If faster-than-light communication is possible, some observers in certain reference frames will see the signal travel back in time – and perhaps arrive at its destination before it is sent. Without this, causality – the sequence of cause and effect – is ruined. Special relativity sets light-speed as the ultimate speed limit for information transmission in the Universe – and with good reason. Anything else, Einstein said, would be “spooky action at a distance.”įaster-than-light communication, if it were possible, would wreak havoc with the laws of physics. Perhaps this phenomenon could be exploited to allow for faster-than-light communication – and, in any case, it violates a physics principle called “locality”, which states that physical changes to a system should be caused by things that are nearby – for example, interactions between particles that are close together. When Alice collapsed the entangled wavefunction by measuring her particle, what happened to Bob’s particle? It seems as if information is instantly transmitted across light years from Alice to Bob, telling his particle’s spin to point downwards. This means that Bob must measure spin down for the total spin to be zero. Now, when Alice measures the spin on her particle, she finds it to be up. We produce these particles and send one to Alice and one to Bob. If you take a measurement of the electron spin along a particular axis, you will find that it’s either down or up with a 50% probability of either outcome.įor tradition’s sake, let’s say that Alice is in Alpha Centurai while Bob is in Copenhagen. Our understanding of spin as a quantum number suggests that, when you take a measurement of the electron’s spin, you “collapse its wavefunction” – thus forcing the spin to be either up or down. Take a classic example – an electron positron pair that are generated in such a way that their total spin (along the z axis) is known to be zero. This idea rests on a phenomenon known as quantum entanglement. Most famous amongst these is the Einstein-Podolsky-Rosen paradox. Einstein, famously, felt that “God does not play dice with the Universe.”Īnd so, amongst others, he attempted to come up with theories that contradicted the interpretation of quantum mechanics that had become mainstream in the physics community. That, in fact, the Universe – and subatomic particles – had no “certain state” until they were observed, at which point they had some probability of ‘collapsing’ into certainty. The quantum world, on the other hand, seemed to argue that this could not be done. You could precisely trace a line between cause and effect: you could extrapolate the state of the Universe backwards to its origins, or forward to its end, with absolute certainty. It promised that, if you were smart enough – if you discovered all of the laws of physics, and simply took the appropriate measurements and made the appropriate calculations – you could, in principle, know everything. The classical world held deep promise for believers in rationality. Yet the conclusions and the phenomena that were discovered in the years that followed – particles that behaved like waves, or that had no fundamentally determined position whatsoever – led to conclusions that were at odds with the way that Einstein viewed the universe. ![]() ![]() When Planck first realized that energy was quantized – it came in discrete little packets, rather than flowing continuously like a fluid – Einstein realized that this would necessitate a complete rewriting of the laws of physics. Einstein was not a fan of quantum mechanics – which is ironic, because his work laid the foundations for the field of study.
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