# Contracting over indices chain rule

1. Nov 27, 2012

### black_hole

1. The problem statement, all variables and given/known data

As part of a problem I am doing I am asked to show uββuα = aα where u is 4 velocity and a refers to 4 acceleration. The way to do this is not immediately obvious to me, especially since the problem implies there should be a chain rule step involved which I am not seeing. I thought ∂ refers to ∂/∂t + ∂/∂x ect.

2. Relevant equations

3. The attempt at a solution

Last edited by a moderator: Nov 28, 2012
2. Nov 28, 2012

### Fredrik

Staff Emeritus
Hints: What is $\partial_\beta(u^\beta u^\sigma)$? What is the definition of $a^\sigma$, and is there a way to rewrite that in a way that involves partial derivatives?

Edit: Oops. You don't have to use the product rule here, so ignore my first hint. Just start with the definition of $a^\sigma$.

Last edited: Nov 28, 2012
3. Nov 28, 2012

### black_hole

Well if I do this (∂/∂t + ∂/∂x...)* each component of uβ there's still the uα I have to worry about. aα = duα/dτ so this seems to imply that uββ = d/dτ which doesn't seem right...?

And I'm not seeing how I can get that?

Oh wait never mind...I think I got it! If say I transform into a rest frame, the only component of the four velocity that is nonzero is the time component which is γc but γ=dt/dτ so uββ = d/dτ which makes the whole thing work! (yay)

Last edited: Nov 28, 2012
4. Nov 28, 2012

### black_hole

yeah you really don't have to use chain rule

5. Nov 28, 2012

### Fredrik

Staff Emeritus
In the (momentarily) comoving inertial coordinate system, we have u=(c,0,0,0), not (γc,0,0,0), and this is only at one specific moment. The four-acceleration is the rate of change of the four-velocity as a function of proper time, and the four-velocity is just the normalized tangent vector to the world line. The four-acceleration is telling us how that tangent vector's direction in spacetime is changing with proper time, so you can't compute it from knowledge of that direction at only one point on the world line.

You do, actually. Recall that the chain rule is about taking the derivative of the composition of two functions. Can you figure out what the two functions are in this case?

Last edited: Nov 28, 2012
6. Nov 29, 2012

### andrien

I replied to this thread by using chain rule which gives answer in one line.someone deleted it,and send me an infraction for it.

7. Nov 29, 2012

### Fredrik

Staff Emeritus
Yes, I noticed. My reply to it was deleted as well. I just said that in the homework forum, we try to give hints, not complete solutions. It can be hard to do this when the calculation is this short.