A while back, someone asked me what my subfield, amplitudeology, is really about. I wrote an answer to that here, a short-term and long-term perspective that line up with the stories we often tell about the field. I talked about how we try to figure out ways to calculate probabilities faster, first for understanding the output of particle colliders like the LHC, then more recently for gravitational wave telescopes. I talked about how the philosophy we use for that carries us farther, how focusing on the minimal information we need to make a prediction gives us hope that we can generalize and even propose totally new theories.

The world doesn’t follow stories, though, not quite so neatly. Try to define something as simple as the word “game” and you run into trouble. Some games have a winner and a loser, some games everyone is on one team, and some games don’t have winners or losers at all. Games can involve physical exercise, computers, boards and dice, or just people telling stories. They can be played for fun, or for money, silly or deadly serious. Most have rules, but some don’t even have that. Instead, games are linked by history: a series of resemblances, people saying that “this” is a game because it’s kind of like “that”.

A subfield isn’t just a word, it’s a group of people. So subfields aren’t defined just by resemblance. Instead, they’re defined by practicality.

To ask what amplitudeology is really about, think about why you might want to call yourself an amplitudeologist. It could be a question of goals, certainly: you might care a lot about making better predictions for the LHC, or you could have some other grand story in mind about how amplitudes will save the world. Instead, though, it could be a matter of training: you learned certain methods, certain mathematics, a certain perspective, and now you apply it to your research, even if it goes further afield from what was considered “amplitudeology” before. It could even be a matter of community, joining with others who you think do cool stuff, even if you don’t share exactly the same goals or the same methods.

Calling yourself an amplitudeologist means you go to their conferences and listen to their talks, means you look to them to collaborate and pay attention to their papers. Those kinds of things define a subfield: not some grand mission statement, but practical questions of interest, what people work on and know and where they’re going with that. Instead of one story, like every other word, amplitudeology has a practical meaning that shifts and changes with time. That’s the way subfields should be: useful to the people who practice them.