If energy has mass, how can it travel at the speed of light?
Unfortunately, there are several concepts all described as mass. A particle will travel at the speed of light if it has zero rest-mass. But that is not the same as its gravitational mass.
I think of mass a physical presence that has itertia. If energy has gravitational mass, does that mean that a larger collection of energy would have a measurable gravitational pull on a body of mass?
If so: How sensitive are our best gravitational sensors and how much energy would be required to be detectable?