We don't yet have flying cars, much less jet packs. I like our Roomba, but domestic robots still have far to go before they can whip up an omelet, set the table, and pour the coffee. In these and many other areas, technology continues to lag behind the rosy sci-fi scenarios of my youth. Allow me, then, to give the future a little kick in the pants by describing how to build a long anticipated and long overdue technology: The force field.
Start with a phase conjugate mirror. A conventional mirror simply reflects light, bouncing it off at an angle in the same way that the bumper on a pool table redirects the motion of a ball. A phase conjugate mirror, in contrast, reflects light in exactly the reverse direction and form as the light comes in. Banking shots would be impossible on a pool table with "phase conjugate" bumpers, as balls would always bounce back in exactly the same direction whence they came.
Add to the phase conjugate mirror this additional ingredient: pumping beams that create a amplified reflection of the incoming wave front. (Read the bit under "phase conjugate mirror" at this source for more details.) To recur to the pool table example, it would be as if you gently tapped a ball at a bumper and it came speeding straight back at you.
Lastly, top off the amplified phase conjugate mirror with an illumination beam—a laser that rapidly scans the protected area, say. This illuminating beam can operate at relatively low power levels, given that it serves only to bounce a few photons off of the target. When some of those illuminating photons find their way from the target to the amplified conjugate phase mirror . . . BAM! Out flashes a blast of electromagnetic energy, automatically aimed on-target.
To better understand how the force field works, consider a ready application: Protecting spacecraft from orbiting debris. A scanning laser would rapidly sweep the area from which space junk would most likely approach the protected craft. Most of the time, of course, that beam would dissipate into empty space and the force field would remain quiet. When the scanning laser illuminated an approaching threat, however, the phase conjugate mirror would bounce a beam of electromagnetic energy right back at the debris (or, what would in practice amount to the same thing, given the speed of light, at the location the debris occupied an flash earlier). With enough amplification, the phase conjugate mirror could alter the trajectory of the approaching junk, directing it away from the spacecraft. With more amplification, the force field could simply vaporize the threat.
I could say more, but readers who have read this far can probably work out other interesting applications of the force field, as well as the problems introduced by non-reflective or highly reflective targets and the remedies afforded by using different frequencies for the scanning and amplified beams. Some readers might quibble that, regardless of its merits, I've not really described a force field, but rather only something that appears to function like one. Given that sci-fi authors don't typically explain how force fields work, though, I don't feel too bad about borrowing the label.
I don't claim this recipe for a force field as any sort of breathtaking innovation, granted. Once you get your head around phase conjugate mirrors, the rest of what I've suggested falls into place pretty quickly. I guess you could call it obvious to one reasonably skilled in the relevant arts—dynamic holography—and, thus, unpatentable. Still, though, I've yet to find any references on the 'net about this method of creating a force field.
Given that I make my living as a law prof, rather than a non-linear optical scientist, you might wonder why I dabble in these topics. It turns out that I've long had an interest in holograms. I set up a home lab to make them when I was in high school, and later developed a holographic information processing system that, in theory at least, answered a challenge that a hero of my youth, Douglas Hofstadter, put to me personally. His response left me so disillusioned that I abandoned my plans to pursue a degree in AI, but that is another, much longer story.