麻豆传媒

Some ideas聽about the quantum聽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聽construct in quantum mechanics called Bell鈥檚 theorem.聽This theorem deals with situations where particles interact with each other, become entangled, and then go their separate ways. It is what鈥檚 called a 鈥渘o-go theorem鈥�, one designed to show that some assumption about how the world works is not true.

Bell鈥檚 theorem rests on three assumptions. First, there鈥檚 local causality, which says that objects can only affect what鈥檚 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 鈥渙ne world鈥�.

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 another鈥檚 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 鈥� but only if there are many worlds.

鈥淓veryone agrees that there鈥檚 a contradiction if you accept all three axioms of Bell鈥檚 theorem and the experimental results, so you鈥檝e got to reject at least one,鈥� says McQueen. But it actually makes most sense to get rid of the requirement for a single world, say McQueen and Waegell.

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 we鈥檒l 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 Charlie鈥檚 measurements. If we know what both Alice and Bob measured, we can predict the exact results of Charlie鈥檚 measurement.聽For example, if聽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. Alice鈥檚 particle must be both spin-up and spin-down when she measures it.

鈥淲e get a contradiction in what Alice measured: she must have gotten one result, and also must have gotten the other result,鈥� says McQueen. 鈥淭hat鈥檚 not possible 鈥� not unless you have two Alices.鈥�

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.

鈥淭he Bob that obtains a particular measurement is only going to meet an Alice that obtains a corresponding measurement,鈥� says Mateus Ara煤jo at the University of Cologne in Germany, who was not involved in the work. 鈥淚t starts as entanglement of particles, but then when you do the measurement it becomes an entanglement of worlds.鈥�

Many physicists are sceptical of the idea because it is difficult to test empirically. McQueen admits as much. 鈥淚 don鈥檛 think I could ever experimentally confirm that you have bifurcated into two versions of yourself,鈥� 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 Einstein鈥檚 theory of general relativity, Waegell says. The mismatch between these is one of the biggest problems in physics.

鈥淚 think Einstein probably would have hated this,鈥� says Ara煤jo. Nevertheless, he says, it鈥檚 just as plausible for the incorrect assumption in Bell鈥檚 theorem to be the one stating there is only one world as it is to be locality, perhaps even more so.