<img src = "https://cdn.arstechnica.net/wp-content/uploads/2021/01/quantum-series-1-800×450.jpg" alt = "Quantum mechanics is complex and folds your brain together. But it does can be explained. "/> Enlarge /. Quantum mechanics is complex and folds the brain together. But it can be explained.
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Exploring the quantum world
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Some technical revolutions occur with drama and bang, others fidget unnoticed into our everyday experience. And one of the quietest revolutions of our current century was the entry of quantum mechanics into our everyday technology. In the past, quantum effects were limited to physics laboratories and delicate experiments. However, modern technology increasingly relies on quantum mechanics, and the importance of quantum effects will only increase in the coming decades.
So it is time to explain quantum mechanics – or at least its basics.
My goal in these seven (!) – sub-series is to introduce the strangely beautiful effects of quantum mechanics and to explain how they influence our everyday world. Each issue includes a guided hike into the quantum mechanical forests, where we will admire a new – and often surprising – effect. Back at the visitor center, we'll talk about how this effect is being used in technology and where to look for it.
Getting into a number of quantum mechanical articles can be intimidating. Few things are more fearful than "a simple introduction to physics". But for the intrepid and brave, I'll make a few promises before we begin:
- No math. While the language of quantum mechanics is written with fairly advanced math, I don't think 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'll leave this discussion in the pub for pints. Here we focus on what we see.
- Everything we encounter is tested experimentally. 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, watch particles mix and match, and see how these effects lead to our current technology as well as advances that are about to emerge to recognize them of the laboratory.
So put on your mental walking shoes, grab your binoculars, and follow me as we set off to explore the quantum world.
What is quantum mechanics?
My mother once asked me, "What is quantum mechanics?" This question has baffled me for a while. My best answer so far is that quantum mechanics is the study of how small particles move and interact. However, this is an incomplete answer as quantum effects can also be important on galactic scales. And it is doubly unsatisfactory because many effects like superconductivity are caused by mixing and mixing multiple particles.
In many ways, the role of quantum mechanics can be understood by analogy with Newton's gravity and Einstein's general theory of relativity. Both describe gravity, but general relativity is more correct – it describes how the universe works in every situation we've tested. 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 using an optical clock to make precise time measurements, Newton's gravity is good enough.
Both classical mechanics and quantum mechanics describe movements and interactions in a similar way. Quantum mechanics is better, but classical mechanics is mostly good enough.
What fascinates me is that "good enough" is increasingly no longer. Much of the technology developed in this century is based on quantum mechanics – classical mechanics is no longer accurate enough to understand how these inventions work.
Let's start today's hike with a deceptively simple question: "How do particles move?"
Kitchen quantum mechanics
Some of the experiments we'll see require special equipment, but let's start with an experiment that you can do at home. Like a cooking show, I'll explain how to do it, but you are encouraged to join in and do the experiment yourself. (Share your photos in the discussion below. Bonus points for setting up the experiment in your cubicle / workplace / other creative environment.)
To study how particles move we need a good particle pea shooter that we can use to make lots of particles to play with. It turns out that not only does a laser pointer keep the cat entertained, but it is also a great source of particles. There are copious amounts of photons, all of which are moving in almost the same direction and with almost the same energy (as indicated by their color).
When we look at the light from a laser pointer, it leaves 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 off). At this point, it's tempting to guess that we know how particles move: they leave the end of the laser like little ball bearings and move in a straight line until they hit something. But as good observers we should make sure.
Let's challenge the particles with an obstacle course by cutting thin slits in aluminum foil with razor blades. I made a few different cuts in the aluminum foil. The first is a single slot that is a few millimeters long. For the second I stacked two razor blades and cut two parallel slits a few tenths of a millimeter apart.
Enlarge /. Horizontal slots made of aluminum foil with razor blades. The top slot is from a single blade while the bottom is from two blades glued together.
In a darkened room, I set up my laser pointer so that it shoots across the room and hits a blank wall. As expected, I see a spot (assuming the cat isn't around). Next, I poke the single slot in the aluminum foil in the path of the laser and look at the pattern on the wall. When we send the light through the single gap, we see that the beam expands dramatically in the direction perpendicular to the gap – not along the gap.
Enlarge /. Laser light falling through the single horizontal slot is distributed vertically
Interesting. But let's move on.
Now let's insert the closely spaced slots into the laser beam. The light is spread out again, but now there is a striped pattern.
Enlarge /. Laser light falling through the two horizontal slits creates the characteristic stripes of quantum mechanics.
Congratulations! You have just discovered a quantum mechanical effect! (whoo hoo animated emoji) This is the classic double slit experiment. The fringe pattern is called interference and is a tell-tale signature of quantum mechanics. We will see many such stripes.
By now you have probably seen such glitches before, as water and sound waves show exactly this type of streak.
Enlarge /. Water waves from two sources (one visible in green, the other hidden behind the moderator). The circular waves overlap into areas of added strength (light stripes) and areas where the waves cancel each other (dark bands). The formation of stripes is a signature of the wave motion.
In the photo above, each ball creates waves that move in a circle. But a wave has both a peak and a bottom. In some places the top of the wave of one of the balls always coincides with the trough of the other (and vice versa). In these areas the waves always cancel and the water is calm. In other places, the tips of the waves of both balls always come together and result in a wave that is particularly high. The troughs are particularly deep at these points.
Does the fact that we see streaks when our laser pointer goes through two slits mean that particles are waves? To answer this question, we need to take a closer look.