# The Speed of Light is NOT Constant

It’s often said that in the context of special relativity, the speed of light is constant in all frames of reference. In other words, everyone will measure the speed of light to be the same speed regardless of how fast they’re moving. This has some important implications, like the helicity of photons being an invariant quantity. However, it turns out that the speed of light *isn’t* constant in all frames of reference. It’s even possible to choose a frame of reference where the speed of light is one quadrillion meters per second. Here’s why the speed of light isn’t constant.

# What’s a frame of reference?

In order to understand what follows, we need to understand what a “frame of reference” is. A frame of reference is really just a coordinate system where we attach physical variables. Say you’re working with a coordinate system where you’re at the origin and you see a car moving to the right at 5 meters per second. We can also choose another coordinate system where the car is at the origin of that coordinate system, and that coordinate system moves along with the car. In this new coordinate system, you would be described as moving 5 meters per second to the left where the car is stationary.