Watching

We thought time travel was impossible, as we had never been visited by anyone from the future, but what we didn't know is that they've always been here - we just can't see them. They are humanity in its final form. They know no illness, and death is centuries away. And then one of them starts watching a 20th century girl dying of an incurable disease. Even the people of her time find this hard to look at. And yet the more he watches the less he begins to pity her. Why can't he turn away?