And sadly for Justice Stewart this is, of
all his writing it's what most of us remember.
But it's I think a useful thing.
When we think about life, that
and many of us have a sense for if we were to encounter life on Mars.
Encounter some of the signs of life elsewhere.
We'll know it when we see it. But, how might we see it on Mars?
How might we find signs of life?
And I think a useful way to think about that problem, is turn it around.
Ask, if you were an alien observing the Earth from afar.
How might you detect signs of life?
Well one way of detecting the signs of our advanced civilization.
Is that we're making movies, we making television.
We're broadcasting television and radio signal out into space.