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So, 2 days ago I learned that quantum entanglement is just about our perception. There is not an active link between particles and they are not communicating with each other. But then, while surfing on the internet this page came up https://www.sciencealert.com/these-4-cosmic-phenomena-travel-faster-than-the-speed-of-light

It says: ""If I jiggle one electron, the other electron 'senses' this vibration instantly, faster than the speed of light. Einstein thought that this therefore disproved the quantum theory, since nothing can go faster than light," Kaku wrote." and then I read about EPR paradox and really confused. If there is not an active link between quantum particles what Kaku is talking about ? Can someone explain?

Norbert Schuch
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Ç.Eti
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This piece is marked as syndicated from Business Insider, itself repeating Kaku himself on Big Think, so the very first step is to go there - and that quickly shows that they've omitted key qualifiers in Kaku's text:

But actually this experiment (the EPR experiment) has been done many times, and each time Einstein was wrong. Information does go faster than light, but Einstein has the last laugh. This is because the information that breaks the light barrier is random, and hence useless. (For example, let's say a friend always wears one red sock and one green sock. You don't know which leg wears which sock. If you suddenly see that one foot has a red sock, then you know instantly, faster than the speed of light, that the other sock is green. But this information is useless. You cannot send Morse code or usable information via red and green socks.)

Whether quantum particles "communicate" with each other is, if you really push it against the ropes, ultimately up to which intepretation of quantum mechanics you choose to believe. However, what's really clear is that even if they do communicate, they cannot communicate any causal information, so they cannot be used to communicate faster than light and cannot give rise to any time-travel paradoxes (which is the real worry with FTL communications). The details of how this comes about are subtle and there are plenty of questions on the subject on this site, but Kaku does a good job of explaining the limitations given the length of the piece.

However, given the inappropriate cutting they've done with the piece, I'll add this as well: as a rule of thumb, if Science Alert has content that is in direct contradiction with other media, it pays to slow down, read around, and think carefully about just how well they are representing the science. They have plenty of reasonable coverage, but they've graced these pages before in ways that didn't leave them looking very good. In this specific case, though, it just takes a bit of pulling on the thread to find the original source along with its more careful hedging of the conclusions and, moreover, pulling on that thread until you find that original source is a key bit of internet information hygiene that needs to be practiced pretty much always.

Emilio Pisanty
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There are multiple ways to describe the same physical phenomena using the English language. A lot of these ways are valid and sensible and yet a lot are mutually contradictory. Your example demonstrates two contradictory English language explanations of the same physical phenomenon. The difficulty of answering a question like yours is that it can't really be answered in terms of physics, it has to be answered in terms of the English language.

The bottom line is: quantum mechanics allows for correlations that are not possible in classical local speed-of-light-obeying physics. This can be interpreted in two ways: you can say quantum mechanics is classical and there is nonlocal instantaneous communication (as in de Broglie-Bohm theory, which is considered ugly for a number of reasons), or you can say that quantum mechanics is "quantum" and local, which requires redefining what you know about position and information, but which leads to a much prettier picture of the world.

You can find an argument that this is the case in Sidney Coleman's lecture Quantum Mechanics In Your Face, but it's definitely not made for a popular audience. He assumes you know about hamiltonians and hilbert spaces and all that.

  • So can we say quantum particles can send information to each other faster than light as a theory? Or can we say quantum particles can warp spacetime like a wormhole and communicate? or those questions aren't making any sense ? – Ç.Eti Jul 04 '17 at 21:40
  • @Ç.Eti You can say they do or they don't, because that's just English and the physics is unchanged! It depends on your meaning of the word "information". I'm not well-versed in ER=EPR (the wormhole thing you're referring to) to comment reliably. –  Jul 04 '17 at 21:51