Wave Particle Duality

A particle or a wave?

Wave particle duality is one of those things that I can't help thinking we haven't got quite right. I think that this may also be the reason why it confuses so many people. It is a subject that has no place in every day experience, there is nothing in every day life that can give you the idea that a particle could actually behave like a wave.

In fact there is not much in life to give you any real experience of particles. It actually took until 1905 for Einstein to provide a method to show that atoms actually existed and as it turned out were very very small indeed.

During most of the 17th, 18th and 19th centuries arguments about the nature of light went back and forth, was it particle like? was it wave like? It seemed to be both yet people tended to side with one idea or the other. Newton was firmly in the camp of the particle.

Eventually a guy called Taylor decided to try and settle it using a variation of Young wave experiment. In 1909 he placed two very small holes right next to each other and shone light through them. They created a wave interference pattern, just like the type of thing you get in water when two waves overlap. So, light is a wave, job done. Not so fast, that was only the first part of the experiment.

Next he got some exceptionally sensitive film and repeated the experiment. For bright light, interference patten as expected. But for really dim light the photos showed tiny pinpoints of light scattering out of the holes, making it seem like light was in fact particles. It gets stranger, when he exposed the film to the dim light for a long time he ended up with an interference pattern again! In other words light was behaving like particles and waves. So was everyone right?!

It gets even odder, Louis de Broglie then said well if light can behave like particles and waves, maybe particles such as electrons can behave like waves. Experiments were performed and lo and behold if Louis isn't right. Electrons can behave just as if they are waves.

So it seems, from experiment that light and particles can be both waves and particles.

What is the physical interpretation of this? Is de Broglie correct? Certainly seems that way.

Has anything changed to alter our opinions? We've had about 80 years or so to ponder this one, so is wave-particle duality still consider the correct interpretation of the observations?

In quantum mechanics we have the good old Schrödinger equation (covered in more detail in another post) which is based on the idea of something called a "wave function". As the name suggests this is based on the idea of waves and can explain things like interference and diffraction pattens. What is cool though is that it can also explain the particle behaviour of particles. So this definitely adds weight to the idea.

That said ... we still don't really know for sure.

It is true that Quantum mechanics has done fantastically well since its discovery and because of this most physicists accept the wave particle duality argument but there are some who are not convinced.

Louis De Broglie extended his original idea to show that there is no duality. Light is not a wave or a particle, but both simultaneously, similarly an electron is both a particle and a wave at the same time, not one or the other.

Some believe that duality can be replaced by wave only and that we are just misinterpreting particle behaviour. Take a look at superconductivity, this seems to be governed by a quantum mechanical wave, same goes for lasers, yet these are what we call macroscopic, in other words they are operating in dimensions far larger than those we normally associate with quantum mechanics.

There is no doubt from experiment that light can behave like a particle or a wave, same goes for an electron. What is in doubt is the physical interpretation of these observations. As yet no one has been able to come up with an experiment, or been able to interpret the results from existing experiments, to conclusively say which argument is correct.

May be the answer is that it is not possible to give an definitive interpretation, I hope not, I'd feel as though the universe had cheated! May be we are still missing some clue that will give us a new insight or explanation of the results. May be, may be, may be.... don't you just love physics.