# Homework Help: About normalized D-velocity :uhh:

1. Dec 11, 2005

### alfredblase

what is meant by the expression "the normalized D-velocity"?

specifically when it is a tensor like $$u^\mu$$

Thanks

2. Dec 12, 2005

### dicerandom

I assume you're looking at something dealing with more than 4 dimensions? I think this just refers to a d-dimensional velocity vector which has been normalized to length one, you can think of it as a unit vector pointing along the particle's world line.

3. Dec 12, 2005

### robphy

From a google search of "normalized D-velocity",
one finds eq (1.2.4) in Polchinski's string theory book
$$u^\mu=\dot X^\mu (-\dot X^\nu \dot X_\nu)^{-1/2}$$
...in agreement with dicerandom's comment. (The idea is not specific to string theory, of course. The minus arises from the choice of convention for the Lorentz-signature of the metric.)

Last edited: Dec 12, 2005
4. Dec 12, 2005

### alfredblase

hehehe, that is precisely the book I am reading and the quote was taken precisely from that page xDD. its just that I had never come across that expression before. I should have guessed what the phrase meant, as it is actually quite obvious. Well reading this book is very good physics training for me =) thx guys. Ill get better and better I´m sure heh, but IT will get harder and harder haha, who will win?... emmm, oops xD