Only postmodernists & creationists think that objective notions of right & wrong are concepts that cannot be provided by the real world.
The use of "Only" is very arrogant of you...
Oh? Who else am I leaving out?
...and "objective notions" seems to be an oxymoron.
I mean concepts of right & wrong as objectively true, as opposed to "socially constructed competing texts" or "womens' truth vs. mens' truth" or somesuch set of equally valid competing subjective "truths". Creationists likewise believe Man will never converge on a single well-supported consensus of what the Truth is.
A concept is a mental construct, occurring in the mind. Since it is in the mind, by definition, it is subjective. To call something objective it must come from outside the human mind. We may try to be "objective" and take our personal biases out of the equation, but how do we do that without assumption? The "real" world is quite different to different people. Some people have such a different view of the world that we call them insane, others are just colorblind, some might even be tetrachromats.