Re: Help with SR time dilation

From: RP (no_mail_no_spam_at_yahoo.com)
Date: 02/07/05 

Date: Mon, 07 Feb 2005 00:08:57 -0600

Androcles wrote:
> "RP" <no_mail_no_spam@yahoo.com> wrote in message
> news:36njg3F51ps6gU1@individual.net...
>
>>
>>Androcles wrote:
>>
>>
>>>"RP" <no_mail_no_spam@yahoo.com> wrote in message
>>>news:36n4d6F5402fmU1@individual.net...
>>>
>>>
>>>>DavidBowman wrote:
>>>>
>>>>
>>>>>>>From the point of view of observers in Frame A,
>>>>>
>>>>>
>>>>>>their own frame is stationary.
>>>>>
>>>>>
>>>>>I think I see the problem.
>>>>>
>>>>>A common misconception is that the word "relative" means that if the
>>>>>universe consists of nothing but two spacecraft, there is no way to
>>>>>tell which one is actually moving. That isn't true.
>>>>>
>>>>>The one moving is the one for whom time runs slower.
>>>>>
>>>>>"Relative" refers to the fact that you can only tell if you're
>>>>>moving
>>>>>by loking at something else. But if you DO look at something else,
>>>>>you
>>>>>CAN tell if you're moving... but only relative to the "something
>>>>>else".
>>>>>
>>>>>And accelleration isn't relative at all! Even if the universe
>>>>>consists
>>>>>of nothing but a single spacecraft, the pilot can still tell that
>>>>>he's
>>>>>accellerating because he's pushed against the real wall of the
>>>>>spacecraft.
>>>>
>>>>Incorrect. It is only wrt the other matter in the universe that the
>>>>ship is accelerating. The force that the passenger "feels" is due to
>>>>his interaction with the ambient field, a field that has no existence
>>>>independently of its many sources external to the ship.
>>>>
>>>>Richard Perry
>>>
>>>
>>>And what evidence do you have to support this claim?
>>>
>>>Androcles.
>>
>>Let your "ship" be "all of the ponderable matter in the universe". How
>>would it accelerate with nothing to even move with respect to? In the
>>argument above the ship was taken to be the only thing in the
>>universe, which means that it was taken to "be" the universe. How
>>would it accelerate? Where might it be off to?
>>
>>Richard Perry
>
>
> I asked you what evidence you had, not what thought experiment you had.
> You have just embarked on Russell's paradox.
>
> http://plato.stanford.edu/entries/russell-paradox/#HOTP
>
> Androcles.

I'll grant that given rocket blast, the ship can accelerate wrt the
center of mass of the system. If we assume "only" this frame, then as
a side effect, velocity also becomes absolute, i.e. wrt it. Wrt the
ship's FoR, OTOH, the ship is immobile, infinitely massive, the
exhaust gasses have accelerated.

Velocity is thus not absolute if we allow as valid any FoR in the
system, but now neither is acceleration absolute. Wrt the gasses it is
the ship that accelerated while the gasses remained at rest. Only wrt
the center of mass is acceleration absolute, and only in that frame is
momentum conserved. OTOH, in the real world there is plenty of
evidence that momentum is conserved in every inertial
frame. Thus the very idea that we can consider and derive arguments
from a statement such as "Even if the universe consists of nothing but
a single spacecraft[...]" is not well founded. It is meaningless in
that it is contrary in its metrics to the structure of the real universe.

Richard Perry

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