That's a good question, actually. I'm not really sure of the answer to it.

I know that five thousand years ago there was an inter-dimensional breach where these people built a portal and were able to come to our world. Their technology was much more advanced than ours was. Definitely closer to yours than us. They had satellites even. But the portal was eventually sealed off... but there's been smaller rips to other worlds that pop up occasionally.

I imagine one could potentially go to Earth, but I've never seen one do that before.

So... honestly, I don't know.