# Electromagnetic tensor

dear all,

I have a question about Electromagnetic field tensor.
As I was reading introduction to relativity by Hobson, I saw this sentence:
"in order that the rest mass of a particle is not altered by the action of the erforce we require the latter to be a pure force, so:
u.f=0
1) what does it mean?

The other question is about the rest of this text:

in the component form this reads:

f_μ u^μ=q F_μσ u^μ u^σ=0
so that F must be an anti symmetric tensor. 2) why?!!!!!

## Answers and Replies

Related Special and General Relativity News on Phys.org
If this holds

Fμσuμ uσ = 0

then Fμμ = 0, and Fμσ = -Fσμ

It seems obvious to me if the full expansion is written out, but I'm not sure if what I've written is rigorous or helpful.

If this holds

Fμσuμ uσ = 0

then Fμμ = 0, and Fμσ = -Fσμ

It seems obvious to me if the full expansion is written out, but I'm not sure if what I've written is rigorous or helpful.
Dear friend,

I agree with he first one but could you please explain the second one more?
Fμσ = -Fσμ

I don't know why it seems so odd to me!

F12u1 u2 + F21 u2 u1 = 0 => F12 = -F21 ?

F12u1 u2 + F21 u2 u1 = 0 => F12 = -F21 ?
U r right!
Thank you!

does anyone know the answer to the first question?
why should it be like this:
u.f=0?!!
what does hobson mean by that?

Bill_K