Talk:Special relativity

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Former good articleSpecial relativity was one of the Natural sciences good articles, but it has been removed from the list. There are suggestions below for improving the article to meet the good article criteria. Once these issues have been addressed, the article can be renominated. Editors may also seek a reassessment of the decision if they believe there was a mistake.
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Next steps[edit]

New sections under consideration[edit]

After adding the section on Graphical representation of the Lorentz transformation, it became possible for me to move Causality and prohibition of motion faster than light from where it had been stuck to a more rational location in the article, and then to fence off the old legacy sections behind a "Warning! There be lions and tigers and bears (oh my) beyond this point!" sign.

There is obviously a lot left to be done. Optical effects ought to include relativistic aberration and maybe the Fizeau experiment??? Dynamics, of course, covers force, energy and momentum, collisions, and relativistic mass (and why the majority of physicists consider it to be a deprecated concept). Relativistic mass is a concept that most lay persons have heard about, and I'm sure that many visitors to this article page have been disappointed not seeing any mention of it.

I'm hoping that my reorganization should make it easier to add these additional topics. Prokaryotic Caspase Homolog (talk) 09:33, 11 November 2018 (UTC)

Hmmm... it looks like I need to cover the magnet and conductor thought experiment as a preparatory step before covering relativistic aberration. Prokaryotic Caspase Homolog (talk) 06:48, 18 November 2018 (UTC)

Should experimental results be blended with the main narrative or kept separate?[edit]

Currently, almost all discussion of the experimental justification for SR is sequestered in the Status section, not that there is very much of it.

Is this a desirable organization? Prokaryotic Caspase Homolog (talk) 20:49, 14 November 2018 (UTC)

Pre-relativistic understanding of length contraction and time dilation[edit]

@JRSpriggs: Please comment on the proposed revised statement+notes+references and suggest changes as necessary. Prokaryotic Caspase Homolog (talk) 07:47, 15 November 2018 (UTC)

  • I had written the following:
In pre-relativistic physics, distance and time were considered to be independent measurements, and despite some puzzling experimental results, physicists had no inclination to believe that measured distance or time between events should change as a result of a shift in frame from which measurements are made.
  • You commented, "distance between events DOES change in prerelativistic physics as a result of a change in the reference frame, unless the time of the events is the same" and changed the wording to the following:
In pre-relativistic physics, distance and time were considered to be independent measurements, and despite some puzzling experimental results, physicists had no inclination to believe that measured time between events should change as a result of a shift in frame from which measurements are made.
  • Pre-relativistic views of length contraction and time dilation are rather complex to describe, and my initial phrasing was an (apparently futile) attempt to avoid going into extensive discussion of Lorentz's and Poincaré's speculations. I propose the following revision with notes and references:
In pre-relativistic physics, distance and time were considered to be independent measurements, and despite some puzzling experimental results, physicists had no inclination to believe that any "true" measured distance[note 1] or time[note 2] between events should change as a result of a shift in frame from which measurements are made.
  • I am quite aware that the final form of Lorentz ether theory predicts, within its domain of applicability, results which are identical to those of special relativity. LET, however, underwent extensive development between 1892 through 1905, and meant quite different things at different times. Just because the final form of the theory does not contradict Newton's third law, does not invalidate my statement in the notes that earlier versions had difficulties in conforming with classical mechanics.

Notes

  1. ^ The results of the Michelson–Morley experiment led George Francis FitzGerald and Hendrik Lorentz independently to propose the phenomenon of length contraction. Lorentz believed that length contraction represented a physical contraction of the atoms making up an object.[1]:62–68 In his view, length contraction should result in compressive strains in an object that should result in measurable effects. Such effects would include optical effects in transparent media, including optical rotation[p 1] and induction of double refraction,[p 2] and the induction of torques on charged condensers moving at an angle with respect to the aether.[p 2] Lorentz was perplexed by experiments such as the Trouton–Noble experiment and the experiments of Rayleigh and Brace which failed to validate his theoretical expectations.[1]
  2. ^ For mathematical consistency, Lorentz proposed a new time variable, the "local time", which depended on the position of a moving body following the relation .[p 3] Lorentz considered local time not to be "real"; rather, it represented an ad hoc change of variable. Impressed by Lorentz's "most ingenious idea", Poincaré saw more in local time than a mere mathematical trick. It represented the actual time that would be shown on a moving observer's clocks. On the other hand, Poincaré did not consider this measured time to be the "true time" that would be exhibited by clocks at rest in the aether.[2] The multiplication of hypotheses led to disturbing conflicts with classical mechanics, including violation of Newton's third law of action and reaction.[1]:39–42

Primary sources

  1. ^ Lorentz, H.A. (1902). "The rotation of the plane of polarization in moving media" (PDF). Huygens Institute - Royal Netherlands Academy of Arts and Sciences (KNAW). 4: 669–678. Retrieved 15 November 2018.
  2. ^ a b Lorentz, H. A. (1904). "Electromagnetic phenomena in a system moving with any velocity smaller than that of light" (PDF). Huygens Institute - Royal Netherlands Academy of Arts and Sciences (KNAW). 6: 809–831. Retrieved 15 November 2018.
  3. ^ Lorentz, Hendrik (1895). "Investigation of oscillations excited by oscillating ions". Attempt at a Theory of Electrical and Optical Phenomena in Moving Bodies (Versuch einer Theorie der electrischen und optischen Erscheinungen in bewegten Körpern). Leiden: E. J. Brill. (subsection § 31).

References

  1. ^ a b c Miller, Arthur I. (1998). Albert Einstein's Special Theory of Relativity: Emergence (1905) and Early Interpretation (1905-1911). Mew York: Springer-Verlag. ISBN 0-387-94870-8.
  2. ^ Darrigol, Olivier (2005). "The Genesis of the Theory of Relativity" (PDF). Séminaire Poincaré. 1: 1–22. Retrieved 15 November 2018.
  • You are over-thinking this. I was not talking about length contraction or any such thing. The sentence which I changed said "... measured distance or time between events should change as a result of a shift in frame ..." (emphasis added). Before special relativity, we had Galilean relativity according to which the transformation law for frames of reference was:
.
The "− v t" term means that the location of an event depends on the time that its position is measured. So if you subtract two such locations to get the x-component of the distance, you will get a value which depends on the times of the events. That is my whole point. JRSpriggs (talk) 20:10, 15 November 2018 (UTC)
Well, that does not represent what Purgy and I intended. Will have to do a major re-write. Prokaryotic Caspase Homolog (talk) 23:49, 15 November 2018 (UTC)
More precisely one could perhaps write about coordinatizing two events
in two Galilean inertial frames in standard configuration
leaving separately both their time lapse
and their contemporal spatial distance
invariant.
Not talking about the boring coordinates, this could be also more detailed to
The space—time interweaving puts an end to identical time as well as to contemporality across non comoving frames. Sorry, I missed from the diff-view the suggestion below, and also had no edit conflict. Use to your liking. Purgy (talk) 09:17, 16 November 2018 (UTC)

  • Let's try this:
In Galilean relativity, length ()[note 1] and temporal separation between two events () are independent invariants, the values of which do not change when observed from different frames of reference.[note 2][note 3]
In special relativity, however, the interweaving of spatial and temporal coordinates generates the concept of an invariant interval, denoted as :
[note 4]
The interweaving of space and time revokes the implicitly assumed concepts of absolute simultaneity and synchronization across non-comoving frames.

Notes

  1. ^ In a spacetime setting, the length of a rigid object is the spatial distance between the ends of the object measured at the same time.
  2. ^ The results of the Michelson–Morley experiment led George Francis FitzGerald and Hendrik Lorentz independently to propose the phenomenon of length contraction. Lorentz believed that length contraction represented a physical contraction of the atoms making up an object. He envisioned no fundamental change in the nature of space and time.[1]:62–68
         Lorentz expected that length contraction would result in compressive strains in an object that should result in measurable effects. Such effects would include optical effects in transparent media, such as optical rotation[p 1] and induction of double refraction,[p 2] and the induction of torques on charged condensers moving at an angle with respect to the aether.[p 2] Lorentz was perplexed by experiments such as the Trouton–Noble experiment and the experiments of Rayleigh and Brace which failed to validate his theoretical expectations.[1]
  3. ^ For mathematical consistency, Lorentz proposed a new time variable, the "local time", called that because it depended on the position of a moving body, following the relation .[p 3] Lorentz considered local time not to be "real"; rather, it represented an ad hoc change of variable.[2]:51,80
         Impressed by Lorentz's "most ingenious idea", Poincaré saw more in local time than a mere mathematical trick. It represented the actual time that would be shown on a moving observer's clocks. On the other hand, Poincaré did not consider this measured time to be the "true time" that would be exhibited by clocks at rest in the aether. Poincaré made no attempt to redefine the concepts of space and time. To Poincaré, Lorentz transformation described the apparent states of the field for a moving observer. True states remained those defined with respect to the ether.[3]
  4. ^ This concept is counterintuitive at least for the fact that, in contrast to usual concepts of distance, it may assume negative values (is not positive definite for non-coinciding events), and that the square-denotation is misleading. This negative square lead to, now not broadly used, concepts of imaginary time. It is immediate that the negative of is also an invariant, generated by a variant of the metric signature of spacetime.

Primary sources

  1. ^ Lorentz, H.A. (1902). "The rotation of the plane of polarization in moving media" (PDF). Huygens Institute - Royal Netherlands Academy of Arts and Sciences (KNAW). 4: 669–678. Retrieved 15 November 2018.
  2. ^ a b Lorentz, H. A. (1904). "Electromagnetic phenomena in a system moving with any velocity smaller than that of light" (PDF). Huygens Institute - Royal Netherlands Academy of Arts and Sciences (KNAW). 6: 809–831. Retrieved 15 November 2018.
  3. ^ Lorentz, Hendrik (1895). "Investigation of oscillations excited by oscillating ions". Attempt at a Theory of Electrical and Optical Phenomena in Moving Bodies (Versuch einer Theorie der electrischen und optischen Erscheinungen in bewegten Körpern). Leiden: E. J. Brill. (subsection § 31).

References

  1. ^ a b Miller, Arthur I. (1998). Albert Einstein's Special Theory of Relativity: Emergence (1905) and Early Interpretation (1905-1911). Mew York: Springer-Verlag. ISBN 0-387-94870-8.
  2. ^ Bernstein, Jeremy (2006). Secrets of the Old One: Einstein, 1905. Copernicus Books (imprint of Springer Science + Business Media). ISBN 978-0387-26005-1.
  3. ^ Darrigol, Olivier (2005). "The Genesis of the Theory of Relativity" (PDF). Séminaire Poincaré. 1: 1–22. Retrieved 15 November 2018.
  • This edit rephrased the footnote defining "length" as a prima vista triviality. A prominent intent in me writing a "bulky" definition for this everyday notion was to introduce the notion of events in their fundamental role of establishing "length", the value of which will turn out as varying from frame to frame, because of varying coordinates of these events. I think the ladder paradox is directly connected here, and a caveat of carrying forward a sloppy notion of length into the realm of STR is appropriate. Purgy (talk) 11:10, 17 November 2018 (UTC)
I know what you meant, but I found your original wording confusing. "In a spacetime setting the length of a rigid object is defined by the spatial distance of the two events made up of the ends of this object at the same time." The ends of the object are not events, but follow world lines. So you intended that the length of the object should mean the spatial distance between two events, having the same time coordinates, selected from the world lines of the two ends. This spatial distance would, in general, be less than the length measured in the rest frame of the object (i.e. its proper length). In the rest frame of the object, however, it is not at all necessary that the selected events have the same time coordinate.
All this amounts to a lot of superfluous detail, especially in a footnote for the Galilean scenario for which length is an invariant. Prokaryotic Caspase Homolog (talk) 13:45, 17 November 2018 (UTC)
To me an end of an object at some point in time makes up a perfect event ("two events made up of the ends of this object at the same time"), and varying time traces out a world line, which is a certain collection of events; and I perceive no problem with getting acquainted to the notion of a time-varying lenght, as observed from different frames, never getting longer than some distinguished length, one might call a proper length. I do not know how fruitful it is to continue measuring lengths at different times, which only works in the rest frame of the object, and I expressed my doubts about leaving the notion of length in its "trivial" setting of Galileian spacetime.
I did not suggest a footnote pertaining to the Galileian notion of length, but a footnote to a sustainable notion of length in general, and to focus the attention to the upcoming change in the notion of length. I think the method of "how to measure length" should not be changed/restricted during migration to STR, and thus I am unaware of a "lot of superfluous detail", however, I consider a footnote, missing to warn about the upcoming subtleties, to be really superfluous. Lifting the wording above confusion (which I miss) to your standards is beyond me, disagreeing is not. :) Purgy (talk) 17:01, 17 November 2018 (UTC)
In terms of English language usage,
  • I do not know what a "spatial distance of two events" means. I do know what a spatial distance between two events means.
  • I do not know what "two events made up of the ends of the object at the same time" means. I do know what two "ends of the object measured at the same time" means.
  • Applying the two "Englishian phrase transformations" essentially converts one phrase to the other, except for your use of "is defined by."
Prokaryotic Caspase Homolog (talk) 18:33, 17 November 2018 (UTC)
An apology for lack in English idiomology is certainly nonsensical, I just regret it. In German the use of a genitive ("of") instead of a preposition ("between") is common habit, I do ask for improving my construction of "making up" events from their spatial and temporal coordinates (but leaving "events" in place), while talking about their "spatial distance", and, thirdly, after applying the EPT (see above) I do miss not only the rigor-effusing "is defined by", but also the mentioned "event", which is at the heart of post-Galilean spacetime.
I repeat the heart of my complaint: "prima vista triviality". If it were not for the last, innocuously sounding phrase ("measured at the same time"), the whole footnote would be absolutely "superfluous". My concern is to transport to the reader this essential condition not as a small closing phrase, but as an essential, concept transforming information, and I tried to do this via the demonstrative use of "defining" and "event". As usually, take what you like. Purgy (talk) 21:13, 17 November 2018 (UTC)

───────────────────────── No, it is not trivial. It is at the heart of the "brain freeze" that led to my oversight. I should have known better. I thank JRSpriggs for correcting me, and I thank you for introducing the original version of the footnote.

Pushed to main space. We can work on improving it later, but there are other topics to add. Prokaryotic Caspase Homolog (talk) 04:19, 18 November 2018 (UTC)

I am afraid of being misunderstood. I never wanted to state that the footnote, as is stands now, were trivial. I want to express my perception that its current formulation lacks emphasis on the changing settings in Galilean and -say- Einsteinian spacetime. To my taste, it evokes a prima vista(=sometimes wrong!)-impression of not being that fundamental as it is, by putting a condition, generating the decisive difference, into a small appendix of the sentence. My pleadings just ask for more emphasis on the change, suggesting the use of the words "define" and "event". There is absolutely no need, for a statement to be correct, of conforming to my taste. Purgy (talk) 09:24, 18 November 2018 (UTC)

Re recent edits[edit]

Re recent edits:

  • Time dilation: To me it was important to hint to the "lifetime" of muons as an "expectation value", to contrast "high"- and "low"-speed muons, to emphasize "equality" in the respective rest frames, and, maybe not so important, to introduce "proper time". All four items negligible?
  • Composition of velocities: I protest. It's about "transformations" of velocities to other frames: u to u'. The LT primarily transform coordinates, their "differential form" is unexplained jargon, the ubiquitous use of the word "addition of velocities" for this situation is abuse of language, be it widespread as it might. "Adding" velocities, as measured in different frames, is a categorical flaw, and each velocity measured in one frame, necessarily introduces another frame: the rest frame of the measured body. This is a delicate topic.
  • Standard configuration: This, imho, had no encyclopedic value at all, if it were not for its necessity in introducing Minkowski diagrams, so I do not really understand the perceived emphasis put on it. My personal preference is to start with showing the boost-equations in (1+1) dimensions, generalize to (1+3) dimensions in matrix notation, and reduce the matrix again to "standard configuration". Maybe this would pay the rent in presenting the Thomas rotation, the transversal Doppler effect, ... Summing it up, I think from an encyclopedic POV it is more seminal to introduce the (3+1)-LT with β and γ, and stuff its matrix with 0s, than to put a priori importance on a "standard configuration", simply gained and well defined by these 0s.
  • Two postulates: I do not understand the emphasis on this, too. It would be my first concern to derive from clunky postulates some most handy equations to proceed with the development of a closed theory. So why should any researcher resort to the postulates, if it were not for writing a bestseller (... any single formula in a book reduces the paid circulation by ... (unknown source)). Isn't this unencyclopedic, a la textbook? ;)
  • Apologies for my off topic curiosity: I know about the constructs of gerunds and infinitives, and I learned back then that "allow" could be used with both constructs (in contrast to "allow for", which is only to be used with the gerund). Was this wrong all the time, has it changed some time in between, or is it a matter of taste? Ignore, if bothering. Thanks.

As usual, something to wholeheartedly disagree to. :) Purgy (talk) 20:19, 23 November 2018 (UTC)

Time Dilation[edit]

Before[edit]

Time dilation explains a number of physical phenomena; for example, the (expected) lifetime of a muon that starts to exist with a collision of a cosmic ray with a particle of the Earth's atmosphere, then moves at very high speed towards the surface, and decays near there, is measured as greater than the lifetimes of slowly moving muons, generated and decaying in a laboratory. Both muons would, of course, measure identical lifetimes, when looking at their respective wrist watches, and both observe the other as moving, and the moving watch as ticking slower than their own, which is said to show the proper time, as measured by an observer (at rest in its frame).

After[edit]

Time dilation explains a number of physical phenomena; for example, the lifetime of high speed muons created by the collision of cosmic rays with particles in the Earth's outer atmosphere and moving towards the surface is greater than the lifetime of slowly moving muons, created and decaying in a laboratory.

Commentary[edit]

Both JRSpriggs and I worked on this one. We halved the number of words and simplified sentence structure. Measuring "(expected) lifetime of a muon" is easily misread from what you apparently intended.

Dragging effects[edit]

Rindler states, on page 54 of his textbook, "From the point of view of special relativity, however, the result (3.1) is nothing but the relativistic velocity addition formula!" He doesn't even use the word "composition." All of the "delicate" nuances that you insist on should already be implied and understood from previous explication of the relativistic composition of velocities, and should not clutter discussion of dragging effects.

Standard configuration[edit]

Shorthand expression "standard configuration" reduces a lot of verbiage. The expression is repeatedly used not just in my writing, but in the legacy "Technical discussion of spacetime" section containing material that I judged to be beyond freshman-sophomore mathematics level. If you wish to start with showing the boost-equations in (1+1) dimensions, generalize to (1+3) dimensions in matrix notation, and reduce the matrix again to "standard configuration" etc. etc. , then you should draft a section for the technical section. It's there for a reason. It sounds to me like your proposal could be a great addition.

Two postulates[edit]

You appear to misunderstand my rewording of the section titles. Historically, this article appears to have been written by two camps of editors, a group of "two-postulates" advocates, and a group of "single postulate of Lorentz invariance" advocates. When I started my series of edits, the article was a mishmash of contributions by the two groups, and was very confusing to me, until I understood the historical development of this article. I am a "packrat" and do not like throwing out material. Rather than delete legacy "two-postulates" material, what I am trying to say with my rewording of the section titles is: Traditionally, special relativity has been presented in terms of Einstein's two postulates. This article does not follow this old tradition, but instead presents special relativity using the single postulate of Lorentz invariance.

Use of "allow"[edit]

You are referring, I presume, to this edit. "Allow to deal with" is wrong, and this wrongness is not a matter of taste. You can write, "allow one to deal with" or "allow dealing with".

General comment[edit]

Please use diffs to pinpoint problematical edits. Otherwise you force me to do a lot of research to pinpoint the exact phrase that you have having issues with. Prokaryotic Caspase Homolog (talk) 12:28, 24 November 2018 (UTC)

Purgy's reply[edit]

Thanks for the grammar, apologies for missing links, Roma locuta, ... We are the faithful. Nevertheless, a few remarks:

- no high speed muons, no equal lifetime in specific frames?

- I did not refer at all to "Dragging effects", I experience the content of this section as delicate. I seriously protest against calling the "transformation" of a velocity from one frame to another frame an "addition", not even a composition, just because the transformation in the Galilean transform is represented by an addition. Velocities are never added, the are transformed in a Galilean setting by simply adding a quantity, which is characteristic for the transformation, and necessarily must be of equal dimension and in a more complex way in STR spacetime. Of course, the transformation, as all LT do, involves a velocity, but I would avoid calling this even a composition. As usual: just to let you know.

- I would prefer not to. Bartleby's English is not firm enough to compose a full section.

- Obviously I am not sufficiently sensitive to experience the nuances of the two sources, or I was trained to ignore all postulate driven derivations after having seen the derivation of the full blown LT from first principles, presented by a real Grandmaster of the Chalk (sort of a W. Lewin, if this is allowed to state wrt lecture quality).

I am around, hopefully not disturbing. Purgy (talk) 18:40, 24 November 2018 (UTC)

Not obvious?[edit]

Never would I simply revert this edit, but ...

- Given two frames in standard configuration (unprimed, primed),
- and having derived that the parameters and of the respective Lorentz transformations of coordinates between these frames, both parameters representing measurements in the respective frames, satisfy the equation in this configuration,
- and having also derived how a velocity transforms under Lorentz transform with parameter (denoted ) to specifically and
- then -using the equation of the premise-

To me the above conclusion in the last step is immediate/obvious/..., but I see no reasoning that to achieve an inverse of a Lorentz transform

- primed and unprimed symbols are interchanged and a parameter is replaced by its negative,

(both recipes only motivated by the structure of two formulae?), but rather this is coherently done by

- performing an appropriate Lorentz transform (from all primed to unprimed), and then
- substituting the derived parameter identity.

BTW, my editorializing term of velocities being incoherent, if they belong to different frames, is based on the fact that in the inverse Lorentz transformation the parameter velocity and the transformed velocity are derivatives with respect to a different time. This reservation also holds for transforming simple coordinates. Validity of any replacements must be justified for each installment. Purgy (talk) 10:32, 4 January 2019 (UTC)

Remember, I consider the primary target audience for the first half of this article to be lower division college science students and senior high school students. Many of your contributions tend to be detailed to the point of being incoherent to the target audience, although entirely appropriate for the second, "technical" half of the article.
If a lower division college science student asks me what a good book for self-study would be, I would recommend Morin or French. I desire the first half of the article to be at the level of those textbooks.
You can be as detailed and precise as you want in the "technical" half of the article.
Prokaryotic Caspase Homolog (talk) 12:25, 4 January 2019 (UTC)

Emission theory of light[edit]

The normal introductory dialog on relativity says "No aether could be found so there must be constant speed of light", and then we change basic constructs of time and space to fit. But the much more obvious explanation is the Emission theory. It occurred to me as a student, and to Newton long before that. And the binary star refutation was not produced until long after relativity, and Fox's refutation of that much later.

I believe this warrant some solid introductory text. Otherwise it simply does not make sense to anyone that does not already understand it. And it encourages the worst type of thinking in the sciences, merely repeating what authoritative people say without thinking about it.Tuntable (talk) 00:51, 4 April 2019 (UTC)