Wave/Particle duality
This post was motivated by an article I just read, and which I’ll comment on at the end, but was also a good opportunity to talk about one of the Big Ideas in quantum physics - something called “wave particle duality”. Normally we think of waves and particles as being very different things. A particle is something like a billiard ball, which has a position, a speed, a mass, etc. It bounces of walls in a very predictable way, bounces off other balls, and is always there. A wave on the other hand is something like sound or a water wave. It’s spread out - it can bend around corners or objects, overlap with itself and even cancel itself out. That last bit is called “destructive interference” and you can sometimes see it (hear it, actually) when listening to music from stereo speakers - as you walk around the room, you might find that at some points the sound almost disappears, while in other places the sound is quite loud. That’s because the sound waves from each speaker take slightly different times to reach you, and sometimes can be “out of phase”, cancelling each other out. (It only works well with good speakers, though, because otherwise the sound waves from each speaker are different enough that they don’t cancel out.) A very cool application of this is noise-cancelling headphones, where a circuit creates a new sound wave to cancel out background noise.
Wave/particle duality, however, is a part of quantum mechanics which says that all of matter - light, atoms, you, me - is both a wave and a particle. The most famous example of this is the “double slit experiment”, where a beam of light is shone at two narrow slits in a screen and projected onto the screen. These slits work just like the speakers I mentioned above - they’re putting out two copies of the same light. If light was a particle, the two beams of light would just go on their merry way and we’d see two bright patches on the screen. But because light is a wave, we instead see this strange banded pattern where sometimes the light waves have destructively interfered with each other. A picture (courtesy of Wikimedia) demonstrates this best:
What’s odd, though, is that I can repeat this experiment for electrons - firing single electrons, one at a time, towards a pair of slits, and looking where they hit a photographic plate. Since my gun isn’t perfectly accurate, the electron might go through the left slit, or it might go through the right slit. If electrons were particles, as we expect, then we would gradually see bright dots building up in two patches behind each slit. But, amazingly, we see the same pattern:
Check out this awesome movie of the pattern building up. It’s from an experiment by Hitachi, and that site’s a good read. There really are individual electrons hitting the screen, one at a time, in specific spots - just like particles. But the pattern says that somehow, that single electron can still interfere with itself and cancel itself out - just like if it was a wave. Whoa! And it also works with heavier particles, atoms, molecules - and even porphyrins, which are biological molecules. This really is a fundamental property of matter. Other experiments, such as the photoelectric effect explained by Einstein, also show that light sometimes acts like a particle.
Therefore, we conclude that all of matter acts as both a wave and a particle. The resolution is something called the “wave function”, which is a like a wave of probability that describes the position of a particle. It tells us the probability of finding the particle at any given position in space. There’s some probability of the electron going through each slit, and so a wave of probability flows through both, interferes with itself, and eventually reaches the screen. When it comes time to hit the screen, the electron must “choose” where it will strike according to those probabilities - the bright spots are high probability, the dark spots are low probability. You can apply exactly the same interpretation to the experiment with light - and we have a unified explanation for the double slit experiment.
Of course, I’ve glossed over heaps of details here, both the mundane and the exciting (although both are probably relative terms!). I didn’t discuss the mathematics of constructive and destructive interference, exactly how the wave function is tied into probability, or what the wave function looks like, which are all well understood topics. But I also didn’t touch on why or how the electron chooses a given position, which is still the subject of some controversy, or whether these double slit experiments will continue to work for bigger and bigger molecules or even objects like ping-pong balls, a subject of even more “discussion”. Practically, the answer is no (the experiments would be impossible to perform) but would there be intereference to see if we could? There’s the question of why we don’t usually see this wave particle duality, and other quantum effects, in our day to day life. These are really fundamental questions, and while quantum mechanics as we currently understand it works brilliantly, it’s still not complete. These are important, fundamental questions and I think it’s exciting to think that there’s so much new physics waiting to be discovered!
I think I’ll leave it here, for now, and discuss my actual motivation for this post tomorrow!