Image via WikipediaRKV (n.) - An accepted abbreviation for relativistic kill vehicle, a weapon that moves at near-light speeds in order to inflict maximum impact damage on its target. Also sometimes called a relativistic bomb, an RKV is a kinetic energy weapon taken to its logical extreme, combining the principles of Newtonian and Einsteinian physics.
Newton's second law holds that force equals mass multiplied by rate of acceleration. Thus, even a small mass moving at sufficient acceleration can generate significant force. "Conventional" kinetic weapons apply this law by simply dropping large, inert masses from planetary orbit (like the tungsten rods dropped from satellites in Warren Ellis's Global Frequency), allowing gravity to accelerate the payload to destructive velocity. RKVs go one step further, using vast interstellar distances to accelerate kinetic warheads to near-light speed, multiplying the force of their impact to catastrophic -- even planet-killing -- extremes.
RKVs are often a favorite plot device for hard sci-fi authors, including Larry Niven in his Known Space series, Charles Stross in Iron Sunrise, Joe Haldeman in The Forever War, and Vernor Vinge in A Fire Upon the Deep. RKVs don't require that civilizations develop faster-than-light travel or communication, as even subluminal propulsion systems can accelerate weapons to relativistic speeds given enough time and distance. Thus, wars fought between planets and stars are an ideal theater of conflict for RKVs, especially if you don't have FTL sensors to see them coming.
I bring it up because: Today is the 54th anniversary of Russell-Einstein Manifesto. On July 9, 1955, Albert Einstein, Bertrand Russell, and nine other noted intellectuals signed an essay highlighting the unconscionable dangers posed by nuclear weapons and implored world leaders to seek other, non-atomic-armageddon means of guaranteeing security and resolving conflict. Many view the manifesto as Einstein's repudiation of the application of his scientific breakthroughs to martial purposes. Unfortunately for Uncle Al, science has always led the way to new and more efficient weapons. Even setting aside the fission/fusion applications of Einstein's theories, his work on relativity can be applied for mass destruction in numerous ways, including the often overlooked brute-force example of RKVs. Food for thought, and some great science fiction.