# Homework Help: Complex Numbers and Vector Multiplication

1. Jun 27, 2011

### iampaul

I have read from my algebra book that the product of two complex numbers is still a complex number: (a+bi)(c+di)= (ac-db)+(bc +ad)i
I was thinking that since complex numbers can be used to represent vectors, the product of two vectors should still be a vector. But I have also read from my physics book that there are two ways to multiply vectors, which are the dot product and the cross product.The dot product is the product of two parallel vectors and results into a scalar, while the cross product is the product of two perpendicular vectors. Why should these products be defined differently? Using complex no.s, two vectors whether parallel or not should still yield a product which is still a vector. Are these dot and cross products different from ordinary complex number multiplication? If so, when do we use the ordinary complex no. or vector multiplication? Am i missing anything? What math topics should i read? Please reply, i'm really getting confused.

Any help will be greatly appreciated!!

2. Jun 27, 2011

### tiny-tim

hi iampaul!
vectors exist in a vector space

a vector space is defined as having addition of vectors, and multiplication of a vector by a by a scalar (to make a vector)

we can also define an inner product (essentially, combining two vectors to make a scalar), a wedge product (essentially, combining two vectors to make a "2-form", which is something like a tensor), and a dual (eg in 3D, the dual of a vector is the 2-form "perpendicular" to it)

we cannot define a combination of two vectors to make another vector (other than addition, of course) unless we abandon the symmetries of the space

for example, in your 2D case, we could define a "complex vector product" to be the result of multiplying the corresponding complex numbers …

but then y times y would be -x (because i times i = -1), but x times x would be x

this is unsymmetric: rotating the whole space 90° should leave everything looking the same, but it makes your very simple "complex vector product" look completely different!
No, the dot product is a product of any two vectors (and results in a scalar), while the https://www.physicsforums.com/library.php?do=view_item&itemid=85" is the product of any two vectors, and results in a pseudovector (which teachers usually wrongly tell you is another vector, since after all it does look exactly like one ).

The dot product is an inner product. The cross product is the dual of a wedge product.
google "vector space" and "wedge product"

Last edited by a moderator: Apr 26, 2017