1 The two postulates of SR

In the book 2 "Spacetime Physics" by Taylor and Wheeler we can read:
All the laws of physics are the same in every free-float (inertial) reference frame (page 55)
The principle of relativity rests on emptiness (page 56) Accordingly to the theory of Relativity, the speed of light must be the same in all free-float frames in uniform relative motion (page 60 - line 1)

In the book 3 "The evolution of scientific thought from Newton to Einstein" by A.d'Abro at page 451 we can read:
"the velocity of light is invariant only for Galilean observers; it is variable from place to place when accelerated observers are considered, and it is also variable in the neighbourhood of matter."

Each of these 3 quotes seem simple at first sight, but become complex when you compare them.

My first thought is what exactly are the laws of physics. The laws of physics can be defined as descriptions of physical processes. This raises the question do we mean all sorts of physical processes or only some.

At page 60 of book 2 we can read:
"All the laws of motion are the same in the one free-float frame as in the other."
Such a sentence is of not much help to answer the question:
"Is the behaviour of a clock at rest the same as a moving clock."

To answer: Yes, based on the first postulate of SR seems too simple. To find the answer based on a real experiment seems to me the primary strategy to follow.

1) Also at page 60 (After page 60 - line 1) we can read:
"Has observation checked this conclusion? Yes, many experiments demonstrate it daily and hourly in every particle-accelerating facility on Earth."

2) Also at page 60 of book 2 we can read:
"Values of the speed of light as measured by laboratory and by rocket observer turn out to be identical."
This defines an experiment.

3) Immediate there after we can read;
"This agreement has cast a new light on light. Its speed rates no longer as a constant of nature. Instead, today the speed of light ranks as a mere conversion factor between the meter and the second. etc. This decree assumes the invariance of the speed of light. No experimental result contradicts this assumption."

All these 3 cases are most probably true, but if you trully want to understand you must know the details of how these experiments are performed. Unfortunate no such detail is supplied.

In fact IMO if you want to do science you should start from bottom up from experiments. Even better from experiments which are slightly different. The problem is to perform experiments involving lightsignals is very difficult. For example, it is very difficult to measure the speed of light accurately in a laboratory and even more difficult in a rocket. In a rocket you need a clock and to measure the speed of light which a clock which also uses lightsignals, IMO seems nearly impossible.

The conclusion, that certain experiments are impossible is of extreme importance, because its creates a type of physical limit. The next step to define a certain number of concepts, we all agree upon, seems to me a healthy solution.

To start from the idea that the universe is empty and that we can have frames, which are in relative linear motion, does physical not make sense because in fact each frame requires at least one object and in order to create movement, forces are required, which introduce accelerations which inturn create frames, which are not in linear motion.

It are the laws which describe the objects (processes), in these non inertial frames which are the most important, to unravel.

Just some thoughts.

Nicolaas Vroom

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2 The two postulates of SR

>	In the book 2 "Spacetime Physics" by Taylor and Wheeler we can read: [... a different PARAPHRASE]

>	In the book 3 "The evolution of scientific thought from Newton to Einstein" by A.d'Abro at page 451 we can read: [... yet another different PARAPHRASE]

>	[... discussion bemoaning the differences among these PARAPHRASES]

All is resolved when you use the ACTUAL postulates from Einstein's 1905 paper:

1. The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion.

2. Any ray of light moves in the "stationary" system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body.

[Earlier in the paper he defined the "stationary" system of coordinates to be an arbitrary inertial frame. So "stationary" is a label without semantic content.]

Note in particular that Einstein's statements are more precise than any of the paraphrases. As I keep saying: physics is subtle and to understand it one must be precise in thought and word.

Tom Roberts

3 The two postulates of SR

4 The two postulates of SR

> >	All is resolved when you use the ACTUAL postulates from Einstein's 1905 paper: 1. The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of co-ordinates in uniform translatory motion.

5 The two postulates of SR

Just what it says implicitly: all of them.

>	Physical systems don't undergo changes as a result of any law.

You have to read a paper written in 1905 in the context of 1905. That's how they spoke and wrote back then. Today we would replace "laws" with "models" and slightly re-phrase the sentence to be modeling the change of state, rather than determining it.

>	The evolution of the universe is not controlled by any law.

No. But in today's nomenclature he is discussing a model of how changes of state occur.

>	The evolution of the universe partly is a cooling process which involves different stages.

He is not discussing that at all. Implicitly he is describing an abstract world that is very much simpler.

As did Newton, and essentially every physicist outside of cosmology.

>	All of this from a physical point of view has nothing to do with any co-ordinate system.

Yes. That's why the model must not depend on one's choice of coordinate system. In today's nomenclature this postulate requires models to agree with this rather obvious property of the world we inhabit. (Though he is limiting the discussion to inertial frames.)

>>	2. Any ray of light moves in the "stationary" system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body.

>	IMO the simplest situation is to consider a "stationary" system and a stationary body both at rest.

Hmmm. This explicitly says that the emitting body need not be at rest, but its emitted light will still travel with speed c relative to the "stationary" system.

Remember that earlier he defined the "stationary" system as an ARBITRARY inertial frame, with "stationary" being just a label with no semantic content.

>>	Note in particular that Einstein's statements are more precise than any of the paraphrases.

>	Specific the first postulate is very difficult to understand, what is meant.

One must read it in the context of 1905. See above.

>>	As I keep saying: physics is subtle and to understand it one must be precise in thought and word.

>	I fully agree with you. The problem it is easy to write that the speed of light is every where the same. It is very difficult to explain why that is true.

Today science does not attempt such "explanations", because we now recognize that we are making MODELS, and such "explanations" are valid only in the context of the model. Attempting to discuss "why" the model works is outside the scope of physics, and enters into metaphysics and philosophy.

>	Experiments to demonstrate the same are even more 'complex'

6 The two postulates of SR

That's a translation of his 1905 paper. It must be read in the context of 1905. It is also a rather archaic discussion of SR, and more modern references would be better -- we have learned A LOT about this theory since 1905.

>	[...] In short the clock which in absolute sense is at rest runs the fastest. All other clocks run slower.

This is wrong, as has been discussed several times in this thread. Note also that this is your own fabrication, because the reference has no such thing as "absolute sense".

Clocks DO NOT "run faster or slower" -- clocks ALWAYS tick at their normal rate. Anything else would violate the first postulate of SR.

This was not made crystal clear in the reference, and Einstein made a mistake related to this (a clock at the equator does NOT "go more slowly" than one at the pole -- it is only MEASURED to do so). Note that here he assumed the earth is exactly spherical.

Moving clocks, however, are OBSERVED/MEASURED to tick slower than identical clocks at rest. This is called "time dilation", which affects OBSERVATIONS AND MEASUREMENTS, not clocks -- it is a geometrical projection, not any "change" in clocks' tick rates. It can, of course, have physical consequences, such as pion beams much longer than the pion lifetime would imply.

Measure the width of a desk with a ruler. The ruler inherently performs a geometrical projection of the distance being measured onto itself -- ALL instruments necessarily make such projections. If you tilt the ruler relative to the desk you will get a different answer, but neither desk nor ruler was "changed". Moving clocks are "tilted" (in spacetime) relative to clocks at rest, but like ruler and desk they are not "changed".

Tom Roberts

7 The two postulates of SR

8 The two postulates of SR

Tom,

I would emphasize that the clocks that are OBSERVED/MEASURED to be runing fast/slow are observed from a different reference frame (either an inertial reference frame or different gravitational "potential"). THe basic confusion is the mistaken idea that the observed clocks are actually different. That is your point, of course, but the key understanding is that observers in DIFFERENT reference frames observe different things even when observing the same object.

Rich L.

[Moderator's note: This thread, if not this specific post, is now going around in circles. Unless there is some extraordinarily original insight, it is probably best to close it. There is no dispute among experts about what observers measure, nor about the cause of the differences. There is also a huge literature on the topic. -P.H.]

9 The two postulates of SR

This posting is still in discussion On Thursday, 29 August 2019 21:02:27 UTC+2, richali...@gmail.com wrote:

>	On Wednesday, August 28, 2019, at 2:31:35 PM UTC-5, Tom Roberts wrote: ..

> >	Moving clocks, however, are OBSERVED/MEASURED to tick slower than identical clocks at rest. This is called "time dilation", which affects OBSERVATIONS AND MEASUREMENTS, not clocks -- it is a geometrical projection, not any "change" in clocks' tick rates.

See below.

>	That is your point, of course, but the key understanding is that observers in DIFFERENT reference frames observe different things even when observing the same object.

The issue is not what observers see in different reference frames. (It is obvious that most observers observe differently) See below.

>

[Moderator's note: This thread, if not this specific post, is now going around in circles. Unless there is some extraordinarily original insight, it is probably best to close it. There is no dispute among experts about what observers measure, nor about the cause of the differences. There is also a huge literature on the topic. -P.H.]

I doubt if all agree about the cause. At least not within this discussion. For me, the cause of the difference in the behaviour of clocks lies in acceleration. This becomes the most obvious if you move a rod of a fixed length l in a straight line over a set of clocks also a distance l apart. At the front (and back) end of the rod, there is an observer and a clock. All the clocks (including the two attached to the clock) are synchronized in the rest frame. The task of each observer is to write down, when his position coincides with the position of a clock at rest, both readings of his moving clock and the clock at rest. At the end of the trip (under the assumption that the speed of the rod varies) when the results of both observers are compared (vertical), they will be the same. The reason is that both moving clocks will undergo exactly the same forces and accelerations, as such their behaviour is the same. Comparing the results of each coincidence (horizontal) the readings of the moving clocks are lower (runs behind) as the corresponding clocks at rest. In general, this is because the forces are different, but specific because the lightpath of the moving clocks is longer. It is physics.