An experiment of cosmic proportions, looking at some of the most distant visible corners in the universe, could help close what may be the last major loophole in quantum physics, or shake it to its very foundations.
In the bizarre realm of quantum physics, two or more particles can get linked so they stay in sync instantaneously no matter how far apart they are. Albert Einstein derisively called this seemingly impossible connection "spooky action at a distance" — scientists nowadays give it the name quantum entanglement. Einstein believed that quantum entanglement could be explained on a deeper level by the more intuitive laws of classical physics.
In the classical picture, two objects in different regions of space could not influence each other faster than the speed of light. Also, every particle would have well-defined properties at every moment in time -- classical mechanics-based, "hidden variables" that would dictate the strangely synchronized entangled behavior between the particles.
Fifty years ago, physicist John Bell devised a mathematical formula that predicted what scenarios would occur if the counterintuitive predictions of quantum physics were governed by these classical hidden variables. Experiments on Bell's theorem have supported a purely quantum physics picture, rejecting the existence of hidden variables and showing particles are linked more strongly than one would expect under the laws of classical physics.
However, researchers have also identified major potential loopholes in Bell's theorem.
Two have been closed, but a third remains, "one known as 'setting independence' or sometimes colorfully called the 'free will' loophole," said theoretical physicist David Kaiser at MIT. "It's a really crazy-sounding loophole, but it turns out it's the easiest way to fake an outcome in tests of Bell's theorem."
In this scenario, the two detectors measuring the entangled particles have a shared history, via an event, information or third party they have in common. This could link them and lead to biased results. Therefore, a scientist testing Bell's theorem would not have complete control in choosing what each detector measures.
To solve this loophole, Kaiser and his colleagues have proposed looking for answers from the most remote corners of the known cosmos.
The experiment they propose relies on the fact that the universe has been expanding ever since the Big Bang happened nearly 14 billion years ago. As such, objects can be far enough away from each other to have been out of contact since the beginning of the cosmos, with no way for any signal to have ever reached from one to the other. This makes it possible to test the last loophole in Bell's theorem for a so-called "Cosmic Bell" experiment.
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