Talk:Principle of relativity

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Talk:Principle of relativity/archive1

I have replaced the existing article with a new article. I wanted to emphasize how special relativity restored relativity in physics. I also wanted to emphasive that observation is the basis of physics.

There is a widespread misconception that general relativity extends the concept of relativity to accelerated motion. Einstein certainly hoped to achieve that, and at first he believed that general relativity had achieved that, but after correspondences with the astronomer Willem de Sitter, and the mathematicians Herman Weyl and Felix Klein, Einstein agreed that it is unclear whether the theory of general relativity does imply Mach's principle, contrary to what Einstein had hoped.
In later years, Einstein formulated the principle of equivalence in a more restricted form than at first.
External link to 146 Kb PDF document: The Einstein-De Sitter debate
--Cleon Teunissen 19:21, 25 Feb 2005 (UTC)


Contents

[edit] THREE principles

Two were stated as essential by Einstein, " These two postulates suffice for the attainment of a simple and consistent theory " reference

http://www.fourmilab.ch/etexts/einstein/specrel/www/

and then he added his personal POV which is "we establish by definition that the ``time required by light to travel from A to B equals the ``time it requires to travel from B to A." which is the basis of his derivation of the cuckoo transformations. (same reference). Claims of one principle only are WP:NPOV violations and WP:OR violations, the average reader should not be mislead. Der alte Hexenmeister 23:34, 11 July 2006 (UTC)

[edit] Mass-energy versus Gravity

Within the section on General Relativity, you state the following:

"Gravity alters the rate of progression of time" and "The deformation of space-time due to gravity ..."

IMHO, 'Gravity' in the above statements should be replaced with 'mass-energy'. In other words, gravity IS deformation of spacetime due to mass-energy. Alfred Centauri 12:26, 2 May 2005 (UTC)

I think you have a good point there. I've done some editing.
Besides that, I'm not sure whether an article on the principle of relativity should mention the theory of general relativity at all. The principle of relativity refers to the principle of relativity of inertial motion. the discussion of GR in the article is so brief that I wonder whether it is doing any good. --Cleon Teunissen | Talk 16:34, 2 May 2005 (UTC)

[edit] Principle of Relativity: range of applicability

I think that the statement that

 "The principle of relativity refers to the principle of relativity of inertial motion."

is an interpretation or a judgement, rather than a definition.

Literally, the principle of relativity is the principle that things are, well, relative. How far that principle can be legitimately be applied (and to what things) is a question that has occupied physicists for centuries.

In Principia, Isaac Newton outlined a model in which inertial motion was relative and "accelerated" or "rotational" motion was not. But a powerful contemporary, Bishop Berkeley, objected on the grounds that Newton's model applied the principle of relativity insufficently broadly. According to Berkeley, the inclusion of arbitrary absolute properties weakened the theory - to Berkeley, there should be no absolutes but God. Although he phrased it in religious terms, his objection was basically the Occam's Razor argument against the inclusion of unnecessary entities, making everything relative produces a more self-contained model.

Newton seems to have been aware of the problem and in an odd passage in Principia, seems to talk about the ability of revolving shells of matter to make enclosed objects partake of their revolution. This sounds very much like an early description of the principle of relativity applied to rotation, and would nowadays be described as a Machian idea, or perhaps as a description of frame-dragging under general relativity. It's difficult to see how this fits with the rest of Principia, so perhaps Newton included the section to pacify Berkeley.

Ernst Mach was a high-profile proponent of the principle of relativity in its most general physical sense, and strongly influenced Einstein. According to Mach, "relativistic" physics meant fully relativistic physics, in which acceleration and rotational effects were due to the interplay of an object with background environmental matter. Without that background reference, acceleration or rotation would not be physical, and if there was no physical cause, there should be no physical effect. If we claimed that the entire universe was rotating, there should be no resulting forces, because there would be nothing physical for the rotation to exist relative to, and the supposed rotation would be a purely mathematical notion with no physical consequences. In his book "Science and Mechanics", Mach explicitly defines a "relativist" as being someone who subscribes to this much more general principle of relativity. When Einstein produced his "special" or "restricted" theory of relativity (aka "Special Relativity" or "SR"), Mach reputedly did not like it at all -- by his standards, it was probably not worthy to be referred to as theory of relativity, although Mach himself did not seem to have anything more constructive to offer.

Einstein then went on to produce his "General Theory of Relativity", which was intended to at last be a fully compliant Machian theory, and to begin with, descriptions of "General Relativity", or "GR" did assume that the theory was a fully successful implementation of the general principle of relativity (see e.g. the content of Einstein's 1921 Princeton lectures, reprinted in "The Meaning of Relativity"). In later years this claimed compliance had to be downgraded somewhat, the theory had allegedly been found to allow rotating-universe solutions, and the fact that Einstein had designed it to reduce to SR meant that certain non-Machian assumptions built into SR were inherited, at least in part, by the general theory. A paper by the Harwell research group described an experiment where they had successfully measured time dilation in centrifuged material, and the paper set the cat amongst the pigeons by mentioning that the same basic result could be predicted either by applying the principle of equivalence and calculating the effect as gravitational time dilation, or by applying special relativity and calculating the effect as the result of velocity-based time-dilation. Unfortunately, although both calcualtions were adequate to explain the result, they were found to be geometrically incompatible, so at least one had to be wrong, and since invalidating SR would also invalidate the current version of the general theory, it was decided that the least worst option was to retain SR and restrict the accepted domain of validity of the equivalence principle only to situations where it did not conflict with accepted SR work. Where Einstein had originally said that acceleration and gravitational effects were wholly equivalent and interchangeable, later writers preferred to say that this had only been an approximate relationship rather than a law, that Einstein hadn't meant it literally, and that of course, one could in practice distinguish between acceleration and gravity.

Towards the end of his life, writing in Scientific American (April 1950), Einstein wrote that he no longer believed that it was correct to use the special theory as a foundation or building block for more advanced gravitational theory. A fully general theory, he said, ought in his opinion to be designed to conform to the general principle from the ground up without these sorts of compromises. Unfortunately he died without being able to produce a third, all-encompassing theory of relativity, and to this day, the true status of Mach's Principle (or, the idea that the general principle of relativity should be truly general) still generates controversy.

--ErkDemon 03:21, 24 Jun 2005 (UTC)

[edit] Pinciple of relativity: range of applicability (2)

A physicist has no choice but to follow where the evidence leads him. We are faced with several layers of counterintuitiveness.

We have that there is, as far as we can tell from measurements, unconditional relativity of position in space. We have unconditional relativity of the first derivative of place: relativity of inertial motion in space. We do not have relativity of the second derivative of position in space: acceleration is locally detectable; it is possible to measure acceleration with respect to local space. (Generally speaking, all motion is motion in space-time, of course)


I should explain why I use the distinction between global measurement and local measurement.
An observer in a space-craft, moving inertially in space, can take measurement readings of the Cosmic Microwave Background Radiation, and thus he can infer his own velocity with respect to CMB. But that measurement is looking at someting that originated a very long time ago, very far away. If you apply the restriction that you are not allowed to look very far, then the finding is that what is detectable is relative velocity. It appears to an observer that only relative velocity can be used in any theory of physics.

Remarkably, acceleration with respect to the local inertial frame of reference is detectable, any accelerometer will tell you how hard you are accelerating with respect to the local inertial frame of reference. (I follow the convention of defining an 'inertial frame of reference' as follows: a coordinate system that is co-moving with a free-falling test mass.)

The relativistic theory of gravitation describes that space-time is not a fixed background. As John Wheeler put it ultra-condensed: "Matter/energy is telling space-time how to curve, space-time curvature is telling matter how to move."

Gravitational curvature of space-time meant that physicists had to retreat from the position of special relativity. The Minkowski space-time of special relativity is quite different from newtonian absolute space, of course, but a form of absoluteness is still present: any inertial frame of reference extends to infinity. Minkowski space-time is not a Euclidian space, but it shares with Euclidian space that it is a geometrically straight space.
Gravitationally curved space-time has the property that inertial frames of reference at different locations can (depending on the circumstances) be accelerating with respect to each other.

So in our solar system we are surrounded with examples of local inertial frames of reference that are accelerating with respect to each other. The Earth, in its orbit around the Sun, is following a geodesic, so the frame of reference that is co-moving with the center of mass of the Earth is a local inertial frame of reference. A satellite, say a space station, orbits its own primary, the space station is orbiting Earth. Etc. etc.

Interestingly, there are very few examples of local inertial frames that are rotating with respect to each other. General relativity does predict the phenomenon of frame dragging but it is an exceedingly small effect, only significant in the vicinity of very very heavy, very rapidly spinning celestial bodies, such is rapidly spinning neutron starts.


[edit] Rotation is absolute

Rotation is absolute, but not in the sense that Newton described.
The difference between velocity and rotation is that while you cannot measure your own velocity with respect to space itself, you can measure your own rotation with respect to your local space. (As noted, frame dragging implies the possibility of local rotational motion of space-time, but frame dragging is exceedingly insignificant.)


It is possible that Newton reasoned as follows: "If I assume that acceleration is absolute, and it appears that observation compels me to make that assumption, then there must be an underlying reference. If the second derivative of position, acceleration, has a reference, then so must the first derivative: velocity, even while it remains hidden from view.

Others have argued: if velocity is relative, then there can be no background reference at all, implying that acceleration cannot possibly be absolute. Then the task for the physicist is to find a theory that explains why acceleration appears to be absolute, while being relative all along.


It seems to me that Nature is even more counterintuitive than anybody expected.
To my knowledge, physicists have, for the time being, come to the conclusion that position and velocity are unconditionally relative, but that acceleration/rotation is absolute.


Hmm, it seems to me that most physicists don't really try to understand Nature but instead positivistically interpret the laws of motion as if they describe not observations but reality itself... (but then, how many physicists are skilled in philosophy?) Harald88 21:28, 29 October 2005 (UTC)

As far as I know, Einstein attempted to implement what he coined as Mach's principle.
What if a theory can be formulated in which there is full reciprocity of rotational transformations? If you have a hollow spherical shell of solar system size that is rotating, will that result in frame dragging effects that result in the inside of the rotating sphere in a space-time deformation that exactly matches the physics of rotation? That is: if inside the hollow spherical rotating shell a planet resides, will observers on that planet be able to figure out whether the planet they are on is rotating, or that some hollow sphere is rotating around them. To my knowledge the general theory of relativity does not satisfy this form of Mach's principle, you can tell the difference, by way of measurement. Hence the assertion: velocity is relative, acceleration/rotation is absolute.


What we see in our universe is relativity of inertial motion. As far as we can tell with our present state of knowledge, Nature has not seen fit to also have the property of relativity of all motion, including accelerational/rotational motion.

[edit] Inertia

So what can inertia be? If space is a physical entity, putting up resistance against acceleration, how then can velocity be without any resistance?

Interestingly, there is a physical parallel between inertia and the phenomenon of inductance. Imagine a current circuit with in it a self-inducting coil. Let the current curcuit be superconducting. This superconducting current circuit will have zero resistance to current. However, when an electric potential is applied to change the current strength, then the change in current strenght elicits induction in the coil and that elicits a counterforce that opposes that change in current strenght.

That does not explain inertia, it just provides an example of a system that offers zero resistance to the first derivative, but that does put up resistance against the second derivative: change of current strenght is opposed.
--Cleon Teunissen | Talk 11:16, 24 Jun 2005 (UTC)

Actually, it partially explains inertia: for the selfinduction of an accelerating electron corresponds to its relativistic inertia increase. You can't regard these two things seperately for then you'd have to add the effects, and that would be erroneous. And funny enough, one century ago this was all well understood, and my 25 year old textbook explains how the electron's magnetic field corresponds to the increase of "relativistic mass" (which is currently out of fashion, and with that also that insight). Harald88 21:39, 29 October 2005 (UTC)

[edit] Principle of relativity: range of applicability (3)

I think that the statement that
"The principle of relativity refers to the principle of relativity of inertial motion."
is an interpretation or a judgement, rather than a definition. Literally, the principle of relativity is the principle that things are, well, relative. How far that principle can be legitimately be applied (and to what things) is a question that has occupied physicists for centuries. --ErkDemon 03:21, 24 Jun 2005 (UTC)


You made a valid point: I suppose the following formulation could have been used:

"The current principle of relativity as used in physics entails a principle of relativity of inertial motion."

Here is how I use the expression 'principle of relativity':
The content that the scientific community attributes to the 'principle of relativity' changes over time. In that sense there is a succession of principles of relativity, starting with the proposal of Galileo Galilei. Invariably the Principle of relativity and the physics of kinematics are completely intertwined. I find it hard to separate them conceptually.


The newtonian principle of relativity involves the following two assertions:
(G1) When two observers are moving inertially with respect to each other, and they are watching the same events, then they will percieve the distances, velocities etc differently, but in the end they will infer from their observations the same body of laws of kinematics. (That is pretty amazing, but that is the way it is.)

(G2) When you want to transform measurements taken as seen from one inertial frame to another frame you perform vector additions; velocity addition is always straighforward vector addition.

(The G's in (G1) and (G2) stand for Galilei, for newtonian relativity is by convention called Galilean relativity.)

Interestingly, (G1) does not need to be altered in any way for special relativity, it is exclusively (G2), the assertion about how to transform measurement data from one inertial frame to another that was modified in the transition from Newtonian kinematics to special relativity.

So if there is such a thing as the principle of relativity, I would say (G1)
--Cleon Teunissen | Talk 14:17, 24 Jun 2005 (UTC)

[edit] Theory of relativity

Why are we including the theory of relativity on this page? I may cause confusion to the reader.--Light current 20:25, 20 January 2006 (UTC)

Why not? The special theory of relativity is based on the principle of relativity; the general theory was an extension of it (but that extension failed). The text on GRT desperately needs rephrasing, that's for sure; nearly everything about gravitation should be removed, it's not relevant and indeed confusing. Harald88 22:19, 20 January 2006 (UTC)
  • It will confuse people.
  • We have a pages dedicated to the special and general theory of relativity
  • Principle of relativity does not depend on relativity

Lets keep thing as simple as possible please!--Light current 22:24, 20 January 2006 (UTC)

It is essential to distinguish Einstein's general principle of relativity from the standard PoR, which he later called the special principle of relativity, exactly to reduce existing confusion even among scientists. See also the first discussion by Cleonis on this page. Of course, if you find a way to make this disambugation more transparent, that would be good. Harald88 12:21, 22 January 2006 (UTC)

[edit] merge with theory of relativity?

[- title inserted Harald88 22:04, 10 July 2006 (UTC)]

I suggest that we redirect principle of relativity to theory of relativity. The two articles are trying to do exactly the same thing, in the same way; they link to mostly the same sections. The only exception are 1) Indian proto-relativity which could easily be added as a link in theory of relativity and 2) general covariance which needs a complete rewrite anyway. Any thoughts? --Michael C. Price talk 23:23, 9 July 2006 (UTC)

Hmm... Yes I agree that they are somewhat double, but in view of the above discussions (which somewhat changed my mind), I suggest rather the opposite: to copy-paste whatever is better from Theory of relativity, and to delete that article. IMO the Principle of relativity article should be reworked so that it discusses more properly the different proposed principles of relativity (as distinct from the related theories). Harald88 22:11, 10 July 2006 (UTC)
I believe I have copied all the useful material from principle of relativity (which it too long, IMO) to theory of relativity. Are you suggesting a straight cut&paste from TOR -> POR? That's sounds OK to me. I'm not sure if I quite follow what content changes you envisage, but whatever they are they will be easier to implement when the duplicate page(s) are removed. --Michael C. Price talk 23:49, 10 July 2006 (UTC)
Ah, I see what you mean (& are the changes I've been makiing): the principles behind the various theories that we want to highlight at the summary level. --Michael C. Price talk 00:24, 11 July 2006 (UTC)]
In my opinion, we should keep the article Principle of relativity, which is a well-defined concept, and turn the article Theory of relativity into a disambiguation page with links to special relativity and general relativity. (I think it's the same thing as suggested by Harald88 above.) Yevgeny Kats 04:26, 11 July 2006 (UTC)
I agree. --Michael C. Price talk 04:51, 11 July 2006 (UTC)
I somewhat disagree. I think that the business on "Indian relativity" and "Galilean relativity" need to be removed from theory of relativity, as both are statements of the principle, and not of Einstein's theories which is what is really being refered to. However, I like the current thumbnail sketches of EInstein's theories. They do not need to be expanded, but neither do I feel that they should be removed.
For this page, we need to establish what the "special principle of relativity" and the "general principle of relativity" are. Note that Galileo essentially stated the special principle in his work. Also note that the difference between Newtonian physics and special relativity is not in the principle of relativity but instead in what that principle is asked to act with. In Newton's mechanics, the added presmises are an absolite space and and absolute time with universal simultaneity. In Einstein's theory, the constancy of the speed of light (given synchronized clocks and standard rods) is the added premise. --EMS | Talk 05:32, 11 July 2006 (UTC)
"Indian relativity" and "Galilean relativity" will be removed from theory of relativity when it is turned into a disambigutaion page as the changes suggested above are implemented. I like the addition of a mention of Newtonian physics. I'm going to make the changes and then we can continue to improve the content. --Michael C. Price talk 05:46, 11 July 2006 (UTC)
Changes done as discussed. --Michael C. Price talk 05:59, 11 July 2006 (UTC)

[edit] Indian Relativity

I remoeved a staement about an 'Ancient Indian theory of relativity' I doubt that any 'ancient Indian theory' proposed exactly what is meant here by principle of relativity. If we could find out what that word meant, then we could add it as a footnote to the article. DJ Clayworth 19:57, 2 February 2006 (UTC)

And I removed it again. The link provided by the author (namely, http://www.crystalinks.com/indiascience.html) does not say anything substantial at all about "Indian Relativity". If you do not know what exactly this 'Indian relativity theory' stated then how can you deduce that it is relevant to the topic of our article? The article is about physics, no need to quote here every ancient sage who used the word 'relative' once or twice in his writings.
—The preceding unsigned comment was added by 212.199.22.14 (talk • contribs).
See also Gravitation for a similar discussion of a dubious invented in India claim. See these claims regarding alleged ancient Indian helicopters and such like, which rather speaks for itself.---CH 03:19, 21 April 2006 (UTC)

[edit] This page needs a rewrite

Please note that the principle of relativity is not the theory of relativity. As-is, this page could be a redirect to theory of relativity. Instead it should have sections entitled "special principle of relativity" and "general principle of relativity", and talk about what those are. Kindly note that the special principle is Galileo's original principle of relativity. Talk of electromagnetism and the speed of light should be kept to a minimum, and at best be part of a quick reference to special relativity.

The question to be answered here is "what is the principle of relativity?". Instead, this is a discussion of the theory of relativity. That needs to be corrected. --EMS | Talk 16:09, 11 July 2006 (UTC)

No, it is a discussion of how the theories of relativity derive from the principle of relativity, as defined in the opening sentence as:
The principle of relativity, refers to Galilean relativity and its subsequent application to Albert Einstein's special relativity and general relativity.
You seem to wish for the principle of relativity page to eschew all 20th century developments. Such a page would then be called Galilean relativity - and it already exists. It is the theory of relativity that needs a rewrite or new title -- the title is singular but the content is plural. --Michael C. Price talk 16:34, 11 July 2006 (UTC)
You are confusing the principle and the theory. You need to get the principles stright, and then do please mention their connections to the theories. However, you do not need to talk about the theories themselves. I do see the theory of relativity page needing some expansion, but only to list major consequences. This page needs to be refocussed of Galileo's principle and Einstein's generalization of it for general relativity. --EMS | Talk 17:08, 11 July 2006 (UTC)
Our viewpoint and motivation seem identical, we are just disagreeing over the means. I see the need to distinguish between the principle and the theories, and for the principle to focus on Galileo; this page needs to split the Galilean section in two (which I'll attend to). But there is a natural overlap between the theories of R (pl) and the principle of R (sing), since all the theories have the same principle in common. However theory of relativity (sing) should just disambiguate onto single theory and not talk about the relationship between theories. I am still unhappy with the duplication between the two pages as they stand; it will lead to endless rewrite confusion. --Michael C. Price talk 17:33, 11 July 2006 (UTC)
I wholeheartedly agree with your concern about duplication. My point is that the theory of relativity page does what I want it to do, and there is no need for this page to duplicate that. This page needs to focus more closely to the principles. i. e. -
  • The speical principle of relativity - The laws of physics are the same for any set of observers in uniform linear motion with respect to each other.
  • The general principle of relativity - The laws of physics are the same for all observers independent of their state of motion.
This article does not even state the general principle. Instead of a discussion of the theories of relativity, what is needed here is a discussion of the principle and their relationship to the theories. Once you focus in, you will find that you are looking at the foundations of relativity, and are paying much less attention to what has been built on that foundation. --EMS | Talk 04:31, 12 July 2006 (UTC)
Actually the way you have stated the general principle is confused, since the general principle is more than just general covariance. This article states that GR exhibits a local Lorentz covariance, which is a stronger statement than just general covariance since it implies general covariance (all observers see same physics) and that the local physics they see is that of SR (i.e. Lorentzian). This muddleness extends through a number of GR article. --Michael C. Price talk 09:58, 12 July 2006 (UTC)

[edit] erroneous statement about Maxwell's equations

Some anon removed "since the invariance of the speed of light is a consequence of Maxwell's equations of [[electromagnetism]".

It is doubtful that any quality publication claims that Maxwell's equations alone suffice.

Maxwell himself used his equations and publicly disagreed with that claim by proposing the MMX (his equations were defined relative to the ether). Maxwell's equations together with the PoR do have that consequence however. It may also be derived form combining his equations with the electromagnetic theory of matter. Harald88 21:09, 9 October 2006 (UTC)

I have restored the claim, adding the PoR requirement, which makes the statement correct. It also means that it leads naturally onto the next paragraph. --Michael C. Price talk 23:46, 9 October 2006 (UTC)
Fine to me Harald88 19:04, 10 October 2006 (UTC)

[edit] Berkeley's General Relativity

What about the general assertion that underlies all relativity? This is equivalent to Berkeley's Idealism. It is the statement that all objects exist as they are only in relation to an observing subject. No object exists as it is in itself, apart from an observing subject.Lestrade 18:02, 8 November 2006 (UTC)Lestrade

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