Re: Twin paradox revisited ll

Dear bill:

"bill" <cosmosco@xxxxxxxxxxxxxxx> wrote in message
news:1185150277.116510.125670@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Jul 21, 11:53 am, "N:dlzc D:aol T:com \(dlzc\)"
<d...@xxxxxxx>
wrote:
Dear bill:

"bill" <cosmo...@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".

On the basis that 'observation creates reality' - that
what one 'measures' (or 'observes' or 'determines')
*is* reality why would a person - who *believes* that
observation creates reality - having determined
something then insist that he does not believe what
he is seeing?

It is a common practice here. You must be new. ;>)

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.

And aren't those observations (determinations)
nothing more than visual illusions generated by
the red shift and blue shift of the light from the
planet?

Problem is, if you total up the number of such full orbits, it
will agree with the stay-at-home's age, and not the traveller's
age.

The original posting was that because the
astronaut sees the light from earth as being
intensely blue shifted he then believes
(determines) that his twin is physically aging at
a faster rate than he is.

For that matter, he should insist that his stay-at-home twin is
being bathed in X-ray and gamma radiation from his Sun... it's
surface temperature is now too high to allow life on a planet as
close as Earth.

My question is - on the basis that the astronaut
sees 'the earth take millions of years to orbit the
sun once' does he truly believe that whilst he is
moving away from us that the earth's orbital
velocity *physically* reduces to a mere 1K-s and,
as he returns and sees it moving 'like a bat out
of h*ll' does he really believe (determine) that the
earth's rate of travel has increased to an
impossible near light speed?

He can measure and assume he is a "virgin", and infer that all
those changes are "physically happening". He can make those
measurements, and assume he is a devotee of classical Doppler,
and infer what he sees as much better, but still not agree with
the stay-at-home twin. Or he can make those measurements, and
use relativity, and determine exactly what the stay-at-home is
measuring for him/herself.

It is not a question of what he is measuring. The question is
what can he do to make sense of it?

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.

My point exactly.

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

So if you measure that the earth is orbiting the
sun at 1K-s are you of the opinion that it is
*physically* orbiting at that velocity?

If it was, it would be pulled into the sun.

Not at all. If you were entirely clueless, and assumed you knew
nothing about the physics of the Sun-Earth system, you would
still be able to make that system work. The mass of the Sun and
Earth would have the same proportion as the stay-at-home twin
woudl say it was, but the total mass would decrease to make that
orbit "work out".

It isn't "physically" anything. If you insist on ignoring
averything you know, you can make the physics work.

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 *physically*
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.

So he sees (determines, measures) the planet
moving at *physically impossible* orbital velocities

.... physically *possible* ...

but then applies relativity and concludes that the earth
is *not* moving at those velocities *in its own reference
frame*

.... in fact is moving no differently than it was when he lived
there.

ergo he must realise that what he *sees* is
nothing more than a visual illusion created by his
relative rate of travel.

Given that the traveller comes back younger than the
stay-at-home, yes that is correct.

The original posting was to the effect that from
the traveler's point of view, the earth *is physically*
moving at those impossible velocities.

Which is both true and false, and ultimately leads to your
confusion.

My argument was that the traveler would
*presumably* have some sort of education *in*
physics including relativity thus that he should, as
Confucius suggested, *apply* (not forget) that
knowledge.

Some of the postings in this discussion imply that
the traveler is *incapable* of applying knowledge
and makes his decisions on the basis of a purely
solipsist, philosophical attitude.

The Galilean 'Principle of Relativity insists that
the traveller cannot know if his ship is moving with
uniform velocity or is at rest *without reference to
an external point* i.e. he cannot *see* the
universe 'rushing past him' at near light speed.

Or the Sun-Earth system.

The original posting

.... in this context, I thing you mean "the original poster", or
OP for short ...

insisted that the faster aging rate of the earth
twin *only* takes place during acceleration following
turn around and that it does *not* apply when the ship
stops accelerating. In other words, at the very instant
that the traveler takes his foot of the gas the earth's
rate of travel around the sun reverts from near light
speed to 30K-s *intantaneously*.

That is incorrect. The values of acceleration are unimportant
*in SR*. And are really outside the scope of SR, except in some
very limited cases. It is "velocity history", essentially "how
fast" for "how long" that matters.

The traveler must *know* that this cannot possibly
occur in reality thus must conclude that what he
*saw* (or measured or determined) was *not* reality
either in his reference frame or the earth's reference
frame.

It *was* reality for him, because that is what he measured.

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".

Are you suggesting that the traveler returns, given the
respective factors, that he actually finds that all life on
the planet *has* been obliterated?

I still don't see how you achieve "obliteration". If you mean
"his twin has aged more than him, including the possibility of
death and meeting his own grand-kin", yes.

If the traveler is of the opinion that he has not aged
at the slower rate

Nothing in his frame will seem slower to him. Only on comparison
of his clock to the Universe will come surprises.

but that his twin ages at the faster rate he is denying
Einstein's 1981

He died in 1955, what did you mean?

insistence that it is the clock (the twin) who experiences
the force of acceleration which is the one that
*physically* ticks over (ages more slowly).

The twin paradox can be perfomed with 3 clocks (3 people if you
like), and no acceleration within the period of the experiment
(departure to final arrival). The acceleration is just part of
the gedanken, and not a requirement (unless you insist on seeing
the same twin arrive and meet his older twin).

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.

Irrelevant to the specific topic , merely a reiteration.

I guess that dpends on what you mean by "obliterate" then.

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".

Those experiments have shown that accelerated
particles

.... no, "fast moving" ...

age more slowly than slower moving particles but they
do *not* prove that the latter, and the universe, ages
more rapidly.

Compared to the fast moving particle, who can insist *nothing*
changed for him, the Universe ages more rapidly.

The concept that the stationary particle ages at a
faster rate than the accelerated particle should be
sufficient for physicists to stop all of those experiments
which cause them to age at a faster rate than
would otherwise occur.

This is a joke, right?

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.

That has nothing to do with the original posting which
insisted that the stay at home twin physically ages
at the faster rate and *only* during the traveler's
period of acceleration following turn around.

Which is in part incorrect.

David A. Smith


.

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