Does Light Have a Measurable Density?

[Bkat2d11]

Welcome to PF!

If you are talking about mass density, no, light does not have mass so it doesn't have density.

If it doesn't have any mass how does light move things?

 

[jbriggs444]

If it doesn't have any mass how does light move things?

It has momentum. Also, to the extent that light is omni-directional, it can have energy density and, hence, mass density. [An ideal laser beam remains massless. It has non-zero energy density but has a matching momentum density according to $E=pc$ so that the 4-vector sum has zero magnitude].

Mass is not additive. Two entities that individually have no mass can form a system with non-zero mass.

A quick trip to Google says that:

"The cosmic (2.7 K) background radiation, for instance, has an energy density of 0.24 eV/cm³"

Edit: Note that a Crookes radiometer (aka "light mill") does not demonstrate light pressure. It operates by a different and more subtle principle. You want a Nichols radiometer to measure light pressure.

 

[vanhees71]

Light is an electromagnetic wave field and as any dynamical "object" it contains and transports energy, momentum, and angular momentum. Any relativistic field can be characterized by two parameters, its mass (and by mass we exclusively mean invariant mass, which is a scalar under Poincare transformations) and its spin. The electromagnetic field is massless and has spin 1. As any massless field it has two polarization degrees of freedom, which can be charactrized by the two possible values of the helicity $h=\pm 1$, corresponding to the left- and right-circular polarized wave modes for any wave vector $\vec{k}$. The masslessness of the em. field implies the dispersion relation $\omega=c k=c|\vec{k}|$ for each field mode.

Energy and momentum of the electromagnetic field are described by the energy-momentum-stress tensor. For details see

https://itp.uni-frankfurt.de/~hees/pf-faq/srt.pdf