Subtitle: Understanding Space, Time, and Gravity
The following comments should be most useful to you if you already have familiarity with special and general relativity.
How can we deepen our understanding of space, time, and gravity? Let’s start with a quote from Einstein himself:
“The principle of relativity is a principle that narrows the possibilities; it is not a model, just as the second law of thermodynamics is not a model.” –Albert Einstein*
*(This statement was made by Einstein in 1911 at a scientific meeting in Zurich. In 1911 Einstein was still using “principle of relativity” to mean theory of relativity.)
Why Special Relativity Should Not Be a Template for a Fundamental Reformulation of Quantum Mechanics
Now, if you want the kind of intuition that can only be provided by a model, you can go with a Lorentzian ether theory*, which can be made to be experimentally indistinguishable from Einstein’s relativity (special and general). At least, I’m sure it can be made indistinguishable, I’ve never seen a definitive proof that it can’t be made indistinguishable, and I’ve been told by a respectable mathematical physicist (John Baez) “I suspect it’s probably easy to make up ‘Lorentzian ether theories’ that are experimentally indistinguishable from special relativity.” Then it could be said there are only metaphysical/philosophical/aesthetic or pedagogical differences between these two perspectives. There are probably domains in which each perspective is preferable to the other, but I feel that the ether perspective makes more physical sense. Just because it’s impossible to find the ether rest-frame doesn’t mean you should throw it away! Occam’s razor cuts differently given different values — for pedagogy, I value intuition and speed of comprehension over “truth”. Physics is a “tool”, it’s not the “truth”. Whether an “ether” truly exists or not is of no consequence to me; what matters is the power of the ether tool to help me describe nature. (You might also ask is space-time a physical object that is actually curved, as so many people seem to imply or assume… and how objectively “true” is that? or is that just a mathematical analogy? and what is the physical interpretation of the metric? If you use curved space-time as your model or to guide your intuition, I’d say that’s even less metaphysically justified than an ether model.)
*(The Lorentian ether theory I have in mind, at its most fundamental level, views the universe as excitations in quantum fields or a lattice of quantum oscillators, where there is a frame of reference at rest with respect to the lattice. However, the properties of the oscillators and their interactions make it impossible to find this rest frame. So an “ether” is a quantum field viewed as a physical entity and the luminiferous ether is the photon field.)
For a pretty good explanation of Lorentzian ether theory/ies, wikipedia-it: Lorentz Ether Theory
To see support for the ether-perspective given by John Bell (of Bell’s Theorem), you can find it in a google-books preview. (That’s where I read it, in Chapter 9 below.) Here are the sources:
John S. Bell, “How to Teach Special Relativity”, Progress in Scientific Culture 1 (1976)
John S. Bell on The Foundations of Quantum Mechanics
Chapter 9. How to Teach Special Relativity
John S. Bell. Speakable and Unspeakable in Quantum Mechanics. Cambridge University Press, Cambridge, 1987
(You can also find relativity-perspective explanations of the spaceship paradox, but they all seem too complicated and far removed from the simple ether-physics.)
Now, for gravito-electromagnetism (GEM). Just as a 1/r^2 Coulomb force generates magnetism when the finite speed of the mediating photon is taken into account, a 1/r^2 Newtonian gravitational force generates “gravito-magnetism” when the finite speed of the mediating “graviton” is taken into account. So magnetism* is fundamentally an electric-force effect (this is even more obvious in the ether perspective), and gravity must have some analogous “magnetic” effects! Einstein showed that gravity should be non-linear, so we know that the graviton should self-interact. (General relativity also implies that the graviton should be spin 2.) Taking that self-interaction (and spin-2) into account should bring us all the way to the equivalent of general relativity. Presumably, in most of the universe (barring black holes, supernovae, et cetera), all you really need to know about gravitation is GEM to have an accurate description. (I could be wrong, though… we should see if GEM explains the previously “missing” precession of the perihelion of Mercury.) So even without an ether, one can speak of a classical spin-2 graviton field (in flat space-time) that generates gravitation. This gives a (more) physical model than talking merely about metrics and so forth.
You should read the Feynman Lectures on Gravitation (or just the foreword/preface and introduction) to see this particle perspective discussed at length. (Brian Hatfield, who wrote the introduction, is the same person who wrote Quantum Field Theory of Point Particles and Strings, the book that helped provide a key insight that went into Chris’s and my research in QFT. For that, see the paper WTF is QFT? [pdf] in my Writings)
I wrote a very detailed (but unfortunately not-pedagogical) paper that showed how you can work backwards from the full Einstein gravitational field equations to get the gravitational analogue of electro-magnetism. See page 11 of the GEM paper and compare equations 8&9 with equations 10&11): Gravito-Electromagnetism [pdf] in my Writings
I think since the ether perspective makes more sense, it will be more useful in developing a quantum theory of gravity (and an intuitive perspective of any quantum field theory). However, it seems that the obstacle to this view is the EPR paradox and Bell’s Theorem, since they seem to rule out “local” theories and ether uses local interactions only.
P.S. When I’m feeling artful, I use “aether”, and when I’m feeling practical and concise, I use “ether”.