Re: lawless physics

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Concerning where John Mallinckrodt wrote:

> 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.]

Although everything that John says here is correct (I think), it should be
pointed out that even though *relativity* does not impose a restriction on
a traveller's travel time between two different points in space, there
still is a *physical* limit on how fast a human traveler can go between
these points.  The limit is due to the finite strength of the human body.
If the accelerations in starting up and stopping for the trip are too large
then the traveller will be crushed by the spacecraft's cockpit/seat/bed/
chamber etc..  This effectively places an upper bound on the traveller's
acceleration wrt a local inertial frame in which the traveller is
instantaneously at rest.  This bound on the acceleration then places a
lower bound on the time needed for the traveller to get to his/her
destination without being demolished in the process.

[NB: I hope I didn't "needlessly confuse matters by raising the question of
the need for acceleration on the part of *somebody*".  I suspect that John's
warning here may have been an attempt to head off a twin paradox discussion.
If so, this is understandable.  If John had my objection/observation here
in mind, then I'm sorry for having brought it up.]

David Bowman
dbowman@gtc.georgetown.ky.us