[paul cotter (OP)]

Right, i'll have a go. My question concerns the historic break with the Newtonian paradigm around the time when relativity was developing(SR). The speed of light,(1) radiating from a moving object and (2) impinging on a moving object are decreed by SR to be exactly c, regardless of the speed of the object. (1) does not, in my opinion, require any postulate from relativity: once emitted the speed of the radiation will be determined solely by μ and ε. (2) However conflicts with Newton as one would imagine that the speeds would be additive/subtractive. What experiments/theory led to the conclusion that light impinging on a moving object at any arbitrary speed( <c) would always arrive at c? I have struggled with the wording of this question and am still not satisfied with it. 

[alancalverd]

Worth remembering the visitor who asked a Dubliner the best route to Cork. "If I was going to Cork, sir, I wouldn't be starting from Dublin".   That theme runs through my discussions with Hamdani on the "twin paradox" and is relevant here.

Simply, if you begin with classical/newtonian physics, you can't derive relativistic equations because the key element v/c (or more often v²/c²) is absent.

But if you start from a relativistic position, you can derive newtonian physics by simply putting v << c.

Consider a blind man running into an oncoming car. Until the instant of impact, he has no knowledge of their relative velocity, but at impact he has a very good idea of the kinetic energy of the collision.

Replace the car with a photon, and replace the man with the textbook "observer" - in this case a spectrometer. Since we have postulated that no information can travel faster than c, we are "blind" before the event and all we know is the energy of the photon/spectrometer event. We know by experiment that if we impose a relative velocity or gravitational field between the source and observer, we observe a proportionate  doppler or gravitational shift of that energy.

[Petrochemicals]

What experiments/theory led to the conclusion that light impinging on a moving object at any arbitrary speed( <c) would always arrive at c?

Or

What experiments/theory led to the conclusion that light on a moving object would always arrive at c?

[paul cotter (OP)]

Thanks Alan, but you have misinterpreted my question, as I feared many would. What I want to know is, in the early developmental stages of relativity(ie the work of Lorentz), when and how it was determined that in the case of light hitting a moving object the speeds would no longer be additive/subtractive. This would have been a major break with existing theory of the time. On a different subject, re Hamdani, maybe we should send him David Hilbert's 20(?) odd unsolved mathematical problems since he claims to be making progress on Riemann?

[alancalverd]

Your response set me off in the direction of Michelson- Morley, and it occurred to me that the remaining proponents of aether point out that you can't measure the one-way speed of light, but the refutation of aether frequently misses a simple trick.

Suppose we are travelling east-west through the aether. So I build a rigid frame that simply projects a beam of light E-W and reflects it back to the source. Now rotate the frame to N-S. Without even measuring the transit time, it is obvious that the beam will be deflected (twice) by the flow of aether and simply will not return to the source. But it does.

To my delight, I find that the Wikipedia entry on "length contraction" starts from  the MM experiment and states that LF contraction is an attempt to reconcile MM with a stationary aether, with the implication that Einstein simply decided not to start from Dublin but to assume the constancy and maximality of c. 

[Halc]

What I want to know is, in the early developmental stages of relativity(ie the work of Lorentz), when and how it was determined that in the case of light hitting a moving object the speeds would no longer be additive/subtractive.

That was derived from the postulates. It is only true under Einstein's postulates, and not under say LET, a form of absolutism which does not accept either of Einstein's premises.

(1) does not, in my opinion, require any postulate from relativity: once emitted the speed of the radiation will be determined solely by μ and ε.

Einstein could have instead just accepted Maxwell's equations as a postulate instead (Maxwell's work conflicts with Newton's) of the frame invariance of light speed since the latter can be derived from the first, but the theory wasn't about electromagnetism, so it was simpler to cut to the chase.

On another note, I don't know how you are using the word 'impinge' in your posts above.

The speed of light,(1) radiating from a moving object

Light does not radiate away from a moving object at c.  If an object is moving at say 0.5c east, then light that is at the object (coming from it or not) moving west will be 1.5 light seconds away one second later.
The postulate says light moves at c relative to any inertial frame, and the object is said to be moving, meaning we're using a different inertial frame than the one in which the object would be stationary.

LET does not accept any of the above. It is an absolutist theory, so light moves at c period, relative to the one and only preferred frame, and not relative to any other non-preferred frame. Velocities are still additive under LET. This doesn't make LET wrong.

What experiments/theory led to the conclusion that light impinging on a moving object at any arbitrary speed( <c) would always arrive at c? I have struggled with the wording of this question and am still not satisfied with it. 

It's a postulate. If there was hard evidence behind it, it wouldn't need to be a postulate. So it is merely an assumption, and one that not all competing theories accept.
I'm pretty sure that Maxwell's work was a heavy influence on the choice, but it's frame invariance does not prove the absence of a preferred frame.

[paul cotter (OP)]

Brief reply to Halc, as i'm working at the moment. I did say I struggled with the wording and I accept it wasn't the best. Just for the record, I fully accept SR(and GR too!) and I am just trying to fill in some gaps in my understanding.

[paul cotter (OP)]

Hi Halc, here we go again! My interpretation, which could well be wrong, is that a lot of the ground work for relativity was done by Lorentz, Fitzgerald, Poincare and others and that Einstein had the vision to knit it all together in SR. I certainly was not considering LET(aether! God forbid!). I am somewhat confused by your statement that light emitted by a moving body does not travel at c: suppose an observer measures a body passing by at 0.5c and then this body emits a pulse of light- surely in both the frame of the observer and the frame of the emitter the speed of said light pulse will be c? Impinge is a word I have often seen used in this context similar to collide, strike or we could use the biblical term "smite". One final request, Halc, refresh my ageing brain as to the conflict between Maxwell and Newtonian mechanics. 

[alancalverd]

A classical interpretation of em propagation requires a medium, Maxwell doesn't.

You can't label it as Newtonian physics since Newton's hypothesis was "corpuscular" - remarkably close to our photon model - and doesn't need a medium. Problem is that it doesn't predict interference, including "Newton's Rings" (!) but his corpuscles did travel in straight lines at a constant speed.

The joy of Maxwell is that c depends only on μ and ε, so in the absence of a medium, it is a constant and hence a reasonable starting point for Einstein.

[paul cotter (OP)]

When I used the term "Newtonian" I meant the pre-relativity paradigm where speeds would add/subtract arithmetically rather than his corpuscular concept of light. Is the lack of a medium for em radiation the only conflict between classical and Maxwell?

[alancalverd]

Let's mix our metaphors a bit. Imagine a wave whose motion is governed by Maxwell's propagation equation. In one unit of time τ its peak propagates from a to a + cτ, then the process begins again and wavelength λ = cτ.

Now imagine I am moving towards the source. I encounter a peak, followed by another peak. I can measure λ' and τ' between the peaks but I have no other information about the source and λ'/τ' = c because the propagation mechanism (dependent only on ξ and μ) inside my measuring equipment is exactly the same as that outside.

[paul cotter (OP)]

Thank you, Alan, that is very clear. All this is quite familiar to me but I lacked a clear mental picture to accompany the formulae. Case closed.

[Halc]

I certainly was not considering LET(aether! God forbid!).

I never tried to imply that you were. I just bring up LET because (never mind the aether part), it is an absolutist way of looking at things, a valid way of looking at things which denies all premises of special relativity. So Einstein making those premises was a choice, not something that was obvious from the evidence.

I am somewhat confused by your statement that light emitted by a moving body does not travel at c: suppose an observer measures a body passing by at 0.5c and then this body emits a pulse of light- surely in both the frame of the observer and the frame of the emitter the speed of said light pulse will be c?

According to Einstein's premises, yet, it will be c, and I didn't say otherwise. But I said that it would distance itself from said moving object at some rate other than c.

I had the object moving east and the light west, so after a second, the two are 1.5 light seconds apart. Sure, in the frame of the object, the pulse is 1 light second away after a second, but the object isn't a moving object in that frame.

One final request, Halc, refresh my ageing brain as to the conflict between Maxwell and Newtonian mechanics.

Maxwell's equations are frame invarint under Lornentz transformations. Newton's equations are not. That's the best I can do. I'm actually not familiar with Maxwell's equations, and I don't do electrical problems because I'd get things wrong.


and I've never detected signs of aging brain from you.

[paul cotter (OP)]

Halc, I don't want to be argumentative as I am very happy with all the replies but you did say "light does not radiate from a moving object at c" in reply #9. Am I missing something?

[paul cotter (OP)]

I don't really care what subforum it is in, as long as I get the required answers, which I have. However I will defer to your opinion, BC, and if the mods want to relocate it, that's fine with me. As you know it started out as a general posting question before evolving into a physics question.

[Halc]

you did say "light does not radiate from a moving object at c" in reply #9. Am I missing something?

Semantics
I was perhaps ambiguous in my reply.
The separation between moving object and the light pulse increases at the rate of 1.5c in my example. The light pulse of course is moving at c, the rate of change in its separation from the point in space where it was emitted.

As for moving the topic, I can do that, but I would like to re-title it at the same time, and that means you have to select a title more appropriate for the main sections. I suggested one, but it wasn't clear if that conveyed your actual question.

[paul cotter (OP)]

Thanks Halc, you may rename it as you choose and move it to whatever subforum  you consider appropriate, I am not fussy about title or location. You may also wish to delete the non-physics posts.

[Halc]

Thanks Halc, you may rename it as you choose and move it

OK, this is the new topic, split off, leaving not much in chat. A post or two here could have been left with the old thread.


As for me bringing up LET, I did so as an example of absolutism (not relativity), and not because aether had anything to do with the points I was trying to make, which is that one can have an equivalent theory without the postulates of SR, which results in a valid interpretation of the same data, and oddly enough, without relativistic velocity addition.
The reason nobody seems to go with any form of absolutism is because it renders it impossible to measure any distance or duration. Lacking a test for the absolute frame, all empirical tools give wrong values.