Understanding Scalar and Vector Products in Geometric Algebra

In summary: It works in any dimension with geometric algebra as well. Also, the 7D cross product is unsatisfactory: no Jacobi identify and it's not unique.
  • #1
smims
7
0
(Scalar)·(Scalar) = Scalar
(Scalar)·(Vector) = Scalar
(Vector)·(Vector) = Scalar
(Scalar)x(Scalar) = Not valid
(Scalar)x(Vector) = Vector
(Vector)x(Vector) = VectorDid I get them right, if not why?

Thanks
 
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  • #2
By "x" do you mean the vector cross-product? And by "." do you mean the vector scalar product? If the answer is yes, then those operations are only applicable to a pair of vectors. I would say there are four common types of "multiplication" involving scalars and 3D vectors:
  1. Scalar times scalar to produce a scalar (ordinary multiplication)
  2. Scalar times vector to produce a vector (scaling a vector)
  3. Vector times vector to produce a scalar (scalar or "dot" product)
  4. Vector times vector to produce a vector ("cross" product)
 
  • #3
5. Scalar cross scalar to denote the pair (scalar,scalar). "Not valid" is misleading here.
6. Scalar cross vector also denotes a pair and no vector. If it is supposed to be a vector, the cross has to be explicitly defined, e.g as scaling.
 
  • #4
fresh_42 said:
5. Scalar cross scalar to denote the pair (scalar,scalar). "Not valid" is misleading here.
You are interpreting AxB for scalars A and B in the sense of the Cartesian product of a pair of sets? [the set of ordered pairs (a,b) where a is an element of A and b is an element of B]. That is rather a stretch since A and B are not sets and their cross product would not be an ordered pair but would, rather, be a [possibly singleton] set of ordered pairs.
 
  • #5
jbriggs444 said:
You are interpreting AxB for scalars A and B in the sense of the Cartesian product of a pair of sets? [the set of ordered pairs (a,b) where a is an element of A and b is an element of B]. That is rather a stretch since A and B are not sets and their cross product would not be an ordered pair but would, rather, be a [possibly singleton] set of ordered pairs.
Agreed. I simply found "not valid" a bit harsh, esp. because the word scalar has been put into brackets. Sometimes I've also read lines like ##2 \times 2 = 4##.
 
  • #6
fresh_42 said:
Agreed. I simply found "not valid" a bit harsh, esp. because the word scalar has been put into brackets. Sometimes I've also read lines like ##2 \times 2 = 4##.
If one insists on intepreting ##\times## in the sense of the Cartesian product and interpreting 2 in the sense of set theory and the Von Neumann construction then it would be more correct to say that ##|2 \times 2| = 4## and that ##2 \times 2## = { (0,0), (0,1), (1,0), (1,1) }

This follows since, in the Von Neumann construction, 2 = {0,1}.

Bringing us back on topic for this thread... One should interpret the ##\times## notation according to context. In the context of ##2 \times 2## and a discussion of scalars and vectors, it cannot reasonably denote either a cross product of vectors or a Cartesian product of sets. The most reasonable interpretation would as an ordinary product of integers.
 
Last edited:
  • #7
Thank you all for your feedback.
The comments certainly help a lot.
 
  • #8
stevendaryl said:
By "x" do you mean the vector cross-product? And by "." do you mean the vector scalar product? If the answer is yes, then those operations are only applicable to a pair of vectors. I would say there are four common types of "multiplication" involving scalars and 3D vectors:
  1. Scalar times scalar to produce a scalar (ordinary multiplication)
  2. Scalar times vector to produce a vector (scaling a vector)
  3. Vector times vector to produce a scalar (scalar or "dot" product)
  4. Vector times vector to produce a vector ("cross" product)
It works for 7D vectors as well.
 
  • #9
Zafa Pi said:
It works for 7D vectors as well.

It works in any dimension with geometric algebra as well. Also, the 7D cross product is unsatisfactory: no Jacobi identify and it's not unique.
 

Related to Understanding Scalar and Vector Products in Geometric Algebra

1. What is the difference between a product of scalars and a product of vectors?

A product of scalars refers to the multiplication of two or more scalar quantities, which are single numbers that have magnitude but no direction. In contrast, a product of vectors involves multiplying two or more vector quantities, which have both magnitude and direction.

2. How is the product of scalars different from the dot product of two vectors?

The product of scalars is a simple multiplication of numbers, while the dot product of two vectors is a mathematical operation that results in a scalar quantity. The dot product takes into account the magnitudes and angle between two vectors, while the product of scalars does not.

3. Can the product of scalars result in a vector quantity?

No, the product of scalars will always result in a scalar quantity. This is because scalar quantities do not have direction, so they cannot be combined to create a vector quantity.

4. How is the product of scalars and vectors used in physics?

The product of scalars and vectors is used in various equations and calculations in physics. For example, velocity (a vector quantity) can be calculated by multiplying a scalar quantity (speed) with a vector quantity (direction). It is also used in calculating work, power, and force, among other physical quantities.

5. Is the product of scalars and vectors commutative?

No, the product of scalars and vectors is not commutative. This means that the order in which the quantities are multiplied affects the result. For example, the product of a scalar and a vector will give a different result than the product of the vector and the scalar, unless the scalar is equal to 1.

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