You don't know the difference between two cars moving at approximately the same speed together down the highway and one moving at a different speed relative to the other????
The 'same approximate velocity' as what?
Each other. That is what moving in relative motion to another system means.... moving at the same approximate velocity....
Can you provide a quote, reference, or link where he says this?
Its the entire postulate behind special relativity.....
Special relativity - Wikipedia
"
Special principle of relativity: If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the
same laws hold good in relation to any other system of coordinates K' moving in uniform translation relatively to K"
Objects moving at widely different velocities must have their physics transformed from one frame to the next. An object moving at a constant velocity of 100 mph and one moving at a constant velocity of 1/2 of light speed, do not share the same physics. They don't even share the same units of time or lengths of measuring rods.....
The clue is in the word 'moving' - this means they are in motion relative to us; i.e. they are in different frames.
But as Einstein explained the major point was how fast they were moving with regard to you....
"Einstein explained that when two objects are moving at a constant speed as the
relative motion between the two objects, instead of appealing to the ether as an absolute frame of reference that defined what was going on. If you and some astronaut, Amber, are moving in different spaceships and want to compare your observations,
all that matters is how fast you and Amber are moving with respect to each other."
Reference frames only have meaning in terms of relative motion; it is nonsensical to talk of '
frames not moving in relative motion'. If something is not in relative motion with respect to you or some other thing, it is in the
same frame as you or that other thing; this is known as a
proper or comoving frame.
I'm pretty sure I explained all this to you months ago.
No, you still have not figured out the difference between frames moving "with" the same uniform translation, and frames moving with relative motion to each other but not the same uniform translation.
Let us repeat:
Only in frames moving at constant speed relative to one another, are the laws of physics the same. I.e. frames undergoing no acceleration or not moving at vastly different velocities.
Frames that undergo acceleration have fictional forces.
This was made clear in his General Relativity.
Einstein's General Relativity Theory: Gravity as Geometry - dummies
"Einstein’s basic principle was that no matter where you are — Toledo, Mount Everest, Jupiter, or the Andromeda galaxy — the same laws apply. This time, though, the laws were the field equations, and your motion could very definitely impact what solutions came out of the field equations."
So the same laws applied no matter your place in the universe as long as the speed was constant relative to one another or did not greatly diverge. And then he made it clear that your motion could very definitely impact what solutions came out of the field equations.
Since those galaxies are accelerating away from us, or in the case of our local group towards or away from us, they are not under constant velocity, but acceleration, and hence their motion impacts what solutions come out of the field equations.
Even the earth is undergoing acceleration, hence the fictitious force of the Coriolis and centrifugal force.
If the laws of physics were the same between an object moving at 100 mph and one moving at 1/2 of c, no transformations would be required between them, they would already be the same.....
You MUST convert their measurements to your measurements. Which if you bother to think about it means they are not measuring the same thing you are. Their second is not the same duration as your second. Their ruler is not the same length as your ruler. The time and distance they measure for light is not the same time and distance you measure for light. It is RELATIVE to the velocity of each frame......not the same in each frame.
They remain constant to each frames point of view, because frame A calls his ticks a second just as frame B calls his ticks a second, even if neither frame is of the same duration.... The second in frame A is not the same as in frame B, hence frame B must be transformed to the same time unit of frame A in order to compare them...
To put it simply 1 light year to frame A is not the same distance as in frame B. Frame A does not measure the same charge of an electron as does frame B, he can't, his clocks and rulers are not the same duration or length. His electron charge is relative to his energy content as B's is relative to his energy content. they are NOT the same...... This is what RELATIVE means, each to its own frame due to its own energy.
If everything was the same and truly constant, there would be no need of Relativity and transformations..... If frame A equaled frame B then conversion would be unnecessary and they would not be Relative, but equal.....
Only in frames moving with the same approximate velocity are things equal. Then no conversions are necessary.....
And such is why you will never find an explanation in mainstream as to why the speed of light always calculates to c. They have no answer, because they treat this frame as absolute, not relative....