What time is it? | Live Science

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

Is time relative?

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

But by building on the work ahead of him, Einstein discovered that the passage of time is relative. In the particular theory of relativity, moving clocks run slowly; the faster you move in space, the slower you go through time. The closer you get to the speed of light, the greater this effect.

Einstein showed in his special theory of relativity that two observers cannot agree on simultaneous events. This can be understood from this diagram. To the left is a train carriage with a person, Alice, inside. Alice lights a candle in the center of the carriage and observes rays of light arriving at the two ends of the carriage at the same time, T2. To the right we see the scenario from Bob’s point of view on the platform, while the train is passing at speed v. He sees the two rays of light emitted at the same time, just like Alice. But because the train is moving to the right, the rear of the train first intercepts the left-hand light, at time T1 < T2. Imens tager lyset lidt længere tid om at ramme fronten af ​​toget, hvilket det gør på tidspunktet T3 > T1. Then from Bob’s perspective, the events that Alice saw as simultaneous arise, 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 several measurements showing this effect. An atomic clock on board a jet will tick slower than one on Earth. A subatomic particle called a muon does not exist long enough to travel from the atmosphere where it is formed when cosmic rays beat air molecules to the ground. But because muons travel close to the speed of light, they appear to exist farther from our perspective, allowing them to complete their journey.

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