The Velocity Illusion. View the image with the centre covered. Then with the sides covered. Which perception of velocity is correct? Both? Neither?

The Unattainable Speed of Light

How does special relativity add velocities? Welcome to the world where 1 + 1 is not quite 2.

A letter poured in

I received a letter with some questions about special relativity. Here is a part of that letter:

…let’s say we hypothetically were able to send a space ship across space at the speed of light minus 5 mph. Couldn’t a person run from the back of the space ship to the front at 10 mph? — Luke Copko

The implication is that the runner has now broken the light barrier. That’s a no-no in the world of special relativity.

An observer (call him Luke) is on Earth, witnessing this whole scenario. Let’s have another observer (call her Leia) sitting aboard the ship, monitoring the dilithium. Won’t the runner (call him Anakin) be moving at 5 mph above the speed of light?

Here’s what we might expect from Luke’s frame of reference.

Our intuition tells us to add velocities, meaning Anakin moves away from Luke at greater than light speed. Special relativity says no.

How do we explain this apparent breach of relativistic protocol?

Now consider Leia’s frame of reference. Wouldn’t she see Anakin moving one way at 10…