🚀 go-pugleaf

In article 3031@galois.mit.edu, jbaez@riesz.mit.edu (John C. Baez) writes:
>In article <1992Jul16.100622.13764@vax5.cit.cornell.edu> cpyy@vax5.cit.cornell.edu writes:
>
>>I asked this same question in a past physics class.  The reference frames
>>are not equivalent.  The one twin actually can feel himself accellerating,
>>while the other cannot.  Funny, I asked, "Then relative speed is not what
>>actually causes the aging--it's the acceleration?"  The physics professor,
>>who was highly respected, replied, "Yeah, it has something to do with
>>the acceleration."
>
>One way to stay highly respected is to know when to be vague.  :-)
>Indeed the aging has "something to do with" with the acceleration, but -
>if our twins were in a closed universe, say R x T^3 with its standard
>flat metric (here I mean that space is a torus and time is a line), and
>one stood still while the other went around the universe (relative to
>the standard coordinates on R x T^3), then the one who went around would
>be younger when they met again, even though NEITHER experienced any
>acceleration.  (Of course I'm assuming that our travelling twin was BORN
>at a high velocity.  Technical details left to NASA.)
>
>Of course this twist is less relevant to reality since nobody knows if
>the univser is closed, and certainly nobody has circumnavigated it.
>Nonetheless, it shows that one has to be careful.


It is amazing how the twins paradox lives on, and causes problems.
Even to "highly respected" physics professors it seems. Of course,
you CAN compute the effect of the acceleration ( it is a General
Relativistic effect ), and this has been done ( for the experiment 
when a couple of 'planes carrying atomic clocks circumnavigated 
in opposite directions ). The GR effect is definitely second-order.
The basic effect being measured is one of relative velocities ; just
draw a space-time diagram and it is obvious what's going on. 
The twins paradox was not intended by Einstein to illustrate GR but
SR effects. The acceleration is for all intents and purposes irrelevant.


	IMHO , usual disclaimers apply,

	Phil.