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The speed of light is constant in all inertial frames of reference.

I've just begun studying relativity and my question is rather elementary.
Well, Einstein did consider this postulate as the basis for his theory but my intuition says something slightly different.
I tried to get to this postulate given that the Theory is true. The latter says that it is impossible to travel at $c$, and as I keep increasing my speed, it becomes more and more difficult to accelerate myself and I end up travelling only at a fraction of $c$(let's say...$c/1000$? although this is practically unfeasible too).
Well, couldn't we say that in my frame, the speed of light(assuming that it's travelling in the same direction as I am), I witness the photon travelling at $\frac{999}{1000}c$. But that's almost as good as $c$!
My point is, what if the second postulate is a result of the fact that one can hardly reach such speeds, so this approximation always comes out to be almost $c$?Have I thought of this correctly?Or have I got it all wrong?

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    $\begingroup$ Postulates, laws, principles,...in Physics theories are extra axioms in order to pick from the more general mathematical solutions the ones fitting the data and observations and define the units and forms, so that the physics theory is predictive of new data. $\endgroup$
    – anna v
    Feb 13, 2022 at 18:29

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"[W]hat if the second postulate is a result of the fact that one can hardly reach such speeds [...]?" But the second postulate doesn't depend on such thought-experiments. Nor was it very strongly supported by real experimental evidence at the time Einstein proposed it. It was a bold postulate that, together with the Relativity Principle, generated a new kinematics and dynamics, cleared up several perplexing issues in electromagnetism in a most elegant way, and made many radical predictions that have been born out by experiment. That's why most physicists accept the truth of the second postulate!

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That's an ingenious suggestion, but I am afraid that your intuition is not supported by the experimental evidence. You might find it interesting to Google 'experimental tests of special relativity', and to read about experiments at places such as CERN where particles are accelerated to very close to the speed of light.

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Albert Einstein's second postulate, that the light speed c is constant in all frames of reference, means in fact that c has no particular physical meaning; it is rather an artifact of our choice of the measurement units for distance and duration. If we chose to measure distances in light-seconds instead of meters, c would drop out of all equations! Special Relativity teaches us that our world is 4-dimensional, and that time- and space-coordinates may mix up, depending on our spacetime viewing angle. What is not mixing up is causality; it can never be reversed through a mere choice of the spacetime viewing angle.

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