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"Spooky" Science Points Towards Quantum Computing
Stony Stevenson writes to tell us that University of Michigan physicists have been able to establish an "entanglement" between two atoms trapped more than a meter apart in different enclosures using light. This shows how two different atoms can have a sort of communication, something Einstein referred to as 'spooky action-at-a-distance'. "By manipulating the photons emitted from each of the two atoms and guiding them to interact along a fibre-optic thread, the researchers were able to detect the resulting photon clicks and entangle the atoms. Professor Monroe explained that the fibre-optic thread was necessary to establish entanglement of the atoms. But the fibre could be severed and the two atoms would remain entangled, even if one were 'carefully taken to Jupiter'."
My arm-chair understand of Entanglement suggests that it should violate causality. Consider the following thought experiment.
We have two pairs of quantum mechanically entangled electrons. We sent a single electron from each pair five light minutes in to space. A long with a small machine that measures that's designed to react when it an electron comes "de-entangled". When it senses this, it immediately the spin of the electron in the other pair.
Here on earth we have a Tsar Bombe linked to one of the electrons from one of the pairs. Five meters away, the other electron is linked to a button. When a person presses the button, it measures one of the electron, thus breaking its entanglement. That instantly breaks the entanglement of the other electron live light minutes away. The machine then breaks the entanglement of the other pair thus instantly triggering the Tsar Bombe destroying the hut and everything in 100 Sq miles.
The problem is that, as I understand it, this would happen ten minutes before I press the button. Whoops! You see, when I de-entangle the first electron the disentanglement on the other side happens five minutes in my past. When the machine disentangles the second electron, the other electron is five minutes in its past. Totalling to ten minutes. Can you see what I'm getting at? I'm assuming this argument isn't new - What mistake have I made here?
Simon.
I've always wondered if we would one day be able to use entangled photons to peer beyond the event of a black hole. Keep one particle in an observable state and send one through the black hole. Something is bound to happen and it might give us some insight into what exists beyond the event horizon. This experiment sounds like a step toward that possibility.
To receive a signal you have to measure something. That can be ones and zeros streaming from a wire or light scattering off a distant smoke signal. To make a measurement you have to collapse the wave function. Once the wave function is to more, you have no chance of sending anything else. So maybe we could send a single bit with a single entangled state. Perhaps the trick would be to get a whole lot of them. The fact that the universe is self consistent lends credibility to causality.
The tugging of the rigid wire isn't an instantaneous transfer of motion. Each atom must tug on the one next to it, etc. At no time does this transfer of motion exceed the speed of light.
BTW, I've heard this question posed more often as a pair of scissors with the blades as long as the Solar System. Close the short end and the tips should be moving faster than light. Except they don't, because as you get further out to the tips it requires more and more energy to move them faster. They'll get close, but never exceed c.
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It's a subtle zing at University of Michigan. The physicists are from there, their football team lost to much-weaker Appalachian State Saturday in what's arguably the biggest upset in college football history. Since U-M is often perceived as arrogant people feel they got their comeuppance.
(Yeah yeah, off-topic. Still a great news item though. Such was the delight of rivals Ohio State and Michigan State that students from there were emailing one of Appalachian State's players, asking to be added to his friend list.)
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I believe the ansible was a device that used entanglement to provide faster than light communication without breaking the laws of physics. It was later proven (60's?) that under existing quantum theory entanglement cannot transmit information, so the ansible fell out of favor with some authors, particularly those trying stay true to science.
The "faster than the speed of light" thing surprises me. Not because of how c functions in relationship to matter and energy, but because the physicists, whose discipline has now had a full 100 years to digest these complexities, and personally, eight or more years of post-secondary education hammering home the need to state things carefully, fail to state that the fact of the violation of the speed of light for an effect can not itself be established at faster than the speed of light.
Two physicists in a similar reference frame measure two entangled particles in different light cones (any interaction would therefore need to travel faster than ligth). The entanglement effect says that if one measures red, the other measures blue. How do they confirm this? The information about their measurements must travel *at the speed of light* until information from the distinct measurements meets up. At *this point in time* they know if the entaglement effect conformed with theory or did not conform with theory. They can't posssibly determine this conclusion faster than the speed of light between the positions where the measurements were taken.
It interests me that the effect can travel faster than light, but the conclusion about the effect can not, yet I've never seen a physicist discuss this. The discussion always goes entanglement, faster than light, spooky, bada bing. It's possible that the entanglement effect doesn't resolve itself until information about the two experimental measurements (which converges in obedience with the speed of light) actually meets up. Perhaps the disentanglement takes place only *after* the results of the two experiments meets up. That would involve the experiment (and experimenters) having become entangled in the experiment. Weird? In the realm of the very tiny, that's never stopped mother nature before.
On a related point, I've never seen a physicist comment on whether it is possible to take two particles of unknown histories and prove they are not entangled. I suspect this can only be done by taking measurements which shuffle the quantum deck. Entangled particles are always introduced as an exceptional state of matter, produced painstakingly only in laboratory equipment for the purpose of conducting this experiment.
Is it not possible that most of the particles in the universe are entangled with most of the other particles of the universe? If there is no physical demonstration that two particles *are not* entangled, on what basis could you answer "no"? As a simpler case, is it possible to construct three particles A, AB, and B where AB is entangled with both A and B?
It just bugs me that the typical account of this effect rarely gets past the word spooky before exposition ceases, as if the very phrase "faster than light" causes some kind of cerebral blood flow trauma in any person who has devoted eight years of higher education in grappling with the consequences of E=mc^2.
If we put entangled photon pairs down different fiber lines, and include a birefringent component to split the beam into polarized components... Each photon ought to essentially split itself. We wouldn't know which path a given photon took until we measured it, but we would know what the properties were supposed to be based on the waveform collapse.
:-)
In this case, the observation would be the exact same as it the photon actually had a discrete property which caused it to choose one path as it hit the crystal.
Note, however, that Heisenberg never suggested that the photon would be both at once. He simply said one could not *know* what state it would have until observation without knowing the exact "state of everything else in the universe" ("Physics and Philosophy"). Most physicists also don't suggest that an electron takes up the entire space of an electron cloud, just that such is a "useful way to think about it."
In short in this case, we cannot know whether the photon *really* took both paths and later collapsed that into a choice, or chose one in when it reached the crystal. Postulating about unknowables seems to be a little like Intelligent Design. On the other hand, you may just be confusing the map with the territory
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Don't usually reply to AC's, but no, the speed limit arises not because of something we noticed in "particle accelerator experiments" it is because of the geometry of space time, which is different than the euclidean geometry that we expereince at low speeds and energies.
If you could send something out faster than the speed of light, then you can truly send things into the past and there by violate causality. If you want to know why this is, study Minkowskian geometry, and particularly its Lorentian coordinate changes which correpond to frame changes arrising from changes in speed, something that is very trival in Euclidian geometry, but not in our world.
So either:
1 - you can't go faster than the speed of light.
2 - you can, but we don't have free will, and something else keeps you from violating causality.
3 - It looks like you can, but somekind of multi-world split resolves the paradox
"Observed" basically means that information about it is present in some other system. So you don't need a human to notice it; a detector completely suffices.
About what observes all the particle around you when you're not looking: The environment does (by simply interacting with them). That's what is called decoherence.
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", in order to separate the two you need to transmit some additional information by classical means"
No, you do not need to transport it seperatly, per se. You only need to have the receive understand how to interpret the spins. This can even be done even if the spin direction is completely random.
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