A wormhole is like a tunnel with a very narrow throat. If you send a sound signal into such a tunnel, part of the echo returns, while part gets stuck in the middle, and will leak out very slowly. That is exactly how scalar particles (the simplest type of spinless particles) behave in calculations for a four-dimensional traversable wormhole. Right after the signal is sent, most of it is reflected or gets stuck in the throat, and only a small fraction passes through. But if you wait, the trapped part gradually “leaks out,” and in the end, about half of what a black hole of the same size would absorb passes through the wormhole.
Charged massless fermions (particles with half-integer spin, similar to electrons but without mass) behave quite differently. For them, the wormhole is nearly transparent at low energies: the transmission probability is close to one. The same mechanism helps these particles be efficiently absorbed by magnetic black holes, and here we find an unexpected connection to the long-known Callan–Rubakov effect, which describes fermion scattering on magnetic monopoles. So the wormhole acts as a perfect conductor for one type of particle and as frosted glass for another.
The takeaway is simple: if we ever learn to create or find wormholes, charged massless fermions would be the best choice for transmitting information through them. Scalar particles, on the other hand, would help us detect the wormhole itself — by how they get stuck and leak out with a delay, like an echo from a deep well.
The Callan–Rubakov effect, which the authors compared to fermion transmission, originally described how a magnetic monopole catalyzes proton decay, as if a tiny magnet makes particles disappear.
In the movie Interstellar, the wormhole serves as a portal to another galaxy, but they didn't specify which particles would get through faster — now we know that for signals, fermions are the better choice.