A recently proposed thought experiment in quantum mechanics suggests that different observers can come up with contradicting conclusions both with 100% certainty. So it suggests that different observers live in different universes. Then how can they interact with each other?

Or alternatively those conclusions are not contradicting after and coin really can come up both tails and heads at the same time. Or, another alternative, quantum mechanics is just not applicable on the macro level.

All three different answers are not welcome because they force us to abandon at least one of three very sensible assumptions of how things work. But it seems we have no other choice.