Students are taught that one of the consequences of Einstein’s special theory of relativity is that moving objects contract in length in the direction parallel to their velocity vector. But does this contraction actually happen or is it some sort of illusion? The answer is yes, length contraction actually happens and no, it is not an illusion.
Einstein in 1904 stated (as quoted in the book One, Two, Three … Infinity by George Gamow, 1961 page 97) “we deal here with the contraction of space itself, and all material bodies moving with the same speed contract in the same way simply because they are imbedded in the same contracted space”.
A common misconception is that the high speed of the object causes its length to contract as if caused by some process in the object. Length contraction occurs due to the high relative speed of the reference frames in which the observation is made.
When you measure the length of a rod that you hold at rest in your hands the length that you measure is called the proper length. A meter rule of proper length 1.0 m has a length of 0.44 m according to an observer moving parallel to it at a relative speed of 0.90c. The rule may be at rest and the observer moves at 0.90c or the rule moves at 0.90c and the observer is at rest, the length of the rule according to the observer's reference frame is 0.44 m in both cases. But, if both observer and the rule move at the same speed in the same direction the observer measures its length as 1.0 m as the relative velocity is zero.
As Einstein proposed, an object is measured in space and time and it is the space-time that contracts due to the high relative speed and so an object contracts when it is observed moving at a very high relative speed as it is locked in Einstein’s space-time geometry.
And why does space contract according to reference frames moving at high relative speeds? It must contract to keep the speed of light the same value (invariant) and the laws of Physics the same in all inertial reference frames. These are Einstein’s postulates.
In summary, Einstein’s theory of special relativity predicts that “a moving object appears to shrink in the direction of motion as measured by a stationary observer”. An “observer on a moving train sees only the motion of the light beam” and so the beam travels a greater distance than according to someone at rest who “sees the rear of the train moving towards the light beam causing it to travel a shorter distance”. “As the train approaches the speed of light its length shrinks to nearly zero”. (The quotations are from Stephen Hawking's article, “A Brief Relativity”, in Time Magazine on December 31 1999)