We have two pairs of interacting charges. They are identical, aside from the fact that the pair on the right starts with an initial velocity to the right. Assume that the right-hand pair is far enough away from the left-hand pair that we can neglect the interaction between the pairs. What we observe, then, is the result of the charges within each pair interacting with one another.

How can we explain what we see based on what we have learned in previous courses?

Can we explain why the charges move away from one another?

Can we understand why the charges on the right move away from one another more slowly?

Now let's add a couple of observers. The blue observer (Jack) starts at rest, like the two charges on the left. The purple observer (Jill) has the same initial velocity as the two charges on the right.

Putting ourselves in Jack's shoes, does everything look all right?

What if we put ourselves in Jill's frame of reference? What will Jill observe?

Does anyone have a problem with this?

I hope some of you do, because there is an apparent contradiction here. We are used to the fact that there is a symmetry in reference frames - it makes sense that when we switch to Jill's frame of reference that what Jill sees is a mirror image of what Jack sees. However, we are also used to the fact that two observers should be able to agree on basic things like which charges are further apart at a given instant. Jack thinks the charges on the left are further apart, while Jill thinks the charges on the right are further apart. Something seems to be wrong with this picture.