Down at the level of atoms and electrons, quantum physics describes the behavior of the very smallest objects. Solar panels, LED lights, your mobile phone and MRI scanners in hospitals: all of these rely on quantum behavior. It is one of the best-tested theories of physics, and we use it all the time.
On the face of it, however, the quantum realm is extraordinary: within it, quantum objects can be “in two places at once,” can move through barriers, and share a connection no matter how far apart they are. Compared with what you would expect of, say, a tennis ball, their properties are certainly weird and counterintuitive.
But don’t let this weirdness scare you off! Much of quantum physics’ odd behavior becomes a lot less surprising if you stop thinking of atoms and electrons as minuscule tennis balls and instead imagine any quantum object as something like a wave you create by pushing your hand through water. You could say that at small scales, everything is made of waves.
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In the spirit of demystifying quantum behavior, here we discuss three key types of “weird” quantum actions that typical water waves can do just as well, along with the one thing that sets the quantum world apart.
NOT WEIRD: HEISENBERG’S UNCERTAINTY PRINCIPLE
Imagine tossing a tennis ball. If we wanted to, we could track the ball’s exact position and velocity throughout its flight. Strangely enough, if we shrink the ball down to the size of, say, an atom, this kind of tracking becomes impossible.
This limitation is called Heisenberg’s uncertainty principle. In quantum physics, it is impossible to know an object’s precise position and momentum (its velocity times its mass) at the same time. A tennis ball’s momentum is just its mass multiplied by its velocity, but for waves, we determine momentum by measuring the distance between successive wave crests, a factor called the wavelength.
Waves are fickle, however, so it is impossible to determine their position and wavelengths with 100 percent precision. In practice, any wave, whether watery or quantum, will always cover a range of positions and consist of a range of wavelengths. The more you restrict one of those ranges, the less you can control the other.
Consider two extreme types of water waves: The first is an infinitely repeating wave of regularly spaced ripples made by the wind on an endlessly long canal. There you could measure the wavelength by identifying the repeating pattern of wave crests and troughs. But you can’t say anything about the wave’s “position” in the canal because it doesn’t have a start or end point. Conversely, for a wave consisting of a single, thin crest in an otherwise calm pond, you can measure its position, but it does not have a well-defined wavelength because it never repeats.
In practice, all waves lie somewhere in between these two limits. Quantum waves are no different.
NOT WEIRD: SUPERPOSITIONS AND ENTANGLEMENT
A quantum object can be “in two places at once” by being in a so-called superposition of states. For waves, this is no surprise. A water wave can be in two places at once. If you send a wave down a forked channel, it will easily split and flow through both channels at the same time.
A related quantum concept is entanglement, which combines superpositions in two waves. In a vinaigrette dressing that has been left to stand, for example, oil will float on top of the vinegar. Carefully making a wave in the oil will then also cause a wave in the vinegar, and ripples will appear at their interface. Measuring the wavelength of the oil wave also tells us about the wavelength of the vinegar wave. In other words, the two waves are linked, and their properties depend on each other.
If we pour the separated dressing down a forked channel, this relation will hold, and the combined oil-vinegar ripples will move down two channels at the same time. If you then measure the wavelength of just the oil wave in one channel, you immediately know all wavelengths in both channels, even if they are far apart. If the dressing were quantum, you would say the waves in the two channels are “entangled” with each other. Quantum technology uses entanglement to create unbreakable encryption or speed up computations. For your salad, breaking the entanglement by shaking the dressing to mix it is probably more useful.
NOT WEIRD: TUNNELING
Another seemingly peculiar feat of quantum objects is that, with some probability, they can pass through barriers. This action is called tunneling. Throw a tennis ball at a wall, and (as long as the wall remains standing) it will bounce back. Do this with an atom, and you might find it on the other side.
In some cases, a water wave can go through a barrier just like a quantum particle. You can demonstrate this movement in your own bathtub: build an underwater wall in the tub that is tall enough that it almost touches the water’s surface but not quite. If you send a wave at this wall at a glancing angle, it will always bounce back. This behavior is analogous to so-called total internal reflection of light rays. It depends only on the height of the barrier and the angle at which the wave approaches the wall.
Although the wave cannot travel over the barrier, a small tail of it can probe the other side. If the wall is thin enough, you will see the tail remembering its original motion and magically reappearing as a traveling wave. Voilà, your water wave has tunneled through a wall! The same phenomenon of “broken” total internal reflection, but with light rays instead of water waves, is used in certain types of touch-screen displays.
VERY WEIRD: QUANTUM MEASUREMENT
We can demystify most weird quantum behaviors by thinking of small particles as waves instead of minuscule balls, but genuine quantum weirdness still arises when you measure a quantum object. Whether it’s a wave traveling through two different channels or one that has tunneled through a barrier, measuring a quantum wave results in the entirety of that wave suddenly appearing in a single location: in one channel and not the other, or on one side of the barrier and not the other. This doesn’t happen with salad dressing.
Funnily enough, the mathematical equations that describe quantum waves do not explain what happens when we measure those waves. Physicists don’t yet agree on how best to describe or interpret this process. Quantum measurement is the one thing that sets quantum behavior apart from water waves, making quantum physics truly strange.
To appreciate how unusual quantum measurement is, imagine someone speaking to a crowd of people. Sound waves spread out across the crowd, and everyone hears the speech. In the quantum world, however, the sound wave would spread out just as expected, but as soon as a single person in the crowd perceived (or measured) it, the entire sound wave would concentrate itself in that person’s ear, and no one else would hear it.
Now that is weird.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.