Re: Are *observed* SR effects real?

On Jul 16, 4:51 am, mluttg...@xxxxxxxxxx wrote:
On Jul 15, 4:44 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:



On Jul 15, 7:22 am, mluttg...@xxxxxxxxxx wrote:

If you have no comments or questions regarding this post,

No comment, I am eager to read the next episode :-)

Marcel Luttgens

Alright, it's fine that you have no comments or questions so far,
though I'm surprised. Some of what I've told you already flies in the
face of common sense -- which is not necessarily a bad thing, but a
hurdle to overcome anyway.

So, as promised, what I will take is a scenario similar to what
Einstein used as an explanatory instrument, and for the same purpose.
I will first tell you what two different observers see (and this is in
fact confirmed in equivalent experiments), and then we'll show that
this is completely consistent with the laws of physics, even though
their observations might be surprising at first.

There is a federal train whipping through a campus town. It's an
electric train and it receives its power from a suspended wire
alongside the track, via electrical brushes mounted near the ends of
the train. As the train passes by a pole supporting the wire,
occasionally a bright spark is thrown at the wire support at the pole
and the electric brush as the brush whisks by it, sometimes a reddish
spark, sometimes greenish, sometimes yellowish.

There's a physicist working in a lab next to the tracks, and there's a
physicist working in a lab on the train. As it turns out, they are
both doing independent and competing light-speed isotropy experiments,
and they know each other. Both of them just about a half-hour ago
completed measurements that showed that light speed is isotropic, and
this is true even though one lab is moving relative to the other.
(This repeats an earlier published test of Einstein's light speed
postulate, but now with higher precision.)

As the train passes by, Stan, the researcher in the lab in town, looks
out and sees the following:
1. Two flashes simultaneously, one yellow from the back of the train
and one green from the front of the train.
2. A short time later still, a bright red flash at the front of the
train.

Tom, the researcher on the train, sees the very same flashes (the only
ones that actually happen that day), but sees them in the following
order:
3. A green flash from the front of the train.
4. A short time later, two flashes simultaneously, one yellow from the
back of the train and one red from the front of the train.

They radio each other to see if this disrupted the other's isotropy
experiment (it didn't) and to relate what they saw.

Stan goes out after the conversation and measures the distance from
the three poles where he saw the flashes to his lab. As it turns out,
the poles where the yellow and green flashes came from are equidistant
from his lab, though the pole where the red flash came from is further
away. He uses his three observations: isotropy, equal distance from
source, and (1) above to CORRECTLY conclude that the yellow and green
flashes are simultaneous. He is using the *definition* of simultaneity
that we cited last time.

He's not sure about whether the red flash was later just because the
flash was coming from further away, but since he's measured the
distance, he can figure out the propagation delay knowing the speed of
light. There is indeed a delay but not as long as what he observed
between the red and yellow flashes. So he CORRECTLY concludes that the
red and yellow flashes are not simultaneous.

Stan also notes that the two poles where the simultaneous (yellow and
green) flashes came from are 600 m apart, and since they came from the
brushes at the front and back of the train, he knows the train is 600
m long. Remember that he also knows the distance to the pole where the
red flash happened -- we may use that later.

Tom, on the other hand, notes that he was standing exactly midway
between the two brushes at the end of the train when any of the
flashes happened. The distance from either end of the train to his lab
is 400 m, so he knows the whole train is 800 m long. So he uses his
isotropy results, and the fact that he was equidistant from the
brushes and observation (4) to CORRECTLY conclude that the red and
yellow flashes are simultaneous and that the green and yellow flashes
are not simultaneous.

So, they radio this information to each other:
Stan: Using the *definition* of simultaneity, the green and yellow
flashes are simultaneous, and the red and yellow flashes are not
simultaneous.
Tom: Using the *same definition* of simultaneity, the red and yellow
flashes are simultaneous and the green and yellow flashes are not
simultaneous.

Note they have both *taken into account* propagation delay, and that
they each use only the *measurements* they individually make.

So, there is obviously a disagreement here about the simultaneity, and
this causes some head-scratching about how it is the other could have
seen what they saw. We'll come back to that next time, using the laws
of physics.

Incidentally, they also disagree about the length of the train, as you
can see. But they both immediately see that this is simply rooted in
the problem of the simultaneity. For instance:
Stan says the train is 600 m long, "...because I used the
*simultaneous* marking of the locations of the ends of the train using
the green and yellow flashes, which is the *definition* of physical
length." But Tom replies, "But those flashes were NOT simultaneous.
The green flash at the front came before the yellow flash at the back.
If you mark the front of the train first and then mark the back of the
train second, OF COURSE you will come up with a number that is too
small." "But that's not what happened with the green and yellow
flashes," says Stan. "Yes, it is!" shouts Tom, "You should have used
the yellow and RED flashes."
Then, after a pause, Stan says, "Look, if you measure the back of the
train first and then the front of the train second, you'll get a
number that's too long for the train. That's why I didn't use the
yellow and red flashes." "But that's not what happened with the yellow
and red flashes," says Tom. "Yes it is!" shouts Stan.

And it appears there is no way to settle this, because they both use
the same definition of physical length and they both use the same
definition of simultaneity and all propagation delays have been
accounted for, and there is still disagreement about both the
simultaneity and, as a result, the length of the train. Finally, note
that there is NO implication whatsoever of something physically
happening to the train to squeeze it or stretch it out. But nor is it
an optical illusion. It all rests on the *definition* of simultaneity,
which we defined last time and which makes perfect sense.

Comments? Questions?

PD

Question:

Which were their measurements that showed that
light speed is isotropic?

They were doing experiments in their labs that were independent of the
green, yellow, and red flashes. As mentioned in the first long post I
gave you, they were performing experiments similar to those found
here:
http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#round-trip_tests

So they already *know* from these other measurements that light speed
is isotropic.


Comments:

Stan says the train is 600 m long, Tom disagrees and
claims that the train is 800 m long.

Yes.

Then Stan said, use your ruler to measure the length,
you will find that it is 600 m.

OK.

Tom did it, and found
600 m,

No, please read what I wrote again. Tom measured the distances from
each of the brushes to where he was standing -- those were each 400 m.
He did this with a ruler. 400 m + 400 m = 800 m, and he was in the
middle between the two brushes.

and claimed that there must be something wrong
with the simultaneity rule.
Stan replied: "Perhaps not if the train is moving at
100 m/s".
We have experimentally found that the speed of
light is 300 m/s (!).  As I determined the length of
the train from the time taken by light to reach me
at 300 m/s, i.e. 1 second, I inferred that that the half
length of the train is 300 m.
After 1 second, the middle of the train, where you
were sitting, travelled 100 m, so you were at 300 m
+ 100 m = 400 m from the yellow flash, and the
red light reached you after 1 second if we add
to its velocity the velocity of the train.

Marcel Luttgens

.

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