Many of us have considered this very same question, and I'll tell you what I know from my looking into it (all who read this, please feel free to correct me on any of this that I write, for I am not 100% sure on the absolute certainty of these ideas).
The only legitimate scientific theory on building a real life lightsaber, involves both plasma and electromagnetism. Basically using a form of cold fusion to produce a plasma blade that would then be focused and contained by the electromagnetic field into a solid beam or "saber' like weapon. The only problem being that "cold fusion" doesn't exist yet, we do not possess the technology necessary to produce cold fusion. We do have regular good old fashioned fusion, and could use this technology to try and build a working prototype, however the heat that this would produce in the hilt, would be like stepping foot on the surface of the sun :o . The plasma sword would burn through your hand, and then the floor and then everything else below it. For the first couple of seconds Robot Chicken comes to mind (Qui Gon and Obi Wan in Episode 1 on the droid control ship in the begging) "Qui Gon" hey hold this for a second "Obi Wan tries to hold, but drops" "saber falls through the floor" "random worker below" Hey look a Lightsaber!!!!, wait OH GOD!!!!!! "random worker killed by lightsaber" "Qui Gon" Shakes head lol :D Ah The Hilarity of Robot Chicken :D
So as soon as someone invents cold fusion, I'm sure someone will give it a go.