Re: special relativity: accelerated frames

["John S. Denker" <jsd@MONMOUTH.COM>]

At 08:20 AM 5/3/01 -0400, Chuck Britton wrote:

> I choose my car to be my RF, and notice that the highway, trees
> etc are going past me at a reasonable constant speed of 20 m/s and I
> suddenly mash down on the middle pedal of my car.

OK.

> I notice that the stuff piled on the back seat of my car 'rushes
> forward' onto the floor,

OK.

> (presumably, somehow pulled forward by the outside world)

It is fashionable to presume that.  That's Mach's principle.

However... AFAICT Mach's principle makes no useful predictions.  Indeed, if
it makes any predictions at all, it predicts that if you removed the
"outside world" the stuff in the car would behave differently -- which is
not true.

It would be better to say that the stuff rushes forward according to its
own inertia and momentum.

> but the things outside my car seem to be indifferent
> to my having stopped the world from moving.

As David Bowman pointed out, stuff inside and outside behaves essentially
the same.  It may help to consider a bird outside the car, initially
comoving at 20 m/s.  When you apply the brakes, the bird will accelerate
forward relative to your reference frame.

The major difference is that the stuff inside the car undergoes complicated
physics-of-materials interactions as it collides with the seats,
floorboards, and other parts of the car.  Gravitation is simple.  Materials
science is complicated.

========

Another issue here:  The "field" that causes stuff to rush forward when you
decelerate your frame of reference affects everything equally.  No Eötvös
experiment could detect a difference in the acceleration of the trees
relative to the highway.  Indeed no experiment could detect anything, which
is why the world is "indifferent".

> (I ALSO have to wonder how my little Saturn is able give the highway
> such a tremendous amount of KE and have the brakes absorb it - but
> Leigh would just say that I am reifying energy and should stop DOING
> that!!  :-)

KE is not frame-independent.  The KE of a northbound baseball is less to a
northbound-moving observer and greater to a southbound-moving observer.  No
energy transfer is required to make this true.  Just as the (X,Y)
components of an objects's position are mixed up by a rotation of the
observer (with no change in the object), the (E,P) components are mixed up
by a boost of the observer (again with no change in the object).

Asking questions about local energy balance is smart and you should NOT
stop doing that.