# Cauchy-Schwarz for two spacelike vectors

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1. Aug 28, 2015

### bcrowell

Staff Emeritus
2. Aug 29, 2015

### martinbn

You can use the idea of the "usual" proof for the Euclidean case. If $v$ and $w$ are space-like then for any real number $t$ consider the vector $x=v+tw$ and its inner product with itself. You have $(x,x)=(v+tw,v+tw)=|v|^2+2(v,w)t+|w|^2t^2$. The inequality holds if and only if the discriminant of the quadratic polynomial is negative, if and only if the polynomial has only positive values. So if the inequality hold if and only if the span of the two vectors consists of space-like vectors.

3. Aug 29, 2015

### bcrowell

Staff Emeritus
Thanks, martinbn! I'll have to work that out and make sure I understand it.

4. Sep 6, 2015

### bcrowell

Staff Emeritus
I have a discussion of Cauchy-Schwarz and triangle inequalities now in section 1.5 of my SR book, http://www.lightandmatter.com/sr/ . The case we discussed here is relegated to a homework problem, where I suggest the idea of martinbn's argument (with credit to martinbn) and ask the reader to carry it out.