Subatomic question: particle pairs and spin

Oh heck. That was the first thought I had when trying to remember this physics principle. Aren’t there particles, which (can) exist in pairs, and when you affect one member of one pair, the other is affected instantaneously? I think the way you affect them is to change the spin, but I can’t remember correctly.

What are the names of these particles? I seem to remember they were leptons or neutrinos.

Are they always paired? Do you take two and “introduce” them, or do they start out stuck together and we split them?

Can we do this, or is this just something that is theoretically possible? I mean, presumably these little buggers are mind-bendingly small. Can you send one member of the pair to L.A., and do something to its mate in New York, without some part of this process being totally impractical? How would you store something that small? EM field?

Why can’t this be used for instantenous communication? I had a really annoying roommate, who was an electrical engineer, so I figured he had to be useful for something other than walking around naked from the waste down. I asked him, and the way he explained it was really weird. He said it “violated causality”, but this seemed to boil down to: we can’t communicate instantaneously, since all of our other theories say light is the fastest thing in the universe and it would be a pain to change all of those theories.

Needless to say, I don’t put much stock in that theory since this guy was a bozo, and since I wasn’t sure I was following him correctly anyway. I don’t have any problem with light as an absolute speed limit, but I didn’t really think speed limits applied to information. Can somebody explain this to me in layman’s terms, preferably with your pants on?

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  • Boris Badenov

You’re talking about “entangled” states, Boris. Take two spin-half particles, say, two electrons, and combine them in an entangled state (beats me how you’re going to accomplish this). According to quantum mechanics, the entangled state collapses when it is measured for one of the electrons, affecting the other electron no matter how far away. So, if you measure the spin of one electron as spin up, then the other electron must be spin down.

At this point I was plannin to launch into a long diatribe about qubits, quantum teleportation, information theory, and all sorts of fun stuff, but I’ve got some work I need to complete first. Sigh --I guess I’ll have to resume tomorrow.

I believe the essential point is that you can’t communicate any information with entangled particles.

Let’s said Alice entangles two particles and sends one to Bob, who lives 1000 miles away. She now measures the spin on her particle, which collapses the wave-front, and finds – glory be! – that it’s “spin up”. That means that Bob’s particle is “spin down”.

Big deal. All that means is that she now knows what Bob knows, but they haven’t conveyed anything of consequence. The information doesn’t mean anything. What has transpired is no more alarming than Alice knowing that Bob knows it’s noon where he lives.

Of course, if you want to be picky, Alice doesn’t really know that he knows it; she knows that he could know it. You know?

The point is, you can’t encode any information on this process.

Whoa, I’m confused now. Do you mean no information could be transmitted
(1) because an up-down dichotomy is too simple to mean anything, or
(2) because Bob can’t affect the spin of his particle, it just goes up or down at whim?

If (2) then, yeah, no communication. But (1) seems kind of weird … aren’t binary codes the whole deal behind Morse code and digital computers? Couldn’t you have a sequence of particles, numbered one through eight, with Bob arranging them up or down for a code? Alice has matched particles at her end, measures them, and has Bob’s simple message instantly even though he is light-minutes away? It would be more worthwhile if he were light-years away, I suppose.

Even if they only had one particle, it seems like it could be useful. “Up” would correspond to the length of the tone in a telegraph wire, “down” would correspond to silence. Just a thought though.

Until this thang gets worked out, I’m not going to stray more than a few light-hours from home, that’s for sure.

Subatomic particles are very strange, but not without a certain charm.

Ok, ok, I’m sorry.

peas on earth

On a more serious note, here’s how I understand the answer to Boris B’s question. (And I’m not a physicist nor do I play one on TV, so take this for what it’s worth).

As far as I understand it, the question you ask is indeed a very good one and is still an area of active research. There are two basic possibilities - one is that one or more of our current theories are wrong, and causality can be violated. The other is that the theories are OK, and for some reason (there are a few possibilities) the entanglement effect doesn’t actually violate locality.

Here’s a web page on the subject by a guy at Lawrence Berkley National Labs. I only skimmed it; I think it would take a fair amount of effort to actually sit down and understand it on a nuts and bolts level. But if you wanna have a go at it:

I again don’t pretend to understand this stuff very well. But one possibility for avoiding FTL as a byproduct of entanglement might be this: you have two entangled particles. You can measure the state of both simultaneously, which conveys no information. If you try to change the state of one, this breaks the entanglement. And you can’t avoid this by saying “ok, it had the value I wanted at time T, so measure it then”, because you have to convey that information to the remote observer at <=C. The net effect of this is that you could use entangled particles to make two distant observers aware of the same random number at a particular time, but not of any arbitrary information that you chose. And as I understand it, all the entanglement experiments have verified so far is the former.

If you can figure out what the hell the LBNL guy is saying, that might provide some more insight :slight_smile:

So my understand is that the jury’s still out, but personally I’m gonna place my bets on C still being the limit for conveying information.

peas on earth

Hmm, that’ll teach me to not use angle brackets. Let’s see if this works any better:

“…information to the remote observer at <=C”

peas on earth

What about using it for uninterceptable commuincation? Let’s say you have ‘x’ numbered elements of an entangled pair and somebody to whom you want to send a message has the reciprocal particles numbered the same way.

Your message is a string of 0s and 1s. The ‘down’ state equals a 0 and the ‘up’ state Collapse one particle, if it’s the equivalent to your first bit, collapse the second ‘y’ second later. If not, collapse the next one ‘2y’ seconds later.

How do you intercept that? Of course it’s a ridiculously convoluted and expensive scheme, but it’d work, wouldn’t it?

Only humans commit inhuman acts.

As for the other point:

While there are ways to do exactly what you are saying, I don’t think the entangled pair method would be effective for that purpose. In this example experiment, we don’t have total control over the content of the information.

Boris B: Okay, since you understand binary, let’s do a thought experiment.

Alice has a machine that flips 8 coins (equivalent to 8 bits or one byte, which could be used to encode an ASCII character).

Bob (located a light-year away) also has such a box. When Alice pushes a button, all the coins flip, in a completely random and utterly uncontrollable way, to either heads or tails. The moment she does that, all of Bob’s coins are flipped the opposite way, even though he’s located one light-year away.

Now that’s a pretty nifty device: it can send a byte of data faster than the speed of light! Fine. Now, tell me how you’d use it to send the message, “Hello there” to Bob?

I thought there was some way a human could affect the spin of the particle directly. I can’t really figure this part out.

Timothy, I like that analogy. :slight_smile:

Boris, no we definitely don’t have the ability to manipulate a single particle in such a specific, controlled way.

While quantum entanglement can’t be used to send information, I wonder if it could be an explanation for so-called “synchronicity”, the idea put forward by Jung that certain coincidences aren’t really random.

Timothy Campbell wrote:

Simple. Each second, Alice can press the “flip” button or not press it. Each second that Bob sees his coins flip, he records a “1”; each second he doesn’t see a flip, he records a “0”. After eight seconds, he’s accumulated one byte, enough to encode one ASCII character. (Heck that’s enough to encode one extended ANSI 8-bit-character-set character, too! She can communicate with all the special Norwegian and Hungarian letters in addition to the regular alphabet. But I digress.)

Ok - but for the entangled particle case, as I understand it (which again isn’t very far at all, so take this with a grain of salt), my impression is that the “flip button” is postulated to break the entanglement, or somehow damage things in a way that prevents information from being conveyed. If that’s true, the analogy would be something like this. Alice and Bob each have a single die, which is flipping about randomly. When the two are entangled, Alice and Bob can drive to opposite ends of town and their dice will keep doing random things, but in sync with each other, such that every time Bob’s die is showing a 5, so is Alices - they tumble in exactly the same way. But if Bob attempts to grab the die and make it show some number of his chosing at some particular point in time, the whole thing breaks down and the dice become independent. So the dice can be used to make both Bob and Alice aware of the same random number at the same point in time (useful for as a one-time-pad cryptogram), but that’s all - it can’t be used to send non-random messages.

I’m sure the analogy is flawed, since my understanding of this is poor at best.

But it seems to me that there are a few possibilities:

(1) What we see as entanglement is really explainable by something else.

(2) What we see as entanglement really is such, but by its nature it can’t be used to xmit information FTL.

(3) What we see as entanglement really is, and can be used to send FTL information. In this case, a lot of our physics needs to be thrown out and the universe is much more bizarre that we currently are thinking.

IMHO, (3) isn’t inconceivable (a lot of quantum physics is counterintuitive), but my bet is still on (1) or (2) until a fair amount of evidence for (3) piles up.

peas on earth

I think both of those analogies are useful, but maybe we can settle this by getting back to the specific experiment.

In the experiment linked above, each time the experiment is tried, there is only a 25% chance that Alice will successfully transfer the entanglement. When Bob receives a photon, he does not know if he is receiving a photon that has been unentangled, or is still entangled, until he confers with Alice by conventional (non-FTL) means.

Also, note that the source entangled pair is not necessarilly sent by Alice. It can be sent by a third party. If it was sent by Alice, the “transfer” would occur at about the speed of light. Depending on where the source of the entangled pair is located, Bob may receive his photon simultaneously with Alice, after Alice, or even before Alice. So if we think of Alice as “sending”, then Bob can “receive” before Alice “sends”. Of course it is really a misnomer to say that Alice is sending anything. She is merely taking the more active role by attempting to transfer the entanglement.

Whatever the situation, it is not known that a transfer has occured until Alice has contacted Bob by conventional means. That prevents Alice from being able to send information via on/off pulses.

All right, that’s it. I was going to take an ice-fishing trip to Pluto, but if I have to wait for my cell phone to transmit signals at “snail speed” (that’s the term us rocket-setters use for light speed), then I just ain’t goin. I’m dialing my travel agent right now.

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  • Boris Badenov

Well, Pluto definitely has enough ice for such a trip. Now, as far as fish go…

Well, I read through the paper referenced above and while interesting, it doesn’t advance or contradict the idea of entanglement. Rather, it sets up a theoretical approach to logically deducing the results of experiments (assuming 100% effective detectors, which are far from present reality). Essentially, he is attempting to compose a set of logical operators that could be applied to experimental results. In a sense, he is outlining what could be coded in a computer to take the experimental parameters through a series of [if…then…not…or] type statements and end up with a “True” or “False” answer to the question, “does this result require faster than light influence?”

So, we are left with my usual answer on this subject. EPR, the theoretical paradox that was formulated in an attempt to debunk Heisenberg’s Uncertainty Principle, has yet to be proven. Some have claimed that they can apply Bell’s Inequality to their experiments and prove, by statistical anomaly, that EPR is true.

One problem is that they make certain assumptions about what “should” have happened, or worse, what “would” have happened had they changed the experimental setup at a different point in the experiments than the one they chose.

The second problem is, there is no such thing as a 100% effective detector, so for the forseeable future, ‘evidence’ of EPR will depend on the statistical ‘weight’ of experiments shifting this way or that. Conclusive proof then, is elusive, since belief in the results lies in whether one accepts that the statistical anomalies (in a truly random system) are significant enough to be incontrovertable.

Personally, I don’t think EPR is a valid criticism of HUP. I just don’t believe that HUP necessarily applies to the ‘twin’ of the particle being measured. If you measure one property of a particle, you effect only that particle IMHO.

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Stephen, I’d love to get your opinion on this, because this was something that bugged me about the experiment, but I don’t understand the experiment well enough to tell if my intuition is on target.

It seemed to me that in that experiment that Alice is not really effecting any change on the entangled pair, but is rather obscuring her own view of the originating photons (by introducing her own photon) just enough to make it so that she only gains information about the photon from the pair under a certain circumstance, but that this information is still the same as it would be if she had done nothing at all.

This might not be an apt representation, but this is the way I think of it:

To borrow from bantmof’s dice example, let’s say that the entagled pair are like 2 4-sided dice, that must total 5, so if photon A is 3, then photon B must be 2. If photon A is a 1, photon B must be a 4.

Alice is sending out her own photon that is always a 2. One out of four times, the photon she receives can become entangled with her photon, just because it happens to be a 3. In this case, Bob’s photon must be a 2. Since she started with a photon that was a 2, and Bob got the 2, and she knows that Bob got the 2, she concludes that she has “transferred” this 2 to Bob.

But of course Bob would still have been getting the 2 even if she did nothing, so she has no causal involvement on what Bob observes.

So this image that I have makes the experiment pretty much meaningless. Am I off base?