A “No Mathematics” (but seven parts) guide to modern quantum mechanics

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Some technical revolutions go in with drama and a bang, others meander unnoticed in our everyday experience. And one of the quietest revolutions of our current century was the advent of quantum mechanics in our everyday technology. It used to be that quantum effects were limited to physics labs and delicate experiments. But modern technology is increasingly relying on quantum mechanics for its basic operation, and the importance of quantum effects will only grow in the coming decades.

As such, the time has come to explain quantum mechanics – or, at least, its basic principles.

My purpose in this seven (!) Sub-series is to introduce the strange beautiful effects of quantum mechanics and explain how it affects our everyday world. Each issue contains a guided walk in the quantum mechanical forest in which we will admire a new – and often surprising – effect. Back in the visitor center we will talk about how the effect is used in technology and where to look for it.

Starting a series of articles on quantum mechanics can be intimidating. Few things cause more fear than a simple introduction to physics. ‘But for the fearless and brave, I will make some promises before we begin:

• No math. Although the language of quantum mechanics is written with fairly advanced math, I do not believe that you need to read Japanese before you can appreciate Japanese art. Our journey will focus on the beauty of the quantum world.
• No philosophy. There was a fascination with the ‘meaning’ of quantum mechanics, but we let the discussion down for pints in the bar. Here we will focus on what we see.
• Everything we encounter will be experimentally verified. While some of the results may 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, see how particles mix and mingle, and see how these effects give rise to our current technology and advances that are about to make it out. of the laboratory.

So put on your spiritual hiking boots, grab your binoculars and follow me as we explore the quantum world.

What is quantum mechanics?

My mother once asked me, “What is quantum mechanics?” This question has been bothering me for a while now. My best answer so far is that quantum mechanics is the study of how small particles move and interact. But this is an incomplete answer, because quantum effects can also be important on galactic scales. And this is doubly unsatisfactory, because many effects such as superconductivity are caused by the mixing and blending of multiple particles.

In many ways, the role of quantum mechanics can be understood in analogy to Newton’s gravity and Einstein’s general relativity. Both describe gravity, but general relativity is more accurate – it describes how the Universe works in every situation we have 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 are taking precise measurements with an optical clock, Newton’s gravity is good enough.

Similarly, classical mechanics and quantum mechanics describe both movements and interactions. Quantum mechanics is more correct, but classical mechanics is usually good enough.

What I find fascinating is that ‘good enough’ is increasingly not. Many of the technologies developed in this century are beginning to rely on quantum mechanics – classical mechanics are 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?”

Quantum Mechanics for Kitchen

Some experiments we will see require specialized equipment, but let’s start with an experiment you can do at home. Like a cooking program, I will explain how you can do this, but you are encouraged to follow the experiment yourself. (Share your photos in the discussion below. Bonus points for setting up the experiment in your hockey / workplace / other creative environment.)

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

If we look at the light of a laser pointer, it goes out of the end of the laser pointer and moves in a straight line until it hits an obstacle and spreads (or touches a mirror and reflects). At this point, it’s tempting to guess that we know how particles move: they leave the tip of the laser like small 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 made a few different cuts. The first is a single slot, a few millimeters long. For the second time, I stacked two razor blades together and cut two parallel slits a few tenths of a millimeter apart.

In a dark room, I set my laser pointer to shoot through the room and hit an empty wall. As expected, I see a place (provided the cat is not in the area). Next, I place the single slot in the aluminum foil in the laser path and look at the pattern on the wall. If we send the light through the single slot, we see that the beam expands dramatically in the direction perpendicular to the slot – not next to the slot.

Interesting. But let’s move on.

Let us now place the careful slits in the laser beam. The light has been spread out again, but now there is a striped pattern.

Congratulations! You just noticed a mechanical effect! (whoo hoo animated emoji) This is the classic double-slot experiment. The stripe pattern is called interference and is a clear signature of quantum mechanics. We will see many such stripes.

Now you have probably seen all such interference, as water and sound waves show exactly these kinds of streaks.

In the photo above, each ball creates waves that move in a circle. But a wave has a peak and a trough. In some places the peak of the wave from one of the balls always coincides with the trough of the other (and vice versa). In these areas the waves always stop and the water is calm. Elsewhere, the peaks of the waves of both balls always come together and add to make an extra high wave. In these places the bowls are also extra deep.

Does the fact that we see streaks when our laser pointer passes through two slits mean that particles are waves? To answer the question, we will have to take a closer look.