The biggest question is what is the point? A gram-scale probe is nice, but how is it going to return <i>ANY</i> useful data at all from even the nearest star system 4ly distant? Skimming the paper seems to make no mention of it<p>Also, they're saying that it is 100GW of total power and the "ground-based laser array will be need to be kilometers in scale". Say we're talking 10 square kilometers, that's 10GW/Sqkm.<p>With 1,000,000 m^2 per km^2, that's pumping 10 kilowatts per square meter up through the atmosphere.<p>That means a crow-sized bird, with a wingspan of ~18"/50cm x 7"/20cm is 0.1 m^2 and will absorb a kilowatt of radiant energy. Like spreading it out on top of ten 100-watt incandescent bulbs, or directing a 1000-watt hairdryer at it. It won't fry instantly, but will rapidly overheat in the few minutes they claim it will take to accelerate the craft to 0.2c.<p>Since they're detecting atmospheric disturbances, perhaps they could route around birds by momentarily turning off beams that would be wasted anyway?<p>Even assuming this is a cover for an array to fry satellites or incoming ICBMs, it seems kind of frivolous... I'd love to be wrong because it'd be cool to generate those kinds of speeds, but...?