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Special Relativity

Physics (Year 12)

Einstein's theory of special relativity

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Dev Lohar

Foundational understanding

Special relativity is the theory of space and time in the absence of gravity. It was published in 1905 by Albert Einstein and is able to explain phenomena that Newtonian physics was unable to explain. These include the dilated half-life of muons (which are created in the upper atmosphere), and the differing value of the measured momentum of high speed particles compared to the theoretical value according to Newtonian physics.

To understand special relativity at a high school level, there are 4 definitions which need to be known since they are mentioned frequently. These are; observer, frame of reference, inertial frame of reference, and event.

  • Observer: Any person who is equipped with measuring devices which allows them to measure various physical quantities.

  • Frame of reference: A rigid body which the observer considers to be at rest. For example; the room an observer is in, the spaceship an observer is travelling on, etc.

  • Inertial frame of reference: A frame of reference which does not rotate nor accelerate,

  • Event: It is something that happens at a particular point in space and time.

Einstein put forward 2 postulates which now define special relativity. They are:

  1. Laws of physics are the same in all inertial frame of references.

  2. Speed of light, c, has the same value for all observers in every inertial frame of references.

Effect of special relativity

There are numerous effects due to special relativity. The 4 main effects; time dilation, length contraction, relativistic momentum and energy will be explained in separate sections. Here, we will state the other effects.

  • There is no such thing as absolute velocity because measurements have to be taken relative to another object. Space isn’t considered an object.

  • Simultaneity is relative. This means that 2 events which seem to occur at the same time (ie. simultaneous) for one observer, may occur at different times for another observer.

  • High-speed particles exist for longer compared to the same particles when stationary


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