|Do we really understand Magnetic Resonance?
by Stanislav Sykora (Extra Byte, Castano Primo, Italy)
delivered at the MMCE 2011 meeting (Magnetic Moments in Central Europe),
March 16-20, 2011, Tatranská Lomnica, Slovakia
Please, e-mail me your comments and questions. I will collect them verbatim and answer any questions on this page.
A true Philosopher's answer to the title question would be "We don't, of course". Science is not about a real understanding of anything, but rather about describing and developing models for predicting the outcomes of experimental and real situations. But I am not a Philosopher, I am a mere MR physicist and I think that I do my job reasonably well. I can tell the engineer how to build an MR instrument of any kind. For the chemist I can simulate nD spectra of fairly large molecules. For the physician I can devise ways to produce MR images. I have nothing to be ashamed of! Yet, all the time, I have this nagging doubt: do I really understand the phenomena underlying my ascertained abilities? And even before the Philosopher starts sneering, I know that I do not.
Which vexes me, because I am sure that if I understood things a bit better, I could do a better job.
So this talk is about those aspects of Magnetic Resonance (MR) which, I am afraid, nobody really understands. I do not address ontological enigmas (like why all protons are exactly the same), just questions for which I feel there should exist pragmatic answers.
Questions like these:
- Why do we have to use different explanations for different MR phenomena?
- Which aspects of MR are undeniably quantum and cannot be described classically?
- Are Bloch equations classical? And is it sensible to try and derive them from quantum theory?
- Is electromagnetic radiation involved in magnetic resonance? If so, is it true or virtual?
- Does spontaneous emission from spin systems exist and, if so, what is its entity and properties?
- Is MR a near or a far phenomenon? Is remote excitation and/or detection possible?
- Can an FID be really described as coherent spontaneous emission? What about CW-NMR?
- Do we understand the interaction of a spin ensemble with a nearby conductor?
- Which phenomena can be described considering an isolated spin, and which can't?
- What is the role of spin relaxation processes in all this? Are they essential or marginal?
- What is the role of time-averaged Hamiltonians in magnetic resonance?
- Can an FID be described as a sum of individual quantum transitions?
- What gaping holes in quantum physics does all this uncover?
- What does all this tell us about the ontology of photons?
- Can MR throw any new experimental light onto basic aspects of natural philosophy?
Since an in-depth discussion of all these topics is impossible in the brief time of a talk, I will present a list of them and then focus on the few where I think I can add some insights.