There are two extreme positions that one can adopt in relation to the nature of reality and our knowledge of it.
Realism [extreme objectivism]: the world is the way it is and we can know it just as it is;
Idealism [extreme subjectivism]: the world is the way we know it to be—what it is apart from our knowledge of it is not something we can know.
Each of these extreme positions encounters its own set of problems.
A more moderate form of realism holds that the world is the way it is but our knowledge of it is invariably coloured by our own species limitations and individual limitations. As Aquinas says, quidquid recipitur recipitur secundum modum recipientis—whatever is received is received according to the mode of the receiver. The human senses are limited to a narrow band of the electromagnetic spectrum and psychologists have demonstrated the extent to which perception is actively constructed rather than passively received. On a higher level, our beliefs are embedded in a web of bias and subjectivity.
On this view of things, our knowledge is real but limited, imperfect but, at least in principle, perfectible. This applies across the board both to physical and to social realities.
In relation to the nature of physical reality, these problems constitute a large part of the philosophy of science. I’m going to confine my subsequent remarks to social reality.
You ask: what is objective law?
Laws are prescriptive rather than descriptive; they tell us what should be done or not done, not what is or is not the case. The realism/idealism divide on law comes to something like this:
The legal realist will argue that laws are objective to the extent that they correspond to our human nature, making it possible for that nature to be developed individually and in cooperation with others. There can only be one law or set of laws that meets this criterion.
The legal idealist will argue that, at best, objectivity in law is possible only formally and not materially, there being no way to know objectively what is or is not constitutive of human flourishing.
The moderate legal realist will agree with the legal realist in part [we can know, broadly, what constitutes human flourishing] but will agree with the legal idealist in allowing for a multiplicity of approaches and instantiations of law, all permitting our human nature to flourish, some better than others.