6 ms·
Here's a SO discussion: https://physics.stackexchange.com/questions/15282/quantum-entanglement-faster-than-speed-of-light https://physics.stackexchange.com/que
by sbf501 4y ago
Here's a SO discussion:
https://physics.stackexchange.com/questions/15282/quantum-entanglement-faster-than-speed-of-light https://physics.stackexchange.com/questions/15282/quantum-en...
"Collapsing an entangled pair occurs instantaneously but can never be used to transmit information faster than light. If you have an entangled pair of particles, A and B, making a measurement on some entangled property of A will give you a random result and B will have the complementary result. The key point is that you have no control over the state of A, and once you make a measurement you lose entanglement. You can infer the state of B anywhere in the universe by noting that it must be complementary to A.
The no-cloning theorem stops you from employing any sneaky tricks like making a bunch of copies of B and checking if they all have the same state or a mix of states, which would otherwise allow you to send information faster than light by choosing to collapse the entangled state or not."
Basically you know a that since they are complements, you know the value of the other after you measure one. No information was transmitted at all.
IANAQP. At all. Not one bit.
- _8j50 4y agoIs there some known property that is keeping the changes random? Or is it pure observation? Because if it is the latter then is it not correct to say that it is not known if FTL transmission can work?
- Retric 4y agoQM says unmeasured quantities are uncertain, which shouldn’t seem odd on the surface. Entanglement means measuring one thing is effectively equivalent to measuring a property of the other. It’s vaguely like randomly cutting a board in half and by measuring how long one board is you can figure out how long the other one is. The odd bit isn’t really measurement it’s all the oddity around what happens before something is measured.
- rubicks 4y ago"...making a measurement on some entangled property of A will give you a random result and B will have the complementary result." "...once you make a measurement you lose entanglement." One entangled pair can transmit exactly one bit of information exactly once. The measurement necessarily destroys the pair. Do I understand correctly?
- convery 4y agoNo, as you can't influence the outcome of the measurement. So both parties reads a bit, which is random, they just happen to be opposite of eachother. No useful information is transmitted.
- ratg13 4y agoI think what the person is struggling to understand is the phrase "lose entanglement". I also wonder what is meant here. Are the particles no longer entangled after they are measured once, or what is being said here?
- b112 4y agoI'm struggling with the idea that reading a bit at A is random, and B is random, but that A is the inverse/opposite of B. That means B is not-random to me. I have a feeling that terms being used are specifically defined terms of art. EG, don't mean what they mean in common english, but instead, have a different technical meaning.
- themoonisachees 4y agoB basically isn't random (that's why quantum entanglement is intersting) but you have 3 options on when to read its state ( which you can only do once): Option 1 is that you read it before the state of A is read, in which case my undestanding is you are effectively A in this scenario. Option 2 is that the state of A has been read, but it is unkown to you yet, because of speed of light reasons. You read B, and can infer the state of A, but because A is random, no information has been transmitted. The "sender" cannot control which state of A they read, and had you not known about it, the state of B might as well be random. Option 3 is that you read B after recieving the state of A, in which case there is something useful in that only you can make sure that only the sender knew of the state of A, meaning you are effectively authenticating a message. But still no information has been transmitted faster than light, it's just a secure channel.
- Viliam1234 4y ago
- downboots 4y agoqubit*
- bcrosby95 4y agoMy layperson understanding is it's like someone giving you a penny and they keep a penny. One is heads and the other is tails. When you eventually peak at your penny you know what the other is.
- jquery 4y agoEntanglement isn’t necessary in that scenario, though. My understanding is that peaking at your penny forces the other penny into the complement state. There’s an element of causality which is why we call it entanglement and not simply spin complements. My understanding is also that causality is instantaneous, i.e., faster than light, but still cannot be used to transmit information.
- diplodocus1 4y agoThe difference being that yours being tails is actually not a fact and a probabilistic outcome? vs. it just always being tails and you not knowing? how would we ever know the difference?
- foota 4y agoI don't know how it applies specifically here, but generally until it's collapsed it will act as a wave, so you could see things occur that aren't possible for an already collapsed states.
- doubleunplussed 4y agoA particle is less like a coin with only two sides, and more like a little arrow which could be pointing in any direction. Quantum mechanics allows you to measure the projection of the arrow along any one axis, and you'll always get +1 or -1. If the two people with entangled particles measure along the same axis, they will always get opposite results. But they are also free to measure along different axes. The statistics of what results the two experimenters get when they measure their particles arrows along random axes in repeated trials are not the same as what you would get if the arrows had a predetermined direction all along. Google "Bell inequality" for details.
- galangalalgol 4y agoThey are explicitly saying this will help them keep the two atomic clocks better synchronized. Which is terribly useful for radio astronomy. But it does seem like that would imply information had been transferred. If it really does work I want to see what happens when you put one in orbit for a while. Donthe differential frames of reference decohere them?
- tbrownaw 4y agoI'm pretty sure that setting up entanglement involves information transfer.
- andreareina 4y agoSure. The article implies that post-entanglement, perturbations of one particle's state will transfer to the other, which is not the case.
- tbrownaw 4y agoI don't think it does? Atomic clocks don't exactly count atomic oscillations the way a grandfather clock counts pendulum swings. They have a more normal oscillator, and then keep that calibrated by checking it against atomic oscillations measured over some relatively short time. Thirs is a way to let those repeated short measurements be more accurate. It doesn't mean that the oscillations of the atoms in both clocks are somehow locked together over the long term.
- andreareina 4y agoFTA (emphasis mine): Two atomic clocks have been connected using quantum entanglement – a property that intrinsically links them so that changes in one instantaneously affect the other. Paywall beyond there so I don't know how they entangled the clocks, I'm just responding to the popsci description of entanglement which the article repeats.
- 4y ago