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Wave-Particle Duality


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I've recently been reading Physics in Minutes: 200 Key Concepts Explained in an Instant by Giles Sparrow. So far a good book but once I had left my "safe zone" in terms of physics (straightforward classic mechanics) and entered the pages regarding quantum Physics I found myself rather lost. To be honest, my questions about Wave-Particle Duality are just the tip of the iceberg (but I thought I'd go one question at a time!). 

So, Wave-Particle Duality. I get how a wave can act as a particle because of photons but what I can't visualise is how it works the other way around. In my mind a wave is, well, wavy. Something which a particle isn't (I don't think so anyway - I may be wrong!). So how can a particle act as a wave? 

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There is no way of explaining it in human terms.

It is just a necessity to accept that sometimes particles sometimes behave like a wave and sometimes as a particle.

I don't think there is any explanation as such..... it is just the way the universe operates.

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I wonder if the problem here is more related to the terminology than anything else.  For example, photons or electrons say, are described as "particles".  But what is "a particle"?  In the macro world we have some sort of intuitive understanding of what a particle is.  Given a pair of tweezers and a magnifying glass you could pick one up and drop it in my hand.  At some point, even if it's squishy, a particle is hard and solid.  In the sub-atomic world however, perhaps that gives entirely the wrong impression.

Let's say instead we decided instead that photons and electrons (and a whole lot of other stuff besides) were actually thozbylks, which may have some or all of mass, energy, momentum, charge and a load of other characteristics.  We can't actually see these thozbylks or even touch or hold one.  We merely know they create certain effects that allow us to determine their presence (or otherwise).

If we do experiments and arrange our equipment in some subset of ways, our thozbylks appear to behave as we'd expect particles to behave in the macro world.  But if we use different kit or arrange what we have another way, we perceive them to behave as we'd expect waves to behave in the macro world.  And in fact if we arrange our experiments in some ways the thozbylks seem to know what we're up to and behave one way when we're looking and another when we're not.  Of course thozbylks don't really change from particles to waves or vice versa and they aren't actually one or the other.  They just do what thozbylks do and have always done, much of which appears to contain a large element of "confusing people who are looking".

That's my take on it, anyhow.  At the subatomic level we (that is, scientists -- personally I wouldn't know a thozbylk from a thyzbolk) appear to categorise "subatomic stuff" by its effects and how we can measure them rather than by the physical manifestation that we're used to in the macro world.  That description may however include some sort of analogy with the macro world that may serve to give a misleading impression of what you're dealing with.  I'm fairly certain for example that superstrings aren't really little bits of vibrating string.

I've probably deeply offended a load of particle physicists here with my gross generalisations and riding roughshod over what passes for reality (at least on Tuesdays), especially the ones who deal with thyzbolks on a daily basis, for which I can only apologise.  I am a lowly computer scientist doing his best to get his head around this stuff too.  I will take pleasure in reading any post that makes a decent job of explaining how waves can appear to be particles should someone care to write one.

James

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I think the problem arises because the science we currently have doesn't adequately describe what we see so we have to flip-flop between the two possible states. I always found it easier to think of particles as a packet of energy. Energy can appear as waves or mass (particles) and that helps us to understand what goes on. Whether they act as waves or particles is purely our way of explaining what is observed.

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The thing that blew my mind with this whole thing was when I heard about the double slit experiment. Basically when you fire a beam of light at a pair of closely spaces slots you see an interference pattern on the other side ( classic example of light acting like a wave), however  incredibly  the same thing happens  if you configure your equipment to emit a single photon at a time ( you could say this is  a particle of light at a time).  But and here's the thing that blew my mind, if you measure which slot each photon actually goes through (observing light acting like a particle), then the interference pattern disappears and you just get two spots  ( one for each slot) as you expect for particles.  This is all down to the peculiar nature of the quantum  world (something to do with the fact that the photon goes through both slots, but if you observe it you collapse the probability function) of which I don't understand, but again shows that how we actually  describe light as a wave or particle isn't quite right but is a suitable explanation  based upon the observations of the experiment you perform.

fascinating stuff this physics :)

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I forget who, but an eminent scientist once said that if you think you understand quantum theory, then you don't.

Trying to account for the behaviour of the quantum universe in human terms is not possible in a way that is 'logical' to us.

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I forget who, but an eminent scientist once said that if you think you understand quantum theory, then you don't.

I believe that was Richard Feynman, though he may not have used exactly those words.

James

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I think the idea (wunderz?) of wave particle duality can be overplayed? Usefulness depends on lifetime, range, energy (virtual mass etc.) of such things? A convenient model, under specific circumstances? I still think of (low energy) photons (infinite lifetime & range, propagator of e-m force) as "waves". I think of (high energy) intermediate vector bosons (Z0, W+/-'s, short lifetimes, (charge!), propagator of the weak force) as particles. The strong force: Quarks as particles, gluons as waves? The graviton... an exercise for the reader? Basically, there is sufficient (Heisenberg?) "uncertainty" for everyone.  ;)

I started out on a high energy photo-production experiment - 80 Gev electrons "skidded past" (analogy!) LEAD nuclei in a target, emitted Bremsstrahlung(!) radiation - A *spectrum* of high energy photons. Wave particle duality (Heisenberg uncertainty) allowed high energy photons to spend time as a "superposition" of *virtual* vector mesons (same quantum numbers as the photon!). In turn, these could be "frozen out" as real particles by (again) interaction nuclei in a liquid hydrogen (nucleus) target. These were then observed by their decay products etc. (Well, basically something like that!)  :p

The interaction could be modeled by various means: The "Pomeron", people such as Veneziano... The models worked rather *well* - But are now superseded? Fair dues Mr. Feynman - It is often said that "most scientists" cannot understand quantum mechanics - But I wouldn't bet on it re. some Theorists! lol. Scientists build on previous knowledge. A few develop theories ab-initio, but most jumb in a calculate (do) stuff! Certainly no one need feel "stupid" by skimming the introduction! Do your research according to your ability? Ask questions. :)

I don't think of Particle Physicists as particularly thin-skinned? lol. I don't think of them as superior to (humble) computer scientists... Or Anyone! In my day, things were more easy going? No Twittering scientists with a "message for humanity"! No desire to suppress "wrong thinkers"! Thing change... :o

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I don't think anyone really can grasp the entire concept.  (See Schrodinger's Cat thought experiment via Google.)  However, I spent many years dealing with light in my career.  The best way for ME to grasp the concept (from a practical, engineering perspective) is that light travels as a wave, but arrives as a particle.

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To me it is not a question of duality, it is what it is.

The "duality" is picking the easiest way to explain an aspect of it's behaviour.

An electron can usually be best explained by biasing the description towards partical characteristics, and then just to keep everyone on their toes a few wave characteristics - double slit. But an electron is an electron, it is our explanation that selects the partical or the wave aspects. The problem is right at the start by using the term "partical", or the term "wave".

Cannot put a picture up here easily but my Physics teacher Doc Harris drew it as an elipse with a sine wave inside, and said that is what you are dealing with.

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I forget who, but an eminent scientist once said that if you think you understand quantum theory, then you don't.

Trying to account for the behaviour of the quantum universe in human terms is not possible in a way that is 'logical' to us.

Neils Bohr. He also said that he 'felt somehow suspended in language.' I think it was John Gribbin who said that Eddington had said, 'You might as well call them slithy toves,' and then added, to my great delight, 'which is exactly what they are...'  This mirriors JamesF's post.

Wave and particle, in quantum theory, are not, I think, words used literally but are really just metophors for two different manifestations of one reality.

The double slit experiment always makes me think about time, which we know we don't understand. We fire 'one photon at once.' Now hang on, what does that mean? It means that, in our everyday macro world time frame, we believe we are separating the firing of photons in a dimension we call time, with past, present and future. But suppose the quantum world doesn't recognise this time frame? Suppose the photons refuse to be bound by this separation we feel we have imposed. There is also the entangled photons experiment which seems to suggest that photons communicate instantaneously. But suppose that separation in space (this time) is meaningless for them.

Here's a separation metaphor. You want to separate into categories two shirts,  four socks and three boxer shorts so you put each in a separate glass drawer in a glass chest and observe it from in front. You have a nice separation. But when you observe this glass chest from above your separation is, in a sense diminished. You will see what appears to be a mixture of all three types of garment. I think what's odd about quqntum theory is that, unlike classical physics, the point of view of the observer is critical. Whether or not we will ever be able to visualize the inside of an atom I don't know, but my bet is that our best hope lies in unravelling the nature of our point of view. And if I were clever enough I'd start by examining time.

Olly

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I am sceptical about entanglement.

If we put a red sock in one box and a blue in another... Then separate them..do we say that when we open a random box and see a red sock that the other must have suddenly become blue, but before that was neither red nor blue? It was blue all the time...we just had not looked.

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Okay, so actually trying to "visualise" a particle acting like a wave is quite futile and it's more an idea that'd be considered during calculations. Is that it? 

Another confusing thing is the idea that a probability function collapses when observed. An issue of S@N a few months back explained how the Sun creates light, To my memory (correct me again if I'm wrong) when a proton passes a deuterium nucleus there is a moment where there is a 50/50 chance of it bonding to the nucleus or not. As a result, it oscillates between the two states creating a changing magnetic field, which in turn makes a changing electric field and so light is given off. My understanding of it is that the proton oscillates because it is in the two states at once (like a certain cat we know of). The crazy part I don't get is how being observed effects this process. How do the particles know they are being observed? If we could see inside  the Sun, would it stop working because we can see the particles fusing? 

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To observe something we have to interfere in some way....... bounce photons off or sense a field...which disturbs the system.

The act of observing means we have to extract information through some sort of physical process.

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To me it is not a question of duality, it is what it is.

The "duality" is picking the easiest way to explain an aspect of it's behaviour.

An electron can usually be best explained by biasing the description towards partical characteristics, and then just to keep everyone on their toes a few wave characteristics - double slit. But an electron is an electron, it is our explanation that selects the partical or the wave aspects. The problem is right at the start by using the term "partical", or the term "wave".

Cannot put a picture up here easily but my Physics teacher Doc Harris drew it as an elipse with a sine wave inside, and said that is what you are dealing with.

I agree, it's an electron, it behaves like an electron, and belongs to a family of 'slithy toves'. 

Question is, how do we communicate such abstract ideas, with such an inadequate vocabulary?

I guess we rely on previous experience to chose what we think are the most appropriate terms, and refine them as our understanding improves. We have used the terms wave and particle to communicate our ideas about electrons to one and other, and would probably not have got this far without them. Duality too, has played it's part in furthering our general understanding of the nature of the electron, but (for me anyway) it has outlived its usefulness, since it doesn't allude to properties such as spin or the exclusion principle. Wave equations, non Euclidean geometries, and probability fields are more useful for describing what a relativistic atom may actually look like (if it could be looked at), but such ideas are more difficult to articulate, and communication of them is limited to those who understand the mathematics.

Poets create words of their own, words which initially are separate from meaning or definition, but when used in a certain context, conjure up beautiful images. I imagine they must experience a modicum of annoyance when physicists with no words of their own, steal them, and give them meaning. 

IMHO, new ideas need new words. I look forward to hearing some.

Ray

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Poets create words of their own, words which initially are separate from meaning or definition, but when used in a certain context, conjure up beautiful images. I imagine they must experience a modicum of annoyance when physicists with no words of their own, steal them, and give them meaning. 

IMHO, new ideas need new words. I look forward to hearing some.

Ray

Dirac didn't think much of poetry; he said something like, 'The purpose of a scientifc theory is to express in the simplest possible terms something which has never been said before. The purpose of poetry is precisely the reverse.'  I have no idea whether or not he was joking but, if he wasn't, he didn't know much about poetry.  :grin:

Olly

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There is no way of explaining it in human terms.

It is just a necessity to accept that sometimes particles sometimes behave like a wave and sometimes as a particle.

I don't think there is any explanation as such..... it is just the way the universe operates.

Feynman on wave/particle duality: http://www.youtube.com/watch?v=hUJfjRoxCbk. Feynman is an excellent teacher.

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Dirac didn't think much of poetry; he said something like, 'The purpose of a scientifc theory is to express in the simplest possible terms something which has never been said before. The purpose of poetry is precisely the reverse.'  I have no idea whether or not he was joking but, if he wasn't, he didn't know much about poetry.  :grin:

Olly

Hmmm, if Dirac were to have joked about anything, would anyone have known for sure?

 I would give him the benefit of the doubt, and say he was joking, if not, I too would disagree.

The scientific truth is there to be discovered. If we do not uncover something now, others in the future probably will. Having an idea for a new theory doesn't prevent someone else from coming to exactly the same conclusion. Indeed, arguments as to who said what first, seem to be common in scientific circles.

A poet on the other hand, may think of a verse but fail to write it down. The lost verse is not there for others to discover.

(Thinks me maybe wandering off a bit, but hey, dunk biscuit and carry on)

Is this one of those areas where science and art overlap? (shudder, did I just say that?). What I mean is, when a physicist puts a name to a completely new phenomena, they get to chose, right? Terms like charm or quark are are more interesting than surnames or 2nd declension Greek. I'm not advocating the use of wild imagination in science, but maybe it creeps in occasionally.

I hope that 'dark matter' is just a 'place holder', and that when we know what it actually is, someone gives it a better name.

Wish that scope and mount would hurry up and arrive :grin: Ray

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These days I've become very sceptical indeed about how we (as an apparently self aware race of beings) are perceiving the universe we appear to exist within. although, we are doing our very best with the tools at hand (our current senses et all), I realise that.

We have such a limited point of view of just about anything at all it's difficult to understand to see how we are ever going to understand the true nature of erm .. any of it really?

Is it a particle, or is it a wave? .. do we even know (or think we know) what we really mean by calling it a particle or a wave?

Perception itself is such a strange beast, strange enough that we have no real understanding of what we think it really means at the end of the day.

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...   Is it a particle, or is it a wave? .. do we even know (or think we know) what we really mean by calling it a particle or a wave? ...

Yes we talk about the " existence" of these waves and or particles. OK. but what does exist?  

Example :

Gamma rays, light and radio waves are all electro-magnetic radiation and each is a form of energy  OK?  

Energy propagated as particles.

Energy in the quanta (photons)  gamma rays, visible light etc. is conveniently described by the simple equation :

 Q = h . nu  ;  where h is plancks constant and nu is the wavelength OK?

And the number of these " particles "  per second (energy per unit time) represents the power being transferred.

Energy Propagated as Electric and Magnetic waves.

Now as this energy may be described as an electro-magnetic wave,  by suitable manipulation of the 4 equations we find

that the propagation of this same energy is described by the Poynting vector:

Q = E X H  where E and H are the electric and magnetic field ans X is the vector cross product.

The question that arises is Where does the photon come into existence? 

Let me describe. Take a dipole antenna operating at 10MHz (wave length 10m).

When this antenna is excited we find that the radiation takes place at a distance of many wavelengths from the antenna.

it is only in the far field that one the  equation Q = E X H apply.

Question at what distance from the antenna does Q = h.nu apply ? Or where does the photon come into existence?  

Jeremy.

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At the end of the day, are we really OK/happy/content with reducing what really is to an equation/set of numbers?

Good point.

I think we have to see the numbers and equations as merely describing the behaviour of something..... whatever that something may be.

Rather like an oil painting being a representation of a scene rather than a complete description of it.

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