The lightsaber

Lightsabers, made of immaterial beams of light, collide whereas they should pass right through one another without a sound. Moreover, laser beams propagate in a straight line as long as they do not meet an obstacle, therefore lightsaber blades should cut right though the hulls of spaceships.
Let us drop the idea of a laser-based saber, imagining instead plasma confined by a magnetic field. Plasma is a super-heated gas, whose color and luminosity vary depending on its temperature and composition. This would explain the variety of sabers, but users would have great difficulty in not burning themselves.
Keeping a gas in the plasma-state requires considerable energy: 40 kW are necessary for a 4 inch (10 cm) saber at 18,000 °F (10 000 °C). It would be difficult to fit the required generator into the saber's hilt. To increase the length, we can confine plasma in a magnetic field in the shape of a stretched out bottle. This newer style saber would behave more or less like those in Star Wars. It is nonetheless foolhardy to confine plasma magnetically. A simple handful of magnets would disrupt the confinement field, and plasma would spill onto the person wielding the saber.