Gravitational Time Dilation

You must’ve heard that clocks tick at different rates, depending on gravity’s strength. Your ground floor neighbour ages slower than people on the tenth floor. We call this gravitational time dilation, and the good news is that you don’t have to know Einstein’s theory of relativity to understand it. Hear us out.

Imagine a speeding up rocket with Alice at the top and Bob at the bottom. Alice’s sending Bob one light pulse per second. As the rocket is moving faster each second, Alice’s every signal travels increasingly a shorter distance. By the way, the speed of light is never changing, strange as it may seem, so Bob must receive signals faster than one per second. Thus, Bob inevitably concludes that Alice’s clock ticks faster than his. And now comes the general equivalence principle: the rocket’s acceleration is indistinguishable from a gravitational pull. So Bob’s conclusion would be the same if he lived on the ground floor and Alice on the tenth! Of course, the time dilation is negligible in ten story buildings or any building on our planet but if you orbit around a black hole and your buddy is closer to its event horizon, you would find out that she’s younger than you when you meet again.