Special Relativity
Inertial reference frames
The first key insight is to approach the problem with the understanding that motion is relative to the inertial reference frame of the observer.
An inertial reference frame is a frame of reference in which a body at rest remains at rest and a body in motion moves at a constant speed in a straight line unless acted upon by an outside force. (source)
In other words, in an inertial reference frame attached to an object that is not accelerating, it is impossible to tell if the object is at rest or moving at a constant velocity.
A blue dot is floating in interstellar space. The only object in sight is a red dot. The red dot is moving at a constant angular velocity around the blue dot.
Problem: Is the blue dot moving around the red dot, or is the red dot moving around the blue dot?
Solution: Trick question—the physics works the same way in both cases!
The speed of light is constant
The distance a photon travels in a given amount of time is constant in every inertial reference frame.
This statement is more complicated than it seems, but that’s a tale for another time.
- is defined as the distance traveled by a photon in 1 second.
- is not a measurement, it is a scientific constant. In fact, the definition of the meter is derived from the distance that light travels in seconds.
- Science has accepted that this value, , is what we have decided to call the speed of light.
It is key to pin the definition of this way because as we will see later, distance and time are not absolute, they are relative. The time or distance can change depending on the observer’s reference frame, but distance traveled by a photon divided by elapsed time is always .