Re: [Phys-L] causation

[Richard Tarara <rtarara@saintmarys.edu>]

I think I'm with Steve here.  First, this is mostly a metaphysical 
discussion and in that light I would go with Bohr's view that our deep 
understandings are based on our experiences.  At the macroscopic level 
we have the experience of causation--I push on the box and then it 
moves--but at the microscopic (quantum) level we are devoid of 
appropriate experiences.  Based on our experiences and our knowledge of 
quantum mechanics, it would 'seem' that there is no causation at that 
level--but do we have enough 'understanding' to be sure.   I also agree 
that just because we can time-reverse an equation doesn't mean that that 
equation relates to reality in that form--as Steve says, we do have the 
'arrow' of time as part of our experience.   Sean Carroll's Great 
Courses lectures on time are worth a listen if you are not really up on 
'time'.

rwt (causing the message to be sent by pushing a button on my computer :-))

On 2/9/2019 10:09 AM, Steve Spicklemire via Phys-l wrote:
> 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.
> _______________________________________________
> Forum for Physics Educators
> Phys-l@mail.phys-l.org
> http://www.phys-l.org/mailman/listinfo/phys-l


--
Richard Tarara
Professor Emeritus
Saint Mary's College

NEW:FREE SELF-PACED COMPUTER BASED
COURSES FOR WINDOWS OR MAC
ENERGY FOR THE 21ST CENTURY
THE PHYSICS OF MOTION
NEW VERSION OF THE ANIMATED CHALKBOARD
THESE AND OTHER FREE PHYSICS INSTRUCTIONAL SOFTWARE AT:
http://sites.saintmarys.edu/~rtarara/software.html