Re: SR cannot determine Contraction

"Simple Simon" <me@xxxxxxxxxxxxxxxxxx> wrote in message
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"Artful" <artful@xxxxxxxxxx> wrote in message
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"Simple Simon" <me@xxxxxxxxxxxxxxxxxx> wrote in message
Dono is saying that simply because an observer is travelling relative
to
a rigid rod doesn't physically change it.

And I agree . But that is NOT what he is saying. he is saying that the
moving rod does NOT take up less physical space (that it isn't physically
shorter) in the frame in which it is moving. It is.
I think that the two of you are primarily arguing semantics

If he was saying what you are saying, there would be no argument.

However, that is not the case.

He disagreed with my correct statement that a rod when moving has a shorter
length and takes up less physical space at a given time than when it is at
rest. He says that there is no length contraction. that the well known
pole-and-barn example is incorrect and that SR does not predict what it does
predict (that the pole fits wholly within the barn at some point in time in
the frame of the barn).

If all he was saying is that relative movement does not make a physical
change to the rod 'itself' (ie in the frame of the rod), I would agree with
him (as I did with you) because this is something I have said also.

The problem is he tried to be 'smart' by pointing out something that he
thinks is an error on my part, which is actually an error on his.

(apart from a
block he seems to have against the relativity of simultaneity which is the
explanation for the 'pole-in-barn' paradox (I believe, I'll have to look
it
up. If it's something like a train travelling on a track with a tunnel on
it,
then I think that I follow it).

Yes .. that is it indeed. The problem is one cannot take length
contraction, time dilation or relativity of simultaneity on their own ..
they come as a package (best known as the Lorentz transform). If you try to
discuss one in isolation, you end up with contradictions. You'll note I
have been careful from the outset to specify that I am talking about how
much space the rod takes up (it length) at a given time in the observers
frame.

For example:
He will say something like: "length contraction does not compress objects,
it simply affects their measurements as viewed by observers in relative
motion".

Indeed it does not compress them in their own frame of reference.. it
compresses them in another frame of reference .. in the sense that they
physically take up less space .. their length is shorter.

If you consider the word compress as a verb, then what he is saying
is that length contraction measured by an observer does not change the
object (by compressing it, for example). This is a valid statement.

It is .. I agree completely. However, that is not what Dono is arguing (or
rather, not the point of contention of his argument).

You will
reply with something like: "That's the same thing. If something has a
shorter length, it is compressed." I interpret that, in response to his
statement, to say that the object HAS undergone some change,

The object hasn't .. its projection in the observers 3D space is different
to how it is in the objects 3D space. Its not that something has happened
to it (ie before it wasn't, now it is) .. its just the nature of the
projection.

even though I
know that's not how you mean it. You are saying that the length
contraction
is real.

It is in the sense that physics considers things as real. ie it is what is
measured and observed. Including measuring a pole in relation to a barn ..
despite Dono's claim to the contrary, SR predicts that the pole will be
shorter in length than the barn. It will fit inside.

To emphasize this, consider the train fits in the tunnel from the FoR of
the
track but not from the FoR of the train.

Yeup .. or pole and barn.

Let's stop the (rigid) train in the
tunnel.

Once you start talking acceleration things get tricky. A uniformly
accelerating object (increasing in speed relative to an observer) physical
expands (in its own non-inertial frame of reference). A rigid body cannot
uniformly accelerate and remain rigid.

To do this, from the FoR of the track you decelerate the front and
back of the train at the same rate.

You can't do that to a rigid train .. it would break.

From the FoR of the train, the back
decelerates more slowly than the front.

The acceleration also happens at different times relative to the train, if
it is simultaneous relative to the track. Not a pleasant thing to try to do
to a rigid body :)

That is, the train actually
compresses! (At least that's the way it seems to me.)

All interesting.. but not really relevant to what was being discussed .. its
an even more confusing derailing of the train.

Another problem, as I see it, is that by explaining the "paradox" with
length contraction, you seem to be saying that the object has been
compressed.

It has .. in the sense that it is occupying less space in the observers
frame of reference. It does not undergo any change in itself (in its own
rest frame). I have pointed that out several time.

I see the proper explanation as being because of the relativity
of simultaneity.

Indeed, that is why it is taking up less space. You have to take all
aspects of relativity together.

The reason that I say this is that from the FoR of the
train, it is the tunnel's length that is contracted.

Yes it is. Which is what confuses many people when they first come across
the train/tunnel or pole/barn 'paradox'.

[snip for brevity]

I think that your looseness with terminology (when you apply verbs to an
object to describe length contraction in inertial frames)

I've been quite precise.

lends itself the
interpretation that you are claiming work is done on the object when it is
length contracted. But I'm probably wrong.

I think you are, or at least not attributing the blame appropriately. I am
*not* the one who used the verb "compress" .. that was Dono's choice. I
said it took up less space at a given time in the observers frame of
reference. He chose to describe that as being compressed. I think he's
confused himself by doing so.



.

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