Special Relativity

The results that flow from the principles of special relativity are remarkable. Using thought experiments, Einstein showed that the speed of light is the same in all frames of reference, that means anywhere in the Universe.
The measures of space and time in one reference frame must be transformed to those in another reference frame. These transformations show us that when an object moves at a high speed relative to an observer, the observer sees LESS of its length - this is know as LORENTZ CONTRACTION. Also, TIME for such an object appears to run more slowly - an effect called TIME DILATION.

TIME DILATION:

Special relativity predicts that an Earthbound observer sees time slow down on board a spacecraft travelling at close to the speed of light relative to EARTH. At 90% of light-speed, the passage of TIME is halved - a clock on the spaceship advances only 10 minutes while more than 20 minutes pass on EARTH. This effect is symmetrical and can be reversed. This is because there is no absolute frame of reference. It's impossible to see everything as it is, when it is/was.

So measurements of time and space vary between moving reference frames. Einstein also showed that an object gains mass when its energy increases and loses it when its energy decreases. This ultimately led him to realise that mass and energy have an equivalence, which he expressed in his infamous equation E=MC2.