Re: [Phys-L] another DIY relativity experiment

[John Denker <jsd@av8n.com>]

On 05/20/2016 06:16 AM, Bob Sciamanda wrote:
> All we can do in "measuring" time or space intervals is to compare
> them - either to each other or, by convention, to some chosen
> "standard".  When I say that a clock "slows down", I can only mean
> that it has slowed in comparison to either another clock, or perhaps
> itself under other circumstances.

I'm not saying that's wrong physics.  It is known that
you can formulate all of relativity (special and general)
in terms of clocks that can't be trusted, rulers that
can't be trusted, et cetera.

I'm just asking, why would you want to?

I say again, it can be done
  http://www.feynmanlectures.caltech.edu/II_42.html
It would be perverse, but not impossible.

In ordinary three-dimensional space, you could define the
notion of arc-length along a path in terms of odometers
that can't be trusted ... but why would you want to?  It's
so much easier to say that the odometers are correct, and
the path-length is just plain different from path to path.

Also note that temperature affects various clocks differently,
so anything that affects all clocks (and all odometers) the
same must be considered a "coincidence" of the kind that is
off-scale implausible ... especially given that a simpler
explanation is readily available.

There was a period in the history of physics when the
state-of-the-art approach to relativity was built around
velocity-dependent rulers, velocity-dependent clocks,
various mutually-inconsistent notions of velocity-dependent
mass, et cetera.  HOWEVER that period ended more than 100
years ago.

This is one of those things that is easier to explain to
the students than to the faculty.  The students are not
heavily invested in the archaic (pre-1908) way of doing
things, and are happy to adopt the modern viewpoint,
including the idea that clocks are like odometers in 
spacetime.

In contrast, those who are heavily invested in the old
way of doing things will have to do a lot of unlearning.
Indeed, Einstein initially did not appreciate or even
understand what Minkowski had done.  Eventually, though,
he figured it out and used it for all of his later work.
Without spacetime, he never would have figured out GR.

I reckon the idea of spacetime is like the idea of horseless
carriages.  Such things have been around for a while now.
We ought to incorporate them into our thinking and our
teaching.