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B How do we know some particles don't exist until measured?

  1. Jun 20, 2017 #1
    I don't know if I am right, but I have read about how some particles do not even exist until measured. How would we know this?
     
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  3. Jun 20, 2017 #2

    ShayanJ

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    We don't. That's part of some interpretations of quantum mechanics which means quantum mechanics itself is silent on it.
    Also, even in those interpretations its not like some particles are like this and some are not. Those interpretations consider this to be a property of any quantity related to any quantum system.
     
  4. Jun 20, 2017 #3
    thank you for clearing that up. But there must me some merit to this interpretation. What is their argument for this? Is this the Copenhagen interpretation? Because I know Niels Bohr believed in the particle disappearance thing.
     
  5. Jun 20, 2017 #4
    btw what interpretation do you believe in? I lean towards the many worlds interpretation.
     
  6. Jun 20, 2017 #5

    ShayanJ

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    This mostly comes from Bell's theorem. It says that nature is either realistic or local, it can't be both. So local theories have to have this property. But of course some interpretations can work around this, like superdeterminism, or maybe many worlds too, I'm not sure.
     
  7. Jun 20, 2017 #6

    ShayanJ

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    At first, its not a matter of belief, its a matter of preferring one over others. Its an important distinction!
    I'm still not that much specific about which interpretation I prefer, but I do know that I don't like Bohmian mechanics and many worlds.
     
  8. Jun 20, 2017 #7
    Is Bohm is kind of like saying that everything follows a deterministic law and there is no free will, along with no measurement problem?
     
  9. Jun 20, 2017 #8

    ShayanJ

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    The no free will part is not part of Bohmian mechanics(BM), its a part of superdeterminism.
    But other things you said are true. And BM doesn't have measurement problem because its a realistic theory, which by Bell's theorem, means its non-local.
     
  10. Jun 20, 2017 #9

    phinds

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    It's really irrelevant since they all conform to the same math and are equal at the level of "shut up and calculate"
     
  11. Jun 20, 2017 #10
    but the way I see it, it is also important to explain why you can get to that math.
     
  12. Jun 20, 2017 #11

    phinds

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    The interpretations happen after the math says what's happening so, yeah, they are an attempt to say WHY the math says what it says but since all interpretations come off the same math, none is preferred (except as a personal preference)
     
  13. Jun 20, 2017 #12
    true *pretends to know what he is talking about*
     
  14. Jun 20, 2017 #13

    ShayanJ

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    But even that is an assumption that is just stated a lot. There may be fringe cases of applications of QM that are still not tested and different interpretations disagree in their predictions about them. It may still be possible to be able to experimentally distinguish different interpretations and we just don't know how or don't have precise enough measurement devices for. One example is quantum non-equilibrium in Bohmian mechanics which I'm not sure in what situation, if any, happens.
     
  15. Jun 20, 2017 #14
    just took the words right out of my mouth
     
  16. Jun 20, 2017 #15

    Paul Colby

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    This is only possible if either the theory differs or the way the theory is applied differs in such a way it yields
    different experimental outcomes. I know of no case where this is the claim. Has anyone made such a claim?
     
  17. Jun 20, 2017 #16
    It's just presented this way to pop science audience to make it more interesting.
    In reality they don't have defined properties until they are measured.
     
  18. Jun 20, 2017 #17
    Yup, Davies has an essay on this topic called "Particles don't exist", talks about particle detectors, and more. I'd have to read the essay again to get his points, and if needbe, I can respond tomorrow. But IIRC, particles are only a utilization of a flat minkowski space, and if you go into curved spaces, you'll have some observers seeing particles being created when in the persons rest frame they see none.
     
  19. Jun 20, 2017 #18

    Paul Colby

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    Look, someone would first need to explain to me what "exists before measurement" even means quantitatively. To get quantitative about it implies some measurement so it would seem that an explanation would be amusing. Exist before measurement isn't even meaningful classically.
     
  20. Jun 20, 2017 #19

    ShayanJ

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    As I said, one example is the quantum non-equilibrium in Bohmian mechanics.
     
  21. Jun 22, 2017 #20

    bhobba

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    We know exactly the why of the fomalism which every interpretation has - it just an extension of probability thoery:
    http://www.scottaaronson.com/democritus/lec9.html
    https://arxiv.org/pdf/quant-ph/0101012.pdf
    https://arxiv.org/abs/1402.6562

    Basicallly QM is part of an area of math called generalized probability models or theories. Ordinary probability theory has distinguishable outcomes (also called pure states) eg we can tell the difference easily between raining and not raining. But what if we relax some of these assumed things about ordinary probability theory - well you get generalized theories and that is what QM is. It has a number of distinguishing features such as you cant tell the difference between pure states. Also in ordinary probability theory you cant continuously go from one pure state to another via other pure states. But in QM you can. If you model a physical system by some generalized probability state space it turns out QM is the simplest theory that will allow you to continuously go from one pure state to another. But physically we expect a theory to be like that ie if its in one pure state at time t1 and another at time t2 it should have gone through some other state while doing that.

    But knowing formally the why of something is entirely different to knowing what it means. We face that in even good old probability theory - the Kolmogorov axioms tell us formally what it all about - but what does it mean. We have all sorts of answers - Bayseanism, frequentest, decision theory to name just a few. In a lot of respects all interpretations of QM is this applied to QM - see the following by John Bayez:
    http://math.ucr.edu/home/baez/bayes.html

    its nothing new and just like probability no one has been able to definitively answer which is correct - maybe it will never be answered.

    Personally I adhere to the Ensemble interpretation because it's just, basically, the frequentest interpretation applied to QM. Most applied mathematicians think of frequentest ideas in probability theory, but some like Bayesianism (those into Bayesian inference) which corresponds to Copenhagen, and some like Decision Theory (ie probability is the weight a rational agent would decide if faced with a deterministic situation but for some reason doesn't have all the information - its used by Actuaries in so called Credibility Theory). The decision theory approach is favored by Many Worlds adherents and maybe even BM.

    Thanks
    Bill
     
    Last edited: Jun 22, 2017
  22. Jun 22, 2017 #21

    bhobba

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    Exactly.

    This whole quantum interpretation thing is loaded with stuff like that - they freely use words like exist before observation, realty etc etc. All of which are very very deep philosophical questions that there is no answer to. Best to avoid them in physics or use them very sparingly. I generally don't like quoting philosophers but IMHO Wittgenstein expressed it best - Whereof one cannot speak, thereof one must be silent. But then again before becoming a philosopher he was an aeronautical scientist. He went to further his studies by doing a PhD in applied math but after attending some lectures by Bertrand Russell (people sometimes forget he was also a mathematician) he switched to philosophy. Just as an aside he published his famous Tractatus which the above comes from. Someone noted he never completed his PhD so he submitted it as his thesis. He said to the examining committee - “Don’t worry. I know you’ll never understand it.”. The examining committees report was masterfully succinct: “I consider that this is a work of genius but, even if it is not, it is well above the standard required for a PhD degree.”

    QM, like ordinary probability theory says nothing about what happens before observations occcur. In flipping as coin probability theory says nothing about whats going on while its flipping. Same with QM - its silent on the whole issue But that doesn't stop people endlessly speculating.

    I wouldn't say its a waste of time because it does help clarify some things in QM. At first sight you might think QM has this thing called wave-function collapse. But we have non collapse interpretations so obviously it doesn't. From that viewpoint its quite a reasonable thing to do.

    Thanks
    Bill
     
    Last edited: Jun 22, 2017
  23. Jun 22, 2017 #22

    Simon Phoenix

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    And that's what drives me too :wideeyed:

    I got interested in QM thanks to my chemistry teacher in high school. He was trying to explain atomic orbitals. When he got to the p-orbitals I had a rather profound 'WTF?' moment. I asked a zillion questions which he couldn't answer and then he said "Don't worry, nobody understands QM properly". That was like a red rag to a bull to me and I was determined that I was going to understand it!

    The foolishness of youth. Over 3 decades later and I still don't understand QM - although I don't understand it to a much deeper level now.

    The greats like Feynman, Weinberg, Gell-Mann, and so on and so on, don't understand it to an even greater depth!

    It's true that 'why' is a never-ending spiral down the rabbit hole - you can just keep on going getting 'deeper' and 'deeper' answers (if they exist). But, personally, the 'why' is important to me - even though I know I will never get to an ultimate why, I would like to travel a bit further down Why Boulevard than I have so far before I stop for coffee and a muffin :smile:
     
  24. Jun 22, 2017 #23

    atyy

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    The chief merit of the Copenhagen is that it is a practical interpretation.

    You shouldn't take this particle disappearing thing too seriously - the Copenhagen interpretation is simply agnostic about whether the moon exists when you are not looking at it.
     
  25. Jun 22, 2017 #24

    Simon Phoenix

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    In my previous reply I didn't really attempt to answer your question. The answer to your specific question is, as others have said, "we don't"

    But that's not really a feature of QM - it's more a feature of the question. I mean how do you know your sofa doesn't suddenly pop out of existence when you get up to make a coffee and then pop back into existence when you come back to watch the second half of Doctor Strange? You don't, strictly speaking. We'll let the philosophers tie themselves up in knots over that one o0)

    It's more interesting to consider what physics, and QM, does tell us about the nature of reality. If we consider the phenomenon of entanglement we find that we are constrained in our choices of how we describe 'reality'. We find that ascribing definite properties (like values of spin in particular directions, or momentum, or polarization, and so on) to the individual objects of an entangled pair is not permitted. It's not permitted because the logical consequence of thinking like that is to predict the wrong answers for experiments with these entangled systems.

    That's really quite profound - if these entangled objects individually really did have definite properties then, sure, we'd be able to list them - write them down, at least in principle. Any theory we construct from such assumed properties is doomed to fail as a description of nature.

    OK - there's a way out - we could have definite properties but then we also need to imagine that there's some mechanism, or effect, we haven't accounted for. This effect must be faster than light and it must allow information about distant things to affect local results (results 'here') in such a way that the answers come out just right. This assumed 'effect' is not hindered by anything - put a mile of concrete, a mile of lead, a mile of neutron star material in between you and the distant things - doesn't matter. It's like magic.

    So either we accept that things at the 'quantum' level don't have definite properties in the absence of measurement, or we accept there's some magical effect that travels faster than light (and probably instantaneously) through anything we put in between that makes things come out just so (and in such a way that we can't use this to communicate). My personal preference is not to give credence to such a magical superfluous effect.

    So discounting any strange faster than light effect we have the situation that QM tells us that it is wrong, in certain cases, to even suppose that objects have definite properties before they are measured. I will probably get into trouble for this, but descending into 'pop science' vernacular it's as if 'reality' is continually distilled from an ocean of potential reality.

    Is it clear just 'why' nature is like this? Not to me it's not - but it's endlessly fascinating :smile:
     
  26. Jun 22, 2017 #25
    So, let's talk about particles overall. From how I understand it, particles are just a utilization of a flat space. Why is that? Well thanks to do the Heisenberg uncertainty principle, we really don't know where our particles are at. This doesn't matter on a flat space, because of symmetry. If we try to apply this to a curved space (for the most part), we can't invoke a symmetry argument. So what's the point of talking about particles at all if we can't generalize their arguments to curved spaces? We shouldn't be talking about whether or not particles exist before our detector detects them (a whole other issue on its own), but rather if we should even WORRY about particles at all! There are far better observables to use that we *can* use, and *will* generalize to curved spaces.

    I think Paul Davies can say it better than I can, so here are some quotes from one of his papers: "Bohr taught us that quantum mechanics is just an algorithm for computing results of measurements. Any discussion about what is a 'real, physics vacuum', must therefore be related to the behavior of real, physical measuring devices, in this case particle-number detectors. Armed with such heuristic devices, we may then assert the following. There are quantum states and there are particle detectors. Quantum field theory enables us to predict probabilistically how a particular detector will respond to that state. That is all. That is all there can ever be in physics because physics is about the observations and measurements that we can make in the world. We can't talk meaningfully about whether such-and-such a state contains particles except in the context of a specified particle detector measurement. To claim (as some authors occasionally do!) that when a certain detector responds (registers particles) in somebody's cherished vacuum state that the particles concerned are 'fictitious' or 'quasi-particles', or that the detector is being 'misled' or 'distorted' is an empty statement."

    -A few paragraphs later after going through some math about Rindler particles, and worldlines-

    "In ordinary laboratory quantum theory, we often talk about 'particles' as though they really exist, in the sense of entities with well defined properties independent of our observations. But in fact we cannot substantiate that image, for a 'particle' is merely an abstract heuristic model that provides an easy mental image of how one type of detector measurement is related to another. There is no need (and I contend it is meaningless) to regard the particle as a really existing thing skipping measuring devices.

    This strongly Copenhagen philosophy receives, in my view, powerful support from the particle analyses of curved space quantum field theory, where any attempt to hang on to the idea of particles being 'really there' is doomed to failure. Is a Rindler particle 'really there' in the state |0_M>? Is a rotating particle 'really there' when a rotating detector responds to |O_M>? Is it the Bogolubov transformation which tells you what particles are present, or a system of detectors? If the latter, which detectors? Which trajectories? The answer is that the idea of 'particles' is just a model, which works well for some conventional situations, but is usually utterly useless as soon as one moves away from inertial observers in flat space-time."
    "
    I honestly think the groundwork laid by Wheeler/Fulling/Davies/Dewitt isn't appreciated enough, and more people should take a look at the papers they've written on these subjects!
     
    Last edited: Jun 22, 2017
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