Axion

The axion is a particle that no one has measured so far. For now, it exists only as a prediction on paper. Physicists invented it in the 1970s to remove an inconsistency in their formulas. If it exists, it would be unimaginably light: estimates put it at a billionth of a billionth of the mass of an electron, the particle in an atom’s shell. It would also be almost completely invisible, since it barely interacts with ordinary matter. This exact combination makes it both interesting and extremely hard to detect.

A particle that solves two puzzles at once

The original motivation was a problem in the theory of the strong nuclear force. This force holds the building blocks of an atomic nucleus together. The equations of this theory actually allow neutrons to have a slight electrical imbalance. But measurements show that this imbalance is zero, or at least immeasurably small. In physics, a law of nature that settles exactly on zero for no apparent reason is considered suspicious.

The proposed solution is a new field that moves on its own into the zero position, much like a ball rolling into a dip. The axion would be the particle associated with this field. It thus eliminates a problem that cannot be fixed simply by tweaking constants.

The second reason for the great interest came later. About 85 percent of the matter in the universe is invisible and reveals itself only through its gravity. This dark matter must consist of something that emits no light. Axions fit this role remarkably well. A particle that was invented for a completely different reason and then turns out to solve a second puzzle as well is considered strong evidence of a good idea.

How to turn an invisible particle into light

Theory predicts that an axion can turn into a light particle inside a strong magnetic field. This is the only practical lever for experiments. So researchers build a kind of trap: a very strong magnet, plus a cavity tuned like a tuning fork to a specific frequency. If an axion flies through it, a tiny radio signal should arise inside the cavity.

The problem is the strength of the signal. It lies far below the noise of ordinary electronics. That is why such detectors are cooled to within a few thousandths of a degree above absolute zero. In addition, amplifiers from quantum technology are used, the same kind found in quantum computers.

On top of that, no one knows the exact mass of the axion. The cavity must therefore be tuned step by step, like searching for a radio station on a very long dial. So far, only a small part of the possible range has been searched. A non-detection only rules out one section of the range, not the idea as a whole.

Why the word shows up in tech news

The best-known experiments are called ADMX in the USA, as well as MADMAX and BabyIAXO in Germany. The latter are being built at the DESY research center in Hamburg. Reports on them usually don’t say axion found, but rather another mass range excluded. That sounds unspectacular, but it is normal progress in this field.

For tech-minded readers, a second point is relevant. The search for axions drives forward measurement technology that is also needed elsewhere: extremely low-noise amplifiers, strong magnets, precise cooling technology. These components overlap heavily with what quantum computers require.

A common misconception: the axion has nothing to do with the Higgs particle, even though both were sought for a long time. The Higgs has been confirmed since 2012 and explains why particles have mass. It’s also confusing that companies and products carry the name Axion, such as chips or software. These names are pure marketing and have nothing to do with the particle.