What is Space?
D.M. Marett (2010)
Is the vacuum of space a real object unto itself, or is it simply emptiness, a nothing that does not exist in the absence of matter? From the viewpoint of Descartes and Leibniz, without physical objects, "space" would be meaningless because space is the framework upon which we understand how physical objects are related to each other. Sir Isaac Newton, on the other hand, argued for an absolute space ("container space"), which can continue to exist in the absence of matter.
In the 19th century, James Clerk Maxwell introduced the idea of an electromagnetic ether - a medium of space capable of supporting the structures of the electric and magnetic fields or light. Maxwell's idea of space was that it had a density and a transverse elasticity - when stressed it would impose a restoring force, much like a solid. This made it suitable for the transmission of transverse waves that waved in the body of the medium. It was Maxwell's challenge for an experiment to detect this medium that led to the Michelson and Morley (M&M) experiment of 1881 and 1887. There is good reason to believe that the M&M experiment was the wrong one for the job, but at the time it confounded the understanding of researchers. This frustration opened the way for the introduction of new theories that might solve the dilemma, such as Einstein's relativity theory.
Initially, Einstein's theory of special relativity considered an ether of space "superfluous" and seemed to side with Leibniz, in that relational space is the space that defines the relations between objects, with the implication that it cannot exist in the absence of matter. It removed the medium of space that Maxwell conceived of for the light waves to wave in, as this would provide a preferred reference frame for light, and in relativity theory there can be only relative reference frames for light. Einstein asserts that the Maxwell-Hertz equations are simply valid in "empty" space. In this interpretation of relativity (Dynamical Relativist Interpretation), light and fields must consist of particles - since there is nothing else. Energy cannot exist in space in the absence of material objects. Space is not a "thing", it is an intellectualization of the relation of objects to one another. It is the ultimate "nothing." But this has a flaw - space has "properties" that cannot be denied - it has non-zero magnetic permeability and electric permittivity, it can support energetic structures such as electric fields, gravitational fields, magnetic fields - and these fields according to Quantum electrodynamics consist of particles that have arisen spontaneously out of this "nothing". Space has foremost the relativistic attribute that light propagates through it at exactly C.
Repeatability (light propagates at C, magnetic permeability is always 4p x 10-7) implies causality (space provides a structure that excludes all velocities for light except exactly C, excludes all magnetic permeability's except exactly 4p x 10-7). This implies that space cannot be "nothing".
Since Einstein was obliged to admit that space has "properties" a few years later. Einstein's general relativity (1917) re-introduced a concept of absolute space, and in particular, he had to try to explain Mach's concept of inertia. This "ether" of Einstein is not the electromagnetic ether of Maxwell but Minkowski's "absolute worlds" hypothesis grafted to Einstein's geometric concept of gravity. This new theory of the medium (Substantivalism) is not in space so much as it is in time. A medium for electromagnetic waves is missing, the only waves conceived of in the fabric of space-time are gravitational waves.
The Substantivalist Philosophy most consistent with Relativity might be defined as follows:
1) Substantivalism - spacetime is a real substance that exists independently of physical objects. Compatible with perdurantism, eternalism and parminedeanism (most compatible with SR).
2) Perdurantism - Objects are 4 dimensional and extend over time.
3) Eternalism - that all times past, present and future are equally real (coexist).
4) Parminedeanism -Being is fundamentally real; change is an illusion.
5) inertia exists due to an objects motion with respect to spacetime. Spacetime resists non-straight motion.
This is distinct from a Dynamic Relativist philosophy, mentioned earlier, which has for its definitions:
6) Dynamical Relativist Interpretation - spacetime does not exist independent of real objects. Compatible with endurantism, presentism and heracliteanism.
7) Endurantism - 3D objects extend over a spatial region only (not across time) but endure through time.
8) Presentism - that only the present exists.
9) Heracliteanism: - becoming (change) is fundamentally real.
10) Relationalism - inertial effects can only be due to an objects motion with respect to other objects.
General relativity says that space-time has a structure, but this structure is determined by the matter density of the universe. This is how the matter of the universe (the fixed stars) can provide inertia.
However, the substantivalists might say that there can be inertial forces in space devoid of matter (Mach's principle does not hold). The metric field does not need to be a matter field, since space-time exists as a thing in and of itself. Thereby gravitational waves can propagate in space-time alone.
On the other hand, a relationalist might say that the metric field is a matter field, and if the matter of the universe is removed, nothing would be left (space is simply a void). Gravitational waves could be viewed as propagating in the metric field (matter field).
Einstein sees electromagnetic forces as relative to the perspective of the observer and thereby do not exist objectively in an absolute space. This goes against Maxwell's original idea that the magnetic lines of force are real objective structures independent of the observer.
Following all this, if Einstein is correct, then Maxwell's wave nature of light could be replaced with an idea of the electromagnetic energy propagating as singularities (points) - very much like discrete particles. To quote Einstein (1909):
"Anyway, this conception seems to me the most natural: that the manifestation of light's electromagnetic waves is constrained at singularity points, like the manifestation of electrostatic fields in the theory of the electron. It can't be ruled out that, in such a theory, the entire energy of the electromagnetic field could be viewed as localized at these singularities, just like the old theory of action-at-a-distance. I imagine to myself, each such singular point surrounded by a field that has essentially the same character of a plane wave, and whose amplitude decreases with the distance between the singular points. If many such singularities are separated by a distance small with respect to the dimensions of the field of one singular point, their fields will be superimposed, and will form in their totality an oscillating field that is only slightly different from the oscillating field in our present electromagnetic theory of light. "
By reducing the EM oscillation to a point, there is no medium or waves in the conventional sense. There is no good explanation offered why light would have a wave character, would travel at exactly C, or be transverse.
Enter the Quantum Electrodynamics Model of Light:
Does QEM offer any further answers? Feynman continues with Einstein's notion that light is a particle -Feynman refers to the particle existing in space time. Feynman implies that the wave-particle duality of the photon was resolved in favour of the particle. He says quite explicitly - "I want to emphasize that light comes in this form - particles. It is very important to know that light behaves as particles, especially for those of you who have gone to school, where you were probably told something about light behaving as waves. I am telling you the way it does behave, like particles." Feynman , QED, P. 15. He argues that even though wave theory correctly predicts the "why" of partial reflection, and particle photon theory does not, QED is only concerned with predicting the probability of the event and thereby the "why" is not something that the theory can or will explain. P.24 of Feynman's "QED".
There seems to be no explanation as to how the photon carries with it frequency and polarization information. The photon is said to be circularly polarized and to have a frequency f, but in what sense? If the photon is a particle, how is the wave impressed upon its body? It can't be a wave in space, since space is a void (Maxwell's ether is dead). Even if we conceive of a sea of virtual particles to propagate the electric and magnetic fields of the EM wave in space, one would have to believe that the only real particle (the real photon that triggers the photomultiplier) contains all of the frequency information to create the field, so it must be oscillating first to guide the virtual particles. In what way is it oscillating?
To describe fields, the "empty vacuum" must be given a rich, spontaneous inner life. The electromagnetic field in space in QED is said to produce virtual short-lived electron-positron pairs that change the distribution of charges and currents that generated the original electromagnetic field. The ether of Maxwell has been replaced by a medium of virtual particles, miraculous in properties - they can be spontaneously generated and then destroyed, can defy the conservation of energy principle, and the virtual photons can be given mass due to their short range and short lifetime. Virtual particles cannot be detected by definition - if they persist long enough to be detected, then they are no longer virtual. This interpretation is sometimes referred to as the material grid of space - it has been attributed a density that is constant in space with no clumping, and stays constant as the universe (allegedly) expands. In fact approximately 70% of the density of the universe has been attributed to this material grid. So like Maxwell's ether, this virtual material grid of space has a density. However, this material grid of QED is a matter grid - the virtual particles have mass - even the virtual photons. In this sense it is more like a Dynamic Relativist interpretation, where there is only matter in space, except that they fill all of empty space with virtual matter! The estimates of the various contributions of these virtual particles to the density of space also doesn't add up - they are much too big. The model cannot be "real" because the energy of spontaneous quantum activity becomes infinitely large - these are the so called "infinities" that Feynman had to sweep under the carpet.
The underlying problem of this new conception of space is that since the virtual particles are by definition "undetectable", we are not much better off than with Maxwell's original ether, which fell from grace precisely because it was undetected. The "condensations in space" could be something completely different from the particulate forms that have been attributed to them, in fact, they would have to be, since their energies don't add up.
To conclude, the modern conception of space remains a contradiction - physicists have many times attempted to throw away the medium of space, only to have to re-introduce it again later - usually with models that are internally inconsistent and contradict known laws of physics. And the most important question to be left unanswered is the "why"? Why is the electron mass the mass that it is, why is the speed of light the speed that it is - the fundamental units of space have very specific values that these modern models just can't answer. Perhaps a radical re-thinking of the nature of space is needed.
6) Richard Feynman, "QED: The Strange Theory of Light and Matter." 1988.