1. Say I have Observer A & B in a nearly empty universe.

Observer A & B believe they are at rest relative to each other. A & B both measure the distance to each other as 1 light minute.

Now This universe has 2 other observers C & D who are also at rest relative to each other. They also measure the distance between themselves as 1 light minute.

Observer A & B measure the distance between C & D and believe that the distance is 22.3 light minutes and that C & D are travelling almost straight at them at .999c

Observer C & D measure the distance between A & B and believe that the distance is 22.3 light minutes and that A & B are travelling almost straight at them at .999c

So who's frame is the correct frame? According to Jeff both sets of observers should have some way to figure out which one of them have the real frame that they both can agree on.

SR says that either frame is valid and the final frame will be dependant on how much each one accelerates.

If A & B don't do anything and C & D accelerate to become at rest with A & B then C & D will see that the distance between them will expand to 22.3 light minutes and that the distance between A & B seems to shrink to just 1 light minute.

If C & D don't do anything and A & Baccelerate to become at rest with C & D then A & B will see that the distance between them will expand to 22.3 light minutes and that the distance between C & D seems to shrink to just 1 light minute.

If A & B and C & D both accelerate at the same rate until they come to rest with respect to each other then do you realise that none of the observers will measure the distance between anyone at 1 light minute or 22.3 light minutes?!?!
Not hard to calculate what they will measure.

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Originally Posted by Jeff Root
It is still the case that neither the GPS satellites nor
the GPS receivers observe any distances.

Okay, I did deny that. But I didn't deny "the basics of
relativity", which is what I was denying that I denied.

I'm not the least bit sure that the length contraction
couldn't be observed. But I asked korjik over and
over again, many, many times, to suggest a technique
by which it might be done, and he refused every time.
Nobody else suggested anything useful either. Maybe
no useable technique was put forward because no
technique was possible, or at least practical. So that
became my stance. I asserted that measurement of
distances by relativistically-moving observers were so
difficult as to be practically impossible. I never once
claimed that length contraction doesn't happen.

Sure there is. It is the proper distance. The distance
measured when at rest relative to the objects being
measured, which every observer will agree is correct.

It is easy as pie to distinguish the correct value from
an incorrect one.

Which I do.

No way. That doesn't even make sense. I am most
definitely not claiming that there is an absolute frame
of reference of any kind, nor that any one frame is in
any way superior to others in general.

I'm just saying that the proper measurement of a
length or distance gives the correct value, while
a measurement made in motion relative to what is
being measured cannot give the correct value.
Everyone will agree with the results of the proper
measurement. In general, everyone disagrees with
results of measurements made in relative motion.

-- Jeff, in Minneapolis
Actually, Korjik repeatedly said that any technique would work. That is slightly different from what you said. It is up to the person who believes there is an error to show the error.

Proper is not real Jeff. You do not measure the proper distance, ever. You measure the real distance, which if the observer isnt moving, is the same as the proper distance.

You are just plain wrong. There have been numerous mentions of observations that cannot work without length contraction. You are also just plain wrong that the proper value is the 'correct' value. Whether you are making the stationary frame special is a bit debatable, because technically making everyone measure the same distances throws all of SR out the window and forces Galliean relativity/transforms which throws frames right out the window.

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When I use a tape measure to measure the length of
my hallway, I am making a direct measurment of length.

When some ancient Greek guys found the distance of
the Moon using geometry and Earth's diameter, the angle
of Earth's umbra, the angular diameter of the Moon, and
the size of Earth's shadow on the Moon, they measured
angles, which are just ratios of lengths. For the purposes
of this thread, I'd say they measured the distance. Not
directly, for sure, but they measured distance, not time.

Similarly when measurement of the Moon's parallax
became possible. Although widely-separated observers
on Earth might synchronize their observations, time is
not a factor in the measurement. What is actually
measured are angles and distances. The result is a
measured distance to the Moon, using geometry.
Not direct, but a measurement of length as opposed
to something else.

My objection in the other thread was to korjik's
assertion that the traveller would think the distance
from the Solar System to the distant star was only
about 1 light-year. That is what I disagree with, and
what I would argue against if I knew how. The 1 ly
figure, even if it could actually be measured, is an
obvious distortion of the true distance. That is the
core of my assertion.

-- Jeff, in Minneapolis

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Originally Posted by korjik
Originally Posted by Jeff Root
over again, many, many times, to suggest a technique
by which it might be done, and he refused every time.
Actually, Korjik repeatedly said that any technique would work.
Yes, and you refused to specify one.

Originally Posted by korjik
That is slightly different from what you said.
Yes, it is slightly different from what I said, and it is
perfectly true, but what I said is also perfectly true.

Originally Posted by korjik
It is up to the person who believes there is an error to
show the error.
Yes. In this case I asserted that you were in error in
saying that some technique could be used by a relativistic
traveller to measure the distance between stars. I wanted
you to specify a technique so that we could analyze it.
But you refused, over and over and over. If you actually
knew of a technique that would work, I would think you
would suggest it.

Originally Posted by korjik
Proper is not real Jeff. You do not measure the proper
distance, ever. You measure the real distance, which if
the observer isnt moving, is the same as the proper
distance.
Does that make sense to anyone?

Does it make sense to you, korjik?

Originally Posted by korjik
You are just plain wrong.
Then show it.

Originally Posted by korjik
There have been numerous mentions of observations that
cannot work without length contraction.
Three, actually. Cosmic-ray generated muons, particles
in accelerators, and GPS measurements. However, none
of them involve measurement of distance by an observer
in relativistic motion, which is what is in question.

Originally Posted by korjik
You are also just plain wrong that the proper value is
the 'correct' value.
If there is a 'correct' value, then it is the proper value.
No other value can claim that status.

Originally Posted by korjik
Whether you are making the stationary frame special is
a bit debatable,
I'll belabor the point that I'm not talking about a
"stationary frame", which of course is a fiction, but
a frame which is common to observer and observed.

Originally Posted by korjik
because technically making everyone measure the same
distances throws all of SR out the window and forces
Galliean relativity/transforms which throws frames right
out the window.
I don't believe that for a millisecond.

-- Jeff, in Minneapolis
Last edited by Jeff Root; 2012-Jul-18 at 10:17 AM.

5. Originally Posted by Jeff Root
... The 1 ly
figure, even if it could actually be measured, is an
obvious distortion of the true distance. That is the
core of my assertion.

-- Jeff, in Minneapolis
The core is that you think one of the measures, in one situation, is the "true distance", and the others are "distortions".

Why?

6. Originally Posted by Jeff Root
Okay, I did deny that. But I didn't deny "the basics of
relativity", which is what I was denying that I denied.
You deny length contraction would be seen by someone travelling at high speed; that is a basic fact of relativity. You are denying it.

I'm not the least bit sure that the length contraction couldn't be observed.
I used to be doubtful about this; but having recently seen a camera that can film things moving at the speed of light (it is quite spooky to see a flash of light passing slowly through a bottle) the only thing we need to do now is accelerate a macroscopic object to a significant proportion of the speed of light. How hard can that be...

The trouble is, even if we did that and showed something being length contracted, at least some people would say that this wasn't a "direct measurement" (there was a digital camera involved, after all). That is the problem with the "no true scotsman" argument: the person making it can just keep moving the goalposts.

But, as the "direct" (however you choose to define that) measurement of this is totally and utterly irrelevant (even if an interesting engineering challenge) can we just drop it?

But I asked korjik over and
over again, many, many times, to suggest a technique
by which it might be done, and he refused every time.
No. He quite reasonably said you can use any technique that you would use if you were stationary. You waffled that this wouldn't work because the results would be "unceratain" for some unspecified reason.

Nobody else suggested anything useful either.
Wayne [and Grey] produced quite a detailed descriptions of techniques using triangulation and radar. I rather lost interest for a while and didn't see your responses to those.

No way. That doesn't even make sense. I am most
definitely not claiming that there is an absolute frame
of reference of any kind, nor that any one frame is in
any way superior to others in general.
Even though only one of them is correct? So the "correct" frame of reference is not superior to the "incorrect" ones?

The thing is, Jeff, I'm sure you don't mean to sound like a relativity-denier, but you do. You sound as if you are denying length contraction ("but there is no evidence for it ... ok, there is no direct evidence for it"). You sound as if you are insisting on a special frame of reference ("only one is correct"). You seem to ignore arguments you don't like ("no one has shown me how it could be measured"; "OK here are three ways ..."; "no one has shown me how it could be measured").

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Originally Posted by WayneFrancis
Say I have Observer A & B in a nearly empty universe.

Observer A & B believe they are at rest relative to each
other. A & B both measure the distance to each other
as 1 light minute.

Now This universe has 2 other observers C & D who are
also at rest relative to each other. They also measure
the distance between themselves as 1 light minute.

Observer A & B measure the distance between C & D
and believe that the distance is 22.3 light minutes and
that C & D are travelling almost straight at them at .999c

Observer C & D measure the distance between A & B
and believe that the distance is 22.3 light minutes and
that A & B are travelling almost straight at them at .999c
You got the numbers backward. The last two measured
distances you refer to need to be shorter, not longer than
the others. The easiest way to fix this would be to make
the proper distance between each pair of observers 22.3
light-minutes, and the distance as measured by the other,
relatively-moving pair, 1 light-minute.

Originally Posted by WayneFrancis
So who's frame is the correct frame? According to Jeff
both sets of observers should have some way to figure
out which one of them have the real frame that they
both can agree on.
Not hardly. I never said or implied anything about a
"real frame". The "correct frame" for a measurement
of a thing is the frame of the thing being measured.

Simple. Obvious. Universally applicable.

All of the observers know that proper measurements
give correct values, while the measurements in relative
motion give incorrect values.

Since you established the distance between each pair
of observers to be the same for both pairs, you limited
potential dramatic effect of your point.

Originally Posted by WayneFrancis
SR says that either frame is valid and the final frame
will be dependant on how much each one accelerates.

If A & B don't do anything and C & D accelerate to become
at rest with A & B then C & D will see that the distance
between them will expand to 22.3 light minutes and that
the distance between A & B seems to shrink to just
1 light minute.
Ouch. At least two mistakes there. First, the same
mistake as earlier: The distance between A & B seems
to increase, not shrink.

Second, the acceleration throws everything out of synch.
You are talking about a really enormous amount of
acceleration. And the distance between C & D is not
trivial. This means that C & D will not be able to stay
a constant distance apart even in their own frame. If
the distance in their own frame can change to anything
(depending on how they choose to synchronize their
acceleration), then the distance in anyone else's frame
is at least equally unpredictable.

Third, I'm not sure that the reason you think "C & D will
see that the distance between them will expand" is the
reason I just gave. You seem to be saying it is due to
the cessation of length contraction. Ouch ouch ouch.

Originally Posted by WayneFrancis
If C & D don't do anything and A & B accelerate to become
at rest with C & D then A & B will see that the distance
between them will expand to 22.3 light minutes and that
the distance between C & D seems to shrink to just
1 light minute.
I agree that the situations are completely symmetric.

Originally Posted by WayneFrancis
If A & B and C & D both accelerate at the same rate until
they come to rest with respect to each other then do you
realise that none of the observers will measure the distance
between anyone at 1 light minute or 22.3 light minutes?!?!
Not hard to calculate what they will measure.
The distances will vary depending on how the accelerations
are synchronized.

the numbers backward, but you may be right in principle
about the change due to acceleration.

-- Jeff, in Minneapolis
Last edited by Jeff Root; 2012-Jul-18 at 11:31 AM.

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Originally Posted by pzkpfw
The core is that you think one of the measures, in
one situation, is the "true distance", and the others
are "distortions".

Why?
Because they are.

I'm sorry, I understand how pathetic that answer is.
I don't know how to explain what is self-evident.

-- Jeff, in Minneapolis

9. Originally Posted by Jeff Root
I'm not the least bit sure that the length contraction
couldn't be observed. But I asked korjik over and
over again, many, many times, to suggest a technique
by which it might be done, and he refused every time.
Nobody else suggested anything useful either. Maybe
no useable technique was put forward because no
technique was possible, or at least practical. So that
became my stance. I asserted that measurement of
distances by relativistically-moving observers were so
difficult as to be practically impossible.
I suggested radar, which is a remarkably common method for measuring distances to objects which are too far away to use a ruler, and showed how two observers moving relative to each other would get different results. I also pointed out that we're remarkably close to having the needed sensitivity to be able to see the movement of stellar positions from relativistic effects due to the motion of the Earth. It's far from practically impossible.

Originally Posted by Jeff Root
But I didn't deny "the basics of
relativity", which is what I was denying that I denied.
...
I'm just saying that the proper measurement of a
length or distance gives the correct value, while
a measurement made in motion relative to what is
being measured cannot give the correct value.
This is "denying the basics of relativity". One of the postulates of special relativity is that all inertial reference frames are equally valid. Measurements made in one frame are not preferred over any other. You can't simultaneously claim that this postulate is false and also insist that you aren't "denying the the basics of relativity". (Well, at least you can't be consistent and claim both of those things at once; you are apparently trying to claim both of those things nevertheless).

10. As an exercise in physics, my inclination is simply to perform the triangulation measurements and make the corresponding calculation of the distance to the target, making sure to specify the frame of reference unambiguously. Then as a thought exercise I could do an exact transformation to the frame of reference of another observer who is moving relative to me. I would just let the language of mathematics do the talking and not get bogged down in quibbling with words concerning what is "correct" or "true".

Once again, this thread has become so labored and convoluted that I cannot tell what is being asserted or denied as the case may be, and I really do not care.

11. Originally Posted by Jeff Root
Because they are.

I'm sorry, I understand how pathetic that answer is.
I don't know how to explain what is self-evident.
I'd say that it is self-evident that there is one correct measurement for any given distance and time. Unfortunately, Einstein discovered that what is "self-evident" turns out to not actually be true! In most of physics, I think we've realized that many of the obvious things we take for granted have turned out to be wrong.

12. Originally Posted by Jeff Root
Because they are.
I'm sorry, I understand how pathetic that answer is.
I don't know how to explain what is self-evident.
Perhaps you could tell us how to determine which person is "true" as opposed to the others that are "distorted".

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Why would you consider proper distance to be "true"? We already have a term to differentiate it from the rest, ie "proper". Isn't that enough?

Also your reasoning falls apart somewhere. You say that all observers will agree on the length of an object in its rest frame, ie its proper length. But the question is, how would those observers, that are themselves not in that rest frame, come to agree upon that? Because they apply the lorentz transformations from their respective frames to the rest frame, and each of them will arrive at the same answer. But that can be said about every frame, not just the rest frame. If some observer A is moving relative to the object, then all observers will also agree on the length as measured by A (as opposed to the rest frame of the object), by doing the exact same thing, applying the lorentz transformations from their respective frames to this observer A's frame. Again all will get the same result and agree upon it.

14. That's a good point: in Jeff's world it seems that the moving observer cannot measure the distance and therefore cannot use the Lorentz transformation to convert it to the distance that the "stationary" observer would measure. And so they cannot agree on the distance...

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Originally Posted by Jeff Root
Because they are.

I'm sorry, I understand how pathetic that answer is.
I don't know how to explain what is self-evident.

-- Jeff, in Minneapolis
It isnt, You are wrong. You are replacing belief with evidence. You miss the entire point of relativity. The whole reason why it is called relativity as opposed to absloutity.

16. I regret I messed up my thought experiments numbers and Jeff focused on that instead on admitting that the basic premiss of the thought experiment was the "correct measurement" would depend completely on how the 4 observers eventually come to rest with respect to each other. And since there was 3 different solution that I can think of that would achieve that end there would be 3 different answers that could be deemed "correct" or more exactly the would be no way to determine which one was "correct" which is the very point of SR.

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Originally Posted by Strange
You deny length contraction would be seen by someone
travelling at high speed; that is a basic fact of relativity.
You are denying it.
That is not a fact of relativity.

Relativity says that length contraction occurs. It does
not say that the contraction will be seen.

I said that I think the measurement is "so difficult as
to be practically impossible." Under the relentless hours
of interrogation I may have sometimes expressed it more
strongly than that, but that is what I meant. And as I
admit in the next quote, even that is too strong. But so
far no evidence has been put forward to refute it.

Originally Posted by Strange
Originally Posted by Jeff Root
I'm not the least bit sure that the length contraction
couldn't be observed.
...
The trouble is, even if we did that and showed something
being length contracted, at least some people would say
that this wasn't a "direct measurement" (there was a digital
camera involved, after all). That is the problem with the
"no true scotsman" argument: the person making it can
just keep moving the goalposts.
I haven't moved any goalpost. My original point in the
other thread was that the traveller would not think the
distance between the Solar System and the distant star
would change from 22.3 ly to about 1 ly. In order to try
to meet the objections to that assertion, and based on
all the evidence I had (and still have), I further claimed
that the traveller simply would not be able to measure
the distance, because the conditions would make it
practically impossible.

Since then several ideas have been suggested about
measuring the times of radar pulses, muons or whatnot.
Those are not measurements of distance. They are
measurements of time. Distance can only be derived
from the time measurement through theory-dependent
calculations. Yes, I accept the theory as correct. But
that doesn't mean I won't object when you call a
measurement of time a measurement of distance.

Originally Posted by Strange
But, as the "direct" (however you choose to define that)
measurement of this is totally and utterly irrelevant (even
if an interesting engineering challenge) can we just drop it?
I don't remember if I'm the one who first used the term
"direct" or if someone else used it first. In any case I
agree that it is irrelevant. A measurement of time is not
a measurement of distance. My objection is that the
traveller would not measure the distance as about 1 ly,
but even if I'm wrong about that, he still certainly would
not think that the distance actually was about 1 ly.

Originally Posted by Strange
Originally Posted by Jeff Root
But I asked korjik over and
over again, many, many times, to suggest a technique
by which it might be done, and he refused every time.
No. He quite reasonably said you can use any technique
that you would use if you were stationary. You waffled
that this wouldn't work because the results would be
"unceratain" for some unspecified reason.
He refused to specify technique, over and over and over.

He wants me to choose one. I would choose a technique
which I think will not work. That is unavoidable, because
I have not been able to think of a technique which would
work. So if I were to end up showing that the technique
I picked wouldn't work, korjik and you and everyone else,
even including myself, would complain that I set the
situation up incorrectly-- I used the wrong technique.
So korjik, or you, or some other proponent of the idea
that the distance can be measured, has to specify what
technique should be analyzed. By asking me to choose
a technique, korjik is requiring me to create a strawman
argument. There is no need for that. Just specify a
technique that can be analyzed, and we can go forward.

Originally Posted by Strange
Wayne [and Grey] produced quite a detailed descriptions
of techniques using triangulation and radar.
Radar measures time. The triangulation possibility was
extremely vague. It wasn't even made clear what was
supposed to be measured. The descriptions were not
detailed at all.

My own reply was not waffling, but it was at least as
vague as Wayne's description of the measurement
technique: I said that the distance measurement would
be difficult or impossible because of the problem of
determining simultaneity. When the two ends of the
tape measure are located is as important as where they
are located, and there will be a huge unresolveable
discrepency in this case.

Grey did give some good descriptions of things, but not
of how the traveller could measure the distance from the
Solar System to the distant star.

Originally Posted by Strange
Originally Posted by Jeff Root
I am most definitely not claiming that there is an
absolute frame of reference of any kind, nor that any
one frame is in any way superior to others in general.
Even though only one of them is correct? So the
"correct" frame of reference is not superior to the
"incorrect" ones?
There is no absolute frame of reference. No one frame
of reference is superior to others in general. The two
statements amount to the same thing.

The frame of a thing being measured is the only frame
which allows correct measurement of the thing's length.

Originally Posted by Strange
The thing is, Jeff, I'm sure you don't mean to sound like
a relativity-denier, but you do. You sound as if you
are denying length contraction ...
This is what I said to korjik back on June 24:

Originally Posted by Jeff Root
I am not saying there is any problem with the
observations or the theory. I am saying there is a
Originally Posted by Strange
... ("but there is no evidence for it ... ok, there is no
direct evidence for it"). You sound as if you are
insisting on a special frame of reference ("only one is correct").
Only a measurement made in an object's frame can give
a correct length measurement of that object. If the
object's length is changing, then no measurement of
its length can be correct because there can be no
general agreement on the simultaneity of the two
endpoint measurements.

Originally Posted by Strange
You seem to ignore arguments you don't like
("no one has shown me how it could be measured";
"OK here are three ways ..."; "no one has shown me
how it could be measured").
No one has shown me how the distance between the
Solar System and a distant star could be measured by
a relativistic traveller.
"Okay, here is a way that the half-life of relativistic
muons is measured, a way that light travel times
might be measured given a magical radar, and a way
involving triangulation that might work if I could say
exactly what it was."

-- Jeff, in Minneapolis

18. Originally Posted by Jeff Root
I haven't moved any goalpost.
I very carefully worded my comment to avoid saying that you had moved any goal posts.

The frame of a thing being measured is the only frame
which allows correct measurement of the thing's length.
So you are simply using the word "correct" as a synonym for "proper" rather than "true" or "right"?

a way involving triangulation that might work if I could say exactly what it was.
I am curious why you think it works from earth but not from a moving spaceship ...

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Originally Posted by Grey
I suggested radar, which is a remarkably common
method for measuring distances to objects which
are too far away to use a ruler, and showed how
two observers moving relative to each other would
get different results.
As I said to Strange, radar measures time, not dstance.
In mundane situations, where relative speeds are low,
such as traffic monitoring or detecting ICBMs, that is
not a problem. At the very high speed of the scenario
in question, it is.

Originally Posted by Grey
I also pointed out that we're remarkably close to having
the needed sensitivity to be able to see the movement
of stellar positions from relativistic effects due to the
motion of the Earth. It's far from practically impossible.
I agree that we may be close to having the sensitivity
needed to see the movement of stellar positions due to
a relativistic effect of the motion of the Earth. I do not
agree that the effect is a change in distance.

Originally Posted by Grey
This is "denying the basics of relativity". One of the
postulates of special relativity is that all inertial reference
frames are equally valid. Measurements made in one frame
are not preferred over any other. You can't simultaneously
claim that this postulate is false and also insist that you
aren't "denying the the basics of relativity".
I agree completely with the postulate. What I disagree
with is your interpretation of it.

All inertial reference frames are equally valid. All inertial
reference frames equally allow observers in those frames
to accurately measure the lengths of objects in their own
frame. All inertial reference frames equally require that
observers in those frames will be unable to accurately
measure the lengths of objects moving relative to their
own frame.

Special relativity has no postulate or equation stating
that an observer can accurately measure the lengths of
things moving relative to him.

Can you pet a kitten without killing it?

How about if the relative speed between you and the
kitten is .999 c ? Yet you can safely pet a kitten
that you take with you aboard your spaceship nomatter
what your speed relative to anything else. Apparently
"all inertial reference frames are equally valid" doesn't
mean that what is possible in one frame is also possible
between two frames. It just means that it is possible
in any frame.

-- Jeff, in Minneapolis

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Originally Posted by Hornblower
Once again, this thread has become so labored and
convoluted that I cannot tell what is being asserted
or denied ...
I'm denying that a relativistic traveller would think
that everything outside his spaceship shrank because
his speed changed.

Everyone else seems to be stuck in interpreting that
as denying the basics of relativity.

-- Jeff, in Minneapolis

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Originally Posted by NEOWatcher
Perhaps you could tell us how to determine which
person is "true" as opposed to the others that are
"distorted".
I think I've said about a dozen times, now:

An observer not in motion relative to the thing
being measured can get an accurate measurement
of true length or distance. An observer in motion
relative to the thing being measured will always
get a distorted measurement.

-- Jeff, in Minneapolis

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Originally Posted by WayneFrancis
I regret I messed up my thought experiments numbers
basic premiss of the thought experiment was the
"correct measurement" would depend completely on
how the 4 observers eventually come to rest with
respect to each other. And since there was 3 different
solution that I can think of that would achieve that
end there would be 3 different answers that could be
deemed "correct" or more exactly the would be no way
to determine which one was "correct" which is the
very point of SR.
Can we try again? That bit at the end surprised me,
and I think you were basically right: The two ships
travelling together, one ahead of the other, would
see the distance between them increase when they
match speed with the other pair of ships. I thought
at first that they'd stay the same distance apart.

That is part of the simultaneity problem which I say
would make distance measurements so difficult as
to be practically impossible.

-- Jeff, in Minneapolis

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Originally Posted by WayneFrancis
I regret I messed up my thought experiments numbers and Jeff focused on that instead on admitting that the basic premiss of the thought experiment was the "correct measurement" would depend completely on how the 4 observers eventually come to rest with respect to each other. And since there was 3 different solution that I can think of that would achieve that end there would be 3 different answers that could be deemed "correct" or more exactly the would be no way to determine which one was "correct" which is the very point of SR.
He isnt interested in what SR actually says, or what evidence there is for it. That is why every time someone tries to give a concrete example, he ignores it, picks at an irrelevant error, or give a non-sequitur reason why it would not work.

24. Here is another take on this topic with a thought exercise. Suppose we have two identical spacecraft, A and B. A is on the ground and B flies by just over it at nearly the speed of light, while we observe from off to the side. With the camera and flash gun of our dreams we take a picture at just the right instant, as B is directly over A and at the same distance from us. From this picture we can get a visual measurement of the apparent length of B in comparison with A. In principle we get an exact visual observation of B's length contraction and can compare it with what is predicted by the SR theory. We also can make an exact calculation of what B's crew would see, which should be the appearance that A is length-contracted. In principle we could ask them what they saw and compare their report with our calculations.

What sort of uncertainties or inaccuracies does anyone think I am missing in my remarks?

25. Originally Posted by Jeff Root
Relativity says that length contraction occurs. It does not say that the contraction will be seen.
But there is absolutely no reason to think it won't be (other than your personal beliefs).

26. Originally Posted by Jeff Root
I'm denying that a relativistic traveller would think that everything outside his spaceship shrank because his speed changed.

Everyone else seems to be stuck in interpreting that as denying the basics of relativity.
Perhaps that is because it is. Unless you can show, using the mathematics of relativity, that he wouldn't?

27. Order of Kilopi
Join Date
Sep 2004
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5,447
Originally Posted by Hornblower
Here is another take on this topic with a thought exercise. Suppose we have two identical spacecraft, A and B. A is on the ground and B flies by just over it at nearly the speed of light, while we observe from off to the side. With the camera and flash gun of our dreams we take a picture at just the right instant, as B is directly over A and at the same distance from us. From this picture we can get a visual measurement of the apparent length of B in comparison with A. In principle we get an exact visual observation of B's length contraction and can compare it with what is predicted by the SR theory. We also can make an exact calculation of what B's crew would see, which should be the appearance that A is length-contracted. In principle we could ask them what they saw and compare their report with our calculations.

What sort of uncertainties or inaccuracies does anyone think I am missing in my remarks?
lets see: B would have to be travelling through the atmosphere at close to the speed of light, so it could not happen. The Flash gun of your dreams would have to be too expensive so it would not actually get built. B's crew would be too busy hanging on for dear life to actually use the radio, and the redshift would make it too hard to use anyway.

At least, that is the quality of the argument so far....

28. Originally Posted by korjik
lets see: B would have to be travelling through the atmosphere at close to the speed of light, so it could not happen. The Flash gun of your dreams would have to be too expensive so it would not actually get built. B's crew would be too busy hanging on for dear life to actually use the radio, and the redshift would make it too hard to use anyway.

At least, that is the quality of the argument so far....
This is a thought exercise with no atmosphere or other mechanical impediments, which I perhaps did not make clear. Cost of the flash is not an issue. I am not asking about real world technical or cost issues. Let's put us in free fall out in space alongside A, while B flies by. We in principle have the means to measure B's relative speed and apparent length exactly. As I think I understand the theory that should make it possible, in principle, to calculate exactly what B's crew would see. Does anyone think I still am missing something that would introduce and inherent uncertainty?

29. okay I think this has been going round in circles long enough, there is no added value to continuing this thread.
I have the impression that Jeff Root is somehow arguing for a perfect rest frame where everything should be measured true. There is no such frame in relativity.
If you would like to discuss this then take it to ATM.

As usual, if there are any good reasons why this should be re-opened, report this post.

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