Show me the "good particle" and then we'll talk. Until then I can only rationally assume something doesn't exist until proven otherwise.
Some of the more interesting (to me) arguments of the realists/naturalists (the justification question rather than the ontological question above), consider there to be emergent properties from the nature of interaction between agents. Basically game-theory. Presume we all were psychopaths, but intelligent self-maximizing agents and not the irrational kind. What behaviors would maximize achieving our ends? What behaviors would we need to hold others accountable to support our ends? You can take this thought experiment pretty far to get an "emergent" moral system with first-order properties similar to those we see in real societies. I find this insufficient though, you still need some base value system. Even the psychopath example presumes a set of ends for individuals like "continued survival" and so boils down to something like a form "utilitarianism."
Not especially: there is an abundance of immanent ethical theories that define the Good (or Right) in terms of basic things in our grasp: the number of people who go hungry, the number of people who die of preventable diseases, &c. These don't require some spooky or transcendental universal fundamental: they're about seeing people suffer in ways that we can measure, seeing that many patterns of suffering are generalizable, and taking actions to countermand that.
(There are also plenty of ethical systems that are both immanent and non-consequentialist. I follow one of them. But it's maybe beyond the point of the original comment to explain them.)
The problem with all those theories, is they can be attacked as simply efforts to define "good" – with the ensuing problem that other people will define "good" in contrary ways, and if "good" is just a definition, then how can a mere definition be, in an objective sense, superior to a competing proposed definition?
That's basically G. E. Moore's argument against naturalistic objective ethics. If one agrees with it, then the only options available are to reject objective ethics, or to reject naturalism.
Neither intersubjective nor Kantian ethics have this problem: intersubjective ethics doesn't admit of an objective Good, and Kantian ethics don't admit of an is-ought distinction (all "ought"s are in fact "is"es that are bound in actions).
You don't have to agree with them, but it's a real phenomenon. It's up to you to decide how you handle it.
I agree, that if naturalism / physicalism / materialism is true, then the concept of "objective good" becomes hard to justify. Certainly, some people have tried, but a lot of people question whether those attempts can be successful – indeed, doubting those efforts is one thing that both naturalists who doubt the objectivity of the good, and non-naturalists who believe in the objectivity of the good but reject the idea that it can be given a naturalist foundation, can agree on.
