So if influences do travel at finite speed then faster-than-light communication also becomes possible. They then showed, also using a theoretical argument, that the system does not satisfy the inequality. Like John Bell they came up with an inequality: if the system is to remain un-exploitable for faster-than-light communication, then measurements must satisfy this inequality. They imagined a system of four entangled particles and supposed that influences travel between them at finite speed. No experiment can rule out the hidden influence idea directly, so the researchers used a theoretical argument. Space and time," says Jean-Daniel Bancal, one of the physicists Observe without sacrificing our sense of things happening smoothly in "We are interested in whether we can explain the funky phenomena we It is this hidden influences idea that the researchers wanted to explore. And the idea would sit much more comfortably with our intuition that a cause must come before its effect. Particles interact through influences that do take some time to travel across space? These influences would have to travel faster than light, but that's ok since they don't allow for communication. Textbook quantum theory holds that the interaction between particles is instantaneous. If this is indeed the case, relativity is safe. But physicists expect that what happens between entangled particles can't be harnessed for true communication, essentially because there is no way anyone can control the "messages", or influences, that pass between particles. What relativity forbids is faster-than-light communication. This seems to put quantum theory in conflict with relativity, but there is a subtlety which irons out the tension. But according to Einstein nothing, and that includes information, can travel faster than light. Experiments have shown that the messages that pass between particles would have to travel more than 10,000 times faster than light. The second explanation gets into trouble with Einstein's theory of relativity. But both theory and experiment have shown, time and time again, that the inequality doesn't hold: nothing in the past can have determined the particles' properties at the time of measurement. He showed that if the behaviour of two entangled particles were down to a past cause, then measurements of their properties would have to satisfy a particular inequality, now called Bell's inequality. The first explanation was ruled out in the 1960s by the physicist John Bell. The other is that the two electrons are communicating with each other, sending messages that travel so fast that the interaction appears instantaneous. One is that something in the past, some cause we have not yet discovered, determined what the spin of the two electrons was going to be. There are two common sense explanations for what is going on here. (For another example of entanglement and Einstein's criticism of it see this article.)
It's a strange effect, but it has been demonstrated in the lab many times. Now if you measure the spin of one of two entangled electrons and find it pointing up, then immediately the spin of the other, if measured, will be pointing down, no matter how far away it is. When you measure the spin of an electron you will find it pointing either up or down. An example is a property of electrons called spin. Have once interacted can remain linked even when they are Image: Timothy Yeo, Centre for Quantum Technologies at National University of Singapore.Įinstein's "spooky action" shows up when we are dealing with so-called entanglement: two particles that
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