back-of-envelope:<p>conservation of momentum: 2<i>(num photons)</i>h&#x2F;lambda = (100 kg)*(c&#x2F;3), so 5.9×10^36 photons needed to propel a 100 kg object to 1&#x2F;3 speed of light. 1.5×10^18 joules of energy, which is 11% of the energy output of the US in 2001 (according to wolfram alpha). That&#x27;s a 5.8 Tera Watt laser running for 3 days straight.<p>I haven&#x27;t even gotten into dispersion of a laser beam.<p>The ludicrousness of this proposal is left as an exercise to the reader.<p>Edit: here&#x27;s their proposal with physics. Just to be clear, I don&#x27;t doubt the theoretical possibility, I doubt the economic and experimental reality. <a href="http:&#x2F;&#x2F;www.deepspace.ucsb.edu&#x2F;wp-content&#x2F;uploads&#x2F;2015&#x2F;04&#x2F;A-Roadmap-to-Interstellar-Flight-15-h.pdf" rel="nofollow">http:&#x2F;&#x2F;www.deepspace.ucsb.edu&#x2F;wp-content&#x2F;uploads&#x2F;2015&#x2F;04&#x2F;A-R...</a>