Michael has said much of what I want to say. The problem is purely
semantic - a question of what to call *real*. I too single out the *proper*
length of a "rigid" object and the *proper* period of a clock as useful
invariant properties of certain objects. I have a problem when "proper" is
used as a synonym for "real", rendering other length and period measurements
unreal and only appearances. Is this real/appearance distinction restricted
to only clocks and "rigid" rods?
I would indeed find this language more meaningful if one could give it a
universal definition - ie. if one could define the "proper/real" time and
distance separations between two arbitrary events in space time. This would
give the concepts "proper/real" a universal philosophical meaning rather
than restrict the "reality" concept to events on the world lines of clocks
and "rigid" rods.
Bob Sciamanda
Physics, Edinboro Univ of PA (Em) http://www.winbeam.com/~trebor/
trebor@winbeam.com
----- Original Message -----
From: "Michael Edmiston" <edmiston@BLUFFTON.EDU>
To: <PHYS-L@LISTS.NAU.EDU>
Sent: Wednesday, September 14, 2005 2:26 AM
Subject: Re: "moving clock runs slower" (yes)
|I don't think I have a problem thinking in terms of proper mass, proper
| length, proper time being invariant and that we could just drop the
| "proper" and call them mass, length, time. I don't have a problem with
| the notion of projections and appearances. I think the problem I have
| is knowing what to call "real."
|
| Let me give some examples.
|
| A colleague begins in my frame and we discuss some measurements we would
| like to make. My colleague then accelerates to a different frame and
| observes the same events I do. We both record our measurements in our
| lab notebooks. She will record different data in her lab notebook than
| what I record... and this includes different lengths, different masses,
| and different times for objects and events that we can agree were the
| same objects and events.
|
| I don't have a problem realizing that upon getting together later we
| notice our lab notebooks really do have different data recorded in them
| even though we were observing the same objects and events. Indeed, I
| could have predicted, based upon my measurements and my knowledge of our
| relative velocity, what her lab notebook would say before I actually
| looked at it. But that doesn't change the fact that when we get
| together to compare our notebooks that they really are different. Thus,
| as a result of my colleague having left my frame, having made some of
| the same measurements I made, then having returned to my frame, there is
| a lasting difference... the data recorded in our notebooks.
|
| Neither notebook is wrong. Neither notebook is better than the other.
| I would be inclined to say they both represent reality even though they
| differ. But I think that bugs people. How can two notebooks describing
| the same events both be "the real data" if they don't agree with each
| other? The common perception of "real" would require us to declare one
| notebook as "the real one." I think the fact we can't do that requires
| a broader definition of "real" than some people want to accept.
|
| The example just given just describes what is written in notebooks. The
| second example describes how we build an instrument. When we build an
| accelerator, the portions of the accelerator that handle the
| higher-speed particles need relativistic corrections. For a linear
| accelerator we either have to build the drift chambers longer than we
| would calculate classically, or we need to slow down the oscillator as a
| burst of particles travels through the classically calculated chambers.
| For a cyclotron we likewise have to slow down the oscillator or we have
| to increase the magnetic field at larger radii. For both machines it is
| common to keep the oscillator constant. Therefore the later drift
| chambers in a linear accelerator are indeed designed longer than a
| classical calculation would dictate, and the magnet on a sector-focused
| cyclotron has the outer high-field regions expanding in size beyond an
| ordinary pie-shape.
|
| It is common to refer to this requirement as an adjustment for the
| increased mass of the particles as they reach higher and higher speeds.
| It's okay with me if we want to say mass is invariant and that our
| measurements of the masses of high-speed particles are different than
| the proper mass because they are in a different frame from us. But we
| still have sitting in front of us the cyclotron with sectors made
| broader to accommodate "the increasing mass." The cyclotron is real,
| the widening sectors are real, it works when designed that way, it
| doesn't work if not designed that way. Thus, it seems to me the
| increased mass is just as real as is the notion that mass is invariant.
| Again, two real descriptions of the same objects even though the
| descriptions differ.
|
| Finally, the "twin paradox." My twin takes a long high-speed trip. We
| know she took the trip (and I did not)because she changed frames three
| times... (1) my frame to an outbound frame, (2) the outbound frame to an
| inbound frame, (3) the inbound frame back to my frame. Upon her return
| here we sit with the lasting difference that she is younger than I am.
| It's okay with me if we want to say her clock did not run slow because
| her clock was her proper time. However, in the end we have the lasting
| difference that she is now physically younger than I am. I like John
| D's description of this that we started at the same place, ended up at
| the same place, but she took a different space-time path to get here
| than I took. As a result of the different paths we arrived here at
| different times in our lives.
|
| These three examples are a kind of progression. The numbers recorded in
| the notebook are just measurements, and you might describe the
| differences as being different projections of a single reality. The
| particle accelerator seems to require thinking of the difference as more
| than just a projection, although I suppose you could still define
| projection to include this. That is, the different projection involves
| more than human perception; it forces a tangible design change in an
| instrument. My twin being younger really seems to stretch the
| projection idea. I repeat...as a result of the different spacetime
| paths we arrived here at different times in our lives. How does
| projection fit into this? Is she not younger but just looks younger?
|
| In the end I have difficulty with reality and projections and
| appearances. If we describe relativistic differences as different
| projections yielding different appearances that seems to connote they
| are not real. Yet here we sit with notebooks having different numbers
| recorded in them (and both are correct)... with accelerators that only
| work when tangible changes are made in their construction to accommodate
| "the appearance of increasing mass"... and here I sit with my twin
| sister whose hair is not as gray as mine.
|
| Are these differences in appearance? Differences in reality? Both?.
| Does relativity ultimately mean we have to accept multiple descriptions
| of reality? Phrased differently... do we have to say that the lack of
| an absolute reference frame also implies an inability to have a single
| description of reality?
|
| Michael D. Edmiston, Ph.D.
| Professor of Physics and Chemistry
| Bluffton University
| Bluffton, OH 45817
| (419)-358-3270
| edmiston@bluffton.edu
|
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