Talk:Special relativity/Archive 4

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Archive This is an archive of past discussions. Do not edit the contents of this page. If you wish to start a new discussion or revive an old one, please do so on the current talk page.

Contents

Interesting

The universe seems to be arranged in hierarchies. There are various levels at which you can understand it. The level we are most famililar with is the level of everyday life. Going down we run into the levels of organs, cells, molecular biology, chemistry. Each level has its own laws which work in certian "special cases" with all violations at the "extreams". Complex Adaptive Swarms exhibit similar layered behavior also, at each layer the swarm is made of smaller complex adaptive systems. This layered behavior doesn't appear in swarms made of simple systems. This all seems to imply that either there is no bottom to the layers of the universe(and no TOE) or, the bottom layer is made of smart, adaptive particles.--SurrealWarrior 05:15, 10 July 2005 (UTC)

Actually, the universe only has one set of laws. It's just that under different conditions, different portions of the laws become the dominant force. Scientists tend to simplify things by only using and studying those portions of the laws that are dominant for any given set of conditions. This leads to the appearance that quantum physics, Newtonian physics, and general relativity are three different sets of laws. --Carnildo 03:41, 29 July 2005 (UTC)

New image and caption

First of all, my thanks to Cyp for that amazing image. It is quite marvelous and very, very much should be in this article. (It could also be on the Lorentz transformation article, but it must be on this one in any case. It has a lot of expanatory power and is a tremendous asset to this article.)

I have framed it and greatly expanded the caption. I hope that the expanded caption will help people to understand it and what is going on. If people can find a way to shorten the cation and/or make it clearer I would not mind, although I doubt that much cutting can be done without losing meaningful content. Similarly if people see something significant that I have not mentioned then please feel free to expand it. --EMS | Talk 21:37, 22 August 2005 (UTC)

Yes, it is a truly outstanding addition to this article. If you are still around Cyp, well done ! (did you make the animation yourself ?). It should definitely be here. The chunk of text in the caption is a little lengthy, but can't really change much there. However, if shortening the caption becomes an issue, then parts of the caption could refer to the main text, where the animation can be discussed further. ---Mpatel (talk) 15:46, August 23, 2005 (UTC)
Yes, I'm still around, and I did make it myself. (See the image description page for the source.) I made it very short, to keep it under ¾ MB. Wasn't sure how to explain the picture with a short caption, so I just dumped it on the page without a caption to let someone else worry about it. Thanks, I'm glad the image is liked. Κσυπ Cyp   11:13, 24 August 2005 (UTC)

the mass dispute

The concept of velocity dependent mass, relativistic mass, is examined and is found to be inconsistent with the geometrical formulation of special relativity. This is not a novel result; however, many continue to use this concept and some have even attempted to establish it as the basis for special relativity.

Actually, velocity dependent mass (although not relativistic mass) was at the basis of SRT, and even the first verified prediction of SRT. IOW, your(?) allegation is known to be historically at fault.
For half a century a large amount of textbooks and authorities on that subject used that concept - I learned it from Alonso&Finn and many others learned it from Feynman. Only in recent years there has been a push (or should I write Putch?!) by some influential people to abandon the concept, partly by spreading misinformation that the uninformed swallow (I call incorrect claims by experts of a certain age lies, because it's humanly almost impossible that they don't know the truth.) The cause can harldy be convention or lack of information, as the debate is too much an emotional dispute - instead it appears to be a clash of ideologies and ontologies, similar as the never fully solved difference between Lorentz and Einstein.

It is argued that the oft-held view that formulations of relativity with and without relativistic mass are equivalent is incorrect.

Hmm, there can be no doubt about them being *by definition* mathematically equivalent, but for reasons of consistency (as already pointed out on the main page) some prefer relativistic mass, while others prefer rest mass to be called "mass" for better compatibility with the now popular geometric reinterpretation.

Left as a heuristic device a preliminary study of first time learners suggest that misconceptions can develop when the concept is introduced without basis.

Indeed, that suggestion is misleading, it's an under-the-belt punch to suggest that people as Feynman had misconceptions about mass, or were stuck at "first-time learner level". I had not yet reflected on it, but now that you put my attention to it: this kind of dirty war is unacceptable for Wikipedia, I demand that phrase to be corrected to a neutral (NPOV) description of the disagreement (which BTW is already correctly indicated in the article).

In order to gauge the extent and nature of the use of relativistic mass a survey of the literature on relativity has been undertaken. The varied and at times self-contradicting use of this concept points to the lack of clear consensus on the formulation of relativity. As geometry lies at the heart of all modern representations of relativity, it is urged, once again, that the use of the concept at all levels be abandoned. PACS numbers: 01.40.Gm, 03.30.+p http://arxiv.org/PS_cache/physics/pdf/0504/0504110.pdf

Hmm, who is Anonymous @bellsouth.net?
Yes I know that people try to impose their POV and even demand the suppression of differing ontologie. I trust that such improper behaviour will have no chance in Wikipedia, as succombing to such propaganda is against the rules. But am I right to notice that the person who copy-pasted the text of that paper doesn't understand NPOV? At least he/she could have presented the two sides of the debate, or indicated that he/she didn't. For example, an internal report of the CERN (publicly accessible) shows that relativistic mass is actually used and accepted by a number of physicists (not a peer reviewed publication, but neither is the above Arxiv doc!). I could just as well give a similarly biased presentation which largely agrees with that overview, but disagrees on the crucial points, mainly by correcting the errors of that paper and by adding my own errors that push the contrary conclusion. (It's a common propaganda method, also used in religions, to "provide a survey" that suggests a NPOV, only to reach one's own POV as "conclusion"). Furthermore, the writer above is at odds with Einstein who admitted that his 1905 paper inconsistently based dynamics on kinematics, while the used instruments on which the units for kinematical derivation are based ("length", "time"), depend themselves on dynamics.
Clearly, the debate must be mentioned in Wikipedia, and IMO it's now reasonably balanced; the most popular(?) stand is in the main text and the threatened POV confined inside a separate paragraph. Harald88 23:18, 19 October 2005 (UTC) and 12:22, 20 October 2005 (UTC)

I now found the likely cause of much misunderstanding: In a paper by Okun in Physics Today (June 1989 p.31, "The concept of mass"), he may have caused confusion between "relativistic mass" and the "transverse"/"longitudual" mass concepts that were used around 1900:

"These are the very expressions with which Lorentz introduced the two masses. Together with the "relativistic mass" in the relation p = m_r v, where m_r = E/c^2 (which is equal to m_t when m =/= 0, but which had a more general meaning applicable also in the case of photons), these masses formed the basis of the language physicists used at the beginning of the century."

People can easily confuse the different concepts from reading that paper (or a later one of him, if he continued obfuscating them), and this misunderstanding may have played a significant role in the crusade against "relativistic mass". In particular, it may have caused the misunderstanding that relativistic mass is another word for transverse mass, or that F=ma is supposed to be generally valid with relativistic mass. However, that same paper has useful references, for example that relativistic mass" was called "the mass" by Tolman in 1912 - to be checked! Harald88 20:17, 23 October 2005 (UTC)

Errors in "second postulate"

It read: "Second postulate (invariance of c) The speed of light in vacuum, commonly denoted c, is the same to all inertial observers, is the same in all directions".

NOT so: SRT is a theory of physics, and Einstein made sure in 1905 to *not* include isotropy of light speed in the postulate, but in the following *operational definitions*, as it can't be verified by experiments, by definition. In SRT the OWLS is freely *chosen* to be isotropical by the experimenters. See Einstein's definition of "light speed" in his 1905 paper (html); also the discussion yesterday and today in sci.physics.relativity (search in Google groups for OWLS in subject and tom roberts , 19 Oct). Harald88 22:39, 20 October 2005 (UTC). Note also that misunderstanding about this point apparently played a role in this article not being a Featured Article in the past, without thorough verification. What made Einstein's approach appealing, was how he derived the LT from a *minimal* number of assumptions. For example: that light speed will look isotropic when clocks are synchronized by means of clock transport, follows from the *first* postulate.

BTW, there is still more wrong with that postulate paragraph: SRT is in particular *not* a theory about metaphysics. Unfounded metaphysical claims by editors have no place in SRT! Instead of that, the postulates should be phrased in a manner that closely follows Einstein 1905, for he chose his definitions with care, he himself closely following what others had concluded before him. Harald88 23:21, 20 October 2005 (UTC)

The actual words that Einstein used in his second postulate (in the English translation) are:
"Any ray of light moves in the "stationary" system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body. Hence velocity = (light path) / (time interval) where time interval is to be taken in the sense of the definition in §1."
"§1" refers to the definition of what we now call Einsteinian synchronisation using light signals. If Einstein were talking about "two-way light speed" (TWLS) then it would make no sense for him to have to explain what he meant by "time interval", as it would, in that case, just be the proper time recorded by a single clock. But he would have had to explain what he meant by "light path" being a there-and-back distance.
The second postulate should indeed be interpreted to refer to "one-way light speed" (OWLS). It contains within it the requirement, within Special Relativity, that clocks be synchronised via Einsteinian Synchronisation. Isotropy of light speed is more than just an "operational definition" for experimenters: it is an intrinsic feature of the theory. If you choose a different synchronisation method, you're not studying Special Relativity, you're studying another theory (for example, the version of Ether Theory by Mansouri & Sexl, which is mathematically equivalent to Special Relativity but has different definitions of coordinate time and velocity).--Dr Greg 18:14, 31 January 2006 (UTC)
I see what you mean: you do have a point about Einstein's presentation. However, principles of physics are based on observation, not on speculation or mere convention or freely chosen procedures (in fact, that timing convention was already in use at that time).
To do a re-take: your above quote is from paragraph 2, after he defined the coordinate "time" which is used in your above quote. He defined light speed operationally as follows:
"In agreement with experience we further assume the quantity:
2AB/(t_A'-t_A) = c
to be a universal constant--the velocity of light in empty space."
From this together with the freely chosen coordinate time follows that also one-way light speed is determined as c, which is what he wanted to get for convenience.
Of course, how to present it is a matter of taste: it's possible to write it in the mathematically simpler way as you propose and as Einstein arrived at presenting it, but it makes the full formulation of the second postulate more complex, without adding anything useful. In that formulation of the light speed postulate, the time interval that is used for defining light speed, is itself defined by a procedure that is based on two-way light speed (c). That definition needs to be mentioned and explained. Eliminating the circularity brings you back to the definition with one clock...
Harald88 19:34, 31 January 2006 (UTC)

Triangle of Velocities proves that invariant speed is mathematical error

The article at http://www.masstheory.org/triangle_of_velocities.pdf presents detailed analysis of Lorentz transformation, it's derivation procedure and formal mathematical proof that introducing "invariant" speed is indeed nothing but mathematical error.

I suppose that you mean light speed, as defined in the second postulate. But what do you mean with "indeed"? I hope you didn't misunderstand my criticism on misrepresenting the second postulate as criticism on the second postulate itself, which as Einstein explained, was based on experimental results. Apart of that, I assume that the NOR rule implies that discussion of your paper on this page is not useful. Harald88 20:41, 28 October 2005 (UTC)

All history moved to History page

I merged the History page with the version that was still on the Special relativity page, and reedited it plus made some additions. In some cases I had to make a choice between different renderings; some confusing/erroneous sentences I deleted as well as some non-relevant material that just didn't fit in. In case I stepped on a sore toe by deleting something, just reinsert any lost phrase that you may consider essential. Harald88 13:47, 29 October 2005 (UTC)

Relativity and unifying Electromagnetism - error?

I think that the main article is practically ready to make it a featured article; but just now my eye fell on the last paragraph, and I suspect that it's flawed. For how can one completely make the magnetic field of a current loop disappear? I don't think so. Note: In the coming days I'll be too busy with other things, so if I'm not mistaken, please correct this without me. Harald88 21:32, 1 November 2005 (UTC)

Its not making the current in a loop disappear, its making the current in a straight wire disappear. To make the current in a loop disappear, you have to be in a spinning (i.e. non-inertial) frame of reference. But still, why would the magnetic field not disappear, and the loop become an electrostatically charged loop?PAR 22:01, 1 November 2005 (UTC)

OK, you already picked up one glitch in the description. But different from the straight-line motion of two parallel moving charges (Einstein 1905), in a wire we have relative motion of negative and positive charges. You can't make that disappear. Thus, in any frame there are moving charges. How could then the magnetic field disappear? My advice: either consult a good (I mean Very Good) textbook on this and use that as source (I saw no reference anyway), or stick to the simple case of Einstein 1905 (available on the web) and which also is not too complicated for an introduction page to SRT. IMO that last option would be really the best, as a current in a wire is more complicated than it looks. Harald88 22:56, 1 November 2005 (UTC)

Now it's almost 2 weeks later, and that a paragraph is still defect (questionable claims without backup from a reliable source). If this week still nothing is done about it, and if I find the time, I'll partly rewrite it using the simple case of Einstein 1905 (but if someone else likes to do that, please do!). Harald88 12:31, 12 November 2005 (UTC)

I'm starting to think that you are right. If the wire is electrostatically neutral with moving negative charges, but, say, equally dense non-moving positive charges, there is no moving coordinate system that will make the current go to zero. For a moving observer the wire will acquire an electrostatic net charge, but the current can't be made to go to zero. If the current is not zero, the magnetic field is not zero. PAR 15:52, 12 November 2005 (UTC)
From a mathematical point of view, the four-current is a four-vector J = (cρ, j). If the charge density ρ is zero, then no Lorentz transformation will annihilate the current j. Trying to use physics language: if the wire carries a current but it is electrostatically neutral, the charge density is zero, so the four-current is space-like. However, every four-current with zero current j is time-like, and it is impossible to transform a space-like vector into a time-like vector. So I agree, the current can't be made to go to zero. -- Jitse Niesen (talk) 18:01, 12 November 2005 (UTC)
I now rewrote it, but it may still be expanded (Wolfkeeper, the conclusion reflected the erroneous claim). Harald88 09:49, 13 November 2005 (UTC)
No.WolfKeeper 20:21, 14 November 2005 (UTC)

Need another diagram or two

The first half of the article is devoid of diagrams that's bad...

Also check out this awesome diagram:

http://origins.colorado.edu/~ajsh/sr/centre.html

It shows how two comoving objects can both be at the center of a lightcone at all times.

We need one of those... WolfKeeper 16:41, 2 November 2005 (UTC)

nominated a daughter article for deletion

I nominated a daughter article Special relativity for beginners for deletion. My gist is basically this should be merged, since forks are bad, and that the parent article needs this kind of language in the first place, and should not have its existence for an excuse for people to continue writing technical details in the current article without elaboration. This is the Wikipedia policy: include as much details as possible, but explain them in the main article in simpler language. Not the other way round, move the simple language off the main article and onto another one! It is discussed at Wikipedia:Articles for deletion/Special relativity for beginners -- Natalinasmpf 16:51, 17 November 2005 (UTC)

Yes, I don't like this separation. And the main page needs some of those diagrams- my feeling is that we need more Minkowski diagrams in the article to help those who are more visual. Readers shouldn't have to plug numbers into Lorentz equations, the diagrams should do it for them.WolfKeeper 17:58, 18 November 2005 (UTC)

The result of the debate on the votes for deletion placed on Special relativity for beginners was: merge (4) Not merge(7). The 7 votes broke down as: Keep(4) Transwiki(1) Rename(2). See discussion on the Special relativity for beginners page. loxley 10:37, 23 November 2005 (UTC)
I am against merger because it will disrupt the Special relativity page and because, in most textbooks the Special relativity for beginners page is summarised as 'according to SR the space-time interval is invariant therefore, as s^2=gxx ...' (ie: one sentence). I am also against merger because there are at least three ways of explaining SR: simultaneous linear equations with Einstein's assumptions, Minkowski/Noether invariance with a geometrical interpretation and advanced differential geometry (cf:general covariance). Should we include full guides to each of these approaches? All three approaches are actually deeply related but explaining the relationship will need a short course on maths. The advantage of using the second approach for explaining SR is that it is geometrical and, as Plato put it, understanding is largely geometrical. Lastly, I am also against merger because SR is widely taught in terms of Einstein's raw assumptions and if we portray it differently in the main article we will confuse intermediate students and not be thanked by their teachers. loxley 10:55, 23 November 2005 (UTC)

Hyperbolic Geometry and Special Relativity

The following got added to the article by an anonymous author and removed by me:

Special Relativity is a physical model of hyperbolic geometry. For example, on the Poincare Disk, set up a polar coordinate system. Then any point on the Poincare Disk can be identified with a uniform motion on a plane. The point (2, 30), for example, could represent an object travelling on a plane with a uniform rapidity of 2 in the direction of 30 degrees north of the polar axis. (The rapidity of an object is the arctanh of its speed as a proportion of the speed of light. An object travelling with rapidity 2, for example, would be going tanh(2) = 96.4% of the speed of light.) The Poincare distance between two points on the Poincare Disk can be identified with the relative speed between two objects travelling in uniform motion on the plane. So every theorem in hyperbolic geometry can be translated into a true statement in special relativity.

This is a very dense and misleading write-up. A constantly accelerating observer is undergoing what in special relativity is called hyperbolic motion, but this does not endow spacetime with a hyperbolic geometry. (I know that this is not what was being said or intended, but it was being implied.) The point being made is also very subtle and trivial. --EMS | Talk 04:59, 20 November 2005 (UTC)

I agree that the addition of hyperbolic geometry was too densely written, and somewhat misleading. (In the last sentence, perhaps the writer should have said something like "...every theorem in hyperbolic geometry can be translated into a theorem in relativistic kinematics, and conversely.") Still, the connection between hyperbolic geometry and relativistic kinematics is pretty cool (well, at least I think so, but then again I'm a geometer!) and do I think it belongs somewhere in Wikipedia. I'm just not sure where! Tuolumne 07:27, 20 November 2005 (UTC)

You indicate it yourself: it seems to be neatly fitting in an article about hyperbolic geometry. Harald88 12:26, 20 November 2005 (UTC)

change sequence?

The sequence of the different paragraphs is, I think, not optimal:

1 Postulates

2 Status

3 Consequences

4 Lack of an absolute reference frame

5 The Lorentz transformations of space and time

6 Addition of velocities

7 Mass, momentum, and energy


Why not introduce the subject matter earlier, thus for example:

1 Postulates

2 Lack of an absolute reference frame

3 Consequences

4 The Lorentz transformations of space and time

5 Addition of velocities

6 Mass, momentum, and energy

[almost at the end:] Status

Harald88 12:52, 20 November 2005 (UTC)

It might even be easier to read in the 4,2,1,3,6,7,status order; since that way you can show how the postulate agrees with lorentz equations and it all makes sense. That's even the historical way it happened. Personally I think starting with the postulates is unduly mystical, but not formally incorrect in any way.WolfKeeper 19:13, 20 November 2005 (UTC)

Historically it started with the impossibility to pinpoint an absolute reference frame, with the conclusion that the PoR seems to hold as if one is always at rest in an ether - from which follow the postulates. To start with the Lorentz transformations is IMO a bit heavy. Anyway, my main point is that "status" should be near the end, and I notice that you agree on that. Let's wait for more opinions. Harald88 20:50, 20 November 2005 (UTC)

I now did the minimum swaps about which we agreed; however, if you or someone else prefers to also swap 1."Postulates" with 2."No absolute ref.", I'm OK with that. Harald88 20:00, 22 November 2005 (UTC)

Isn't the lack of an absolute reference frame really a consequence of the postulates? Masud 17:09, 23 December 2005 (UTC)

diagram 1

It's a nice illustration, but the text can certainly be improved by removing jargon and confusion. Notably, two accelerations are confused, and I'm thinking about how to avoid that confusion without making it too long. And isn't the animation a bit too fast? Harald88 09:29, 26 November 2005 (UTC)

OK I'll try to improve it a little more, and we'll see. Harald88 19:38, 2 December 2005 (UTC)

Galilean relativity discussion

An anonymous author added this to the Consequences section:

However, applying Galilean relativity to electromagnetic phenomena leads to serious problems and was one of the starting points when Einstein laid down the principles of Special relativity. Rotors of electric motors and generators move with velocities far below the speed of light. Contrary to mechanical movement there is no low velocity approximation for electromagnetic induction.

I have removed it because:

  • The text lists no consequences of SR that I can see.
  • The first sentence is correct and is a part of the history of special relativity, but I cannot see the point the point of the rest of this write-up, or how it fits into a discussion of special relativity.

--EMS | Talk 22:45, 26 November 2005 (UTC)

While I agree with the removal, it did make me realize that there is nothing about the history in this article (except the one sentence refering to history of special relativity). What do you guys think about writing one or two paragraphs about this, and if you think that's a good idea, where in the article would this fit best? -- Jitse Niesen (talk) 10:46, 27 November 2005 (UTC)
I don't really care: I think that history is unnecessary for a first understanding of SRT. Harald88 20:36, 2 December 2005 (UTC)

A question

Dears contributors

I want to pose a question, and I hope that any of you can answer.

There is any plausibility on assert that all particles, and so the all the other bodies in Universe, travels to the future?? Can I say that the "light speed can be considered as the velocity by what an observer moves yourself towards future". This is (as long as I can translate adequately from Portuguese) an free translation from an quote excerpted from the Portuguese version of Wikipedia (http://pt.wikipedia.org/wiki/Espaço-tempo). This is an article to treat of Spacetime. Would any of you please answer me about this kind of reasoning? I would be happy if any commments can be send to eduardolauande@aol.com. Thank you all in advance.

Greetings


Note:Appologies on my "bad English writing"

EDULAU

a Brazilian (Portuguese speech) contributor of the portuguese Wikipedia

I don't think that would be a scientifically correct statement, but it does have intuitive appeal. In a Minkowski diagram, the set of spatial points as a function of time could be seen as a "movie" by moving the spatial axes along the corresponding time axes at the speed of light. I think that explaining the Minkowski diagram would be better. Then the idea would have some meaning, instead of being a vague and untestable idea. PAR 03:43, 27 November 2005 (UTC)

Principle of special relativity

I made a redirect for the principle of special relativity to this article. Is that appropriate? Would it be better if it were directed to postulates of special relativity or if it had its own article? -- Kjkolb 00:19, 28 November 2005 (UTC)

I think that's OK... however: I know the PoR, and that it later was called the Special principle of relativity by Einstein. But I never heard of the label "principle of special relativity". Harald88 21:44, 28 November 2005 (UTC)
Thanks. Another encyclopedia had the equivalent of a Wikipedia redirect under that name and we're trying to make sure that Wikipedia covers everything they do. The other encyclopedia has many inappropriate redirects, so I wanted to make sure. -- Kjkolb 08:35, 29 November 2005 (UTC)

Tensors in SR

I was thinking it'd be a good idea to have a small section of 4-vectors such as position, energy-momentum, etc. and the lorentz transformation tensor, EM tensor etc. and describe how we use the lorentz transformation tensor to transform these quantities from one inertial frame to the other. This would also help in linking from the Classical field theory page when that's done. Masud 05:56, 1 December 2005 (UTC)

I was writing an article for Classical field theory and felt it necessary to put in a section on relativistic invariance, but later realised that it probably belongs in the SR article. I would like to ask people to see if they think we should put it in (or at least in modified form). Masud 08:53, 23 December 2005 (UTC)
Looks like I'm just talking to myself; I assume no one has a problem with this. I'm assuming there are little or no objections. However, in order not to bloat the SR article, I may add a little subsection or a link to a new Tensors in Special relativity page. Masud 12:51, 23 December 2005 (UTC)
I think its a good idea - I had it in my mind to do it myself eventually. I think a section in the SR article is a good start. Then if it gets too big, we can move it. "Tensors in relativity" sounds like "Real numbers in Newtonian mechanics" - its not really the subject. Maybe "Spacetime physics" or "spacetime electrodynamics". Just thinking out loud. PAR 16:48, 23 December 2005 (UTC)

I have added the section. Lots of editing still to be done. My knowledge of various articles on wikipedia that we can link to is limitied, so I was hoping someone could help out by adding some in. There are invariably faults in my work, and the speedy remedy of these faults on an otherwise quality page would be much appreciated. Masud 17:47, 23 December 2005 (UTC)

Notation

Some editing is in order: somehow, the prime indicator of S-prime got deleted. Trekphiler 14:45, 2 December 2005 (UTC)

where? Harald88 20:32, 2 December 2005 (UTC)

Physics 101

Is it possible to rewrite somewhat? As is, the article seems to be aimed at physicists (or students of physics), not general readers. Fewer equations? More explication? Certainly, fewer assumptions of knowledge not (necessarily) in hand. Trekphiler 16:13, 2 December 2005 (UTC)

Your request looks almost opposite to that of Masudr here above. But if you do some suggestions or at least point out what could benefit from some more simple clarification, maybe someone here will try it. You could also try yourself of course. And what do you think of starting with the Lack of absolute frame paragraph, and next the Postulates? Harald88 20:23, 2 December 2005 (UTC)
PS What do you think of Special relativity for beginners, does that help? Harald88 20:29, 2 December 2005 (UTC)

Clock Comparisons

These effects are not merely appearances; the time in the different frame of references essentially do travel at different rates to each other and the lengths of objects really are physically changed whilst in relative motion.

a) Read it carefully and this sentence looks pretty garbled. b) In particular 'the time....do travel at different rates' actually, it's worse than the usual cliches make out. If Mrs Stripe is moving (inertially) at an appreciable fraction of c with respect to Mr Plain then after they have met, her clock runs slower than his on events in her immediate neighbourhood, and faster than his in his neighbourhood. Before they meet, exactly the opposite is the case. In short some processes observed by both run faster according to Ma Stripe, some run faster according to Daddy Plain, and a sufficiently convoluted and widespread process will look like a bit of both to both at different times. Clocks don't 'run at different rates' - that suggests that they are comparable. They're not. c) Even if Ma Stripe has a sufficiently powerful telescope to read Plain's clock, the moment she reads 4 am, most events she sees as simultaneous with that reading aren't simultaneous with it for Daddy Plain. The reading is meaningless. Some of the events Plain thinks happened at 4am are already past for her and some are in a far future.