"practically-rigid body"

J

John Locke

Registered Member
The following should appear absurdly basic to most of you, so forgive my incompetence.

I currently frequent the pages of Relativity by Albert Einstein, and have encountered some difficulty with the terminology, as I haven't studied sciences or geometry ever before. I've begun to understand most of it, but still have confusions over one term.

I basically cannot find a satisfactory definition for the term "practically-rigid body." I understand the meaning of "rigid body" (though maybe some of you would like to define it for me, just in case?). But I find suspicious the use of this adjective "practically" (at any rate, I think it functions as an adjective). I assume its use intends to further specify the term "rigid body," but I do not know to what end.

Thanks a lot.
 
Perhaps if you gave us the entire sentence, so we could see the term in context, we could help. I have no idea what it means just from what you've said.

Generally speaking, talk of relativity deals in absolutes. So I would assume "rigid body" would refer to an object which, for the purposes of whatever concept or thought experiment was under discussion, was so strong it could not be bent or compressed in the slightest degree.
 
In a gendanken thought experiment we can postulate anything we want. However, when we try to translate gendanken results into an accurate representation of the real world, we sometimes find that our postulates do not define reality.

A rigid body may be a helpful ingredient in working toward understanding some principle or theory or whatever. But in the real world, impulse ( in the strict physics sense of the word ) is transmitted through a body at the speed of sound ( which depends upon several variable conditions ) within that body.

So, we can possibly see an impulse delivered to one part of a body and then it will be a significant amount of time before the impulse travels all the way to the other end of the body, whereupon the entire body will be seen to finally begin to change in movement in accordance with the impulse.

A "practically" rigid body is in the same category as the "kind of" pregnant woman. She either is or she ain't.

A body is either rigid due to gendanken postulization or it, in the real world, ain't.
 
A practically rigid body, is a body which is, for all intents and purposes, rigid. The difference is, it does not exclude the possibility that at some level that the body is not rigid, but it would be at a level which is beyond the scope of the argument.
 
Raphael said:
A practically rigid body, is a body which is, for all intents and purposes, rigid. The difference is, it does not exclude the possibility that at some level that the body is not rigid, but it would be at a level which is beyond the scope of the argument.
Click to expand...

Thanks. Just what I needed.
 
Here's another way of thinking about it:

A perfectly rigid body is one in which the bulk modulus of elasticity is infinite.
This means that the speed of sound in that body would also be infinite.

Investigating such a body in the realm of Einstein's relativity produces interesting effects, and leads to the violation of causality: I.e. if (1) the special theory is a good representation of reality, and (2) the speed of sound in some body is infinite, then (3) some effects may precede their causes.

This means that Einstein's relativity together with the principle of causality predict that perfectly rigid bodies are impossible, and that the most rigid body theoretically possible would have a speed of sound equal to the speed of light in vaccuum.

This prediction seems to pose no problems in practice. The highest speed of sound in Earth materials is (I think) in diamond, at 12km/s. The highest suggested speed of sound in the Universe (as far as I can tell with 5 minutes looking :)) is in the core of a neutron star, at (perhaps) several tens of percent of the speed of light.
 
Even though all this has been, as Data once said, "enlightening", perhaps the thread starter would provide a referenced exact quote from Einstein concerning the alledged "practically" rigid body so that we may then try to learn exactly what the father of our relativy may have been trying to convey to us.
 
See for yourself:
Chapter 1
Chapter 3

Looking at the context, my previous post is pretty irrelevant.

Bear in mind that Einstein wrote in German. "Practically rigid body" is a translation.
 
Pete:

My apology to you: you seem to react as if you were the thread starter and had been misunderstood. I assure you that was not my position.

I had believed that John Locke was the thread starter, and perhaps John's providence of specific information would help us all give help in his questioned matter.

As you imply, is it true then that Einstein never wrote in English?
 
My apologies in return; my terse tone was not intended.

The implication you correctly point out was also unintentional - I have no idea if Einstein wrote anything in English, and I only meant to indicate that he wrote this particular work in German.
 
My point was, if Einstein, or an interpreter, had some significant language fluency problem, how do we know what in the hail he really meant to be telling us?

Perhaps he was as delusional as a doper on LSD.

Perhaps he was a true hypergenius with universal insights that even the most fervent of Relatyvyist has not yet fully understood.

But, if we are not, perhaps, communicating in exactly the same language, how do we know?
 
Raphael said:
A practically rigid body, is a body which is, for all intents and purposes, rigid.
Click to expand...

The problem is with the translation. The more widely-used term used in English is <i>quasi-rigid body</i>. A quasi-rigid body does exhibit non-rigid behavior such as flex and deformation, but that behavior is very slow compared to the rotation rate of the body. One does not have to go to the extent of detailed modeling of the non-rigid behavior of such a body with finite element analyses to get a good understanding of the dynamics. Quasi-rigid bodies can be modeled as slowly converting some of their rotational energy into internal heat through dissipative processes.

Raphael said:
The difference is, it does not exclude the possibility that at some level that the body is not rigid, but it would be at a level which is beyond the scope of the argument.
Click to expand...
The difference between a perfectly rigid body and a quasi-rigid body is very real. For example, consider the problem of the stability of rotation about the principal axes of an asymmetric body (i.e., a body with three different principal moments of inertia). It is a standard upper-level undergraduate physics exercise to show that rotation about the middle axis is unstable but that rotations about the major and minor axes are stable.

This is not true for quasi-rigid bodies. It is a fairly standard upper-level undergraduate/lower-level graduate mechanical engineering exercise to show that rotation about the <i>major</i> and middle axes is unstable for a quasi-rigid body; only rotation about the minor axis is stable.
 
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D H,

On translations:
When a translation says something like, "If, in pursuance of our habit of thought, we now supplement the propositions of Euclidean geometry by the single proposition that two points on a practically rigid body always correspond to the same distance (line-interval)...", a clear example of intent of usage is established. Given this example, it can be inferred that any other uses of the same term within a topic in a single writing also refer to the same usage. In this regard, I stand fully by my statement.


Otherwise, I don't deny anything else you wrote, I will however clarify that my answer was intended for someone who hasn't "studied sciences or geometry ever before."
 
John Locke has left the building and does not wish to properly and exactly clarify their original post?
 
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