# Real Lorentz–FitzGerald contraction

• heldervelez
In summary, the conversation discusses the existence of a real length contraction and the viewpoints of Lorentz, Einstein, and others on the subject. It also presents a thought experiment by Barceló and Jannes that suggests a Lorentz-FitzGerald contraction can occur in a condensed matter system. The conversation concludes with a visualization of this contraction based on the assumption that there is no instant action at a distance.
Mentz114 said:
Hi Hans,

as far as I can see you have only 'proved' that an observer from a moving frame will see a wave packet contracted. This is not surprising because its momentum observed from the moving frame is different. Lorentz contraction is a function of the relative velocity between the frames and so cannot be real in the sense that in the rest frame of the wave packet something changes.

Suppose I am in my kitchen eating a shami kebab, and being watched by someone from a spaceship at rest wrt to me. They zoom off and reach a high velocity; according to you my kebab shrinks to half its previous length. But what about all the other 'observers' in the universe. Are you saying that my kebab will vary in size according my relative velocity wrt them also ? That's more like a sort of Machian kebab principle than Lorentz contraction.

The only things I can regard as real are those that are agreed by all (IRF) observers. Einstein's principle of covariance explicitly states that anything that can be altered by a change of coordinates ( eg LT) is not a real physical effect !
Realize that the characteristic speed is only 1 meter/second in the example with
the spring/mass grid.

Another example is that of the rubber sheet: If you press your finger in a rubber
sheet then it deforms in a circular symmetric way. However, if you move your finger
in a straight line then the deformation becomes a "Lorentz contracted" oval.

Again, this happens with respect to the slow mechanical propagation speed in the rubber.
Mentz114 said:
There was a young man called Hans
who thought he could tell at a glance
that things on the move
(it is easy to prove)
are somewhat in length a bit sans

Sorry I had to use French.

Sans problème Regards, Hans

Thanks Hans de Vries. I've already cited your delicious book at #28 and #45.

I like and surprised to acknowledeg Poincaré historical precedence on SR concepts (simultaneity, syncronism, Lorentz group, etc...).

"REAL"
Quoting Poincaré (1906) On the dynamics of the electron:
"How do we go about measuring?
The first response will be: we transport objects considered to be invariable solids, one on top
of the other. But that is no longer true in the current theory if we admit the Lorentzian
contraction. In this theory, two lengths are equal, by definition, if they are traversed by
light in equal times."

As we see (rsrsrs: 'not see'), it can exist anisotrophy of light speed.

The consequences of a 'real' length contraction can lead us to unsuspected conclusions.
The paradoxes could go away easily. Realize that spacetime is a feature of Einstein analisys pov, not present on others pov analisys. Questionable, yes.

A paradox is, by definition, an handicap in any theory.
A good theory has no paradoxes.

(*) does someone know where to find (netfree) a modern form of Poincaré's papers?
(the notation is not the usual one)

Hi Hans,

I'm not denying that these effects can be observed between IFRs, but I don't think they are real in the covariant sense.

M

Mentz114 said:
matheinste,

since Rindler's name is associated with accelerated frames, are you sure that the quote you gave isn't referring to those, rather than IFR's ?

The book only treats SR, in fact acceleration is not even in the index, and the quote is from a very early chapter dealing with relativistic length contraction. Almost immediately above the quote, on the same page after the length contraction formula are the words---

"This shows quite generally that the length of a rigid body in the direction of its motion with uniform velocity v is reduced by a factor----."

Matheinste.

jtbell said:
This cries out for a relativity limerick that starts with something like "There was a young fellow named Bob".

There was a young fellow named Bob,
Who was eating a shami kebab.
His twin, on a trip, in a fast rocket ship
Saw the shami contracted, by jeepers!

Einstein's principle of covariance explicitly states that anything that can be altered by a change of coordinates ( eg LT) is not a real physical effect !
Length contraction is not a changing component of a covariant quantity like a vector (time dilatation is). It is the comparison of the length of two different vectors. These vectors really have different length, but I don't think that it's appropriate to speak of one and the same thing really contracting.
Nothing contracts, one simply compares different things, depending on the reference frame.

GRDixon said:
There was a young fellow named Bob,
Who was eating a shami kebab.
His twin, on a trip, in a fast rocket ship
Saw the shami contracted, by jeepers!

Gotta make the rhyme come out right... hmmm... last line:

Saw the kabab had shrunk to a knob!

Not quite up to the one about the fellow named Fisk, but it will do.

heldervelez said:
Thanks Hans de Vries. I've already cited your delicious book at #28 and #45.

Hi, Helder.

Special Relativity is generally teached first using only space and time and the
postulate that the laws of physics should be the same independent of the boost
or rotation of the reference frame. Live as we know would be quite unlikely if
this wasn't the case. So, somehow nature manages to create the conditions
we need.

In my book I work the other way around. The aim is to derive the postulates, as
well as the invariance under boosts and rotation from the relativistic wave equations.

Interesting is that the symmetry isn't perfect. Physics isn't the same under parity
inversion. The world we see in the mirror can not physically exist in each and every
detail because the electroweak force is not the same in a mirrored world.

But then, we don't need this symmetry to survive. We can rotate, we can change
velocity but as long as we don't mirror our self we can't get problems because of
different laws of physics. Nature does a wonderful job in creating the necessary
symmetries but it doesn't care about the ones we don't need.

Regards, Hans

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Hans de Vries said:
All relativistic wave equations exhibit physical Lorentz contraction. ... The proof can't hardly be any simpler.
It is a very simple proof that I like very much except for one thing. You have not defined the word "physical" so it is unclear to me that you have proved physicality. For example, the same proof would be valid for a computer simulation of the relativistic wave equation, would that imply that the computer simulation is physical? Or, you could do the exact same proof for waves in other dimensions, would those then be physical?

You have certainly derived length contraction in a particularly pleasing and elegant way, and you have therefore shown that any system which obeys the relativistic wave equation will demonstrate length contraction. But without a solid definition of the word "physical" you cannot say that you have proven that length contraction is physical.

By the way, unlike "real" I do have a definition of "physical" which I like:
Physical - of or pertaining to physics.

Length contraction pertains to physics so it is physical. (my proof is even simpler)

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DaleSpam said:
...
I do have a definition of "physical" which I like:
Physical - of or pertaining to physics.

Length contraction pertains to physics so it is physical.
circular definition. is not a definition at all (pt- tautologia, en - ??)

how could you, mr. Dalespam, have such a profound misunderstanding of 'real' versus 'frame dependent', artifact,... since the beginning of this discussion?

I use the Poincaré version of 'measuring' that I quoted above.
'rigid rods' are dead, I beleived since GR , but I found that Poincare' did it sooner.
-----------------

mr. Hans de Vries : Einstein paper has a move where a symetry appears, it was when he made psi(V)=psi(-V)=1, and ... ,a generalized lorentz contraction with some nuances, ...can appear someday

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heldervelez said:
circular definition. is not a definition at all (pt- tautologia, en - ??)
My definition of physical is not circular.
heldervelez said:
how could you, mr. Dalespam, have such a profound misunderstanding of 'real' versus 'frame dependent', artifact,... Since the beginning of this discussion?
If you believe that you have a good definition of "real" then I would be glad to hear it and thereby clear up my profound misunderstanding.

To those that do not know the concept of circular definition, see: http://en.wikipedia.org/wiki/Circular_definition" , (tautology , from greek)

The concept of 'real' is a philosophical concern. Plato, Kant, Berkeley ...
It is a never ending story if you wish to adress those issues. I skip.
I am a simple minded one and can use the sketch/art work at post #2 to understand the level of reality I meant.
---------------------------------------------------

In post #59 I've said "OK, there is no way to distinguish experimentally the existence of an 'absolute referential'.
By the rules of this forum I will not try to show you differently.
But the its existence is a different issue and, in another thread, having the time, I will try to show why I consider it mandatory."

Mr. A.T. in post #61 replied : "Then it is not physics. " to the sentence "...no way to distinguish experimentally..."
and replied "In physics it is the same." to the sentence "But the its existence is a different issue".

I'will focus my attention to clarify what I meant in post #59 and to show that it is a misconception the usual perception that it is fruitless and not meaningful the pursue of 'absolute rest frame'.
I remember that it is extremelly difficult IMO, unreasonable, to maintain sentences that deny some outcome at 'the end of a path' without actually explore the 'path'.

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Ich said:
Length contraction is not a changing component of a covariant quantity like a vector (time dilatation is). It is the comparison of the length of two different vectors. These vectors really have different length, but I don't think that it's appropriate to speak of one and the same thing really contracting.
Nothing contracts, one simply compares different things, depending on the reference frame.
Thanks, Ich.

heldervelez said:
The concept of 'real' is a philosophical concern. Plato, Kant, Berkeley ...
It is a never ending story if you wish to adress those issues. I skip.
I completely agree with this.

However, now I have a hard time understanding why you chose to place a thread entitled "Real Lorentz–FitzGerald contraction" in the physics sub-forum when you seem to agree that it fits better in the philosophy sub-forum. I also do not understand your previous rather dismissive comment about me having "a profound misunderstanding of 'real'" when it turns out that not only do I correctly understand the issue but you share my understanding.

Your present statement seems wholly at odds with your entire previous approach.

copy/paste from post #72

"REAL"
Quoting Poincaré (1906) On the dynamics of the electron:
"How do we go about measuring?
The first response will be: we transport objects considered to be invariable solids, one on top
of the other. But that is no longer true in the current theory if we admit the Lorentzian
contraction. In this theory, two lengths are equal, by definition, if they are traversed by
light in equal times.
"

Can I presume that no important paper can be presented against the "Real Lorentz-Fitzgerald-Poincaré' length contraction, nor experiment?

That is a definition of "length", not a definition of "real". How can you possibly have evidence either for or against "real Lorentz contraction" if you cannot even define "real"?

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