Chances are you’re reading this from quarantine, and if you’re anything like me, you’re probably looking to break out of the monotony. I see the same things day in and day out. I noticed all the strange scuffs on my walls for the first time today and just realized that all the outlets in my apartment are upside-down. You might have noticed new details in forgotten corners of your quarantine bunker too. Maybe you discovered a new squeaky drawer or a bird living in the tree outside your window. I’m here to tell you monotony can be exciting if you know where to look.
I also began to notice the exact times of day when the morning sunlight makes the best rainbows through my East facing windows. Sunlight is made up of all the colors of the rainbow mixed together, which is called “white light”. My window bends the sunlight and separates all its components. Red light particles bend less than yellow which bends less than green and so on with blue and violet. The window splits up the spectrum of sunlight into a full rainbow. This happens because light is a particle, and each single particle travels a single path through my window.
But is it really a particle?
My friend Jack Lubin, a fellow graduate student at the UCI Physics Department, is also quarantining and shared a photo with me the other day. It’s a picture of sunlight passing through a crack in a door jamb:
Since light is a particle, you might think that each ray of sunlight follows a single path through the gap, creating a clear and crisp shadow on the opposite wall, but that isn’t what the pattern on the wall looks like. So then where is the door’s shadow? As light passes through the doorway, all of the beams of light must pass through the small gap in the door. Then they spread out over multiple paths before they hit the wall. This is how a pinhole camera works. In a pinhole camera, light passes through a small gap, and then gets projected onto a light-sensitive material in the back of the camera which records the image. In fact, the stripe pattern you see is actually an image of the blinds in Jack’s bedroom window.
This process is called diffraction, and it is a characteristic of waves. While single particles can only travel along a particular path, waves can spread out in multiple directions and take multiple paths. The key is that the gap needs to be small. You would see a shadow of the door if the gap were big, but since the gap is small, you get this diffraction effect instead. When light, or any wave, squeezes through a small gap, it spreads out. Water waves do the same thing.
It turns out that light can act as both a particle and a wave. This is called particle/wave duality, and it is a crucial part of quantum mechanics, which describes the properties of light and other particles in the universe. This happens because in quantum mechanics, there are some very strange rules. The most important rule is that no matter how hard we try, we can never know exactly where something is and its momentum at the same time. Particles have well-defined positions and waves have a well defined momentum. But we can never know both position and momentum for particles and waves perfectly. We can only know position and momentum within a margin of uncertainty. In the case of light, if we try to constrain its momentum, it will act more like a wave and if we try to constrain its location it will act more like a particle. This effect is much more pronounced in small things, where the margin of uncertainty is large compared to the objects themselves. Other particles like electrons, protons, and neutrons behave somewhat similarly too. In fact, I am a collection of electrons, protons, and neutrons, and all combined I am also a particle and a wave. I’m only ever in one place at a time and only follow a single path wherever I go, so it seems I act like a particle. I also have my own wavelength (approximately 8×10^-36 meters when I’m walking, which is about 6 septillion times smaller than the size of a hydrogen atom). That doesn’t mean I can diffract through a doorway. Unlike light, my wavelength is incredibly small and the doorway is way too large, so the margin of uncertainty is far too small.
This might all sound crazy, but we are all going a least a little crazy in quarantine. So if you need something interesting to think about, I urge you to look around your house because you might find something even crazier than you are.
Post by Alex Broughton, graduate student working on his PhD blending High-Energy Particle Physics and Astrophysics at UCI.