize the electrons on the antrons, and there'll be another burst of radiation and a lot of neutrons left over. So the net result will be a mob of neutrons and a flock of radiation. What do you think? Is that thing out there"—he gestured toward the anomalous planet they were leaving behind—"out of a reverse universe?"
Before the word 'antimatter' was even widespread, Jack Williamson was writing stories about it. He called it "contra-terrene" (CT) matter and it was called the Seetee Series. This was mid-to-late 40s/early 50s.
E.E. Smith is using "negative matter" in Gray Lensman from his Lensman series. It was published about 1939-1940.
His description of the reaction of negative matter with normal matter is not scientifically correct:
Negative matter and normal matter annihilates each other and creates massive radiation, which burns living beings, but doesn't create heat/visible light/explosions (what should happen when such amounts of energy is released).
But that is probably because there wasn't much known about antimatter then.
From Olaf Stapledon's Last and First Men, published around 1930:
Unlimited power, derived partly from the artificial disintegration of atoms, partly from the actual annihilation of matter through the union of electrons and protons to form radiation
Superficially this sounds like matter-antimatter annihilation. But it talks about the wrong particles: protons and electrons rather than protons and antiprotons. Nonetheless it has a very specific feature of the theory of antimatter: that oppositely charged particles annihilate each other and release radiation.
By around 1929 Dirac had derived a model of physics in which particles came with oppositely charged pairs that would annihilate each other on contact, emitting the combined energy of the particles in the form of radiation. These oppositely charged particles later came to be known as antimatter. At the beginning, however, Dirac thought that the positively charged partner of the electron in his theory must be the proton. It's only by 1931 that he was convinced that it didn't make sense for the proton to be the antiparticle and Dirac instead proposed there must be a new particle, the positron, which was later found in the lab.
In other words, the theory Stapledon describes is precisely the state of the theory of antimatter at the time of publication of the book.
It's hard to imagine where else Stapledon could have got the idea from. AFAIK The prediction that positively and negatively charged matter should annihilate on contact isn't simply some idea that was floating around at the time. It's a very specific prediction that Dirac was forced to reach as a result of his theoretical work. And Stapledon was in regular contact with at least one person on the fringes of academia at Cambridge where Dirac worked.
It's used as a weapon in the book. A device is invented which can cause the remote explosive collapse of atoms. It is used, for example, to destroy aircraft.