Re: lawless physics (fwd)

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

On Mon, 16 Mar 1998, Bob Sciamanda wrote:

> I would add that there is a restriction on getting from one "event" to
> another;  something which might well "possibly matter to an individual".
> A space traveler leaving earth might well be interested in what earth
> events he can return to, for example.

Bob,

Of course you are right.  When one departs from home at high 
relative speed for any appreciable amount of time (with 
"appreciable" approaching zero as the relative speed approaches 
c!), one forfeits the ability to return within the life span of 
loved ones.  I illustrate this point in class as part of a 
discussion of what life might be like if the speed of light were 
60 mph.  Kiss your spouse goodbye in the morning, get on the 
freeway to go to work, and when you return that evening ...   

By referring to "all that can possibly matter to a traveler," I 
was addressing *only* the question of whether we are subject to a 
"speed limit."  IMO it is highly misleading, the source of much 
confusion, but still common to infer that relativity imposes a 
speed limit on *us.*  It does no such thing.  If anything, it 
teaches us to discard entirely the notion of "personal velocity."  
We don't "travel" to a distant galaxy; it travels to us.  
Relativity imposes a limit only on the speed that we can *infer* 
for the galaxy based on *our measurements* of how far *it* moves 
(relative to *our* measuring rods) in a given amount of time 
(relative to *our* clocks.)  It imposes no limits whatsoever on 
how long it might take the galaxy to get here.  [NB: I hope nobody 
will needlessly confuse matters by raising the question of the 
need for acceleration on the part of *somebody.*  I acknowledge 
this fact, but it is not relevant to the point I am trying to 
make.]

> > ... The fact that
> > no observer can *measure* a material object to be traveling faster than
> > the speed of light is a statement about *measurements* of space and time,
> > not about "how fast something can move."
>
> John, IMHO this is more confusing, than enlightening terminology.  Is not
> "how fast something moves" a measurable, involving measurements of space
> andtime intervals and the creation of the concept of velocity?    Your 
> wording seems to imply that "how fast something moves" is something in 
> itself, distinct from, independent of, and existing quite apart from any 
> space-time measurements.

I hope it is evident from my remarks above that I *certainly* 
didn't mean to imply that.  Sorry if I wasn't clear before.  My 
quotation marks were intended to imply that I find the phrase 
itself highly misleading unless the words "relative to ..." are 
appended or at least *very* clearly implicit.

> [...]
> The distance and time of travel as measured aboard a space ship are
> contracted as compared to earth measurements, and the pilot can, by his
> clocks, get anyWHERE in an arbitrarily small time,  but he will not make
> lunch served on a distant star if that EVENT is "space-like" separated
> from his present existence.

Right; but this fact follows directly from the previously 
acknowledged limit on what *other observers* can determine the 
*pilot's* speed to be relative to them.  And *that* limit can be 
connected with the reassuring idea that relativity preserves 
causality.  As I said at the end of my first message, "one 
person's "faster than light" is another person's "backward in 
time."

John
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