off the top of my head...
Einstein's theory of special relativity states that the speed of light is constant in all frames of reference, therefore any observers in any frame of reference will agree on the speed of light but not length, mass and time measurements.
oh and also... this is c&p from hsc online for that specific dot point;
•The principle of relativity was first stated by Galileo and embodied in Newton’s first law. It states that it is not possible to perform an experiment within an inertial frame of reference to detect the motion of the frame of reference. The only way to detect the motion of an inertial frame of reference is by referring to another frame of reference. For example, if you are in a spacecraft far from any planet, star or other object, then you cannot tell if you are moving. Your rockets have long been turned off and you are coasting along to your destination with uniform velocity. However, without referring to outside objects (for example, triangulating off certain stars over a long time period), it is impossible to measure, or even detect, your velocity.