A defensive shield, commonly referred to as simply a shield and sometimes as a Holtzman shield, was a protective energy field that could surround the person who wore it. Shields were produced by a Holtzman generator, the field deriving from Phase One of the suspensor-nullification effect...
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Dune Shields block anything physical moving above a certain speed (I don't know that it's ever exactly defined), which blocks just about any projectile weapon but can by bypassed by knives and swords, hence everybody fighting with those in Dune. Notably you can actually move your arm faster than a shield's limit, as Paul Atreides mother notes during one of his duels with the Fremen that he's moving too slowly out of habit of fighting against shields. The main counter that isn't a melee weapon is the
Maula pistol, which fires slow-moving poison darts and still only works at such short range that swords are a viable weapon.
Of great concern in this scenario is that a Dune shield cooks off like a nuke when hit by a laser. It's not clear how powerful the laser has to be to do this, but if any random cat-toy pointer will work then every nation, criminal gang, and random hobbyist on the planet now has access to nukes. If it takes an industrial-strength laser same scenario except random civilians can't do it, but most countries and even well equipped civilian groups will be able to throw a shield+laser+egg timer combo to set up nuclear timebombs. The balance of terror just got a lot more terrifying.