A constant speed of light in all reference frames? Surely you can't be serious.
in [Relativity]
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From: PD on 13 Feb 2010 15:17 On Feb 13, 1:40 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 13 Feb, 18:44, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 13, 7:29 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > I've been absolutely racking my brain (to the point of getting a > > > headache) for the last few days about this issue, and it's clear that > > > the speed of light (where light is either considered in the form of a > > > ballistic photon, or a wave-cycle) cannot, physically, be constant in > > > all relative frames, and at the same time be constant when travelling > > > between two objects in two different frames. It's a physical and > > > logical impossibility. > > > > It's also clear that velocities cannot be additive (in the form of > > > speed of bullet+speed of gun), and nor can they be subtractive > > > relative to a background medium (in the form of speed of propagation > > > in medium-speed of source). > > > > Take an illustration: > > > > A C > > > B > > > > Where A and B are atoms that pass infinitely close to each other. In > > > the illustration, A and B are separated from C by a distance L. A and > > > C are stationary relative to each other. B is moving, and approaching > > > C at a speed S. A pulse is emitted from both A and B simultaneously > > > towards C, at the point when A and B are equidistant from C. > > > > Now, clearly, if velocities were additive, then light from B would > > > reach C much quicker than light from A. We don't see that, so we can > > > dismiss that immediately. > > > > Next, if velocities were subtractive, like sound, well that seems like > > > a compelling explanation for what we see, which is that light from > > > both A and B travel towards C at the same speed. But the presence of > > > an absolute medium seems to fall down when one considers that, to be > > > consistent with observation, the speed of propagation orthogonal to > > > the direction of travel must be the same as the speed in the direction > > > of travel. > > > > A speed (i.e. a mesure of distance traversed within a period of time) > > > cannot possibly be measured constant in all directions within a frame, > > > *and* constant between frames, where the frames themselves are moving > > > at a speed relative to each other. So how the hell does one reconcile > > > this physically? > > > First of all, it's a mistake to say that velocities must be either > > additive or subtractive, as though those are the only two > > possibilities. > > I think you're taking it too literally - by "additive" and > "subtractive, I mean the alternatives are that a object's velocity > must cause either an increase or a decrease in the speed of light in a > particular direction relative to something. And that statement is wrong. To see this, you should try the exercise of seeing what happens using the expression I gave you. v' = (v+u)/(1+vu/c^2). Since you like working in the concrete, I suggest you take particular numbers and punch them into a calculator. If you start with a v that is less than c, you will find out that no matter how big u is, v' will be different than v. This is the "must" case you were alluding to above. But if you start with v that is equal to c, you will find that no matter how big u is, v' will again be exactly c. This is a surprising result, but the truth nonetheless. It may come as a surprise to you that the *same* velocity combination rule can give two different results: velocity that changes value when the original velocity is sub-light-speed, and velocity that doesn't change value when the original velocity is light-speed. But here is yet another example where algebra gives you a physical insight that is not obvious without it. > > > > > The reality is that velocities combine this way: v' = (v+u)/(1+uv/c^2) > > or this way: v' = (v-u)/(1-uv/c^2). > > Now then, the right question might be, why on earth would it be this > > way rather than simple addition or subtraction. > > > Secondly, if you're looking for a diagram that makes sense of this, > > you need to be Googling first for something like "worldline in > > Euclidean space". > > This will show you what the *meaning* of velocity is on the worldline > > diagram. > > This will also show you *diagrammatically* what it means to transform > > the velocity to a different frame and WHY the additive rule would be > > expected if the universe had that geometry (disconnected time and > > space dimensions). > > Then you can find out *diagrammatically* what it means to transform > > the velocity if the universe has connected time and space dimensions, > > and just a little playing around with the diagram will reveal the > > reason for the odd-looking sum rule above. > > > Robert Geroch's book that I've mentioned to you previously has some > > good presentations of these diagrams. > > Tell you what Paul, to clarify my thinking, consider this simple > setup: > > S1 D2 > > D1 S2 > > We've got sources S1 and S2, paired with detectors D1 and D2. They're > all mechanically connected, so that a movement in one of them produces > a movement in all the others - in other words, their relative > distances are always maintained. Each source is transmitting a regular > pulse of light to its counterpart detector (so S1 is transmitting to > D1, etc.), and both sources are transmitting simultaneously with each > other. > > Now, we calculate that a pulse has just been emitted from both > sources, and we suddenly accelerate the whole setup "upwards" (i.e. > relative to how it's oriented on the page now) to near the speed of > light, and we complete this acceleration before the signals reach > either detector. > > Now, do both detectors *still* receive their signals simultaneously, > or does one receive its signal before the other? And are the signals > identical, or do they suffer from Doppler shifting, etc? In which reference frame are you looking for the answers? In the reference frame that is the one that the set-up accelerates INTO (so that they end up at rest in that frame), or in the reference frame that the set-up accelerates FROM (so that they end up in motion in that frame)? The answer to your question depends on this choice. How does this help clarify things for you? PD
From: Ste on 13 Feb 2010 15:19 On 13 Feb, 19:48, eric gisse <jowr.pi.nos...(a)gmail.com> wrote: > Ste wrote: > > I've been absolutely racking my brain (to the point of getting a > > headache) for the last few days about this issue, and it's clear that > > the speed of light (where light is either considered in the form of a > > ballistic photon, or a wave-cycle) cannot, physically, be constant in > > all relative frames, and at the same time be constant when travelling > > between two objects in two different frames. It's a physical and > > logical impossibility. > > Experiment says otherwise. Alter your preconceptions. Experiments say our *observations* are otherwise. The question is to reconcile observation with physical reality.
From: PD on 13 Feb 2010 15:24 On Feb 13, 1:43 pm, Ste <ste_ro...(a)hotmail.com> wrote: > On 13 Feb, 19:11, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 13, 1:01 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > On 13 Feb, 18:45, PD <thedraperfam...(a)gmail.com> wrote: > > > > > On Feb 13, 11:51 am, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > Not really. I'm still struggling to understand what is happening > > > > > physically to explain these phenomena (which is not helped by the > > > > > dearth of interest in physics in physical, rather than mathematical, > > > > > explanations). > > > > > Oh, come now. You appear to have bailed on the discussion of > > > > relativity of simultaneity, which I was doing with purely physical > > > > explanations and a complete lack of math. > > > > I think you're being just a bit disingenuous here. > > > > I didn't bail on it. I said I felt that your train analogy had a lot > > > of extraneous concepts, such as clouds and tracks, > > > On the contrary, I *agreed* with you that the clouds (which I never > > brought up -- you did) are extraneous, as are the tracks, which is > > precisely why the velocities of the train with respect to the tracks > > are irrelevant. > > Indeed, and that is where the analogy ended as I recall. > > > > and then you didn't > > > really go on to say anything more about that analogy or about > > > simultaneity. > > > I'm sorry, read again. I laid out the plan for where we were going > > next. Did you not see that? > > I did read it again before replying to you, to make sure I hadn't > missed anything. I couldn't see your response to me that dealt with > the analogy any further. Here is where I think we last left things: http://groups.google.com/group/sci.physics/msg/26e91b6493e277e1? and http://groups.google.com/group/sci.physics/msg/4a126a6622bac8b1
From: mpalenik on 13 Feb 2010 15:28 On Feb 13, 2:11 pm, Ste <ste_ro...(a)hotmail.com> wrote: > I'm afraid I do not accept that my "choice of mathematical coordinate > system" is the explanation for this.- Hide quoted text - > Think of it as your orientation in spacetime. For example, if you're facing north, a natural coordinate system is that north is foward and east is right. If you walk in a straight line, you'll end up going north. Now, turn 45 degrees. A natural choice of coordinate system is northeast is forward and southeast is right. If you walk forward, you'll go northeast. Are you going to tell me that you won't accept that your natural choice of coordinate systems is responsible for you ending up in different places when you walk forward? It has to do with your orientation, whether it's your orientation on the surface of the earth or your orientation in spacetime--your orientation depends on what happens when you move forward.
From: Androcles on 13 Feb 2010 15:33
"dlzc" <dlzc1(a)cox.net> wrote in message news:03939c75-b124-4c35-aa59-a1bee75f2725(a)u19g2000prh.googlegroups.com... Dear Ste: On Feb 13, 6:29 am, Ste <ste_ro...(a)hotmail.com> wrote: .... > So how the hell does one reconcile > this physically? Simple. All you have are light speed signals, one way or another, being correlated at a single point. Not mental model constrains Nature, only the human brain struggles with his / her preconceived notion of "what is going on out there". David A. Smith ======================================= Nice preconceived notion of another way of light speed signals you have, Smiffy. Are you struggling with it? |