Physics Challenge! Explain "entangled particles" to me

AlmostAtheist

Hey All,

I'm totally fascinated by the concept of entangled particles. As I understand it, you take a proton (or something tiny like that), split it apart, but don't observe the resulting particles. (Quarks?) The particles now exist in some sort of limbo state, neither "on" nor "off". You separate the particles, then observe one of them. This causes its waveform to collapse or some such thing, and it is now locked into a state, say "on". The other particle will immediately assume the other state of "off". The thinking goes that there is a hard link of sorts between them, so that as soon as one is forced into choosing a state (by the act of your observing it) the other is immediately forced into the opposite state. Sort of like having a 10-foot-pole lying on the ground, and pushing one end an inch. The opposite end of the pole also moves an inch at the exact same moment.

Ok, do I understand what happens? (If I've got that much wrong, jump in now and correct me)

Now, a few questions:

1) If we only know this happens by observing one particle, then confirming that the other particle is in the opposite state, how do we know they weren't in those states all along? How do we know they were in this limbo state to begin with? I can imagine putting a black ball and white ball into a bag, randomly selecting one without looking at it, then glancing in the bag. If there's a white one in the bag, I know I have a black one in my hand, but they didn't magically change colors at the moment I looked. They were white and black all along.

2) Assuming there's a reasonable answer to 1) -- and I have no doubt that there is -- how is this thought to be happening? Any theories?

I've tried to research it myself, but all I find are "wow! entangled particles are sooo cool!" type articles, with no information to explain any of it.

Thanks for your help!

Dave

AlanF

Think of multi-dimensional incestuousness.

AlanF

Satanus

Yes, it an interesting phenomanon.

AlmostAtheist

Here's one article I found that is reasonably readable. It doesn't go into the entangled particles stuff much, but it gives a little background.

http://library.thinkquest.org/3487/qp.html

IP_SEC

AA my understanding, which is probably inacurate, is that there doesnt actually exist two ghost particles until one is observed. It is a mathmatical formula of probability (50/50) which doesnt become 100/0 until you have observed the outcome.

It's closely related to Shroedinger's Cat *google it* The cat is not both alive and dead at the same time. Its just that the probability of either state is 50/50 until it is observed.

Maybe?

AlmostAtheist

is that there doesnt actually exist two ghost particles until one is observed

Theoretical experiment (perhaps actually performed with neutrons):

Fire an electron gun at a wall, firing randomly. The wall has two slits through which electrons can pass. Don't observe the particles coming through the slits, just look at the result on the wall behind the slits. You find that the particles coming through the wall's slits behave like a wave, that is, the evidence suggests that each particle goes through BOTH SLITS on its way to the wall. Like a wave that goes out in all directions, it appears that each single particle manages to get through both slits each time.

Change it a bit. Observe the slits during the experiment so you can see each particle as it comes through, thus seeing if they actually do go through both. They don't. They go through one or the other.

But in the first experiment they clearly DID go through both. The only difference was you observed them.

Ok, so we're suggesting that observation determines the state of the electron, its direction in this case. Rather than existing in all possible directions, it exists in just one. We didn't control which one, but we forced it to pick one for purposes of our observing it.

So what counts as "observing"? The slits were there all along, they "saw" the electron go through. Why wasn't their presence counted as an observation? What if we set up a camera to watch them, but promised never to look at the tape? Would the pattern of hits on the opposite wall suggest the wave pattern or the particle pattern? If after observing the wave pattern, we watched the tape, what would happen to the opposite wall?

If the camera's presence counts as observation, why? What if the camera didn't work? Would it still count?

And of course the big question -- has the two-slits-in-the-wall experiment ever actually been carried out? It's spoken of as if it has, but I can't see any evidence of it.

Dave

AlmostAtheist

The cat is not both alive and dead at the same time. Its just that the probability of either state is 50/50 until it is observed.

And the difference in the quantum world is that the cat really is both dead and alive. But it only works at the quantum level, so no real cat could ever experience it. The theory goes that the atomic particle in the box with the cat has a 50/50 chance of decaying. But until we observe it, it is both decayed and not decayed. Only by observing it do we force it into one of those states.

My problem is that I just can't yet understand the difference between flipping a coin and not knowing if it's heads or tails until I observe it, and this supposed limbo world of unobserved particles. The claim is that there is a very real difference. Honestly, I believe them, I really do. Some very smart people have spent a ton of time and money researching this stuff, and if it was so easily dismissed they just wouldn't do it.

But I don't get it. Yet.

Dave

stillajwexelder

Gods = Heisenberg,Plank,Einstein,Schroedinger, Curie,Rutherford, GLEN SEABORG

AlmostAtheist

I am SO never going to get this. I have been reading non-stop since I first posted. If I see the word "gluon" again, I'm going to burst.

How can anybody get their head around this stuff?

There are tons of references to these entangled particles, but nobody ever says what exactly they ARE. One phrase that pops up alot is "two entangled particles are generated, and then...", but I've yet to see anybody explain how that entanglement takes place. I have two particles (A and B) that are not entangled with each other, and I have two particles that are entangled (C and D). I cause A to interact (whatever that means) with C, and B to interact with D, then poof, C and D are now "entangled". Ok, but how? And what does it actually mean?

One point that was made in a few places is that the entanglement idea has been shown experimentally to produce results outside of what probability would suggest. In other words, if I flip a coin 100 times, and it comes up heads 85 times, I can reasonably assume something is going on. It ought to be 50/50, or close to it. But I can't figure out what it is that they are testing that they expect to come out 50/50. If it's the state of the particle ("up" or "down"), and the entangled particle A is generally (always?) coming out in the opposite state of B (upon observation of B), how is that showing anything? It doesn't sound like a probability thing, it sounds like looking at the heads side of a coin and finding that lo-and-behold, the opposite side is tails. Every time. Well, yeah.

I don't mean to deride the physicists, I'm trying to explain how thoroughly I'm not getting it, despite trying so very hard to do so. Let me reiterate, I fully believe that they know what they're talking about, that they are right, that this limbo-state-until-observed business is on the up-and-up. I'm just trying to understand it. Or at least be pointed in the right direction toward understanding it.

So my fundamental question is: "How is it known that the quantum state of entangled particle A and the state of entangled particle B is in a limbo state until one of them is observed? In contrast, how is it known that they were not in those states all along?"

Elsewhere? You out there?

Dave

Quotes

I had a semester of Quantum Physics at University.

Let me see if I can help....

The De Broglie wave equation collapses into....

no, wait a sec...

The Heisenberg Uncertainty Principle demands that the Planck constant must....

Nope, that's not helping either.

Um... uh...

OK. There's these particles, see. And sometimes they date other particles. And they want to get out of the relationship (so they can spend their weekends chasing other particles); but they can't split up because they're entangled. So sometimes the particles will drift away from the other particles, so they can get out of the entanglement but avoid being called "Opposer Particles" or "Apostate Particles".

Got it?

~Quotes, of the "Didn't understand it back in school, either" class

Physics Challenge! Explain "entangled particles" to me

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