Some ideas about the quantum world appear to suggest there are many versions of you spread out across many parallel universes. Now, two scientists have formulated a proof that attempts to show this is really true.
The proof involves a fundamental construct in quantum mechanics called Bellâs theorem. This theorem deals with situations where particles interact with each other, become entangled, and then go their separate ways. It is whatâs called a âno-go theoremâ, one designed to show that some assumption about how the world works is not true.
Bellâs theorem rests on three assumptions. First, thereâs local causality, which says that objects can only affect whatâs near them and an effect must occur after its cause. Next, a lack of superdeterminism, which rules out the idea that everything is predetermined by some external force. The last is every measurement has only one outcome, a stipulation that researchers simply call âone worldâ.
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Tests of this have already shown that all of these assumptions cannot be true at once. Measuring one partner in a pair of entangled particles seems to affect the other, even when the two are separated by vast distances and the measurements are made far too quickly for any signal to travel between them.
Traditionally, physicists say that this means local causality is violated, and it proves that entangled particles can change one anotherâs measured states. But Mordecai Waegell and Kelvin McQueen at Chapman University in California interpret it differently. They argue in a paper submitted to the British Journal of Philosophy of Science that local causality can be preserved â but only if there are many worlds.
âEveryone agrees that thereâs a contradiction if you accept all three axioms of Bellâs theorem and the experimental results, so youâve got to reject at least one,â says McQueen. But it actually makes most sense to get rid of the requirement for a single world, say McQueen and Waegell.
Read more: How to think about⊠The multiverse
They worked through a classic thought experiment in which three entangled particles are sent to three detectors that are far away from one another. The people at the detectors, who weâll call measurers, are Alice, Bob, and Charlie. First, Alice makes a measurement of a quantum property of her particle called spin. Then Bob measures the same thing for his particle, followed by Charlie for his particle. Each measurement will either return a spin of up or down.
Based on the rules of entanglement, if we know what Alice measured, it narrows down the possible results from Bob and Charlieâs measurements. If we know what both Alice and Bob measured, we can predict the exact results of Charlieâs measurement. For example, if Alice and Bob both get spin-up, Charlie must get spin-down.
But when the researchers calculated every possible outcome in a scenario including local causality, they found that Alice would have to get two different results from one measurement. Aliceâs particle must be both spin-up and spin-down when she measures it.
âWe get a contradiction in what Alice measured: she must have gotten one result, and also must have gotten the other result,â says McQueen. âThatâs not possible â not unless you have two Alices.â
The solution, they say, is a hypothesis called semi-local worlds. In this scenario, when Alice makes a measurement, she splits into multiple Alices who get different results. The same goes for Bob and Charlie. The worlds of each of the measurers continues separately until they compare their results, at which point their worlds merge.
âThe Bob that obtains a particular measurement is only going to meet an Alice that obtains a corresponding measurement,â says Mateus AraĂșjo at the University of Cologne in Germany, who was not involved in the work. âIt starts as entanglement of particles, but then when you do the measurement it becomes an entanglement of worlds.â
Many physicists are sceptical of the idea because it is difficult to test empirically. McQueen admits as much. âI donât think I could ever experimentally confirm that you have bifurcated into two versions of yourself,â he says.
Waegell, however, says there may be a way to test it by taking extremely fast measurements of systems in the process of splitting into different worlds. But he is not sure if we will ever have the equipment to do so.
Many worlds might also make it easier to reconcile quantum mechanics with Einsteinâs theory of general relativity, Waegell says. The mismatch between these is one of the biggest problems in physics.
âI think Einstein probably would have hated this,â says AraĂșjo. Nevertheless, he says, itâs just as plausible for the incorrect assumption in Bellâs theorem to be the one stating there is only one world as it is to be locality, perhaps even more so.
