 So let's say I'm sitting in a train. I reach into my pockets and I pull out Two positive charges. I hold these charges next to each other like this What's gonna happen? They fly apart and repel each other. Okay. Now, let's look at the situation again from Bob's reference frame Now Bob is standing on the platform watching this train was by and when I hold the charges like this From Bob's point of view, these charges are moving forward with the train Now moving charges Constituted an electric current and two currents moving in the same direction attract each other So even Bob's reference frame if he goes through the calculations and the laws of electromagnetism There should be a very very weak attractive force between these two charges And as the train gets very very very fast the current gets stronger and stronger because the charges are moving more quickly and This attractive force should become quite noticeable It wouldn't be enough to overpower the repulsion But it'll be enough to make it noticeable that they're repelling each other more slowly And in fact if this train train were to accelerate all the way to the speed of light The two charges wouldn't repel each other at all. The attraction due to the current would perfectly cancel The repulsion due to their positive natures So what this means is me sitting on the train. I just see two stationary charges together. They repel But Bob standing on the platform Sees two charges either repel each other slowly or maybe even not even repel each other at all How can this be right? I mean who is right? Bob, right? Or am I right? Now as we established in the last section the laws of physics should be valid in any reference frame There's no reason to pick Bob's reference frame of a mind You might think look we can definitely say me on the train. I'm moving while Bob on the platform He's not but Bob standing on the earth and an astronaut in space would say no Bob spinning around the Sun at 30 kilometers per second How can he possibly claim to be the one reference frame? That's correct So who's correct Bob on the platform or me on the train to find out we need to derive special relativity