Re: Twin paradox revisited ll

Dear bill:

"bill" <cosmosco@xxxxxxxxxxxxxxx> wrote in message
news:1184981064.527191.20310@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jul 20, 7:11 pm, "Martin Hogbin"
<goatREMOVETHIS...@xxxxxxxxxx>
wrote:
....
That is really two questions. On the basis of the
best measurements he can make, and allowing
for all effects that he can think of, the traveller
calculates that the other twin's clock is running
more slowly than his own during the cruise phase.
I such circumstances I would believe that this is
what is 'really' happening. Would you come to
the same conclusion?

No I would not. I cannot accept that the traveler
*really* believes that the earth is orbiting the sun
at around 1m-s nor do I believe that this is what
would 'really' be happening.

It is not about "believe" but about "measure". You can go
outbound fast enough that you could see the Earth take millions
of years to orbit the Sun once. But it will move like a bat out
of h*ll on your return journey.

During the acceleration the situation is much more
complicated but the answer is essentially the same.

As regards whether it is 'physically' happening, I
cannot answer this question unless you define
exactly what you mean 'physically'.

By 'physically' I mean the concept that the earth is
'really' orbiting the sun at 1m-s as distinct from
'apparently' as determined by the traveller.

One expects that the Earth really could care less how fast the
traveller is moving.

But relativity is about what you measure, and what you can
correctly infer about what another frame might measure (based on
your own measurements).

It has nothing whatsoever to do with what *we*, as
stay at home observers observers, think but what
is claimed the *traveler* determines is reality.

Yes, for the traveller.

So he *really* believs that the earth is *physicaly*
orbiting the sun at 1m-s?

If that is what he measures *in his own frame*. If he forgets to
use relativity to calculate how fast the Earthlings would
calculate it was moving.

....
Other than what one 'could argue' I fully agree with
those comments but I cannot agree, as expressed
above, that the stay at home *physically* ages at
the faster rate thus that the traveler could
obliterate all life on earth by taking his foot off the
gas pedal.

You need to define 'physically'.

That the traveler destroys all life on the planet. When
he returns home he learns - hopefully - that this has
not *physically* taken place.

It physically *has* taken place. And the traveller had squat to
do with the stay-at-home aging, only to do with his own "lack" of
aging... with his "gas pedal".

The fact that the traveler finds on his return that
everything is 'normal' back here - that life
continues - should indicate to him that the earth
had *not* been orbiting the sun at near light
speed, that what he saw or determined was
nothing more than a visual illusion generated
by his rate of travel.

No, it indicates that the passage of time is not
universal. Of course, on his return, the traveller
will be aware that, from the earthbound twin's
point of view, nothing unusual has happened.

Having 'believed' that all life on the planet has
been obliterated it would not only be 'from the
earthbound twin's point of view, nothing
unusual has happened' but also from the
*traveler's* point of view.

Except that the traveller is younger thant eh stay-at-home.

The bit you have not grasped is that the passage
of time is not universal. This is very
counterintuitive but it is the inescapable conclusion
of experiment.

Or rather, in the *interpretations* of those experiments.
As far as I am aware there has been no experiment
which proved that from the traveler's point of view it is
his twin that ages at the faster rate than himself.

Yes, exactly that has been experimentally determined. Slow
particles with short lifespans age more rapidly than faster ones.
And it has nothing to do with "accleration" or "accelerators" or
"magnetic fields" or "new and unexplained physics".

The length of a journey between any two points depends on the
path you take. This applies equally well if the "points" are
elapsed time on a clock, and relative motion provides the
different path between start and end of journey.

David A. Smith


.

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