Einstein radically changed our perception of space and time with his two postulates of special relativity. The first postulate states that the laws of physics are the same for observers in any inertial reference frame. The second postulate states that the speed of light in a vacuum is the same in all inertial reference frames.

An event is specified by giving its space and time coordinates. Observers who are in different inertial reference frames will assign different coordinates to the same event.

Observers in a reference frame where two events occur at the same place measure a time interval between those events called the proper time. Observers moving relative to that frame will always measure a longer time interval. This effect is known as time dilation.

The Lorentz transformation equations are used to relate these sets of coordinates. Special relativity predicts that different observers may not agree on the time interval between two events.

The inability to agree on time intervals led to Einstein’s thought experiment about the concept of simultaneity. In this thought experiment, he showed that observers in reference frames moving relative to one another would not agree that there was zero time interval between separated events − in short, they would disagree about whether events occurred simultaneously or not.

Special relativity also correctly predicts that different observers may not agree on the spatial interval between two events. The length of an object at rest in an inertial reference name is known as its proper length. Observers who are moving relative to that frame will always measure a shorter length than the proper length. This effect is known as length contraction.

The Doppler effect applies to light. The astronomer Hubble used this effect to argue that the universe is expanding.

The effect has a purely relativistic aspect, the transverse Doppler effect, due to time dilation.

Measurements of any object’s velocity will vary depending on the inertial reference frame from which the velocity is measured. The Lorentz velocity transformation equation relates velocity values along a line, reported from different frames that move relative to each other along that line.

The intertwining of space and time means that a relativistic definition of linear momentum must be accepted if consistent relations among them are to be retained at both low and high speeds, and for observers in all inertial reference frames.

Special relativity reveals a previously unseen connection between two fundamental concepts: mass and energy.  A mass has a tremendous amount of rest energy locked up, ready to be released either in a terrifying chain reaction or in a controlled, useful manner. Meanwhile, the converse is also true: Energy has a mass equivalent and may be transformed into matter. 

Special relativity also forces a new perspective on the kinetic energy of an object.