Special Relativity

To begin, we develop some intuitions around the consequences of the speed of light being constant and that all velocities are measured relative to some frame of reference (aka, relativity).

To narrow the scope and really dial in our understanding of the fundamentals, consider the special case where all objects are moving at constant velocity, far from any gravity well (aka, special relativity). This is where Einstein started, too.

Let’s follow the logic and see where it takes us.

Einstein’s postulates

Einstein began with two postulates (source):

1. The laws of physics are the same in all inertial reference frames.
2. Light travels in a vacuum with the same speed $c$ in any direction in all inertial frames.

The “laws of physics” in our analysis are then only those that satisfy the first postulate. This also means that there is no “special” inertial reference frame with special properties. There is no “rest frame” or “absolute space”, only motion of one frame relative to another.

The second postulate states that the speed of light has the same definite speed for any observer, regardless of the motion of the source.

So, like Einstein, we will try our best to forget about what we think we know and instead focus on the consequences of the postulates.