[Steely Tom]

This is most interesting.

Something I've learned recently is that a photon has spin. This seems very strange to me, because a photon has no mass. How can it have spin?

But that is no stranger (I guess) than the fact that a photon carries momentum, even though it has no mass. Perhaps its momentum is what's spinning, even though there's no mass there to spin.

It has energy though, so its mass-energy (E/c²) not zero.

I learned about this from the YouTube lecture series of David Butler, which is the best I've ever seen... and I've watched a lot of them.

If you watch all five of his "How small is it?" videos in order, he will take you from what you see through a magnifying glass right down to the Higgs boson, and explain everything with amazing insight. As I said, the best I've ever seen.

[LibWhacker]

Thanks for your tip on Butler’s lecture series. I’m going to check him out.

And no, I can’t answer your question about spinning objects that have no mass. It never would have occurred to me to question whether or not such things exist. That’s what I love about FR; there’s always someone who asks the question that stops you in your tracks and makes you go, “Whoa,” but in the end, after much head scratching, can lead you closer to understanding, thank you!

[Olog-hai]

Photons have zero rest mass. But anything that has energy also has mass per Einstein’s equation.

As far as small things, that is also relative; to the naked eye, a human cell would be infinitesimally small (invisible), and on average an adult has 300 trillion of those. Bit hard but not impossible to wrap one’s mind around going smaller, to subatomic particles.