In the Introduction to Electrodynamics book, by David J. Griffiths, 4th edition, page 60, the author makes the following statement:
"it is not the position, velocity, and acceleration of Q right now that matter: electromagnetic "news" travels at the speed of light, so what concerns Q is the position, velocity, and acceleration Q had at some earlier time, when the message left."
In Example 7.9, page 320, he also makes the same statement:
"electromagnetic "news" travels at the speed of light, and at large distances B depends not on the current now, but on the current as it was at some earlier time (indeed, a whole range of earlier times, since different points on the wire are different distances away)."
- In fact, I am struggling with the idea of "electromagnetic news" and its meaning.
- In addition, what do we mean by "at some earlier time," because Griffiths always make this statement throughout his book?
- How could electromagnetic waves depend on quantities, such as the current in Example 7.9, took place "at some earlier time?"
Any help is much appreciated. Thank you so much.