Imagine two friends, Alice and Bob, are observing a fireworks display from different locations. Alice is on a hilltop, while Bob is in a valley. Both claim they saw the fireworks explode at different times. How can Einstein's Theory of Relativity explain their different perceptions of the event?

Einstein’s Theory of Relativity tells us that time is not absolute; it depends on how fast you are moving and how strong gravity is where you are. Alice, standing on the hilltop, is farther from Earth’s gravity, so her clock runs a tiny bit faster than Bob’s clock in the deeper valley, where gravity pulls harder and slows time slightly. Because Alice’s clock ticks faster, the fireworks fire at a time that appears later to her than to Bob, who sees the event earlier in his slower clock. Thus the two friends can legitimately disagree on the fireworks’ timing simply because gravity makes their clocks run at different rates, even though the fireworks themselves exploded at the same instant in the universe.