My physics is a little rusty, so please forgive me if I'm wrong. I think the law you're referring to is the Law of Conservation of Mass. As energy has mass and subatomic particles have mass, you can convert back and forth between them all day (good luck with that), and never violate it. In particular, Einstein's E=mc^2 is applicable here.
It was a surprise to me to find that the term 'matter' is actually poorly defined. At least according to Wikipedia.
Ah yes - I did mean conservation of mass, thank you.
I think most people use 'matter' to refer to the regular objects we interact with that are composed of atoms, and that's how I intended to use it in the above post. However, if I had my way, electromagnetic radiation would also be consider 'matter', because it's the same stuff, just a different form.
>> As energy has mass
Is that generally accepted, btw? Because I've always heard people talk about light as having no mass, and it never made sense, because that what I understood e=mc^2 to represent. I think it does have mass, it's just so minute (smaller than anything we know by such an order of magnitude that the speed of light squared is used to express it) that it doesn't seem like it.
I'm no expert, but my understanding is a little different. Its not that energy "has" mass but that energy and mass are two different forms of the same thing, i.e. they are equivalent. If some process is able to convert mass to energy then you could find out much mass you'd get but multiplying the energy by c^2 (hence E = mc^2).
It was a surprise to me to find that the term 'matter' is actually poorly defined. At least according to Wikipedia.