Final answer:
In order for time on a rocket to slow down to half its rate as measured by Earth-based observers, the rocket must travel at a significant fraction of the speed of light. This is due to time dilation, a phenomenon predicted by Einstein's theory of relativity.
Step-by-step explanation:
In order for time on a rocket to slow down to half its rate as measured by Earth-based observers, the rocket must travel at a significant fraction of the speed of light. This is due to time dilation, a phenomenon predicted by Einstein's theory of relativity. As an object approaches the speed of light, time slows down relative to a stationary observer.
Present-day jet planes do not approach speeds close to the speed of light, so their time dilation effects are negligible. However, the concept of time dilation has been experimentally confirmed through high-speed particle accelerators and in space travel.