I think that pop culture has led a lot of us to believe that quantum physics is the stuff of geniuses: how could I, a normal person with an average IQ, begin to comprehend it? Isn’t that the “big ball of wibbley-wobbley-timey-wimey stuff” that the Tenth Doctor notoriously described in that episode (my favourite episode, BTW) of Doctor Who? To be honest, I don’t think quantum physics has had a very fair chance. Of course, my hat goes off to those who can study it in depth — it is complicated and difficult, and you have to be very dedicated in order to devote your career to research in this field. That being said, I haven’t been able to find very many sources that can really break down the concepts into language that anyone can understand, even if the reader doesn’t have an advanced grasp on scientific or mathematic principles. I don’t think the basics should be out of reach to all but an elite group of ~*Master Scientists*~. So, I’m going to try to break down some of the foundational concepts over the course of a few posts, and hopefully I can help introduce a few people to a topic that they wouldn’t otherwise be exposed to. Today, I’m going to start with a particularly puzzling concept called wave-particle duality.
I’d like to make a little disclaimer here: I’m a scientist, but I’m not an expert in quantum physics by any means. So, if you are a quantum physicist and you notice any mistakes that I make in this series, feel free to comment and let me know. We’re all learning together! So, with that, let’s start with some definitions:
Wave: how energy moves around. Light travels to us in light waves, and sound travels to us in sound waves. Waves can be mechanical, meaning that they have to move through actual matter, or electromagnetic, which can travel through a vacuum. Waves are defined based on a wave equation, and will propagate back and forth in either a longitudinal manner (like sound waves) or a transverse manner (think of your classic sine wave). Waves have lots of cool properties like refraction, interference, and polarization.
Particle: a small unit of matter. Particles can be defined by their volume and mass and can be identified under a microscope or with the human eye, in the case of some macroparticles.
Basically, particles exist at a specific point in the universe, while waves propagate and can’t be specifically pinned down. Except, when scientists took a closer look at both of them, they discovered that these definitions aren’t always necessarily true. That’s lesson number one in physics: everything that you think you know is a huge lie, and universal truths become more and more blurry as you look closer and closer.
Thomas Young was a British scientist who first performed what is now known as the ‘double-slit’ experiment in the early 1800s. He studied pretty much everything a person can study, but is best known for his studies on the nature of light. Basically, he blasted a stream of light at a plate with a single slit, hypothesizing that if light were made of particles, as Newton had previously suggested, then the only light that would make it through the plate would be that which made it through the slit, in the size and shape of the slit. However, the light appeared in a different pattern known as a diffraction pattern, which occurred due to the light waves interfering with each other. Where two light waves hit, they superposed and formed a new wave where the amplitude was equal to the sum of the original two waves’ amplitudes. He repeated the experiment with two slits in the plate, showing a much more obvious diffraction pattern. The conclusion was that this pattern couldn’t have happened if light were made of particles, since particles don’t display interference and diffraction. Therefore, light had to be a wave.
Meanwhile, there was another problem that was baffling scientists: black-body radiation. In short, a black-body is something that absorbs all wavelengths of electromagnetic radiation that it comes into contact with. The concept of a black-body is theoretical, but the closest example that we have in nature is a black hole. When there is no heat energy exchange between the black-body and its surrounding environment, it is in a state called thermal equilibrium, and it begins to emit radiation. So, black-body radiation is just electromagnetic radiation that is coming from a black-body. This is actually really cool when it comes to black holes; if you’re interested, check out Steven Hawking’s research on Hawking radiation!
A scientist named Max Planck was commissioned to figure out what was happening with the black-body radiation and to come up with an equation to describe it (there is a mathematical equation to describe all laws of physics, even if we haven’t quite figured out the equation yet). He eventually came up with the equation, but there was one big problem. The equation couldn’t work unless the energy from the black-body was only being emitted in specific amounts at a time. Basically, the equation relied on the assumption that the energy wasn’t a constant stream of energy, but it was emitted in discrete amounts called quanta. It was impossible to emit energy in an amount smaller than this quantum, and anything higher than one quantum had to be a multiple of that quantum. Mathematically, this assumption is described as E=nhv, which is now known as Planck’s postulate and is one of the fundamental assumptions in quantum physics.
At the same time, Albert Einstein didn’t quite buy the theory that light was simply a wave. To expand on Planck’s work and understand light quanta better, Einstein took on the project of understanding something called the photoelectric effect. The photoelectric effect describes how electrons are emitted from materials when hit with light. If light were emitted in a constant wave, then the energy of the electrons should be proportional to the light intensity. However, experimentally, this just wasn’t happening. The electrons were still being emitted even when the light beam was quite low intensity. To account for this, Einstein tried using the same assumption that Planck had used (E=hv). This works because light is a form of electromagnetic radiation. Using this assumption, he found that the electrons were being emitted based not on the intensity of the light, but on the frequency of the quanta, which were named ‘photons’. Basically, light was a wave, but the energy wasn’t evenly spaced out. Rather, the energy existed in dense yet massless clusters that had momentum. The light was showing properties of both waves and particles.
To summarize what we have so far: Thomas Young showed that light is a wave. Planck and Einstein showed that the energy in a light wave exists in particles. Thus, light has been established as showing properties of both waves and particles.
This led Louis de Broglie, another Nobel Prize winner, to his big hypothesis. He thought that, if light existed as both a wave and a particle, then maybe everything could exist as both a wave and a particle. In essence, he suggested that all matter exhibits ‘wave-particle duality’. He came up with an equation to find the wavelength of a particle, where λ=h/p. This seems like it would be difficult to prove. I mean, when I look at my hand, it certainly seems like it shouldn’t have a wavelength, or else I would have noticed it by now. But, quantum physics describes the physics of things so small that you can’t notice them — that’s the whole point. Actually, some scientists discovered that you can, in fact, see the wave property of matter through a technique called electron diffraction! A scientist named George Thomson, as well as a pair of scientists named Lester Germer and Clinton Davisson, shot electron beams at certain materials and noticed that the electrons were diffracting in a pattern not dissimilar to the light diffraction observed by Thomas Young. So, it was determined that matter does display wave-particle duality at the smallest scale. My hand itself is definitely a discrete object, but the electrons inside it are particles that have wavelengths.
So, there you have it. Through a series of experiments and proposed equations, scientists came to the conclusion that matter exhibits wave-particle duality, meaning that it behaves as both a wave and as a particle. With this understanding, the first research on quantum mechanics has begun, and the stage is set for an entirely new group of physical laws to be defined. Pretty cool, huh?