It's fun to toss around theories about how there might be other universes out there, and there may even be some evidence that at least four of them have smashed headlong into our universe. You'd think proving that this is the case would be tricky, but neutrons may be taking trips to other universes all the time.
A few years ago, physicists showed that it might be theoretically possible for neutrons (particles of zero charge and one of the basic constituents of atoms) to make the leap from our universe to a different one if the gravitational potential of an entire galaxy were involved. Lucky for us, we happen to live in a galaxy (yay!) so if we look closely enough, it should be possible to spot these neutrons jumping out of our galaxy.
The odds of any one neutron pulling this trick are very, very low: worse than one in a million, if it happens at all (and that's a big if). But by watching a bunch of neutrons all at once, we might be able to experimentally verify whether any of them are disappearing off to alternate universes.
The way we'd do it is by filling up a bottle with ultracold neutrons, sealing it up, and then letting it sit there for a year and watching what happens. Neutrons undergo beta decay at a rate that we're familiar with, and if there's no other way for them to get out of the bottle but we notice that some extras are missing, there has to be an explanation as to where they went, and another universe might just fit the bill. Furthermore, since the gravitational potential changes as we orbit the sun, we should notice annual changes in the number of neutrons that vanish.
We're the first to admit that this theory seems a little, um, exotic, but it's also something that we're able to go and test with current technology. And if it does show something suggesting that neutrons are taking vacations off to alternate universes, well If nothing else, it would be one of the most conceptually mind-blowing discoveries ever made ever.