Schrödinger’s Cat Was Never Both Alive and Dead
If you are completely confused by quantum mechanics, you do not understand it.
Schrödinger’s cat is a quantum physics thought experiment that has become a meme. A politician who clings to power while thought to have resigned is “Schrödinger’s Minister”. A movie star who attracts both acclaim and derision is “a Schrödinger’s cat of pop culture”. In fact, in a typical two-week period, you can find news articles comparing Schrödinger’s cat to an uncertain council election, hypocritical geopolitics, undiagnosed COVID-19, false-flag bombings, and patchy internet coverage. Whatever happened in Schrödinger’s box 87 years ago, today Schrödinger’s cat seems more alive than ever.
Schrödinger introduced his famous cat in 1935. He argued that it showed that if the contemporary mainstream understanding of quantum mechanics were correct, then it would mean cats can be both alive and dead at the same time. In the media, this is extended as a metaphor for ambiguity or coexisting opposites. But in the literal case of a real cat, it is patently absurd. Schrödinger, therefore, concluded that this understanding of quantum mechanics must be wrong. Instead, Schrödinger agreed with Einstein’s view that quantum mechanics is an incomplete theory. However, experiments have since shown that this view is incorrect. For this reason, the popular understanding is that, absurd or not, Schrödinger’s cat really is both alive and dead.
If you think this makes no sense, then you are right. In fact, we have known for over fifty years that it is not true. The truth is that Schrödinger’s cat was never both alive and dead. To understand why, we must learn about the concept of decoherence. Once we do, we will see that quantum mechanics is not quite so strange as we have been led to believe. Contrary to popular belief, it actually makes quite a lot of sense.
Quantum Superposition
Imagine flipping a coin in a pitch-black room. The coin lands, but you cannot see it because it is dark. As soon as you turn on the light and see the coin, you immediately see that it shows heads. How do you interpret this? Simple – before you turned on the light, the coin already showed heads, but you did not know. Turning on the light allowed you to find out what was already there. There is nothing mysterious about this.
However, quantum mechanics introduces a new concept – quantum superposition. An example of a physical entity that can be in quantum superposition is a radioactive atom. Such atoms have a chance of decaying into different atoms, and emit nuclear radiation if they do so. For example, if you leave a radioactive atom of uranium-229 for one hour, there is a 50% chance that it will decay into thallium-205 and a 50% chance that it will not. Imagine putting an atom of uranium-229 in a box. After an hour, you come back and measure whether the atom in the box is still uranium-229 or is now thallium-205. There is a 50% chance of either possibility. Therefore, this experiment is similar to the coin flip above – until you open the box, you do not know whether the atom decayed or not, but you find out when you examine it.
However, according to quantum mechanics, there is a catch. Unlike with the coin, it is not true that the atom is either uranium-229 or thallium-205 before you measure it. Instead, it is in a quantum superposition of the two possibilities. It is only when it is measured that this superposition collapses and the atom becomes one or the other possibility. This is because radioactive decay is a quantum mechanical process, and such processes place quantum particles like atoms into quantum superpositions. We can show this with experiments that give outcomes that are only possible if such quantum superpositions exist (such as those that won the 2022 Nobel Prize in physics). Informally, it is said that the atom in the box is both uranium-229 and thallium-205 before it is measured because it retains the potential to be observed to be either. It is only once the atom is measured that it becomes one or the other. While different from coins, the quantum superposition of atoms is not inherently mysterious either. Individual atoms are so small as to be undetectable in our everyday lives, so it is not altogether problematic for them to follow different rules from what we are familiar with.
However, the logic of Schrödinger’s cat is to amplify such quantum superpositions to the large-scale world, where they do become a problem. Specifically, Schrödinger imagined taking a radioactive atom (say, uranium-229) and placing it in a box with a cat, a Geiger counter, and a vial of poisonous gas. A Geiger counter is a device that clicks if it detects nuclear radiation. So, the Geiger counter will click if the atom decays, and will not if the atom does not decay. The counter is wired up so that if it clicks it activates a mechanism that releases the poisonous gas, which will kill the cat. Therefore, if the atom decays then the cat will die; if it does not decay then the cat will live. However, remember that before it is measured the atom enters into a quantum superposition of decayed and not decayed. This seems to imply that the cat must enter a superposition of dead and alive. This is the mystery of Schrödinger’s cat – it seems to be both alive and dead.
Decoherence Killed the Cat
However, there is a leap of logic hidden in the Schrödinger’s cat experiment. It is assumed that the quantum superposition survives the transition from the atom to the cat. Imagine instead that quantum superpositions are destroyed when they are amplified from the microscopic scale to the everyday scale. Then, the cat would never be in a quantum superposition of both alive and dead; it would simply live or die like a normal cat.
Indeed, we often talk about quantum mechanics with the tacit assumption that it applies only to the microscopic world. For example, quantum mechanics is often described as the physics of the very small, which describes atoms and subatomic particles while classical mechanics describes the everyday world. This is partially justified by the fact that many quantum properties diminish with increasing mass and size. This indeed means that they naturally become negligible for cats, which are a trillion billion billion times larger and heavier than an atom. However, this argument does not work for Schrödinger’s cat; since the superposition of the atom is directly transmitted to the cat, there is no natural diminishment from the increasing scale. In order for Schrödinger’s cat to avoid its mortal limbo, there must be some specific effect that destroys the superposition on its journey from atom to cat.
Fortunately, in 1970 Heinz-Dieter Zeh discovered exactly such an effect – decoherence. We can understand quantum decoherence by considering the analogous effect in an everyday phenomenon – sound. We can create a (classical) superposition of sound by playing two different pitches at the same time. If these pitches are sufficiently close to each other then this produces a beating effect – the volume of the sound pulses between loud and quiet. This effect is called interference and is only possible because more than one sound is present. The two different sounds interfere with each other to produce an effect that is not present in either single sound. However, while two pitches can produce vibrant beating, adding more pitches tends to diminish rather than amplify the effect. The beating of each pair of sounds occurs at a different rate, with the effect that they drown each other out. If you played a billion sounds of different pitches all at once you would not hear even a hint of beating – you would hear white noise. The combination of so many sounds causes decoherence – the destruction of the interference effects that are characteristic of superposition.
Sound Demonstration: Two slightly different tones (frequencies 450 Hz and 451 Hz) are first played separately. The two tones are then played together, and beating can be heard as a result of interference. Finally, ten tones are played together and white noise is heard, since the combination of many sounds causes decoherence.
Quantum interference – which is analogous to the interference of sound – is at the core of quantum mechanical phenomena. It is a process by which two parts of a quantum superposition interact. Interference is how we know that quantum superpositions are real; they can be used to produce effects that would otherwise be impossible. In quantum physics, a physical entity is described as coherent if it can show interference effects. Everyday objects like cats are made of immense numbers of atoms that would each individually be coherent. However, just as with sound, when so many atoms are combined, the interference effects of the individual atoms are swamped by the noise of the other atoms. This is quantum decoherence, the result of which is that everyday objects show no evidence of being in quantum superposition.
Where does this leave us with Schrödinger’s cat? Inside the box, the radioactive decay of the atom is detected by the Geiger counter. A Geiger counter consists of an immense number of atoms and so has decohered. This means that the Geiger counter never enters a superposition; it either clicks or it doesn’t. So, the measurement of the Geiger counter destroys the superposition before it ever reaches the cat. The cat never enters a superposition; either the Geiger counter clicks and the cat instantly dies, or it does not and the cat lives. The fate of the cat is not determined by our opening the box. If the cat is found dead, it was not our curiosity that killed the cat. It was decoherence.
Confusion is Not Understanding
Ultimately, Schrödinger’s cat does not demonstrate the incurable weirdness of quantum mechanics. The cat behaves perfectly sensibly; it is never both alive and dead. It either lives or dies, and we find out which has happened when we check the box. For all the hype, Schrödinger’s cat turns out to be quite mundane. Yet, this is exactly why it teaches us so much. Science is not supposed to be mysterious. Science should explain. Phenomena should make more sense after scientific analysis, not less. Schrödinger’s paper was entitled “The Present Situation in Quantum Mechanics”. He saw his thought experiment as a contemporary problem to be solved. Decades later that problem was solved, and our present understanding of quantum mechanics is far better than that available in 1935.
The great physicist John Wheeler once said, “if you are not completely confused by quantum mechanics, you do not understand it.” This is just one quote in quantum mechanics’ long history of glorifying confusion and dismissing attempts at understanding. This attitude persists today, everywhere from the media to university classrooms. But Wheeler was wrong. The example of Schrödinger’s Cat shows us how confusing claims about quantum mechanics, like cats can be both alive and dead at the same time, are actually signs of incomplete understanding. When we probe such oddities, we find a more sensible understanding that resolves our confusion. In fact, Wheeler’s quote is antithetical to the scientific spirit. No scientist should be proud of a theory being confusing or incomprehensible. When an aspect of quantum mechanics seems confusing, there are two explanations. First, it may have been poorly explained to you, as is often the case with Schrödinger’s cat. Alternatively, there may really be unsolved problems that need further scientific research to resolve. In either case, the correct conclusion is the much more sensible opposite of Wheeler’s quip: If you are completely confused by quantum mechanics, you do not understand it.
The sound demonstration really helped clarity how the largely reliable everyday position and condition of things emerges out of quantum superposition. Does this rule any possible “evolutionary use” of quantum superposition (or other quantum phenomenon) at a biological level (i.e. is the most micro level of biological still too macro for any quantum effects)?