[Phys-L] causality

[John Denker <jsd@av8n.com>]

On 2/7/19 12:49 PM, Anthony Lapinski wrote:

> Still having some issues with causality.

It's tricky.

> To me, a change in area should make the fluid velocity change, not
> the other way around.

That's not what the equations say.
That's not how the fundamental physics works.

> Like a net force makes an object accelerate

Again, that's not what the equations say.
That's not how the fundamental physics works.

Useful counterexample:  Consider a centrifuge.  You control
the acceleration.  Knowing the acceleration allows you to infer
the force on the payload.  Does this mean the acceleration
"causes" the force?

I insist that F=ma means exactly the same thing as a=F/m.
Neither has anything to say about causality.
The force and the acceleration happen at the same time,
whereas a cause normally precedes the effect.

In a great many situations, when somebody asks "why", the
question is at best ambiguous.  Very often it is better
to steer the conversation away from causality.  Usually
the interesting question is /how do you know/.
 -- Sometimes if you know the force, you can infer the acceleration.
 -- Sometimes if you know the acceleration, you can infer the force.
 ++ Neither of those has anything to do with causing the force or
  causing the acceleration;  if anything, they have to do with causing
  the *inference* ... but that's a completely different issue.

> or a voltage produces a current.

Same song, different verse.
 -- Sometimes you hook a resistor to a constant-voltage supply.
  This allows you to infer the current.
 -- Sometimes you hook a resistor to a constant-current supply.
  This allows you to infer the voltage.
 ++ Neither of those says anything about causation.


For the next level of detail, see:
  https://www.av8n.com/physics/causation.htm