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Time is the apparent progression of events from past to future. While it’s impossible to completely define the nature of time, we all share many common experiences bound by time: Causes lead naturally to effects, we remember the past but not the future and the evolution of time appears to be continuous and irreversible.

Is time relative?

Einstein’s theory of special relativity revealed that the experience of the flow of time is relative to the observer and their situation. Previously, the work of Isaac Newton had assumed the existence of a “master clock” that kept synchronized time throughout the universe. This clock wasn’t really thought to exist, but the concept allowed Newton’s equations to work. The key idea was that all observers could agree on the exact same moment of time, according to the Internet Encyclopedia of Philosophy (opens in new tab).

However, building on work before him, Einstein discovered that the passage of time is relative. In special relativity, moving clocks run slowly; the faster you move in space, the more slowly you progress through time. The closer you get to the speed of light, the greater this effect becomes.

Einstein's theory of special relativity shown with trains. Mark Garlick/Science Photo Library via Getty Images

Einstein showed in his theory of special relativity that two observers cannot agree on simultaneous events. This can be understood by this diagram. On the left, a train carriage is shown with a person, Alice, inside. Alice turns on a light in the middle of the carriage and observes light beams arriving at the two ends of the carriage at the same time, T2. On the right we see the scenario from Bob’s point on view on the platform as the train moves past with velocity v. He sees the two light beams emitted at the same time, just like Alice. However, because the train is moving to the right, the rear of the train intercepts the leftward light first, at time T1 < T2. Meanwhile, the light takes a little longer to strike the front of the train, which it does at time T3 > T1. So from Bob’s perspective, the events which Alice saw to be simultaneous occur one after the other. (Image credit: Mark Garlick/Science Photo Library via Getty Images)

In the decades since Einstein first proposed this concept, physicists have made multiple measurements that demonstrate this effect. An atomic clock aboard a jet airplane will tick at a slower rate than one on the ground. A subatomic particle called a muon doesn’t exist long enough to travel from the atmosphere, where it’s generated when cosmic rays strike air molecules, to the ground. But because muons travel at close to the speed of light, they seem to exist longer from our perspective, allowing them to complete their journey.

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