Truly Immovable Rod
The Truly Immovable Rod (also called the Universal Stillpoint Rod) is a meme derived from a fa/tg/uy's story about how his dickish DM applied some very dodgy (but accurate if it was the real world and not the D&D world) physics to an Immovable Rod, judging that it remained immutably fixed in the universe while the planet they were on was moving (and thus setting it off, relative to the players, at several thousand miles an hour when activated). The very same fa/tg/uy later used an Immovable Rod as a siege weapon to destroy a castle, pissing off his DM.
It was considered that, since every celestial body is moving very quickly, anybody who placed such an Immovable Rod would never see it again, as their system left the stationary rod floating in the void. If this had happened a lot, there could be any number of Immovable Rods out there, simply biding their time until some unsuspecting and unfortunate planet happens to pass through the same region of space an*CLANG!*
WHAT THE FUCK WAS THAT?
Oddly enough, this exact thing happened in Lucifer's Hammer, a book written by Larry Niven and Jerry Pournelle in 1977, when a space station orbiting the Earth was suddenly hit.
Dickish GMs who wish to avoid such shenanigans should utilize a setting that is not based on planets, but instead, on an infinite plane of existence consisting solely of land extending in all four prime directions, and without proper "space" as we know it in this crapfest universe that we type this shit in. Or to use a Ptolemean universe, with a planet in the center of the Universe that doesn't move or spin, and sun, moon, stars and planets orbiting around it. Only then will the Immovable Rod function as advertised, thus avoiding the collision of physics and magic, and consequently, rendering all attempted castle demolitions using this method as invalid schlep comedy and copy-cat antics to be derided and punished with style. A final alternative is to claim it's relative to the nearest celestial body, or anchored to a manasphere a la Shadowrun.
Why This Wouldn't Work
Einstein's Theory of General Relativity (as well as many of the theories it superseded) tells us that there can never be a true "fixed point" in the universe, as all movement is based on the observer. For example you can cause a mountain to move simply by sailing past it on a moving object. To your perspective, you're sitting still, and the mountain is moving! This also applies to planets and entire star systems.
Why this would work, but not how you expected it to
Relativity aside, the only way an object could be immovable would be if it had infinite mass. Any object of infinite mass would subject everything in the universe to infinite g-forces, thereby pulling everything in the universe toward itself at the speed of light. This would be extremely bad for anyone trying to use the immovable rod as a weapon. The good news is that, since these g-forces are infinite, they are equal regardless of distance from the immovable rod, which means there are no tidal forces and no spaghettification; people will live long enough to be crushed into neutronium instead.
In short: the moment you activate the rod, the universe as you know it ceases to exist. You won?
Well, actually only your local galaxy would be at any risk for the next million years, since inter-galatic space is so huge that even rushing toward the immovable rod at the speed of light will take ages.
(1) Immovable rods can, in fact, be moved via the application of 8,000 pounds of force. As soon as the force being exerted against them drops under 8,000 pounds, they stop moving. This implies that they are being kept in place not by inertia, but by some kind of cosmological, gravitational, or magnetic drag.
(2) A comment on the original thread mentioned that immovable rods would also be indestructible: "if any part of the rod can be damaged it implies the molecules are being broken apart, dented in, etc ... moved. If the rod is immovable then the whole rod should be immovable on a molecular scale." As a corollary of this, immovable rods also maintain a temperature of absolute zero.
(3) Immovable rods are stated to be made out of metal. Many metals become superconductors when brought near absolute zero. Therefore, immovable rods are probably superconductors when activated.
(4) Many planets have magnetic fields. Some of these magnetic fields are much stronger than Earth's. If you'd like to know what happens when you bring a superconductor and a magnetic field near each other, look up "magnetic flux pinning" or "quantum locking" on Youtube.
- http://suptg.thisisnotatrueending.com/archive/1709686/ - suptg archive of the original thread.
- https://www.youtube.com/watch?v=9eKc5kgPVrA - a more scientific explanation of immovable objects.