# Time Machines

#### What time is it? That depends on how fast you're traveling.

Thanks to Einstein, we know that the faster you go, the slower time passes--so a very fast spaceship is a time machine to the future. Five years on a ship traveling at 99 percent the speed of light (2.5 years out and 2.5 years back) corresponds to roughly 36 years on Earth. When the spaceship returned to Earth, the people onboard would come back 31 years in their future--but they would be only five years older than when they left. Indeed, Einstein himself could be alive today! If he could have hopped aboard a spaceship traveling at 99 percent the speed of light in 1879--the year of his birth--he would be only 17 years old upon his return to Earth today.

##### Slowing time

Imagine that each of the six clocks shown started measuring time on the day Einstein was born: March 14, 1879. Five of the clocks were placed in spaceships while one remains on Earth. Each spaceship travels at a different speed and as a result, time has been progressing at different rates on each spaceship. The faster the rate of travel, the more slowly time elapses.

If the people onboard the speeding ships returned to Earth now, they would travel in time from the date shown on the clocks to today--our present, but their future.

Einstein born: March 14, 1879

clock 1: 0% the speed of light

clock 2: 25% the speed of light

clock 3: 50% the speed of light

clock 4: 75% the speed of light

clock 5: 99% the speed of light

clock 6: 99.99999999% the speed of light

CLOCK 1: 0% speed of light (Earth)

Speed (relative to Earth): 0 kilometer/hour

Length of second (relative to Earth): 1.00 second

CLOCK 2: 25% speed of light

Speed (relative to Earth): 25% the speed of light

(75,000 km/sec or 47,000 miles/sec)

Length of second (relative to Earth): 1.03 seconds

CLOCK 3: 50% speed of light

Speed (relative to Earth): 50% the speed of light

(150,000 km/sec or 93,000 miles/sec)

Length of second (relative to Earth): 1.15 seconds

CLOCK 4: 75% speed of light

Speed (relative to Earth): 75% the speed of light

(225,000 km/sec or 140,000 miles/sec)

Length of second (relative to Earth): 1.51 seconds

CLOCK 5: 99% speed of light

Speed (relative to Earth): 99% the speed of light

(297,000 km/sec or 185,000 miles/sec)

Length of second (relative to Earth): 7.09 seconds

Einstein is 17 years old.

CLOCK 6: 99.99999999% speed of light

Speed (relative to Earth): 99.99999999% the speed of light

(299,999.997 km/sec or 185,999.998 miles/sec)

Length of second (relative to Earth): 19.6 hours

Einstein is one day old.