Tim88

It's not clear to me which is A and which is B, but anyway: "does not constitute a valid relative aging comparison [..] in an absolute sense" is an understatement. No matter what can be read into a diagram, length contraction and time dilation go hand in hand in SR and are a function of speed only. As was elaborated in the spun-off thread starting from http://www.scienceforums.net/topic/98048-relativity-and-shared-realities-split-from-clocks-rulers/?p=941488 , pretending that a clock remains length contracted but temporarily ticks faster due to what is done to another far away clock is not a valid physical description in SR. If you think otherwise, please comment there as it relates to claims about physical reality. That's a very nice presentation of the Block universe interpretation of SR. I notice that we use different (and incompatible!) definitions of "real"; onlookers should keep our definitions in mind. And about the idea that folks can disagree yet all be correct, that is for me a self contradiction of terms. If I correctly understand what you try to say, I would say that folks can disagree and still all are somewhat right (but technically, all or most are wrong). That could be another thing to elaborate in the thread on "shared realities". I may have overlooked that part, thanks to Cerelitas putting it in bold; I now put a single word in bold. In the parallel thread I hope to have clarified why the word "only" is erroneous. Consequently it leads to the inconsistency that you arrived at. For example, if I see a turn to the left, from another perspective it may have turned to the right. The turning is not only apparent, although it depends on which observer is doing the looking. The phrase "being only apparent - i.e. dependent on which observer is doing the looking" is a logical error.

That is better, however as I understand from Neumaier's book on QFT, it's inaccurate to put "the standard model particles and fields"; particles are assumed to be phenomena caused by fields. Maybe if you want to go beyond pure GR, you have to choose your favourite quantum theory! And if I'm not mistaken, even in absence of such particles or fields, the speed of light and the lengths of rulers are supposedly determined in GR. Further, "outdated" could merely refer to fashion; that does not suffice for making pertinent claims as if giving factual statements. So, now I am curious what of Einstein's paper has convincingly been disproved by later research. BTW, a similar paper stems from 1924 but it's not yet available on Wikisource, so for onlookers I chose the more accessible version. Similarly I often cite Einstein's 1905 paper for SR as it's in some points clearer and more precise than a number of later papers by other authors.

The green underlined sentences are an overly simplified version of what I stated in my post: according to our measurements their radioactive "clock" is time dilated, so that some can reach the Earth before falling apart. As reckoned from their rest frame, instead the height of the atmosphere is strongly reduced, which has the same explanatory effect. You noticed correctly that the muon is contracted according to the Earth's rest frame, and that clocks on earth are slowed down according to the muon's rest frame. However neither plays a role in that discussion; they are irrelevant for explaining why so many muons can reach the earth. However, for explaining how the same speed can be measured "both ways", it can be useful to also look at it from "the muon's perspective"! The Earth's rulers are then just as much length contracted as the atmosphere [edit, elaboration: this results, from the muon's perspective, in an exaggerated distance measurement!], while the Earth's clocks appear to be slowed down by a factor 5. You may think that now v = distance / time doesn't work out. However a clock high up in the atmosphere is, from that perspective, incorrectly synchronized with a clock on the surface. According to the muons, it's thanks to this incorrect clock synchronization that Earth observers still measure the same, correct speed v when clocking a muon between both clocks. As a matter of fact, we have now started a variant of the calculation exercise that I advised to do in the other thread. [Edit:] It's worth the effort to plug in numbers and verify this for yourself. And possibly this makes you reconsider the sketch that you made in a following post.

I did not notice a direct contradiction to my "bold" statement in this long thread; however I did not read all comments. Note that SR, just like QM, is a theory about phenomena; the popular version (Einstein's) is on purpose stripped of speculations about "reality". In that sense, strictly speaking, no direct answer on the questions about "reality" "are" SR. However, in the sentences that led to that statement I briefly indicated that both the "Lorentzian" and the "Minkowskian" interpretation agree that nothing (or at least, nothing that we are here concerned with) happens to the inertial system, but that something happens to the non-inertial system that affects mutual measurements (if before and after the change of state the system is used as inertial reference system). And I know no other interpretations that consistently match SR. Maybe you can identify posts that seem to disagree with that? [edit: glitches in phrasing]

My mind boggles over "rates through space-time" ... Consequently I cannot know if a sentence containing that phrase is correct or not. Can you rephrase your question using phrasing like "clock rates according to" ?

The space-time interval of the un-accelerated clock is in a space-time plot depicted as a straight line: this straight line between two events is shorter than a curved line between those same events, independent from your perspective. See post #18 : in common SR jargon, "frame" stands for a collection of universal inertial reference systems with virtual rulers and clocks that are in relative rest - and assumed to be in true rest by convention. It is therefore perfectly correct to say that we "change frames" if we used to treat our former state of motion as being in rest, and if after acceleration we next treat our new state of motion as being in rest instead. Although the word frame is a common English word, it can therefore cause confusion to use it for a car or a rocket in arbitrary motion (but as you say, proper context is everything!). One can also use frame, in an extension of SR, for accelerated reference systems; however there are issues with that and needlessly introducing such frames creates unnecessary complexity. Objects are predicted to contract if they change velocity on the way from A to B. Obviously their contraction, as measured on Earth, cannot contract for example the distance between London and Paris! We even would have different contractions at the same time of that distance if objects with different speeds were flying in-between. However you are almost right: it's just a matter of picking the right reference frame. According to measurements in a moving frame, the distance between London and Paris is contracted. Thus, in the example with muons from space: according to our measurements their radioactive "clock" is time dilated, so that some can reach the Earth before falling apart. As reckoned from their rest frame, instead the height of the atmosphere is strongly reduced. And then your calculation works: v = less time / less distance. See for a nice sketch http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/muon.html PS. You may be interested in Bell's "spaceship paradox" which was designed to show how physical length contraction (but pure length contraction only from one perspective) can be, if it is prevented. Some of his colleagues thought that he was wrong and that space must contract, a bit like you suggested. But that's a topic by itself. [edit: corrected an error in phrasing, happily without consequences]

I notice that we are now zooming in on the issue that relativistic effects cannot be a mere illusion. "The question here is to convince me (& others) that the effect of contracting is real, [..] in the sensibility of the observed" is a faulty question - similar to asking someone to convince you that a chameleon is brown. IMHO, the main reason why it is difficult to really understand SR, is because people often debate two erroneous interpretations, neither of which matches SR: 1. all measurements of length contraction and time dilation of a measured object inform us about the physical reality of that object 2. all measurements of length contraction and time dilation of a measured object are mere illusions. 1) can easily be disproved by the fact that it depends on with which reference standards they are compared; and we cannot prefer one over the other. 2) can easily be disproved by the fact that clock retardation is measurable as a physical fact about which everyone has to agree; mere illusions cannot do that. The solution, of course, has to be found in the fact that every measurement is a comparison between the measured object and a reference object. We then find that a change of velocity must affect the object that changed velocity. With the Lorentz ether interpretation, true changes happened to lengths and clock rates, but the true values are not accessible to us. With the Minkowski Spacetime (or Block Universe) interpretation, the object's trajectory in Spacetime receives a bend at that location in Spacetime. Consequently when next things such as lengths and frequencies are compared, they will be measured differently from before due to a physical effect on the accelerated object. Thus, if it is the observed that changes velocity, there is a real effect on the observed (although measured differently from different "perspectives"); while if it is the observer that changes velocity, the apparent effect on the observed is due to a real effect on the observer. A most useful exercise from which I personally gained much understanding, is to do a calculation of the effect of velocity on the mutual observations of lengths and clock rates, as calculated from an assumed "rest system" S0, e.g. S0 C1-----L----C2 S1 C'1-----L----C'2 ->v A more elaborated calculation that leads to additional insight, is to analyse what happens if S1 starts at rest and then attains velocity v.

I don't think that anyone has a problem with the reality of observation of phenomena; for sure the phenomena are real. For example, at turnaround the traveler observes an instant increase of the light and radio frequencies coming from Earth, compared to the ship's own clock frequency. Disagreements only arise when claims are made that go beyond the phenomena themselves.

I have the same beef as some others here in the first posts. You may define space as volume only, but the claim that it is "not some kind of ether", is controversial at best. It directly contradicts Einstein's interpretation of GR. He argued that necessarily space is not physically empty. Of course, you could say that space is not some kind of ether, but instead contains some kind of ether; however most people would probably consider that a word game. Even more, "space" as a locally defined volume depends on the physical properties of that volume. In a nutshell: " "empty space" in its physical relation is neither homogeneous nor isotropic [..] The ether of the general theory of relativity is a medium which is itself devoid of all mechanical and kinematical qualities, but helps to determine mechanical (and electromagnetic) events. " - https://en.wikisource.org/wiki/Ether_and_the_Theory_of_Relativity Of course, it's easier to spot wrong or controversial formulations than it is to give right or uncontroversial ones. You could simply leave out the phrases "It is not something form of ether" and "not the volume of space", but that would be a little poor in view of the title. So, you could maybe state that space can be considered as a map (or part of a map, in view of space-time) that relates to a physical reality, some effects of which we describe with the map. Just my 2 cts. Another issue is with point 2: according to QFT, if I'm not mistaken, space is filled with fields. Probably it's better not to include "space time creation" in the topic "what is space made of". And please correct typo's: feilds ->fields Which is a topological space that is resembles Euclidean space at beach point. -> Which is a topological space that resembles Euclidean space at each point. (right?) [edit: a few additions]

Concerning Mr Magoo I disagree with VandD's phrasing: distant simultaneity is a mere convention, and thus I would not use the word "real" there. However I do agree with his(/her) description of phenomena! I'll let him elaborate. 1. Further we certainly agree that in no way an observer can make things happen in an object just by looking at it; that was exactly my point (note that it appears that QM disagrees somewhat; but it's definitely so in SR). Sorry if my phrasing was ambiguous for you! I even wrote a paper about that (correcting an accepted error in the literature) in the European Journal of Physics; I can send it to you if you like. 2. Now, concerning your preference of using proper measures only: I can understand that, as it looks "cleaner". However, that preference could possibly hinder understanding of how there could be any absolute effects, and how to correctly account for effects from motion. More importantly, it could result in conflict with the just established point 1. here above, if you think that the"proper" point of view is always what should be considered "physical reality". Because that textbook fell in the trap of suggesting (although not explicitly claiming) that the "proper Observation" of the traveler during turnaround makes physical sense, despite the fact that it implies instant magical action at a distance (which is not allowed in SR), and despite the fact that it even demands the traveler to quickly and continuously modify clock synchronisations aboard the ship, and then to pretend that nothing happened to her clocks, but that instead a huge instantaneous effect happened to the Earth's clocks! [edit: typo's]

I would say that such an observation is merely the effect of acceleration of the traveler. "Her point of view", according to which the earth twin ages very rapidly during turnaround, is not physically valid in SR. As the related switch of reference system implies the claim to be in rest before and after the turnaround, it is just as "possible" that the far away earth clock instantly ticks faster due to the traveler's rocket engines firing, as the corresponding instant change of velocity of the rest of the universe (and zero change of the traveler's speed!). Messy accounting is avoided by either: - sticking to one reference system or - explicitly accounting for the effects from acceleration and the switch from one reference system to the next one The observed phenomena during and after acceleration are due to things happening to the accelerated object, and not due to things happening to the non-accelerating objects. (slight improvement in phrasing)

Oops I slightly missed the target when I wrote that relativity "already was not "relative" in the sense of "nothing happens""; I intended to refer to your preceding post, by emphasizing that relativity already was not "relative" in the sense of "all observations are symmetrical". Then concerning "there can be millions of S (S2,S3,S4,S5,...) for which the contraction will be different", that is exactly the point. All those perceived contractions are mutually exclusive; at best one of those viewpoints can correspond to reality. In Newtonian mechanics this also played, but it was less striking (according to different reference systems, the kinetic energy increases or decreases; who is right? and the rotating bucket physically rotates relative to which frame?). Thus I did not say that the electromagnetic equilibrium positions of its atoms change in many ways at the same time; to the contrary, one can at most accept one such view as reality, only we cannot know which one, if any. Newton therefore postulated the existence of absolute space, and built up his theory from that assumption. The SR equivalent of that is the Lorentz ether; did you know that the Lorentz transformations were first derived based on that concept? It works and it makes SR perfectly understandable, without anything looking paradoxical. However, the Lorentz ether implies a hidden physical difference between inertial frames, so that the mathematical symmetry of observations between inertial frames would not correspond to hidden reality. A way out is Minkowski's 4D Spacetime: in that view of reality, all inertial frames are truly on equal footing. An acceleration nevertheless corresponds to curved trajectory, so that symmetry is also naturally broken by acceleration with that view. A disadvantage is that time must then be regarded as somewhat the same physical thing as length; some people even would like to give time and length the same units. We cannot know from measurements which worldview is correct (and possibly they are both wrong), but either of them works as a functional model of the "relativistic" world so that we can make sense of the phenomena. Regretfully neither of them fully solves the riddles of quantum mechanics, but it's already great to end confusions of SR. Your "zero configuration" is usually called "proper length, rate etc; it corresponds with the viewpoint that the object is in rest. However I doubt that you think that everything is all the time in rest! And yes, although not in that article, but in one of the year 18xx by Lorentz, it is shown that similarly the moving Earth must be contracted relative to the stationary ether. Replace the term "stationary ether" by the frame which you by chance called for example S3, and you arrive at SR. [edit: additional remark] The original point of the Lorentz transformations by Lorentz and Poincare, was to explain how motion could affect a reference system in such a way, that if one assumes it be not in motion but in rest, the observed phenomena do not show any error from making that assumption. It appears as if rulers of other systems (even those in "true rest") are length contracted, and clocks of other systems are slow. Next, the same philosophical change occurred as with Newton's mechanics: textbooks often fail to mention conceptual models that are not necessary for describing the phenomena, as they are arguably not part of physics. Thus usually no mention is made of the absolute or true rest model (or even it is falsely claimed that it was disproved!), and neither is Minsowski's physical Spacetime model discussed, according to which the separate concepts of "time" and "space" should be abandoned. "Shut up and calculate"!

Sorry, I realize now that my clarification was too terse. Call S0 the ECI system of the Earth, and S1 the rest system of the traveler when he is on the outbound leg, traveling at constant speed away from Earth. As determined with S1, a ruler of the traveler while still on Earth is approximately at rest in S0; it is contracted in the direction of motion. When the traveler comes at rest in S1, his ruler expands to its rest length. According to other systems it changes lengths in other ways. Nevertheless, according to all reference systems in which the laws of physics are valid, its length changes along the direction of change of motion. This is necessary in order to be consistent with the measured effects on clock counts, such as with atomic clocks, and the measured lack of effect of rotating Michelson-Morley type of instruments when in different states of motion. I now coloured part of your text in blue; and I dare say that your interpretation of SR disagrees with SR. It's probably an interpretation bug. According to SR, the laws of optics hold perfectly in the "rest" system of your choice; there is no optical distortion due to some magical SR influence, if that is what you have in mind with "observe the world with paramorphosis". One of the papers that I recalled when I wrote that some people argue that relativistic effects are "physically real", analysed reflecting light rays from a moving mirror (moving at an angle with respect to its reflecting surface). By means of the Huygens construction the author found, unsurprisingly, that in order to obey the PoR, the moving mirror (in rest in S1) has to be length contracted according to the rest system S0. S0 and S1 disagree which one, if any, corresponds to reality; but they agree that a mirror that accelerates from one state of motion to a new state of motion, also undergoes a change in equilibrium shape. And that shape change is not mysterious, it can be understood as due to a change of electromagnetic equilibrium positions of its atoms (Bell elaborated on that in his physics lectures). As a matter of fact, that this could be the case was one of the first hypotheses that lead to SR, and it was inferred from calculations by Heaviside which showed that the electric force range of electrons is reduced in the direction of motion (he found, based on Maxwell, the exact formula that in modern textbooks is derived from SR). Edit: I forgot to comment on the last part. I consider the papers by Einstein and Langevin to which I referred earlier, very much "what relativity tells us". And those papers already describe precisely that absolute effect! Maybe the problem is just related to the name "relativity theory". Einstein attempted to make all forms of motion "relative" by means of GR, but it did not totally catch on and already was not "relative" in the sense of "nothing happens". Nevertheless that name stuck, and may be the prime cause of misunderstanding.

Sorry I did not look in detail at the paintball games posts. It just sounded (and still does) as if you claimed that in SR a clock can somehow move outside of a reference frame. A reference frame in SR spans the whole universe; it's impossible to not be present in it.

It sounds to me that you are rephrasing the following remark by Langevin on p.48, 49 of his paper (I already gave the link): We will see the appearance of this absolute character of acceleration in another form. [..] In particular, in this reference system [that is in uniform motion] the two events considered may be taking place at the same point, in relation to which a portion of matter has traveled a closed cycle and has come back to its starting point thanks to its non-uniform motion. And we can say that for observers related to that portion of matter, the time period elapsed between the departure and return, i.e. the proper time of the portion of matter will be shorter than for observers who would have stayed connected to the reference system in uniform motion. That portion of matter [ will have aged less between its departure and its return than if it had not been accelerating, i.e. as if it had remained stationary relative to a reference system in uniform translation. What is your issue with that?

I cannot parse that sentence; however Einstein's 1905 paper may be useful for clarification. He used just one frame for two clocks, see §4 of http://www.fourmilab.ch/etexts/einstein/specrel/www/ And although this is slightly off-topic, by chance the title there is "physical meaning [..]"

An "Observer" in SR is in fact a reference system that is supposed, by definition, to be stationary. Each Observer, by convention, assumes the other to be in motion. The Observers must admit (in fact they already claim) that they are not both really stationary; however none of them is obliged to accept that his conventional assumption is wrong. Once more, that is no different in Newtonian mechanics. Bold face mine. I think that I already replied to that in post #31. According to some the ruler contracts while according to others it expands; thus I'm an exception to "everybody". Observation follows physical laws, in particular the laws of optics. How do you explain that the observation from another state of motion results in a shortening effect? Or that clocks really retard on others, if nothing really happens? PS I know of two explanations for "one physical entity" that are consistent with SR: 4D block universe (which is a kind of 4D ether) and Lorentz ether (which is 3D). We are free to choose the one that appeals most to us.

Interesting total disagreement! On this I agree with bvr, and I suspect that the disagreement with VandD is about language, or philosophical (in which case it cannot be fixed by physics). When the ships are put side by side at the same velocity then they are measured to be the same length. Expanding on my posts #29 and #31: According to SR the length of objects is contracted when they are in motion (in itself that is a *relative" observation); and this is consistent with the measured clock retardation due to motion of atomic clocks. By the way, relativity of simultaneity is irrelevant for that absolute effect, as the final comparison is done at the same location. I replied to your "the change in frequency of a pendulum clock, which explicitly depends on g". g is the gravitation constant. However, I now take it that you merely intended to introduce it as a kind of spring. And then we totally agree on that.

You introduced gravitation in this thread, not me; and it's not "moot" at all. We can tell who is accelerating just as we can tell who is at a different gravitational potential. I would be surprised it anyone here was thinking about mechanical effects; but one never knows of course! Ah well spotted, that was somewhat sloppy phrasing - thanks for the correction! Similar nitpicking: Once more, all clocks are in all frames. To avoid confusions and calculation errors one has to either stick to one frame, or consciously do a transformation from one frame to another.

Yes, agreed; however: There the discussion would come into muddy (and GR) waters, as it's *somewhat* like that. Both changes don't depend on the kind of material or assembly. In Einstein's original version of GR it was even pretended to be the same effect (edit: from a different perspective), substituting acceleration with gravitation.

Although I agree with your statement, I was contemplating to add a clarification to my earlier post which, by pure chance, disagrees with the bold statement with which you agreed. The reason is subtle but essential for understanding. I consider "physical change" to mean a change according to measurements with any reference system in which the laws of physics hold - called "inertial frames" or "Observers". As words like "reality" are insufficiently predefined for discussions as this one, I label such a physical change as "real". Now -please forgive a little nitpicking- in SR it is impossible for things to change inertial frames; no doubt what was meant, is that when something changes velocity relative to inertial frames, it is not physically changing. However, we know from SR that according to measurements with any inertial frame, the length and emission frequency or clock frequency of things change when they change speed. The change of clock frequency has been confirmed by experiments, and if we assume that the speed of light is unaffected by the motion of the source, then necessarily this implies a similar change of length. This is important for physical insight: the different "view" of Observers in different state of motion is immediately understood as due to their different states of motion - there is no magical difference in view without physical cause for that difference in view. That also immediately clarifies why it is not the same if you accelerate, or if the other accelerates: if you change velocity and recalibrate your instruments to the new "rest system" (indeed, your clocks are not automagically synchronized to that system!), then -using the above definition of "real"- the change in lengths and clock rates of the other system are merely appearance due to real changes in your measurement system. However, the changes in your measurement system remain "relative" in the sense that different Observers attach different values to them.

Thanks People who claim that "nothing physically happened" will have a difficult time to explain how after "nothing happened" there is a difference in clock readings - by pure definition something physical happened. And by straightforward definition too, the average clock rates were different during the separation according to all reference systems. Note that, nevertheless, this has historically been the subject of much debate; I recall having searched for and found, in the peer reviewed literature, articles that claim that "nothing happens" and articles that "length contraction is real" (from memory, I don't recall the exact phrasings but they were definitely in disagreement!). also, neglecting a possible effect of the force of acceleration, of course the proper length does not change, and neither does the proper clock rate.

"the frame at rest" is moving according to other frames. However for consistency I reckon with relativistic mass as well as rest mass. Other words for rest length and rest frequency are proper length and proper frequency. Further I agree with you on several points. We cannot "move" in 4D spacetime (a concept that originates not with Einstein but Minkowski). Moreover, I'm not part of the "we all" of VanD, as the "reality" that I observe is not 4D spacetime; instead, the "reality" that I observe consists of many physical phenomena such as temperature, frequencies, distances and forces. However, such things are measured with a consistent standard measurement system. According to all consistent measurement systems in which the accepted laws of physics are valid, the clock did not continue ticking as usual. Instead, its changes of velocity made it loose ticks compared to clocks with which nothing physical happened. On the other hand, I agree that the clock of course did not simultaneously tick at multiple rates, just as it did not simultaneously have multiple speeds or multiple kinetic energies. The equivalent reference systems are mutually exclusive: when one is chosen, for convenience, as assumed reality, then the others are at the same time assumed to be non-rest systems in which measurements are affected by motion. That already was the standard way of dealing with reference systems in Newtonian physics: you pretend that one reference frame is in rest, and calculate consistently from that perspective. A different opinion leads to a different calculation and corresponds to a different assumed reality. Calculations mess up if we assume that everyone is all the time in rest so that nothing happens (Parimedes again)! Edit: slight improvement in phrasing

I suspect that the lack of understanding all has to do with how you phrase it. As Langevin already indicated, the central issue is a change in velocity. Using the term "acceleration" instead of "change in velocity" obscures a key point, and disconnects the description of the calculation; the result is a lack of insight. I strongly suggest to try to rephrase your account accordingly. We then find, depending on your choice of inertial reference system, that the acceleration implies that the traveling clock undergoes a change of speed which causes a corresponding change in clock rate. If we use for convenience the handy but somewhat misleading jargon "Observer" for "standard SR reference system", then his is qualitatively agreed on by all Observers. What also all Observers agree on, is that at the end the clock is again moving at the same speed as before (neglecting planetary motion). Note also that the clock always remains in any chosen reference system; its speed is changed with respect to the chosen system. BTW, of course we can jump reference systems midway, but that merely leads to mathematical exercises which confirm that the Lorentz transformations work. If you choose for example the stay-at-home reference system (approximating it to be inertial and neglecting effects from gravitation) then the traveler accelerates to a speed which is maintained most or all of the time (in Langevin's example the traveler doesn't even stop but slings around a star); in comparison, the speed of the stay-at-home is zero. The accelerations themselves are only important insofar as they bring about these speed changes. Thus, from the perspective of the stay-at-home frame, the traveler clock had a constant higher clock rate all or most of the travel period. The "the two opposite changes" in clock rate correspond perfectly with the two opposite changes in speed. It is instructive to consider other reference systems, for example the one in which the travel is in rest in the outgoing leg. From that perspective the clock rate first is increased to its proper rate, and then on the way back home it is strongly decreased as the traveler tries to catch up with the Earth. Then when the traveler decelerates, the clocks are again ticking at the same slowed down rate. Is there still something counter-intuitive in that description? PS. it may also be enlightening to carefully read the first description and the related physical insight concerning time dilation by Einstein in 1905 - § 4 of http://www.fourmilab.ch/etexts/einstein/specrel/www/ (we now know that it needs to be corrected for the effect of gravitational potential, but that's not important for the physical insight)

What is contentious about that?