If parallel universes exist, here's how we could actually find the evidence
Every few seconds, the spin direction of a random particle in the room is measured. If the particle is spinning one direction then the gun goes off and the physicist dies. If the particle is spinning in the opposite direction, there's just a clicking sound and the physicist survives.
She has a 50/50 chance of surviving, right?
It might not be that simple if we live in a multiverse - the idea that multiple universes, apart from the one we call home, exist.
This scenario with the physicist and the gun is the start of a famous thought experiment called "quantum suicide," and it's one way for physicists to consider if we really are living in just one of many (and potentially infinitely many) universes.
This thought experiment relies on quantum mechanics and the idea that there is no single objective reality. Everything that we see around us is just one possible configuration of all the probabilities of any one thing happening. One interpretation of quantum mechanics is that all the other arrangements of probabilities could exist in their own separate universe. So if you follow the thought experiment all the way through with this idea in mind, then the second that first particle is measured, the universe splits into two universes, based on two possible outcomes: one in which the physicist lives and one in which she dies.
Her survival is now tied to a quantum probability, so she'll be both dead and alive at once - just in different universes. If a new universe splits off every time a particle is measured and the gun either fires or doesn't, then in one of those universes, the physicist will end up surviving, say 50 particle measurements. You can think of this as flipping a coin 50 times in a row. You have an extremely low likelihood of getting heads each of those 50 times - less than a 1 quadrillion chance, but it is possible.
And if it happens, that's enough for the physicist to conclude that the multiverse is real, and effectively she becomes immortal in the universe in which the gun never goes off. But she also becomes the only person who knows that parallel universes exist.
This probably sounds like the plot of a sci-fi movie, but there are other, more reasonable-sounding versions of the multiverse that are backed up by math and are potentially testable.
"Some people conflate parallel universes with jumping through a portal into another world, or something like that," Matthew Johnston, a physicist at the Perimeter Institute, told Business Insider. "But it's not really like that at all."
Actual observational evidence of a multiverse will be tricky to find, but it is possible. Here's how physicists will do it:
There are actually many different multiverse theories, and the multiverse from the quantum suicide thought experiment, where every possibility becomes reality, is one of the most radical.
A convenient way to think about different multiverse theories is MIT physicist Max Tegmark's multiverse hierarchy where he organizes them into four different levels.
We will focus just on level one multiverses - the version with the most traction in physics, and thankfully the easiest to wrap the mind around. Level one is also where we have the best chance of actually finding evidence that proves the multiverse is real.
Multiple universes are predictions of the math behind existing theories, and the level one multiverse idea is predicted by a very well-respected and central concept in physics: inflation.
So it's an idea that physicists have to take seriously, Johnson said.
What we mean by 'universe'
To think about the idea of multiple universes, we first have to pin down what we mean by universe. Our definition of "the universe" has been changing since the invention of the first telescope when we peered out into the cosmos and learned that the Earth is not the totality of existence.
But the universe is a lot bigger than what we could ever see with a telescope, Johnson said. Our universe is just the spherical amount of light that has had time to reach us. If we wait another billion years for more light to reach us, our definition of the universe would change, Tegmark told Business Insider.
Someone standing on a planet trillions of lightyears away would have a completely different picture of "the universe" based on how much light has reached their planet.
By definition there's no way to get to these other bubble universes because we'd have to travel faster than the speed of light. While we can't see them, physicists think we could still find traces of them from their birth.
Where's the evidence?
The idea of inflation holds that our infant universe experienced a period of rapid expansion (right after the Big Bang) where a nanometer of space suddenly exploded into over 250 million lightyears of space in less than a trillionth of a second.
Once inflation starts, it never completely stops. It does stop in some regions of space-time where chunks of space pinch off into bubbles like the universe we see around us today, but space continues expanding every where else. If expansion is infinite, which many believe it is, then new bubble universes are constantly forming. You can almost think of it as running with a bubble wand held out behind you: you'd leave a trail of bubbles.
So essentially we might be drifting through space-time in a frothy bubble bath of universes.
Again, there's no way to get in touch with any of these other bubble universes because we can't travel faster than the speed of light. That being said, we should theoretically be able to prove they exist. Here's how:
When our own bubble universe first formed, it's possible that we collided with other bubble universes forming around ours. We're probably not near any of those bubble neighbors anymore because the continued expansion of space-time is carrying us farther and farther away.
However, the impact of an early collision could have sent ripples through the cosmic microwave background radiation (heat that is left over from the Big Bang). Theoretically, we should be able to spot those ripples with telescopes, Johnson said. It would show up as a disc of discoloration - like a bruise.
Johnson is looking for those bruises, but a lot depends on how fast other bubble universes sprang into existence and how many there are. If only a few other bubbles exist, they might have not hit us.
The Planck space telescope is currently listening to the skies, searching for the evidence of these multiverse collisions.
A multiverse hiding in the LHC
Some physicists have theorized a different version of the multiverse. This version arises from string theory and the idea that there are many more dimensions that we don't have access to (think Matthew McConaughey in the fifth dimension in "Interstellar"). Some physicists think parallel universes lurk in those extra dimensions.
This multiverse idea is testable too.
Physicists will be searching for mini black holes when the Large Hadron Collider turns back on this month. It's impossible for the LHC to produce any type of black hole that would be remotely dangerous, but this theory speculates that microscopic black holes that disappear almost immediately could be generated from the high-power particle collisions in the LHC. The presence of black holes would indicate that gravity from our universe is seeping into extra dimensions.
"As gravity can flow out of our universe into the extra dimensions, such a model can be tested by the detection of mini black holes at the LHC," physicist Mir Faizal told Phys.org. "We have calculated the energy at which we expect to detect these mini black holes in gravity's rainbow [a new theory]. If we do detect mini black holes at this energy, then we will know that both gravity's rainbow and extra dimensions are correct."
That would be compelling evidence for both string theory and parallel universes, and it would help explain why gravity seems to be so much weaker than the other fundamental forces.
Still, there's no hard evidence yet. And some still doubt that these universes exist.
"I only believe in things with concrete, verifiable experimental evidence supporting them, and that's not the case right now with the concept of parallel universes," Brian Greene, a theoretical physicist at Columbia University, said in a video discussing the multiverse.
The key, though, is that physicists are getting away from just philosophical discussions of the multiverse, Johnson said. They're actually putting the idea to the test.
Some are still betting on the more radical and so far untestable versions of multiverse theory. Tegmark has joked that he'll try the quantum suicide experiment when he's old and no one will miss him when he's gone.
We hope that he doesn't.