LDM wrote:
- “If, as in those days, you believed in Maxwell and the wave equation, it was a VERY well motivated question to ask ‘what is it that is waving’ when light waves (not photons) propagate.”
Dear LDM,
the question “what is waving” was never physically motivated and only the people who misunderstand not only relativity but also the rudimentary philosophy of physics can say something else in 2006.
The aether is recognized as a gigantic dead end, a useless superconstruction, and a wrong theory, and it has always been one. The only positive thing about the aether is that some of the people who believed it in the 19th century have also done some extremely serious and important physics, unlike most of their followers in the 20th and 21st century.
The reality of Nature is encoded in the set of mathematical equations - Maxwell’s equations in this case - and they are the main story and the full story. Naive mechanistic ways to imagine “what is waving” should only be created for children in the kindergarden or other people who have some intellectual limitations that prevent them from understanding that equations themselves can be fundamental and they are fundamental and that the fields can live in empty vacuum.
If someone really needs to hear it, we may always say: Yes, there are gummi bears everywhere that are waving. The gummi bears must be there because some kids just can’t live without them.
Adult physicists should be able to live without gummi bears. The electric and magnetic vectors exist directly at each point of the vacuum, without requiring any additional substrate, and it must be so, otherwise the basic postulates of special relativity would be compromised. In fact, Hendrik Lorentz, despite being known as one of the advocates of aether even after 1905, was credited by Einstein as one of the heroes who have really cured science from the wrong idea of the aether.
In his essays, Einstein argued that it was Lorentz who has explained that exactly one electric vector (E) and one magnetic vector (B) exist at each point of space, even in the vacuum, and all other vectors that had been used to describe the electromagnetic fields were derived concepts.
In other words, Lorentz separated the "clean" description of the electromagnetic field from the "messy" complications that arise from complicated materials; it was a similar act as Newton's decision to set the friction equal to zero in order to go beyond Aristotle. It was Lorentz's emphasis on the vacuum Maxwell's equations that allowed Einstein to see the pure truth: there can't be any aether that fills the vacuum because the beauty and symmetry of the equations, something that is also known to hold experimentally, would otherwise be compromised.
Bert Schroer and the aether
Bert Schroer suggested that noncommutative geometry revives the concept of the aether.
The noncommutative parameter, described by an antisymmetric tensor "THETA^{ij}", indeed breaks the Lorentz symmetry, like the aether. But it also breaks the rotational symmetry, unlike aether, because the tensor is not invariant under rotations of three spatial coordinates. Noncommutativity carries no entropy, unlike aether. The reason of the vanishing entropy is that noncommutativity has no nontrivial microscopic structure, unlike the aether. A noncommutative parameter may be viewed as a spontaneous symmetry violation in a theory that preserves the symmetry - and the original theory already carries all the excitations, unlike the space without the aether.
To summarize: noncommutativity is not the same thing as the aether, unless we want to abuse our terminology.
Motivated and unmotivated theories
Let me be more general about the unmotivated ideas that can be classified as regressive philosophical preconceptions. Take the sound and light. Sound is made of waves in the air. Is it natural to expect the same thing from light?
The answer is No. We only conjectured that there was a material that is waving - whenever there is sound - because we wanted to be able to:
- explain that the sound can have different speed and other properties in different environments and in different inertial frames; the extra material whose properties can change helps
- unify sound with some previous physical theories that rely on the concept of the air, such as the theory of breathing and the theory of winds and hurricanes
Neither of these motivations existed in the case of light. Light has the same properties in each point of vacuum, and as one can figure out by analyzing Maxwell’s equations, it also has the same properties in all inertial reference frames.
The second point is not applicable either because there is no independent experimental reason to think that there exists something such as the aether; for example, there is no aether wind. To summarize, the aether does not explain anything and it violates the knowledge of physics at the end of 19th century. It was never motivated. It was always an unscientific philosophical dogma - or a psychological paradigm that allowed old-fashioned physicists educated as "corpuscular physicists" to think about a new class of phenomena that follow slightly different rules.
It may have been hard for them to learn about the "first known kind of waves" that are not carried by any material with a microscopic structure, but the expectation that every wave must be carried by such a material never had any scientific or explanatory justification.
The absence of experimental or robust theoretical evidence in favor of the aether was a fact that Einstein realized pretty well which allowed him to overcome the limitations of other physicists of his time and the limitations of the laymen and to propose and complete his special relativity.
Hidden variables
Similar philosophical conclusions hold not only for the aether but also for dozens of other fantasies that billions of people believe for purely irrational reasons. Instead of the intelligent creator, consider hidden variables in quantum mechanics. Much like in the case of the aether, there are absolutely no indications that the correct predictions need some extra garbage (hidden variables) or that they depend on some additional assumptions that could change as a function of the position in space. Quantum mechanics works the same way in the whole Universe.
In both cases, we have some obviously universal laws - Maxwell’s equations with the principle of relativity and the probabilistic character of physical predictions - that seem to hold in the whole Universe. This has always strongly indicated, to any rational person, that it was a very bad idea to try to find a more “microscopic mechanism” for these phenomena. Any new structure that one adds beneath these systems - Maxwell’s equations or the probabilistic structure of quantum mechanics - will ruin the universal validity of these laws. It adds arbitrariness. It adds chaos. It makes the predictions less accurate. It is likely to break the symmetries that are known to hold exactly. It should be removed by Occam's razor.
Of course, today we have much more specific ways to prove that hidden variables can’t exist (unless we want to believe that effective locality as we know it is just a gigantic cosmic hoax and an unexplainable global conspiracy) - but the previous paragraph was meant to settle the fact that the aether or the hidden variables were never motivated by scientific arguments - they were always unsubstantiated dogmas of philosophers or other people who wanted to impose their naive ideas on Nature.
But Nature does not care how we think that She should behave. Nature does not care that some people believe in Intelligent Design, the aether or the spin foam, or the indistinguishability of male and female brains. It is scientists’ task to listen to Her and see how She actually looks like instead of telling Her how to behave.I chose the example of the hidden variables because of the name whose meaning is rather general: it is exactly the adjective "hidden" that should always make you think. If something is so perfectly hidden, especially if it is hidden at every point of the Universe, does it really exist? The aether and the hidden variables were so perfectly hidden that they don't exist. And there are arguably many other popular concepts with a similar fate.
