Development of the idea of the
Vector Cross Product
Engineering mathematics uses the vector dot product and cross product,
often as mathematical definitions with useful properties. The dot product
gives the vector amount that one vector contributes along the same line as another
vector, and that straightforward result may serve as sufficient explanation
for the dot product. The cross product, however, is partly the result
of multiplying different components of two vectors to get a product vector
that is at right angles to both of the original vectors and that has a magnitude
equal to the area of the parallelogram that the two vectors frame. The reason
for multiplying the components of the vectors in such a manner is usually not
explained in terms of how the mathematics was designed to account for
properties of nature. The usual explanation is that the mathematical relation
will be found useful.
The dot product is
a · b = |a||b|cos(theta)
The magnitude of the vector cross product is
|a X b| = |a||b|sin(theta)
And the vector cross product is
a X b = (a2b3 - a3b2)i -(a1b3 - a3b1)j + (a1b2 - a2b1)k
The magnitude of the cross product is equivalent to the area of the
parallelogram that the two vectors (a and b)
describe, and the formula for the vector product is the same as the determinant
for the array shown here.
| i j k|
det |a1 a2 a3|
|b1 b2 b3|
The vector product is perpendicular to both of the original vectors.
A few questions
- How was the cross product defined?
- Was it defined to account for how nature works?
- Why does the cross product (and the natural
field behavior) act at right angles with the
plane that the original vectors define?
- Is there a reason that the order of cross product
coefficients is given in the determinant?
While trying to find more about the origin of the cross product in
particular, I found an advertisement for a book about determinants
in a 1882 book about quaternions. The advertisement promoted the book as
increasing interest in the discussion through showing how natural
considerations led to the development of the determinant. More generally,
it seems that including
the historical development or even the cause for the development of
a tool shows an appropriate use of the tool or property.
The following information is from A History of Vector Analysis; The
Evolution of the Idea of a Vectorial System, Michael J. Crowe, 1967,
republished in 1985.
Early influences
- Parallelogram of forces
Bernard Lamy and Pierre Varignon
- Leibniz' Concept of a Geometry of Situation
I believe that by this method one could treat mechanics
almost like geometry, and one could even test the qualities of materials,
because this ordinarily depends on certain figures in their sensible
parts. Finally, I have no hope that we can get very far in physics until
we have found some such method of abridgement to lighten its burden of
imagination. (Quoted from Leibniz "Studies in Geometry of Situation
with a Letter to Christian Huygens" in Philosophical Papers and Letters,
1956)
- The Concept of the Geometrical Representation of Complex Numbers
Creative Design
- Sir William Rowan Hamilton (1805-)
-
- After his third year at Trinity College of Dublin University,
he was "offered the honor of becoming Andrews' Professor of Astronomy
at the University of Dublin and Royal Astronomer of Ireland". (MJC)
- While seeking "the extension of the complex number system to three
dimensions", Hamilton discovered quaternions on October 16, 1843. Quoting MJC
Thus in a very dramatic manner Hamilton discovered and announced the
discovery of quaternions. These are hypercomplex numbers of the form
w + ix + jy + kz, where w, x, y, and z are real numbers, and i, j, and k
are unit vectors, directed along the x, y, and z axes respectively. The i,
j, and k units obey the following laws: ij=k jk=i ki=j ji=-k kj=-i ik=-j
ii=jj=kk=-1
It is to be noted that for two quaternions q and q', qq' does not in
general equal q'q. The loss of commutativity in quaternions, while it is
very important historically, is also significant mathematically, because
this complicates calculations in which quaternions are used. All the other
properties discussed above are satisfied by quaternions. Thus it may be verified
that quaternion multiplication is associative and quaternion division is
unambiguous. These are two important properties which bear special mention,
since they are not preserved in the algebra of modern vectors.
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- Hermann Grassmann (1809-1877)
-
- Attended no mathematical lectures at the University of Berlin.
- Grassmann wrote Theorie der Ebbe and Flut in 1840.
(The book was published in 1911).
- He said that the idea of geometrical multiplication was from his father's
books. His father had written, The rectangle itself is the true
geometrical product, and the construction of it, as it appears in
section 53, is really geometrical multiplication. If the concept
of multiplication is taken in its purest and most general sense, then
one comes to view a construction as something constructed from elements
already constructed and in fact constructed in the same way. Thus
multiplication is only a construction of a higher power. In geometry
the point is the original "producing" element; from it through construction
the line emerges; if we take as the basis of a new construction the finite
line constructed from the point, and if we treat it in the same manner as
we formerly treated the point, then the rectangle emerges. Just as the line
came from the point, so the rectangle comes from the line.
- Included philosophical reasoning based on space in his early work.
- Quoting MJC, Grassmann's geometrical product is quite similar
to the modern vector (cross) product. Another quote in the Grassmann
multiplication of two vectors the product is not another vector,
but rather a directed area. It is true that this area (or the set of
geometrically equal areas) determines a vector (or set of vectors)
perpendicular to that area, and that the vector (or vectors) so defined
is precisely that vector which is the product in the modern form of the
multiplication.
- Grassmann defined a linear product that "is identical to the modern
scalar product".
A History of Vector Analysis, James Walter Joiner, 1971, (doctoral
dissertation at George Peabody College for Teachers)
The history of the development of vectors and the cross product involved
more than one person and the consequent choices and practice of the
community of workers who heard about and used the developing set of tools.
Hamilton was one of the early developers, notably from his discovery of
quaternions in 1843. His idea seems to have been based on analogies with
the theory of complex or imaginary numbers (numbers having a component
with a factor of the square root of -1.) Hamilton was the first to use
the word vector, based on the Latin veher, "to draw".
Oliver Heaviside promoted the use of vectors (including cross products)
in his 1893 book "Electromagnetic Theory". He said that vector mathematics
were considerably easier to use than quaternions and that vector mathematics
could be discovered while using Cartesian mathematics through noting groups
of calculations that repeat when working with the math for certain kinds of
engineering applications.