[Airthumbs (OP)]

I know this is obviously a very abstract question, allow me to explain more of my angle so to speak..

Newtonian Physics was seen as the breakthrough in Gravity being generated by mass itself and everything was all huncky dory until Mercury started raising more questions...

Following the subsequent toppling of this endorsed theory at that time Einstein was able to produce the answer which led to our little Mercury fitting nicely back into the scheme of things.

This doesn't stop progress, it accelerates it to new boundaries of science for which now we are in the same place before.  Using analogy I could say our little Mercury has returned to haunt us again. 

We know there is dark matter and dark energy out there, we know there is a quantum world where the commonly used expression "all laws of physics break down" seems to apply. 

Coincidentally at the same period of scientific evolution we have discovered some remarkable things about our Universe that even Einstein himself supposedly refuted at the end of his illustrious career.  Black holes and gravity waves for example.

Now we are being told that there are galaxies that should not exist, particles that can be in two places at the same time, the possibilities of a bouncing universe banging along like an engine ticking over, expanding and contracting until it finally runs out of energy in a truly vast amount of time from our perspective, is more than even a science fiction novel could contrive.

And so I ask the question, if E=mC2 fails to describe the quantum world then it is wrong?

[Kryptid]

And so I ask the question, if E=mC2 fails to describe the quantum world then it is wrong?

In what way does it fail?

[alancalverd]

It is wrong in the same sense that "yellow, soft, curved and edible" describes a banana but not a motorbike.

People who get upset when we discover something that isn't described or predicted by current models and prejudices are not scientists. The "laws of physics" are nothing more than mathematical statements of what we have observed to date, and there's no reason why they shouldn't evolve as we investigate bigger and smaller phenomena, or (as in Einstein's case) simply review our axioms. 

[Origin]

Following the subsequent toppling of this endorsed theory at that time Einstein was able to produce the answer which led to our little Mercury fitting nicely back into the scheme of things.

Newtonian physics was not toppled.  Every physic student starts out learning Newtonian physics.  In the Newtonian gravity model gravity is treated as a force.  This model of gravity works quite well in most circumstances.

We know there is dark matter and dark energy out there, we know there is a quantum world where the commonly used expression "all laws of physics break down" seems to apply.

I don't know where you heard something like this but the reality is that the "quantum world" is governed by quantum physics which is obviously physics.

And so I ask the question, if E=mC2 fails to describe the quantum world then it is wrong?

Of course not.
E=mc^2 also fails to describe the trajectory of a cannon ball, but that does not mean the equation is wrong.

[evan_au]

"all laws of physics break down"

This expression is usually applied to:
- The singularity at the centre of a black hole. Einstein's General Relativity breaks down at this point. Einstein saw this as a sign that his theory did not apply there [perhaps some day, Quantum Theory may eventually provide a solution?].
        - But this is a bit academic, because:
        - The singularity is surrounded by a black event horizon
        - you can enter the event horizon, but you would be spaghettified before you got close enough to examine the singularity
        - there is no way to communicate your results to anyone outside the event horizon
- The infinities you get when you try to express any Feynman diagram with a loop in it
        - This has been solved by the mathematical process of Renormalisation, and is no longer considered a breakdown. https://en.wikipedia.org/wiki/Renormalization
        - These infinities do not appear when computers simulate quantum field theory, because (as I understand it) the finite grid prevents high-order terms from the Feynman diagram from appearing. [This may point to some new form of quantisation?]
- The infinities you get when you try to quantise gravity near a black hole. These do not go away through renormalisation, and this is currently an unsolved problem in Quantum Field Theory.

here is a quantum world where the commonly used expression "all laws of physics break down" seems to apply

Quantum Theory and General Relativity apply quite reliably throughout almost all of the universe (except close to the centre of a black hole).
- With our current level of knowledge, Einstein's Cosmic Constant seems to describe the behaviour of Dark Energy pretty well. ESA's Euclid spacecraft is collecting more data from distant galaxies to see if there is some historical divergence from constancy in this Cosmic Constant. See https://en.wikipedia.org/wiki/Euclid_(spacecraft)
- We don't know what Dark Matter is, but at present it doesn't seem to violate any fundamental laws of Physics. Scientists were hoping to discover some candidates at the LHC, but that hasn't happened (yet) - but it may yet turn out to be some form of weakly-interacting subatomic particle (which is what most cosmologists guess at present).

Einstein himself supposedly refuted at the end of his illustrious career.  Black holes and gravity waves for example

I think "refuted" is the wrong word.
- Within 10 years of Einstein's publication of the General Theory of relativity in 1915, various people pointed out that this could predict a black hole solution. It's true that as late as 1939 Einstein was dismissive of such ideas; he died in 1955, before Black Holes became established in theory and observation. But it was Einstein's theory which predicted them.
https://en.wikipedia.org/wiki/Black_hole#General_relativity
- Again, it was General Relativity which predicts Gravitational Waves. But these effects are very subtle, and Einstein in 1939 thought they could not exist. But he subsequently published a paper showing that they were feasible. https://en.wikipedia.org/wiki/Gravitational_wave#History

I think it is fair to say that it took a long time for the implications of Einstein's General Theory of Relativity to be developed - indeed, it is only with recent supercomputer simulations that various black holes merger scenarios of  have been modelled with any accuracy (eg different masses, different spin axes, etc). That would not have been possible in the slide-rule era, when Einstein did most of his work...
https://en.wikipedia.org/wiki/Slide_rule