Re: [Phys-L] causation

[Steve Spicklemire <steve@spvi.com>]

Thanks John,

I get what you’re saying about Newton’s Laws. I’m obviously not a philosopher, so I guess I’m somewhat ill equipped in terms of the underlying philosophical background, but I feel that this is mixing two different things:

1) The direction time itself (forwards and backwards, which can be unambiguous determined by looking at entropy)
2) The interactions responsible for “causing” momentum to change (as time progresses in the direction established by “1”).

Maybe the argument is really just about the definition of “cause” ;-).

If I throw a ball in the air, I feel it’s reasonable to say that something “causes” it to come down. Yes, the trajectory can be computed by time reversible equations of motion that involved only the momentum “now” and the force “now", neither preceding the other. But it seems ridiculous to claim that the gravitation attraction between the Earth and the ball is “caused" by it’s changing momentum. Yes, you can infer either if you know the other, but that’s just a simple mathematical consequence of the nature of the relationship. It seems on the face of it what I mean by “cause” here is obvious, and not dependent the time ordering of the “F” and the “dp/dt” which are of course simultaneous. Does that make any sense? Where am I going wrong here?

thanks,
-steve

> The math is not wrong as far as it goes, but it's only half of
> the story:
> 1) Yes, using the differential equation, you can integrate forward in time.
> 2) You can equally well integrate backward in time! Like this:
>
> p_past = p_now - Fnet_now*dt
>
> Bottom line: Newton's laws tell us just as much about the past
> as the future. Since causes must precede effects, Newton's laws
> cannot be statements about causation.