Wave-particle duality question

[badgerchap]

Sorry, this one is physics rather than astronomy, but it's been bugging me all night so I have to get it out.

Everyone seems to think it amazing that light exhibits properties of both waves and particles, and in fact there were, historically, huge controversies regarding this apparent dual personality of light. My question is:

Isn't this duality an obvious product of something that is solely a particle?

To my mind, any object that vibrates, as all matter does, will exhibit a wavelike property while travelling at speed. Any object that moves from side to side or in a circular motion will describe a wave when propelled.

Am I being dense here, or have I missed something?

[George Jones]

any object that vibrates, as all matter does

I think I know what you have in mind, but I am not completely certain that I do. Could you expand on this a bit?

[badgerchap]

As far as I understand it a photon is a quantum - a discreet packet of energy. To my knowledge matter cannot remain perfectly still, unless it it at absolute zero, which it cannot be whilst it contains energy. I may be wrong here, but I'm pretty sure a photon, which is energetic, should vibrate....?

[Wurzil]

i have this book and find it explains all quantum stuff quite well. i like it.

Quantum: A Guide for the Perplexed: Amazon.co.uk: Jim al-Khalili: Books

w

[George Jones]

An individual photon or electron or atom or molecule doesn't have a temperature, as temperature is a bulk property.

Suppose we have a source of single-colour photons (a "torch") that is so dim that it sends out one photon every several minutes. Shine the torch on wall that has two, thin parallel slits. Some of the photons are blocked by the wall, but some of the photons make it through the slits, and each slit acts as a source of individual photons. Some distance beyond the wall with the slits, there is a second wall on which there is a sensitive photographic film. Since the torch only sends out a photon every few minutes, and the first wall the blocks some of the photons, photons marks accumulate on the film very slowly and individually. You watch the photon marks accumulate individually over many hours.

After many hours, you notice that the indvidual photon marks have arranged themeselves in a pattern characteristic of a wave!

[blinky]

I remember seeing this on the BBC Open University and found it a fascinating experiment. I really gets the idea of wave/particle duality over to you.

[Wurzil]

.........unless you put a detector on each slit to see which one its going through - in which case the atoms just pile up like particles. atoms are sneaky and dont like being watched!!

[badgerchap]

I'm familiar with the meaning of the wave particle duality, very much so, but I don't seem to be able to find a reason for it. I've gone pretty deep into quantum entanglement, probability, energy states etc, but nowhere can I find an explanation of why the duality exists!

[badgerchap]

.........unless you put a detector on each slit to see which one its going through - in which case the atoms just pile up like particles. atoms are sneaky and dont like being watched!!

Lol lucky we're dealing with photons then not atoms (tongue in cheek )

[Paxo]

The wave function represents the probability of finding a given particle at a given point. Some interesting commentary on it here:

The Dual Nature of Light as Reflected in the Nobel Archives

Steve..

[Wurzil]

Lol lucky we're dealing with photons then not atoms (tongue in cheek )

works the same with atoms or protons or photons i think.

w

[badgerchap]

Atoms tend to behave in different ways than photons because they are not elementary particles. I think. I could just be waffling!

[badgerchap]

The wave function represents the probability of finding a given particle at a given point....

Isn't this in reference to the probability wave?

[brianb]

works the same with atoms or protons or photons i think.

Well you can't actually do the experiment with atoms - but you can with electrons, or with atomic nuclei.

Yes it is the same - whatever you're using - if you look for particles, you see particles not waves; if you look for waves, you see waves not particles. It's all linked by Heisenberg's Uncertainty Principle.

[Wurzil]

I'm familiar with the meaning of the wave particle duality, very much so, but I don't seem to be able to find a reason for it. I've gone pretty deep into quantum entanglement, probability, energy states etc, but nowhere can I find an explanation of why the duality exists!

i just had a search on the web, and it appears that the scientists dont know why!! Its all open to interpretation. The most popular is the copenhagen interpretation.

Copenhagen interpretation - Wikipedia, the free encyclopedia

w

[George Jones]

Well you can't actually do the experiment with atoms - but you can with electrons, or with atomic nuclei..

And buckeyballs .

Wave-particle duality of C(60) molecules. [Nature. 1999] - PubMed result

[brianb]

it appears that the scientists dont know why!!

The anthropomorphic principle is sufficient to explain. You can work out the physics of a universe where there is no wave/particle duality i.e. Planck's Constant is zero, and it turns out that particle/particle interactions become totally deterministic (shades of Laplacian dynamics). This makes the physics rather boring, you either get no nucleosynthesis at all (hence no stars, hence no observers since their energy comes indirectly from nucleosynthesis in stars) or very rapid runaway nucleosynthesis (and there is nothing but iron-56, which makes chemistry and therefore life as we know it impossible). We exist, therefore Planck's Constant is non-zero, therefore there is wave/particle duality.

BTW the Copenhagen Interpretation is somewhat hard to swallow (hence Schrodinger's Cat, the "thought experiment" which was designed to show that the Copenhagen Interpretation is ridiculous.

The multiple universe interpretation is much easier to understand. In Schrodinger's Cat experiment, there is a fork, in one universe the radioactive particle decays and the cat dies, in another the particle does not decay and the cat remains alive. The observer "chooses" (unconsciously of course) one of these universes at the instant the box is opened ... or, if you prefer, one observer records "cat dead" and one "cat alive" but we can be associated with the universe containing only one of these observers ... the Copenhagen Interpretation of the cat being simultaneously alive and dead until the box is opened is simply unnecessary.

[badgerchap]

BTW the Copenhagen Interpretation is somewhat hard to swallow (hence Schrodinger's Cat, the "thought experiment" which was designed to show that the Copenhagen Interpretation is ridiculous.

Yeah, that's kind of what I thought, seems o little bit lumpy. If my interpretation were correct, however, it would neatly explain Heisenberg's Uncertainty principle. Like this....(God, why does no-one slap me and tell me to shut up when I start rabbiting like this?).....

If a photon, as a particle, vibrates whilst travelling in a straight line, it will describe a wavelike course throughout space, and will behave as a wave.

There are two ways in which the wave-particle can be observed:

1. The wave-particle is observed at a single moment of time, appearing motionless. Of course at this point, it would appear as a particle.

2. The wave-particle can be observed in motion, in which circumstance it will appear as a wave.

In this way, a photon (and so the rest of matter) needs not be a wave AND a particle, but, depending on the method of observation, will be a wave OR a particle.

Now going outside to bang my head against a wall.

[brianb]

There are two ways in which the wave-particle can be observed:

1. The wave-particle is observed at a single moment of time, appearing motionless. Of course at this point, it would appear as a particle.

2. The wave-particle can be observed in motion, in which circumstance it will appear as a wave.

In this way, a photon (and so the rest of matter) needs not be a wave AND a particle, but, depending on the method of observation, will be a wave OR a particle.

Not quite. When you stop thinking in terms of a wave or a particle, and start thinking in terms of a probability density function, you're starting to get the hang of it. The probability density function has both wave properties (a characteristic wavelength, or energy, or momentum) and a position (taken to be the point at which the function gets its highest value). The position is of course always changing. Shine a photon or particle beam to try to find out where the object is, and the beam will change the position and/or momentum of the object.... and, the more accurately you want to measure its position, the shorter wavelength & so the higher energy of the beam, and the more you will disturb it. For items like billiard balls, the effect is insignificant - you can shine a torch onto a rolling billiard ball without changing its position or speed noticeably - but for objects as small and light as electrons, the reactions are likely to overwhelm the original state.

[badgerchap]

So the wave described by light does not actually exists as a wave per se? Are we saying that the 'wave' is just the probability of the photon being at a certain point, i.e. the photon is most likely to interact with other matter at some point along a wave shaped path? Probability always confused me a bit lol

[George Jones]

In many ways, the questions

"What causes wave-particle duality?"

"What causes apples to fall?"

are similar, and the answers

"Quantum theory causes wave-particle duality."

"Gravity causes apples to fall."

are similar.

"Gravity" is the name we give to a collection of physical effects and quantitative rules (mathematical laws). "Quantum theory" is the name we give to a collection of physical effects and quantitative rules (mathematical laws).

In the case of gravity, however, many of the effects are everyday experiences that we have observed since we were born. Consequently, its is easy to say "Oh, that makes sense." once we see the theory of gravity. In the case of quantum theory, the effects are not everyday experiences, and we have to work hard (often by using subtle and difficult experiments) to observe these effects. Consequently, it is easy to say "Oh, this seems mysterious; why is it true? what is the mechanical model that produces these effects?" when we think about quantum effects.

It seems reasonable that as we move further and further from everyday experience, the physical effects we observer become more and more more counter-intuitive. Also, the theories needed to describe our observations become more mathematical, as they describe a wider range of quantitative phenomena. This can be frustrating (I know this from personal experience!), but can also be, "Wow, isn't it cool that the world is like this!"

Often, "Why?" with respect to a fundamental theory can only be answered by a more fundamental theory. Sometimes, a reinterpretation of a theory can answer "Why?" in a satisfactory way, but this is sometimes only personal satisfaction, as different people might favour different interpretations (as in quantum theory). No interpretation can conflict quantitatively with what is already known. From 1926 on, Schrodinger worked hard to express his waves as vibrations of something "physical", but he was unsuccessful, and probability waves won the day.

The classical/quantum interface is still an issue, but most physicists believe in many worlds (brianb) and decoherence together with brain state entanglement. We only experience the world that is quantum-entangled with our brain. This interpretation has some problems, particularly with respect to cosmology, where quantum fluctuations, magnified by inflation, become superclusters of galaxies.

I don't mean that you should become complacent. Never stop trying to answer questions, through thought, discussion and study, that you ask yourself! I have studied physics seriously and quantitatively for over half of my life, and, for me, the Wow! factor (and associated frustrations) is as strong as ever.

In their day-to-day work, physicists usually take the shut-up-and-calculate approach to quantum theory. When taking breaks form their day-to, their thoughts sometime turn to the meaning of quantum theory. A couple of physicists who haven't stopped trying to understand the meaning of quantum theory are Chris Isham and Roger Penrose.

[badgerchap]

Why, thank you George, that was excellent. I'm still (lovingly) banging my head against a wall though. I think I need to start somewhere near the beginning and take a look at the mathematics of the whole thing. Quantum mechanics has always sat a bit oddly with me because I've never been given the opportunity to look at the numbers. Do you know anywhere online (for god's sake please don't say wikipedia) where I can start to hunt down the core mathematical principles of quantum theory? My maths is OK, so I'm quite prepared to have equations thrown in my face!

Thanks for all your help though! I've read a load of popular science regarding this (e.g. Brian Greene, Timothy Ferris and some others), but the explanations are always just short of being able to clarify things, I suppose because we can't quantify the effects without using maths.

Guy

[George Jones]

I'm still (lovingly) banging my head against a wall though.

I hope to post more, but I thought a quote from the preface of The Structure and Interpretation of Quantum Mechanics by R.I.G. Hughes might amuse you:

Having thus outlined my program and declared my allegiances, I leave the reader to decide whether to proceed further, or to open another beer, or both.

[brianb]

Having thus outlined my program and declared my allegiances, I leave the reader to decide whether to proceed further, or to open another beer, or both.

And/or have an attack of the screaming abjabs.

Nice one.

[badgerchap]

And/or have an attack of the screaming abjabs.

Nice one.

Yup, that sounds about right!

Nice quote though George, lets have more!