Wave/Particle Duality

[chris_uk_83]

Further to the suggestion in the "I wish people discussed more sciency things" thread, I'm going to write one or two pieces about bits of physics that I find fascinating, hopefully so people will ask questions or mention other things that interest them and we can get a nice bit of QA going, maybe even some scientific debate (which is actually possible).

I'll start with Wave / Particle Duality.

We all know that light is a wave right? We’ve all done the experiments in high school physics that show that a beam of light will spread out when you pass it through a narrow slit. This effect is known as diffraction and is a property that only a wave can exhibit.

Another way we can show that light is a wave is through interference. This is where we point two coherent beams of light at the same target and we see a series of light and dark fringes. Coherent means that the wavelength of the two beams is exactly the same (you can only do this using lasers, since light bulbs give out a large range of different wavelengths). The following diagrams show examples of constructive and destructive interference


Constructive interference occurs because peaks in one wave hit the target at the same time as peaks in the other wave.
Destructive interference occurs because peaks in one wave hit the target at the same time as troughs in the other wave.

By thinking about this for a while, you can see that at different distances from the source (or different locations on a target screen), the amount and type of interference will change. This means you get what is known as an interference pattern on the screen. It looks a bit like this:



The only way this pattern can occur is if light is a wave. If you’re still not convinced have a look at this lovely Wikipedia article until you are.

However, light can also be thought of as a particle because of something called the Photoelectric effect. In the photoelectric effect, light is shone onto a metal surface causing electrons to jump off the surface. This is because light gives energy to the electrons in the atoms that make up the metal, allowing them to leave the captivity of their host atom. If light were a wave, then increasing the intensity (brightness) of a beam of light would increase the energy given to each electron, and would cause more tightly bound electrons to jump off the metal too. As it turns out, experimentally this doesn’t happen. The only way to increase the energy of the light beam is to increase the energy of the individual photons that make it up. A photon is effectively a particle of light. You increase the energy of a photon by decreasing the wavelength (or increasing the frequency) of the light beam (i.e. going from visible light to ultraviolet).

The only way the photoelectric effect can happen (and it is proven experimentally) is if light is a particle. Still not convinced? Look at this article to convince yourself.

So wave is both a wave and a particle depending on the particular circumstances of the experiment your carrying out. But what if we force it to behave like both at the same time? Let’s see.
Enter Young’s Double Slit experiment. This is similar to the way you show interference that we discussed above. Send a beam of coherent photons (a laser beam) through two closely spaced slits to produce two coherent beams of photons, then observe the interference pattern created on the screen. What happens though if you send the photons through one at a time (and this can actually be done experimentally)? Well, each photon has an equal chance of going through either slit, but common sense dictates that it must choose a slit i.e. it can’t go through both. This should stop any interference, because how can a single photon interfere with itself? Therefore no interference pattern should build up on the screen.

The funny thing is that when you do this, an interference pattern still builds up on the screen. The only way this can happen is if the single particle goes through both slits at the same time! What!? ‘Right’, you think ‘I’ll find out which slit it’s going through’, so you attach a detector to the slits. Now you can measure which slit the photon goes through, to see if it really is going through both.

What happens when you do this is that you find that each photon goes through only one slit, and roughly half of the photons go through each slit. What you also find is that the interference pattern on your screen does not happen any more; the photon is now behaving exclusively like a particle and all you did was look at it!

This shows that a photon can be a particle or a wave, and indeed will be both if you don’t pay too much attention to it. As soon as you start trying to look too closely though it takes on one behaviour over the other. Confused yet? You should be!

What gets even weirder is when you start getting particles like electrons and protons to behave like waves! Which they do! You can create an interference pattern on a screen by firing a beam of electrons at the double slit described above! :hmmm:

[Lunar4]

Oh yes, I have read about this in a book, called "The Elegant Universe" by Brian Greene. A good read, I recomend it if you have an interest in Physics

[Drexxus Maximus]

Theres a great documentary on this very subject called "What the bleep! Down the Rabbit Hole." Blockbuster has it in my town so I imagine most others do as well.

[chris_uk_83]

Another good thing to look up is the Schrodinger's Cat experiment, which demonstrates that a cat can be both dead and alive at the same time. Slightly mind boggling stuff really.

[allthesedamnnamesaretaken]

Michael Jackson can also be both male and female, black and white, human and alien. Triduality! They should really do more sciency experiments on him/her/it

[MaximiIian]

I have a question about wave/particle duality, but it's probably a stupid one:
Since all things move in a wave and particle form, and since sound moves in a wave form, then what is the particle transmitting sound?

[chris_uk_83]

I have a question about wave/particle duality, but it's probably a stupid one:
Since all things move in a wave and particle form, and since sound moves in a wave form, then what is the particle transmitting sound?

That's a good question and not at all stupid.

There are two parts to this answer so bear with me. Sound travels in a different way to light and other bodies (such as electrons): sound moves through vibrations in the material in which is is travelling. This is difficult to explain without the air of a diagram, but it's similar (NOT the same though, just similar) to water waves: they need water to travel through (obviously), and they travel by moving the water around; sound needs a medium, either a gas or a solid body to travel through and sound waves travel by moving the particles within that medium. Look at this page for an animation of how sound travels, using the immortal slinky spring! Go here for an overview of how sound travels too.

So your answer to part one is that sound travels by manipulating the medium around it and there is no one 'sound particle'. This is the classical answer. However, there is another, slightly better, answer; the quantum answer.

We've heard about a photon being a 'particle' of light. Well it's better to think of a photon as a 'packet of energy'. It's kind of a small portion of a light wave with a fixed energy. We call it a particle because it exhibits particle-like behaviour. (Giving a name to something because it behaves like something else is quite commonplace in physics and is generally quite useful). Now in the quantum picture (physics on a really small scale) sound waves travel in these finite 'energy packets' too, in exactly the same way as a photon (though they still need a medium to travel in as they are sound after all). These 'sound photons' are called 'phonons' and are considerably less well known than their electromagnetic counterparts.

So there are two answers to your question:
1) it doesn't make sense to talk about 'sound particles' because sound travels by moving particles all the time anyway.

2) It's called a phonon.

Both answers are correct, it all depends on what scale you're talking about. Incidentally, a lot of physics depends on what scale you're talking about and I'd launch into a large discussion of that now but I think this post is long enough!

[Darkragnar]

Q.What is an Particle and why does it "Die" when you look at it and yet if you dont why are there 2 of it, what changes when you observe a particle.

[chris_uk_83]

Q.What is an Particle and why does it "Die" when you look at it and yet if you dont why are there 2 of it, what changes when you observe a particle.

I'm glad I've managed to make people curious about this. There is a lot of speculation and complicated maths surrounding this question, but the truth is that the quantum world (for simplicity purposes 'quantum' just means really really small) behaves in a way that is incomprehensible to our understanding. The whole idea is based on the probability of something happening and not on the fact that it will or won't happen. The Heisenberg uncertainty principle tells us that you can know either exactly where something is, or you can know its momentum, but you can't know both becasue the act of observing one changes the system.

It also seems to be true that, until you actually look, the electrons orbiting an atom exist in a superposition of every possible location they can be in (this is actually known as a wavefunction but I'll spare you the maths!).

'Ah', you say 'That just means that you don't know exactly where something is until you check it's there. That makes sense.' Well not quite, because you can show, through indirect effects, that the orbital electrons are actually in a combination of all places at the same time!.

This boggles your mind for at least the first two weeks of a quantum mechanics course. Then the horrendous maths just drags you down and you stop caring about what it is you're working out in the first place!

[Darkragnar]

^aye but what changes during an act of observation, i mean its not like your sending some kind of undiscovered waves or anything towards your particle, so does observing actually change the probability of that particle on a quantum level.

1 more question that i always had about quantum mechanics , when every particle in the quantum level is phasing in and out of existence then why don't bigger things follow the same quantum laws that work so well on the small scale ?

btw Which college are you studying from ?

[chris_uk_83]

I'll start with the easy to answer question: I was at Lancaster University (UK) for 4 years taking a Masters degree in physics, but I've finished now and this is the only way I get to keep my physics knowledge current in my head

what changes during an act of observation?

That's a difficult one. The cop out answer is that you can only observe the particle by hitting it with another particle, be that a photon, and electron or something else. After all, we only see things because photons bounce off them and enter our eyes. Bouncing a particle off something will clearly change its momentum and therefore alter its state. This is the main argument for the Heisenberg Uncertainty Principle.

I don't buy this explanation for fixing the wavefunction of an orbital electron though. It seems that maths makes things happen. What I mean by this is that before you look, there is a finite probability that the particle can be in a number of different states; afterwards the probability that it is now definitely in one particular state is 1. All the weird effects come about from this manipulation of probabilities and I have absolutely no idea what physically happens! I don't think there are many people in the world who do know what physically happens!

Your next question is slightly simpler to answer, though the implications are just as difficult to believe.

1 more question that i always had about quantum mechanics , when every particle in the quantum level is phasing in and out of existence then why don't bigger things follow the same quantum laws that work so well on the small scale ?

Again we look to the Uncertainty Principle. This time though, we use a different incarnation of it that says the uncertainty in energy multiplied by the uncertainty in time is less than or equal to Plancks constant divided by 2 pi (also known as h bar, written as h with a bar through the top of it). The implication of this is that you can 'borrow' energy from the universe as long as it's a small enough amount and for a short enough time so that the product of the two does not exceed 'h bar' (1.055 x 10^-34 Js). It's like the universe checks its books every so often and as long as you can sneak the energy out and back again before it notices, then you get away with it.

This will only worok for small particles becasue they require relatively small amounts of energy to create (see E = mc^2 for how mass relates to energy). Now it's theoretically possible that something big could be created spontaneously, but it could only exist for a really really really really small amount of time before vanishing again so we never see it happen. Just think: the bigger the object, the smaller the time it can exist.

So in answer to your question, we do see quantum mechanics at work in the macroscopic (big) world but their effects are so comparitavely small that they are never ever noticed.

[pseudocaesar]

i actually just did wave/particle duality in my first semester of year 1 Chem. Interesting stuff!

[MaximiIian]

Another question related to the sound-wave thing:
so, provided that you have medium through which the phonons can vibrate and travel, such as air or water, is it possible, once you have harnessed and collected pure phonons, to accelerate them out of a weapon barrel in pulsed stream?
I ask this because I'm trying to think of a way that a medium-ranged sonic-based small-arm would work, as part of my sci-fi story.

[chris_uk_83]

It doesn't make sense to talk about sound in terms of "pure phonons". A phonon is more a measurement of sound energy than a physical particle (in the same way that a photon is more a measurement of electromagnetic energy than a pure particle). The reason they exist at all is because there is a minimum possible energy that a vibration (sound wave) can have and this value is not infinitessimally small. So you can't 'collect phonons' as they always travel at the same speed - the speed of sound in the particular medium.

Your directional sound gun however, is quite possible and actually exists. Check out this Wikipedia article and follow its links for more information on how they work. I'm not an expert on fluid mechanics though, which is what you need to know about to create sonic weaponry. I'll do my best if you come up with any more questions though!

[Diogenes_of_Sinope]

"...you think ‘I’ll find out which slit it’s going through’, so you attach a detector to the slits..."
Hi, what kind of detector? (it seems that detection/observation is what screws things up :hmmm: )

[Simetrical]

Coherent means that the wavelength of the two beams is exactly the same (you can only do this using lasers, since light bulbs give out a large range of different wavelengths).

You can still use a light bulb, you just have to take a very narrow sliver of its light and then split it in two. The split beams won't be monochromatic, but they will be reasonably coherent. This is how Young's experiment was initially performed in the beginning of the 19th century, over a century before lasers were invented.

[chris_uk_83]

You can still use a light bulb, you just have to take a very narrow sliver of its light and then split it in two. The split beams won't be monochromatic, but they will be reasonably coherent. This is how Young's experiment was initially performed in the beginning of the 19th century, over a century before lasers were invented.

Quite true, but lasers work better

Hi, what kind of detector? (it seems that detection/observation is what screws things up

Quite right, this is one proposed explanation for what happens and it does applpy if you're using a beam of light that is going through two slits; the seperate photons go through physical detection processes (interacting with some particles, which causes a current to be generated, which is then detected, I imagine it's scintillation but I'm not 100%). But if you send one photon at a time, classical physics says there is no way one photon can interfere with itself and yet it does, by going through both slits simultaneously.

The "Quantum Version of the Experiment" section of This Article, does a slightly better job of explaining things. Also see the Copenhagen Interpretation

So yes, physical interference by detection can be blamed, but there are other experiments that can be done to show that this isn't always the case.