A “no math” (but seven-part) guide to modern quantum mechanics

A “no math” (but seven-part) guide to modern quantum mechanics

Enlarge / Quantum mechanics is complex, fold-your-brain stuff. But it can be explained.

Aurich Lawson / Getty Images

Some technical revolutions enter with drama and a bang, others wriggle unnoticed into our everyday experience. And one of the quietest revolutions of our current century has been the entry of quantum mechanics into our everyday technology. It used to be that quantum effects were confined to physics laboratories and delicate experiments. But modern technology increasingly relies on quantum mechanics for its basic operation, and the importance of quantum effects will only grow in the decades to come.

As such, the time has come to explain quantum mechanics—or, at least, its basics.

My goal in this seven(!)-part series is to introduce the strangely beautiful effects of quantum mechanics and explain how they’ve come to influence our everyday world. Each edition will include a guided hike into the quantum mechanical woods where we’ll admire a new—and often surprising—effect. Once back at the visitor’s center, we’ll talk about how that effect is used in technology and where to look for it.

Embarking on a series of quantum mechanics articles can be intimidating. Few things trigger more fear than “a simple introduction to physics.” But to the intrepid and brave, I will make a few promises before we start:

  • No math. While the language of quantum mechanics is written using fairly advanced math, I don’t believe one has to read Japanese before you can appreciate Japanese art. Our journey will focus on the beauty of the quantum world.
  • No philosophy. There has been a fascination with the ‘meaning’ of quantum mechanics, but we’ll leave that discussion for pints down at the pub. Here we will focus on what we see.
  • Everything we encounter will be experimentally verified. While some of the results might be surprising, nothing we encounter will be speculative.

If you choose to follow me through this series of articles, we will see quantum phenomena on galactic scales, watch particles blend and mix, and see how these effects give rise to both our current technology and advances that are on the verge of making it out of the lab.

So put on your mental hiking boots, grab your binoculars, and follow me as we set out to explore the quantum world.

What is quantum mechanics?

My Mom once asked me, “What is quantum mechanics?” This question has had me stumped for a while now. My best answer so far is that quantum mechanics is the study of how small particles move and interact. But that’s an incomplete answer, since quantum effects can be important on galactic scales too. And it is doubly unsatisfactory because many effects like superconductivity are caused by the blending and mixing of multiple particles.

In many ways, the role of quantum mechanics can be understood in analogy with Newtonian gravity and Einstein’s general relativity. Both describe gravity, but general relativity is more correct—it describes how the Universe works in every situation we’ve managed to test. But 99.99 percent of the time, Newtonian gravity and general relativity give the same answer, and Newtonian gravity is much easier to use. So unless we’re near a black hole, or making precision measurements of time with an optical clock, Newtonian gravity is good enough.

Similarly classical mechanics and quantum mechanics both describe motions and interactions. Quantum mechanics is more right, but most of the time classical mechanics is good enough.

What I find fascinating is that “good enough” increasingly isn’t. Much of the technology developed in this century is starting to rely on quantum mechanics—classical mechanics is no longer accurate enough to understand how these inventions work.

So let’s start today’s hike with a deceptively simple question, “How do particles move?”

Kitchen quantum mechanics

Some of the experiments we will see require specialized equipment, but let’s start with an experiment you can do at home. Like a cooking show, I’ll explain how to do it, but you are encouraged to follow along and do the experiment for yourself. (Share your photos in the discussion below. Bonus points for setting the experiment up in your cubicle/place of work/other creative setting.)

To study how particles move, we need a good particle pea shooter to make lots of particles for us to play with. It turns out a laser pointer, in addition to entertaining the cat, is a great source of particles. It makes copious amounts of photons, all moving in nearly the same direction and with nearly the same energy (as indicated by their color).

If we look at the light from a laser pointer, it exits the end of the laser pointer and moves in a straight line until it hits an obstacle and scatters (or hits a mirror and bounces). At this point, it is tempting to guess that we know how particles move: they exit the end of the laser like little ball bearings and move in a straight line until they hit something. But as good observers, let’s make sure.

Let’s challenge the particles with an obstacle course by cutting thin slits in aluminum foil with razor blades. In the aluminum foil I’ve made a couple of different cuts. The first is a single slit, a few millimeters long. For the second I’ve stacked two razor blades together and used them to cut two parallel slits a few tenths of a millimeter apart.

Horizontal slits in aluminum foil made with razor blades. The upper slit is from a single blade, while the lower is from two blades taped together.
Enlarge / Horizontal slits in aluminum foil made with razor blades. The upper slit is from a single blade, while the lower is from two blades taped together.

Miguel Morales

In a darkened room, I setup my laser pointer to shoot across the room and hit a blank wall. As expected I see a spot (provided the cat’s not around). Next, I put the single slit in the aluminum foil in the laser’s path and look at the pattern on the wall. When we send the light through the single slit, we see that the beam dramatically expands in the direction perpendicular to the slit—not along the slit.

Laser light passing through the single horizontal slit is spread vertically
Enlarge / Laser light passing through the single horizontal slit is spread vertically

Miguel Morales

Interesting. But let’s press on.

Now let’s put the closely spaced slits into the laser beam. The light is again spread out, but now there is a stripey pattern.

Laser light passing through the two horizontal slits produces the distinctive stripes of quantum mechanics.
Enlarge / Laser light passing through the two horizontal slits produces the distinctive stripes of quantum mechanics.

Miguel Morales

Congratulations! You’ve just spotted a quantum mechanical effect! (whoo hoo animated emoji) This is the classic double-slit experiment. The stripey pattern is called interference, and is a telltale signature of quantum mechanics. We will see a lot of stripes like these.

Now you have probably seen interference like this before, since water and sound waves show exactly this kind of striping.

Water waves from two sources (one visible in green, the other hidden behind the presenter). The circular waves overlap into regions of extra strength (bright stripes) and regions where the waves cancel each other out (dark bands). The formation of stripes is a signature of wave motion.
Enlarge / Water waves from two sources (one visible in green, the other hidden behind the presenter). The circular waves overlap into regions of extra strength (bright stripes) and regions where the waves cancel each other out (dark bands). The formation of stripes is a signature of wave motion.

In the photo above, each ball creates waves that move out in a circle. But a wave has both a peak and a trough. In some places the peak of the wave from one of the balls always coincides with the trough from the other (and vice versa). In these areas the waves always cancel out and the water is calm. In other locations the peaks of the waves from both balls always arrive together and add up to make a wave that is extra tall. In these locations the troughs also add up to be extra deep.

So does the fact that we are seeing stripes when our laser pointer goes through two slits mean that particles are waves? To answer that question, we’re going to have to look more closely.

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