Special Relativity and Angle Grinder Disks

© Eugene Brennan

The theory of special relativity was postulated by Einstein in his 1905 paper, "On the Electrodynamics of Moving Bodies". While things behave seemingly "normal" at speeds we experience in our daily lives, bizarre things happen at near-light speeds. One of these is time dilation. So for an object that's moving at super-high velocity, time slows down. You may have heard of the thought experiment in which there are two twins and one of them goes off on an interstellar space journey while the other twin remains on Earth. When the first twin returns home years later, he finds that his brother has aged much more than him. The theory was proven by placing an extremely accurate atomic clock on an aircraft while keeping a similar clock stationary on the ground (though it was slightly more complicated, with two aircraft, one travelling east and the other travelling west). Before the experiment started, both clocks were synchronised. On checking the clocks after the flight ended, it was discovered that the clock on the aircraft had run slower than the clock on the ground by a tiny fractions of a second. For a 4 1/2" angle grinder disc, it works out that if a disk were running constantly for 10 years, the perimeter of the disk would age slower than the centre by 7 microseconds. This may seem minor, but relativistic effects must be accounted for when doing GPS calculations to ensure accuracy, due to the satellites' velocity.