Every time you make a choice, you spawn a multitude of universes, leading to umpteen other yous – some of them living very different lives. This raises a myriad of moral conundrums, from what we owe our other selves to the death of hope.
It sounds like a concept from a philosopher's fevered imagination, but many physicists believe the multiverse is real. And they've got evidence – here are four here are four ways that multiverse may show itself in our everyday world.
1 The wave function
This mathematical entity describes the properties of any quantum system. Such properties –– an atom's direction of spin, say –– can take several values at once, in what is known as quantum superposition. But when we measure such a property we only get a single value: – in the case of spin, it is either up or down.
In the traditional Copenhagen interpretation of quantum mechanics, the wave function is said to "collapse" when the measurement is taken, but it isn't clear how this happens. (Schrödinger's famous cat, neither alive nor dead until someone looks inside its box, illustrates this.) In the multiverse, the wave function never collapses: rather, it describes the property across multiple universes. In this universe, the atom's spin is up; in another universe, it's down.
2 Wave-particle duality
In the landmark experiment, photons are were sent one at a time towards a pair of slits, with a phosphorescent screen behind them. Take a measurement at either slit, and you'll register individual photons passing particle-like through one or the other. But leave the apparatus alone, and an interference pattern will build up on the screen, as if each photon had passed through both slits simultaneously and diffracted at each, like a classical wave.
This dual character has been described as the "central mystery" of quantum mechanics. In the Copenhagen interpretation, it is down to wave function collapse. Left to its own devices, each photon would pass through both slits simultaneously: the measurement at the slit forces it to "choose". One way to explain the interference pattern through many worlds, by contrast, is that each photon only ever goes through only one slit. – Tthe pattern comes about when a photon interacts with its clone passing through the other slit in a parallel universe.
3 Quantum computing
Though quantum computers are in their infancy, they are in theory incredibly powerful, capable of solving complex problems far faster than any ordinary computer. In the Copenhagen interpretation, this is because the computer is working with entangled "qubits" which can take many more states than the binary states available to the "bits" used by classical computers. In the multiverse interpretation, it's because it conducts the necessary calculations in many universes at once.
4 Quantum Russian roulette
This amounts to playing the role of Schrödinger's cat. You'll need a gun whose firing is controlled by a quantum property, such as an atom's spin, which has two possible states when measured. If the Copenhagen interpretation is right, you have the familiar 50-50 odds of survival. The more times you "play", the less likely you are to survive.
If the multiverse is real, on the other hand, there always will be a universe in which "you" are alive, no matter how long you play. What's more, you might always end up in it, thanks to the exalted status of the "observer" in quantum mechanics. You would just hear a series of clicks as the gun failed to fire every time – and realise you're immortal. But be warned: even if you can get hold of a quantum gun, physicists have long argued about how this most decisive of experiments would actually work out.