The great vectors-versus-quaternions debate

What?  You've never heard of it?  A big knock-down, drag-out fight between great minds of its day over, more-or-less, the philosophy of how to go about mathematical physics.  None of this "let's do an experiment to distinguish between these two theories" stuff; that's for wimps.  This was the deep stuff:  nuts-and-bolts versus mathematical elegance; generic versus well-behaved; even, so we're told, particles versus waves (I kid you not).

Old paradigms get crushed; it's part of how new paradigms establish and maintain themselves.  History gets buried.  But that doesn't mean we have to like it, or stand for it.  As I write this, here's the sum total of what Wikipedia's article History of quaternions has to say about this colorful event in the history of mathematical physics:

From the mid-1880s, quaternions began to be displaced by vector analysis, which had been developed by Josiah Willard Gibbs and Oliver Heaviside.  Both were inspired by the quaternions as used in Maxwell's A Treatise on Electricity and Magnetism, but — according to Gibbs — found that "... the idea of the quaternion was quite foreign to the subject."  Vector analysis described the same phenomena as quaternions, so it borrowed ideas and terms liberally from the classical quaternion literature.  However, vector analysis was conceptually simpler and notationally cleaner, and eventually quaternions were relegated to a minor role in mathematics and physics.

Yawn.  (Also, so much for Wikipedian neutrality; but that's a different can of worms.)

In this post, I resurrect a paper about the vectors-versus-quaternions debate, written in the Long Ago when we used things called typewriters, and wrote-in special symbols by hand.  It's been languishing for years in a file folder, one of those physical things that are the models for the icons on your phone.

Here's how the paper came about.  I learned about the vectors-versus-quaternions debate from my father.  In fact, I learned about quaternions from my father.  I inherited his enthusiasm for them.  And then, in my third year at WPI, I seized an opportunity to study the debate in depth.

One of the requirements for the BS degree at WPI was the Humanities Sufficiency project.  The idea was that tech students should be well-rounded, so they should take (and pass) a bunch of humanities classes and then, building on those classes, write a term paper on a subject that bridges the gap between humanities and sciences.  WPI had an undergraduate grade called "NR", short (I think) for "Not Recorded":  if you didn't pass a course, it didn't go on your transcript (though you didn't get your tuition back, naturally).  This resulted in students taking some classes because they were interested, and being sometimes more concerned with learning than with getting high grades.  So you'll understand when I say, it took me till my third year to accumulate the class credits I needed for the Sufficiency because I only passed about 50% of the humanities classes I took.  Though a bunch of people, including me, were rather bemused when I not only passed, but got high marks in, Philosophical Theories of Knowledge and Reality.

I chose vectors-versus-quaternions as my topic, with the no-nonsense title "Quaternions: A Case Study in the Selection of Tools for Mathematical Physics".  The Sufficiency was ordinarily a half-semester project, but wasn't required to be, and with such a juicy topic and personal interest, of course I took a bit over that.  Professor Parkinson actually apologized for not giving me a top grade on it, explaining that he had a strict rule never to give a top grade to a Sufficiency that took more than the basic half-semester.  At the time, what I thought (but at least had enough tact not to say to him) was that I was doing the work for its own sake, not for a mere grade.  Later, when that grade caused me to graduate With Distinction instead of With High Distinction, I understood why he'd apologized.  And was belatedly a bit put out, after all.

The things I learned from this paper have ever after informed my understanding of how scientific paradigms are chosen — a really major theme in my life since, after all, my master's thesis (pdf) brushed against a rejected paradigm (extensible languages), while my dissertation outright resurrected one (fexprs).  The influence from this is also deep in the foundations of my thinking on memetic organisms, which I blogged on some time back.

The writing style is a bit stiffer, here and there, than I strive for now.  (I was even worse in highschool.)  All in all, though, I'm still fairly pleased with the piece.

The original had both footnotes meant to be read inline, and endnotes with bibliographical details.  Here, I've put both in sections at the end, using letters for the erstwhile footnotes, numbers for the endnotes.  While I'm being pedantic, this version of the paper has three changes from the original version submitted in Spring 1986.  (And, just to prove what a nerd I am, the changes were made in 2002.)  Footnotes [m] and [y] have been added; and where Hamilton's nabla is first defined, I've corrected it to use full derivatives, having originally miswritten it with partial derivatives.

Associative memory is a strange thing.  Certain details stick with us.  I remember worrying about one partial sentence from Crowe that I just couldn't think how to put differently, and in the end deciding to let that passage stand; though at this late date I've no idea which passage that would be.

Contents: Title Body Footnotes Endnotes Bibliography

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Quaternions: A Case Study in the Selection of Tools for Mathematical Physics

John N. Shutt

Presented to: Professor Parkinson
Department of Humanities
Term D, 1986

Submitted in Partial Fulfillment
of the Requirements of
the Humanities Sufficiency Program
Worcester Polytechnic Institute
Worcester, Massachusetts
Corrections and additions: 11 March 2002

---

Quaternions are a form of hypercomplex number with four components.  Mathematically, they are the next most well-behaved algebra after the complex field.  The extent of their usefulness for mathematical physics has been in doubt since their discovery.

This paper examines historically the principal issues in the use of quaternions for mathematical physics.  The historical and mathematical background of quaternions is examined, followed by their application first to classical physics, and then to modern physics.  The paper concludes with an analysis of some of the major issues.
 

Vectorial analysis, in its general sense, is the mathematical treatment of directed magnitudes.  It arose in the first half of the nineteenth century as a synthesis of two major trends of thought, one in physics and the other in mathematics.¹

It has been pointed out² that the geometry of physics after Newton differed intrinsically from that of the ancient Greeks.  The difference is that, while Newtonian physics is set on a Euclidean stage, many of the principal players are (implicitly) vectors, which are not present in Euclid.

This left physics by the early nineteenth century working under a handicap.  Vectors underlay most of physics, but could not be handled gracefully.  The primary mathematical tool for handling geometry was the Cartesian coordinate system; Cartesian coordinates are flexible in principle, but in practice they are apt to be unwieldy and generally opaque.  A natural language for vectors was needed.^a

Mathematics by this time had become overextended.³  The problem was with the concept of number.  The formulae of algebra, originally developed using only positive numbers, were now being successfully applied to negative and complex numbers.  Since mathematicians had traditionally grounded their work on intuition, the more extensive number systems left many mathematicians uneasy.

The problem led George Peacock, in 1833, to postulate the principle of permanence of form.  This principle said that "Whatever algebraical forms are equivalent when the symbols are general in form but specific in value [positive integers], will be equivalent likewise when the symbols are general in value as well as in form."⁴  By 'general in form' is meant that properties of a particular number cannot be generalized; for example,  14 mod 7 = 0  doesn't imply that  x mod 7 = 0.  'General in value' is meant to refer to fractional, irrational, negative, or complex numbers, but leaves a question almost as big as the one to which the principle is addressed.  Despite its shortcomings, the principle was important because it did recognize that algebra is based on rules.

At least six people^b had independently devised the geometrical representation of complex numbers before Gauss finally published the idea in 1831.⁵  Several of these people used this representation as a justification for the complex number field.  (Of the first two of the six to make the discovery, Wessel embraced this justification but Gauss did not.)  It was Gauss's publication that finally drew general attention to the idea.

However, William Rowan Hamilton (1805–1865) was not aware of Gauss's 1831 paper until 1852.  He was influenced instead by John Warren, in whose work he would have been exposed to the concepts of the associative, commutative and distributive laws.  Hamilton did not consider the geometrical approach a sufficient justification.  In 1837 he presented a fresh approach, interpreting complex numbers as algebraic couples of real numbers.⁷  He defined addition, subtraction, multiplication and division of couples and then derived from them the primitive properties of complex numbers.

These mathematical developments suggested to many of the mathematicians involved that a further extension of number might be analogous to (3-dimensional) space.⁸  It was generally expected^c that the sought-after extension would have three terms, and obey all the laws of complex algebra (associative, commutative, and distributive), as well as having close ties to spatial geometry.

The ties to spatial geometry take many forms.  The two that Hamilton eventually settled on are (1) the Law of the Norms,^d and (2) unique division.  In retrospect, only the real and complex numbers satisfy both all the usual laws and (2), and (1) is simply impossible in three dimensions.^e 9

The idea that triplets (Hamilton's term) might not satisfy all the usual laws had occurred to Hamilton as early as 1830.  He was acquainted in particular with non-commutative multiplication from some speculations in set theory.¹⁰  On 16 October 1843, as he walked to a Council of the Royal Irish Academy, several of the above ideas converged in his mind to produce quaternions.  He had been working recently with triples of the form  a+bi+cj, with  i² = j² = −1; he now realized that he could satisfy (1) above by making the assumptions that  ij = −ji, and that this product yielded a third imaginary component  k = ij, with  k² = −1.¹¹

The resulting quaternion has the form  a+bi+cj+dk, with  i² = j² = k² = ijk = −1.  Quaternion multiplication is distributive over addition, and associative, but not commutative.⁹  The norm is a modulus of multiplication, and right-division and left-division are unique.  Real and complex numbers and quaternions are the only three possible division algebras — that is, algebras with associative and commutative addition, distributive and associative multiplication, and unique division.^f

Hamilton created a plethora of new terms for use in his new algebra.¹⁴  A quaternion q is made up of a real part, called the scalar of q and denoted  Sq, and an imaginary part, called the vector of q and denoted  Vq.  Alternatively, it can be expressed as the product of a positive real number (the 'length,' or square root of the norm of q), called the tensor of q and denoted  Tq, and a quaternion with tensor equal to one, called the versor of q and denoted  Uq.^g  Thus  q = Sq + Vq = TqUq.

A versor u has a unique decomposition  u = cos θ + v sin θ  with angle  0 ≤ θ ≤ π  and unit vector  v = UVu.^h 15  If p is a vector perpendicular to v,  p' = vp  is the rotation of p by angle θ about v.  This allows great-circular arcs in space to be represented by quaternions, leading to elegant proofs in spherical trigonometry.

Any non-zero quaternion q has a unique inverse  q⁻¹  such that  qq⁻¹ = q⁻¹q = 1.  Left- and right- division by q are defined respectively as pre- and post- multiplication by  q⁻¹.  If q is a versor  q = cos θ + v sin θ, then for an arbitrary vector p,  p' = qp(q⁻¹)  is the conical rotation of p by angle 2θ about v.

Another useful decomposition is that of the quaternion product of vectors into its scalar and vector parts.  If u,v are vectors separated by angle θ,  Suv = −(TuTv) cos θ  and  Vuv = (TuTv) (sin θ) n  where  n = UVuv  is a unit vector perpendicular to u and v.ⁱ  The scalar part is commutative  (Suv = Svu), and the vector part anticommutative  (Vuv = −Vvu).  Suv  and  Vuv  were later to form the basis for modern vector analysis.

Quaternions were the subject of a debate at the British Association meeting of 1847.¹⁶  George Peacock, who favored quaternions, did not come forward, but Sir John Herschel did, and called quaternions "a cornucopia of scientific abundance." Against quaternions it was objected that owing to their complexity, quaternion calculations are overly prone to mistakes.  There was also at the meeting at least one representative of the status quo; in Hamilton's words,

Mr. Airy, seeing that the subject could not be cushioned, rose to speak of his own acquaintance with it [quaternions], which he avowed to be none at all; but gave us to understand that what he did not know could not be worth knowing.

The background to this paper would be incomplete without some mention of Hermann Günther Grassman (1807–1877).¹⁷  In 1844 Grassman published a work, monumental in both size and scope, entitled Die lineale Ausdehnungslehre (calculus of extension).

The ideas behind the Ausdehnungslehre began in 1832 with the interpretation of a parallelogram as the geometrical product of two lines.  Grassman generalized this insight to other shapes and an arbitrary number of dimensions, and placed it on an abstract mathematical basis.  The system of the Ausdehnungslehre was a very broad mathematical generalization originating from these geometrical concepts.  Several types of multiplication were defined, the only requirement for a multiplication being distributivity over addition.

The Ausdehnungslehre of 1844 was written in a strongly metaphysical style, and was also highly abstract at a time when mathematics was based on concrete intuition.  Grassman was unknown.  Consequently, the Ausdehnungslehre went unnoticed by the world at large.

Despite Grassman's efforts, including a revised Ausdehnungslehre in 1862, his work remained obscure throughout his life, only beginning to attract interest about the time of his death.  One by one, Grassman's discoveries were remade by others, with Grassman's anticipation unveiled in a subsequent question of priority.

Although Grassman's inner and outer products are similar respectively to the scalar and vector parts of Hamilton's quaternion product of vectors, Grassman was conceptually distant from quaternions.  The significance of the Ausdehnungslehre here is that it encompasses an n-dimensional system of vectorial analysis.^j

An account will now be given of three significant figures in the application of quaternions who worked within the quaternion tradition in the nineteenth century.  These three figures are Hamilton, Tait, and Maxwell.  Following this, the circumstances will be described by which quaternions were abandoned in favor of vector analysis.

The first major publication on quaternions was Hamilton's Lectures on Quaternions of 1853.¹⁹  The text ran to over 700 pages.  It was difficult to read; by 1859, Herschel — a great enthusiast of quaternions and an able mathematician — had only managed to read through 129 of its pages.

In 1859 Hamilton began work on the Elements of Quaternions.  It was originally to be an elementary treatise, but became a reference work longer than the Lectures — though without the metaphysical emphasis of the earlier work.  The Elements developed quaternions mathematically in great detail, but did not add to their physical application.  By his own admission, Hamilton was by this time out of touch with contemporary physics.

Hamilton was convinced of the value of quaternions to physics, and had published scattered such applications.  In 1846 he had defined a (nameless) operator  ᐊ = i ^d⁄_dx + j ^d⁄_dy + k ^d⁄_dz.  However, he never did concentrate his own efforts on applications to physics, choosing instead to develop quaternionic theory.  He had planned for a major section of his Elements on the ᐊ operator, but the section was never written because of his death in 1865.  The Elements was published posthumously in 1866.

Peter Guthrie Tait purchased and read Hamilton's Lectures in 1853, out of general curiosity.^k 21  In 1857 he encountered an application that reminded him of Hamilton's ᐊ operator; pursuing this, he shortly became a devoted quaternionist, ultimately succeeding Hamilton as their chief advocate after the latter's death.

Tait's interest in quaternions was for their physical applications.  His Elementary Treatise on Quaternions, published in 1867, was the first accessible introduction to quaternions.  This work went into some detail on the operator ∇,^l using it to express several important theorems (e.g. Green's and Stokes').

Tait did much to further quaternionic applications to physics.  Oddly, he seems to have scrupulously avoided quaternions in his other work, including all of his lectures at the University of Edinburgh.  Quaternions are also omitted from Tait's collaboration in mechanics with Lord Kelvin, the Treatise on Natural Philosophy; speaking of this later, Kelvin said,

We [Kelvin and Tait] have had a thirty-eight years' war over quaternions....  Times without number I offered to let quaternions into Thomson and Tait [the Treatise], if he could only show that in any case our work would be helped by their use.  You will observe that from beginning to end they were never introduced.

It should be understood that Kelvin was throughout his life resolutely opposed to all vectorial methods.

James Clerk Maxwell originally derived his equations in the 1860's using component notation.²²  He began to study quaternions in 1870.  In his Treatise on Electricity and Magnetism of 1873, he presented both component and quaternionic notation.

Maxwell was a firm believer in physical analogy.  He favored quaternions as an aid to thinking, because the notation corresponds more closely than does that of components to physical reality.  For calculation, however, he considered component notation superior.  He made this distinction in the preliminary chapter of the Treatise, where he advocated "the introduction of the ideas, as distinguished from the operations and methods of Quaternions."

Maxwell's use of quaternions in the Treatise was accordingly limited primarily to the restatement of important results in quaternionic form.  Nonetheless it led some physicists who had never done so before to study quaternions.

In particular, this was the case with Josiah Willard Gibbs of America and Oliver Heaviside of England.²³  These two men proceeded independently along very similar lines.

From Maxwell's Treatise, both went to Tait's Elementary Treatise on Quaternions.  Both observed that, as actually used by Maxwell and for the most part even by Tait, the vector/scalar partition of quaternions was more important than the quaternions themselves.  Both then developed systems that treated vectors and scalars as entirely separate entities,  V ∇  and  S ∇  as separate operators,^m etc.

Heaviside went no further than this.  His notation was not entirely compatible with Tait's, but he never introduced concepts outside the quaternion tradition.  Thus his system was essentially a subset of quaternion analysis.

Gibbs however, broke all ties with Hamilton, even to citing Grassman as the main precedent for his system.ⁿ  His notation is substantially different from Tait's; in particular he replaced the prefix operators of Hamilton with infix operators.^o (∇ was an exception to this.)  Most significantly, he introduced a concept totally alien to quaternion analysis — that of the dyad.  (A dyad is neither a vector nor a quaternion, but a tensor.)

Neither Gibbs nor Heaviside shared Tait's scruples about using their systems in their other work.  Gibbs applied his vector analysis in periodic courses at Yale starting in 1879, and in some of his physics papers.  It was Heaviside who did most to disseminate vector analysis; he made heavy use of his system in several important electrical publications, such as his Electromagnetic Theory, permanently linking vectors to that rapidly growing field.

A debate took place in the early 1890's, on the proper vectorial system for mathematical physics.^p 25  This debate involved more than thirty letters and articles over five years (1890–1894) in eight leading scientific journals, as well as a scattering of other published writings.  It was primarily between Gibbs and Heaviside on one side and the English quaternionists on the other.

Superficially, a prominent characteristic of the debate was its colorful verbiage.  Heaviside was the contributor to this for the vectorists, while considered vectorist ideology was supplied primarily by the more dignified Professor Gibbs.  Metaphors and the odd pot shot are scattered through the quaternionists' writings, which are often pervaded by a tone of bitterness.  Particularly fiery are the literary antics of Alexander McAulay, an unknown youngster who joined the ranks of the quaternionists in 1893 ^26,26j with what Tait called "the perfervid outburst of an enthusiast."

The figures of Grassman and Hamilton became weapons in the debate.  The quaternionists played heavily on Hamilton's fame.  The vectorists dissociated themselves from Hamilton entirely, and placed themselves firmly behind Grassman,^q for whom they built a reputation.  As a result of this and the ultimate triumph of vector analysis, Hamilton's fame was tarnished.  (It was later resurrected through Hamilton's characteristic function in quantum mechanics.)

Gibbs was conversant with the systems of Hamilton and Grassman as well as his own.  This served him well on several occasions in the debate, for Tait was ill acquainted not only with Grassman's Ausdehnungslehre but with both Gibbs' and Heaviside's vector analyses.^r

The question recurs throughout the debate, of why quaternions had not been more widely accepted.^s This was generally not itself an issue (an exception is Gibbs' letter to Nature of 16 March 1893 ^26g), but served as a focal point for other issues.

The opening shots of the debate were fired by Tait,^26a and were aimed principally not at vector analysis but at component notation.  His arguments centered on expressiveness; no detail need be given, as the principal interest of the current discussion is with issues between vectorial systems.^t  However, it is significant that Tait apparently failed to appreciate the coming threat of vector analysis.  He seems to have repeatedly underestimated his opponents for several years into the debate.

What touched off the controversy was the following passage from the preface to the 1890 (third) edition of Tait's Treatise.

Even Prof. Willard Gibbs must be ranked as one of the retarders of Quaternion progress, in virtue of his pamphlet on Vector Analysis; a sort of hermaphrodite monster, compounded of the notations of Hamilton and Grassman.

The issue of notations, which Gibbs early subordinated to that of notions,^u 26b nevertheless was addressed repeatedly.  Gibbs disliked the prefix operators of quaternions, because infix operators were the existing norm.^26b  Tait countered that the prefix operators allowed the use of fewer parentheses, thus enhancing brevity of expression.^26c

C. G. Knott objected to the large number of operators in Gibbs' notation^26e, illustrating his point with Gibbs' abbreviations 'Pot,' 'New,' 'Lap,' and 'Max,' all of which represent various combinations of the Nabla operator.  Gibbs argued convincingly²⁶ⁱ that the quaternionic equivalents of these operators were too complicated to be intelligible.

Alexander Macfarlane, like Knott a former student of Tait, brought out another issue in 1891:^26d the sign of the scalar product.  In quaternion analysis,  Suv  for (positive) vectors u,v is negative because  i² = j² = k² = −1  but the vectorists had no stake in  √−1, so for convenience they defined the scalar product to be positive.

Macfarlane had his own solution to this.  He distinguished versors from vectors.  Since two right turns produce a reversal  (−1), he set  i² = j² = k² = −1  for versors; but for convenience he set  i² = j² = k² = 1  for vectors.  Thereafter in the debate he represented a third faction.  The vectorists never specifically addressed his system, so that his net influence was simply to undermine the quaternionists.

The vectorists never addressed the issue of the sign of the scalar product; in any case, there was no need for them to do so, since Macfarlane did it for them.  The response to Macfarlane's innovation came from Knott.  In December of 1892, Knott objected^26e that without  i² = j² = k² = −1, multiplication ceases to be associative.  Macfarlane answered in May of 1893 ^26h that, just as with commutativity, associativity is only a convention and can be given up with impunity if it is convenient to do so.

The crux of the controversy was the issue of notions.  Everyone in the debate (except Cayley, as noted earlier) agreed that vectors and scalars are important for physics.  The vectorists maintained that the quaternion has no notable physical interpretation (other than rotation, Gibbs acknowledged, but dyadics serve this purpose satisfactorily^26b).  The burden of proof throughout the debate thus lay on the quaternionists, although Tait did once make the same accusation of artificiality against dyadics.^26c

The quaternionists did little during the debate to prove their case.  Knott did address the question twice.  In 1892 he argued simply that the ratio of two vectors is a fundamental concept, so that since this ratio is a quaternion, quaternions are fundamental.^26e  In 1893 he added to this an analogy:

Although  sin θ  and  cos θ  occur more frequently than θ itself, we should not conclude that θ plays no fundamental role.  Similarly we should not infer that  αβ  [the full quaternion product] is not fundamental simply because  Vαβ  and  Sαβ  occur more frequently.³¹

There is another issue which recurs periodically throughout the debate.  It is generalizability to higher numbers of dimensions.  From his first article in 1890, Tait praised quaternions for being "uniquely adapted to Euclidean [i.e. three-dimensional] space."^26a  Gibbs in his first letter of 1891 praised vectors for being generalizable "to space of four or more dimensions."^26b  These views are representative of the positions taken on this issue by the respective sides in the debate.  There is one exception: in February of 1893, Dr. William Peddie, Tait's assistant at the University of Edinburgh, attempted to show that "quaternions are applicable to space of four or any number of dimensions."^26f

If there was a winning side to the debate, it was the vectorists.  They consistently outmaneuvered (Gibbs) and outspoke (Heaviside) their opponents.  More fundamentally, the quaternionists did little to justify their position on the crucial issue of notions.  In any event, the decisive factor in the ultimate acceptance of vector analysis was Heaviside's active use of it in his published work.^v

Passing mention may be made of the International Society for Promoting the Study of Quaternions and Allied Systems of Mathematics.³²  It was organized shortly after the debate by Shunkichi Kimura, residing at Yale at the time, and Pieter Molenbroek, and published a bulletin from 1900 to 1913.  The Society was plagued with difficulties from its inception, the first of which was that Tait declined the presidency due to ill health. (He died in 1901.)  In 1913, all the offices were due for election at the same time, with no one to arrange it, and the Society slipped quietly into oblivion.

There was a tendency among quaternionists, which surfaced on several occasions in the debate, to think of the vectorists as ungrateful children.  This is presumably the source of the bitter overtone in their writings that was mentioned earlier.  It may be interesting in relation to this to consider the following excerpt from a review by Heaviside written in the early 1900's.

... as time went on, [after the controversy] ... it was most gratifying to find that Prof. Tait softened his harsh judgments, and came to recognize the existence of rich fields of pure vector analysis, and to tolerate the workers therein....  I appeased Tait considerably (during a little correspondence we had) by disclaiming any idea of discovering a new system.³³

Quaternions will now be considered in their application to twentieth-century physics.  It is appropriate in this regard to recount the early history of the idea of using the real part of a quaternion to represent time.  Priority in this idea belongs to Hamilton.

In the hierarchy of thought, Hamilton placed mathematics above physics, and metaphysics above mathematics.³⁴  An important element of his philosophy was the metaphysical importance of the number three.³⁵  He attached much significance to the tridimensionality of space, and this was a major impetus for his search for algebraic triplets.

After arriving at quaternions by purely algebraic means, Hamilton struggled to reconcile them with his philosophy.  At least since 1835 he had used time as a metaphysical justification for the real numbers; this may have contributed to his later identification of quaternions with the four dimensions of space and time.

He clearly failed to pass on the idea to the physicist Tait.  Recall that in the 1890's controversy the quaternionists represented quaternions as "uniquely suited" to three-dimensional space.  Tait did speculate on the possibility of a fourth dimension, as did Maxwell.³⁷  However, it is not clear that either of them associated a fourth dimension with time.

In 1896, Kimura pointed out that ∇ is not a full quaternion operator because it does not include the derivative with respect to the real component.³⁸  He introduced a full quaternion differential operator  _q∇, and used the scalar component to represent the derivative with respect to time.  He had physical applications in mind.

The possibility of using quaternions occurred to Hermann Minkowski when he was formulating his space-time in the 1890's (naturally enough, since this was during the vector-quaternion controversy).  He rejected them completely as "too narrow and clumsy for the purpose."³⁹

To understand what might have motivated Minkowski to make this judgment, consider some basic four-dimensional properties of quaternions.  Cayley showed in the 1850's that the general rotation in Euclidean four-space may be expressed by the quaternion formula  p' = upv, where u,v are arbitrary versors.^w 40  Also in Euclidean four-space, multiplication by an arbitrary versor  u = cos θ + v sin θ  may be understood as a relative turning by angle θ through 4-space.

Unfortunately, Minkowski space-time is non-Euclidean.  While quaternions have 'length' (modulus of multiplication)  Tq = (w² + x² + y² + z²)^1⁄₂, interpreted as a space-time vector q should have length  (w² − x² − y² − z²)^1⁄₂.⁴¹

A solution to this problem was found in 1912 by Ludwig Silberstein.⁴²  He let the scalar component, representing time, be imaginary by introducing a fourth  √−1  independent of the three of quaternions.  He was thus working with what Hamilton called biquaternions, quaternions whose four coefficients are complex numbers.  Biquaternions do not, of course, have unique division, although Silberstein did define a biquaternion "inverse."^x

By this device, Silberstein was able to express the Lorentz transformation in the form  q' = QqQ, where q is in frame S and q' is its equivalent in frame S'.  Q is a complex versor (i.e.  TQ = 1); further, the coefficient of  SQ  is real, and the three coefficients of  VQ  are imaginary.  Thus  Q = cos θ + v sin θ  for imaginary unit vector v and angle θ.  The resulting transformation is a rotation in Minkowski space-time by angle  2θ √−1.  Compare this with the general Euclidean three-space rotation mentioned earlier.

A different solution to the problem was given in a 1945 paper by P. A. M. Dirac.⁴³  Dirac submitted that the value of quaternions lies in their special algebraic properties, and that therefore resorting to biquaternions is not productive.^y  Restricting himself to real quaternions, he derived the general quaternionic linear transformation  q' = (aq + b)(cq + d)⁻¹, with quaternion coefficients a,b,c,d.^z

He used this equation to describe a transformation in five-dimensional projective space, and restricted it to describe the Lorentz group.  He then derived a one-to-one correspondence between the quaternions q and q' in his transformation and space-time vectors, through a comparatively involved set of equations.  Finally, he used his quaternionic transformation to derive a general quaternionic formula for the relativistic addition of velocities.  The elegance of this formula provoked the only non-mathematical comment in the paper, "The quaternion formulation appears to be the most suitable one for expressing generally the law of addition of velocities."

Dirac's treatment is contrary to the traditional usage of quaternions.  Ever since their discovery, much of their perceived value has been in the physical interpretability of quaternionic formulae.  This perception is evident in the following, written to Hamilton by John T. Graves.

There is still something in the system which gravels me.  I have not yet any clear views as to the extent to which we are at liberty arbitrarily to create imaginaries, and endow them with supernatural properties.  You are certainly justified by the event....  but I am glad that you have glimpses on physical analogies.⁴⁴

Physical interpretability was the quaternionists' main argument against component notation.

Dirac's approach was to set up a correspondence between quaternions and space-time vectors; but the correspondence was unintuitive.  This abandonment of physical interpretation is consistent with the general philosophy of much of modern mathematical physics.  However, it is not the way others have applied quaternions to modern physics.

Papers applying quaternions to modern physics are not as rare as one might suppose, numbering (as nearly as I can determine) at least in the dozens.⁴⁵  These papers deal with a wide range of topics.  It is not within the scope of the current paper to examine all their areas of application;^aa for example, the use of quaternions to describe elementary particles will be omitted.⁴⁶  For modern quaternionic ideology, two representative examples will be used.

The first example is a 1964 paper, "Quaternions in Relativity," by Peter Rastall.⁴⁷  The paper begins with a brief historical account of quaternionic application to relativity, along with commentary on why quaternions have been (up to 1964) repeatedly passed by in favor of other formalisms.^bb  His own interest in quaternions is for field equations in curved (Riemannian) space-time.  He argues that for this general case, neither matrix nor spinor notation yields any clear physical interpretation because neither can easily be understood in terms of tetrads of real coordinates  (x,y,z,t).

Quaternions, by which he means complex or bi- quaternions, are to provide this physical interpretability.  He uses Silberstein's form for Lorentz transformations in flat space-time.  (Recall that Silberstein's quaternions have only four non-zero real coefficients, corresponding directly to coordinates  x,y,z,t.)  He describes Riemannian space-time using tetrad formalism, i.e. with each tetrad of event coordinates he associates a set of four axes, which need not be orthogonal.  By combining these tools he then derives his general field equation.

It is important to such quaternionic treatments of relativity that a quaternionic equivalent is possible for any matrix formula, and vice versa.^cc 48  The most basic quaternion-matrix equivalence, demonstrated by C. S. Peirce in 1881, is between a real quaternion  w + xα + yβ + zγ  (imaginaries α, β, γ) and a  2 × 2  complex matrix

w+ix    y+iz −y+iz    w−ix

=

w

1    0 0    1

+

x

i    0 0    −i

+

y

0    1 −1    0

+

z

0    i i    0

The four matrices equivalent to 1, α, β, γ are essentially the Pauli spin matrices.

The second example consists of two papers written in 1962–63 by a group of physicists on quaternion quantum mechanics.^dd  These papers take advantage (respectively) of two basic properties of quaternions: their close ties to 3-/4- vector spaces, and their lack of commutativity.

The first paper presents the fundamentals of the theory.⁴⁹  Their starting point is that

a propositional calculus exists that we can call general quantum mechanics (as distinguished from complex quantum mechanics) in as much as no number system or vector space at all is assumed in its formulation....

It is always possible to represent the pure states of a system of "general quantum mechanics" by rays in a vector space in a one-to-one manner...

and that the only number system over which this can be done for all such systems is 𝓠, the quaternions.  They suggest that while real and complex quantum mechanics are very similar, "quaternion quantum mechanics has many new features that make it much richer."

The second paper capitalizes on a difficulty that arises from the non-commutativity of quaternions.^ee 50  Multiple mathematical descriptions arise that should be equivalent.  By postulating invariance of the physical laws over these different descriptions, they arrive at a new field of which electromagnetism is a special case.

Perhaps the most extensive example of quaternionic application in the twentieth century (if not for all time) is the work of Otto Fischer.  Fischer was a Swedish civil engineer who became interested in quaternions in the years before World War II.  In the 1950's he published two substantial books on the application of quaternions.⁵¹

In Universal Mechanics and Hamilton's Quaternions (I have not had access to his other book), Fischer set himself a rather ambitious goal.

This is a book written by a civil engineer on universal mechanics with an attempt to introduce a certain order in its mathematical structure by means of Hamilton's Quaternions.  The term "universal mechanics" refers to the mathematics of ordinary physics of motions, elasticity, hydrodynamics, aerodynamics, electromagnetism, together with relativistic and cosmic physics as well as quantum mechanics.

Fischer's aim is to create a close correspondence between concepts and explicit mathematical structure.  In pursuing this goal, he correlates several types of mathematical structural hierarchies to branching specialities within universal mechanics.  One such hierarchy is the "potential pyramid," which expands by repeated differentiation from an 'apex' potential.  Closely related is an "affinor pyramid",^ff also formed by repeated differentiation.  The potential pyramid is static, consisting of functions of simple, quadric or double quadric quaternions,^gg while an affinor pyramid consists of operators on such functions, and is therefor dynamic, taking its shape from the function to which it is applied.

The three different types of quaternions are the basis of Fischer's other major unifying structure.  After introducing and doing much work with real quaternions in imaginaries i₁, i₂, i₃, he proceeds to quadric quaternions by introducing "superdirections" ï, j₁, j₂, j₃ that correspond roughly to different specialities in physics.  Ultimately he applies this technique in the more general case of double quadric quaternions to his goal of unifying universal mechanics.^hh

Fischer appears to have been widely competent in the specialities of "universal mechanics."  He surely did not, however, have a talent for presentation.  The superficial appearance of the Universal Mechanics is that of the numerology of the 'crackpot fringe.'

Fischer was part of what has often been called the "Cult of Quaternions"⁵² — a tradition of enthusiastic devotees that began with Hamilton and continues to the present day.ⁱⁱ  In 1910, C. S. Peirce wrote of his brother James Mills Peirce, who had been a noted quaternionist until his death in 1906, that he "remained to his dying day a superstitious worshipper of two hostile gods, Hamilton and the scalar  √−1."  An earlier reference to the 'cult' is found in the chapter on vector analysis in Heaviside's Electromagnetic Theory.

"Quaternion" was, I think, defined by an American schoolgirl to be "an ancient religious ceremony."  This was, however, a complete mistake.  The ancients — unlike Prof. Tait — knew not, and did not worship Quaternions.⁵³

The members of the 'cult' who have been mentioned in this paper all exhibited rational reasons for their enthusiasm.  Yet, their reputation as a semi-religious community is not entirely unsupported in their own writings.  A comparatively recent example is E. T. Whittaker's statement (1940) on quaternionic research after the turn of the century, that "the good work went on."^jj In 1871 Maxwell had observed, with no sarcasm intended, that "The unbelievers are rampant."⁵⁴

Throughout this paper, the motivating philosophies of applied quaternionists have been noted.  Definite trends are visible in these philosophies.

From their discovery, the claims of quaternions for physical application have been interpretability and utility.  Their interpretation is certainly an improvement over component notation; nevertheless, as F. D. Murnaghan has observed, to the general public quaternions were the archetype of a baffling abstract theory until they were supplanted in this role by Einstein's General Theory of Relativity.⁵⁶  The quaternionists simply continued to assert that quaternions are meaningful, and as a tactic against component notation this was sufficient.

Against vector analysis, however, the tactic failed.  Having extracted the utilitarian part of quaternion analysis, the vectorists discarded the remainder and branded it meaningless.  The result was that the quaternionists were left clinging forlornly to their claims of interpretability while practical-minded physicists flocked to vector analysis.

In the twentieth century, this parting of the ways has led to a peculiar twist of fate.  Following their vectorist and anti-quaternionist traditions, physicists have adopted the utilitarian notation of matrices.  This has led them away from interpretability, and ultimately even away from utility into increasing abstraction.  Quaternionists have continued to emphasize interpretability, finally coming to be its uncontested claimants; but, being an acknowledged fringe group, have had no general acceptance to date.

Rastall's paper of 1964 concludes with the following paragraph.  It illustrates clearly the underlying ideology of the modern physical application of quaternions.  It is also a fitting note on which to end the current paper on quaternions.

The movement towards abstract algebraic and coordinate-independent formulations of physical theories, and away from particular matrix representations and special coordinate systems, is an increasingly popular one, and our work is in accord with it.  Less popular, and seemingly opposed to this rarefied mathematical spirit, is our desire to make abstract concepts more concrete and imaginable.  To pure mathematical minds the aim is unsympathetic.  They are happy in their complex spaces, and would prefer to postulate an affine connection rather than to align tetrad vectors.  It is a matter of taste.  Those, however, who are prepared to exploit the accident of having been born in space-time may find this paper useful.

---

Footnotes

• [a] This need increased greatly with the development of electromagnetic theory in the last third of the century.⁶
• [b] The six are Wessel (1799) and Gauss (about the same time), Argand and Buée (1806), and Mourey and Warren (1828).
• [c] Almost everyone looked for a superset of the complex numbers.  An exception was Servois, who came close to quaternions in 1815.¹²
• [d] In modern terms, the Law of the Norms says that the norm (the sum of the squares of the real coefficients of the terms) should be a modulus of multiplication.  A modulus of multiplication is a function  M(x)  such that  M(a) * M(b) = M(a*b).  The Law of the Norms holds for real and complex numbers.
• [e] In 1844 Augustus de Morgan presented a number of triple algebras that do not satisfy the Law of the Norms but do have moduli of multiplication.  Given the distributive law, the Law of the Norms and the uniqueness of division are equivalent, so naturally de Morgan's triple algebras do not have unique division either.
• [f] If the associative law is also dropped, only one further algebra, Cayley's octonions, is possible.¹³
• [g] 'Scalar,' 'vector' and 'tensor' seem to be the only three of Hamilton's original quaternionic terms that have developed non-quaternionic usages.  The modern meaning of 'tensor' is completely different from Hamilton's.
• [h] Obviously, v is indeterminate for angles 0 and π.  In the following description of quaternions, special cases such as this are generally omitted.
• [i] u,v,n always have the same sense. If a right-handed coordinate system is used, u,v,n form a dextral set.  Hamilton used a left-handed system, while Tait, Maxwell, Gibbs all used a right-handed system.¹⁸
• [j] Also implied in Grassman's calculus of extension are matrix theory and modern tensor analysis.²⁰
• [k] Remember, this is the same book that so effectively stymied Herschel.
• [l] This operator was apparently written as ᐊ by Hamilton but as ᐁ by Tait.  In Tait's form it has been variously given the names nabla (after a visually similar Assyrian harp), del, and atled (delta spelled backwards).²⁴
• [m] In replacing  V ∇  and  S ∇  by  ∇×  and  ∇·  (Gibbs' notation), it is also necessary to redefine ∇ itself from quaternionic operator  i ^d⁄_dx + j ^d⁄_dy + k ^d⁄_dz  to vectorial operator  ı ⃗ ^∂⁄_∂x + ȷ ⃗ ^∂⁄_∂y + k⃗ ^∂⁄_∂z  a subtle and profound (not to say confusing) change that may serve to suggest the size of the intellectual gulf between quaternion analysis and vector analysis.
• [n] There is, nevertheless, substantial evidence that Gibbs' system was inspired by quaternions, and not by the Ausdehnungslehre.²³
• [o] 'Infix notation' means that the operator symbol appears between the operands, as in  u × v.  'Prefix notation' means that the operator appears before the operands, as in  Vuv.  Prefix notation was later rediscovered by Jan Łukasiewicz,²⁷ and is now generally called Polish notation.
• [p] The two sources used for the following discussion of the 1890's' controversy both handle the debate on a roughly chronological paper-by-paper basis.  My discussion is a substantially different organization of the material.
• [q] Gibbs provided the initiative that led to the publication of Grassman's collected works.²⁸
• [r] In a letter to Nature of January 1893, Tait wrote:

  I found that I should not only have to unlearn quaternions (in whose disfavor much is said) but also to learn a new and most uncouth parody of notations long familiar to me....  There I was content to leave the matter....  Dr. Knott [Cargill Gilston Knott, a former student of Tait and a staunch quaternionist] has actually had the courage to read the pamphlets of Gibbs and Heaviside; and, after an arduous journey through trackless jungles, has emerged a more resolute supporter of Quaternions than when he entered.²⁹

• [s] This lack of acceptance was exaggerated.  Actually, 168 works were published in the quaternion tradition in the 1890's.³⁰
• [t] There was only one paper in the debate that argued against the use of all vectorial systems.  This was written by Arthur Cayley in 1894.  It named quaternions in particular, and was answered by Tait.^26k
• [u] In his response to Tait's preface, Gibbs' wrote,

  The criticism relates particularly to notations, but I believe that there is a deeper issue of notions underlying that of notations.  Indeed, if my offense had been solely in the matter of notation, it would have been less accurate to describe my production as a monstrosity, than to characterize its dress as uncouth.

• [v] Of course, quaternions didn't vanish overnight.  They were in the final stages of disappearance in about 1910;³⁶ and, as will be seen, they have never vanished entirely.
• [w] Cayley considered this result in a purely geometrical context — i.e. he didn't have time as a fourth dimension in mind.  Note that a special case of this formula is when the rotation is orthogonal to the real axis; then  v = u⁻¹, giving the general three-dimensional rotation described earlier.
• [x] For a real quaternion q, the inverse is often defined as  q⁻¹ = Kq/Nq, where  Kq  is the complement and  Nq  the norm of q.¹⁵  Silberstein simply carried this definition over to biquaternions.
• [y] Taken in the context of 1945, Dirac's dismissal of biquaternions is reasonable.  However, in the context of modern theoretical physics, which favors such rarified creatures as Clifford and Lie algebras, biquaternions actually do have some interesting properties.
• [z] The form using left-division is  q' = (qa + b)⁻¹(qc + d), with (of course) different quaternion values for a,b,c,d.
• [aa] I have not discussed any specific areas of application to classical physics.  It has not been necessary to do so, since the vector-quaternion controversy was independent of both area and method of application.
• [bb] Discussions of the reception (or rather lack of reception) of quaternions in modern physics appear in most of the modern works I have examined.  Rastall's observations are interesting; but his historical interpretations do not take into account the vectorist and anti-quaternionist traditions.
• [cc] According to A. W. Conway, this equivalence is itself equivalent to wave-particle duality.
• [dd] These papers appeared in Journal of Mathematical Physics.  I find no subsequent papers in that journal by this group.
• [ee] The trouble with the non-commutativity of 𝓠 is that there is no unique tensor product of the Hilbert space  𝓗_𝓠  with itself.
• [ff] Fischer explains this terminology on page 4 of the Universal Mechanics:

  It is more common in literature to use the term "tensor" for the general non-commutative "affinor" and speak of symmetric and antisymmetric tensors instead of tensors and axiators.  But Spielreins terms apparently are more elucidative.

• [gg] Quadric quaternions are quaternions whose four coefficients are themselves real quaternions; equivalently they are sums and products of quaternions in two independent sets of imaginaries.  They have 16 real coefficients each.  Double quadric quaternions are quadric quaternions whose 16 coefficients are themselves quadric quaternions, or equivalently sums and products of quaternions in four independent sets of imaginaries.  Double quadric quaternions have 256 real coefficients each.
• [hh] In gathering the above high-level description of Fischer's method I have made repeated forays into the Universal Mechanics.  I found the book extremely difficult to read.  It is highly concentrated and moves very rapidly, which is natural considering that it covers a very large quantity of material.  This is compounded by the erroneous omission from the printing of scattered pages from the first two chapters of the book.
• [ii] The most recent representative of the 'cult' in my bibliography is James D. Edmonds (1974).
• [jj] Whittaker qualifies as a 'cultist' because of a passage in the same article (which appeared in the Mathematical Gazette):

  ... those who were in the outer circles of Hamilton's influence — e.g. Willard Gibbs in America and Heaviside in England — wasted their energies in devising bastard derivatives of the quaternion calculus...

  This editorial comment resulted in a brief correspondence in the Mathematical Gazette between Whittaker and E. A. Milne in the spirit of the controversy of the 1890's.⁵⁵

---

Endnotes

• [1] Cf. Michael J. Crowe, A History of Vector Analysis (Notre Dame: University of Notre Dame Press, 1967), p. 1.
• [2] Crowe, pp. 127–128.
• [3] On the mathematical ancestry of quaternions, see Morris Kline, Mathematical Thought from Ancient Through Modern Times (New York: Oxford University Press, 1972), pp. 772–779.  On extensions to the concept of number prior to 1800, see E. T. Bell, Development of Mathematics, 2nd ed. (New York: McGraw-Hill, 1945), pp. 172–178.
• [4] Quoted in Kline, p. 773.
• [5] Crowe, pp. 5–11.
• [6] This is remarked on by Crowe, p. 220.
• [7] Crowe, pp. 23–27.
• [8] On Hamilton's attempts, see Crowe pp. 26–28; on other attempts, see [5].
• [9] Edmund T. Whittaker, "The Sequence of Ideas in the Discovery of Quaternions," Proceedings of the Royal Irish Academy 50 (1945) sect. A: 97–98.
• [10] Encyclopedia Britannica, 11th ed., s.v. "Quaternions," by Alexander McAulay, p. 720.  The relevant part of the article is the historical profile, which is taken from the corresponding article in the 9th edition, by Peter Guthrie Tait.
• [11] William Rowan Hamilton, "Quaternions," Proceedings of the Royal Irish Academy 50 (1945) sect. A: 89–92.  This is the first publication of some notes made by Hamilton on the day of his discovery of quaternions.
• [12] See Crowe, p. 10.
• [13] Kenneth O. May, "The Impossibility of a Division Algebra of Vectors in Three Dimensional Space," American Mathematical Monthly 73 (1966): 289–291.  On Cayley's octonions, see Kline p. 792.
• [14] On 'scalar' and 'vector,' see Crowe pp. 31–32.  On 'tensor' and 'versor,' see Felix Klein, Elementary Mathematics from an Advanced Standpoint (New York: Dover Publications, 1945), p. 138.  For some examples of others of Hamilton's terms, see Crowe p. 36.
• [15] The basic properties of quaternions are taken from Louis Brand, Vector and Tensor Analysis (New York: John Wiley & Sons, 1947).  Brand devotes the last chapter (chapter X, pp. 403–429) of his book to quaternions.
• [16] Crowe, pp. 34–35.
• [17] Crowe, pp. 54–96.
• [18] Crowe, p. 155.
• [19] On Hamilton's Lectures and Elements, see Crowe pp. 36–41.
• [20] Bell, pp. 200, 204.
• [21] On Tait's quaternionic work see Crowe pp. 117–125.
• [22] On Maxwell's use of quaternions, see Crowe pp. 127–139.
• [23] On the development of Gibbs' and Heaviside's systems of vector analysis, see Crowe pp. 150–177.
• [24] See Crowe pp. 124, 146.
• [25] The 1890's' controversy is described in some detail in Crowe pp. 182–224 (chapter 6).  Much of the controversy is also covered in Alfred M. Bork, " 'Vectors versus Quaternions' — The Letters in Nature," American Journal of Physics 34 (1966): 202–211.  Crowe's treatment is more comprehensive; however, Bork goes into more detail on the contents of the articles he discusses, and makes frequent use of quotations.
• [26] General reference notes for specific papers in the controversy are ordered chronologically, and indexed by letter under number 26 (hence notes 26a–26k).  References particularly to one or the other secondary source are numbered separately.
  • [26a] Tait, Philosophical Magazine, January 1890, and the preface to his Elementary Treatise on Quaternions, 1890 edition.  See Crowe pp. 183–185 and (less) Bork pp. 202–203.
  • [26b] Gibbs, Nature, 2 April 1891.  See Crowe pp. 185–186 and Bork p. 203.
  • [26c] Tait, Nature, 30 April 1891.  See Crowe pp. 186–187 and Bork pp. 203–204.
  • [26d] Macfarlane, Proceedings of the American Association for the Advancement of Science, published in July 1892.  See Crowe pp. 190–191 and Bork p. 205.
  • [26e] Knott, Proceedings of the Royal Society of Edinburgh, read 19 December 1892.  Crowe pp. 201–203 and Bork p. 207.
  • [26f] Peddie, Proceedings of the Royal Society of Edinburgh, read 10 February 1893.  See Crowe pp. 208–209.
  • [26g] Gibbs, Nature, 16 March 1893.  See Crowe pp. 198–200 and Bork p. 206.
  • [26h] Macfarlane, Nature, 25 May 1893.  See Crowe pp. 203–204 and Bork p. 207.
  • [26i] Gibbs, Nature, 17 August 1893.  See Crowe pp. 204–205 and Bork p. 208.
  • [26j] McAulay, Utility of Quaternions in Physics, 1893.  See Crowe pp. 194–195.
  • [26k] Arthur Cayley, "Coordinates versus Quaternions," and Tait, "On the Intrinsic Nature of the Quaternion Method," both read before the Royal Society of Edinburgh on 2 July 1894.  See Crowe pp. 211–215.
• [27] Bork, p. 204.
• [28] Crowe, p. 161.
• [29] Bork, p. 206.
• [30] Crowe, p. 111.  The supposed slowness of acceptance is discussed in Crowe pp. 219–220.
• [31] Crowe, p. 208.
• [32] A brief account of the history of the International Society is given in Hubert Kennedy, "James Mills Peirce and the Cult of Quaternions," Historia Mathematica 6 (1979): 425–426.
• [33] Crowe, p. 123.
• [34] Kline, p. 778.  These priorities are evident in much of his work.
• [35] On Hamilton's metaphysics, see Thomas L. Hankins, "Triplets and Triads: Sir William Rowan Hamilton on the Metaphysics of Mathematics."  Isis 68 (1977): 175–193.
• [36] Crowe, p. 240.
• [37] Alfred M. Bork, "The Fourth Dimension in Nineteenth Century Physics," Isis 55 (1964): 328–330.
• [38] The article in which he wrote this is mentioned in ibid., p. 338.  The original article is Shunkichi Kimura, "On the Nabla of Quaternions," Annals of Mathematics, 10 (1896): 127–155.
• [39] James D. Edmonds, "Quaternion Quantum Theory: New Physics or Number Mysticism?" American Journal of Physics 42 (1974): 221.  Edmonds derives his information from a 1914 book by Ludwig Silberstein.  These same sentiments are attributed to Minkowski in Otto F. Fischer, "Hamilton's Quaternions and Minkowski's Potentials," Philosophical Magazine (7) 27 (1939): 375.  Fischer does not identify the source of his information.
• [40] Ludwig Silberstein, "Quaternionic Form of Relativity," Philosophical Magazine 23 (1912): 790.
• [41] This observation is made in P. A. M. Dirac, "Application of Quaternions to Lorentz Transformations," Proceedings of the Royal Irish Academy 50 (1945) sect. A: 261.
• [42] Silberstein, pp. 790–809.
• [43] Dirac, pp. 261–270.
• [44] Crowe, p. 34.
• [45] For a list of such papers, see Edmonds, p. 220.
• [46] For references on this topic, see David Finkelstein et al., "Foundations of Quaternion Quantum Theory," Journal of Mathematical Physics 3 (1962): 217, and Peter Rastall, "Quaternions in Relativity," Reviews of Modern Physics 36 (1964): 820.
• [47] Rastall, pp. 820–832.
• [48] A. W. Conway, "Quaternions and Matrices," Proceedings of the Royal Irish Academy 50 (1945) sect. A: 98–103.  On the generality of quaternions, see also William Kingdon Clifford, "Applications of Grassman's Extensive Algebra," American Journal of Mathematics 1 (1878): 350–358.
• [49] Finkelstein et al., "Foundations," pp. 207–220.
• [50] David Finkelstein et al., "Principle of General Q Covariance," Journal of Mathematical Physics 4 (1963): 788–796.
• [51] Crowe, pp. 254–255.  The two books are Universal Mechanics and Hamilton's Quaternions, 1951, and Five Mathematical Structural Models in Natural Philosophy with Technical Physical Quaternions, 1957.  I have not seen the latter book.  It may be relevant that Silberstein used the name "physical quaternions" for his specialized biquaternions corresponding to space-time vectors.
• [52] For example, the title of Kennedy's paper cited in [32].
• [53] Crowe, p. 171.
• [54] Crowe, p. 133.
• [55] Edmund T. Whittaker, "The Hamiltonian Revival," Mathematical Gazette 24 (1940): 153–158.  The associated correspondence appears in Mathematical Gazette 25 (1941): 106–108 and 25 (1941): 298–300.
• [56] F. D. Murnaghan, "An Elementary Presentation of the Theory of Quaternions," Scripta Mathematica 10 (1944): 37.

---

Bibliography

• Bell, E. T.  The Development of Mathematics.  2nd ed.  New York: McGraw-Hill Book Company, Inc., 1945.
• Bork, Alfred M.  "The Fourth Dimension in Nineteenth-Century Physics."  Isis 55 (1964): 326–338.
• —.  " 'Vectors versus Quaternions' — the Letters in Nature."  American Journal of Physics 34 (1966): 202–211.
• Brand, Louis.  Vector and Tensor Analysis.  New York: John Wiley & Sons, Inc., 1947.
• Clifford, William Kingdon.  "Applications of Grassman's Extensive Algebra."  American Journal of Mathematics 1 (1878): 350–358.
• Conway, A. W.  "Quaternions and Matrices."  Proceedings of the Royal Irish Academy 50 (1945) sect. A: 98–103.
• Crowe, Michael J.  A History of Vector Analysis: the Evolution of the Idea of a Vectorial System.  Notre Dame: University of Notre Dame Press, 1967.
• Dirac, P. A. M.  "Application of Quaternions to Lorentz Transformations." Proceedings of the Royal Irish Academy 50 (1945) sect. A: 261–270.
• Edmonds, James D.  "Quaternion Quantum Theory: New Physics or Number Mysticism?"  American Journal of Physics 42 (1974): 220–223.
• Finkelstein, David; Jauch, Josef M.; Schiminovich, Samuel; and Speiser, David.  "Foundations of Quaternion Quantum Mechanics."  Journal of Mathematical Physics 3 (1962): 207–220.
• —.  "Principle of General Q Covariance."  Journal of Mathematical Physics 4 (1963): 788–796.
• Fischer, Otto F.  "Hamilton's Quaternions and Minkowski's Potentials."  Philosophical Magazine (7) 27 (Jan.–June 1939): 375–385.
• —.  Universal Mechanics and Hamilton's Quaternions. Stockholm: Axion Institute, 1951.
• Hamilton, William Rowan.  "Quaternions."  Proceedings of the Royal Irish Academy 50 (1945) sect. A: 89–92.
• Hankins, Thomas L.  "Triplets and Triads: Sir William Rowan Hamilton on the Metaphysics of Science."  Isis 68 (1977): 175–193.
• Kennedy, Hubert.  "James Mills Peirce and the Cult of Quaternions." Historia Mathematica 6 (1979): 423–429.
• Kimura, Shunkichi.  "The Nabla of Quaternions."  Annals of Mathematics 10 (1896): 127–155.
• Klein, Felix.  Elementary Mathematics from an Advanced Standpoint. Translated by E. R. Hedrick and C. A. Noble.  New York:  Dover Publications, 1945.
• Kline, Morris.  Mathematical Thought from Ancient through Modern Times.  New York: Oxford University Press, 1972.
• McAulay, Alexander.  "Quaternions."  Encyclopedia Britannica. 11th ed.  1911.
• May, Kenneth O.  "The Impossibility of a Division Algebra of Vectors in Three Dimensional Space."  American Mathematical Monthly 73 (1966): 289–291.
• Murnaghan, Francis D.  "An Elementary Presentation of the Theory of Quaternions."  Scripta Mathematica 10 (1944): 37–49.
• Rastall, Peter.  "Quaternions in Relativity."  Reviews of Modern Physics 36 (1964): 820–832.
• Silberstein, Ludwig.  "Quaternionic Form of Relativity." Philosophical Magazine (6) 23 (Jan.–June 1912): 790–809.
• Whittaker, Edmund T.  "The Hamiltonian Revival."  Mathematical Gazette 24 (1940): 153–158.
• —.  "The Sequence of Ideas in the Discovery of Quaternions."  Proceedings of the Royal Irish Academy 50 (1945) sect. A: 93–98.

Preface to Homer

But of the tree of the knowledge of good and euill, thou ſhalt not eate of it: for in the day that thou eateſt thereof, thou ſhalt ſurely die.

— Genesis 2:17 (King James Version)

I'm going to suggest a hypothesis on the evolution of societies, natural languages, and memetic organisms.  Relating human language and human thought to programming is a straightforward exercise left to the student.

Eric Havelock in his magnum opus Preface to Plato (fascinating stuff) describes two profoundly different stages in the evolution of human societies, which he calls oral society and literate society.  Greek society, reckons Havelock, was transforming from oral to literate around the time of Plato; the epics of Homer are characteristic of oral culture.  I suggest that there is a stage of societal evolution before that of oral poetry such as Homer's — and that just as Afghan culture is a surviving example of oral society, the Pirahã culture recently studied in the Amazon is a surviving example of... verbal society.  (I can't bring myself to call it "pre-oral", since language is still spoken; but it does have words, and is missing oration, so "verbal" will do till something better comes along.)

Scientific organisms need a literate environment to survive; religious organisms don't need literacy, and were, I reckon, the giants that roamed the ideosphere in the age of orality.  But in the age before Homer's, religions could not have survived either.  If ever there were an actual event that fits beautifully with the myth of Adam and Eve eating of the fruit of the Tree of Knowledge, the transition from verbal to oral society would be it.

Oral thought

In an oral society (in Havelock's vision — I've also read parts of Walter Ong's Orality and Literacy, which follows Havelock's paradigm), knowledge is preserved through an oral tradition.  The form of the account matters; facts are mutated as convenient to put them in a form that will be successfully replicated over many retellings.  Standard patterns are used.  Repetition is used.  A good story also always has an actor:  things don't just happen, they are done, by somebody, which may favor a polytheistic society, with deities personifying what we (in a literate society) would call "abstract" forces.  One might well end up with some such pattern as

And God said, let there be x.  And there was x.  And God saw the x, and saw that it was good.  And the morning and the evening were the n^th day.

And God said, let there be y.  And there was y.  And God saw the y, and saw that it was good.  And the morning and the evening were the n+1^st day.

(Note the repetition of each item, repetition of the pattern, and the actor.)  For a more concrete example, here is the start of Thomas Hobbes's 1676 translation of Homer's Iliad:

O goddess sing what woe the discontent
Of Thetis’ son brought to the Greeks; what souls
Of heroes down to Erebus it sent,
Leaving their bodies unto dogs and fowls;
Whilst the two princes of the army strove,
King Agamemnon and Achilles stout.

Notice that things here don't happen, somebody/something does them.  The goddess sings.  The discontent (of Thetis' son) brings, sends, leaves.  The two princes strive.

The things that are acted on are concrete as well; nothing is abstract in our literate sense.

Such oral tradition can be written down, and was written down, without disrupting the orality of the society.  Literate society is what happens when the culture itself embraces writing as a means of preserving knowledge instead of an oral tradition.  Once literacy is assimilated, set patterns are no longer needed, repetition is no longer needed, pervasive actors are no longer needed, and details become reliably stable in a way that simply doesn't happen in oral society — the keepers of an oral tradition are apt to believe they tell a story exactly the same way each time, but only because they and their telling change as one.  When the actors go away, it becomes possible to conceive of abstract entities.  Plato, with his descriptions of shadows on a cave wall, and Ideal Forms, and such, was (Havelock reckoned) trying to explain literate abstraction in a way that might be understood by someone with an oral worldview.

Note that science can't possibly survive in an oral environment.  It relies on an objectively fixed record of phenomena, against which to judge theories; and it centrally studies abstract forces.  Religion, on the other hand, is ideally suited to an oral environment.  I suggest that religious organisms were the dominant taxon of memetic organisms in oral society, and the taxon of scientific organisms evolved once literate society made it possible.  Leading to a classic Darwinian struggle for survival between the two taxa.

Verbal thought

Those who study natural languages have a luxury not afforded to artlangers, those who create languages artistically:  When a natural language crops up that violates the "natural" rules that linguists thought they understood, new rules can be invented to fit.  But, if an artlanger creates a language that violates the rules linguists think they understand, their creation is likely to be ridiculed.  This was the observation of David J. Peterson in an essay in 2007 (he gives a "Smiley award" to a conlang each year, and does so in essays containing some brilliant deep insights into conlanging).

What is it to a linguist if Pirahã exists? "That sounds totally fake," says the skeptic. Says the linguist, "Yeah, doesn't it?" But in a world where Pirahã doesn't exist, imagine the conlanger who created it. "I just made up a language with no temporal vocabulary or tense whatsoever, no number system, and a culture of people who have no oral history, no art, and no appreciation for storytelling. Oh, yeah, and the language can just as easily be whistled, hummed or drummed as spoken. Oh, and the men and women have different phonologies. Oh yeah, and it's spoken in an area with a dominant language, but nobody speaks it, because they think their language is the best. Oh yeah, and it's supposed to be a naturalistic language." Suddenly when someone counters and says, "That sounds totally fake," the conlanger is put on the defensive, because they do have to account for it—in other words, "Yeah, doesn't it?" isn't going to fly.

— David J. Peterson, The 2007 Smiley Award Winner: Teonaht

Which is interesting for a conlanger, but fascinating in light of Havelock's notion of oral society.  That list of features is pretty much explicitly saying the language doesn't and can't support an oral society:  "no oral history, no art, and no appreciation for storytelling" (doesn't), "no temporal vocabulary or tense whatsoever, no number sytem" (can't).  And for a verbal society to survive in a world where the main Darwinian memetic struggle is between literacy and orality, of course it would have to be an extraordinarily compelling instance of verbality — "nobody speaks" the dominant language of the area, "because they think their language is the best."

The fruit of the Tree of Knowledge

The Pirahã were studied by Daniel Everett, who originally approached them with an evangelical mission — the point of such efforts is to learn an isolated people's language, then translate the teachings of Christianity into that language.  Of course this failed miserably with the Pirahã, with their compellingly verbal language and culture (J.R.R. Tolkien criticized the international auxilliary language Esperanto as sterile because it had no mythology behind it (one could argue Esperanto now does have a mythology of a sort (but I digress))).  Within a few years after completing his dissertation, Everett became an atheist.  (Everett's 2008 book on his experiences with the Pirahã is Don't Sleep There are Snakes: Life and Language in the Amazonian Jungle.)

All of which goes to show that the myth of the fruit of the Tree of Knowledge can be handily applied to the transition from verbal to oral society.  However, as I pointed out in my earlier post on memetic organisms, religious teachings are naturally selected for their ambiguity, their ability to be given a wide variety of different interpretations.  The plausibility of an interpretation of the myth is, therefore, pretty much meaningless — the myth is a contextual chamelion, expected to blend plausibly into different belief systems.  But it is interesting to consider how the myth might have evolved.  The early Judaic written tradition is evidently a written record of an originally oral tradition, and an oral tradition mutates events into a good story (i.e., a highly replicable one).  If the verbality conjecture is somewhere in the general neighborhood of the truth, there may once have been a vast cultural upheaval as orality supplanted verbality, perhaps (or perhaps not) on a par with the modern struggle between scientific and religious thinking (a major theme of current geopolitics).  Such an upheaval might be expected to make a lasting impression on the oral societies that emerged; the lasting impression would be a myth; and the myth would be shaped into the forms of orality, with concrete actors and objects.  What sort of myth do you think might result?

[Update:  I further discuss the timing of the verbal/oral transition in a 2015 post Sapience and language.]

Memetic organisms

When Richard Dawkins coined the word meme (back in 1976, in The Selfish Gene — a must read), I suggest he made one understandable mistake, an oversight that, as far as I can tell, has lingered ever since.  It might even explain why memetics hasn't become a viable field of scientific research.

A meme is roughly an idea that makes copies of itself, which compete with copies of other memes for available resources (basically, human hosts).  When a class of things self-copy and compete, they evolve; Dawkins used the general term replicators for any such things.  Genes are replicators, and he used memes as a second example of replicators, showing that the concept has some generality to it.  All good so far, but he also wrote that this second kind of replicator was "still in its infancy, still drifting clumsily about in its primeval soup".

Um, no.  It's very easy to think that, because the world that memes inhabit —the ideosphere— isn't directly visible to our senses.  Ask yourself, if the memetic equivalent of a tyrannosaurus were (metaphorically) standing right next to you, how would you know?

Memes are, I suggest, nowhere near the primeval-soup stage.  For thousands of years, memetic organisms have roamed the earth, with reproduction (as opposed to replication), death of individual organisms, inactive memes, and something like differentiated organs.  You are surrounded, at this moment, by memetic organisms.

What's an organism?

Some large groups of memes get copied together; the name memeplex  has been been suggested for such groups.  Astrology, say, or algebra.  But a biological organism isn't just a set of genes — it is, as Dawkins put it, a vehicle for genes.  The replicators have evolved the trick of building these vehicles for themselves.  Here are some things to expect of any kind of organisms.

• Each organism carries a reasonably stable set of replicators, some of which influence the organism's fitness.
• Organisms reproduce.  A child is created by a process involving one or more parents, from which the child inherits many (most?) of the replicators it carries.
• Organisms die.  When they do, some of the replicators they carried may be carried on by their descendants.
• Replicators with the potential to induce certain organism traits may be carried by organisms that don't exhibit those traits.  (Both recessive genes and junk genes spring to mind.)

So, when looking for memetic organisms, we want a class of entities that carry sets of memes; that exhibit reproduction and death, inheriting from their parent(s) and so preserving memes from deceased ancestors; and that carry along some memes across generations in some sort of "inactive" form.

Seeing a memetic organism

In 1994, there was much fanfare about the twenty fifth anniversary of the first moonwalk.  Footage of astronauts on the moon was replayed on television.  In one of these clips, an astronaut stood on the moon and did the classic experiment of dropping a light object and a heavy object to see if the heavy object fell faster.  (A feather and a wrench, I think they were.)  It wasn't well controlled; the point was evidently public education about science, for which it was preceded by a verbal explanation of the experiment — so it taught about scientific method as well as about universal gravitation.  Great stuff.

What left my jaw hanging was that the explanation started with (iirc) "Aristotle said".  Nobody was supposed to believe Aristotle's theory about falling objects, but this guy on the moon was deliberately teaching the general public about what they weren't supposed to believe.  Inherited memes systematically preserved in a sort of "inactive" form.

Thomas Kuhn described some aspects of this species of memetic organisms in 1962, in The Structure of Scientific Revolutions (another must read).  Notably, he described its reproductive process, which is what he called a scientific revolution.  Here's a rough portrait of a paradigm scientific field as a memetic organism.

The meme set carried by the organism includes a mass of theories, some of which contradict each other.  At the center of this mass is a nucleus of theories that are supposed to be believed (part of what Kuhn called a paradigm).  Surrounding the nucleus are theories that are meant to be contrasted with the paradigm and rejected, together with memes about how to conduct the contrast; one might call this surrounding material the co-nucleus.  While the organism thrives, the contrast with the co-nucleus strengthens belief in the nucleus, thus recruiting and retaining members of the organism's scientific community.  When the organism falters (the scientific community loses faith in the paradigm), eventually a new paradigm emerges, forming the nucleus of a new organism, while the new co-nucleus may contain both some nuclear and some co-nuclear material from the parent(s).

During reproduction, fragments may be drawn for the new nucleus (as "inspiration") from pretty much anywhere, even from non-sciences.  It seems that a thriving science may deliberately surround itself with a sort of third ring of memes, outside the co-nucleus and perhaps somewhat loosely coupled with the science itself (symbiotic?), that provide raw material for new co-nuclear or nuclear formations.  This third ring may include alternative, pseudo-, and fringe science; and science fiction, which can provide a venue for scientists within the community to explore new ideas without the ridicule or ostracism that would result if they prematurely proposed the same ideas in a scientific forum.

[I am, btw, not only let down but also rather fascinated, to find my memory of the moonwalk gravity demo does not match the footage I've found on YouTube [link]; besides being a hammer rather than a wrench, this footage doesn't have the explanation, nor the name Aristotle, that waxes so prominent in my recollection.  Either what I'm remembering is dominated by the epiphany I had while watching it rather than what was shown, or, not impossibly, there could be other footage floating around, either from a separate incident or (less expensively) with some sort of dubbed-over narration.]

Second example

Religion seems to be a second species (or genus, or some taxon anyway) of memetic organisms.  I may do even more poorly here, as I'm practically unread on comparative religion, but I'll take a stab at it anyway; hopefully, it will suffice to make the taxon plausible, even if my specific suggestions don't hold up at all.

The best fit for an organism seems to be below the scale usually called a sect, though conceivably somewhat above the scale of a congregation.  Part of the carried memetic material is a large mass I'll call a religious tradition, which may be written or oral.  The tradition is augmented by further memes, which I'll call an interpretation, determining what different parts of the tradition are supposed to mean.  The tradition and interpretation should be able to recruit and retain followers.  When changing societal environment makes those memes less effective, it becomes increasingly likely that the community will either splinter, or adjust its carried meme set, creating a new organism with perhaps some deletions or even additions to the tradition, but especially, changes to the interpretation that make it work better in the societal environment.

What makes for a successful religious organism?  A successful scientific organism features highly persuasive contrast between nucleus and co-nucleus, and there is presumably some of that in the religious case too, practices of other religions preserved as persuasive examples of what not to do; likely the scientific species is partly descended from the religious.  But there is also an interesting implication from the suggested religious model, that over many generations, a religious tradition will evolve to be amenable to a very wide range of interpretations, as this will allow the tradition to facilitate successful reproduction in a wide variety of societal environments.  A successful tradition would therefore be an ambiguous one.

Can memetics become a normal science?

There seem to be two problems with memetic research that have held it back.

One is that efforts in memetics have been dominated for decades by attempts to define what a meme is.  The definition of gene was arrived at after extensive study of biological organisms; so presumably, one should expect extensive study of memetic organisms to be a prerequisite for arriving at a really good definition of meme.  Identifying the organisms is a start.

The other has to do with what Kuhn called normal scientific research.  This is the sort of research that takes place within a paradigm scientific field (a thriving scientific organism, that is).  The paradigm usefully constrains the sorts of questions scientists are to ask and the sorts of answers they are to give (a nuance I didn't even try to capture in my rough portrait of scientific organisms, above).  Kuhn describes such research as "puzzle solving", and its narrow focus is its strength, allowing a very great deal of focused work to be done so that, eventually, flaws lurking in the paradigm become impossible to ignore and a reproductive event is triggered — a scientific revolution, shifting things to another paradigm better describing reality.

But memetics hasn't provided that sort of structure.

There seems to be some potential, in the ideas I've proposed here, to define how memetic organisms are to be identified and analyzed, sufficiently that researchers might proceed methodically to find and study organisms.  In other words, these suggestions might be developed into a functioning paradigm that could guide normal scientific research.

Maybe.