Black-and-white photography carries three associations. It’s generally considered more real, more serious, and more artistic. Serious and artistic are inherently subjective qualities. Real, though, is hardly subjective. Viewed in objective terms, color is far more real than black and white is - if only because color is such a defining element of reality. It’s fitting, then, that the 42 images which make up “Hymn to the Earth: Photographs by Ron Rosenstock’’ are in black and white. The show runs at the Worcester Art Museum through March 18.