# Q.Entanglement & Superposition

## Recommended Posts

Regarding Quantum Entanglement and superposition, I am wrong somewhere, I want to know where!

1) I thought that to entangle 2 particles you set it up in a special way so that one would have an up spin and the other a down spin.

2) I have been told that when a particle is in a superposition it is in both states at once, only once the particle has been observered does this wavefunction collapse, the superposition is destroyed and we know the true spin of the particle.

Now I see a problem with that.

If the two entangled particles (which are currently in a superposition) are both in 2 states (up/down) at once, then when you measure particle 1 and discover it has an up spin then instantaneously the 2nd particle becomes down spin.

I used to think that quite simply entanglement worked whereby 1 particle was up and the other was down but we didn't know which was which, but if they are both in a superposition and a superposition means both states then neither we nor the 2nd particle can know which particle has which spin until 1 is measured, at this point particle 2 must come out of its superposition accordingly.

Whether a particle should be up or down spin depends on the 1st particle. At the time of entanglement the 1st particle has both an up and a down spin, so the 2nd particle has both as well. When particle 1 is measured particle 2 has to automatically and instantaneously mirror this.

The flaw being nothing is instantaneous, so where's the fault?

##### Share on other sites

I suggest you look into Bell's Theorem and the EPR Paradox.

##### Share on other sites

1) I thought that to entangle 2 particles you set it up in a special way so that one would have an up spin and the other a down spin.

That's one example; there are many ways systems can be entangled.

2) I have been told that when a particle is in a superposition it is in both states at once, only once the particle has been observered does this wavefunction collapse, the superposition is destroyed and we know the true spin of the particle.

Personally I avoid using the word "collapse," as did my QM textbook. Say instead that when measured, the wave function is now such-and-such. This is a matter of philosophy, but in my opinion using the word "collapse" suggests physical connotations, some of which may be indefensible.

Now I see a problem with that.

That's okay. You can see a problem with the change in state function being instantaneous, but that doesn't mean there actually is one. The suggestion of studying EPR and Bell is a good one. Also consider Bohmian mechanics.

##### Share on other sites

The flaw being nothing is instantaneous, so where's the fault?

The information about both particles was contained in the measurement you made that collapsed the wave function, so you didn't get information faster than c. And you can't communicate the information anywhere else faster than c. No causality violations.

##### Share on other sites

The information about both particles was contained in the measurement you made[/b'] that collapsed the wave function, so you didn't get information faster than c. And you can't communicate the information anywhere else faster than c. No causality violations.
(I added bold) which measurement? The one where I determind what spin that particle is?

I think my problem here is that superposition says that both particles have both spin.

So if you seperate the entangled particles by a distance and measured the particle's spin at the same time but in different locations one would measure up and the other measure down.

How is it that these particles which have both spins know that when observered particle1 will be up and particle2 will be down?

Afterall the particles aren't already in that spin, they have both, quite how it can spin both ways I'm not sure, but it's more the fact that these particles which have both spins will always, when observed, reveal one to have an up spin whilst the other down.

This was my problem with superposition and why I thought that superposition was that it was in one unknown state, not that it was in both.

This isn't about the person observing particle2, it's about the actual particle knowing whether it is up or down.

##### Share on other sites

This isn't about the person observing particle2, it's about the actual particle knowing whether it is up or down.

You aren't the first to be uncofortable with this. Consider some routes taken by physicists in the past:

1) Say that there must be a better theory than QM in the near future for this regime, but that QM is the best theory available now. Use QM, but never entirely accept it.

2) Say that QM is simply inferior and accept another system, such as bohmian mechanics, because of their successes and despite their failures. You would then claim this new theory was the better and that QM was obsolete - but you'd still use QM, because the fact is there is no better theory available for solving problems in the quantum mechanical regime. Including bohmian mechanics.

3) Admit that this system you've created where a particle knows it is spin up or down somehow predicts nothing, is philosophically difficult to defend, and appears (currently) to be invalid. You would then use QM, and leave the worrying for a time when experiment invalidated the theory.

Maybe there are more choices to add?

##### Share on other sites

(I added bold) which measurement? The one where I determind what spin that particle is?

I think my problem here is that superposition says that both particles have both spin.

So if you seperate the entangled particles by a distance and measured the particle's spin at the same time but in different locations one would measure up and the other measure down.

How is it that these particles which have both spins know that when observered particle1 will be up and particle2 will be down?

Spooky' date=' isn't it? Einstein thought so too.

Afterall the particles aren't already in that spin, they have both, quite how it can spin both ways I'm not sure, but it's more the fact that these particles which have both spins will always, when observed, reveal one to have an up spin whilst the other down.

This was my problem with superposition and why I thought that superposition was that it was in one unknown state, not that it was in both.

This isn't about the person observing particle2, it's about the actual particle knowing whether it is up or down.

It can be viewed as oscillating between the two states - that's the interaction that works in atomic clocks - but experiments have shown that it's not a matter of being in one state, with the particle knowing but we don't. (the phenomenon aka "hidden variables")

##### Share on other sites

I know Einstein's "spooky at a distance" quote, but am not yet at the bottom of the problem, is there a bottom? Did Einstein reach it (if his comment was for the identical reason as mine).

So a particle is less physically in 2 states at once, but more osciallating between two, ok, so that still doesn't help to the "how does a particle know which state it will be in when observered" question! It must know somehow, otherwise entanglement wouldn't work.

1) What physical experiments have been done showing that "it's not a matter of being in one state, with the particle knowing but we don't"?

2) When observed how would a particle know to stop oscillating between 2 states and reveal only 1, and in cases like entanglement a specific spin, not random?

##### Share on other sites

I know Einstein's "spooky at a distance" quote' date=' but am not yet at the bottom of the problem, is there a bottom? Did Einstein reach it (if his comment was for the identical reason as mine).

So a particle is less physically in 2 states at once, but more osciallating between two, ok, so that still doesn't help to the "how does a particle know which state it will be in when observered" question! It must know somehow, otherwise entanglement wouldn't work.

1) What physical experiments have been done showing that [i']"it's not a matter of being in one state, with the particle knowing but we don't"[/i]?

2) When observed how would a particle know to stop oscillating between 2 states and reveal only 1, and in cases like entanglement a specific spin, not random?

That's one of the things that sets the quantum world apart from the classical world. We're used to one set of behavior, but physics isn't really about why the rules are the way they are. There are various "interpretations" which tie in with philosophies, but ultimately physics is an attempt to describe how nature behaves, and not why.

I know at some level it can be unsatisfying to not know why the rules are the way they are, but the first order of business is to learn the rules. And one rule is that superposition is a valid description of a state of a system. The wave function is not localized to one particle - because they are entangled, it describes both, and the whole wave function "collapses" at the same time.

##### Share on other sites

That's one of the things that sets the quantum world apart from the classical world. We're used to one set of behavior' date=' but physics isn't really about why the rules are the way they are. There are various "interpretations" which tie in with philosophies, but ultimately physics is an attempt to describe how nature behaves, and not why.

I know at some level it can be unsatisfying to not know why the rules are the way they are, but the first order of business is to learn the rules. And one rule is that superposition is a valid description of a state of a system. The wave function is not localized to one particle - because they are entangled, it describes both, and the whole wave function "collapses" [b']at the same time[/b].

Which, if the particles are not at rest wrt each other, is "relatively" difficult.

##### Share on other sites

Which, if the particles are not at rest wrt each other, is "relatively" difficult.

Good and interesting point. Maybe that's the crux of the problem - the collapse that is simultaneous in one frame (the frame of the wavefunction, if one can define it that way) is not going to be simultaneous in other frames. Have to think about it.

##### Share on other sites

1) What physical experiments have been done showing that "it's not a matter of being in one state, with the particle knowing but we don't"[/i']?

The following links have information on this topic, including referenced experimentation.

Dr. Chinese's site.

Zz on the Physicsforums blog.

Baez on Bell-Aspect

##### Share on other sites

Good and interesting point. Maybe that's the crux of the problem - the collapse that is simultaneous in one frame (the frame of the wavefunction, if one can define it that way) is not going to be simultaneous in other frames. Have to think about it.
I don't follow.

The wave function collapses simultaneously for both of the particles, why does it matter if they are moving relative to one another?

I've never studied Bell's Theorem, nor EPR or anything else which answers my question, I just kinda thought of the question when thinking about the topic, I never knew it was obviously such a big debate! I'll have to now look into it, that top (of 4) sites Locrian linked to looks great, thanks.

##### Share on other sites

I don't follow.

The wave function collapses simultaneously for both of the particles' date=' [b']why does it matter if they are moving relative to one another?

[/b] I've never studied Bell's Theorem, nor EPR or anything else which answers my question, I just kinda thought of the question when thinking about the topic, I never knew it was obviously such a big debate! I'll have to now look into it, that top (of 4) sites Locrian linked to looks great, thanks.

In accordance with SR a particle's definition of simultaneous depends on the inertial frame it is at rest in. It is in motion wrt all other inertial frames and will "disagree" with the definition of simultaneous of any particle at rest in those other frames.

##### Share on other sites

OK, I'm not really at the advance enough level to fully understand Bells' Theorem so, without the proof, I want the conclusion!

Is it that:

The 2 entangled particles are both in a superposition and each individually can be thought of as osciallating between the two spins. When observered the two particle's single wavelength will collapse revealing each particle's spin.

And the fact that superposition is the combination of both spins and that the two particles break down to always mirror each others spin is just how it works, there's no "only the particle knows" and no probability (well, other than a 100%) it just happens, we can't really say why.

---------

And then about superposition, we say that before being observed a particle has 2 spins, whereas after being observed it has 1 spin.

Soo... lets talk about particle x, it gets created and goes along having 2 spins, then one day a scientist observes its spin and sees that particle x has "up" spin. Then he lets the particle get on its way, particle x carries on his day and the scientist and all his knowledge randomly disappears out of all existence.

Is particle x now back into a superposition (possesing 2 spins)?

Or does it still only have 1 spin (up) (but nobody knows)?

##### Share on other sites

Or does it still only have 1 spin (up) (but nobody knows)?

When measured, it's state function changes. It is now in the new state. It will not go back to being in a new superposition of states unless something occurs that causes it to.

This issue of what is in a superposition and what isn't is a deep subject. The process by which a quantum system ceases being in a superposition of states and begins to act classically is called decoherence.Obviously, when you look around, nothing appears to be in a superposition - so why is anything ever in one at all? It is actually quite a bit of trouble to keep large systems from decohering, because something in the lab is very likely to interact with them, thereby changing their state function to one that is no longer a superposition. This is why work done on quantum systems tend to be done in incredibly controlled environments at very low temperatures.

Actually, understanding decoherence may help you understand the issue here. Let me refer you to a GREAT paper on the subject:

Decoherence.

I love that paper...though I admit, I have yet to actually read all 41 pages . It is well written and I think you may get some further information out of it.

##### Share on other sites

Anyone have anything else to say on the rest of my post #15???

I'll continue this thread (if needed) once I fully understand decoherence, Bell's Theorem and the EPR Paradox, which may take a while as I don't have that much time free and the later too seem quite complex!

## Create an account

Register a new account