[Phys-L] Re: "moving clock runs slower" (yes)

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

Moses Fayngold asked (me?),

>   Is the speed of a moving car a real characteristic of the process or not?

I'm not sure why you are addressing this or your other questions to
me, but I would have hoped that my message made clear that, at least
for pedagogical purposes in introducing the concepts of relativity, I
would like to refer to them as elements of MY reality, a reality that
is, however, different from the reality of those in other reference
frames.  Nevertheless, I also appreciate the very important kernel of
truth in the position of others who might say that a car cannot
really "have" a speed and that we shouldn't therefore consider it to
be "real."

> If to be real means to be proper, then what is its proper speed?
> Zero, as measured in its rest frame?

I don't understand what you are asking here.  If there is ANY meaning
to the phrase "proper speed" its value surely must always be zero.
So I think I agree with everything you say here and you seem to as
well.

>   Then why does the car smash into smitherenes if it (God forbid!)
> crashes into a concrete wall? Only because the wall runs into it?

That would most certainly be its view of "reality" and it seems to me
that it would be right about that.

>  But, again, the wall's motion is not real either, because its
> proper speed is also zero.

Now I think you are being facetious.  There is no proscription
against two things crashing into each other whether or not you want
to hold that "proper speed" is a useful concept.

>   Will you say that the crash itself was not real because of this?

Only an inveterate nihilist would say something as silly as that.

> If you do not like the example with the speed as I use it here to
> make it more vivid, you can recast the discussion in more rigorous
> terms of relativistic energy E and momentum P suggested by Bob
> Sciamanda, but the question I ask will not disappear.
>   There is still a big confusion between real and absolute (or
> invariant, for that matter). Many people think that what is not
> absolute, but "merely" relative, is thereby not real.

I am not one of them, but I understand their point of view and that
it has its advantages for some purposes.

John Mallinckrodt
Cal Poly Pomona
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