Joan-Claude van Dirk Helps to Trivialize Special Relativity
in [Relativity]
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From: PD on 17 Mar 2005 15:04 cadwgan_gedr...(a)yahoo.com wrote: > PD wrote: > > cadwgan_gedrych(a)yahoo.com wrote: > [snip] > > > Why don't you tell us how SR can correctly measure the length of > > > even a rod that is at rest wrt our frame? > > > > > > In fact, why don't you tell us how SR can even correctly measure > > > the one-way, two-clock speed of anything? > > > > > > In order to do that, here are the hurdles which SR must overcome: > > > > > > [1] SR must prove that its clocks are correctly synchronized. > > > [2] SR must prove that its clocks are not physically slowed. > > > [3] SR must prove that its rulers are not physically contracted. > > What happened to your defense of relativity? You are under the mistaken impression that I *need* to defend relativity. I've asked you a bunch of questions about *your* understanding of relativity. PD
From: Daryl McCullough on 17 Mar 2005 15:21 cadwgan_gedrych(a)yahoo.com says... >Here is an example of its blatant circularity/triviality: > >SR sez: >"If we manually force all clocks in all frames to >obtain the same one-way light speed, then, by George, >all clocks in all frames will obtain the same one-way >light speed." > >Can you, even your blinded eyes, see the silly circularity? SR says more than that---it says that *every* experiment done in the "moving" frame, using clocks synchronized Einstein's way, produces the same results as the same experiment performed in the "stationary" frame. In particular, consider the following experiment: In the moving frame, we synchronize a clock at rest at position x'=0 and a clock at rest at position x'=L using light signals. Then we slowly (at a speed much slower than the speed of light) move the first clock from its position x'=0 until it is side-by-side with the clock at postion x'=L. Then we compare times. The prediction of SR is that the clocks will continue to be in synch. That is a nontrivial empirical prediction. More generally, SR predicts that any clock transported along any path will show an elapsed time of tau = Integral of square-root(1 - v(t)^2/c^2) dt where v(t) is the instantaneous velocity of the clock at time t. That doesn't follow circularly from the definition of Einstein synchronization. Your basic point is correct---some predictions of a theory are not true predictions, they are more like conventions; Einstein's synchronization convention is such a convention (that's why it's called a "convention"). In order to extract the physical content of a theory, it is helpful to rephrase predictions so that the result is independent of any such conventions. The constancy of the speed of light is *not* a physical prediction, unless you operationally define how you are measuring it. -- Daryl McCullough Ithaca, NY
From: Harry on 18 Mar 2005 05:39 "PD" <pdraper(a)yahoo.com> wrote in message news:1111081433.010734.73380(a)o13g2000cwo.googlegroups.com... > Harry wrote: > > "PD" <pdraper(a)yahoo.com> wrote in message > > news:1111075406.192734.177260(a)g14g2000cwa.googlegroups.com... > > > Harry wrote: > > > > "PD" <pdraper(a)yahoo.com> wrote in message > > > > news:1111004650.489408.138750(a)l41g2000cwc.googlegroups.com... > > > > > > > > > SNIP > > > > > > > > > > This asynchronousness is easily proved, as follows: > > > > > > Given two events, we know that they must physically occur > > > > > > either absolutely simultaneously or not, > > > > > > > > > > And how do you know that? This is not only not obvious, but > also > > > > > incorrect. > > > > > Define simultaneity and how you would determine whether two > things > > > are > > > > > simultaneous. > > > > > > > > It depends on one's starting assumptions. > > > > In science (natural philosophy) the starting assumption used to > be > > > that one > > > > single observervation independent reality exists. > > > > Based on that assumption, two events can only be either > simultaneous > > > *or* > > > > not simultaneous. > > > > > > The difficulty with this, SR holds, is: > > > 1. Given two observers in relative motion, there will be a > disagreement > > > as to whether two events are simultaneous or not. There is no > > > physical way to determine which observer is correct. > > > > That is not a problem for SRT, just as it doesn't matter for > Newtonian > > mechanics not to know which observer is moving, and as it doesn't > matter for > > electronics not to know which conductor is at zero potential. One may > just > > assume whatever and it works. > > There is a difference. In the other cases that you mentioned, the > physical laws are quite explicit in depending not on the absolute scale > of those quantities but on the *difference* only. Acceleration (which > enters into the dynamics) does not depend on the velocity of the > reference frame; potential *difference* does not depend on which point > is chosen to be zero. In the case of simultaneity, critics of SR often > assert that simultaneity MUST be relied on as an absolute property. The > fact is, as SR points out, there is nothing in physical law that > demands simultaneity being an absolute property. The physical laws describe relative measures of speed and potential. Which does not prevent models in which they are absolute, but measured relatively. Your above "difficulty" only exist for certain philosophers, because of their personal taste. > > > 2. Given a set of observers all in relative motion and three events > A, > > > B, C, it will be the case that A and B will be simultaneous to only > one > > > observer among the set of observers, and B and C will be > simultaneous > > > to only one observer among the set observers, and these two > observers > > > will not necessarily be the same observer. The association of > > > simultaneity with the observer-independent reality (i.e. absolute > > > simultaneity) would imply that one of these choices is correct, > > > > Exactly, > > > > > which > > > would mean there would have to be a way to physically distinguish > > > that choice as being correct (and the other incorrect). > > > > Certainly not, that's an erroneous argument. Why on earth would one > think > > that whatever exist must be measurable in principle, or worse, that > > it must be measurable with our existing instruments? > > I don't think anyone worries too much about whether it is measurable > with existing instruments. Certainly, Einstein proposed some > measurements to test the theory that involved experimental apparatus > not yet developed, but he DID suggest some ways to test it *in > principle*. My objection was against your argument that all that exist must be measurable by us, as if we are gods who can measure all that exists. > SR does NOT say that simultaneity is in principle a > verifiable property, but we don't have the instruments to detect it > yet. > > As to whether something can exist but be unmeasurable in principle, > even indirectly, this becomes a subject of metaphysics, not physics. Exactly. You made a metaphysical claim about physics, with which I disagreed. > Personally, I feel strongly that physics makes a representation of the > universe, but that there are limitations to our representation that > have to do with semantics (particle-wave duality, for example) or > intractibility (perturbation theory and renormalization), for example. > To that extent, there may well be a reality that is outside our > understanding even in principle, but that lies outside physics. > Personally, I also have faith (and I mean that term) that nature does > not exhibit things that are inherently unmeasurable or undetectable. > > For a long time, folks were sure that neutrinos were massless and > left-handed. As such, right-handed neutrinos would interact with > *nothing* in the universe and would, by their very nature, be > undetectable and would have no implications for the rest of reality. So > it was perhaps interesting to ask the question whether right-handed > neutrinos could exist *anyway*. The dominant thinking has been, of > course they *could*, but they are not part of physics. For physicists, > massless, right-handed neutrinos simply do not exist. This is clearly > based on a faith that the universe does not feel compelled to produce > "useless" particles. Hmm, that is not physics but philosophy. Physics is neutral about such issues - and physicists the same. > There are some ether theories that are identical in saying that the > ether is present but it is by nature completely undetectable, even in > principle. Such theories are generally rejected on the principle of > simplicity. Given two experimentally indistinguishable models A and B, > where B involves something that A doesn't but whose existence cannot be > verified, the choice is usually A, and that's a defendable choice. OK, we agree that that is an optionally choice. OTOH, what do you think of neutrinos? Likely a theory can be divised in which they don't exist, but instead we just introduce a correction factor in calculations. IMO, Okham's razor is easily misapplied. > Similarly here, if absolute simultaneity is a property of two events > but is completely and by nature undetectable, even in principle, then > for the purpose of physics, absolute simultaneity is not a needed > concept. Right - except maybe if physics is based on the premise that its purpose is to investigate nature by observations, and not just observations. AFAIK, such used to be its purpose. Harald > > > 3. No test has been able to distinguish which choice is correct. > > > > Correct, AFAIK. > > > > > > If OTOH one adopts the pov that somehow everyone establishes > his/her > > > own > > > > reality -- effectively everyone's personal universe -- then > indeed > > > everyone > > > > can decide on his/her own truth. "Physical" can then be put in > the > > > domain of films such as Matrix, it's just personal perception. > > > > > > > > Harald > > > >
From: Harry on 18 Mar 2005 06:15 "Bilge" <dubious(a)radioactivex.lebesque-al.net> wrote in message news:slrnd3jpt7.reo.dubious(a)radioactivex.lebesque-al.net... > Harry: > >"PD" <pdraper(a)yahoo.com> wrote in message > >news:1111075406.192734.177260(a)g14g2000cwa.googlegroups.com... > > >> > >> The difficulty with this, SR holds, is: > >> 1. Given two observers in relative motion, there will be a disagreement > >> as to whether two events are simultaneous or not. There is no physical > >> way to determine which observer is correct. > > > >That is not a problem for SRT, just as it doesn't matter for Newtonian > >mechanics not to know which observer is moving, and as it doesn't matter for > >electronics not to know which conductor is at zero potential. One may just > >assume whatever and it works. > > Pay attention, harry. Your reply has absolutely nothing to do with > what he just said. It is posible that I misunderstood him, or in fact you. For him to say - and from his reply, it appears that you are mistaken. Probably this is because you snipped and forgot (or didn't read) the text on which he commented. > And yes, it does matter whether or not two observers > agree or disagree which events are simultaneous. Of course, that was not the subject (which was if we use the starting assumption that an observervation independent reality exists or not). > If simultaneity is > absolute, then two observers will _always_ agree on which events > are simultaneous, in which case relativity would be wrong. Relativity > can only be correct if simultaneity is not absolute. [This has been > experimentally tested, by the way, in bell type experiment using > moving beam splitters. It was found that there existed no intrinsic > time ordering of spin measurements made over a spacelike interval. > That doesn't mean that there could have been an intrinsic time ordering > in some frame - their objective was to test exactly that - the existence > of a preferred frame in order to salvage bohm's theory which requires > such a frame.] That's a slightly different subject but if you have a link to a paper on that, yes please, - it's the one at the bottom, right? > >> 2. Given a set of observers all in relative motion and three events A, > >> B, C, it will be the case that A and B will be simultaneous to only one > >> observer among the set of observers, and B and C will be simultaneous > >> to only one observer among the set observers, and these two observers > >> will not necessarily be the same observer. The association of > >> simultaneity with the observer-independent reality (i.e. absolute > >> simultaneity) would imply that one of these choices is correct, > > > >Exactly, > > And the aforementioned experiment shows that there is no > ``correct choice.'' Events which are simultaneous in one frame > are simultaneous in _every_ frame. You mean, they are *out of sync* according to the calibration of *every other* frame - or your experiment is different from this one of PD. > By definition, simultaneous > events aren't time ordered. If events are time ordered, then > their separation is timelike, not spacelike. Knock off the > semantics dance. > > >> which > >> would mean there would have to be a way to physically distinguish that > >> choice as being correct (and the other incorrect). > > > >Certainly not, that's an erroneous argument. > > It's a perfectly valid argument. See PD's comments. > If one choice defined a time ordering, > then every event in that frame wold be time ordered, in which case, the > separation between the events would be timelike, not spacelike and the > transforms would be the galilean transforms. This discussion with PD transcends transforms. If you knew the difference between Newton's mechanics and galilean transforms, you'd know what we're talking about. > >Why on earth would one think that whatever exist must be measurable > >in principle, or worse, that it must be measurable with our existing > >instruments? > > You who have suddenly become a philosopher Bilge, you amaze me. I joined this group after I discoverd that I'm interested in philosophy, and that physics is affected by it in a hidden way. Regretfully I am not learned in philosophy, as I always thought that philosophy is useless. > ought to be able to > grasp the reason why. Anything which can have any effect on anything > in this universe, has to be measureable in principle, since by definition, > an effect is the outcome of an experiment. Now that's too the point. There is a difference between measuring something with our toolbox, and inferring cause and effect. And it seems that you agree that what we measure are effects, from which we infer causes. But necessarily the causes of what we can measure can logically not be measured, except if they themselves are effects. In the end we may expect that causes exist that can't be measured, but that can be inferred. > Explain how something can affect physical objects, yet conspire to not affect the physical objects > of an experiment. See above. Nothing is suggested to exist and that is useful for physics but that has no effect. SNIP flagrant misunderstanding. > >> 3. No test has been able to distinguish which choice is correct. > > > >Correct, AFAIK. > > Incorrect. The article below reports an experiment that demonstrates > quantum correlations over a spacelike interval using moving beam > splitters, are completely independent of any time ordering assigned > to the measurements over that spacelike interval. "Incorrect" would imply, if I read him well, that an absolute reference frame has be measured. But I'll read the blow paper with interest. Thanks. Harald > A. Stefanov, H. Zbinden, N. Gisin and A. Suarez. > Physical Review. A, volume 67, 042115 > > A related article is available at the arxiv: quant-ph/0311004 >
From: cadwgan_gedrych on 18 Mar 2005 09:07
PD wrote: > cadwgan_gedrych(a)yahoo.com wrote: > > Here is an example of its blatant circularity/triviality: > > > > SR sez: > > "If we manually force all clocks in all frames to > > obtain the same one-way light speed, then, by George, > > all clocks in all frames will obtain the same one-way > > light speed." > > > > Can you, even your blinded eyes, see the silly circularity? > > You confuse a conclusion with an assumption. Read the 1905 paper. It > states at the outset that the constancy of the speed of light is an > *assumption* of the theory, not a conclusion of the theory. See my response to Daryl. (It cannot be an assumption because it is given up front via definition.) > Now, if you have any experimental evidence that the speed of > light is NOT constant for all inertial frames, then let's have > that, because that would be a truly interesting result (and it > would immediately dispense with a whole bunch of physics, > including SR, GR, the Dirac equation, quantum field theory, > QED, QCD, the standard model Lagrangian, nuclear physics, etc.). > > PD (The proper word is "invariant," not "constant." These things matter when discussing the in's and out's of physics.) The only "physics" that one-way invariance would "blow away" would be SR. And, ironically, Einstein himself told us exactly how to have one-way variance. In his little popular book "Relativity," he stated mathematically that if the assumed-to-be-absolutely- synchronous clock of classical physics were used, then light's one-way speed would be c +/- v in all frames. But I do not need such clocks to answer your challenge to provide experimental evidence that the speed of light is NOT constant [sic] for all inertial frames because all I need is the following simple experiment: Let two observers meet in passing as a light ray approaches them. (Oa = Observer A and Ob = Observer B) .........Oa .......................................<~~~~~~~~~light ray .........Ob When the two observers briefly meet, they know that the light ray's tip is equidistant from them because they are at a single point in space, and the ray's tip is also at one point in space. We can qualitatively label this distance "X". After the observers separate, the light ray will reach one of them, as shown below: ....Oa .............<~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~light ray ............Ob Since the tip of the light ray cannot be in two places at once, the observers will see it sequentially at absolutely different times. (Here is a down-to-earth example: If I see the real you in both Texas and New York, then this proves that I saw you at absolutely different times because you cannot be in two places at once.) We can - again purely qualitatively - label the ray's absolutely different arrival times "Ta" and "Tb." (All we care about here is the fact that these times are absolutely different.) The observers can now compare one-way light speeds. (Having no rulers or clocks, they must do this qualitatively.) Here are their extremely simple results: Light's speed wrt Oa = X/Ta Light's speed wrt Ob = X/Tb |