Question about relativistic terms ?

Question

If a(vector) is the proper acceleration vector of an object, what is |a| is called in relativistic terms ?

Answer 1

The magnitude of the proper acceleration is a good name for it.  I don't know if there is an official name for it.  It's equal to the magnitude of the 3 dimensional spatial acceleration in the rest frame of the object.  

|v|^2=v*v=-1 (where * means dot product), and taking the derivative we find that v*a=0.  In the rest frame of the object, v=(1,0,0,0), so a=(0,a_x,a_y,a_z) to ensure that v*a=0.

From this we find that |a| is the magnitude of the 3-d acceleration in the rest frame of the object.

____

Answer 2

(This is a private joke between the asker and myself)

I am throwing popcorn at your head.

Answer 3

it represents the magnitude of that acceleration.

example:

A(vector) = 3meters/sec^2 at 30 degrees north of east.

|A| = 3 meters/sec^2 only.