[triangulation at speed]

[Grey]

However, length contraction has not been observed by experiment which is all I was stating. It's theorised so it has not been shown in experiment after experiment.

Well, there have been no experiments where we take a meter stick, accelerate it to near light speed, and then measure the length. That's not feasible with our technology (or with any foreseeable in the near future). However, that's certainly not the only way to test Lorentz contraction. Indeed, the original Michelson-Morley and Kennedy-Thorndike experiments require both time dilation and Lorentz contraction to explain the results. There are other ways to measure contraction as well. For example, we can't directly measure the length of heavy ions moving at relativistic speeds as they zip through particle accelerators. However, from relativity we'd expect them to be shaped something like pancakes rather than little spheres from the perspective of the lab frame. And indeed, if you try to treat them as spheres, you get the wrong scattering results, but if you apply Lorentz contraction and treat them as compressed in the direction of motion, you get results that match observation.

Relativity makes a very long list of predictions, some of which involve time dilation alone, some of which involve Lorentz contraction alone, and some of which involve both. Some of those predictions cannot be tested easily, but many of them have been. I stand by my statement that Lorentz contraction has been clearly shown by experiment many times in the century since Einstein introduced relativity.

[WayneFrancis]

I don't believe there were any observers in the muons frame to report on their obsevations during that journey.

This type of argument shows a very basic lack of understanding about science and observations. It is like saying no one was around a 150 million years ago so their for the Jarassic period is suspect.
We know muons decay very rapidly
We know how they are generated in the upper atmosphere
We know their speeds when the reach the surface of the Earth
We know that given their decay rates that without SR they wouldn't reach the ground before they decay.
We know SR is correct by multiple independent methods to a very high degree of accuracy.
Therefore we can reasonably conclude what is happening to the muons in their frame of reference and that matches observation.

[Jeff Root]

I don't believe there were any observers in the muons
frame to report on their observations during that journey.

Are you denying that we have measurements of the half
life of muons?

It doesn't look like it to me. He's denying that any
measurements have been made by somebody moving
at sufficient speed to measure length contraction of
everything else in the Universe. I agree with Webbo.
I think that such a measurement is so difficult as to
be practically impossible. It will never be done.

Length contraction due to the observer's motion can
only be inferred from observations of other things,
such as half-lives of muons.

-- Jeff, in Minneapolis

[WayneFrancis]

It doesn't look like it to me. He's denying that any
measurements have been made by somebody moving
at sufficient speed to measure length contraction of
everything else in the Universe. I agree with Webbo.
I think that such a measurement is so difficult as to
be practically impossible. It will never be done.

Length contraction due to the observer's motion can
only be inferred from observations of other things,
such as half-lives of muons.

-- Jeff, in Minneapolis

OK for a second say we didn't have any tests to show length contraction. How do you and webbo then propose that Special Relativity has been confirmed with such a high degree of precision if there is no length contraction?

You can't throw out half of a theory and then just say the other half gives the exact same results as the whole theory combined. I'm sorry Jeff but this sounds like more of your "I don't think we'll ever be able to measure it so I doubt it" without you seemingly thinking through all the things that already confirm it.

It just boggles my mind that you can dismiss away half the theory and all its implications and expect to get the same result. Let me use a cooking analogy.

Here are the basic ingredients for a very basic lasagne

flour
egg
olive oil
garlic
beef mince
tomatoes

cheese
milk
parsley
butter
salt
pepper

Now remove half of them. I'll divide the ingredients up 3 different ways and you tell me how you'd get a lasagne from just those ingredients

Just first half	Just 2nd half	Alternating
flour	cheese	flour
egg	milk	olive oil
olive oil	parsley	beef mince
garlic	butter	milk
beef mince	salt	butter
tomatoes	pepper	pepper

Tell me in what way any of those three recipes would make a lasagne. Then when you are done with that. Please reformulate SR so that it works without length contraction. It isn't something that is just a by-product of the theory. So if you want to remove it you have to do it in such a way that your modified version of SR would still get all the same results as has been confirmed by numerous observations.

[NEOWatcher]

But what has that got to do with observed length contraction? Obvious or otherwise.

I suggest you read some history on all the experiments leading up to GPS. Even Sputnik had an element of reletivity experimentation.
Then, I would like you to explain how the measurements made can be done without measuring any kind of movement between two known differences.
I see no way to measure time without involving a known distance, whether that be the length of a pendulum swing, or the difference between waves in an atomic emission.

[cjameshuff]

It doesn't look like it to me. He's denying that any
measurements have been made by somebody moving
at sufficient speed to measure length contraction of
everything else in the Universe. I agree with Webbo.
I think that such a measurement is so difficult as to
be practically impossible. It will never be done.

But it was done, with the muon observations. The figure for muon half life is for an observer in the muon's rest frame. Webbo's claim is equivalent to claiming such things are unknown.

Length contraction due to the observer's motion can
only be inferred from observations of other things,
such as half-lives of muons.

Time dilation and length contraction are the same phenomena. What we measure as time dilation in the frame of a detector counting muons at the bottom of Earth's atmosphere is length contraction in the frame of the muons. Both are required for relativity to have any internal consistency, you can't get one without the other. Time dilation is just the easier aspect to measure directly, though as Grey said, some experiments do directly depend on length contraction as well.

[Jeff Root]

He's denying that any measurements have been made
by somebody moving at sufficient speed to measure
length contraction of everything else in the Universe.
I agree with Webbo. I think that such a measurement
is so difficult as to be practically impossible. It will
never be done.

But it was done, with the muon observations.

No it was not.

The figure for muon half life is for an observer in the
muon's rest frame. Webbo's claim is equivalent to
claiming such things are unknown.

Not if my expression of his claim is accurate. We can
call it my claim. I am responsible for prompting Webbo
to post (in a different thread) the original post here.
I disagreed with korjik when he claimed that someone
travelling for one year at .999 c relative to the nearby
stars would think they travelled a distance of about
one light-year. I still disagree with that.

Let's see if these new links work.

http://cosmoquest.org/forum/showthre...33#post2031033

My claim, for the current discussion, is that no
measurements have been made by anyone moving
at sufficient speed to measure length contraction of
everything else in the Universe. Such a measurement
is so difficult as to be practically impossible. It will
never be done.

Time dilation and length contraction are the same
phenomena.

Different aspects of the same phenomenon.

What we measure as time dilation in the frame of a
detector counting muons at the bottom of Earth's
atmosphere is length contraction in the frame of the
muons. Both are required for relativity to have any
internal consistency, you can't get one without the
other. Time dilation is just the easier aspect to
measure directly, though as Grey said, some
experiments do directly depend on length
contraction as well.

I have never disagreed with any of that, or implied
that I disagreed with any of it.

No measurements have been made by anyone moving
at sufficient speed to measure length contraction of
everything else in the Universe. Such a measurement
is so difficult as to be practically impossible. It will
never be done.

-- Jeff, in Minneapolis

[Grey]

No measurements have been made by anyone moving
at sufficient speed to measure length contraction of
everything else in the Universe. Such a measurement
is so difficult as to be practically impossible. It will
never be done.

Even this isn't true. That is, you're correct that such a measurement has not been done, but it's not "practically impossible". The Space Interferometry Mission, designed to measure the position of stars very precisely, would be sufficiently sensitive to detect changes in apparent positions due to the changing Lorentz contraction effects as Earth travels in it's orbit. Sadly, budget cuts at NASA have postponed this mission indefinitely. However, there's a lot of focus these days on finding exoplanets, and many methods for doing that involve measuring the positions of stars precisely enough to detect the wobble from an orbiting planet. We're very close now to the point where improvements in resolution of any such mission will demonstrate Lorentz contraction of the rest of the universe as a side effect of those precise measurements. Even if SIM never launches, some other mission will probably give similar results within a decade or so.

So, suggesting that the experiment is beyond technical feasibility is false. We could do it now if we had the funding. But there's no real doubt of the result, even so. As I noted above, there are plenty of other instances where we have to take Lorentz contraction of fast moving objects into account, or we get clear disagreement with experimental results. Lorentz contraction is a real result, just like time dilation, confirmed by numerous experiments. If you seriously want to propose a replacement to relativity that does not include Lorentz contraction, the place to do that would be the AtM section of the board, but honestly, I don't think you'd be able to do so and still have a consistent theory that also matches all the experimental evidence to date.

[caveman1917]

No it was not.

But the muon is the instrument. What difference does it make if an observer uses a swiss watch or a batch of muons as a clock? What difference does it make if the observer is human or robotic (ie consisting of instruments only, ie consisting of simply the muons).

Suppose that you accept time dilation but not length contraction. Suppose that we want to test length contraction.

We put an observer with a watch somewhere and let him go at relativistic speed. We agree that the observer will give us a signal after he has been some predefined time in his own frame. Because we know time dilation and thus the rate on his clock and his speed we can predict where that will be in both cases, with length contraction and without.

Do you accept that this is an experiment that directly tests length contraction?

Now suppose that instead of an observer we simply use a timebomb with a set timer. Now suppose that instead of a normal watch, we use a bunch of muons (which constitutes a clock by their known decay time). And suppose that the signal is simply "decay".

Do you accept that this is still essentially the same experiment? Because that is exactly what the muon experiments do. The muons should decay before reaching the ground if length contraction in their frame is false, they should not if it is real. The experiment shows them hitting the ground*, ergo length contraction is real.
[*] strictly speaking we're talking about which fraction of them hit the ground before decaying

[Webbo]

Are you denying that we have measurements of the half life of muons?

No. Are you implying that the muons had observers travelling with them?

I was referring specifically to your proposed experiment, not to experiments testing relativity in general. Due to practical reasons in accelerating instrumentation to a large fraction of c, and the complete lack of a need for doing so in order to test relativity, it hasn't been and almost certainly won't ever be done. However, my statement was somewhat incorrect, as it's actually been done in principle by the muon observations, and similar observations in particle accelerators.

I was also talking about my experiment. Grey's quote was "I agree that it seems completely bizarre and counterintuitive that people who are moving at different speeds will measure distances and time differently from each other, but in experiment after experiment, it's been shown to be true.". And you stated "Such experiments are unnecessary" which did not refute what I stated, and now you are agreeing with me.

Saying it hasn't doesn't change the fact that it has. The universe doesn't care how uncomfortable you are with reality. Experiments have verified relativity time and again, time dilation and length contraction are indisputably real no matter how much you dislike them.

And now you introduce a straw man. I am very specifically only talking about observations of length contraction. What experiments do we have where observers have observed length contraction while travelling?

[Webbo]

I stand by my statement that Lorentz contraction has been clearly shown by experiment many times in the century since Einstein introduced relativity.

Your claim was;

I agree that it seems completely bizarre and counterintuitive that people who are moving at different speeds will measure distances and time differently from each other, but in experiment after experiment, it's been shown to be true.

What experiment can demonstrate your claim? You didn't claim that experiments show Lorentz contraction, you claimed travellers moving at different speeds will measure distances differently.

[Webbo]

This type of argument shows a very basic lack of understanding about science and observations. It is like saying no one was around a 150 million years ago so their for the Jarassic period is suspect.
We know muons decay very rapidly
We know how they are generated in the upper atmosphere
We know their speeds when the reach the surface of the Earth
We know that given their decay rates that without SR they wouldn't reach the ground before they decay.
We know SR is correct by multiple independent methods to a very high degree of accuracy.
Therefore we can reasonably conclude what is happening to the muons in their frame of reference and that matches observation.

You are reasonably concluding that. Grey did not resonably conclude that a moving observer would measure distances differenty, he stated "experiment after experiment" has shown it to be true.

[Webbo]

I suggest you read some history on all the experiments leading up to GPS. Even Sputnik had an element of reletivity experimentation.
Then, I would like you to explain how the measurements made can be done without measuring any kind of movement between two known differences.
I see no way to measure time without involving a known distance, whether that be the length of a pendulum swing, or the difference between waves in an atomic emission.

What element of the system experiences length contraction that it needs to account for or correct?

[cjameshuff]

No. Are you implying that the muons had observers travelling with them?

The muons are the observers.

And now you introduce a straw man. I am very specifically only talking about observations of length contraction. What experiments do we have where observers have observed length contraction while travelling?

Once again...the muon observations, among others.

[NEOWatcher]

What element of the system experiences length contraction that it needs to account for or correct?

Did you read the post. I point it out very clearly at the end.

[WayneFrancis]

You are reasonably concluding that. Grey did not resonably conclude that a moving observer would measure distances differenty, he stated "experiment after experiment" has shown it to be true.

Well either length contraction is 'true' or you need to explain how SR comes up with accurate prediction despite half of it being wrong. I mean you might as well look at this 2 + 4 = 6 then say yea that is right but I don't believe in the number 4 and ignore that 2 != 6.

[Jeff Root]

Wayne,

Do you understand what Webbo said in the quote you
just replied to? It doesn't look like it.

Webbo said that you reasonably conclude what is happening
to the muons. I agree. But nobody has ever measured any
length or distance in the muon's frame, and that is the
disagreement. Length contraction has never been observed.
It has only been calculated using relativity theory. In your
math analogy, 2 + 4 = 6 is easy to calculate given 2 and 6,
but 4 has never been observed, only calculated.

-- Jeff, in Minneapolis

[WayneFrancis]

Wayne,

Do you understand what Webbo said in the quote you
just replied to? It doesn't look like it.

Webbo said that you reasonably conclude what is happening
to the muons. I agree. But nobody has ever measured any
length or distance in the muon's frame, and that is the
disagreement. Length contraction has never been observed.
It has only been calculated using relativity theory. In your
math analogy, 2 + 4 = 6 is easy to calculate given 2 and 6,
but 4 has never been observed, only calculated.

-- Jeff, in Minneapolis

I realise that is what webbo said but webbo, and you, go beyond just saying the nobody has ever measured the distances in the muon's frame. Webbo has been saying that if you could and did do measurements that they don't believe the distance would not be length contracted. You keep using arguments from incredulity that we can't even in principal do measurements while in the frames at relativistic speeds. Both of you seemingly trying to raise doubt on something that has more evidence for it then against it.

You've missed my point. If you even if you don't believe in the number 4 and never observed it you have to ask why it fits the equation perfectly. If you want to claim that length contraction isn't real then you HAVE to replace it with something else that would cause us to get the EXACT same results we have observed.

Honestly saying we don't know because we've never been in the muon's frame is on par with creationist saying that the Earth isn't 4.5 billion years old because none of us were around 4.5 billion years ago to see it first hand.

Now if you want to continue on your claim that length contraction isn't real and that you can't measure distances in accelerated frames then back it up with more then hand waving. Give us a valid reason beyond the staw men and hand waving you've done up to this point. Show me I'm wrong. Show me how the maths of SR still produce such accurate predictions and results, in so many cases, if you remove half of the theory.

Length contraction is currently the ONLY answer that fits and makes SR work. Claiming it isn't real without providing a replacement that is just as bad as creationist saying "I don't like evolution! Intelligent design explains it." but constantly refusing to say what ID actually "is"

[Jeff Root]

Wayne,

The problem is that you keep arguing for length contraction.
Over and over and over and over again, you argue for length
contraction.

But I haven't argued against length contraction at all. I've
said several times that I have no disagreement of any kind
with length contraction. I just say that length contraction
due to motion of the observer has never been measured.
As Grey said at the beginning of post #181 (at the top of
this page).

More specifically, I claim that the relativistic traveller in
the thread which spawned this one would not think that the
distance from the Solar System to the target star changed
from 22.3 light-years to about 1 light-year. He would know
that the distance was still 22.3 light-years, even though he
gets there in just one year of his time, in part because he
would understand how special relativity works.

-- Jeff, in Minneapolis

[Strange]

But I haven't argued against length contraction at all. I've
said several times that I have no disagreement of any kind
with length contraction. I just say that length contraction
due to motion of the observer has never been measured.

And when examples of length contraction happening are provided, you say "but it hasn't been directly measured". So what? How is this "no true scotsman" argument relevant? GPS has to take into account the difference in distance observed by the satellite and the receiver due to their relative velocities. But I assume that is not "direct" enough for you.

More specifically, I claim that the relativistic traveller in
the thread which spawned this one would not think that the
distance from the Solar System to the target star changed
from 22.3 light-years to about 1 light-year.

And yet you have provided no reason, evidence or justification of this denial of the basics of relativity.

He would know
that the distance was still 22.3 light-years, even though he
gets there in just one year of his time, in part because he
would understand how special relativity works.

No. He would know, from special relativity, that a different observer would think it was 22.3 ly, even though he sees, measures and knows it as 1 ly.

[Jeff Root]

Strange,

You breathed yesterday.

How could I know that? I don't even know what continent
you are on, or even whether you are on a continent.

Is it a mystery to you how I did it?

No, of course it isn't a mystery. You know that I didn't
observe you breathing yesterday. I deduced that you
breathed yesterday, from other information.

Observing something and deducing it are two entirely
different things.

But I haven't argued against length contraction at all. I've
said several times that I have no disagreement of any kind
with length contraction. I just say that length contraction
due to motion of the observer has never been measured.

And when examples of length contraction happening are
provided, you say "but it hasn't been directly measured".
So what?

So it hasn't been directly measured. It has never been
observed. It has only been deduced from other things
which can be and have been observed.

How is this "no true scotsman" argument relevant?

It isn't an argument. It is a statement. You and others
here have been trying to argue against it. All I have
done is repeated the statement and asked for a better
argument against it. I have not made any argument
relating to it. The statement stands by itself:
Length contraction due to motion of the observer has
never been measured. It has only been deduced from
observations of other things.

GPS has to take into account the difference in distance
observed by the satellite and the receiver due to their
relative velocities. But I assume that is not "direct"
enough for you.

The satellite doesn't observe any distances.

Neither do the ground stations.

More specifically, I claim that the relativistic traveller in
the thread which spawned this one would not think that the
distance from the Solar System to the target star changed
from 22.3 light-years to about 1 light-year.

And yet you have provided no reason, evidence or
justification of this denial of the basics of relativity.

What denial? I didn't deny anything!

:P

I certainly didn't deny any "basics of relativity".

He would know that the distance was still 22.3 light-years,
even though he gets there in just one year of his time, in
part because he would understand how special relativity
works.

No. He would know, from special relativity, that a
different observer would think it was 22.3 ly, even
though he sees, measures and knows it as 1 ly.

He would be smarter than that. He would understand
special relativity better than that. He would know that
the actual distance is 22.3 light-years, and that if he
could measure the distance, he would get a distorted
value. And he would know how to deduce from that
the value that everyone agrees is the correct value.

-- Jeff, in Minneapolis

[Strange]

The satellite doesn't observe any distances.

The GPS receiver triangulates using the broadcast positions of the satellites. These have to be adjusted to take into account the fact that the satellite and the receiver have different frames of reference and therefore observe distances as being different.

What denial? I didn't deny anything!

You deny that someone travelling at significant speed would see length contraction.

He would be smarter than that. He would understand
special relativity better than that. He would know that
the actual distance is 22.3 light-years, and that if he
could measure the distance, he would get a distorted
value. And he would know how to deduce from that
the value that everyone agrees is the correct value.

There is no "actual" distance or "correct" value. There is one value seen from one frame of reference and a different value in another frame of reference. These are equally correct. By insisting that only one of these is correct and real, you are claiming that there is a special (absolute) frame of reference.

[Hornblower]

I don't know what Jeff is trying to argue or not argue as the case may be, and I am not going to wade through his posts throughout the thread to try to find out. We all acknowledge that we have no way of getting a passing spacecraft to fly by fast enough to visually observe the length contraction that relativity theory predicts. We have inferred the occurrence of such contraction from the observations of high-energy particles we are able to observe and evaluate, and we have confidence in the validity of these inferences. The success of relativistic adjustments to the GPS system further strengthen that confidence. The fact that Webbo apparently does not have such confidence is of no adverse consequence to the advancement of physics. Physicists do not need unanimous consent from the public at large to proceed with their work.

[korjik]

I don't know what Jeff is trying to argue or not argue as the case may be, and I am not going to wade through his posts throughout the thread to try to find out. We all acknowledge that we have no way of getting a passing spacecraft to fly by fast enough to visually observe the length contraction that relativity theory predicts. We have inferred the occurrence of such contraction from the observations of high-energy particles we are able to observe and evaluate, and we have confidence in the validity of these inferences. The success of relativistic adjustments to the GPS system further strengthen that confidence. The fact that Webbo apparently does not have such confidence is of no adverse consequence to the advancement of physics. Physicists do not need unanimous consent from the public at large to proceed with their work.

Slight amplification: The behavior we see of high energy particles does not match theory without the length contraction. The particles cannot behave the way they do if they have to travel the lab frame distances. This means we have an observation that cannot work in the lab frame, yet matches the theory nearly exactly.

This also applies to the GPS system. Without the frame corrections mandated by GR, the GPS system would drift to tens of meters of error in weeks. Again, the observation does not match classical theory yet matches what GR says quite well.

[Strange]

This also applies to the GPS system. Without the frame corrections mandated by GR, the GPS system would drift to tens of meters of error in weeks. Again, the observation does not match classical theory yet matches what GR says quite well.

It is not even a matter of drift (which is mainly due to the time dilation due to the different height in the gravitational field) it is the instantaneous corrections that have to be made because of the relative velocity of the satellite and receiver; without this the calculated position would be many meters out.

[grapes]

So it hasn't been directly measured. It has never been
observed. It has only been deduced from other things
which can be and have been observed.

I think this may be the central point to your argument.

However, in the same sense, it is valid to say that no measurements have ever been directly observed. All measurements are deduced, in some fashion. The distance to the moon has not been measured, it has been deduced from time of flight observations. My BMI has not been measured, just deduced from other calculations.

Such an objection can be frustrating, and the general level of discussion here reflects that. We should move off that point.

But measurements are repeated all the time--using the same, as well as different, techniques (and deductions). Which new (old) measurements should be made (repeated)? Why not discuss that?

[Strange]

However, in the same sense, it is valid to say that no measurements have ever been directly observed.

Exactly. That is why it is such an .... <bites tongue> ... "unhelpful" comment. It adds nothing at all to the discussion.

[pzkpfw]

... He would know that
the actual distance is 22.3 light-years ...

Are these two objects that are "actually" 22.3 ly apart, motionless with respect to each other?

[Jeff Root]

Are these two objects that are "actually" 22.3 ly
apart, motionless with respect to each other?

Yes.

I think I made that clear in reply to you in the other
thread, and agreed that if the objects are in relative
motion, then there is no "actual" distance that can
be generally agreed on, because there can be no
agreement that the observations of the positions
of the two objects were simultaneous.

The proper distance between two objects which are
motionless relative to each other will be agreed by all
observers. It is the simplest and easiest measurement,
the only one which is not distorted, requiring correction.

-- Jeff, in Minneapolis

[Jeff Root]

The satellite doesn't observe any distances.

The GPS receiver triangulates using the broadcast
positions of the satellites. These have to be adjusted
to take into account the fact that the satellite and
the receiver have different frames of reference and
therefore observe distances as being different.

It is still the case that neither the GPS satellites nor
the GPS receivers observe any distances.

What denial? I didn't deny anything!

You deny that someone travelling at significant
speed would see length contraction.

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.

He would be smarter than that. He would understand
special relativity better than that. He would know that
the actual distance is 22.3 light-years, and that if he
could measure the distance, he would get a distorted
value. And he would know how to deduce from that
the value that everyone agrees is the correct value.

There is no "actual" distance or "correct" value.

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.

There is one value seen from one frame of reference
and a different value in another frame of reference.
These are equally correct.

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

By insisting that only one of these is correct and real,

Which I do.

you are claiming that there is a special (absolute)
frame of reference.

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