Re: Why General Relativity

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

Larry Smith wrote:
> My question is about Einstein's original motivation for working on GR.  Is
> Greene right [GR = gravitation]
> or did GR start out as an effort to extend SR to accelerated
> frames?

I can't answer the question, but let me suggest a third
hypothesis, namely that he didn't "start out" to do either.

You've heard the story of the movie star who spent 10 years
becoming an overnight sensation?

I've invented a few things, and usually there is a long-drawn-out
process of gathering bits and pieces, not really knowing where
they will lead.

Einstein probably started out like this:

        accelerated     ?      gravitation     ?     .....
          frames

                ?              ?         ?

                    special
                   relativity       ?      .......

i.e. with questionable relationships between all three items,
plus equally-questionable relationships to a zillion other
items not shown.

He must have suspected pretty early on that there was something
fishy with SR in accelerated frames (twin "paradox").  And he
must have suspected pretty early on that Newtonian gravity
and SR didn't get along (instantaneous action at a distance).
And you don't need to be a genius to suspect that gravitation
has something to do with acceleration.

Eventually, bit by bit, the links get filled in.  The
intermediate state looks like:

        accelerated -- --   - gravitation
          frames                /
               \
                \             /

                  \         /
                   \
                    special
                   relativity


At this point life gets a lot easier.  The overall outline
becomes clear.  No more groping in the dark;  there is a
goal.  Progress speeds up dramatically.

The final picture looks like:

        accelerated -------- gravitation
          frames                /
               \               /
                \             /
                 \           /
                  \         /
                   \       /
                    special
                   relativity


To repeat:  I don't know this to be what happened with GR, but
it is a hypothesis that should be considered.

==================

This makes contact with a message in another thread:

John Clement wrote:
> ...  the last step might be using a memorized equation.

Usually, in a problem of the sort you find in the real world, there
is no "last step".  There is an iterative process, going around and
around the problem (e.g. the triangle above) and chipping away at
it from all directions.  Memorized equations might be used at the
7th step, the 23rd step, the 92nd step, and ......