I came across [this video](https://www.youtube.com/watch?v=lAAmAbJvvJg&ab_channel=minutephysics) and towards the end of it, the video creator says that neutrinos don’t have mirror images.
What does this mean?
What is the ‘mirror image’ of something else, like an electron?
In: 3
It means that neutrinos do not interact with electromagnetic radiation, and therefore do not have a reflection.
Particles have a property called *spin*. This is kind of like the rotation around an axis seen on a spinning object like the Earth, except that the particle doesn’t have a “size” for it to be spinning *around* – it’s just an innate property of the particle like it’s charge or rest mass or the like.
The angle of that spin can lie in one of two directions, relative to the particle’s motion. We say that the particle is “right-handed” if in spins in the same direction as its motion, and “left-handed” if it spins in the other direction. (When we talk about the “direction” of spin here, we mean the direction from which the spin would appear to be counter-clockwise.) This direction – or more properly, a version of it that accounts for some weirdness around relative motion – is called *chirality*, and it’s an inherent property of particles too.
It turns out that the direction of spin matters to the weak interaction, one of the four fundamental forces and one of the two (along with gravity) in which neutrinos are known to participate. In other words, the Universe distinguishes between left and right for some purposes. All known neutrinos are left-handed, and all known anti-neutrinos are right-handed. All other known particles have been detected in both forms, so this is kind of weird.
One theory is that right-handed neutrinos (and left-handed antineutrinos) don’t exist at all. Another, more interesting, theory is that they do, but that they don’t participate in the weak interaction (in effect, their “charge” for the weak interaction is zero, so they don’t interact with it the same way a non-electrically-charged particle doesn’t interact with the electromagnetic field). That would leave them interacting only by gravity and, because gravity is very weak, we wouldn’t be able to detect them in particle physics experiments. There is some evidence to suggest that this may be the case; it would among other things explain the observation that neutrinos have (incredibly tiny) masses.
Not ELI5, but I’m reading the book, “The theory of almost everything” and it goes into a whole lot of detail if you’re interested in physics, and these kind of crazy sounding interactions. It’s dense, but approachable.
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