Time Dilation: Description, Explanation & Examples

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  • 0:05 Relativity
  • 0:34 Is Time Absolute or Relative?
  • 1:41 Effect of Speed on Time
  • 3:07 Time Dilation Is Relative
  • 3:47 Light Speed Is Not Relative
  • 6:32 Do We Observe Time Dilation?
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Time slows down for objects in motion. While this effect is not noticeable in everyday experience, it becomes apparent at speeds approaching the speed of light. This lesson defines this phenomenon as time dilation. Examples are used to help explain how time is relative to speed.


The theory of relativity is used to account for observations from different perspectives. For example, objects appear to be smaller when they are viewed from a distance. Additionally, motion is relative to the perspective of the observer, as objects in a car appear to be still from the perspective of within the car.

Is Time Absolute or Relative?

Atomic clocks on fast-moving planes show time moving slower than do atomic clocks sitting on earth
Time on Plane Moves Slower

What about time? Is time an absolute value? Or does time change depending on how we look at it? In other words, is a second the same for every observer, regardless of their perspective? Well, as it turns out, the answer to this question is no. Rather, time is relative - time slows down for objects approaching the speed of light.

In the 1970s, scientists did an experiment on the effect of speed on time. They put atomic clocks on fast-moving planes and had them fly around the earth twice. When compared with the same atomic clocks sitting on earth, the clocks on the planes slowed down. In other words, not as much time had passed on the planes. Once again, time slows down when you're going really fast (close to the speed of light). Humans have not come anywhere close to approaching the speed of light, so we have not had the opportunity to experience this phenomenon directly.

Effect of Speed on Time

How can time slow down at really fast speeds? Well, to answer this question, we need to consider how we measure time. Time is measured by the observation of repeating events. All clocks record time with some repeating event, such as a pendulum moving back and forth in a grandfather clock.

More sophisticated clocks use light beams bouncing between two mirrors as the repeating event to measure time. Each time the light bounces off the mirror, the clock ticks. Because light maintains the same speed, you don't have to wind up this clock - it keeps on ticking.

As long as your clock is not moving, the light will move the same distance between the mirrors with each pass. However, if the clock moves, the beam of light will travel a greater distance with each pass between the mirrors. If we accept that each pass of light between the mirrors is the same unit of time - a second, for example - then the same unit of time now takes longer to pass when the clock is in motion. In other words, time slows down. This slowing of time at high speeds is referred to as time dilation.

Light beams used to measure time in clocks travel a greater distance if the clock moves
Movement Affects Time

Time Dilation is Relative

It is important to note that time dilation is relative to the perspective of the observer. In other words, if you were to travel in the spaceship with the clock, you would notice no time dilation. Everything would seem normal to you. From your perspective, the beams of light would be traveling back and forth between the mirrors just as if you were sitting still. However, from a stationary perspective, time slows down. In other words, only the stationary observer observes time dilation in the moving ship.

Light Speed is Not Relative

Time dilation helps explain how the speed of light doesn't change with perspective. The speed of light is abbreviated by the letter c, and it's approximately 300,000 kilometers per second (kps) - that's pretty fast. Whether the observer is moving at speeds close to the speed of light or stationary, the speed of light is always measured as c. To explain how this works, let's look at an example.

Imagine you are positioned on earth and you observe two spaceships. One ship is not moving, relative to the earth, while the other is traveling close to the speed of light - let's say 250,000 kps. If each ship fires a laser of light, how fast would you record each laser of light?

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