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Time Dilation at Relativistic Speeds

Updated on May 2, 2016

Time Dilation and Light Speed

Have you ever read Ender’s Game? Were you ever confused on how Ender stayed young in space travel, but everyone else on Earth got old? This is using Einstein’s Theory of Relativity, and that scenario may become completely possible in years to come.

Einstein first made the Theory of Relativity to explain why the speed of light remains constant for all people, no matter how fast they are moving. For example, if a bus is moving 20 mph, then to a person standing still, the bus seems to move at that same speed- 20 mph. However, if I am riding a bike at 10 mph in the same direction as the bus, then relative to me, the bus is only moving at 10 mph. The speed of light is not like that. The speed of light is 186,000 miles per second. If a person is standing still, they see it at that speed. If I am riding a rocket ship in the same direction as the light at 100,000 miles per second, then it should seem to move at only 86,000 miles per second. However, it is still moving at 186,000 miles per second relative to me and the stationary observer at the same time. In order to explain this strange phenomenon, Einstein came up with his Theory of Relativity which states that if you are moving, then time slows down. Many different people have tested this theory and proved it correct. For example, one scientist named Joseph C. Hafele teamed up with another scientist named Richard E. Keating. In order to test the effects of this time dilation, they put one atomic clock on the ground (for a constant), and they put two more atomic clocks in two different airplanes travelling around the world. One airplane was going west and the other one was going east. What he found after was that the clock on the airplane going east (with the rotation of the earth) was on an earlier time than the clock staying on the ground, and that the clock on the airplane going west (against the rotation of the earth) was at a time that was ahead of the other two clocks. Since it was going against the rotation of the earth, the stationary clock had a greater velocity. This time difference was so slight it had to be tested to make sure they were different, but there actually was a difference. This is called the Hafele-Keating experiment.

Some of you are probably wondering, “How can the time on the clocks change?”. The answer is called time dilation.. Let’s start with a basic truth: the speed of light has to remain constant for all people. It’s a law of nature. It is impossible from your own perspective to reach the speed of light. No matter how fast you are going, light will always be moving 186,000 miles per second faster than you. Since it has to remain constant for all people, then if you are moving, time slows down for you. Usually, this time dilation is infinitesimally small, much smaller than can be measured. However, the faster you go, the more that time slows down. This happens because, as I said, the speed of light has to remain constant. For example: You are in a spaceship moving at 50,000 miles per second. Since the speed of light is 186,000 miles per second, then to you, the speed of light should seen to move at 136,000 miles per second. However, that defies that natural rule. So, time slows down for you. To you, it still seems that you are moving at 50,000 miles per second, but when you measure the speed of light, you find something strange; it is still going 186,000 miles per second. In order to make up for this, (because as of now, the speed of light is supposedly moving at 236,000 miles per second) nature makes time slow down for you. Let’s say that your spaceship lands on another planet after your journey. To you, the journey only took 2 hours. However, after you land, you find that your best friend, who was the same age as you, is now several months or years older. The journey only took 2 hours to you, according to the clocks on board the spaceship, but to them, it seemed to take you months or years to reach them. This brings up an interesting phenomenon. Which clock is right? The one on the spaceship, or the one on the planet? The answer: they’re both right. That shows how amazing time dilation is. You were moving through two different times at the same time! To you, two hours passed; to others, more than a year passed. Both of you are right. You have lived through more than a year in the space of 2 hours. You have basically moved through a time wormhole.

Is it possible to travel at the speed of light? Not according to your perspective. Lots of movies get this wrong. In some movies, they move at light speed and emerge happy on a different planet. However, in real life, this isn’t even possible. Even if you got up to speeds that motionless people see light traveling at (186,000 miles per second from your own perspective), then once you came out of hyperdrive by a different planet, everyone you know would probably be dead. Even their children would have grown up a lot, perhaps to your same age. In real life, theoretically, you can get up to huge amounts of speed, but from others’ perspectives, you aren’t moving as fast as you see yourself moving. This is one of the complicated things about time dilation. It always depends on perspective and your speeds relative to something. From the example above, from your perspective, you were moving very fast. You got to another planet in two hours. However, to other people, more than a year passed. Let’s compare this to a superhero. Suppose you had super speed. If a bad guy tried to punch you and you went into your super speed, the bad guy would seem to slow down, which gives you time to run away. From the bad guy’s perspective, however, he just threw a punch and you zoomed away at super speed. No time change occurred for him whatsoever. Same thing with the example above. You believe that you are moving at a very fast speed, but to others, you seem to be moving slower than you perceive. Let’s say your rocket travels 100,000 miles. To you, this would take about 2 seconds. However, other people who are seeing you move at a slower speed see you at, let’s say, 1/100 your speed. They would see you cross that 100,000 mile gap in 200 seconds, or 3 minutes and 20 seconds. So, this short voyage took up 2 seconds of your life, but 200 of theirs. This isn’t a huge difference, but imagine if you were travelling that fast for an extended period of time. A day, perhaps? Or a year? If you spend a year at those speeds, then everybody else would have aged 100 years (remember, all this is just a rough example).

All these things are essentially the basics of why time dilation occurs while moving at relativistic speeds (or speeds that are very, very fast). None of this is quite possible as of yet, but who knows! Humans might achieve these kinds of speeds in the future- but to us, we will only see them in slow motion.


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