David Kipping

The second one would be unbounded avoidance capacity.

So I might see a UFO tomorrow and then the next day and then the next day and then predict I should see it on Thursday at the end of the week, but then I don't see it.

But I could always get out of that and say, well, that's just because they chose not to come here.

They can always avoid future observations fairly easily.

If you survey an exoplanet for biosignatures and you don't see oxygen, you don't see methane, that doesn't mean there's no one living there.

They could always be either tricking their atmosphere, engineering it.

We actually wrote a paper about that, how you can use lasers to hide your biosignatures as an advanced civilization.

Or you could just be living underground or underwater or something where there's no biosignatures.

So you can never

really disproved there's life on another planet or on another star.

It has infinite avoidance.

And then finally, the third one is that we have incomplete physical understanding of the universe.

So if I see a new phenomena, which Boyajian star was a good example of that, we saw this new phenomena of these strange dips we'd never seen before,

It was hypothesized immediately this could be aliens.

It's like a god of the gaps, but it turned out to be incomplete physical understanding.

And so that happens all the time.

In the first pulsar that was discovered, same story, Jocelyn Bell somewhat tongue-in-cheek called it Little Green Man 1 because it looked a lot like the radio signature that was expected from an alien civilization.

But of course, it turned out to be a completely new type of star that we had never seen before, which was a neutron star with these two jets coming out the top of it.

That's a challenge.

Those three things are really, really difficult in terms of experimental design for a scientist to work around.