Talk:Spacetime/Archive 19

Derived from thought experiments
(We don't seem to be using this section, so I've deleted the body so that we focus on one issue at a time. We can always restore it later.) Prokaryotic Caspase Homolog (talk) 23:01, 3 April 2018 (UTC)

New lede
I have just one strong objection to your suggestion. This is against ascribing to Einstein the whole "relativity". I am convinced that all notable physicists assume and assumed that "the" physics holds throughout the universe, see e.g. Galilean relativity. Einstein revolutionary thought was keeping this up and postulating (newly!) the invariance of speed of light. His legendary achievement is melting this into a consistent apparatus. Summing it up, I object to your explicit (1) & (2); constancy of lightspeed is, imho, the one topic to focus on.

In the following I'll try to motivate my effluences that you kicked out, or reordered.
 * I want to guide early on to treating spatial and temporal dimensions on equal footing for coordinatizing events (therefore the long -balancing with space- parenthesis for time), but also guide to the subtle differences, in e.g. metric, of the "where"-dimensions and the "when" dimension, i.e., in a first step naming these. You are right that the 3-D geometry is "expressed" in certain formulae, but this standard "expression" is the wrong way (contraction!) in spacetime, that's why I retracted to "perceived" (we agree upon STR being counterintuitive).
 * I think simply claiming that Einstein "framed his theory in kinematics" is inconceivable to an uninitiated reader. Therefore I explained this by the (rather famous) thought experiments on trains, tracks and flashes. I do not think that this is too far away from a geometric target of the article. Furthermore, the current place of "framing" is rather void of connection.
 * The violated linear additivity of velocities, as an immediately obvious consequence of the constancy postulate, has a better place near establishing the postulate, than later on amongst the many "counterintuitive consequences". My personal preference is to concentrate as much as possible of all the counterintuitivity of facts within STR at one place, within the postulate of constant speed of light, because I believe that logical consequence helps to deal with wrong intuitions.

Cheers, Purgy (talk) 08:34, 5 April 2018 (UTC)


 * 1) In his 1905 paper, Einstein started with TWO well accepted principles, neither of which would be considered revolutionary or surprising. The surprising, counterintuitive version of the invariance of light principle was a consequence.
 * 2) Except for non-controversial definitions, etc. anything mentioned (without references) in the lede needs to be expressed in fully referenced form in the main body of the article.
 * 3) A brief mention of what kinematics means in the body of the article would be appropriate.
 * 4) I need to check out from the library several of his semi-popular books on relativity before I can add to the body of the article the type of mention of his thought experiments that you want. I'll go online to reserve them after I finish this post.  — Preceding unsigned comment added by Prokaryotic Caspase Homolog (talk • contribs) 11:57, 5 April 2018 (UTC)
 * 5) I do insist that the body of the article remain focused on the spacetime interpretation of relativity.
 * 6) See Talk:Special_relativity for a discussion that I initiated inspired by your ideas.

Until the turn of the 20th century, the assumption had been that the three-dimensional geometry of the universe (its spatial expression in terms of coordinates, distances, and directions) was independent of one-dimensional time. However, in 1905, Albert Einstein began his seminal work on special relativity with two simple, completely non-surprising postulates (1) the laws of physics are the same in all inertial frames of reference (this is the principle of relativity), and (2) the speed at which light propagates through empty space has one definite value&mdash;a constant&mdash;that is independent of the motion of the light source.

The logical consequence of taking these postulates together (both of which represented widely accepted principles) is the inseparable joining together of the four dimensions, hitherto assumed as independent, of space and time. Many counterintuitive consequences emerge: in addition to being independent of the motion of the light source, the speed of light has the same speed regardless of the frame of reference in which it is measured; the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity); and the linear additivity of velocities no longer holds true. Einstein framed his theory in terms of kinematics (the study of moving bodies). Einstein's theory was a breakthrough advance over ...

Prokaryotic Caspase Homolog (talk) 11:16, 5 April 2018 (UTC)

A good compromise leaves neither of the people involved in a dispute completely happy. If we can both get to a 90% level of happiness, the revised lede should be good to go. Personally, I think that the lede is in danger of getting too long... for example I would remove entirely "(its spatial expression in terms of coordinates, distances, and directions)", etc. However, I'm at the 90% level. After getting the books from the library, we can expand the History section with a bit of illustration of what "kinematics" means. There are also some unsourced statements in the History section that I happen not to like... I wonder who added them... Prokaryotic Caspase Homolog (talk) 12:42, 5 April 2018 (UTC)

Here is an article that may be of interest to you: 'Relativity' Speaking, discussing points raised by Ralph Baierlein. Einstein did not use the "strong form" of the second postulate that is commonly presented in introductory physics textbooks. Prokaryotic Caspase Homolog (talk) 13:29, 5 April 2018 (UTC)

You wrote, "I want to guide early on to treating spatial and temporal dimensions on equal footing for coordinatizing events (therefore the long -balancing with space- parenthesis for time)". But an essential point is that in the 19th century, space and time were not considered on an equal footing. So the "balancing with space" is not, in my opinion, necessary, and adds to the verbosity. Prokaryotic Caspase Homolog (talk) 13:37, 5 April 2018 (UTC)


 * First, I do not understand the juxtaposition of the "invariance of light" of (1) in your list of arguments and the "light propagation" (2) in the numbered postulates.
 * I never wanted to express any doubt that Einstein used the two postulates you address to derive its STR from; e.g., I am fully d'accord with the formulation at special relativity. But I still strongly oppose to formulations, which insinuate that it was Einstein to set up the principle of equal physics everywhere. It may be really a "completely non-surprising" postulate (if it is really "simple", I dare to doubt) and it was also "widely accepted". Einstein used this postulate in his derivation, but he certainly did not establish it (whatever maybe said in certain sources). The second postulate in your counting was to my knowledge not widely accepted in this rigor before Einstein. It is this postulate, that is genuinely ascribed to Einstein, and that I estimate to be counterintuitive, mind boggling, ... It is this postulate, that causes together with the well adopted first postulate and all the necessary mathematical apparatus all these other logical consequences which bother the uninitiated and confuse the beginners. Mentioning these two postulates on par is imprecise about which of the fundamental facts is new and counterintuitive, and is not helpful for the reader, especially, since this article is about "spacetime" and not about how Einstein derived it (I fully agree with you on this).


 * Helping the newbie reader by telling what makes the postulate "counterintuitive" (addition of velocities), and what it means to "frame something in kinematics", are assets, easily to afford already in the lead, not leaving there some quak-speak.


 * I am not after a compromise, saving some ridiculous percentage of my doggerel verses, but after eliminating, as far as possible, misleading appeals. Purgy (talk) 11:15, 6 April 2018 (UTC)


 * 1) The second postulate as expressed by Einstein in his 1905 paper was not surprising and would have been easily accepted, because the propagation rate of every other known form of wave (compressional sound waves in air and other materials, transverse waves in solids, water ripples...) was independent of the velocity of the emitter.
 * 2) I can modify the first statement to read "the laws of physics are the same in all inertial frames of reference (this is the principle of relativity, first expressed by Galileo)"

Let me try another rephrasing later, after work, removing words like "simple".

Prokaryotic Caspase Homolog (talk) 13:12, 6 April 2018 (UTC)


 * Feel free to phrase to your likings, I am just reporting that I do not think much like Baierlein, however reliable he is considered to be, neither about the number of observers in one frame of reference, nor about any speed of such a frame (that's why he falls for a -to me- wrong interpretation of Einstein's statement). I do not follow his beliefs about "implausibilities" derived from plausibilities, and find his musings (snails) about simultaneity even -sorry!- ridiculous. BTW, I am not sure about Aristotle et al. thinking like Galileo, already. Good luck. Purgy (talk) 14:53, 6 April 2018 (UTC)

At least regarding the second postulate, Baierlein is correct. Einstein clearly used the following approach in 1905: a) the principle of relativity and b) the independence of light velocity from the source. The second postulate is justified by Lorentz's theory, as Einstein explained in 1909: http://einsteinpapers.press.princeton.edu/vol2-trans/393
 * "Upon superficial consideration, it seems impossible to reconcile the gist of Lorentz's theory with the principle of relativity. For according to Lorentz's theory, if a ray of light is propagated in vacuum, this always occurs with the fixed velocity c with respect to a coordinate system K at rest in the ether, independent of the state of motion of the emitting body. We will call this proposition the principle of the constancy of the velocity of light.....From this it follows that the currently used transformation equations for the transition from one coordinate system to another one moving uniformly relative to it are based on arbitrary assumptions. If these are dropped, then the foundations of the Lorentz theory, or, more generally, the principle of the constancy of the velocity of light, turn out to be reconcilable with the principle of relativity."

Regards, --D.H (talk) 15:57, 6 April 2018 (UTC)

Thanks, D.H!

Purgy, since you state that you are "d'accord with the formulation at special relativity", I will drop my attempt to merge our respective wordings in favor of a direct copy/paste from that article.

Until the turn of the 20th century, the assumption had been that the three-dimensional geometry of the universe (its spatial expression in terms of coordinates, distances, and directions) was independent of one-dimensional time. However, in 1905, Albert Einstein based his seminal work on special relativity on two postulates: (1) The laws of physics are invariant (i.e., identical) in all inertial systems (i.e., non-accelerating frames of reference); (2) The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

The logical consequence of taking these postulates together (both of which represented widely accepted principles) is the inseparable joining together of the four dimensions, hitherto assumed as independent, of space and time. Many counterintuitive consequences emerge: in addition to being independent of the motion of the light source, the speed of light has the same speed regardless of the frame of reference in which it is measured; the distances and even temporal ordering of pairs of events change when measured in different inertial frames of reference (this is the relativity of simultaneity); and the linear additivity of velocities no longer holds true.

Einstein framed his theory in terms of kinematics (the study of moving bodies). Einstein's theory was a breakthrough advance over ...

You stated: "But I still strongly oppose to formulations, which insinuate that it was Einstein to set up the principle of equal physics everywhere." As a matter of fact, Einstein really was the first to express the principle of relativity in its modern form. The earlier statement of Galilean relativity implicitly assumed (1) There exists an absolute space, in which Newton's laws are true. An inertial frame is a reference frame in relative uniform motion to absolute space. (2) All inertial frames share a universal time.

Einstein found it necessary to relax the principles of absolute space and absolute time before he could reconcile the first and second postulates. Prokaryotic Caspase Homolog (talk) 23:14, 6 April 2018 (UTC)

Incidentally, taking examples of trains, embankments and lightning bolts from Einstein's semi-popular works would be referencing primary sources. Secondary sources such as Isaacson, Hoffman, Pais, and Stachel have some very interesting things to say about Einstein's thought processes. It can be a lot of work just to drop in one or two sentences in the History section! Prokaryotic Caspase Homolog (talk) 23:26, 6 April 2018 (UTC)
 * Not having something to object to in the first paragraph, my opposition focuses, of course, now on the second.
 * I do not hold true that both postulates were "widely accepted". The first inherently not, since –similar to higher order quantification– it always referred to a specific set of laws (Galileo, early on), and just then referred to the properties of the "spacetime under construction" (Einstein); and the second postulate was obviously under extreme scrutiny (ether or not, propagation in itself, Michelson, Lorentz for what reasons, ...). Acceptance in a scientific community is only meaningful for fully understood concepts (Wittgenstein); Highschool kids (and in part their teachers, too) "accept" a lot.
 * I cannot separate the postulate about "constancy of x" from "measuring x in different frames" (it's one of the Baierlein's I disagree with).
 * I am astonished about the increasing (instead of decreasing!) focus on Einstein's and contemporaries' saids and said-nots, when this article is or wants to be -at least outside the History section- about a modern view on spacetime.
 * All together, please, really feel free to phrase to your likings, I am neither sufficiently involved to dig in "secondary sources", nor sufficiently erudite to discuss historic POVs in a citeable fashion, and I think I mentioned my motivation and all items, I hold relevant to this discussion about the spacetime-space. &mdash;I won't interfere. BTW, I am enchanted by seeing  trains and flashes introduced to kinematics. Purgy (talk) 07:00, 8 April 2018 (UTC)


 * 1. Although I disagree with you on your first point, I have struck out "(both of which represented widely accepted principles)" in deference to your objections. For me, the impressive thing about Einstein's the Kinematic Portion of his 1905 paper is how, starting with totally reasonable premises, he leads you by inexorable logic to his fantastic results.
 * 2. Einstein presents a "weak form" of the second postulate in his 1905 paper:
 * "Every ray of light moves in the 'stationary co-ordinate system' with the same velocity c, the velocity being independent of the condition whether this ray of light is emitted by a body at rest or in motion."
 * A common "strong form" presentation of Einstein's second postulate (from Halliday and Resnick) is
 * "The speed of light in free space has the same value c in all directions and in all inertial reference frames."
 * I simply do not understand why you believe that the weak form of the second postulate "cannot [be] separate[d]" from the strong form of the second postulate. In the 1905 paper, Einstein derived the strong form of the second postulate by applying the first postulate to his statement of the weak form.
 * 3. The focus on the lede is to distinguish the spacetime presentation of special relativity from the kinematic presentation of special relativity, where the kinematic presentation is the dominant manner in which special relativity is taught in the textbooks most commonly used in the United States. It is extremely difficult to describe the differences between these approaches without some mention of their historical development.
 * 4. I believe we have reached the point I mentioned above: "A good compromise leaves neither of the people involved in a dispute completely happy." With your permission, I will apply the product of our joint discussion to the article.
 * Prokaryotic Caspase Homolog (talk) 15:12, 8 April 2018 (UTC)

As soon as I am able, I will add a sentence about trains and lightning flashes to the history section. Right now, all I have are primary sources by Einstein himself. Appropriate secondary sources are all "on hold" at the public library, so I will have to wait until they get checked in. Isaacson's biography will likely be a secondary source that I can cite, since he has written online about this. However, I would rather not use such sources as, for example, a NY Times opinion article, but a published book. Prokaryotic Caspase Homolog (talk) 18:52, 8 April 2018 (UTC)

Isaacson's book came into the library, so I checked it out and added a note about Einstein's use of thought experiments. Unfortunately, putting these observations in the main text interrupted the narrative, which is why I made them into a note. Prokaryotic Caspase Homolog (talk) 05:00, 13 April 2018 (UTC)