Talk:Principle of relativity

message: 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.<BR>
In later years, Einstein formulated the principle of equivalence in a more restricted form than at first.<BR> 
External link to 146 Kb PDF document: The Einstein-De Sitter debate<BR>
--Cleon Teunissen 19:21, 25 Feb 2005 (UTC)
thread_title: Untitled

message: 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)
thread_title:  THREE principles 

message: 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)
thread_title:  Principle of Relativity: range of applicability 

message: 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.<BR>
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. <BR>
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.
=== Rotation is absolute ===
Rotation is absolute, but not in the sense that Newton described.<BR>
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.<BR>
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)
replies: text: 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)
thread_title:  Pinciple of relativity: range of applicability (2) 

message: Rotation is absolute, but not in the sense that Newton described.<BR>
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.<BR>
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)
thread_title:  Rotation is absolute 

message: 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. <BR>
--Cleon Teunissen | Talk 11:16, 24 Jun 2005 (UTC)
replies: text: 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)
thread_title:  Inertia 

message: :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)
replies: text: "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': <BR>
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: <BR>
(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)<BR>
--Cleon Teunissen | Talk 14:17, 24 Jun 2005 (UTC)
thread_title:  Principle of relativity: range of applicability (3) 

message: 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)
replies: text: 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)

text: 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)
thread_title:  Theory of relativity 

message: 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)
replies: text: 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)
replies: text:  Fine to me Harald88 19:04, 10 October 2006 (UTC)
thread_title:  erroneous statement about Maxwell's equations 

message: 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)
thread_title: Berkeley's General Relativity

message: The general principle of relativity states that physical laws should be the same in all reference frames -- inertial or non-inertial. An accelerated charged particle might emit synchrotron radiation, though a particle at rest doesn't. If we consider now the same accelerated charged particle in it's non-inertial rest frame, it emits radiation at rest. 
Indeed physical laws are clearly not the same in non-inertial reference frames. As an example Newton's laws of motion are not valid in non-inertial reference frames. Therefore, we can decide, whether the Earth is rotating or not as demonstrated by the Foucault pendulum. I conclude, general relativity is proven to be ridiculous or The Emperor's New Clothes. 84.59.51.81 08:42, 5 April 2007 (UTC)
replies: text: Please see Wikipedia:Talk_page_guidelines and in particular "Talk pages are not a forum for editors to argue their own different points of view about controversial issues".  Alfred Centauri 12:48, 5 April 2007 (UTC)
thread_title: General principle of relativity

message: A statement to correct (if not soon corrected, I'll remove it):
Nonsense:
"Many physicists, including both Larmor and Lorentz, discarded the special principle of relativity in favor of a luminiferous aether -- a plenum that fills space and is a medium for electromagnetic radiation."
Fact: Lorentz tried to develop a theory that obeyed the PoR and achieved it (almost) in 1904. See also the book "The Principle of Relativity" by Einstein and Lorentz. I cite Lorentz:
"Poincare has objected to the existing theory of electric and optical phenomena in moving bodies [...] It
would be more satisfactory if it were possible to show by means of certain fundamental assumptions and without neglecting terms of one order of magnitude or another, that many electromagnetic actions are entirely independent of the motion of the system. Some years ago, I already sought to frame a theory of this kind"
Harald88 08:46, 30 October 2007 (UTC)
thread_title:  erroneous statement 

message: Uh. I think the history section might need to be, you know, rewritten? It only reads:
Aristotle
Newton
Einstein
Dbutler1986 (talk) 01:56, 16 February 2008 (UTC)
replies: text: Agreed.  This makes no sense whatsoever. Borg Sphere (talk) 17:06, 28 February 2008 (UTC)
thread_title:  History. 

message: The general principle of relativity states that "local laws of physics are the same in all coordinate systems". The current article is incorrect, because it does not state local laws, and because it confuses coordinate systems with reference frames. A reference frame is not a coordinate system. It is the physical matter from which a coordinate system may be empirically defined.  —Preceding unsigned comment added by RQG (talk • contribs) 10:37, 1 April 2010 (UTC)
thread_title:  Incorrect expression of general principle of relativity 

message: The article could use a section on the cosmic microwave background radiation.  —Preceding unsigned comment added by NOrbeck (talk • contribs) 07:50, 25 July 2010 (UTC)
thread_title:  No mention of the Cosmic Microwave Background  CMB 

message: Why is the time line out of order? Hansholler (talk) 07:26, 26 July 2011 (UTC)
thread_title:  order of the timeline 

message: The article speaks of Newton's using an assumption of absolute time. The article on Absolute time says Newton flatly stated that absolute time existed.  — Preceding unsigned comment added by 216.104.121.229 (talk) 12:23, 19 March 2014 (UTC)
thread_title:  Assumption 

message: can anyone tell me with a straight face that this is written for a general audience ?
this is not
General relativity was developed by Einstein in the years 1907 - 1915. General relativity postulates that the global Lorentz covariance of special relativity becomes a local Lorentz covariance in the presence of matter. The presence of matter "curves" spacetime, and this curvature affects the path of free particles (and even the path of light). General relativity uses the mathematics of differential geometry and tensors in order to describe gravitation as an effect of the geometry of spacetime. Einstein based this new theory on the general principle of relativity, and he named the theory after the underlying principle.  — Preceding unsigned comment added by 2601:192:4701:BE80:8409:3056:A236:A59F (talk) 02:22, 31 May 2019 (UTC)
thread_title: not written for a general audience

message: It would be helpful if the authors of this article could clarify the following statement regarding the relativity principle and the principle of the independence of the speed of light (in vacuum) from the motion of the source:    
"These two principles were reconciled with each other (in Einstein's treatment, though not in Poincaré's) by a re-examination of the fundamental meanings of space and time intervals."   
 
Why exactly are these principles not reconciled within Poincare's treatment?
: I agree with this anonymous comment, and removed the parenthetical remark. Roger (talk) 06:36, 26 December 2021 (UTC)
thread_title: A very unclear (possibly, erroneous) statement