&gt; <i>Lubin supposes that the system could propel crafts to an &quot;unheard of&quot; 30 percent the speed of light.</i><p>&gt; &quot;<i>There is no known reason why we could not do this</i>&quot;<p>Yeah, no. There is a known reason. What do you think happens to a spaceship hitting something at 30% c? What do you think happens to the front of a spaceship or craft hitting something the size of bead or pebble at 30% c? The ship will disintegrate.<p>Granted, the ship could be shielded or employ some sort of plasma&#x2F;magnetic deflector technology (still in proof of concept stages of development), but now you&#x27;re talking technology that&#x27;s just as far away, if not further than the propulsion system they&#x27;re talking about using. It&#x27;s also why things like project Orion really never got off the ground or are feasible in their current forms. Sure, we can get up to those speeds, it&#x27;s surviving at those speeds that&#x27;s the challenge.<p>Also, they talk about the weight of a ship being ultra light. If you had to shield it to survive the impact of tiny particles and other space debris you&#x27;ll inevitably encounter on your trip, it&#x27;s no longer going to be ultra light.