From: Edward Green on
On Feb 1, 6:42 pm, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote:
> Juan R. González-Álvarez wrote:
> > eric gisse wrote on Mon, 01 Feb 2010 08:08:39 -0800:
> >> kens...(a)erinet.com wrote:
> >>> Is Length Contraction in SR physical??
> >> No, just as it has been explained to you repeatedly since the mid 90's..
>
> > From Tom Roberts page:
> > [... attempt to discredit Eric by quoting my web page]
>
> This is a LINGUISTIC issue.

That much is spot on.

> When someone asks "Is length contraction in SR
> physical?", they invariably don't have a definite meaning of "physical" in mind
> (if they did, they could answer the question themselves). Generally, they would
> consider a "physical length contraction" to mean that the object ITSELF gets
> physically shorter. This is manifestly not so in SR.<...>

You perhaps unintentionally ran a circle there; disambiguating
"physical" by an emphatic phrase containing "physical". A rod doesn't,
as we well know, get "physically shorter" by any measure we take in
the rest frame of the rod purely. It does in other frames of
reference. That behavior is real enough to break strings, as Bell
noted in his famous thought problem.

If it someday turned out that Lorentz symmetry were subtly broken, we
might change our tune here.
From: Ste on
On 9 Feb, 21:02, PD <thedraperfam...(a)gmail.com> wrote:
> On Feb 9, 12:26 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > Two trains are on adjacent tracks, going in opposite directions,
> > > though I say that only to deliberately reinforce an ambiguity here. It
> > > doesn't matter whether the trains are going at different speeds, and
> > > in fact it isn't even important if one of the trains is stopped, or in
> > > fact whether they are going in the same direction but one faster than
> > > the other. All that matters is that there is a relative velocity
> > > between them.
>
> > > Two lightning strikes occur, drawn to the trains because of the
> > > friction of the air between the trains. In fact, one lightning strike
> > > leaves a scorch mark (a 1 cm spot, if you want to be precise) on
> > > *both* trains as it hits. The other strike leaves a scorch mark
> > > somewhere else on *both* trains.
>
> > > The question now is, were the strikes simultaneous or not?
>
> > > There is an observer on train A, and an observer on train B, and they
> > > are both looking out the window when the strikes occur.
>
> > > They make the following observations:
> > > 1. The observer on train A sees the two lightning flashes
> > > simultaneously.
> > > 2. The observer on train B sees the flash from the front of his train
> > > before he sees the flash from the rear of his train.
>
> > > Now, it is not yet possible to determine whether the strikes were
> > > simultaneous originally. We have more work to do. But I want to see if
> > > you have a picture in your head of what has transpired.
>
> > I have a basic picture, yes.
>
> OK, then.
> Let's now follow up these two observations above and couple them with
> more observations.
> 3. After the strikes, the observer on train A runs a tape measure from
> his location to the scorch mark of one strike and makes note of the
> number. Then he runs a tape measure from his location to the scorch
> mark of the other strike and makes note of the number. These numbers
> are equal. Note the scorch marks are on his train, but that's an
> undeniable marker of where the event WAS when the signal propagation
> began.

Not really. If his train is moving, then the scorch marks will have
actually moved from the location of the event.



> 4. After the strikes, the observer on train B runs a tape measure from
> his location to the scorch mark of one strike and makes note of the
> number. Then he runs a tape measure from his location to the scorch
> mark of the other strike and makes note of the number. These numbers
> are equal. Note the scorch marks are on his train, but that's an
> undeniable marker of where the event WAS when the signal propagation
> began.

I'm not sure I agree with this.



> 5. The observer on A runs some experiments to measure the speed of
> light and the isotropy of the speed of light (that it is the same in
> either direction), and finds that the signal speed is the same. (Note
> this isotropy would NOT hold if the signal were sound, for example.)

And *how* does he measure this?



> 6. The observer on B runs some experiments to measure the speed of
> light and the isotropy of the speed of light (that it is the same in
> either direction), and finds that the signal speed is the same. (Note
> this isotropy would NOT hold if the signal were sound, for example.)
>
> Given these *observations* 1, 3, and 5, what would the observer on
> train A conclude about the simultaneity of the original strikes?

I must admit I don't have a clear enough picture of what is happening.
This gedanken seems to presuppose the very thing in question, that is,
relativity.

Let's refine it a bit by stipulating that the Earth is stationary, the
track is stationary, and the clouds are stationary, and we'll also
stipulate that the lightning strike happens in an instant (even though
it doesn't), and marks all locations at that instant.

Now, where are the trains on the tracks when the lightning strikes,
and are they moving?
From: Ste on
On 10 Feb, 01:05, Edward Green <spamspamsp...(a)netzero.com> wrote:
> On Feb 1, 2:03 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > After all, if I have an
> > elastic meter ruler, and apply various amounts of tension to it, then
> > the measurement of an object can be changed without any "physical"
> > change in that object. On the other hand, if the object to be measured
> > is also elastic, and I apply the same amount of tension to the object
> > as well as to the meter ruler, then there are "physical" changes even
> > though there is no change in the measurement.
>
> In other words, the output of the measurement process is a function of
> both the state of the measuring device and of the measured object.
> Agreed.  But both are surely part of "physical reality".  If you take
> my point.

Yes, but the important point is about ascertaining which variables are
physically changing. There is little point an observer claiming that
he sees pink elephants, if the cause is that the observer is drugged,
as opposed to the elephant being truly present.
From: kenseto on
On Feb 9, 5:54 pm, PD <thedraperfam...(a)gmail.com> wrote:
> On Feb 9, 4:25 pm, kenseto <kens...(a)erinet.com> wrote:
>
>
>
>
>
> > On Feb 9, 11:30 am, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > On Feb 8, 3:09 pm, kenseto <kens...(a)erinet.com> wrote:
>
> > > > On Feb 8, 3:09 pm, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > > On Feb 8, 12:38 pm, kenseto <kens...(a)erinet.com> wrote:
>
> > > > > > On Feb 8, 11:42 am, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > > > > On Feb 7, 7:34 am, kenseto <kens...(a)erinet.com> wrote:
>
> > > > > > > > On Feb 6, 10:57 pm, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > > > > > > On Feb 6, 9:02 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > > > > > > > On 6 Feb, 17:16, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > > > > > > > > On Feb 5, 8:14 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > > > > > > > > > On 5 Feb, 16:00, PD <thedraperfam...(a)gmail.com> wrote:
>
> > > > > > > > > > > > > On Feb 4, 9:57 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > > > > > > > > > > > On 5 Feb, 02:48, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au>
> > > > > > > > > > > > > > wrote:
>
> > > > > > > > > > > > > > > "Ste" <ste_ro...(a)hotmail.com> wrote in message
>
> > > > > > > > > > > > > > >news:3979bca1-a35e-4399-bc71-75d2fc680757(a)21g2000yqj.googlegroups.com...
> > > > > > > > > > > > > > > On 5 Feb, 01:12, artful <artful...(a)hotmail.com> wrote:
>
> > > > > > > > > > > > > > > > On Feb 5, 11:49 am, kenseto <kens...(a)erinet..com> wrote:
>
> > > > > > > > > > > > > > > > > On Feb 4, 6:04 pm, mpalenik <markpale...(a)gmail.com> wrote:
>
> > > > > > > > > > > > > > > > > > On Feb 4, 5:59 pm, "kens...(a)erinet.com" <kens...(a)erinet.com> wrote:
>
> > > > > > > > > > > > > > > > > > > It it does violate the PoR. You made the contradcictory claims that
> > > > > > > > > > > > > > > > > > > the pole can fit into the barn physically (materially) an at the
> > > > > > > > > > > > > > > > > > > same
> > > > > > > > > > > > > > > > > > > time you claim that the pole cannot fit into the barn physically
> > > > > > > > > > > > > > > > > > > (materially)......that a violation of the PoR.
>
> > > > > > > > > > > > > > > > > > No. The doors are not closed simultaneously in the pole's frame, nor
> > > > > > > > > > > > > > > > > > are the two ends of the pole simultaneously in the barn in the pole's
> > > > > > > > > > > > > > > > > > reference frame. In the barn's frame, the two ends of the pole are in
> > > > > > > > > > > > > > > > > > the barn simultaneously and the doors shut simultaneously. In the
> > > > > > > > > > > > > > > > > > pole's frame, the two ends of the pole are in the barn at different
> > > > > > > > > > > > > > > > > > times and the doors shut at different times.
>
> > > > > > > > > > > > > > > > > Sigh..You are making the contradictory claims:
> > > > > > > > > > > > > > > > > 1. The pole can fit into the barn with both doors close
> > > > > > > > > > > > > > > > > simultaneously.
>
> > > > > > > > > > > > > > > > In the frame of the barn
>
> > > > > > > > > > > > > > > > > 2. The pole cannot fit into the barn with both doors close
> > > > > > > > > > > > > > > > > simultaneously.
>
> > > > > > > > > > > > > > > > In the frame of the pole
>
> > > > > > > > > > > > > > > > Two different meanings for 'simultaneously'.. So they are not
> > > > > > > > > > > > > > > > contradictory
>
> > > > > > > > > > > > > > > > You really are not very good at thinking or arguing logically.
>
> > > > > > > > > > > > > > > He seems perfectly reasonable to me.
>
> > > > > > > > > > > > > > > He's saying that, if one stands in the middle of the barn equidistant
> > > > > > > > > > > > > > > from the doors on each side, and one closes the doors simultaneously
> > > > > > > > > > > > > > > (i.e. what I would call "absolutely simultaneously", but which for
> > > > > > > > > > > > > > > conveience we'll say "simultaneous according to an observer standing
> > > > > > > > > > > > > > > equidistant from each door), then is it possible that both doors can
> > > > > > > > > > > > > > > appear closed while the ladder is observed to be inside the barn.
>
> > > > > > > > > > > > > > > It's a simple question, and the answer is "no!".
>
> > > > > > > > > > > > > > > ______________________________
> > > > > > > > > > > > > > > Its a simple question, and the answer is "yes". It is quite easy to devise
> > > > > > > > > > > > > > > an inertial frame in which the doors both appear closed and the ladder is
> > > > > > > > > > > > > > > fully contained in the barn.
>
> > > > > > > > > > > > > > The question wasn't whether "an inertial frame can be devised...".
> > > > > > > > > > > > > > Even I can think of situations where it would *appear* that the ladder
> > > > > > > > > > > > > > was inside and both doors closed. The question is whether this is an
> > > > > > > > > > > > > > artefact of observation, or whether the ladder is actually inside with
> > > > > > > > > > > > > > both doors closed. The answer remains "no, the ladder does not fit
> > > > > > > > > > > > > > inside with both doors closed".
>
> > > > > > > > > > > > > There is a simple test.
> > > > > > > > > > > > > We can determine whether the doors closed simultaneously. We do this
> > > > > > > > > > > > > using the test of simultaneity I indicated to you above:
> > > > > > > > > > > > > 1. Have the door-closings generate signals that travel at identical
> > > > > > > > > > > > > speeds from either event.
> > > > > > > > > > > > > 2. Have a detector positioned at equal distance from the barn doors.
> > > > > > > > > > > > > 3. If the detector receives the signals from the barn door-closings at
> > > > > > > > > > > > > the same time, then the two doors closed simultaneously unambiguously.
> > > > > > > > > > > > > You and I have agreed on this procedure.
>
> > > > > > > > > > > > If the detector is stationary, then yes we agree that this determines
> > > > > > > > > > > > simultaneity.
>
> > > > > > > > > > > > > Secondly, we can determine whether any part of the pole was outside
> > > > > > > > > > > > > the barn when those doors were closed by looking for the marks the
> > > > > > > > > > > > > ends of the pole would have made on the door. In the absence of those
> > > > > > > > > > > > > marks, we can safely conclude that the entire pole was inside the barn
> > > > > > > > > > > > > at the time the doors were closed.
>
> > > > > > > > > > > > Agreed. I would expect this test is definitive, were it possible to
> > > > > > > > > > > > carry out.
>
> > > > > > > > > > > And this has been tested in equivalent experiment. Documented.
>
> > > > > > > > > > Has it really? Which experiment was that, because my sources say that
> > > > > > > > > > length contraction has *never* been experimentally observed.
>
> > > > > > > > > I already told you the answer to that. Please look again.
>
> > > > > > > > > > > Seto does not read experimental papers, because he finds them
> > > > > > > > > > > confusing. So instead he believes what makes sense to him, rather than
> > > > > > > > > > > letting experimental results tell him something different.
>
> > > > > > > > > > You have to concede at least the possibility Paul that the reason Ken
> > > > > > > > > > finds them confusing, and so do I, is because *we* can see that they
> > > > > > > > > > rest on certain assumptions. That is, assumptions that you've
> > > > > > > > > > internalised, and assumptions which me and Ken reject.
>
> > > > > > > > > In Seto's case, it has more to do with thinking that SR says something
> > > > > > > > > it does not. I haven't yet figured out whether you are in the same
> > > > > > > > > boat.
> > > > > > > > > The only assumptions I make are those that have testable consequences.
> > > > > > > > > Which ones of those do you think I have that you reject?
>
> > > > > > > > So are you saying that SR doesn't say that the pole can be completely
> > > > > > > > inside the barn with both doors close simultaneously?
>
> > > > > > > SR *certainly* does not say that this statement holds true in both
> > > > > > > barn frame and pole frame.
> > > > > > > If you thought it did, then you never understood what SR says..
>
> > > > > > I didn't say that that statement hold true in both barn and pole
> > > > > > frame.
>
> > > > > Yes, you did. Two days ago you said just this: "The pole observer have
> > > > > to agree with the barn observer that the pole
> > > > > is completely inside the barn with both door close simultnaeously.."
>
> > > You said: "The pole observer have to agree with the barn observer that
> > > the pole is completely inside the barn with both door close
> > > simultnaeously."
>
> > > You THEN said "So are you saying that SR doesn't say that the pole can
> > > be completely inside the barn with both doors close simultaneously? I
> > > ask this question because that's what I think SR says.
>
> > > I told you No, SR does NOT say this, and neither does the PoR.
>
> > > You THEN said, "I didn't say that that statement hold true in both
> > > barn and pole frame."
>
> > I was referring to the obsolete SR interpretation. I disagree with
> > this old SR interprestion. I agree with Tom Roberts interpretation
> > that length contraction is not physical. It is the geometric
> > projection of the physical length of the pole is shorter and this
> > projection is able to fit into the length of the barn.
>
> SR has not changed, Ken. What Tom is telling you is how SR has
> described the events for decades.
> This does not account for your misunderstanding what you've read.- Hide quoted text -

Yes SR has changed. You and most of your SR brothers insisted that
there is physical length contraction which gives rise to the paradoxes
of SR....whereas Tom Roberts said that there is no physical length
contraction and that there is geometric projection shortening of the
pole. Geometric shortening is not physical....it is like I see you to
be shorter from a distant but you are not shorter physically.

Ken Seto
From: PD on
On Feb 10, 12:21 am, Ste <ste_ro...(a)hotmail.com> wrote:
> On 9 Feb, 21:02, PD <thedraperfam...(a)gmail.com> wrote:
>
>
>
> > On Feb 9, 12:26 pm, Ste <ste_ro...(a)hotmail.com> wrote:
>
> > > > Two trains are on adjacent tracks, going in opposite directions,
> > > > though I say that only to deliberately reinforce an ambiguity here. It
> > > > doesn't matter whether the trains are going at different speeds, and
> > > > in fact it isn't even important if one of the trains is stopped, or in
> > > > fact whether they are going in the same direction but one faster than
> > > > the other. All that matters is that there is a relative velocity
> > > > between them.
>
> > > > Two lightning strikes occur, drawn to the trains because of the
> > > > friction of the air between the trains. In fact, one lightning strike
> > > > leaves a scorch mark (a 1 cm spot, if you want to be precise) on
> > > > *both* trains as it hits. The other strike leaves a scorch mark
> > > > somewhere else on *both* trains.
>
> > > > The question now is, were the strikes simultaneous or not?
>
> > > > There is an observer on train A, and an observer on train B, and they
> > > > are both looking out the window when the strikes occur.
>
> > > > They make the following observations:
> > > > 1. The observer on train A sees the two lightning flashes
> > > > simultaneously.
> > > > 2. The observer on train B sees the flash from the front of his train
> > > > before he sees the flash from the rear of his train.
>
> > > > Now, it is not yet possible to determine whether the strikes were
> > > > simultaneous originally. We have more work to do. But I want to see if
> > > > you have a picture in your head of what has transpired.
>
> > > I have a basic picture, yes.
>
> > OK, then.
> > Let's now follow up these two observations above and couple them with
> > more observations.
> > 3. After the strikes, the observer on train A runs a tape measure from
> > his location to the scorch mark of one strike and makes note of the
> > number. Then he runs a tape measure from his location to the scorch
> > mark of the other strike and makes note of the number. These numbers
> > are equal. Note the scorch marks are on his train, but that's an
> > undeniable marker of where the event WAS when the signal propagation
> > began.
>
> Not really. If his train is moving, then the scorch marks will have
> actually moved from the location of the event.

With respect to what is the train moving? In this reference frame, the
train is not moving at all, though the other one is. I remind you that
it is not stated, nor is it clear, whether both trains are moving or
only one is. Nor does it matter, because even if the train is moving
relative to the track doesn't guarantee that the train is moving in
any absolute sense. For example, if the track itself were moving (say
because the surface of the earth is moving) and the train is moving in
the opposite direction, one could easily visualize that the train is
not moving at all, even if the train is moving relative to the track.

This is a crucial point about reference frames. We are making
statements about observations made IN THIS REFERENCE FRAME, and in
this reference frame, the train is not moving, the scorch marks are
not moving, and we can measure the speed of light in this reference
frame.

>
> > 4. After the strikes, the observer on train B runs a tape measure from
> > his location to the scorch mark of one strike and makes note of the
> > number. Then he runs a tape measure from his location to the scorch
> > mark of the other strike and makes note of the number. These numbers
> > are equal. Note the scorch marks are on his train, but that's an
> > undeniable marker of where the event WAS when the signal propagation
> > began.
>
> I'm not sure I agree with this.

It is exactly the symmetric situation with train A. Since the strikes
left marks on both trains, there is no reason to rule it out here if
it was permissible on A.

>
> > 5. The observer on A runs some experiments to measure the speed of
> > light and the isotropy of the speed of light (that it is the same in
> > either direction), and finds that the signal speed is the same. (Note
> > this isotropy would NOT hold if the signal were sound, for example.)
>
> And *how* does he measure this?

A variety of ways. You could, for example, follow the procedures used
by experimenters as documented in the papers referenced on the first
Google search return on "experimental basis for relativity".

>
> > 6. The observer on B runs some experiments to measure the speed of
> > light and the isotropy of the speed of light (that it is the same in
> > either direction), and finds that the signal speed is the same. (Note
> > this isotropy would NOT hold if the signal were sound, for example.)
>
> > Given these *observations* 1, 3, and 5, what would the observer on
> > train A conclude about the simultaneity of the original strikes?
>
> I must admit I don't have a clear enough picture of what is happening.
> This gedanken seems to presuppose the very thing in question, that is,
> relativity.

No, it doesn't presuppose anything other than what is *actually
observed* in experiment. I cannot underscore this enough. For example,
the claims that both (5) and (6) are both true may seem
counterintuitive. How can both trains measure the speed of light to be
the same from both directions, if the trains are moving relative to
each other? Certainly an aether-based theory would not hold this is
true. Does this mean we are *assuming* relativity is true so that
these statements are both true? No. Statements (5) and (6) are the
results of *experimental observation*. Nature really does behave this
way, even if we find it counterintuitive.

>
> Let's refine it a bit by stipulating that the Earth is stationary, the
> track is stationary, and the clouds are stationary,

On what basis would you make such an arbitrary stipulation, when you
KNOW that this is not the case?
You may be tempted to say, "Because we have to have an absolute
reference for stationary *someplace*, and we might as well make it
Earth because we live here." A moment's thought will tell you this is
foolish. Physical laws don't care where we live. Then, in the search
for finding an absolute reference for rest, you may eventually ask
yourself why such an absolute reference would be needed at all,
especially if there is nothing you can clearly identify that would fit
the bill...

> and we'll also
> stipulate that the lightning strike happens in an instant (even though
> it doesn't), and marks all locations at that instant.
>
> Now, where are the trains on the tracks when the lightning strikes,
> and are they moving?

You see? You are trying to establish an absolute reference frame, even
if it means doing so completely arbitrarily, JUST SO you can say
whether the trains are absolutely moving or not.