Re: What are relativists?

Baugh wrote:

jem wrote:

Baugh wrote:

jem wrote:

Baugh wrote:

Pentcho Valev wrote:

Bryan Wallace http://www.ekkehard-friebe.de/wallace.htm :

"The true scientist must have faith and believe in the scientific
method of testing theories, and not in the theories themselves.






It isn't a faith it is a dicipline.





It isn't a dicipline, it's a discipline. :) 



Thanks, I've got to spell check more often.  My keyboard
spelling is very bad.  (Type too fast and don't spell things out, just
type the "whole word" as per habit).

Good post, but does a relativist take the position that "absolute quantities cannot be defined"? Doesn't any invariant quantity qualify as an absolute quantity? 




What? Like c?  That is a relation between our chosen units for measuring
 spatial distance and our chosen units for measuring time.  It is
comparable to "12 inches per foot".

Consider the mass of the electron?  Again mass is relative to our chosen
common units of space-time, i.e. relative to scale.

But this is I think getting off the point.  Relativists view any
observable, any variable, as relative to the method of determination.
Relative position, relative momentum, relative "state" in QM.

Those quantities such as total spin, or rest energy are not so much
observed as fixed by convention or by assumption.  E.g. total spin
say of an electron is based on the convention of measuring total angular
momentum about a specific point.  (like rest mass is energy in a
specific inertial frame).

What actually goes on in the laboratory is that we note that the
components of total angular momentum of a "spin-1/2" particle
may change by units of hbar/2 rather than say hbar as for "spin-1"
particles.  One then in preparing the beam of electrons which will
travel through the Stern-Gerlach magnet so that their total
angular momentum about some point does not vary by more than one unit
and so you see two output beams reflecting electrons whose total angular
momentum in the x-y plane about any point differ by one half unit.

Remember that to set up an spin measurement the width of the beam (delta
x) needs to be small *and* the spread in the momentum of the beam (delta
P_y) needs to be small.  It is when you idealize an electron as a
classical point particle that you get the illusion that spin is
"intrensic" and thus total spin is an absolute quantity of the electron.

Finally let me say I don't speak for "all relativists"
only for myself with regard to your question.  To answer
your question directly, I don't know.  I would never say
never but give me an example and I'll parse the definition
and see if it is truely "absolute" or if its "absoluteness"
is a matter of convention or restriction in cases.



Your original characterization was that a relativist "takes the position that absolute quantities cannot be defined". Now it appears you've qualified that to "cannot be defined except by convenetion". Since all definitions are conventional, that would tend to make a relativist of everyone who's rational. 


If you define a quantity by convention then it isn't "absolute".
I think we are mixing semantics here.

Re. mixing - I agree, since AFAIK the only way to define *anything* is by convention.

Let me give a sharp example:

We may define lengths as say multiples of the Bhor radius. This as an "absolute measure" presupposes that physical constants are constant. The constancy of hbar, electric constant, electon mass and charge are conventionally fixed and this fixes the Bhor radius.

However you can express the same physical laws by allowing physical constants to vary over (observer) time and so the Bhor radius varies over time. You can do this for example in such a way that the universe
is "not really expanding" but looks that way because of the changing
physical "constants". The red shift of distant stars is "really because atoms where bigger then". It doesn't matter. In the end when
you extrapolate back to the "Big Bang" the universe was small relative
to the size of atoms "because atoms are so darned big with the initial physical 'constants' " and the temperature was "damned hot".

When we measure lengths we measure using solid state rulers, or using
light wavelengths calibrated to atomic spectra and atomic clocks.
These all uniformly depend on the values of the same physical constants.
They are constant by convention and so it becomes from a relativist's
position moot to argue over whether some of these constants "are really varying over time". One need only re-express that variation as an
added variation of the reciprocal observed quanntities such as the
size of the universe and its age.

I.e. the constants are dependent on which model is chosen to represent Nature. So are you distinguishing a relativist as someone who believes Nature can be represented by multiple models, whereas a non-relativist believes there is just one "true" model?

If not, how about giving an example of a hypothetical situation in which an "absolute quantity" would be definable.

Now of course there are other constants such as mass ratios which
may not be varied in this way. However unification theories seek
to reduce such ratios to mathematical constants rather than physical
ones, i.e. one may suppose that there would be in some future theory
a mathematical relationship between the mass of an electron and that
of a proton. This is of course speculation but note that renormalization methods do in fact vary this ratio. 


.

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