Daniel B. answered • 05/11/21
A retired computer professional to teach math, physics
There are at least two questions here.
Question 1:
How long would it take between turning on an electro-magnet and
the magnetic force being experienced by an object.
Answer:
The propagation of magnetic forces happens via electro-magnetic waves,
which propagate just like light, which is also an electro-magnetic wave.
Therefore the effect of the magnet cannot be experienced faster than
the speed of light.
Question 2:
Once the magnetic force reaches the object, could it cause the object
to move faster than the speed of light.
Answer:
No, for the following reason.
Force does not cause speed -- it causes acceleration.
In Newtonian physics, any acceleration after sufficiently large amount of time
can cause arbitrarily large speed. And constant force would maintain constant
acceleration independently of speed.
That is not so in special relativity.
As the speed of the object increases, so does the need for ever increasing
force to maintain the original acceleration.
In the limit, as the speed of the object approaches the speed of light,
the force needed to accelerate it would approach infinity.
