Date: Mon, 2 Dec 2002 16:26:41 -0800 (PST)
From: Jerry Friedman 
Subject: RE: (urth) probable distance from Urth to Bleen


--- Dan'l Danehy-Oakes  wrote:
> > Um, no, Jerry . . . when the Whorl is at rest, the Whorl 
> > people experience time at the same flow as people on Urth or 
> > Blue or Green, but at near-light speeds subjective time (the 
> > passage of time experienced by the crew) is compressed 
> > compared to the objective time of the rest of the universe 
> > (or technically "any other location that is not accelerated 
> > to NAFAL velocities," which happens to encompass most of the 
> > universe).

This "technically" comment is more nearly correct than your
"subjective" and "objective" comments.  You're forgetting the
expansion of the universe, though.  To be nit-pickingly correct,
a great deal of the matter in the universe *is* moving at
relativistic velocities in our reference frame.  So your "any
other location" happens to encompass our galaxy, indeed our
local supercluster and much more, but not the whole universe.

> > This is the basis of the famous "twin paradox."  
> > (I'm not coining the use of "objective" and "subjective" 
> > here: that is how it is done.)  Look into it, if you like, 
> > since it is fascinating!  Or just take my word for it.
> 
> Um, no, Mantis ... in a world that obeys Einstein's rules, 
> "time" is equally [ob|sub]jective in any inertial reference
> frame; more specifically, "time" on the Whorl cruising at
> NAFAL velocities is no more and no less objective than "time" 
> on the surface of Urth. This is a consequence of "time" not 
> being an independent entity in such a world; it is one 
> dimension of a four-or-more-dimensional manifold which we 
> (more) properly refer to as "spacetime."
> 
> The "objective time"/"subjective time" terminology you use
> comes, not from relativistic physics, but from SF writers 
> who play with these concepts in their novels but either 
> do not comprehend what I oversimplify in the previous
> paragraph, or else assume (and I think this quite likely)
> that most "users" of NAFAL ships would think this way, the
> non-absolute nature of spacetime in an Einsteinian universe
> being kind of difficult to wrap one's head around. (I do
> not claim to have my head all the way around it by any
> means!) 

Thanks, Blattid.  I probably shouldn't mention that I have a
Ph. D. in physics, since then people might stop arguing with me
about physics, and as my students could tell you, I do make
mistakes.

There's an interesting and comprehensible discussion of the twin
paradox at .  It contains the comment,
'Relativity here pays the price of permissiveness. It says to us,
"Pick whichever frame you like to describe your results, or use
spacetime diagrams and don't choose a reference frame at all.
They're all equivalent, I don't mind."  No wonder that one
explanation ends up looking like three or four.'

And nothing saying that one reference frame is subjective and
another is objective.

Jerry Friedman

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