Can we build a lightsaber?

Dave - Depends exactly what you mean by lightsaber. Producing a kind of beam of light which is coming out of an object which is incredibly powerful and can cut through things, that's not impossible. But we certainly couldn't get anything nearly as powerful as that. We can't get laser that powerful in anything like that size of container, but we can get lasers which will cut through things within the size of a house, but we'll ignore the size issue at the moment. But the problem is making light suddenly stop at the end of the sort of 2-foot length, unless you put a mirror there it just wouldn't and also, light doesn't interact with other light, so you can't have sword fights with them. They would just go straight through each other and not affect each other. You might conceivably be able to make a plasma sabre so you could somehow constrain some very, very hot plasma into a tube magnetically or something conceivably. I think it would be difficult but just about possible, but certainly not with light.