What does Max Planck mean in this quote?

[nonequilibrium]

In view of all these results - a complete explanation would involve the inclusion of many more well-known names - an unbiased critic must recognize that the quantum of action is a universal physical constant, the value of which has been found from several very different phenomena to be 6.54 × 10-27 ergs secs. It must seem a curious coincidence that at the time when the idea of general relativity is making headway and leading to unexpected results, Nature has revealed, at a point where it could be least foreseen, an absolute invariable unit, by means of which the magnitude of the action in a time space element can be represented by a definite number, devoid of ambiguity, thus eliminating the hitherto relative character.

out of "The Origin and Development of the Quantum Theory" given somewhere in the 1920's. If you type the title in google, you can read a html copy

Spoiler

If you type "max Planck history.mcs.st development quantum theory" into google, the 2nd hit is a free html version of the text (it's not long)

As a fun historical sidenote, the following quote seems amusing in hindsight:

A question, from the complete answer to which we may expect far-reaching explanations, is what becomes of the energy of a light quantum after perfect emission? Does it spread out, as it progresses, in all directions, as in Huygens's wave theory, and while covering an ever-larger amount of space, diminish without limit? Or does it travel along as in Newton's emanation theory like a projectile in one direction? In the first case the quantum could never concentrate its energy in a particular spot to enable it to liberate an electron from the atomic influences; in the second case we would have the complete triumph of Maxwell's theory, and the continuity between static and dynamic fields must be sacrificed, and with it the present complete explanation of interference phenomena, which have been investigated in all details. Both these alternatives would have very unpleasant consequences for the modern physicist.

 

[unusualname]

He means that his constant is the same for all observers in the universe.

Planck liked the idea that alien intelligences anywhere in the universe would all agree on the value of this fundamental constant, there's an entertaining essay which highlights Planck's obsession with Absolutes by J Heilbron in the collection "Quantum Theory at the Crossroads" (Evans & Thorndike, Spinger 2007)

 

[nonequilibrium]

Hm, but the way he says it, it's like he's implying relativity made us believe our physical constants were less "constant" than we thought, but isn't it quite the opposite? The theory of relativity says all constans in the laws are constant (unlike the speed of light was thought to be before the theory of relativity)

Then again it does make me think: imagine you're looking at an object passing you and thus being contracted length-wise: if you were to measure his "planck's constant", would you get the same as "your own" constant?

 

[unusualname]

That's an issue with theories predicting a minimal length scale (eg planck length), since then, as you point out, you have to specify which reference frame the length is defined in. (You can cheat and propose it as a separate postulate, eg Doubly special relativity)

But I think Planck was just proud that he had discovered a universal invariant constant just like the speed of light (and perhaps the Gravitational Constant) which seemed to fix some properties of the microscopic in exact discretised quantities, at a time when relativity of the observer was such an important new idea in macroscopic physics.