Is Length Contraction in SR physical??
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From: Tom Roberts on 5 Feb 2010 00:23 kenseto wrote: > Fitting a ladder through a narrow door way is not the same as fitting > a long pole into a shorter barn with both doors close simultaneously. No. But the situations are ISOMORPHIC in SR. Both are examples of geometric projection. Tom Roberts
From: Bruce Richmond on 5 Feb 2010 00:27 On Feb 4, 10:10 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 5 Feb, 02:16, mpalenik <markpale...(a)gmail.com> wrote: > > > > > > > On Feb 4, 8:56 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 5 Feb, 00:19, artful <artful...(a)hotmail.com> wrote: > > > > > Don't you realize that EVERY reference frame encompasses the whole > > > > universe? > > > > No, an reference frame only encompasses that which is in the reference > > > frame. By definition, if there is more than one reference frame, then > > > you can't be using all of them at once. As I say, in my "universal > > > reference frame", an observer does not undergo a change in reference > > > frame when accelerating. > > > This is wrong on so many levels. > > > First of all, in the framework of GR, you actually can describe an > > accelerating observer with a single reference frame in the geometry of > > curved space time. > > I know, which is why I keep saying "I don't dispute the maths of > relativity", although it seems I may as well bang my head against the > wall. Indeed, I wish I was able to credibly threaten to shoot dead the > next person who says "but all of this is mathematically compatible > with GR". > > > But we don't need to get into that right now, we > > can have a meaningful discussion simply within the framework of SR. > > Indeed. > > > Second of all, replace the term "reference frame" with "point of > > view"--meaning your viewpoint of the velocities and distances to > > objects around you. You're essentially saying that an observer does > > not undergo a change of "point of view" when accelerating. > > Yes, I understand the concept of "point of view". Surely that was > proven elsewhere in my rebuttal to Paul who suggested that there were > no two reference frames in which two events can occur simultaneously. > > > Third of all, a single reference frame DOES encompass all objects in > > the universe. From your reference frame, you can assign 6 coordinates > > to every object in the universe--3 for position and 3 for velocity. > > Indeed. > > > This is how every object is encompassed by a single reference frame. > > Every object has coordinates within every reference frame. It is > > simply a matter of different reference frames assigning different > > coordinates to each object. But all frames are equally valid. > > Indeed, because "a change of reference frame" doesn't involve anything > physical, and so you can use whatever terms you like. After all, I can > point to my computer keyboard and say "keyboard", or I can point to it > and say "hammer", but it doesn't change physical reality.- Hide quoted text - > > - Show quoted text - One thing that does change when you change frames is your reference for making measurements. If you are at rest in frame A you measure the speed of light to be c relative to an object at rest in frame A, and to have a closing speed of c+v relative to object B which is moving at v. If you are at rest with respect to B then you measure the speed of light to be c relative to B and a closing speed of c+v relative to A. The frames agree that the speed of light is c, but they do not agree on what c is measured relative to. The answer to that question determines simultaneity.
From: mpalenik on 5 Feb 2010 00:36 On Feb 5, 12:11 am, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > mpalenik wrote: > > Yes, proper length is only invariant under transforms in the Lorentz > > group. > > Not true. In GR it is invariant under any coordinate transform. > > Tom Roberts You're right, I guess I should have specified that. But since we were talking about situations with people running into barns with ladders, the challenge was more for him to come up with a situation of that sort that could not be achieved through a Lorentz transform. I didn't mean for it to sound like there were actual physical situations where properlength wasn't a meaningful quantity.
From: mpalenik on 5 Feb 2010 00:44 On Feb 5, 12:23 am, Tom Roberts <tjroberts...(a)sbcglobal.net> wrote: > kenseto wrote: > > Fitting a ladder through a narrow door way is not the same as fitting > > a long pole into a shorter barn with both doors close simultaneously. > > No. But the situations are ISOMORPHIC in SR. Both are examples of geometric > projection. > > Tom Roberts Following in Peter's line of thought, do we really expect kenseto to understand what an isomorphism is?
From: Ste on 5 Feb 2010 01:55
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. > > > But there's another way you could rotate it, as well. You can rotate > > > it into "time". If you run with the ladder, you're "rotating" the > > > front of it a little bit into the future and the back of it a little > > > bit into the past. > > > Haha! I've nearly spat my tea out! I've never heard such a ludicrous > > statement before I came to sci.physics.relativity! > > The fact that you find it ridiculous has no bearing on physical > reality. What I do and do not find laughable has *full* bearing on physical reality. > 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? > > 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. > > 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. 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. > > 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. 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. > > > What a moving observer percieves as "time" is the direction of his > > > motion through 4 dimensional spacetime. What he percieves as "space" > > > is the 3 dimensional volume perpendicular to that direction. Thus, > > > "space" in the moving frame is not the same as "space" in the rest > > > frame. It extends part way into the future along one direction and > > > part way into the past along the other (in the rest frame). > > > Indeed. What's most worrying for me is that I can understand exactly > > what you're describing, and yet you can't understand what me and Ken > > are asking. > > > Incidentally, if you understand the maths of SR (I don't), perhaps you > > should try testing this one out. Have the observer in the middle of > > the barn, the doors close simultaneously according to that observer, > > and the ladder travelling at something just less than the speed of > > light. Can the observer possibly observe the ladder inside the barn > > with both doors closed in this situation?- Hide quoted text - > > This is what the original scenario represents. An observer, at rest > with respect to the barn, in the center of the barn, sees both doors > close at the same time with the ladder inside the barn. The person > running with the ladder does not see them close simultaneously, but > sees the front and back doors close and open at just the right time > for him to run through. This is what the math says and is a very > basic problem that might be given to a student who has just started > studying relativity. Listen, the question isn't what a moving observer would observe. The question is what the observer stationary inside the barn would observe. He cannot possibly observe the ladder move into the barn, and then for the doors to close simultaneously, and the ladder physically fit inside. If the maths seems to be suggesting this, then you're misinterpreting the maths. Incidentally, see this paper: http://arxiv.org/PS_cache/arxiv/pdf/0710/0710..3489v2.pdf As you can see, the effect that you allege, of physical length contraction, has never been experimentally observed. |