Think about an electric charged object moves linely, it will produce current hence the magnetic field. But if the obserser moves together with the electron charged object, so there is no relative movement between them, that means the electric charged object is static to the observer, hence there is no current and no magnetic field to the observer. What does that mean? It means that if you bound a magnetic field detector with the object, the detector will not detect the magnetic filed, but if you seperate them, the detector will detect the magnetic field! That is unbelievable! In fact it was an done experiment by a group of researchers and it proved the bounded detector could detect the magnetic field. I think that means "ether" may be exist, and a lot of concepts might be different as defined, like space, speed, time etc.