Physics Challenge! Explain "entangled particles" to me

by AlmostAtheist 41 Replies latest jw friends

  • Satanus

    Physicists don't make mistakes or get confused. The get entangled neurons. But until it is observed, it has not happened yet.


  • Terry

    What it comes down to is really quite ordinary. No, really.

    For us to "observe" something (detect it somehow; measure it) we have to BOUNCE something off of it.

    Radar, for example, is bouncing a signal off of something and measuring how long it takes to return to us and how it has altered from the collision.

    If you were blind, in a room, and had a pocket full of tennis balls you could take the balls out and throw them in different directions one by one and listen and feel the various collisions and returns. You'd develop a visualization of how far the walls were from you.

    Now on the microscopic fundamental particle scale of things imagine what you have to bounce off a quantum particle to detect it. Perhaps a photon or an electron. Do you think at that tiny scale the collision will not affect the path of the particle once it is struck? Yes!

    It is like throwing a bowling ball at baby penguins!

    So, when we measure things we perforce change them. And, we measure that rather awkward change and call it the ACTUAL state of the fallen penguin.

    No way.


  • AlmostAtheist

    Sierra (my 7-month-old) refuses to go to sleep. She's probably in a quandry over this whole thing, too. Can't blame her, really.

    Terry's point is certainly a good one. So the point is less that the particle is "observed" and more that something has collided with it. Going back to my walls and slits (which of course I lifted badly from those sites I didn't understand) the slits didn't observe the passing of the electrons because they didn't shoot anything at them hoping to collide with them and get some information back. The camera would most certainly do so, as it would have to send something out to bounce back from the particle. (We're assuming here that the room is a vacuum, no light, no energy -- impossible, but hey?) So the camera would observe the particles, never mind if any sentient being bothered to ever look at the recording.

    That makes the "observe" aspect a ton clearer. Thank you, Terry.

    So here's our particle, existing all over the room like a bubble extending from the emitter. Then one of my observational particles (light, death rays, something) impacts the particle. Where does it do that? For lack of any understanding of it, I'll say I send out a stream of observation particles and the first one to touch the bubble causes it to collapse to a single particle. No impact, no particle. Does the particle "appear" at the point where my observation particle impacted the bubble? Or can it appear anywhere? It doesn't make sense that it would appear just anywhere, or my observation particle wouldn't have impacted with it. But then at the quantum level some things are caused by their own effect, right? Something all backward like that?

    In talking about this with Gina, I told her I feel like a guy from the 1st century hauled forward to the 21st century, demanding to know how a cell phone works. Never mind that I don't know what a "wire" is, or plastic, or radio waves. I just want to know how that little box you're holding is able to let you talk to people. How can you get that 1st century dude up to speed enough to even begin to "get it"?

    Or can you? Maybe you just can't. I don't want to accept that possibility, but I have to admit it's a possibility.

    Anybody else got a Terry-like revelation to share?


  • iggy_the_fish

    Goodness, this thread has woken me up a bit! To misquote Richard Feynman,

    I think it is safe to say that no one understands Quantum Mechanics.

    AA - You've heard of Richard Feynman right? He was smarter than God, and what he didn't know about quantum theory wasn't worth knowing, and even he didn't understand it! I did mathematical physics at university, and that single electron slit experiment really freaked me out, and continues to freak me out really.

    Here's as far as I got with the problem.

    I'm reasonably happy with the particle "existing" as a probability wave between the emitter and the detector, and I can even just about stretch my brain to thinking about that probability wave going through two gaps simultaneously and continuing on thereafter to the detector screen, creating on its way the interference pattern that eventually describes that banded detection pattern you get out of the experiment.

    It's the mechanism of the detection bit that I continue not to understand. If memory serves, it's the collapse of the wavefunction (the thing that controls the probability distribution) into a certain state. Let's remove the slits for a minute, we have an emitter and a detection screen that's giving us definite locations of collisions of electrons with the screen. So, the way my poor human brain imagines it, the wavefunction of the electron evolves as waves do, and then starts interacting with the detector screen (which also has a wavefunction, which is much more complicated than the single electron of course) when it reaches the screen. Then, these two wavefunctions do some sort of funky mojo, which is like some kind of quantum committee meeting, where they decide whether the emitter is happy for the electron to have been emitted in the first place, and then if this is OK, the electron and the detector discuss where the electron would like to be detected (anywhere the electron wavefunction was non-zero when it hit the screen would be OK).

    What are the exact mechanics of this collapse of probabilities into certainties? I don't know, and unless some kind of amazing breakthrough has happened in the last 10 years since I was at university, physicists don't really know either. I'll have a google, see what I find...


  • New Worldly Translation
    New Worldly Translation

    I got really interested in entanglement after reading a book called Entanglement by Amir Aczel. It's more of a history of how the idea developed from back in Einstein's time (he called it spooky science) to recently. It's a good read but it doesn't go too deep into the fundamental science and I was left with an understanding of what happens but no clearer of why it happens.

    One of the best websites I found was

    It's a dept at Oxford Uni trying to develop quantum computing. They have lots of resources listed for entanglement and quantum physics in gerneral too.

  • iggy_the_fish
    So here's our particle, existing all over the room like a bubble extending from the emitter. Then one of my observational particles (light, death rays, something) impacts the particle. Where does it do that? For lack of any understanding of it, I'll say I send out a stream of observation particles and the first one to touch the bubble causes it to collapse to a single particle. No impact, no particle. Does the particle "appear" at the point where my observation particle impacted the bubble? Or can it appear anywhere? It doesn't make sense that it would appear just anywhere, or my observation particle wouldn't have impacted with it. But then at the quantum level some things are caused by their own effect, right? Something all backward like that?

    Right - with nothing up my sleeve, and without the aid of a safety net, I'll try to answer this! I have to put a disclaimer on my answer - I'm not an expert in this field, but I'll give you the best answer I've got. If there's any real physicists out there, feel free to chip in, fill in the holes in my argument.

    Yes, the particle evolves from the emitter, like a bubble if you like, although it's probably better to think of it as a spreading ripple, like when you drop a stone at the edge of a pond. It's the next bit of your question that's not quite right. You see, your observational particle is also spreading out from its own emitter in the same ripple like way. So now you've got to imagine dropping two pebbles into your pond, at different points on the pond's perimeter, and the result is a combined ripple field accross the surface of your pond, or in our experimental chamber, two "superposed" ripple fields of our particles. This combined ripple field now gives us the probability of finding the particle at any given point.

    Well, now I start to get a bit hand-wavey, because I don't understand this bit myself (see my earlier post). How does the system decide where it ends up finding the particle?

    Have you read anything about string theory? The basic idea is that all fundamental particles can be thought of as different modes of vibration on very small loops of string, and that therefore the whole of the natural world is a gigantic symphony played by the great universal chamber orchestra. To simplify a bit, imagine a guitar string. Without putting your fingers on the fretboard, there are a restricted number of notes you can get out of it, corresponding to what's known as the normal modes of the string (harmonics to you and me). So, if we think of our two particle system as like a vibrating system, there are only a certain number of "harmonics" the system can settle to.

    If we do our two particle experiment (emitted particle and detector particle) 1000 times, we get 1000 different answers. What made each experiment settle to the particular quantum "note" it did? Who knows? Not me that's for sure as mustard! Just remember though that the wavefunction of the detector particle is coupled to a whole load of detection equipment. There's lots of stuff on the internet talking about things like: once a system gets sufficiently complex (lots of vibrational fields interacting with each other) it forces the original system to choose which note it's singing.

    Maybe it's a bit like, when you're uncertain about something (where to go on holiday? France or America this year? Decisions, decisions...) and you talk to enough people about it, eventually you have enough information to make up your mind, and then your probabilities collapse into definite certainties. Perhaps the emitted particle chats to the detector particle, the detector particle talks to the particles in the detector, the particles in the detector talk to the particles in the wiring and computer part of the detector, and when the system becomes complex enough (probably just in the detector bit) our probability waves collapse into a definite "note" which the system "sings", which is then measured by the detector somehow.

    Why is the answer different every time? That I don't know.

    Hope this helps! I know I'm more confused than ever now...


  • AlmostAtheist

    Maybe I should just wait on the Awake! series on Quantum Mechanics? "I Found God in the Tiny World of Quantum Mechanics" by Physicist and Full-Time Pioneer Angus Wallaby

    Maybe not.

    At any rate, thanks for the links and descriptions. All very helpful. I know much more now than when I started. Which means I now know less, actually, but I at least know what it is that I don't know. Progress!

    The opening of this article confuses me:

    It says that in the following experiement, a single photon is fired at a half-reflective mirror. Like throwing a ball at a wall full of holes, we should expect that it would either pass through a hole, or bounce off the wall, depending on where it struck. And indeed, that's what the photon appears to do. It is detected by either detector1 or detector2:

    So far so good. The theory goes, though, that had we not had the detectors in place, the photon -- as a quantum particle -- should have taken BOTH paths, as its course was not determined until it is observed. Or more correctly, it did in fact take both paths, but ultimately resolved to one path or the other at the point of detection. To illustrate that this is what actually happens, the following experiment is performed.

    Two fully reflecting mirrors are placed where the detectors were, and a new half-reflective mirror is placed at their junction. The two detectors are then placed to receive photons from either side of the new half-reflective mirror:

    In the experiment, the photon is only detected at detector1. Now this makes no sense to me at all. It would seem that the photon hitting the new half-reflective mirror is in exactly the same circumstance as it was in the first experiment. It should randomly pick one detector or the other. But instead it always goes to detector1. Can anyone explain why this is? The article says it as if it makes perfect sense, like it was the expected result.

    To make matters worse, they place an obstruction into the mix and then it DOES go to both detectors. And again, this is described as expected behavior:

    Can anybody help me understand why these last two experiments make sense?


  • iggy_the_fish

    humm, I'll have a think about the mirrors. In the meantime, if you want to blow your head apart even further, have a read about the Copenhagen interpretation of quantum mechanics, and the transactional interpretation of quantum mechanics. There's also the many worlds interpretation, but that's a bit weird, even for quantum mechanics!

    The transactional one is interesting because it talks about the emitter and the absorber setting up a standing wave between them, one going forward in time and the other going backward in time! Spooky. Don't quite understand it yet, still looking for a sufficiently dumbed down description on the internet...

    Right, back to the mirrors....


  • AlmostAtheist

    >> still looking for a sufficiently dumbed down description on the internet...

    I tried turning down the "brightness" on my screen, but it didn't help...

    (With apologies to Galagher)


  • Terry

    If you have ever watched one of those "reality" shows on tv, such as Survivor, you will follow this.....

    People are placed in a situation in which they must achieve some goal and overcome obstacles. At the end of those interactions between people, clashes with challenges and conflicts with self, the audience has come to "know" these people at a fundamental level. How? By OBSERVING THEM.

    But wait!

    How do we observe them?

    Well, there are lots of cameras following them around and sound technicians with boom microphones recording everything. (We don't see these technicians; it would spoil the illusion of the main characters.)

    It is apparently a matter of becoming accustomed to camermen and mike booms swarming over you day and night that enables the subjects of these shows to act "normal" and get on with it.

    At the end of the game we have a winner and the audience has its favorite moments.

    In view of the above I'll now build my analogy.

    1.The premise of these "reality" shows is a false premise from the outset. Why? Because the persons who are subjects of the tv show would NEVER have come together____naturally___for such interactions. It is artificialy constructed.

    2.The enviornments of these interactions are manmade and exist purely for the purpose of creating a background of interest for the interactions of personality.

    3.The clashes are real enough IN THEMSELVES up to a point. But, they are goaded by the artificiality of the premise. (Example: in everyday enviornments people don't starve themselves in hostile climates and get insect-bitten 24 hours a day while exerting themselves physically to achieve food rewards.)

    4.The person-to-person and audience-to-person DATA is compromised for the sake of entertainment. What you learn about these people may be something that would never happen in day to day life.

    What is my analogy?

    The experiments that Physicists construct are very similar to these "reality" shows. What we learn about particles in these artificial experimentation environments is about as quirky and hard to understand as the antics of people stranded in hostile enviornments who must compete for food and a million dollar reward.

    The data is constructed from EXTREMES and not natural (at-a-distance) observation.

    Just as our Survivors are pelted with insects, food challenges and competing back-stabbers, so too are our fundamental particles pelted with out-of-nature train wrecks and collisions that render them suspect as to behavior.

    It is the best science has come up with. It is the best TV has come up with. It is interesting and mysterious. But, it isn't TRUTH with a capital "T". It is meta-truth with a tiney "t".

    Perhaps we should moderate our conclusions about the quirkiness of so-called "nature" upon reflection that we are really talking about melodramatic tv.

    If you take your wristwatch and smash it with a hammer you will not learn about it as a FUNCTIONING TIMEPIECE. You will only learn about it as damaged particles incoherently disarranged.

    Think about that. You can't really KNOW a watch by gazing at and speculating on smashed and disarranged smithereens. (Not without having experienced a functioning watch before the smash.)


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