Well E=MC2 can only be “objectively true” when compared with a similar concept that would be expressed as “subjectively true” such as “OMG energy and might, mass and light!”

So, to your point, objective is relational. Relational to what? Well what ever it’s complimentary opposite could be, because without that complimentary opposite, then any label we give it, “objective” or otherwise, would be trivial with a capital T.

Science is only science when compared with processes that are not science, so “art” is quite distinguished there.

E = MC2 however is an objective math formula.

And math of course is an extraordinary objective language when compared against its opposite, poetry.

The underlying reality is irrelevant. What ever the underlying reality is, it appears to be elusive to all of us quite equally…or should I say objectively?