Thank you! I have to think about lensing and horizons in general.

I wanted to select both answers, but this seems to be impossible. Thank you! This was very helpful.

Sergey, Thank you. I am reading everything I can on slice knots. Allow me to ask one additional aspect. As the previous poster and you write, the closed trefoil is not slice, but the connected sum of a trefoil and its mirror is. Is there a simple way to see which knots can appear for a second observer if the first one sees an open trefoil? Probably this would be the "concordance class" of the open trefoil (but maybe this is the wrong term). Is there a way to get an idea for which closed and open knots are contained in it? (Say the simplest elements.) This would help a lot!

No, I am interested in a closed trefoil.