Understanding Radiation Pressure and Its Role in Star Dynamics

[TrickyDicky]

I would like to understand better the concept of radiation pressure, my main confusion comes from the fact that if the Stress-energy tensor for electromagnetic radiation is traceless, that would imply the pressure components of the tensor equal zero, and yet it's obvious radiation exerts pressure when absorbed or reflected and radiation pressure plays an important role in star dynamics.

I must be missing something really basic here, can someone explain this to me?

Thanks

 

[nicksauce]

The relativistic EM stress tensor in 4 dimensions is traceless. But there are 4 components to the trace... one energy density and three pressure. If you put that together, you find that the pressure is one third of the energy density.

 

[TrickyDicky]

The relativistic EM stress tensor in 4 dimensions is traceless. But there are 4 components to the trace... one energy density and three pressure. If you put that together, you find that the pressure is one third of the energy density.

So, I still don't get it, isn't it supposed to have the four components of the tensor vanishing, how can we make a proportion with quantities that are null? how can a zero pressure exert pressure? I'm stuck here.

 

[phyzguy]

What nicksauce is saying is that the stress-energy tensor of radiation looks like:
$$\begin{bmatrix}\rho&0&0&0\\0&-p&0&0\\0&0&-p&0\\0&0&0&-p\end{bmatrix}$$

With p = rho/3, the trace is zero.

 

[TrickyDicky]

What nicksauce is saying is that the stress-energy tensor of radiation looks like:
$$\begin{bmatrix}\rho&0&0&0\\0&-p&0&0\\0&0&-p&0\\0&0&0&-p\end{bmatrix}$$

With p = rho/3, the trace is zero.

Thanks, I see it now.