Absolute Rest

I was thinking of a series of dreams,

Where nothing comes up to the top,

Everything stays down where it's wounded,

And comes to a permanent stop.

                                                        Bob Dylan

In classical physics an elementary particle of substance is considered to have a persistent and contiguous identity that can be tracked through time, and can be associated with a rest frame, i.e., a system of space and time coordinates in terms of which the particle is stationary. (We also typically take advantage of the fact that the coordinates can be assigned so that inertia is homogeneous and isotropic, with standard units of internal energy.) The very fact that such a particle can be known to us is due to its interactions, meaning that it has some palpable effect on other entities, and by the principle of action and reaction the particle is, in turn, affected by those other entities. By these interactions, any entity that has a persistent identity through time with an associated rest frame can be moved away from a certain region of space and time. (See, for example, the corollaries to Proposition 6 in Book 3 of Newton’s Principia.) This leads to the following question:  If we remove from a region of space and time every palpable thing (i.e., everything with which we can associate a rest frame), does there remain anything with which we can associate a rest frame? When the question is posed in this way, the answer seems to be clearly no, essentially by definition. (Only if there exist palpable but unmovable entities could the answer be different.) Nevertheless, throughout the history of physics there has been a persistent tradition asserting that we not only can but must assign a rest frame – called absolute rest – to a region of space that is devoid of all ordinary palpable substance.

Isaac Newton is often cited as having espoused the concepts of absolute position and absolute velocity, and it is sometimes even claimed that the theory of mechanics described in the Principia is based on those concepts. However, the Principia itself makes clear that Newton espoused absolute position and velocity only as conventions or else as metaphysical concepts related to his religious beliefs, and that these concepts played no operative role in his theory of mechanics, as is obvious from the fact that Newtonian mechanics is relativistically invariant under Galilean transformations. This invariance is not accidental; it was built into Newton’s conceptual structure from the basic definitions and postulates. The Principia begins with eight Definitions and three Laws (postulates). Definition 2 defines “the quantity of motion” as the product of mass and velocity (so it actually represents what we call momentum), although the word velocity is not defined. Definitions 3 and 4 and Law 1 refer to "rest", but in all three cases Newton carefully qualifies this by conjoining rest with uniform motion in a straight line:

Definition 3: Inherent force of matter is the power of resisting by which every body, so far as it is able, perseveres in its state either of resting or of moving uniformly straight forward.

Definition 4: Impressed force is the action exerted on a body to change its state either of resting or of moving uniformly straight forward.

Law 1: Every body perseveres in its state of being at rest or of moving uniformly straight forward except insofar as it is compelled to change its state by forces impressed.

This is followed by Newton’s second Law, which refers not to motion, but only to the change of motion:

Law 2:  The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.

These definitions and laws (along with the others, which don't mention "rest" or “motion”) are the basis of Newton's mechanics, and they very explicitly stipulate that “rest” and “uniform straight motion” are interchangeable. They refer only to changes in motion (i.e., acceleration), so obviously Newton's mechanics is not based on and does not contain any operationally meaningful notion of absolute rest or absolute velocity. On this explicitly relativistic foundation Newton’s Corollary 5 (essentially borrowed directly from Galileo) is inevitable:

Corollary 5: The motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forwards in a right line without any circular motion.

Compare this with Einstein’s statement of the principle of relativity in his 1905 paper on the electrodynamics of moving bodies:

The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of coordinates in uniform translatory motion.

In an early draft of Principia, Newton actually included the relativity principle (the content of Corollary 5) as one of the basic Laws, but then realized it was redundant and trimmed the Laws to just three, moving the relativity statement to a Corollary. Thus it’s clear that Newton’s theory of mechanics is relativistic, indifferent to the choice of reference frame, as confirmed by the invariance of the laws of motion under Galilean transformations. In view of this, why is Newton counted as an advocate of the concepts of absolute position and absolute velocity?

The answer is to be found in the first Scholium of the Principia, where Newton labored to establish the epistemological foundations of his theory. Recall that he had defined “the quantity of motion” in terms of velocity, but had given no definition of velocity, nor had he identified suitable measures of space and time. As noted above, his definitions and postulates make no distinction (in fact they positively deny any distinction) between absolute and relative positions and velocities, but they do depend on absolute acceleration as distinct from relative acceleration. Thus Newton needed to explain the meaning of absolute acceleration. As part of this discussion he asserted the existence of absolute position and absolute time, from which we get definitions of absolute velocity and absolute acceleration. However, there are difficulties with Newton’s assertion. He tells us (in the General Scholium) that God constitutes duration and space, and that God is eternal and infinite, meaning that “he endures from eternity to eternity, and he is present from infinity to infinity”. This precludes any boundaries or center to space. Furthermore he says “the parts of space cannot be seen and cannot be distinguished from one another by our senses”, which denies any physical significance to his notion of absolute position.

Despite these admissions, Newton continued to assert that motion had absolute significance, but his arguments in support of this assertion reveal that he was really only referring to absolute acceleration, not to absolute motion (velocity). He gives two specific examples, one involving a spinning pail full of water, and the other involving two revolving globes connected by a cord. According to his own Corollary 5, the phenomena in both of these examples would be exactly the same, whether they are referred to a putative absolute rest or to a frame moving uniformly in a straight line. Likewise Newton says specifically “the causes which distinguish true motions from relative motions are the forces impressed upon bodies to generate motion”, but his own second Law specifically says that impressed forces produce not motion but changes in motion, i.e., acceleration. So throughout his discussion of absolute motion we can only understand him to be referring to absolute acceleration.

The first Scholium serves to make the case that absolute acceleration cannot be judged by reference to relative positions. In other words, Newton needed to refute relationism (as advocated by Leibniz and other contemporaries of Newton), recognizing that “it is possible that there is no body truly at rest to which places and motions may be referred”. In this sense, Newtonian mechanics is indeed founded on absolute space and time, as distinct from relationism. (Special relativity is founded on absolute space and time in exactly the same sense.) Recall that Newton begins the first Scholium by saying that the meanings of “time, space, place, and motion are well known to everyone”, but then he continues

It must be noted, however, that these quantities are popularly conceived solely with reference to the objects of sense perception, and this is the source of certain preconceptions, for the dispelling of which it is useful to distinguish these quantities into absolute and relative, true and apparent, mathematical and common.

We can see in this passage that the preconception Newton wished to dispel was simply the idea that absolute acceleration can be judged with reference to some material body. His comments in the general Scholium about absolute space and time (as distinct from absolute rest or absolute motion) are not referred to in the rest of the Principia, until Book 3, where Newton identifies the center of mass of the solar system (which he called the system of the world), about which not only the planets but also the Sun revolve. Even here he acknowledges that we can't call the center of mass of the solar system absolutely at rest except by convention, since it could just as well be moving uniformly in a straight line. We simply hypothesize that it is at rest for talking purposes (but of course we now know that hypothesis is wrong - the center of mass of our solar system is not even perfectly inertial, and hardly anyone would claim it is absolutely at rest or the center of the universe, as it was thought to be in Newton's day). This is discussed further in Immovable Spacetime.

Newton did not present any arguments in support of the physical significance of the concept of “absolute rest” in Principia, but this isn’t surprising, because in those days the argument about absolute rest was expressed in different terms, viz., it was cast in terms of whether space was a substance or a void. It was understood that a substance could have a meaningful definite state of motion, but the void (nothingness or emptiness) could not. Since ancient times, some philosophers have argued that the world consists of an empty void (the vacuum) occupied by distinct entities, whereas others have argued that there can be no such thing as empty space, and every location must be substantial, which implies a definite state of motion for locations. Ironically, in this debate Newton was (for most of his life) on the side of the vacuum. For example, in Book 3 of the Principia, Corollaries 3 and 4 of Proposition 6 assert the existence of the vacuum. Just as ironically, the relationist Leibniz denied this, and advocated the plenum and vortices of Descartes, a view which today we might call “etherism”. There are actually several different sub-versions of etherism. In some forms the ether is an ordinary substance, but not yet detected for one reason or another, e.g., our instruments are not sensitive enough. In these theories the ether could be removed from a region of space, leading us back to the original question of whether space devoid of all substance has a rest frame. Hence this materialistic form or etherism has no bearing on the discussion, and in fact it is somewhat misleading to even call this a form of etherism. (We mention it only because many avowed etherists seem to espouse an ether of this form.)

More relevant are the forms of etherism that admit the ether is not an ordinary substance, and that it does not interact as an ordinary palpable substance does with other entities.  In these forms the ether is not palpable and cannot be “moved”, so it is sometimes claimed that it represents absolute rest. The difficulty is that, if it is not palpable, it would seem the constituent parts of this putative ether have no identities that can be tracked through time, so it isn’t clear what it even means to talk about its “rest frame”.  At the very least it would have to violate the principle of action and reaction. The advocates of this form of etherism sometimes argue that the ether could conceivably influence ordinary substances without exhibiting a rest frame, and this is certainly true, but it hardly helps to make the case that such an ether must possess a rest frame. Quite the opposite.

One possible use for an ether is to provide an absolute reference to determine the inertial frames and absolute acceleration. Recall that according to a purely relational account only the relations between material entities are significant. Against this conception Newton pointed out that although absolute position and motion are unobservable, absolute acceleration is quite observable, and cannot readily be attributed to the relations between bodies. We say “not readily” because it’s always conceivable that the distant masses in the universe are somehow responsible for the inertial forces exhibited by local objects – such as the water in Newton’s famous thought experiment of the spinning pail - when subject to accelerations relative to the distant objects. It is sometimes said that Ernst Mach originated this line of reasoning when he wrote (in his The Science of Mechanics) “try to fix Newton’s bucket and rotate the heaven of fixed stars and then prove the absence of centrifugal forces”. However, Newton actually anticipated this line of thought, and attempted to give an answer. In Section 4 of The System of the World (an alternate version of Book 3 of the Principia that was found among Newton’s papers and published separately in 1728) he considered whether a purely relationist account of rotation (for example) was plausible. If so, then we ought to be able to give an equally intelligible account of phenomena by assuming either that the earth rotates and the fixed stars do not, or vice versa. Newton argues that this is not the case. He wrote

If the earth is supposed to stand still, and the fixed stars to be revolved in free spaces every 24 hours, the forces by which the fixed stars are held in their orbits are not directed to the earth, but to the centres of those orbits, that is, of the several parallel circles… Then, because the periodic times are equal, it follows that the centripetal forces are proportional to the radii of the orbits. That forces should be directed to no body on which they physically depend, but to innumerable imaginary points in the axis of the earth, is an hypothesis too incongruous.

It is more incongruous still that those forces should increase exactly in proportion to the distances from this axis; for this is an increase to immensity, or rather to infinity; whereas the forces of natural things commonly decrease in receding from the fountain from which they flow.

But, what is yet more absurd, the forces cannot even be directed precisely to the axis of the earth, because of the twofold motion that is observed in the fixed stars, one diurnal round the axis of the earth, the other exceedingly slow round the axis of the ecliptic. The explication thereof requires a composition of forces so involved and so variable, that it is hardly to be reconciled with any physical theory.

Thus, although he does not claim that a symmetrical relationist account of acceleration is logically impossible, he argues that the description with a stationary earth requires us to postulate an extremely contrived and unreasonable set of physical forces, in contrast to the simple inertial forces sufficient to describe the phenomena in terms of an absolutely rotating earth. Under a superficial interpretation we might think this argument assumes the very thing it is trying to prove, namely, the absolute inertia of material entities, by claiming that the distant stars must be subjected to forces to keep them on circular paths around the earth. However, the argument doesn’t essentially depend on the precise form of the force laws, but merely on the assumption that some causal reciprocal interactions between the earth and the stars are responsible for the effects of the relative rotation, including both the equatorial bulging of the earth and the (approximately) circular paths of the distant stars. On the other hand, the case of the stationary earth and revolving field of stars is perhaps not as implausible as Newton suggests, if we accept the possibility of (for example) velocity-dependent forces, such that the rotating field of stars induces effective forces between the stars themselves. In other words, it isn’t necessary to assume the force on each star has its origin on the axis of rotation; they might have their origin in the aggregate of the other revolving stars, and it wouldn’t be surprising for the resultant force to be directed toward the axis of rotation of the axially symmetrical field of stars.

Regardless of whether we find Newton’s argument persuasive, it’s impressive that he actually did think about the rotation of local bodies in the context of the frame of the distant stars, and considered the possibility of a relational account of the phenomena of inertia, two centuries before Mach. As noted above, this discussion appears only in an early draft of Book 3 of the Principia, a version that Newton never published. It appears in a section entitled “The Certainty of the Proof” (odd because it doesn’t refer to any particular proof). In the final version of Book 3 there is no such discussion, perhaps because Newton decided it was too far-fetched to even be worth mentioning. It’s also interesting that, two centuries later, one of the greatest difficulties Einstein faced in developing general relativity was to understand how generally covariant field equations could accommodate rotation.

The modern theory of special relativity is ordinarily understood to consist of the proposition that physics is locally Lorentz invariant, which means that all physical phenomena satisfy the same simple set of equations (homogeneous and isotropic) when described in terms of any one of an infinite class of coordinate systems related to each other by Lorentz transformations. (This is understood to apply locally, since one can obviously find globally distinguished systems of coordinates in various cosmological models.) The complete symmetry and reciprocity of the Lorentz transformation leaves no justification for regarding any one particular system of coordinates (within that infinite class) as the “true” system. As soon as this reciprocity was noticed, the scientific community very quickly discarded the concept of absolute rest, at least as it pertains to local physical phenomena.

Nevertheless, some have continued to espouse a neo-Lorentzian interpretation. One rationale offered for this view is the idea that if “changes in motion” have absolute significance in a theory, then motion itself must also have absolute significance. Indeed many advocates of the Lorentzian view seem to regard this as self-evident. However, this is clearly insupportable. Consider, for example, the fact that Newton’s force of gravity has the property that we can assign a scalar number (which we may call the “potential”) to each point in space, so we have a continuous scalar field of numbers, such that the force of gravity exerted on a test particle at any given point is the gradient of that scalar field of numbers at that point. In other words, the force of gravity is given by changes in the potential. Now, for any potential field we can obviously add any constant number to all the values without affecting any of the forces, because the force doesn’t depend at all on the absolute value of the potential, only on changes in the potential from one place to another, i.e., only on the spatial derivative of the potential. This is precisely analogous to the fact that inertial forces depend only on changes in velocity from one time to another, i.e., only on the temporal derivative of velocity. We can add any constant number to all the velocities without affecting any of the forces. So, is it self-evident that there must be some absolute value of the potential? Must we even assign any ontological status to the potential?

Surely the mere fact that we can numerically equate the value of some physical function to a derivative of some other function doesn’t necessarily confer ontological status onto that other function, let alone assure us that there must be some absolute value of that function. Given any variable function with absolute significance we can always integrate to arrive at the anti-derivative of that function - with an arbitrary constant of integration. But if we insist that this new function also has absolute significance, then we can integrate again to create yet another variable, and so on, ad infinitum. For example, we can integrate acceleration once to give velocity, and a second time to give position, and a third time to give the integral of position, and a fourth time to give the integral of that, and so on. Now, one can argue that all of these functions have absolute values (i.e., that there is some meaningful and unique constant of integration in each case), but surely this is far from self-evident. Most people can grasp this when speaking about an abstract function like a potential, whose ontological status has always been hotly debated, but they have difficulty seeing that the same applies to the concepts of velocity and position, because of their strong pre-conceptions about those concepts.

Another rationale offered by neo-Lorentzians is the idea that, although the Lorentzian ether interpretation and the spacetime interpretation are empirically equivalent for all presently known phenomena, they stand differently in regard to falsifiability in the face of any future phenomena that might be discovered. In the etheristic interpretation, Lorentz invariance represents a large number of independent coincidental facts: electromagnetism happens to be Lorentz invariant, the strong nuclear force happens to be Lorentz invariant, mechanical inertia of every known elementary particle happens to be Lorentz invariant, and so on. There is no conceptual link between these (once the electromagnetic view of the world was ruled out), so for any new class of phenomena that might be discovered, the ether interpretation really gives no warrant to believe it would be Lorentz invariant. The ether can be given whatever properties it needs to conform with any new facts. Indeed this was Lorentz’s professed reason for continuing to prefer his interpretation. He said we shouldn’t relinquish the language of an absolute rest frame, because we might need it someday.

In contrast, the spacetime interpretation takes all those coincidences and removes them from the individual phenomena, and accounts for them in terms of the Minkowskian structure of spacetime itself. In this interpretation, any new particle or interaction that might be discovered tomorrow is constrained to be (at least locally) Lorentz invariant. The only way the spacetime interpretation is viable is if (local) Lorentz invariance is universal and complete. If it fails for any phenomenon, then the single unified spacetime interpretation fails, and we must go back to treating the Lorentz invariance (or lack thereof) of each phenomena as an independent fact, as it is in the ether interpretation. In this sense the spacetime interpretation is much more exposed to falsifiability than is the etheristic interpretation. Think of all the new phenomena, interactions, and particles that have been discovered subsequent to 1905, not to mention the increase in the range of parameters explored by experiment. Any one of these new classes of phenomena or observations might have been found to violate (local) Lorentz invariance (recall the reports of superluminal neutrinos at CERN in 2011...) and rendered the spacetime interpretation unviable, but none of them did – not even the entanglement aspects of quantum mechanics. But the Lorentzian framework would not have been invalidated by whatever might have been found – essentially because the Lorenztian framework lacks the underlying conceptual coherence of the spacetime interpretation.

It’s interesting to review some comments from a well-regarded physicists early in the 20th century. In 1911 Langevin acknowledges that absolute rest is a purely metaphysical concept (no experimental sense):

The employed reference systems are supposed to possess uniform translational motion: for such systems only, observers associated with them cannot experimentally detect their collective motion, and for such systems only the equations of physics must hold their shape when switching from one to another. For such systems it is thus, as if they were stationary relative to the aether: a uniform translation in the aether has no experimental sense.

But just as with Newton he notes the existence of absolute acceleration, which, for reasons not explained, he equates with the existence of an aether:

It should not be concluded, as has sometimes happened prematurely, that the concept of aether must be abandoned, that the aether is non-existent and inaccessible to experiment. Only a uniform velocity relative to it cannot be detected, but any change of velocity, or any acceleration has an absolute sense. In particular it is a fundamental point in the electromagnetic theory that any change of velocity or any acceleration of an electrified center is accompanied by the emission of a wave that propagates in the medium with the velocity of light, and the existence of this wave has an absolute sense…We therefore have hold on the ether through accelerations…

Unfortunately Langevin was mistaken about the example he chose, because although it obviously is possible to experimentally sense absolute acceleration (using an accelerometer, for example), the appeal to the production of electromagnetic radiation as a clear indication of such acceleration is actually false. Not only is it problematic to assert that a uniformly accelerating charge radiates, it is now understood that radiation does not have an absolute sense. In classical electrodynamics this is made explicit in the rather Machian interpretation of radiation reaction given in the Wheeler-Feynman absorber theory, in which the effects of electromagnetic radiation are modeled as interactions (advanced and retarded) between charges in relative (not absolute) acceleration. Whether or not radiation is present at a given event depends on whether we view it from an accelerated reference system. (In fact, even in vacuum nominally devoid of electric and magnetic fields, Unruh radiation is the name given to radiation that appears in terms of an accelerating system of reference.) This is fairly obvious to anyone acquainted with the distant-retarded-action view of electromagnetism, but it’s very surprising to people who hold to the aetheristic view. This is an example of why the classical aether models fell into disfavor, especially as an ontological foundation for electromagnetism.

Incidentally, it is sometimes said that Langevin’s 1911 paper was the first appearance in print of the now famous twins paradox – at least to the extent that he explicitly discussed the reciprocity of the Lorentz transformation, and why an accelerating organism would exhibit a lesser lapse of proper time than an unaccelerated one between two given events. He presented this as another example of observable effects of absolute acceleration. It isn’t clear whether Langevin’s paper appeared before or after Einstein’s 1911 address to a gathering of naturalists in which he said

If we placed a living organism in a box ... one could arrange that the organism, after any arbitrary lengthy flight, could be returned to its original spot in a scarcely altered condition, while corresponding organisms which had remained in their original positions had already long since given way to new generations. For the moving organism the lengthy time of the journey was a mere instant, provided the motion took place with approximately the speed of light.

Of course, this is really just re-casting Einstein’s discussion in his 1905 paper on the two clocks, one at the Earth’s pole and one moving with the Earth’s rotation in a circle around the equator – everything else being equal (i.e., neglecting any effects of gravitational potential). Both the reciprocity of the Lorentz transformation and the significance of absolute acceleration relative to inertial coordinate systems are clear in Einstein’s original paper, but Langevin may have highlighted the potential misunderstanding, perhaps because he originally fell prey to it himself. In any case, Langevin revealed another limitation in his thinking when he concluded his 1911 paper by saying

The concept of energy itself loses its absolute sense: its measurement varies with the reference system to which the phenomena are related, and physicists are currently, in the expression of the laws of the universe, looking after the real elements that possess an absolute sense, i.e. the elements that remain invariant when changing from one reference system to another, and which will play the role in the electromagnetic conception of the world, that which was played by time, mass and energy in the mechanistic synthesis.

Presumably in the first part of this passage he was referring to internal energy, since extrinsic energy (e.g., kinetic energy) has always been frame-dependent. With this understanding this is indeed a crucial aspect of relativistic physics – the inertia of energy. However, his final comment reveals two fundamental misunderstandings of the new physics. First, he was mistaken in thinking that the electromagnetic conception of the world was even viable. It was already well known in 1911 that the stability of matter requires something other than electromagnetism itself. Second, he was mistaken in thinking relativity represented an electromagnetic conception of the world as opposed to the mechanistic synthesis. Special relativity actually represents the unification of electromagnetism with mechanics.

Even after the power and scope of the relativity principle had become apparent, Hendrik Lorentz continued to advocate (albeit somewhat equivocally) a belief in the value of the concept of absolute rest. In an interesting letter to Einstein in January of 1915 he asserted that “we have a completely clear notion of consecutive moments and also of simultaneity” (contrary to Poincare’s famous statement that those who think we have a direct notion of simultaneity are “dupes of an illusion”), and to explain this he considered group of clocks U all mutually stationary and evenly spaced as measured by rulers that are also mutually stationary, and then another similar group of clocks U’ moving uniformly relative to U.

Now, two pictures can be drafted of the phenomena taking place within the system. In one it is clocks U which simultaneously reach the same hand position, in the other one it is clocks U'. In general, both pictures are such that we have no cause to prefer one over the other. This equivalency is precisely the assertion of the principle of relativity… Now, should it afford me satisfaction to adorn, or shall I say to disfigure, one or the other image with an "ether," then I am naturally free to do so. 

He goes on to say that “The issue cannot be whether the ether exists; just whether it is permissible and useful to include it in our picture…as long as I do not dream up too much about it”, and then in his characteristically candid way he admitted that “obviously in one picture I am going to have the ether at rest with reference to U, in the other picture, however, with reference to U’”. These statements reveal that Lorentz not only recognized that the adornment with an ether was purely metaphysical, he also acknowledged that this etherism was shifty, because the believer in the absolute ether frame somehow feels at liberty to take any convenient frame as the ether frame in any particular circumstance “in order to keep everything as simple as possible”. This clearly highlights the inherent hypocrisy of etherism: while insisting that a single absolute ether frame is essential for an intelligible account of phenomena, the etherist routinely gives his account of phenomena without ever making use of it. Lorentz then adds that “when speaking about the equivalency of both pictures” (i.e., when asserting the principle of relativity), “this should obviously mean as far as our experience allows”. In other words, he held open the possibility that Lorentz invariance may actually be violated, and it may turn out that we can distinguish one state of motion from another. This is indeed obvious, although there was hardly a hint of any possibility of violation of Lorentz invariance in those days, and much less today. To the contrary, the history of physics since then has been a story of unbroken confirmations of Lorentz invariance to ever greater precisions over an ever widening range of phenomena.

Perhaps sensing that he really hasn’t made a very persuasive case for his absolutely resting ether, Lorentz went on to he confide his underlying motivation for belief in an ether, although he qualified this by enclosing it in brackets “because I am stepping beyond the bounds of physics”. With that caveat, he wrote

A "universal spirit" which, without being tied to a specific place, permeates the entire system under consideration, or "of which" this system is composed, and which could "feel" all events directly, would naturally immediately distinguish one of the systems U, U’, etc., over the others. Although we are no such universal spirits, if we retain the common notion of "spirit" and "body," surely we are not so vastly different to it. For according to this view, we must feel material processes occurring in the brain; and since we can say only with uncertainty that the intellect has its seat at a specific point in the brain, it looks as if it really could perceive what occurs at different locations of the brain and (with sufficient powers of discernment) examine it directly for "simultaneity."

This is remarkably similar to Newton’s famous comments in Query 28 of the Opticks:

Is not the Sensory of Animals that place to which the sensitive Substance is present, and into which the sensible Species of Things are carried through the Nerves and Brain, that there they may be perceived by their immediate presence to that Substance? And these things being rightly dispatch'd, does it not appear from Phenomena that there is a Being incorporeal, living, intelligent, omnipresent, who in infinite Space, as it were in his Sensory, sees the things themselves intimately, and thoroughly perceives them, and comprehends them wholly by their immediate presence to himself: Of which things the Images only carried through the Organs of Sense into our little Sensoriums, are there seen and beheld by that which in us perceives and thinks.

Both Newton and Lorentz seem to be basing their belief for presentism on the idea that God (or the universal spirit) inhabits and perceives the separate parts of the world at individual instants of some absolute time (“by their immediate presence to himself”). They also both argue that the workings of living brains may approximate this to some small degree, by virtue of an individual perception occupying some extended region of space within the brain.

Oddly enough, in the General Scholium of Principia, Newton himself presented a somewhat different conception of how sentient beings (including God) inhabit the world, occupying not only space, but also time. He wrote

God is eternal and infinite, omnipotent and omniscient, that is, he endures from eternity to eternity, and he is present from infinity to infinity; he rules all things, and he knows all things that happen or can happen. He is not eternity and infinity, but eternal and infinite; he is not duration and space, but he endures and is present. He endures always and is present everywhere, and by existing always and everywhere he constitutes duration and space…  each and every particle of space is always, and each and every indivisible moment of duration is everywhere…  [Likewise] each sentient soul, at different times and in different organs of senses and motions, is the same indivisible person. Each man, insofar as he is a thing that has senses, is one and the same man throughout his lifetime in each and every organ of his senses. God is one and the same God always and everywhere. He is omnipresent not only virtually but also substantially… In him all things are contained and move…

This can easily be interpreted as a description of a block universe of space-time (stated even more explicitly by Laplace), with each identifiable entity comprising a worldline within this eternal and infinite sensorium of God. As such, it is actually more consistent with the relativistic view, despite Newton’s anti-relativistic pronouncements elsewhere. (The passage also contains another intriguing sentence, one that I omitted from the above quotation because it’s meaning is opaque to me:  “There are parts that are successive in duration and coexistent in space, but neither of these exist in the person of man or in his thinking principle, and much less in the thinking substance of God.” It could be taken as a denial of the worldline as an existent entity, but only for parts of space, which would actually argue against the attribution of a state of motion or rest to the parts of space.)

In summary, when pressed to articulate their conceptions of absolute rest, both Newton and Lorentz ultimately appealed to their notions of God, extrapolated from some rather vague and tentative ideas about perceptions existing in sentient beings, e.g., the separate parts of our own brains. There is no support for the speculation that brains involve instantaneous apprehension of events, and whatever one may think of the comments about God (or Lorentz’s universal spirit), they are clearly metaphysical and religious, to which the actual scientific theories of Newton and Lorentz were perfectly indifferent.

Of course, in a certain sense, one can always assert the existence of an ether, if the word is taken to mean simply the medium of spacetime, especially considering that we need something to account for absolute acceleration and the effects of inertia. But the events of relativistic spacetime can have no trajectories through time, so they possess neither a state of rest nor a state of motion. The question is whether we can meaningfully maintain the existence of a concept of absolute position and/or absolute velocity. In the January 1915 correspondence Lorentz and Einstein addressed this question explicitly. In one of his papers (“The Formal Foundations of the General Theory of Relativity”, Oct 1914) Einstein had written about two relatively moving systems of standard inertial coordinates, saying

We look in vain for a sufficient reason for why one of these systems ought to be more suitable than another to serve as a reference system in the formulation of the natural laws; on the contrary, we feel compelled to postulate the equality of status of both systems.

To this Lorentz replied

Are you not going a bit too far here by presenting a personal view as self-evident? As a matter of fact, prior scientists have thought it possible to find the “sufficient reason” you speak of in that both systems move in a different way in reference to the ether. You are correct in your observation only because you are not at all interested in the ether.

Einstein answered

You say sufficient cause for preferring [one reference system over another] can be found in that both systems move in different ways relative to the ether. I understand “cause” in this connection as an observable fact which distinguishes [the two systems], not as a merely conceptual characteristic.

In other words, rather than saying Einstein was “not at all interested in the ether”, Lorentz ought to have said Einstein was not at all interested in regarding as “causal” any concepts that have no observable significance. Of course, with the completion of the general theory of relativity at the end of that year, the entire debate was recast, because curved spacetime itself can possess energy, albeit not localizable, so we retain Lorentz invariance locally, but we blur the distinction between substance and spacetime.

By the way, in other correspondence Lorentz made the interesting observation that the difference between relationism and etherism is not very great, because one could regard them as two different ways of encoding the same information. This is similar to how electrodynamics can be conceived as a field theory or as a distant action theory (with suitable retarded action).

In the early 1950’s Paul Dirac became disenchanted with the direction in which quantum field theory was progressing, and entertained the idea that the concept of an ether might be useful in providing a more satisfactory foundation for progress. However, the ether envisaged by Dirac would not violate relativity because he required that its parts not have definite velocities but rather a distribution of velocities. He wrote

Let us imagine the aether to be in a state for which all values for the velocity of any bit of the aether, less than the velocity of light, are equally probable…

He argued that this would make the aether Lorentz invariant, but there is a problem with this conception. We would not achieve Lorentz invariance by uniformly distributing the values of v such that the probability of v falling within any interval of size Dv between –c and +c is equal. Instead, Lorentz invariance requires that the value of log[(1+v)/(1-v)] is uniformly distributed over the range from –∞ to +∞. The problem is that, mathematically, such a distribution does not exist. There is no uniform distribution over the real numbers. And even if there was, an ether of this form would (by definition) not entail absolute velocity.

In more recent times, advocates of the concept of absolute place and absolute rest have replaced Newton’s and Lorentz’s ruminations about a universal spirit with similar (and similarly vague) notions about a universal wave function. They argue that the quantum correlations appearing between the observed states of entangled particles could be given a more satisfactory conceptual basis (viz, more satisfactory than the current orthodox view provided by relativistic quantum field theory) if we postulate absolute time and absolute rest. The basic idea – although rarely articulated with much clarity – seems to be that an absolute frame would allow us to regard the quantum wave function as an ontological entity and to suppose that it everywhere reacts instantaneously to any measurements performed anywhere. Thus the quantum field (like the universal spirit) is regarded as non-local. This is supposed to help account for the observed non-classical statistical correlations between measurements performed on entangled entities at spatially separate locations.

Such ideas are widely regarded as unfounded and misguided, but one notable physicist who argued that there could be some merit in this approach was John Bell. Of course, Bell is known to have favored a Lorentzian approach to special relativity for pedagogical purposes, based on the fact that most of his colleagues at CERN in the 1970s were unable to correctly answer simple questions about special relativity. The reason for the anecdotal failure of the CERN scientists is impossible to know at this range, but Bell attributed it to an inherent flaw in the usual (Einsteinian) interpretation of special relativity. According to Bell, the “radical break with more primitive notions of space and time” represented by Einstein’s interpretation “often destroys completely the confidence of the student in perfectly sound and useful concepts already acquired”. He goes on to say

The special merit of Lorentzian pedagogy is to drive home the lesson that the laws of physics in any one reference frame account for all physical phenomena, including the observations of moving observers.

This is all somewhat strange, although typical for advocates of etherism. How anyone could ever imagine that the laws of physics in any one reference frame do not account for all physical phenomena is difficult to understand. In fact, the very first postulate of in Einstein’s 1905 paper (quoted previously) consists of the proposition that “The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems of coordinates in uniform translatory motion.” So the special merit of Lorentzian pedagogy (according to Bell) is to drive home Einstein’s first postulate. It appears from Bell’s discussion that he and his CERN colleagues were all taught special relativity very badly, so on that point we can agree with Bell that the existing pedagogy is poor, but he mis-identified the source of the problem. It isn’t that the spacetime interpretation is misleading, it’s that Bell and his CERN colleagues mis-understood the spacetime interpretation.

It’s also strange that Bell is annoyed about how the radical revision of the primitive (un-examined) notions of space and time destroyed people’s confidence in “useful concepts already acquired”. After all, the physicists at CERN weren’t the first to ever have incorrect expectations of what would happen in various relativistic situations. The ether models of Lorentz and others led to numerous incorrect expectations, which is precisely why those models had to be repeatedly revised and amended until finally adopting the full principle of relativity, with the recognition that the standard inertial coordinate systems in terms of which the homogeneous and isotropic laws of mechanics hold good are related to each other not by Galilean transformations but by Lorentz transformations. This implies that the loci of simultaneity of relatively moving systems of standard inertial coordinate systems are mutually skewed. If this realization causes people to adopt a more cautious and critical attitude toward their primitive intuitions and prejudices, then that’s as it should be. It is inevitable that, when we discover that some long held and never previously questioned belief was wrong, we will become more cautious and have to think more carefully about things – at least until our intuitions are adjusted and we become complacent again.

But all of this is just related to Bell’s ideas about pedagogy. More relevant to our discussion of absolute rest is Bell’s 1980 article on Bertlmann’s Socks, in which he tried to convey an understanding of the perplexity of quantum entanglement, and considered various possible ways of accounting for it. One of the ways he enunciated was described as follows:

It may be that causal influences do go faster than light. The role of Lorentz invariance in the completed theory would then be very problematic. An "ether" would be the cheapest solution. But the unobservability of this ether would be disturbing. So would the impossibility of "messages" faster than light, which follows from ordinary relativistic quantum mechanics in so far as it is unambiguous and adequate for procedures we can actually perform.

In 1984 he was still espousing the same idea, in similar terms, as can be seen in an interview appearing in the book “The Ghost in the Atom”, although it should be noted that he explicitly disavows having any actual explanation for quantum entanglement based on the concept of an absolute rest frame. When asked how he would explain quantum entanglement he said

Well, you see, I don't really know. For me it's not something where I have a solution to sell! For me it's a dilemma. I think it's a deep dilemma, and the resolution of it will not be trivial; it will require a substantial change in the way we look at things. But I would say that the cheapest resolution is something like going back to relativity as it was before Einstein, when people like Lorentz and Poincare thought that there was an aether - a preferred frame of reference… and in this preferred frame of reference things do go faster than light… Behind the apparent Lorentz invariance of the phenomena, there is a deeper level which is not Lorentz invariant.

The interviewer pointed out that Lorentz invariance is supported by a vast amount of experimental evidence, and asked Bell if he really thought it was possible that violations of Lorentz invariance would be found. Bell unfortunately seems to have mis-understood the question, because he answered that the etheristic Lorentzian interpretation was empirically indistinguishable from special relativity – which of course is Lorentz invariant by definition. That answer contradicts his statement that the phenomena are not Lorentz invariant. But then in the very next statement he contradicts himself again, by saying that the violation of Lorentz invariance he has in mind is unobservable. It should go without saying that, from a scientific standpoint, violations of Lorentz invariance that are unobservable are not actually violations of Lorentz invariance, they are simply metaphysical assertions.

To this point Bell has not given any indication of why or how a belief in an unobservable absolute rest frame resolves the puzzle of quantum entanglement. He asserts that it is the cheapest resolution, but never establishes that it is actually a resolution. Incidentally, it’s ironic that Bell repeatedly touted Lorentzian relativity as the “cheapest” solution, because Einstein’s reaction to the non-local quantum theory of David Bohm (admired by Bell) was that it was too cheap. (It’s also interesting that Bell used the term “shifty split” referring to the Copenhagen interpretation of quantum mechanics, because Bell seems to have been a somewhat shifty thinker himself, as can be seen when trying to draw a coherent interpretation from his comments.) But finally he does (in the 1984 article) spare a few words to explain why he thinks an absolute rest frame would help to explain quantum entanglement. He says

The reason I want to go back to the idea of an aether here is because in these EPR experiments there is the suggestion that behind the scenes something is going faster than light. Now, if all Lorentz frames are equivalent, that also means that things can go backward in time. This introduces great problems, paradoxes of causality and so on. And so it's precisely to avoid these that I want to say there is a real causal sequence which is defined in the aether. Now the mystery is, as with Lorentz and Poincare, that this aether does not show up at the observational level… And I agree that that's extremely uncomfortable.

There are several problems here. First, Bell has not actually articulated any explanation for quantum entanglement based on “faster than light” communication. He says only that “in these EPR experiments there is the suggestion that behind the scenes something is going faster than light”, but this is hardly an explanation. (What exactly is going faster than light? And why do EPR experiments suggest this unidentified thing?) Second, he really needs to say “instantaneous”, unless he wants to argue that the usual quantum correlations would not appear if the two measurements were performed absolutely simultaneously. The distinction between “faster than light” and “instantaneous” is important, because Bell wants a causal account of the phenomena with absolute temporal ordering, whereas instantaneous communication undermines any causal ordering. It is simply “spooky action at a distance”. Third, Bell is espousing an asymmetry that is not inherent in the phenomena. The two space-like separated ends, A and B, of an EPR experiment are perfectly symmetrical, and are simultaneous relative to one particular standard system of inertial coordinates, and yet we are asked to believe that the explanation of what is “truly” happening is totally different, depending on which end is considered to have happened first. In one case a measurement at A collapses the entire wave function throughout space, and this affects the outcome of the subsequent measurement at B. In the other case, it is just the reverse. And yet from an empirical standpoint the two situations are indistinguishable. Surprisingly, even Roger Penrose has lent support to the notion that quantum entanglement conflicts with special relativity – at least under some interpretations of quantum mechanics. In his 2002 book “The Road to Reality” he notes that the joint probabilities in the situation just described come out the same, either way, and yet he says

If we think of the state reduction as a real process, then we seem to be in conflict with the principle of special relativity, because there are two incompatible views as to which of us effected the reduction of the state vector and which of us observed the reduced state after reduction.

But it isn’t even necessary to invoke the mysteries of quantum entanglement to make this argument. At the very birth of special relativity, in the very first paragraph of Einstein’s 1905 paper, an essentially equivalent situation was discussed, involving unipolar induction, and how the customary view (i.e., the etherist view of an absolute reference frame) draws a sharp distinction between what is really happening, depending on which of the two bodies (the magnet or the conductor) is considered to be in absolute motion. The observable phenomena depend only on the relative motion, so the “customary view” introduces asymmetries not inherent in the phenomena. An etherist would say that unipolar induction suggests the need for an absolute rest frame, to enable us to give a causal account in terms of either an electric field or a magnetic field. But the situation is immensely clarified by recognizing that electric and magnetic fields are just different projections of the same unified electromagnetic field, which depends only on the relative motion between the conductor and the magnetic. Surely no serious person today would argue that unipolar induction suggests the need for an ether, and yet the argument that quantum entanglement suggests the need for an ether is almost identical. In neither case is it persuasive, and in both cases one would expect the relativistic view to give the clearest account of the phenomena.

We should note that most discussion of this kind take place in the context of non-relativistic quantum theory, which of course is non-relativistic. For example, the non-relativistic Schrödinger equation is clearly not invariant under Lorentz transformations. So, if this was the only known form of quantum mechanics, one could well argue that it was in conflict with the principle of special relativity. But of course the current theory of quantum mechanics actually consists of relativistic quantum field theories, such as quantum electrodynamics. These theories are explicitly Lorentz invariant – in fact, the requirement to satisfy Lorentz invariance was indispensable in the formulation of these theories, which are our most successful theories of physics. There is no place in these theories for any (local) absolute rest frame.

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