From: bhv@areaplg2.corp.mot.com (Bronis Vidugiris)
Subject: Re: Understand...!!!!
Organization: Motorola, CCR&D, CORP, Schaumburg, IL
Date: Mon, 28 Sep 1992 21:00:54 GMT

In article <1992Sep26.204345.1200@prim> prim!dave@germany.eu.net (Dave Griffiths) writes:
)In article <25SEP199209055315@utahep.uta.edu> sawyer@utahep.uta.edu (Lee Sawyer) writes:

)> Posts like this really piss me off. Take a couple of physics
)>courses, then ask "What is energy ?"
)>
)
)That's better than studying the subject for a few years and believing you
)now _know_ what these words mean. I hate to go metaphysical on you, but
)aside from the mathematical definition of time as a variable in your
)calculations, what _is_ time? Given that most physics is time reversible,
)how do you know that you're going forward in time?
)
)Do you believe that fields really _exist_, or are they just a mathematical
)device? If the former, how do you imagine something that is infinite?
)
)Just curious.
)
)Dave Griffiths

I've got a great answer for the question of 'what is time'.  I don't recall
where it came from, but it's really simple:

Time is what you measure with a clock.

Philosophically, this is the viewpoint that once you have something defined,
(such as a way to measure it, for example) it has *some sort* of reality.

When you define a 'clock', you are defining 'time'.  There are lots of way
to build clocks, and an 'ideal' one may be an abstraction as opposed to
any particular implementation, but those are the breaks.  We'll just have
to say that people have some idea of 'clockness', and that we can build
clocks that people generally agree are pretty good ones  nowadays (rubidium
or cesium standards, for example).  People agree that grandfather clocks
with weights aren't really so good - they can be affected by the environment
in ways that 'better' clocks are not.

Anyway, the idea of time as something that is measured by a clock clears up
some of the problems many people have with relativity.

All one has to do is to realize that what a clock measures depends on how
it moves.  This is counter to practical every-day experience, where one
expects _ideal_ clocks to stay synchronized no matter how they move around. 
However, at sufficiently high velocities (and/or gravitational fields), this
difference becomes measurable even for a 'perfect' clock.  (I'm not sure
exatly what practical confirmatory evidence is availble, but I know there is
at least some, - i.e. from particles that wouldn't be expected to live long
enough to penetrate the atmosphere otherwise, but are observed to do so in
practice).

This leads right to the idea of the 'relativity of simultanaeity'.  If
the time elapsed on a clock depends on how it moves, you can no longer
synchronize distant clocks simply by moving them together - you'd have
to specify _how_ they were moved together.  Before relativity, one could
assume clocks would always stay synchronized no matter what - after
relativity, one no longer has this view.

Similar things happen with fields as far as 'reality' goes. In principle,
there is a way to measure an electric field, for instance (the acceleration
of a unit charge), though I'm not aware of any practical absolute methods
(more practical ones measure changes in electric fields).  In this sense, a
field is 'real'.

Fields also turn out to be a useful way to look at reality, (at least IMO)
but that's another issue.
Xref: sparky sci.physics:16008 sci.math:12772