## Quantum Entanglement

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### Quantum Entanglement

I'm not a physicist, so this'll probably just seem stupid to a lot of you how understand more about this than I do (most here I'd guess). Anyways, I was talking with my AP Physics teacher about quantum entanglement and how, once you collapse the wave function of one electron the other electron instantaneously becomes the opposite spin if they're entangled. My question isn't about the faster than light information traveling stuff, but just a hypothetical question.

Let's say you have two entangled electrons in a 'box', each a significant amount apart. There is one observer at each box, and each observer experiences the exact same time so we can get rid of relativity. Time dilation is equal for both of them where they are. So, let's say that they both open the box and observe the electron inside each of their respective boxes at the exact same moment. Does that not matter, and the electrons just do what they'd do normally if there was only one observer opening one of the boxes? Because I know that if you open it up and observe one electron and it spins up, then the other starts spinning down. So, if two people open two different boxes, and the electrons inside are entangled, how do the electrons know which wave function to break down to since both are being observed at the exact same moment?

I'm guessing it's the exact same with or without the two people?
Wodashin

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### Re: Quantum Entanglement

The electrons are entangled whether someone is looking at them or not. Even without observation one will be spin up, the other will be spin down but there is no way of us knowing which is which until we observe them. Opening the box collapses what WE see to either spin up or spin down, but the electron will have been in that state to begin with BEFORE we opened the box. There's no 'starts spinning' involved. The electrons are in the states before we observe them.

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### Re: Quantum Entanglement

Ah, I see. I thought it was a statistical 50/50 uncertainty type thing. At least that's what my teacher was talking about since that's what my Paul Davies book said. Idk, it was made back in the 90s so maybe it's wrong. Idk. I really don't, it's why I'm asking.

So the observation doesn't change anything then?
Wodashin

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### Re: Quantum Entanglement

It's still a 50/50 statistical thing, either the particle is in one state or the other. We have no way of knowing until we observe it. What I'm trying to say is that the electrons are already in one of those states, but until we observe the particle and 'collapse' it's wave function we have no way of knowing which state it's in. Us observing it doesn't change anything about the entanglement of the 2 electrons. They will be 'connected' per-se by the quantum entanglement phenomenon such that they will always be in opposite states. Us 'opening the box' to see it merely solidifies which one is in which state.

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### Re: Quantum Entanglement

I see. Problem solved then.
Wodashin

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### Re: Quantum Entanglement

Not exactly. Particles really *are* in fuzzy states until the wave-function collapses, and whether we know about it doesn't really make a difference. In the two-slit experiment, the particle really goes through both slits and interferes with itself, and doesn't have a specific position until it hits the screen behind.
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gmalivuk
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### Re: Quantum Entanglement

Yeah, the idea that the electrons have a specific spin before we open the box is called a hidden-variable theory and is entirely wrong.

As for "how do the electrons know which wave function to break down to since both are being observed at the exact same moment," each person sees the other person as entangling themself with the electron rather than measuring a specific state, so each person only sees one measurement happen. From the perspective of electrons is a bit trickier, since I have never consciously noticed my wavefunction collapsing, this is probably because my memories are stored in the state of my brain, and so a wavefunction collapse is not something we can experience. So the electrons I guess would say "what measurement? I was always in this state."
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Charlie!

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### Re: Quantum Entanglement

Charlie! wrote:I have never consciously noticed my wavefunction collapsing, this is probably because my memories are stored in the state of my brain, and so a wavefunction collapse is not something we can experience. So the electrons I guess would say "what measurement? I was always in this state."
That is a pretty succinct description of a cool story I read recently: http://lesswrong.com/lw/ps/where_physic ... xperience/
Moose Hole

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### Re: Quantum Entanglement

Charlie! wrote:Yeah, the idea that the electrons have a specific spin before we open the box is called a hidden-variable theory and is entirely wrong.

Not really what I was referring to with my posts. I was trying to get him to realize the subtle mechanics behind entanglement. I guess I failed with my description. I know the electrons are not in a specific spin state prior to observation. I was merely trying to state that entanglement forces them to be in opposite states whether we are observing them or not.

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### Re: Quantum Entanglement

Before the spin is measured, its not that we dont know. The particle truly does not have a defined spin in whatever projection we are looking at.

BlackSails

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### Re: Quantum Entanglement

Okay, thanks for bringing me back from confusion. I knew that they would be opposite no matter what, but I also know about the 'fuzzy' physics of quantum mechanics a bit. I get what you were trying to get across now.

So, like I was asking, since observation affects the property of the electrons, what happens when two observers independently observe each end of the entanglement at the exact moment? Relativity trickses aside, the same moment. If the boxes were a lightyear apart, the information would still travel instantaneously, I understand that. What I want to know though is what the implications of observing either ends at the same moment, causing the collapsing wave functions. So, since the information travels instantaneously, but both ends are being viewed at the exact moment, thus collapsing the wave function, how does the function collapse? Because there's a 50/50 chance I'm assuming in it being either/or, so if you were to view both ends at the same time, how would the electrons know which way to spin to be opposite?

I guess all I'm asking is a simpler, stupider version of twin photon experiments but with entangled electrons.
Wodashin

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### Re: Quantum Entanglement

So, like I was asking, since observation affects the property of the electrons, what happens when two observers independently observe each end of the entanglement at the exact moment? Relativity trickses aside, the same moment. If the boxes were a lightyear apart, the information would still travel instantaneously, I understand that. What I want to know though is what the implications of observing either ends at the same moment, causing the collapsing wave functions. So, since the information travels instantaneously, but both ends are being viewed at the exact moment, thus collapsing the wave function, how does the function collapse? Because there's a 50/50 chance I'm assuming in it being either/or, so if you were to view both ends at the same time, how would the electrons know which way to spin to be opposite?

I don't quite understand what puzzles you. Leaving aside for a moment the impossibility of two observers making a measurement at exactly the same moment, I still don't see why there should be anything troubling or perplexing about the situation. If the two observers make the measurement at the same time, one will find a spin-up electron and the other a spin-down electron - just as would happen if they measured them at different times.

I would also be careful about using the word 'information'. Strictly speaking, information never travels instantaneously. The measurement of the spin of one electron does (in the standard formulation) result in an immediate collapse of the spin of both electrons into well-defined and opposite states - but this process doesn't carry any information. If it did, you could use the effect to send a faster than light message.
Aiwendil

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### Re: Quantum Entanglement

What puzzles me is my lack of knowledge I guess. Idk. I know you can't do it at 'exactly' the same time because there is no universal time, it's all relative, but I guess I was just thinking that if you were to see both ends independently, then why would the electrons know how to collapse properly?

Idk, I'm probably mucking everything up.
Wodashin

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### Re: Quantum Entanglement

Wodashin wrote:...then why would the electrons know how to collapse properly?

...

That my dear friend is the big question. We know it does happen. But "how" is still not fully understood.
In that, we know there are no hidden variables (like hidden properties we have not found yet. IE colour of a box, size, weight etc). It's an almost given that there is a "communication" between the particles.

However, AFAIK (from wiki ) no one can pin point the method at which this information is passed. Is the waveform stretched between particles? Is it FTL? Is it through other dimensions? Is it magic?
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Technical Ben

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### Re: Quantum Entanglement

Technical Ben wrote:
Wodashin wrote:...then why would the electrons know how to collapse properly?

...

That my dear friend is the big question. We know it does happen. But "how" is still not fully understood.
In that, we know there are no hidden variables (like hidden properties we have not found yet. IE colour of a box, size, weight etc). It's an almost given that there is a "communication" between the particles.

However, AFAIK (from wiki ) no one can pin point the method at which this information is passed. Is the waveform stretched between particles? Is it FTL? Is it through other dimensions? Is it magic?

Oh, don't give him silly ideas. There is no movement of information from one electron to the other. This is why we didn't ditch quantum mechanics because of the EPR paradox.
Some people tell me I laugh too much. To them I say, "ha ha ha!"

Charlie!

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### Re: Quantum Entanglement

why would the electrons know how to collapse properly?

The wavefunction collapses when either electron's spin is measured. It doesn't matter whether the measurement is made on electron 1 or electron 2 or both at the same time.
Aiwendil

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### Re: Quantum Entanglement

I know the wave function collapses, but there's a 50/50 chance, and if each are viewed at the exact same 'instant', and each has a 50/50 chance of being one or the other once collapsing, how is it possible that they will ALWAYS be opposite in that situation? Probably mucking it up.

Or does this question not even matter because relativity means it can't happen because there are no exact 'moments' of universal time, maybe?
Wodashin

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### Re: Quantum Entanglement

Wodashin wrote:I know the wave function collapses, but there's a 50/50 chance, and if each are viewed at the exact same 'instant', and each has a 50/50 chance of being one or the other once collapsing, how is it possible that they will ALWAYS be opposite in that situation? Probably mucking it up.

Or does this question not even matter because relativity means it can't happen because there are no exact 'moments' of universal time, maybe?

It's actually even worse because of relativity How can it come out the same way in all reference frames, where in different reference frames different measurements will be the first one to collapse the wavefunction?!

The answer is that collapse isn't something special and sudden happening to the system that gets collapsed, but rather an entanglement between the observer and the system, so that the observer is always in the correct state to see one state of the system and not the other - at least that's what it looks like from the outside. If you put a physicist in a box and made her do measurements on entangled electrons, you would get an entangled electrons-physicist system

Extending this a bit further we can think about relativity using the many-world interpretation, where all the possibilities "really happen," but we only see some of them because we're entangled with the world so that we only see one state of it. This makes the above problem less weird - it doesn't matter which measurer goes first because both possible measurement really happened and we just happen to be remembering one of them.
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Charlie!

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### Re: Quantum Entanglement

Technical Ben wrote:However, AFAIK (from wiki ) no one can pin point the method at which this information is passed. Is the waveform stretched between particles? Is it FTL? Is it through other dimensions? Is it magic?

From what I understand, as good a bet as any is that space-time is actually an emergent phenomenon, and at that at some level, concepts like distance and interval have no meaning. Certainly (uhh.. good enough for most theorists, anyway!) it was like this early enough in the big bang, before gravity split off from the other forces. Plank Epoch, I think it's referred to.

It really isn't such a crazy idea. Spacetime is emergent in Loop Quantum Gravity, and ought to be in M-Theory too, when a properly background-independent formulation is found.

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thoughtfully

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### Re: Quantum Entanglement

Ah, I think I finally understand what your concern is. You're worried that if both electrons were measured at exactly the same time, then they won't have a chance to "tell" each other what state they collapsed into - is that right? In other words, electron 1 could collapse to a spin-up state but it doesn't have a chance to tell electron 2 about its decision, because electron 2 was measured at the same instant.

If this is indeed the source of your question, I think the problem is that you are taking some of the loose and quasi-metaphorical language used to discuss wave function collapse too literally. There is no actual "telling" going on. It's easy to fall into the habit of saying things like "electron 1 collapses to a spin-up state, so electron 2 knows that when it's measured, it has to be spin-down" - but this can be a misleading picture.

It's better to think of it as follows.

Let's label quantum states like this: |u> is the state of an electron with spin up and |d> is the state of an electron with spin down. In quantum mechanics, an electron can also be in a "superposition" of states, like this: |u> + |d>. If you measure an electron in the state |u> + |d>, you cause the electron to collapse - that is, it must "choose", so to speak, one of the two states; in this case there's a 50% chance it will collapse to the state |u> upon being measured and a 50% chance it will collapse to |d>. That's what the collapse of a wavefunction really is - a particle in a state which is some combination of the fundamental states it's allowed to be in has to choose just one of those states to be in after the measurement.

The state of the system of two electrons in the entanglement example is something like |u1 d2> + |d1 u2>. Here "1" and "2" represent electrons 1 and 2. This is a superposition of two states, just like in the single particle example. Those two states are "electron 1 up and electron 2 down" and "electron 1 down and electron 2 up". Now, when you measure the two-electron system (and measuring the spin of either electron counts as measuring the system), it has to choose one of those two states. So it can either end up in |u1 d2> or in |d1 u2>. And that's all that happens. Now that it has collapsed into one of those two states, the system "knows" perfectly well what answer to give for a measurement of the spin of either electron.

So you shouldn't think of it as involving two steps (first, the measured electron chooses a particular state; then, it instantaneously tells the other one what it chose). Instead, there's just one step: the two-electron system chooses a particular state. And once it's made that choice, there's nothing left to do - there's no need for one electron to tell the other one anything, because the whole system of both electrons has already collapsed to a particular state.

Does that make sense?
Aiwendil

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### Re: Quantum Entanglement

Notwithstanding that he explicitly tried to sweep relativity under the rug, simultaneity is frame-dependent, anyway.

Whether and which two events are observed as simultaneous depends on your state of motion, and doesn't even require acceleration.

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thoughtfully

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### Re: Quantum Entanglement

Ah, I see. I was looking at this the wrong way I guess, but I get what 'entangled' means now. Single system, with multiple parts? So my question is even more moot than before.

Thanks though, everyone. My understanding of quantum mechanics is limited.
Wodashin

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### Re: Quantum Entanglement

And if you want to ask relativistic questions about quantum mechanics, you need quantum field theory. And if you want to use the word "simultaneous" seriously, you are about to be asking a relativistic question. I recommend Haag's book "Local Quantum Physics: Fields, Particles, Algebras." Such a delightful one.
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### Re: Quantum Entanglement

A much easier explanation for how the two electrons "know" to be opposite each other, even when they're measured at "the same time" while wildly separated in space, can be found by adopting the Many-Worlds Interpretation of quantum physics.

In MWI, a quantum event of any kind causes the universe to split into multiple worlds: in each one, the event definitely occurred one way or another. In some cases, a quantum event has only one outcome, so only one world results. In others, the event may have two (such as an electron being generated with a random spin, or a photon with a random polarization), in which case multiple worlds are produced. This 'splitting' occurs at the location of the quantum event, and spreads outward at the speed of light. When two worlds are almost identical, they bleed into each other, causing interference - this is most of the "quantum weirdness" that you know of, including things like the two-slit experiment. Two worlds that are very different bleed into each other very little, so any interference that occurs is too small to be measured. This is how the macroscopic/classical/non-quantum worlds comes into being - if a photon can have a random polarization in two directions, that's only one difference. But if this different polarization causes it to strike an atom in a different way, which causes it to emit another photon in a different way, which causes different atoms to absorb the new photon, etc., the differences cascade and very quickly the universes are different in billions (or more often, much much larger numbers) of ways, and the interference effects disappear.

So, back to the entangled electrons. We make a pair that are entangled to have opposite spin, and don't observe them. There are now two worlds, which differ only in the spin of the electrons - in one world, electron A is spin up and electron B is spin down, while in the other world they're the opposite. These are very close, and thus the interference causes the "superposition" effects where we can measure one of the electrons as being in both states.

Now, we separate the two electrons by a light year. We have to use slower-than-light travel for this, so even if we started out the moment we created the pair, the world-split would still spread out faster and further than we can go. Thus, no matter how far apart we are, we're in a region of space with two worlds.

The scientists at each point now measure their electrons. This "collapses the wavefunction" of the electron; in more accurate terms, the giant mass of quantum variables that make up the detection apparatus has now interacted with the electron, and reacts in different ways based on which world it is. This cascade of different reactions separates the two universes, so that they no longer interfere with each other, and we now measure the electron to be in a definite state, either spin up or spin down, and not in a superposition. Then they send a message to the other scientists, and lo and behold, the other camp measured the opposite spin. This is guaranteed because the scientists were *always* in a world where the other camp had an opposite-spin electron; the only thing uncertain was which world they happened to be in (was their electron spin up and the other's spin down, or the other way around?).

Note the difference between this interpretation and the "collapse the wavefunction" interpretation. In the latter, measuring the electron changes its state - it goes from "in a superposition" to "spin up" (or "spin down"). The other electron, potentially halfway across the universe, then also instantly changes its state, seemingly passing a message faster than light. We handwave away the problem by stating that you can't influence what state the electron will end up in, and thus "information" can't travel faster than light - the fastest thing that we can control (and thus send a message with) is still a light beam.

In the MWI, on the other hand, there's no faster-than-light communication. The electron doesn't change state when we measure it, it was *always* spin up (or spin down). However, there was a very closely related world with the opposite orientation that caused interference, preventing us from measuring the state of the electron precisely. We can't predict which world we're in (and neither can the rest of the universe), so the act of measurement still looks like the electron assumes a random spin (when, in reality, we've just established which world we were randomly placed into when the split occurred). However, no matter which state the electron "randomly" ends up in, the other electron is guaranteed to already be in the opposite state. We send the message about our electron to a *particular* other camp of scientists (the one in our world), not a random one.

(If you're clever, you may note that this sounds a bit like the hidden variable hypothesis that was already said to be wrong. We get around this with the fact that the rest of the universe can't tell which world it's in, either, and the interference makes sure of this. No measurement you make can distinguish between the two worlds until you actually measure the electron, at which point it appears to be a random choice. This is handwaving as well, but I believe it's a simpler handwave, and results in less hard-to-answer questions. There are, of course, still hard-to-answer questions that will require further theoretical advances.)
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### Re: Quantum Entanglement

Charlie! wrote:
Technical Ben wrote:
Wodashin wrote:...then why would the electrons know how to collapse properly?

...

That my dear friend is the big question. We know it does happen. But "how" is still not fully understood.
In that, we know there are no hidden variables (like hidden properties we have not found yet. IE colour of a box, size, weight etc). It's an almost given that there is a "communication" between the particles.

However, AFAIK (from wiki ) no one can pin point the method at which this information is passed. Is the waveform stretched between particles? Is it FTL? Is it through other dimensions? Is it magic?

Oh, don't give him silly ideas. There is no movement of information from one electron to the other. This is why we didn't ditch quantum mechanics because of the EPR paradox.

Sorry, would a better term be "mechanism that allows the collapse to be simultaneous"?
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Technical Ben

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### Re: Quantum Entanglement

Xanthir wrote:
...
In the MWI, on the other hand, there's no faster-than-light communication. The electron doesn't change state when we measure it, it was *always* spin up (or spin down). However, there was a very closely related world with the opposite orientation that caused interference, preventing us from measuring the state of the electron precisely. We can't predict which world we're in (and neither can the rest of the universe), so the act of measurement still looks like the electron assumes a random spin (when, in reality, we've just established which world we were randomly placed into when the split occurred). However, no matter which state the electron "randomly" ends up in, the other electron is guaranteed to already be in the opposite state. We send the message about our electron to a *particular* other camp of scientists (the one in our world), not a random one.

(If you're clever, you may note that this sounds a bit like the hidden variable hypothesis that was already said to be wrong. We get around this with the fact that the rest of the universe can't tell which world it's in, either, and the interference makes sure of this. No measurement you make can distinguish between the two worlds until you actually measure the electron, at which point it appears to be a random choice. This is handwaving as well, but I believe it's a simpler handwave, and results in less hard-to-answer questions. There are, of course, still hard-to-answer questions that will require further theoretical advances.)

Excellent explanation, haven't read a description this concise and to the point before. The 2nd last paragraph is what I was thinking, you just explained it 10x better than I did. I guess a career in writing isn't my future *damn!*

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### Re: Quantum Entanglement

Xanthir wrote:A much easier explanation for how the two electrons "know" to be opposite each other, even when they're measured at "the same time" while wildly separated in space, can be found by adopting the Many-Worlds Interpretation of quantum physics.

In MWI, a quantum event of any kind causes the universe to split into multiple worlds: in each one, the event definitely occurred one way or another. In some cases, a quantum event has only one outcome, so only one world results. In others, the event may have two (such as an electron being generated with a random spin, or a photon with a random polarization), in which case multiple worlds are produced. This 'splitting' occurs at the location of the quantum event, and spreads outward at the speed of light. When two worlds are almost identical, they bleed into each other, causing interference - this is most of the "quantum weirdness" that you know of, including things like the two-slit experiment. Two worlds that are very different bleed into each other very little, so any interference that occurs is too small to be measured. This is how the macroscopic/classical/non-quantum worlds comes into being - if a photon can have a random polarization in two directions, that's only one difference. But if this different polarization causes it to strike an atom in a different way, which causes it to emit another photon in a different way, which causes different atoms to absorb the new photon, etc., the differences cascade and very quickly the universes are different in billions (or more often, much much larger numbers) of ways, and the interference effects disappear.

So, back to the entangled electrons. We make a pair that are entangled to have opposite spin, and don't observe them. There are now two worlds, which differ only in the spin of the electrons - in one world, electron A is spin up and electron B is spin down, while in the other world they're the opposite. These are very close, and thus the interference causes the "superposition" effects where we can measure one of the electrons as being in both states.

Now, we separate the two electrons by a light year. We have to use slower-than-light travel for this, so even if we started out the moment we created the pair, the world-split would still spread out faster and further than we can go. Thus, no matter how far apart we are, we're in a region of space with two worlds.

The scientists at each point now measure their electrons. This "collapses the wavefunction" of the electron; in more accurate terms, the giant mass of quantum variables that make up the detection apparatus has now interacted with the electron, and reacts in different ways based on which world it is. This cascade of different reactions separates the two universes, so that they no longer interfere with each other, and we now measure the electron to be in a definite state, either spin up or spin down, and not in a superposition. Then they send a message to the other scientists, and lo and behold, the other camp measured the opposite spin. This is guaranteed because the scientists were *always* in a world where the other camp had an opposite-spin electron; the only thing uncertain was which world they happened to be in (was their electron spin up and the other's spin down, or the other way around?).

Note the difference between this interpretation and the "collapse the wavefunction" interpretation. In the latter, measuring the electron changes its state - it goes from "in a superposition" to "spin up" (or "spin down"). The other electron, potentially halfway across the universe, then also instantly changes its state, seemingly passing a message faster than light. We handwave away the problem by stating that you can't influence what state the electron will end up in, and thus "information" can't travel faster than light - the fastest thing that we can control (and thus send a message with) is still a light beam.

In the MWI, on the other hand, there's no faster-than-light communication. The electron doesn't change state when we measure it, it was *always* spin up (or spin down). However, there was a very closely related world with the opposite orientation that caused interference, preventing us from measuring the state of the electron precisely. We can't predict which world we're in (and neither can the rest of the universe), so the act of measurement still looks like the electron assumes a random spin (when, in reality, we've just established which world we were randomly placed into when the split occurred). However, no matter which state the electron "randomly" ends up in, the other electron is guaranteed to already be in the opposite state. We send the message about our electron to a *particular* other camp of scientists (the one in our world), not a random one.

(If you're clever, you may note that this sounds a bit like the hidden variable hypothesis that was already said to be wrong. We get around this with the fact that the rest of the universe can't tell which world it's in, either, and the interference makes sure of this. No measurement you make can distinguish between the two worlds until you actually measure the electron, at which point it appears to be a random choice. This is handwaving as well, but I believe it's a simpler handwave, and results in less hard-to-answer questions. There are, of course, still hard-to-answer questions that will require further theoretical advances.)

mind = blown

but.. yeah, that does make sense.. that is an excellent explanation! what really blows my mind thinking about is that, that means that the universe is absolutely awash in trillions of world splits spreading out at the speed of light.. anytime a truly stochastic event occurs.. the most common event I can think of is radioactive decay.. anytime any nucleus anywhere in the universe underdergoes radioactive decay it creates a split.. so given the amount of radio active substances on the earth alone.. thats billions and billions [/sagan] of splits a second.
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Aelfyre

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### Re: Quantum Entanglement

Aelfyre wrote:thats billions and billions [/sagan] of splits a second.

Billions? Try "enough to make the xkcd number seem infinitesimally small."

On second thought, that might be a tad too large. Still, billions is far too low.
douglasm

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### Re: Quantum Entanglement

And it's part of why we don't all find the MWI compelling.
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doogly
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### Re: Quantum Entanglement

Bah, if the thought of reality-juice being an infinitely divisible substance is the hardest thing you have to accept about quantum physics, you're doing pretty good. ^_^
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Xanthir
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### Re: Quantum Entanglement

doogly wrote:And it's part of why we don't all find the MWI compelling.

Supposedly, a split happens, but it makes no predictions of what happens to the universe as we see it, and has no feasible experiments to prove it (and most thought experiments are mired in "if this is even possible, do this, and then you MIGHT be able to prove it possibly"). In fact, a large part of the MWI is that we can't determine what universe we are in after the split. It doesn't seem a better hypothesis to start with then "God did it".
collegestudent22

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### Re: Quantum Entanglement

doogly wrote:And it's part of why we don't all find the MWI compelling.

The "billions and billions of universes" doesn't bother me, if that is what the simplest theory predicts. I don't feel a multitude of universes is against Occams razor, if we have to add something to the theory to get rid of them.

Just as the billions and billions of stars and galaxies pretty much like our own is less against Occams razor than saying that the Earth is the center of the universe and all the rest we see are just dots on the firmament which happens to match what 10^22 stars would look like.

I know there may be some trouble explaining the absolute square probabilities in the MWI without extra postulates here too, so I am not completely convinced, but I find it more compelling than the Copenhagen Interpretation which I know require ugly extra postulates. (Sorry Bohr, you were a great countryman and we all admire you, and I have enjoyed working at your institute, but scientific integrity comes first. )
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Tass wrote:Nice to see another person sharing my views of quantum mechanics. Use Occam's razor, cut out the wavefunction collapse.
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Tass

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### Re: Quantum Entanglement

Xanthir's Posting is very eloquent (Xanthir should be proud )

Having read this thread, I learned quite a bit about the dominant theories out there involving quantum entanglement, but stepping back I am convinced we are dealing with a situation of lots of theory and not enough solid scientific data to back things up. It is good to have an inventory of different unsupported theories ready so that we are less likely to look stupid when real data comes our way, but too much theorizing without hard data isn't very productive and can even distract from real discoveries that may be possible. We run the risk of believing things that are not established empirically. (like multiple universes) In spite of what I think are good intentions, I've seen so much hand waving throughout my search to understand quantum entanglement, that one should be able to flap one's self up to the moon. The situation has the feel of a potbound plant, desperately trying to grow out roots but instead being stuck growing round and round the inside of the pot. We need some good soil and lots of it to branch out and grow our knowledge. We need more experiments and their results to look at. It all starts with the data. We should build our understanding form there. Theory is fun and keeps people off the streets where they can be more dangerous, but we need to keep our feet grounded.

foxpup

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### Re: Quantum Entanglement

collegestudent22 wrote:
doogly wrote:And it's part of why we don't all find the MWI compelling.

Supposedly, a split happens, but it makes no predictions of what happens to the universe as we see it, and has no feasible experiments to prove it (and most thought experiments are mired in "if this is even possible, do this, and then you MIGHT be able to prove it possibly"). In fact, a large part of the MWI is that we can't determine what universe we are in after the split. It doesn't seem a better hypothesis to start with then "God did it".

Because it's an interpretation not a theory. See the what is/isn't MWI thread; no-one seriously claims it gives different predictions from Copenhagen, just that it can provide a more elegant and less arbitrary way of extracting predictions from Schroedinger-like evolution (although, as discussed in the MWI thread, how to do this is far from clear).

foxpup wrote:In spite of what I think are good intentions, I've seen so much hand waving throughout my search to understand quantum entanglement, that one should be able to flap one's self up to the moon.

I think the main reason for the hand-waving is that we know perfectly well how to treat it mathematically but, as even theoreticians are physicists not mathematicians, they are concerned with the real world and require a philosophical interpretation of that maths in order to feel that it is justified. This is the reason why there are so many different interpretations of QM, physicists all want an explanation of why the formalism works, but because they have different aesthetic values, they each prefer different ones.
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eSOANEM

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### Re: Quantum Entanglement

soggybomb

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### Re: Quantum Entanglement

Or, to sum all such interpretations of quantum physics up "We don't know. We probably don't have enough actual data to really know, so we've come up with as many theories as we can; and then strung them up to their logical breaking points. We can't even test the theories we have now, some of them we don't even know if we can test! Point is we need bigger particle colliders and telescopes so we can have more data damn it!"

Of course maybe there is ftl, or breaking of causality, or something. That recent experiment of a multiple entangled system in, China was it? Ok, doesn't SEEM likely, probably some other cause for the results. But maybe we're all part of a giant computer simulation. It wouldn't be impossible, and then you could sort of just make whatever "physical" rules you wanted. If there's not a formal name for this theory I just called it, we're calling it "Reeve's Theory" in honor of Keanu.

Seems as likely as "multiple worlds" at times, at least to me Of course I love the mathemetician attitude that "the math works out so we've got it!" attitude as well. It's the same attitude espoused to Max Planck when he first entered physics. That of "the math works, there's nothing else to find except cleaning up a few things, we're all good!" Math is wonderfully useful as a self consistent tool for modeling logical systems, I mean that's basically it's definition.

But the universe isn't math, it is, for lack of a better word "stuff". And the point of physics is to come up with a self consistent model that tracks as closely to this actual "stuff" as possible, not just to that which we have observed so far. And since, historically, previous self consistent mathematical models have been upended it would be hubris to assume that our current ones couldn't be as well. Of course, eventually we WILL reach the end, where our model is as perfect as can be made and there really isn't anymore "stuff" to find. But we certainly haven't now.

Err... getting back on topic. Yes, the seeming use of entanglement to do exactly what so many here propose it can't do (not that I'm a big fan of this either). Namely, seeming to violate causality (or some such). So quite relevant. Here's the paper in specificity: http://www.nature.com/nphys/journal/vao ... s2294.html . Behind a paywall For those of us without \$32 to spare on a paper that seems like it would need a followup of one kind or another here is a nice summary: http://arstechnica.com/science/2012/04/ ... eforehand/
Frenetic Pony

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### Re: Quantum Entanglement

Frenetic Pony wrote:Or, to sum all such interpretations of quantum physics up "We don't know. We probably don't have enough actual data to really know, so we've come up with as many theories as we can; and then strung them up to their logical breaking points. We can't even test the theories we have now, some of them we don't even know if we can test! Point is we need bigger particle colliders and telescopes so we can have more data damn it!"
I think you're mixing up different interpretations with different theories. They aren't the same thing.
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gmalivuk
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### Re: Quantum Entanglement

Frenetic Pony wrote:Or, to sum all such interpretations of quantum physics up "We don't know. We probably don't have enough actual data to really know, so we've come up with as many theories as we can; and then strung them up to their logical breaking points. We can't even test the theories we have now, some of them we don't even know if we can test! Point is we need bigger particle colliders and telescopes so we can have more data damn it!"

...

But the universe isn't math, it is, for lack of a better word "stuff". And the point of physics is to come up with a self consistent model that tracks as closely to this actual "stuff" as possible, not just to that which we have observed so far. And since, historically, previous self consistent mathematical models have been upended it would be hubris to assume that our current ones couldn't be as well. Of course, eventually we WILL reach the end, where our model is as perfect as can be made and there really isn't anymore "stuff" to find. But we certainly haven't now.

As gmalivuk says, you're misinterpreting the term "interpretation". All an interpretation is is a way of relating the mathematical formalism to the "stuff" as you describe it. By definition, one interpretation cannot be proven more accurate than another because they both use the same formalism and so produce the same predictions.

And no-one with even the slightest bit of knowledge about modern physics would suggest our theories are perfect (the fact we don't have a quantum theory of gravity seems the most obvious flaw) and so would conclude that our theory will be upended. You must also remember that, until the early 20th century, mechanics looked pretty much the same as when Newton first wrote it down (or at least, produced the same results even if through different formalisms), for a lot of the 19th century, it looked very reasonable to say that physics was pretty much done after all, it hadn't changed its predictions for 200 years.

Now though, we have the experience of the turn into the 20th century and our physics is under 50 years old and is known to have major flaws. Physics is in a very different state than when the previous lot of theories were overthrown and to say that any such replacement will follow a similar pattern is exaggerating the few similarities.

Furthermore, there is no compelling reason to assume that there will ever be a point when we reach the end of physics and our model will be perfect. In fact, it looks very much like our physics will merely asymptotically tend towards the truth. The peak science thread has much more discussion on this point however.
Gear wrote:I'm not sure if it would be possible to constantly eat enough chocolate to maintain raptor toxicity without killing oneself.

eSOANEM

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### Re: Quantum Entanglement

Frenetic Pony wrote:Or, to sum all such interpretations of quantum physics up "We don't know. We probably don't have enough actual data to really know, so we've come up with as many theories as we can; and then strung them up to their logical breaking points. We can't even test the theories we have now, some of them we don't even know if we can test! Point is we need bigger particle colliders and telescopes so we can have more data damn it!"

Lack of data is not the problem. There are more experiments verifying the principles of quantum mechanics in question than one could hope to study in a lifetime- and while it has its (speculated) breaking points, they're nowhere near what you're talking about.

Err... getting back on topic. Yes, the seeming use of entanglement to do exactly what so many here propose it can't do (not that I'm a big fan of this either). Namely, seeming to violate causality (or some such). So quite relevant. Here's the paper in specificity: http://www.nature.com/nphys/journal/vao ... s2294.html . Behind a paywall For those of us without \$32 to spare on a paper that seems like it would need a followup of one kind or another here is a nice summary: http://arstechnica.com/science/2012/04/ ... eforehand/

First of all, here's a free copy of the paper: http://arxiv.org/abs/1203.4834 . The Ars Tecnica article isn't bad, though. An important thing to understand about this experiment, however, is that in terms of theory it is absolutely unremarkable: it uses nothing beyond the quantum state manipulations that we've had for 80+ years.

A brief outline of the experiment: Ma et al create two entangled photon pairs, (12) & (34). It turns out that if you preform the right operation on (23) together you can entangle (14) remotely- this is deeply related to the idea of 'quantum teleportation.' Now one interesting thing about QM is that measurements made in different places can be done in any order, as long as they are independent, and you'll end up with the same result. In fact, this is what keeps entanglement from breaking causality. So, you could do this experiment two ways. In the first, you entangle the two pairs, manipulate (23), and then measure everything. In the second, the one that they choose, you entangle the two pairs, measure (14), and then manipulate and measure (23).

The confusion here is the following: someone might explain the first version of the experiment as "you entangle two pairs of photons, then you manipulate two of the photons (23) together which sometimes changes the state of their entangled partners (14) to be themselves entangled, and then you measure everything to see what happened." If you tell the story this way, then the second version where (14) are measured first seems very strange, and you have to use something like 'steering backwards in time' to make the same description fit. But there's also a more sensible story for the second experiment. You can tell it this way: "you create two pairs of entangled photons, and then measure two (14) which affect the other two (23) in such a way that you can use (23) to pick out times when (14) look like they were entangled." If you explain it like this, then it seems like nothing very mysterious or time-travel-y is going on.

With that in mind, I would say that the real question is why we need different explanations for different orders of measurements, even though the physics seems like it is really the same in both cases. I would guess that there are two reasonable lines of arguments. One could say that this is a sign we are giving too much physical reality to the quantum state, or simply that nature allows this duality in interpretation, analogous to how a force on a charged particle might be seen as caused by electric or magnetic fields from different observers.
Rococo

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