Is Length Contraction in SR physical??
in [Relativity]
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From: Ste on 5 Feb 2010 03:35 On 5 Feb, 07:21, mpalenik <markpale...(a)gmail.com> wrote: > On Feb 5, 1:55 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > Also, even though this has nothing to do with the analogy as I > > presented it, an observer moving along an axis that is always > > equidistant from both doors *can* be moving and also observe the doors > > to be closing simultaneously. > > I misunderstood your wording the first time, but yes, you are actually > correct here--if an observer is moving perpendicular to the axis > between the two doors, he can see them open and shut at the same time > as in the rest frame. When I read it before, I read it as "a moving > observer who is equidistant between the two doors," in which case, the > statement is false. Ok.
From: mpalenik on 5 Feb 2010 03:42 On Feb 5, 3:24 am, Ste <ste_ro...(a)hotmail.com> wrote: > On 5 Feb, 07:08, mpalenik <markpale...(a)gmail.com> wrote: > > > On Feb 5, 1:55 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > All it demonstrates is that you are not familiar with the > > > > theory of relativity. > > > > Then why is it that I seem to be able to understand it so well, even > > > though I have no idea how to apply it mathematically? > > > You have repeatedly expressed an incorrect understanding of the theory > > and refuse to acknowledge it, despite the fact that multiple people, > > including myself, have explained that your interpretation is flawed. > > Why you think you understand it "so well" is beyond me. > > I think you mean you've insisted that my interpretation is flawed - > you haven't explained that argument at all. I'm perfectly willing to > explain my arguments if you don't understand mine, but you can't > seriously expect me to agree with you just because you *say* I'm > wrong? > Well, lets go through the points below where you disagree with me. > > > > > > > > > Of course, I understand what you're getting at, although I understand > > > > > the concept differently. What you're getting at is the effect of > > > > > propagation delays, > > > > > No, this has nothing to do with propagation delays. > > > > What *does* it have to do with then? Bear in mind that a propagation > > > delay is a *physical* concept. A "rotation in time" is a purely > > > geometric concept. > > > > And incidentally, you're not the first person to have said "it has > > > nothing to do with propagation delay", and yet no one who has said > > > that has yet described *any* effect (in SR) that is not adequately > > > explained by propagation delays. > > > If you work through the math, you'll see that the effects I'm > > describing are *after* propagation delay has been accounted for. It > > is *not* adequately accounted for by propagation delays. Those must > > be accounted for *separately*. > > Then give me a concrete example of the difference between what would > be predicted with 'mere' propagation delay, and the actual predictions > of SR, and then we can discuss it. > In this particular thought experiment with the barn and the ladder, I've tried to explain the different to you but you refuse to believe. In the full relativistic framework, an observer at rest in the center of the barn can see both doors shut at the same time while the ladder is entirely inside of the barn. This cannot be accounted for with mere propagation delay. There are other effects of relativity that cannot be accounted for with propagation delay, such as increased half lives of particles moving at high speeds (which have been measured), but they're not related to this thought experiment. > > > > > > > > > whereby both doors can appear to close > > > > > simultaneously when, in physical reality, the distant door has already > > > > > started to open before the near door closed. > > > > > No, the doors close simultaneously in the frame of the barn but NOT in > > > > the frame of the moving ladder. This is *after* correcting for any > > > > "propagation delays" in the observation of the two doors. > > > > I'm afraid I don't accept this. If it is necessary to make the analogy > > > more complex, then consider this. The ladder has an observer sitting > > > on each end of the ladder (in addition to the person stood in the > > > middle of the barn watching the ladder go by). > > > > The observer at the fore of the ladder naturally sees the front door > > > close before the back door, and the front door will start to open > > > again before the back door has even shut. The observer astern of the > > > ladder sees the back door close before the front. The observer stood > > > still in the middle of the barn equidistant from each door sees both > > > doors close simultaneously. > > > No, this is not how special relativity works. You must *first* > > account for the propagation delays. The observers at the front and > > rear of the latter, after accounting for propagation delays, will each > > see the door open and shut at the same time. > > > The relativistic effects have to do with differences measured due to > > VELOCITY, NOT POSITION. > > I accept that there are effects due to velocity. But in this context, > what of them? The fact that the discrepancy in simultaneity between the closing and opening of the two doors only occurs for observers at different velocities. It does NOT occur for observers at the same velocity in different locations. Once you account for propagation delay, there is no descrepancy in simultanaity for observers at the same speed. But the discrepancy REMAINS for observers at different speeds. > > > > Now *of physical necessity*, the observer in the barn who sees the > > > doors close with the ladder inside, *must* be able to answer our > > > question. If the ladder is inside the barn, then this observer cannot > > > *possibly* observe both doors to be shut, unless the ladder is > > > *wholly* and *physically* inside the barn. > > > > The question that remains outstanding, is whether the ladder *actually > > > fits* inside the barn, or whether it crashes before it even gets all > > > the way into the barn. > > > It fits in the barn in the barn frame, where both ends are in the barn > > simultaneously. > > So what you're saying is that the stationary observer in the middle of > the barn could see that the ladder was in the barn and the doors are > both closed, even though the ladder has a "proper length" that is > longer than the distance between the two doors? Yes. > > And if you say it does, then I'm going to ask you, are you *sure* it > does? Have you done the calculations? Yes. Proper length (squared) is dx^2 + dy^2 + dz^2 - cdt^2. The fact that the proper length is greater than that of the barn is accounted for by the fact that some of the ladder is rotated in time, which is what I've been trying to explain to you. > > > > > > > > > > But that's why Ken wants a simple answer to the *physical reality*, > > > > > which is why I devised a setup where the doors are equidistant from an > > > > > observer (and therefore both must truly close simultaneously), > > > > > This is what you don't understand, in this scenario, they are ONLY > > > > closing simultaneously in the barn frame. > > > > Yes, and the observer is *in* the barn frame. > > > > > To a moving observer (at > > > > ANY location, even a moving observer located at the center of the > > > > barn) they do not close simultaneously. *THIS* is what the theory of > > > > relativity states. > > > > Bollocks! The doors cannot possibly close simultaneously "for the > > > barn", unless they also *appear* to close simultaneously for the > > > stationary observer positioned inside the barn and equidistant from > > > each door. > > > They DO appear to close simultaneously for the stationary observer at > > the center of the barn. I have never said otherwise. > > I had not mentioned a "moving observer" at the centre of the barn, > although you are wrong in any event - see next. No, I'm not. > > > > Also, even though this has nothing to do with the analogy as I > > > presented it, an observer moving along an axis that is always > > > equidistant from both doors *can* be moving and also observe the doors > > > to be closing simultaneously. > > > Only if you fail to take into account relativity. Under Newtonian > > physics, you are correct. Under relativistic physics, you are not. > > I don't believe you. Are you sure you have done the calculations and > verified this for yourself? Did you read my second post where I corrected this? I misread your original statement as "a moving observer equidistant between the two doors," in which case, it is in general false. But yes, if the axis of motion is perpendicular to the line between the two doors, then you are correct. > > How can the doors possibly not shut simultaneously with each other if > you are, by definition, always maintaining equidistance from both, and > you are moving with the same velocity relative to both? This is the difference between Newtonian mechanics and relativity. In relativity, when you move, as I've tried to explain before, some of your "space" axis gets rotated into your "time" axis, and so events in front of you are slighty in the FUTURE of the stationary frame, which events behind you are slightly in the PAST of the stationary frame.
From: Ste on 5 Feb 2010 03:49 On 5 Feb, 07:32, mpalenik <markpale...(a)gmail.com> wrote: > > All of this follows logically from the postulates of relativity, which > only require that the speed of light is measured at the same value in > every inertial reference frame and that the laws of physics are the > same in every inertial reference frame. There is ample evidence for > both of these postulates. I've actually just started another thread called "Photon and the speed of light" which might have implications for this. You might want to have a look. > Experimental evidence includes: > Failure to measure a difference in light speed in any direction > Increased half-life of particles moving at high speeds > All the predictions of quantum field theory, which are done on a 4 > dimensional Minkowski spacetime background (where space and time can > be rotated into each other as I've described) including: > particle production observed when colliding particles in an > accelerator > the energy released in these collisions > Quantum chemistry on heavy elements--to accurately predict the energy > spectrum of heavy elements and compounds containing heavy elements, > relativity must be used. > The electromagnetic fields of moving particles > > All of these measurements require transformation laws that take place > in 4 dimensional Minkowski spacetime (Minkowski spacetime being the > thing I'm trying to describe to you). > > The fact is, though, you can demonstrate that for consistancy of the > physical world, if the farmer running with the ladder measures the > same light speed as the man sitting in the barn, then the ladder must > contract in the barn frame and the barn must contract in the ladder > frame. The only way around that is for the two observers to measure > different light speeds. Ah, so does this leave a loophole where I might be right if I can plausibly attack the evidence relating to the invariance of the speed of light?
From: mpalenik on 5 Feb 2010 03:52 On Feb 5, 3:49 am, Ste <ste_ro...(a)hotmail.com> wrote: > > Ah, so does this leave a loophole where I might be right if I can > plausibly attack the evidence relating to the invariance of the speed > of light?- Hide quoted text - If you can attack the invariance of something moving at the speed c (it doesn't have to be light specifically. For example, if it were found that light moved at less than c, it wouldn't matter), then the entire theory as I've described it would fall apart.
From: Ste on 5 Feb 2010 03:59
On 5 Feb, 07:56, "Peter Webb" <webbfam...(a)DIESPAMDIEoptusnet.com.au> wrote: > "Ste" <ste_ro...(a)hotmail.com> wrote in message > > news:ba85d7b7-90c7-47b9-a9a1-6e04278969c1(a)k19g2000yqc.googlegroups.com... > On 5 Feb, 04:31, mpalenik <markpale...(a)gmail.com> wrote: > > > On Feb 4, 11:22 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > Yes, I'm sure I've employed such a similar wily device when getting a > > > sofa through a doorway before. But we're assuming that the ladder > > > *isn't* rotated. > > > See below. It is rotated not in length and height but in length and > > time. > > It is not "rotated" in any physical sense. > > ____________________________________________ > Perhaps if you defined what you mean by "physical sense" this statement > would have some meaning. Speaking of "physical", I'm starting to wonder whether I've actually died and gone to hell for my sins, where the Devil has decided that I am to be tormented by having to explain the meaning of "physical" for all eternity. |