Hi ICANT,
As promised I'm going to deal with your position on postulate 2. On several occasions you've asked questions along the line of the one below.
ICANT writes:
Do you believe the pulse of light can add the forward motion of the train as the basketball does of the player bouncing the ball?
No, I don't.
As I understand it, you believe that postulate #2, as expressed by Einstein, does not allow the motion of the light source to affect the speed and direction of a light beam. Accordingly to you, if we mount a laser pointer vertically on the floor of a high speed moving vehicle, the beam will not be vertical.
Further you've suggested that earth's motion through space might result in the light clock failing to function on earth. You definitely do not agree that a light clock can function in a vehicle moving at a constant velocity of 0.5c. You ignore experimental results contrary to this postion as if they were not even pointed out.
I agree that light beam's do not take on the motion of the source, but the consequences of re not what you suggest.
Let's take peak at postulate 2 from the translation of Einstein's 1905 paper, and then your interpretation of it.
Quoted from your post in
Message 653
quote:
2. Second postulate (invariance of c)
As measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body.
Quoted from a 1923 translation of Einstein's paper
quote:
...also introduce another postulate, which is only apparently irreconcilable with the former, namely, that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.
You interpret the second postulate to require that both the speed and direction of a light beam must be constant in all reference frames.
As a consequence, you indicate that if the were different in a moving vehicle, that the light must be taking on the motion of the source. This you say is incompatible with the postulate
On the other hand, I insist that the postulate requires that the speed alone be constant, and that the same light beam will have different directions in different inertial frames exactly as might a slow moving apple.
Let's consider a situation in which we seemingly must agree. We can examine a case in which we remove direction as an issue.
If a space ship traveling at 0.5c emits a light along the same direction as its motion, postulate #2 clearly requires that the speed of light be a constant in all reference frames. Direction is of course not at issue here, so constant velocity requires only constant speed.
But the speed of light of the transmitted light must be exactly "c" in the inertial reference frame in which the ship's velocity is zero to postulate #2. (I'll call that reference frame the ship frame). That's what "any inertial frame" as stated in postulate #2 must require.
But how does this work? Does it mean that the light beam is "taking on the motion of the ship". Well, apparently not, because postulate #2 also requires that the velocity of light
also be exactly "c" in an inertial reference frame in which the ship is moving at 0.5c. (Let's arbitrarily call this second frame the "stationary" frame.)
Of course the combined results are highly non-intuitive. We cannot resolve the issue by saying that light "takes on the velocity of the source". That position would seem to explain the results as observed from the ship frame, but it would be completely contrary to the result observed in the stationary frame.
It should be quite apparent that it is the attempt rationalize the result, using the "taking on the motion of the source" rationale, is folly. What seems intuitive is clearly not correct. This is the basis for the error you make in forming your interpretation of postulate #2. While postulate #2 does require that the light source not take on the motion of the source, the coordinate systems for the respective observers
must take on the motion of the observers. And those motions, along with the requirement that the speed of light be constant in any reference frame produce the different observations of angles, duration, and distance in different inertial frames. Of course I haven't yet demonstrated that latter proposition.
So what of situations where light is not along the direction of motion, and what of Einstein's use of the word "velocity" rather than speed. I'm present here several arguments all suggesting that Einstein's postulate does not require that the direction of a light beam be the same in all inertial frames.
The first argument I've used before. Einstein says that the velocity of light in empty space is "c", but c is clearly a non-directional quantity, namely 299,872,458 meters/second. The only reasonable interpretation is that Einstein means that it is the speed of light that is constant as measured in any reference frame.
Secondly, Einstein says the following in his paper:
quote:
Examples of this sort, together with the unsuccessful attempts to discover any motion of the earth relatively to the light medium, suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.
In other words, Einstein says it is impossible to determine earth's motion through space by looking at the paths of light beams. If your explanation were correct however, we could tell whether a space ship were "actually" moving relative to an absolute ether by making careful measurements of the angle of a light beam. Whether your interpretation is right or wrong, it is clearly not consistent with Einstein's postulates.
Finally, Einstein developed relativity from the postulates 1 and 2, and showed that SR was a direct consequence of only those two postulates. We can show using the coordinate equations derived from those postulates that the path of a light beam is not constant in direction for all inertial observers. (Ask for a demo if you need one). Accordingly, we know that this prediction must be consistent with postulate two.
Putting all this together, let's look at an example.
Let's reconsider your mirror/train thought experiment (train moving at 0.25c). You seem to agree that a vertical light beam directed from the ground would not reach the top of the frame at a a point directly above the point it reached the bottom of the frame due to the trains motion. You instead ask me some silly diverting questions about whether I can see the light beam from 10' away. Maybe I could see the path of the beam if the train cabin were full of cigar smoke, but isn't enough that I can deduce what the path might be by detecting the point where the light beam hits the top frame? Seriously, do these types of questions have any point at all?
But let's consider things from the coordinate system of an observer inside the train. In that frame of reference, the coordinates of rear and front walls of the train have fixed values, yet the light beam reaches the upper frame of the train at a distance further from the front wall than the point at which the light beam reaches the bottom frame. In the train observers coordinate system, the light beam travels at a diagonal, yet the same beam also travels exactly vertically from the ground frame of reference.
I suppose there is yet another argument to consider. It's one thing to disagree with experts about whether SR is correct. It's quite another thing to suggest that those physicists never actually understood SR at all, but that you, who still struggle to understand the coordinate systems of inertial frames, really do understand Einstein's 1905 paper. ICANT, who thought Einstein's labeling frames as "K frames" meant that those frames worked differently that k and k' frames, despite Einstein's own words to the contrary. I find the prospect that you are a singular expert on SR to be not worth serious consideration and so should you.
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