Re: Are *observed* SR effects real?

On Jul 30, 6:16 am, mluttg...@xxxxxxxxxx wrote:
On Jul 29, 6:53 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:



On Jul 29, 11:02 am, mluttg...@xxxxxxxxxx wrote:

On Jul 29, 3:00 pm, "Dirk Van de moortel" <dirkvandemoor...@ThankS-NO-

SperM.hotmail.com> wrote:
PD <TheDraperFam...@xxxxxxxxx> wrote in message

  ee91f5fe-fca0-450b-8665-2b8507364...@xxxxxxxxxxxxxxxxxxxxxxxxxxx

[snip]

 > At this point in a classroom discussion, when we've moved onto time

dilation, somewhere in somebody's head the wheels about length
contraction are still turning, and something falls softly into place,
and I will usually hear a voice interrupting the discussion:
"You mean that's it?"
And then the conversation goes something like this:
"What do you mean, 'that's it?' "
"I mean, there's nothing more to this special relativity stuff than
that?"
"What were you expecting?"
"To be honest, I was expecting some weird interaction between space
and matter that was making matter shrink. I thought that special
relativity was an effect... the result of some new physics process
that no one was talking about. And I was waiting for you to tell me
about this new physics process. But there's no new physics process,
you're telling me?"
"No, nothing beyond what we've already talked about."
"And so length contraction is nothing more than an artifact of its
definition?"
"More or less. The same is true for momentum."
"But that's DIFFERENT! I mean, we've been using mass times velocity
for momentum. And ANYONE can see that velocity will depend on the
frame of reference, and so momentum will too. I mean, that's obvious.
Momentum is frame dependent because of its definition, and anyone can
see that."
"Yes, exactly."
"And you've just said that the same is true for length, that length is
frame-dependent because of its definition, and nothing more. And
that's IT??"
"Yup. It took us a few more steps to see it, but that's it in a
nutshell."
"Well, forgive me, but what did Einstein get a Nobel prize for? I
mean, where's the new physics in that?? I thought Einstein invented a
way for space to twist matter and squeeze it and slow down process and
stuff."
"As it turns out, he didn't get the Nobel for relativity, but let's
set that aside for a moment and take a step back. Just two days ago,
you were convinced that length was a frame-independent quantity, much
different than momentum, right?"
"I suppose..."
"And we had to get through several steps to even get to this point. We
had to recognize that the laws of physics should be the same in every
inertial reference frame. We had to recognize that this meant that the
speed of light from ANY source had to be the same in ANY frame, which
is a surprise we'd better check experimentally. We had to then
recognize that this meant that simultaneity is a frame-dependent
condition, which is a surprise we'd better check experimentally. And
then we had to recognize that length by its very definition relied on
simultaneity, which is something we probably didn't think about
before. And then we had to recognize that length, then, is also frame-
dependent by its definition, which is a surprise we'd better check
experimentally."
"Yes, I get all that now..."
"Well, at the time Einstein was working on this, many people got some
of the bits and pieces of that, but they chickened out before
following it all the way through. They saw some of the surprises and
said, 'that doesn't seem possible', and they chickened out. Einstein
didn't chicken out."
"And not chickening out is what Einstein won a Nobel Prize for?"
"Not quite. Those many surprises that needed to be checked out
experimentally, were in fact checked out experimentally. And he was
right. And THAT'S when Einstein became famous. Not chickening out was
important. But not chickening out AND being right is the unique
combination that few people get. And that's what wins people prizes."
"But length contraction isn't HARD. Shouldn't it be hard to
understand?"
"No. The best leaps in physics are always obvious in hindsight and
nearly impossible to see in foresight. The point in teaching you
physics is not so much to teach you the stuff that is easy to see in
hindsight. It's to teach you how to blaze a trail toward something
that is nearly impossible to see in foresight. But on that we've
hardly even begun..."

PD

Alas, no students like *that* remain in *this* classroom.
Either they leave the class when they understand - and there
isn't much of interest here beyond this, or they are insuffienctly
gifted (and more than sufficiently blockheaded) to understand,
so they stay here for years or even decades :-)

Dirk Vdm

There are different ways to derive the Lorentz
transformation, but Paul Draper's approach is so
illustratively interesting, that I consider as
a priviledge to remain in his classroom.

I appreciate the compliment. Keep in mind though, that I'm telling you
nothing that isn't already in some wonderful books and isn't already
the subject of a well-taught university class. It's just that it's
much harder to even find it on the internet, and it's even harder to
isolate it from the mounds and mounds of crap on the internet. You get
what you pay for. What you don't get when you don't pay for it is
*quality control*. It is virtually impossible for a novice on the
internet to exercise that QC by himself -- otherwise he wouldn't be a
novice.

PD

This is a digression:

Almost everybody accept that the observed
velocity of light is independent from the
motion of its source.
But is its velocity independent of the
motion of the observer?

Yes.


For instance, the observer is moving
directly toward the light source.

Then, one can consider that the velocity
of light relative to the observer is c-v
instead of c (contrary to Einstein's
postulate),

I'm not sure why you would consider it to be so, but if we do, let's
be careful about the consideration.

First of all, from whose reference frame is the velocity of the light
relative to the observer c-v? Let's take a more concretely posed case.
There is a beacon, emitting brief flashes of light. There is a
platform nearby, at rest relative to the beacon, and an observer
(Francois) on the platform. Then there is a vehicle traveling at speed
v toward the beacon with another observer (Ginny) aboard. Now, let me
ask you this: Is the speed of light's approach to Ginny c-v according
to Francois or according to Ginny? Which did you have in mind? Pause a
moment to answer this to yourself before reading on.

Keep in mind the difference between *measurement* and *calculation*.
Francois *measures* the speed of light from the beacon to be c. He
*calculates* the closing speed between the beacon's light and Ginny to
be c-v, but he doesn't have any way to *measure* that. All he's doing
is taking two *measured* numbers, the light's c and Ginny's v, and
subtracting them.

What does Ginny actually *measure*? Ginny measures the approach of the
light to be at c. I know this because of experimental measurements of
light from stars to the Earth. The Earth varies it's speed relative to
star because it goes around the sun, so in March it is approaching a
star at 67,000 mph and in October it is receding from the same star at
67,000 mph, and if there were a difference in the light speed from the
star due to the motion of the earthly observer, there would be certain
absolutely noticeable effects that, as it turns out, are not there.
(And there is a multitude of other experiments that show the same
thing.)

Now, what may bother you still is this: If Francois, calculating or
not, sees a relative speed of c-v between the light and Ginny, doesn't
Ginny HAVE to see a relative speed of c-v between the light and
herself? No. Relative velocity is not something that stays the same
between reference frames. For example, suppose you were riding on a
gun that fired a really massive shell, so that when it fires, you and
the gun recoil backwards. Let's suppose the shell's velocity relative
to you and the recoiling gun is 2500 m/s. Now take someone else (say,
Paul) standing on the ground who sees you recoil and the shell take
off in the other direction. Paul can measure your recoil speed Vm and
the shell's speed Vs. Doesn't |Vm| + |Vs| = 2500 m/s? No.
Interestingly, it does not. If I measured Vm and Vs I would find the
sum to be somewhat less than 2500 m/s. I realize this is a surprising
result, but it is ALSO abundantly confirmed in experiment -- for
example, in the beta decay of neutrons, which is very similar to the
recoiling gun conceptually.

In *Galilean* physics, relative velocity between two objects is frame-
independent. And for small speeds, this is an excellent approximation.
But it is wrong. Relative velocity between two objects is frame-
dependent.

thus c / (c-v) = 1 / (1-v/c).
Taking into account the SR factor
sqrt(1-v^2/c^2), one gets
f(obs) / f = sqrt(1-v^2/c^2) / (1-v/c),
where f(obs) is the observed frequency
and f the frequency of the light emitted
by the source.

Marcel Luttgens

.

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