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 13:44 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. 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.
From: PD on 13 Feb 2010 13:45 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.
From: dlzc on 13 Feb 2010 13:59 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
From: Ste on 13 Feb 2010 14:01 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, and then you didn't really go on to say anything more about that analogy or about simultaneity. And on a slightly separate matter, I admitted that I couldn't make sense of the speed of light in terms of a stationary medium.
From: PD on 13 Feb 2010 14:11
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. > 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? > > And on a slightly separate matter, I admitted that I couldn't make > sense of the speed of light in terms of a stationary medium. |