Re: What happened between Newton and Einstein?
- From: "PD" <TheDraperFamily@xxxxxxxxx>
- Date: 1 Mar 2007 12:56:12 -0800
On Mar 1, 1:57 pm, "kk" <mr_kurt_kings...@xxxxxxxxx> wrote:
On Mar 1, 10:19 am, "PD" <TheDraperFam...@xxxxxxxxx> wrote:
I'm sorry, I don't see how absolute motion is
the *only* possible cause of the age differences
between twins and triplets.
In the case of the twins, A and B, A and B *both
agree* that A takes a straighter path through
spacetime than B. However, this in no way implies
anything about absolute motion.
You don't see it because using twins obscures the
truth. You must use triplets to clearly see what
is happening.
Unlike the twin case, the triplet case contains
no turnarounds and no accelerations.
This leaves only motion through space (aka absolute
motion) as the physical cause of the triplets'
age differences.
Perhaps. I take it that you've posted this "triplet paradox"
someplace. Perhaps you'd like to give it another go in a separate
post. Until then, I really can't comment. You were the one that
brought up the twins.
[kk wrote:]
Re your second claim, I reply that since Einstein's
2nd postulate is purely a mere definition (of clock
synchronization), it cannot pertain to anything in
nature.
I completely disagree. The second postulate is an
explicit statement about what will be the result of
a measurement of the speed of light,
Please describe how this "measurement" can be made.
Why? A postulate does not need a direct measurement. The
*implications* of the postulate are what's tested. Didn't I say that
already?
There is NOTHING in the statement of the second
postulate that says anything about clock synchronization.
Please describe how one can measure light's one-way
speed between points A and B without using two clocks
that have been "synchronized" in some way.
I can synchronize clocks without using light. I simply note the time
at clock A, walk all the way to clock B and note the time there, and
then walk at the same speed back to clock A and note the time again.
If the time difference on clock A is twice the time difference between
clock B and clock A readings, then the clocks are synchronized. No
light needed. I can do that in the dark.
Why would I have to make that measurement without clocks synchronized
at all?
But, if it makes you feel better, here's a way that doesn't require
two displaced clocks at all, so that any appearance of circularity is
removed: Receive a light signal from a source, any source. Measure the
wavelength by an instrument solely and directly sensitive to
wavelength -- a diffraction grating will serve. Measure the frequency
independently by an instrument solely and directly sensitive to
frequency -- a frequency comb will serve. Then multiply the measured
wavelength and the measured frequency and the result is a measured
speed of light. It is expected that the answer will always be c.
After you have tried to provide this description and
failed, it will then dawn on you that the 2nd postulate
is indeed purely a definition of clock synchronization,
as Einstein said.
Sorry, still don't follow.
That is, you will then see that since it is not
possible to measure light's one-way speed from
point A to point B without two clocks, and you
will also see that the clocks must be related
in some way, you will see that someone must
provide a prescription for how the clocks are
to be related, and this is a definition of
clock synchronization.
However, in Einstein's case, the following problem
arises:
Einstein cannot really (or absolutely) synchronize
clocks,
Nor does he claim he's getting an absolute synchronization. He's just
using a prescription that works, even though it is purely a
synchronization local to that reference frame. There is no workable
synchronization procedure that generates an absolute (across all
reference frames) synchronization, so we take what we have.
and even if he could, he would not get the
"answer" that he wishes to get, namely, a one-way
"null result."
The only way Einstein could obtain a one-way "null
result" was by forcing clocks to read x/c when a
light ray traveled the frame distance x.
But this is clearly not a measurement; it is only
a stipulated "result," given at the start before
any measurement is made.
This is why you cannot describe the procedure for
the "measurement" that you claimed can happen.
Gee, I dunno. Use my synchronization procedure. What's wrong with
that?
However, the postulate is not dependent on clock
synchronization in any way. In fact, using light
for clock synchronization is not even a *requirement*.
You can synchronize two spatially separated clocks
by *any* procedure that carries a signal at the same
speed in both directions -- including walking. Einstein
just happened to use light because he had just
postulated that it satisfies this criterion as a usable
signal.
Here's Einstein's 2nd postulate:
"Any ray of light moves in the 'stationary' system of
coordinates with the determined velocity c, whether
the ray be emitted by a stationary or by a moving body.
Hence
velocity = light path/time interval
where time interval is to be taken in the sense of
the definition in section 1."
Here's the definition from section 1:
"Let a ray of light start at the 'A time' Ta from A
towards B, let it at the 'B time' Tb be reflected at
B in the direction of A, and arrive again at A at
the 'A time' T'a.
In accordance with definition the two clocks synchronize
if
Tb - Ta = T'a - Tb "
No mention of walking or bullets or baseballs; only light.
Also, note Einstein's use of the word "definition."
Note that this definition works for *any* signal that you can
guarantee travels at the same speed from B to A as from A to B.
The fact that Einstein chose light does not mean that it is restricted
to light. Are you reading more into it than what's there?
The time portion (Einstein's "time interval") of
the 2nd postulate is dependent upon Einstein's
definition of clock "synchronization."
I see where you're going, but the circularity is not there.
Note that Einstein did not experimentally measure
light's speed during any part of the 2nd postulate.
Nor does he have to. That's why it's a postulate and not a measured
fact.
In fact, no one has ever measured light's speed
from point A to point B.
Einstein merely stipulated that the clocks must
read equal travel times. That is, they were
forced by Einstein to read equal travel times,
so they did not get this result experimentally.
But if you synched by walking and then measured, then there wouldn't
be any connection, would there?
This means that the 2nd postulate really has
nothing to do with physics - it is merely a
definition. It says nothing at all about any
thing in nature. It merely reflects Einstein's
belief that we will never be able to detect
absolute motion. It is a mere artificial
null result, given entirely by man.
In fact, it is impossible to experimentally
obtain a null result in the one-way case.
("one-way case" here means "the direct and
simple measurement of light's one-way speed
between two points without using clocks that
have been transported or rotated because such
clocks run slow.")
[kk wrote:]
Real clock slowing and real rod shrinkage entered
SR not via Einstein's postulates, but via his
upfront (pre-postulation-era) acceptance of the
Michelson-Morley experiment null result. (Actually,
Einstein simply accepted upfront full round-trip
nullness, which included both the MMx and the KTx,
with the former having rod contraction, and the
latter having clock slowing.)
Actually, if you read the histories of this, Einstein was pretty
steadfast in being unaware of the MMX result. However, he was *quite*
aware of the form of Maxwell's equations, and he was *very* aware of
that factor of c that appeared everywhere in them, and he was
*painfully* aware that no absolute speed appears anywhere in any of
the Newtonian laws of motion and that this fact ensures their
invariance with choice of inertial reference frame. And so he simply
tried to figure out how it is possible that c could appear in
Maxwell's equations and still have those equations be invariant with
choice of inertial reference frame. It was Maxwell's equations that
demanded the invariance of c, not the MMX.
I have read the history, and here it is:
[From the 1905 relativity paper]
"In agreement with experience we further assume the
quantity 2AB/(t'a-ta) = c to be a universal constant --
the velocity of light in empty space."
Einstein's "in agreement with experience" means "in
agreement with experiment." And the only experiment
that said "round-trip invariance and isotropy" was
the Michelson-Morley experiment.
Sorry, leading the witness. There was ample experimental evidence for
Maxwell's equations. He was not referring to direct measurement of 2AB/
(t'a-ta). Sorry. When I said read the history, I meant accounts by
Einstein of his thinking *preceding* this paper. In these accounts, he
was quite firm in being unaware of the Michelson-Morley experiment.
You seem to be trying to convict a dead man of perjury, for what gain
I don't know.
(footnote: actually,
the MMx said only isotropy; the later KTx added
invariance; this is why Einstein used the word
"assume"; he knew that he was talking about both
isotropy and invariance.)
Maxwell did not measure light's round-trip speed.
Nothing in Maxwell's equations tell us anything
about either light's one-way or round-trip speed.
Sure it does. It involves the absolute constant c. It's relatively
unheard of for a theory to include a velocity (as opposed to a
relative velocity), because to do so makes it depend on inertial
frame. That's precisely the point. It says that light's speed will be
c. It is assumed (by Einstein) that, like Newton's laws, this law
pertains, including the value of c, for any inertial frame looking at
the same event. That's what Newton's equations do: even though all the
values of the individual velocities change, depending on the frame of
reference, the form of Newton's laws (including any general constants)
remain completely unchanged. It doesn't matter whether you view the
law of gravity from a frame locked to the sun or one traveling at
100,000 mph with respect to the sun; the law governing th motion of
the Earth, including the value of the gravitational contant G, does
not change one bit. Ensuring that Maxwell's equations exhibit the same
behavior is what Einstein was after.
PD
.
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