# Retarded positions, extrapolation vs. hitting a wall

1. Apr 19, 2012

### isthisrealife

I'm a compsci guy reading Feynmann's lectures leisurely. I find the retarded-field equations somewhat shocking due to the consequences of nature "extrapolating" trajectories to find the fields. Is my understanding of the following scenario correct?

Let's say I have an "electrostatic compass" (like a compass, but the needles have opposite charges). An object with large net charge is moving around me in a circle. It need not be moving very fast at all; all that really matters is that the radius of its motion is very large. Say the object also emits light. Now according to the equations for the field produced by this moving charge, the coloumb force will appear to originate from a point extrapolated by the most recent position, velocity, and acceleration I was able to "see" (pg 21-1, vol2 of feynmann's lectures)

Thus, I will "see" my compass needle actually pointed ahead of where I actually see the object (since I see the object at its retarded position, while the coloumb force originates from this extrapolated position).

Already this is weird, but now suppose someone comes and puts a massive wall in that objects path. I can see the object is going to collide with the wall. But on the other hand, the compass can't see the wall. So, in the moment just before I actually see the object collide with the wall, I will see the compass pointing past the wall, as if the object went straight through it. Once I see the collision, the new velocity has been propogated to me and the compass will correct itself. But for the moment just before I see the collision, my compass will point past the wall as if the object went straight through it. Is this correct?

I find that stunning -- is there some published experiment which has actually observed something like that?

Last edited: Apr 19, 2012