"The only thing that makes gravity is energy."
Correction, the only thing that we "know" makes gravity is energy.
"Light (which is an electro-magnetic phenomenon) has no mass, but has gravity."
Does light have gravity? Light can be effected by gravity, example black holes are black because it sucks in all the light. Lets nothing escape the event horizon.
But if light had gravity itself you'd be able to counteract or "turn off" gravity with a strong light source.
Or double it, increase it by reversing the direction of the light. Thus a mirror on and strong sunlight should cause a local gravity distortion. I don't believe it does.
We really did not understand Electronics and Magnetism until we had the capability to create, and store electricity. Once we were able to do that in a fairly short period of time we had electromagnets, those gave us electric motors etc. People figured out crude meters, leading to better tools, etc.
How do we build a device to create, store gravity? And with so much gravity all around us, how do we test things? How do we build the test equipment capable of measuring minor gravity fluctuations? How do we put gravity in a bottle, or battery to be released when we choose?
When I was still in school back in the 60's we took a field trip to see the big mainframe computer recently installed in the local college. Ran off punch cards I believe. Everyone got the chance to play tic tac toe with it.
(most lost, I fought it to a draw twice, then won the 3rd, tech's were going, "i thought it wasn't supposed to lose?)
Now I'm sitting at a PC that has so much more computing power available to be used per second that its not even funny. Where will that lead us?
Well, we do know how to create gravity. The only thing that makes gravity is energy. It can be in the form of mass (E=mc^2) which is a large amount of energy in a small volume and it can be the energy stored in an electro-magnetic field, does not really matter. Light (which is an electro-magnetic phenomenon) has no mass, but has gravity. However, we are no where near the capability of concentrating the energy of an electro-magnetic field to that of solid matter on the macro level (i.e. more than the mass-energy of a single elementary particle).