Above: Thomas Young's sketch of two-slit diffraction of light. Narrow slits at A and B act as sources, and waves interfering in various phases are shown at C, D, E, and F. Young presented the results of this experiment to the Royal Society in 1803 (Wikimedia Commons)
It’s 1803 and you’re an English scientist. Your name is Thomas Young. You’ve just slumped down into your kitchen chair, thoughtfully gazing at the apparatus you built earlier in the day out of a very thin piece of cardboard, a mirror, and a window shutter. You may not know it yet, but your experiment will change the future of science. Along with some other pioneering researchers, you will give birth to a new branch of physics called quantum mechanics.
But for now, you just sit and replay the experiment in your mind.
Fast fact: The word “physics” comes from an ancient Greek word meaning “nature”.You were trying to determine whether light moves as if it were made up of tiny chunks of matter (distinct “particles”) or whether it moves like a wave. Until now, the European physics community has been split down the middle: some believe that light travels only as a wave (as Christiaan Huygens proposed in the 1690s), while others believe that it travels only as a particle (as Isaac Newton proposed in the 1670s).
Your “double slit” experiment involved setting up a mirror outside your kitchen window so that a sunbeam was directed horizontally through a tiny gap in the window shutter. Inside the house, you placed a thin piece of cardboard edgewise into the beam. The card split the beam into two, with each part hitting an observation screen (Figure 1).
Figure 1. Click image to enlarge (Jenny Kliever)
If light were made up of millions of tiny particles, you would have observed some particles hitting the observation screen on one side of the cardboard, with the rest hitting on the other side. Only two lines of particles would have been visible on the screen (Figure 2).
Figure 2. Click image to enlarge (Jenny Kliever)
However, what you actually observed was very different (Figure 2). The pattern on the screen was what you would expect from a wave. Because waves interfere with one another as they move, they sometimes cancel each other out and sometimes amplify each other (add together). The result is a variety of lines, some indicating high intensity and others indicating low intensity (Figure 3)
Figure 3. Click image to enlarge (Jenny Kliever)
So you’ve proven that light moves as a wave, not a particle. Take a moment to let that sink in… Not only have you proven Isaac Newton wrong, but you’ve settled a centuries-old debate among physicists. Amazing!
Fast forward to present day. Although Young’s results have been verified by other physicists, it has also been shown by Albert Einstein that light is made up of tiny particles called photons.
Wait, what? Particles? How can particles move as a wave?
This is where things get weird. After Young published his findings, physicists all over the world started playing around with his ideas. To see whether shooting one particle at a time would still result in a wave pattern, they shot single photons through an apparatus that was similar to Young’s but involved splitting the path of the photons using a barrier with two holes in it rather than a piece of cardboard. They found that the pattern on the observation screen was still that of a wave, not a particle! And through more experimentation, they found that this result holds true for all matter, not just light.
But shouldn’t a particle only be able to hit the screen in one spot or the other spot? How come they hit the screen in so many different places?
To figure out how, physicists placed a camera near the holes and watched as one particle was shot at a time. They saw that each particle was going through one hole or the other hole and the interference pattern on the observation screen was that of a particle (with two bands), not a wave (with many bands). Wait, what? Observing the electrons changed their behaviour? Now things are getting really weird.
These results have led to a theory called wave-particle duality. According to this theory, light and other substances behave both as waves and as particles. Einstein described it like this: "It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they do."
Fast fact: Theoretical physicists have been trying to find a “grand unified theory” or “theory of everything”: a set of equations that explains everything in our universe, both past and future.This concept causes all kinds of trouble in physics! In fact, a whole new branch of physics was born to explore the trouble: quantum mechanics.
Physics has come a long way since the 1800s. Thomas Young’s double slit experiment demonstrated that light can travel as a wave, and other physicists have proven that it can also travel as a particle. And if that isn’t weird enough, simply watching single particles move through two holes can change the way they behave! But this is only the beginning. There are many other mysteries of quantum mechanics for you to explore!
Light (Glenn Stark, Encyclopaedia Britannica) Discovery of Photon (History of Science) Dr. Quantum Double Slit Experiment (YouTube/Angel Art)
Young T. 1802. The Bakerian Lecture: On the Theory of Light and Colours. Philosophical Transactions of the Royal Society of London. 92:12-48.