I highly doubt it, but it might deliver some memorable zingers about nude aliens.

There is no relation between the ability to communicate and a shared understanding of our concept of quantum transitions - case in point, our invention of the technology we use to communicate with deep space far predates us learning these concepts ourselves.

I'd also hold that the only thing this plaque could ever give is clear sign of artificial creation, and by virtue the (possibly past) existence of some entity capable of creating it. Maybe they'll get a vague idea of what we look like, but if "their" culture does not commonly depict themselves in 2D as we do, or "they" have vastly different morphologies, even that would be unclear. The context needed to understand our attempt at showing our location might also be lost if the thing went far enough.

Physics is a model of reality. Reality is objective, but the model we have chosen is very much "subjective" (maybe arbitrary is a better term).

It's easy to imagine that another species might have never conceptualized electrons as little balls orbiting around a nucleus. They are neither balls nor are they flying in circles, those are simply abstractions we like because they appeal to the way we perceive reality. The way we conceptualize electrons leads to issues like the wave-particle duality, so it's likely just a local optimum we got stuck in. Another species might not even think of Electrons as being distinct entities, maybe they think of the electron field as one large ocean with some waves in it, or they subscribe to the single electron theory, or something we have never thought of and might never imagine from our perspective.

As a theoretical physicist, yes physics could definitely be subjective between different species. Physics is the way HUMANS describe nature to themselves. I don't doubt that it describes some greater nature outside of us that is invariant, but it is only a description - not the thing itself. Like mathematics it is an anthropocentric conceptualization that has many arbitrary and historically contingent choices in its choice of representation and its chosen objects of study.

How could we ever be certain than another intelligence (whatever that means) would be capable of understanding the intended message? Unless of course we are already starting off with the major assumption that the only things that can be intelligent are things like us. I'm not even sure that intelligent has any meaning aside from denoting behavior "similar to us".

Our understanding evolves, course corrects, spins off etc., we can use some static value as purported from the dark ages or by newtonian or later einstenian points of view. They all are measurably correct for the problems that they are trying to solve for the people who lived during those times. A million years from now would these values still be relevant or be considered as having the same value of importance or will they be replaced by even more finer and precise and contextually different values that could be more precise and more accurate etc.,

The prevalence of the 21 cm wavelength is something they would likely discover should they be in a part of the universe similar to ours. What I find laughable is the means of trying to communicate this wavelength and using it as a means to indicate our size.

Now if you go the other way, referenced 21 cm as a well known quantity (say, by making the plaque 21cm wide and referencing its diameter) and used that to describe the hydrogen spin flip in order to teach or communicate our level of physics understanding rather than depend on a shared understanding, then I'd say it makes more sense.

I really wonder how future archaelogists are ever going to decode our timeline. Imagine a meteor strikes, civilization falls apart, and in 20,000 years they dig up a data centre. Even if they get the computers to work and the hard drives are still readable, everything will be encrypted.

What if extraterrestrial "intelligence" didn't have a reason to "evolve" functional equivalents of a visual cortex. Even on earth where having vision gives a distinct evolutionary advantage over non-vision based living forms, species without vision far outnumber those with vision.

Making the data fault tolerant to the discovery by another civilization, its collapse and later rediscovery by another civilization seems a bit of a stretch goal. :)