Super Science Dad Proved General Relativity To His Kids

I was amazed at Project GREAT, in which Tom Van Baak took his three children and three atomic clocks about 1.6 kilometers up Mt Rainier, stayed up there for two days, and showed that the clocks gained about 20 nanoseconds because of the high-altitude's weaker gravity.

When I told people that each person's time flows faster or slower depending on how fast that person moves or where that person is in a gravitational field, most people were incredulous. This effect is real and important enough that GPS satellites must take into account the different time flow rates for people on Earth and for the clocks on the satellites so that positions given by GPS receivers are accurate.