An unspoken assumption was made early in the September
1905 paper that each emitted light pulse was not material,
in that it carried energy but had no intrinsic mass.
Throughout his work on the special theory, Einstein maintained
that light was other than matter. As he put it in 1911,
“the comparison of light with other ‘stuff’ is not
permissible.”14 Thus, Einstein avoided a more complicated
calculation15 by conjecturing, without discussion or proof,
that light was massless. But of course he could not prove
such a thing, one way or the other.