Time Dilation

Relative Clocks

Time runs slower in anything that is moving relative to you.


Time runs more slowly in anything that is moving relative to you. This effect is called time dilation. The faster the object is moving the slower time will pass in that object relative to your time.

Gravity also affects the flow of time and so we are considering a region of space far from the influence of strong gravity.

The proof for time dilation relies on the velocity of light being a constant. The following are a list of points to be mindful of when studying the animation.

  • There are two identical clocks. One stationary relative to you and the other on the spacecraft.
  • The two clocks mark off time using photons. Set the spacecraft speed to zero to see that the two clocks mark off time at the same rate.
  • Each time a photon hits a mirror an event is created and the respective clock increments one unit of time.
  • An event is anything with a precise time and location. Like a photon hitting a mirror.
  • The spacecraft velocity has no effect on the photon speed relative to you. The direction of the photon changes, the speed does not.
  • The photon, in the spacecraft, has further to travel between mirrors the faster the spacecraft travels.
  • The spacecraft clock ticks slower because the photon speed is always c, while the distance travelled by the photon between mirrors increases with spacecraft velocity.

animation controls

The animation has a number of controls to highlight the components that combine together to create the dilation of time in the spacecraft. After selecting a control vary the velocity of the spacecraft to observe the effect.

  • photon trail: Each photon leaves a trail. This makes it easy to measure how far the photons have traveled between events.
  • velocity vector: Vectors are draw to illustrate the direction of travel. The photon vector is always c in length, while the spacecraft vector length can be changed.
  • show events: The events created by the photon reflecting off the mirrors are highlighted. These events must be maintained irrespective of the spacecraft velocity.
  • length contraction: Length contraction is a relativitist effect that contracts the length of the spacecraft. This will be dealt with later in the course.


Time dilation is the slowing of time in anything that is moving relative to you. The animation illustrates the effect by using hypothetical clocks that use a photon to create the events that mark off time. The slowing of time is applicable to this inanimate photon clock, and to animate objects as well. A person in the spacecraft would also age more slowly relative to you. Like the events created by the photon reflecting off the mirrors, the aging process is also a sequence of events at a microscopic level. The communication between atoms in an aging person is all achieved through the exchange of photons. Events can be said to be created at the creation of a photon and the absorption of a photon. The process is not unlike the photon clock. The conclusion is if the photon clock slows then the aging process slows.

In a relative universe someone in their spacecraft floating in space will feel stationary and the laws of physics for them should apply to every other spacecraft floating in space. However their may be a relative velocity between spacecrafts and so everyone in their spacecraft will say that every other clock, in every other spacecraft, is running slow. This seems to be a paradox. How can everyone say that every other clock is running slow? The paradox is resolved when the relativistic effects of lose of simultaneity and length contraction are applied.