[Physics] the physical definition of causality


Maxwell's equations give a physical relationship between the electric and magnetic fields $\vec E$, $\vec B$ at the same time, which some interpret as changes in one causes changes in the other etc. I find this confusing because to me, the cause of both is charge and cause should precede effect.

Therefore, how do physicists determine if there is a causal relationship between two physical quantities?

Best Answer

As far as the electric and magnetic fields go, neither one is the cause/effect of the other changing, since the very presence of an electric or magnetic field depends on your frame of reference. Rather there is a single quantity changing, the field strength tensor, $F_{\alpha\beta}$. A change in one doesn't cause a change in the other, simply one thing changes.

Causality is a extremely gigantic subject, and people have different interpretations of what it means. I think a fairly classical description is that events can only be causally connected, if the event under consideration as the cause is in the past light cone of the event that is considered the effect, and visa versa (except with the future light cone). Then causation is determined primarily through inductive reasoning and correlation, though the arrow need not be $\iff$, rather I believe the necessary condition is correlation, that is $\text{causation}\implies\text{correlation}$. More frankly, if you flip the light switch and a light comes on and those two events are in each others light cones, and you inductively verify after numerous different experiments that those two events are correlated, then you imply causation (note it's a deductive logical fallacy, but its completely legitimate as an inductive method, vis-'a-vi by the method explained above). Though, you wouldn't do that with the light would you...

I hope this helps.

Related Question