Re: Are *observed* SR effects real?

On Jul 18, 4:00 am, mluttg...@xxxxxxxxxx wrote:
On Jul 18, 2:41 am, PD <TheDraperFam...@xxxxxxxxx> wrote:



On Jul 17, 6:32 pm, mluttg...@xxxxxxxxxx wrote:

On Jul 17, 4:42 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 17, 8:06 am, mluttg...@xxxxxxxxxx wrote:

On Jul 17, 12:21 am, PD <TheDraperFam...@xxxxxxxxx> wrote:

On Jul 16, 2:48 pm, mluttg...@xxxxxxxxxx wrote:

On Jul 16, 4:16 pm, PD <TheDraperFam...@xxxxxxxxx> wrote:

Yes, you wrote that, but I added to the scenario that
Stan asked Tom to also measure the length of the train by
using the ruler on the train itself. Then, Tom found
600 m.

The ruler that Tom used was a ruler on the train itself. It yielded
400 m from where he was standing to the brush on the train, and 400 m
from where he was standing to the other brush on the train. I am
reporting to you what he *observed*.

This is not possible in Tom's rest frame.

Yes it is. As I told you, and according to the plan of attack agreed
upon, I am telling you what the *observations* ARE. These observations
turn out to be completely consistent with the actual documented
observations in real, equivalent experiments done by real people and
recorded in real articles in real journals in real libraries.

What I am going to do NEXT is show you that this *observation*, as
surprising as it is to you, is completely consistent with the laws of
physics.

Moreover, it appears that you've already lost a grip on the train of
thought. Let me see if I can recap for you:
1. Stan knows that the yellow flash marks the back of the train, and
the green and red flashes both mark the front of the train. But he
chooses the green flash because *he* says the yellow and green flashes
are simultaneous, and using *simultaneous* marks is essential to the
definition of physical length. He could use the location of the red
flash, but that wouldn't satisfy the definition of length because the
red and yellow flashes are not simultaneous. So for him, the green and
yellow flashes are the simultaneous marks that mark the length of the
train.
2. Tom agrees that the yellow flash marks the back of the train, and
that the green and red flashes both mark the front of the train.. But
he uses the red flash, because *he* says the yellow and red flashes
are simultaneous, and using simultaneous marks is essential to the
definition of physical length. He could use the location of the green
flash, but that wouldn't satisfy the definition of length because the
green and yellow flashes are not simultaneous. So for him, the red and
yellow flashes are the simultaneous marks that mark the length of the
train.

So you see, what you say is "impossible" is quite possible, given what
they actually *observe*, given the *definition* of simultaneity, and
given the *definition* of physical length. Start with what they
*observe* as given FACTS, and then think through the conclusions of
that, rather than starting off by insisting what is possible or
impossible in your head.

The FACTS (the observations) as I stated them are FACTS. They echo
what is actually seen in equivalent experiments. Start with those as
GIVENS, and find out what consequences follow from that. GIVEN those
observations, and using the *definitions* of simultaneity, then one
cannot escape the fact that different sets of flashes are simultaneous
for Stan and Tom. And then given that fact, and the *definition* of
length, then it is immediately obvious that the length of the train
will be different for Stan and Tom, because they're using different
flashes to mark the ends of the train simultaneously.

Let's see if you have any questions about this again, before we go on
to show that the FACTS (observations) are consistent with the laws of
physics.

Nevertheless, according to SR, in his rest frame,
Tom would find *with his ruler*  that the length of the
train is 600 m, not 800 m.

No, SR does not say this at all. I don't know where you got the
impression this is the case.
(If it helps, let's leap ahead a little bit and note that the 600 m
that Stan measures for the length of the train is the Lorentz-
contracted length. What Tom measures is the rest length, because of
course in Tom's frame, the train is at rest, and that is of course
longer than what Stan measures.)

Let me know if this helps and we're ready to go on.

If, on the train, he measured the distance between
the yellow and red flashes that he observed, I agree
that he found 800 m.

Marcel Luttgens

You are not *adding* to the scenario. You are *changing* the scenario.
The observations that are in my scenario are consistent with setups
and results in equivalent experiments recorded in the literature. The
*altered* scenario you just proposed is inconsistent with experimental
results in the literature.

It is nevertheless interesting, because it gives you the
opportunity to show that the addition c + v (v being the
the velocity of the train) must be rejected.

But just to bring us back on track, the key thing is that, according
to the observations made, the yellow and green flashes are
simultaneous for Stan but not for Tom, and the yellow and red flashes
are simultaneous for Tom but not for Stan.

Yes, that's the main point. I am looking forward to reading
the following episode.

Marcel Luttgens

Before continuing, I have a further question:
What are the distances between the 3 poles, as
measured in the Earth frame?

Well, since this is a learning exercise, I tell you what: I'll tell
you the distance between the poles that gave off the green and yellow
flashes in the Earth frame: 600 m.
Now, there's enough information to know what the distance between the
poles that gave off the yellow and red flashes. Do you know what that
distance is in the Earth frame? Hint: You know what it is in the train
frame, and you know that track distances are going to be contracted in
the train frame.
(If you were new to the subject, you might not know how to find this
at this point, but since you have some familiarity, you should
probably be able to do this.)

PD

I am trying to find the gaps in your scenario.
I noted the length of the train:
600 m when contracted (as seen by Stan)

Remember Stan is in the Earth frame. That's right.

800 m at rest (thus in Earth's or Tom's frame)

No, the train is at rest in the TRAIN frame. It is moving in the Earth
frame. Tom is riding on the train.

It appears we need to backtrack just a bit and remind you what a frame
of reference is. I'm guessing from your statement above that you were
under the impression that the train's rest frame means looking at the
train sometime when it isn't moving with respect to the Earth. That is
not the case.

The train moving relative to the Earth exists at that moment in a
zillion reference frames at once. We have been considering just two
out of those zillions.
1. The reference frame in which the Earth and Stan are stationary, and
the train and Tom are zipping by at high speed.
2. The reference frame in which the train and Tom are stationary, and
the Earth and Stan are zipping by at high speed in the opposite
direction.
We are looking at the *same* events, the train's brushes and certain
poles zipping past each other and throwing colored sparks, in *both*
frames. There is no repeating of these events.

Is this clear?

Btw, such train has a terrific velocity!

Now you are saying that the distance measured
on Earth between the "yellow" and "green" poles
is 600 m, so I presume that between the "yellow"
and "red" poles, the distance is 800 m.

No, this is not correct. That's OK, as I said, novices to this
material shouldn't be expected to figure this out just yet.


A few days ago, you wrote:

"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."

I agree that Stan saw simultaneously two flashes,
a yellow one and a green one, and a short time later,
he saw a red flash at the front of the train, which
is physically possible, as the train has a rest
length of 800 m.

I also agree that Tom saw simultaneously a yellow and
a red flash, but he couldn't see a green flash,

The green flash definitely happened. The front brush and a certain
pole zipped past each other and threw the colored spark when they did
so. What would prevent Tom on the train from observing that flash
happening?
(The only thing that's interesting is *when* he saw that flash
happen.)

because
one should then assume that the moving train
was "physically" contracted.

No, there's no assumptions needed here. All we are doing is recording
OBSERVATIONS -- that is, the events that Stan and Tom saw and what
they wrote down in their little log books. There is no disputing of an
observation. What they see, they see. It's not like Tom has to think
things out and say to himself, "Well, if Stan saw that, and if length
contraction holds, then perhaps I should say I saw the green flash
THEN." He doesn't *reason* out what he *should* see. He writes down
what he *does* see, and THEN he asks Stan what he saw.

But till now, nobody
has indubitably proved that length contraction is real.

Yes, they have. These observations are perfectly consistent with other
observations in equivalent experiments done in real life. You seem to
think this is all an exercise in assumptions and logical argument.
It's not. It's based on real-life experiment.

PD
.

Relevant Pages

  • Re: Are *observed* SR effects real?
    ... Stan asked Tom to also measure the length of the train by ... the green and red flashes both mark the front of the train. ...
    (sci.physics.relativity)
  • Re: Are *observed* SR effects real?
    ... Stan asked Tom to also measure the length of the train by ... the green and red flashes both mark the front of the train. ...
    (sci.physics.relativity)
  • Re: Are *observed* SR effects real?
    ... Stan asked Tom to also measure the length of the train by ... the green and red flashes both mark the front of the train. ...
    (sci.physics.relativity)
  • Re: Are *observed* SR effects real?
    ... Stan asked Tom to also measure the length of the train by ... the green and red flashes both mark the front of the train. ...
    (sci.physics.relativity)
  • Re: Are *observed* SR effects real?
    ... Stan asked Tom to also measure the length of the train by ... the green and red flashes both mark the front of the train. ...
    (sci.physics.relativity)