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## TIME

We use time to say when an event happened or how long it lasted. Time seems to pass at the same rate for everyone, but Einstein’s theory of RELATIVITY shows that time is not constant throughout the Universe.

## HOW DO WE MEASURE TIME?

The steady ticking of a clock marks the passing of time. An accurate clock is controlled by something that repeats at a precise, unchanging interval. Early clocks were set by the swing of a pendulum. Modern clocks are set by the vibrations of a quartz crystal.

## DOES TIME BEGIN AND END?

Time began when the Universe was created, in the Big Bang about 14 billion years ago. The Universe is currently expanding. Scientists are not sure if the Universe will expand forever, so that time never ends, or if it will collapse in a “Big Crunch,” stopping time for good.

## BIOGRAPHY: ALBERT EINSTEIN German, 1879–1955

Albert Einstein did not do very well in school, but was fascinated by mathematics and science. The Special Theory of Relativity (1905), the General Theory of Relativity (1916), and his work on the quantum theory of light established him as one of the most original and creative thinkers of all time. He was awarded the Nobel Prize for Physics in 1921.

## RELATIVITY

Einstein’s theory of relativity states that time is not the same for everyone. Time passes more slowly if you are traveling very fast, almost at the speed of light.

### HOW DOES RELATIVITY WORK?

If a person watches two identical clocks, one stationary and one traveling at high speed, the moving clock ticks more slowly. To another person traveling with the moving clock, the other clock appears to be moving, and ticking, more slowly.

#### SHORTCUT THROUGH SPACE AND TIME

Scientists have shown that, in theory, two distant parts of the Universe could be linked by a tunnel through space and time called a wormhole. A wormhole might work as a time machine. By making a round-trip journey through the tunnel, you could arrive home before you left.

## FIND OUT MORE

 Light Quantum Theory Space Universe