If you run 10mph inside of a train that is moving 100mph, are you moving 110mph or 100mph?
In: Physics
Look at this Einstein over here! No, seriously, this is what bugged Einstein.
To an observer in a reference frame where the train is going at a 100mph you will appear to be going 110 mph. To an observer on the train you will appear to be going 10 mph. To an observer in a car going 50 mph in the same direction you will look like you’re going 60 mph. Conversely, you will see the person standing next to the tracks zooming back at 110 mph and other passengers in the train going back at 10 mph.
This is Galilean (or classical) relativity. Perceived speed is 100% relative to the observer’s reference frame.
But then Einstein looked at [Maxwell’s equations ](https://en.wikipedia.org/wiki/Maxwell%27s_equations)and realized that for all those observers, *the speed of light is constant.* So if you are a photon running at the speed of light on that train, you are going at the same speed for all those observers! To have this make any sense you need to accept the fact that as you accelerate time dilates and space contracts, but that’s above my ELI5 abilities.
But, as a result of this, if you run 10 miles per hour on the train, the “stationary” observer will see you as going VERY slightly less than 110 mph.
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