Relative velocity
Maybe, instead of asking about firing out of a spaceship, you should ask about firing off the end of a train...backwards!
Imagine a rifle that fires a projectile that travels at 3,000 fps, and the train itself moving at 3000 fps. Would the bullet travel 0 units of distance and fall to the ground? Hmmmmm...
True… but only with respect to a stationary observer standing alongside the train track -- not with respect to an observer in the train. With respect to a passenger in the train the bullet would leave the rifle
and the train at a velocity of 3000 fps. The bullet would indeed fall straight down to the ground as viewed by the stationary observer, but as viewed by a passenger in the train the bullet would fall down in a long gentle arc.
Let us assume that it takes the bullet 3 seconds to fall to the ground in front of the trackside bystander (we are ignoring the element of friction). With respect to a train passenger the bullet would travel in the air for 3 seconds, and would be 9000 feet behind the train when it fell to the ground right at the feet of the bystander at trackside.
Ditto with the similar space shuttle example. The velocity of an object is not an absolute. It can be measured only with respect to some other point.
Even the term “stationary observer” can be very misleading, as there is in theory no such thing as a stationary observer on our little earth. The earth is moving around the sun, the sun is moving in the galaxy, which is moving in space, etc. Some would say there is nothing at all which is stationary. Even if there were, the question then becomes, “stationary with respect to what?”
An example often used to point out this law of physics looks at a roadside sign which announces the speed limit to be 70 mph. That sign does not make any sense at all. According to relativity, the sign should read, “The magnitude of the relative velocity between your car and the pavement must be not more than 70 mph.”
Explain this to the judge next time. Good luck.
