I would call it new physics. Getting a relativistic electron bunch to coherently radiate is pretty novel.

I forget who said it, but there is a quote in physics along the lines of "we already have a theory of practically everything, but it is so complicated that we can use it to predict almost nothing."

All of basic principles that underlie our everyday experiences don't need quarks and gluons. They're governed by standard quantum mechanics which was fleshed out in the 20's. However, just because we have Newton's laws of the subatomic world doesn't mean we can answer interesting scientific questions with them. I'd even say that people that call something like the standard model a "theory of everything" are a little naive since you can never use to it make predictions outside of the most simple systems.

That area (pushing ahead away from model systems) is the complexity frontier of physics and is where I'm most excited for advancements. It's where we can write out the equation that describes high-TC superconductors, but it has so many dimensions that we can't solve it and make one that works at room temperature. Is that engineering or physics? I'd call the scientific problem of figuring out the laws that emerge from quantum mechanics for these complicated systems physics.

In a similar vein, Maxwell "discovered" the free electron laser in the 1800s. He wrote down the laws of E&M and then everything in the world was solved. That's clearly a dumb way to think of it though, and the new physics that was discovered were the emergent laws that fall out of it when you apply those laws to a system of relativistic particles.

In particular, the emergent phenomena that had to be discovered for the free electron laser (FEL) was the micro-bunching instability. It's a subtle interaction between relativistic particles and a field of photons that will feedback and cause them to bunch together while dumping their energy into the field of light producing the coherent laser radiation. It was that phenomena and the realization that it could be used to create a powerful laser that are the new physics here.

On the other side, the advancement of the FEL to the point that it could make x-rays requires a lot of other new physics to create the machine. This is where there is a bit of a blurred line between physics and engineering. However, I'd still call what we're doing (developing new laws of physics that govern complex systems) physics. Just because we use them to build a machine doesn't make them engineering any more than a solid state physicist discovering a new phenomena in semiconductors and then Intel taking advantage of it in their next generation of processors.

In order to REALLY understand our physical world, we can't write down reductionist laws and say that we're done. It's also notable that the two developers of the free electron laser are rumored to be in the running for the Nobel prize in physics, so it's not just me who thinks that they are physicists, not engineers.

Edit: BTW, here is a great paper that reviews the physics that makes a free electron laser work if anyone is interested.

Huang, Z., & Kim, K.-J. (2007). Review of x-ray free-electron laser theory. Physical Review Special Topics - Accelerators and Beams, 10(3), 034801. https://doi.org/10.1103/PhysRevSTAB.10.034801

> I forget who said it

Paul Dirac. The quote is:

"The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact application of these laws leads to equations much too complicated to be soluble. It therefore becomes desirable that approximate practical methods of applying quantum mechanics should be developed, which can lead to an explanation of the main features of complex atomic systems without too much computation."

Engineering is more about process and process improvements. If an engineer is breaking new ground they are really acting in a capacity closer to being a physicist.

Consider the famous case of Student's T Distribution - it was discovered via engineering, but it is clearly work that was done in the domain of mathematics rather than engineering.

That which is new is science. That which is re-learning an old lesson is engineering.

Science is the process of producing new knowledge and new questions from old questions, under consumption of coffee.

That seems rather dismissive of engineering practices and ignorant of scientific development.

If we want to get into the philosophical difference between engineering and physics, engineering at its core is solving problems and science is answering questions. There is obvious overlap and no clear distinction.

What is dismissive about it? There are a great many brilliant engineers who never do anything particularly novel.

In contrast, it is quite hard to be a brilliant scientist without doing something novel.