Sure, we’ve got the iPhone’s Siri, and the Food and Drug Administration just approved a prosthetic arm controlled by signals from the brain — but where are our smooth-gliding flying machines, our Landspeeders (“Star Wars”) and airborne DeLoreans (“Back to the Future”)?

You may think that the absence of such cars speaks to a failure of engineering or distorted incentives in the marketplace. But the humbling truth is that we don’t have these vehicles because we still don’t know, even in principle, how to directly manipulate gravity. Indeed, the cars missing from our skies should serve to remind us that, to a degree rarely appreciated, we have surprisingly poor control over most of nature’s fundamental forces.
—Adam Frank, The New York Times