What we know about the EmDrive and Cannae Drive
This is not worth getting excited about. Really.<p>Extraordinary claims require extraordinary evidence. These drives would violate conservation of momentum.[0] Conservation of momentum is built in to the known laws of physics at an extraordinarily deep level. It is obviously not impossible that our known laws are wrong, but the alternative would be to somehow produce a <i>radically different</i> set of physical laws that nevertheless reproduce every test of existing theory to high precision (as much as 14 digits of accuracy in some cases). And you probably also need to explain why the existing theory worked so very well if its bedrock foundation doesn't describe our universe.<p>So really, don't get excited. And don't send these folks money that could do real good elsewhere until they give at least some small reason to expect that it's not a wild goose chase.<p>(Edit to add: I wish this were real, too. With all my heart. But it's not a realistic wish.) [Re-edited to cut some less-appropriate phrasing.]<p>[0] Yes, I know the EmDrive site FAQ (<a href="http://emdrive.com/faq.html" rel="nofollow">http://emdrive.com/faq.html</a>) claims that it doesn't violate conservation of momentum. The explanation of why it doesn't actually indicates that it does: "The electromagnetic wave momentum is built up in the resonating cavity, and is transferred to the end walls upon reflection." But the wave's momentum has to come from somewhere, too. So either this is just equivalent to a photon drive (propelled by shooting light out the back), or their device is pulling momentum out of nowhere.