A popular thought experiment in quantum physics, known as the Schrödinger’s cat experiment, goes something like this: A cat is in a box. Also inside the box are a hammer, a vial of poison, a radioactive atom, and a radiation measurer called a Geiger counter. The radioactive material has a 50/50 chance of decaying. If the Geiger counter detects that it has decayed, it triggers the hammer to hit the vial of poison, which spills and kills the cat (cat lovers, rest assured: this experiment is merely theoretical). Because the chances of each outcome — the cat living and the cat dying — are equal, the theory goes, the cat is both alive and dead until it is observed. 


Understanding how this could possibly be requires a bit of knowledge of another (real) experiment in quantum physics: the double-slit experiment. In this experiment, physicists fired photons — the particles that make up light — through two slits in a plate. After looking at where the photons ended up on the screen behind that plate, they noticed that the particles behaved like waves: The screen displayed interference patterns, as if the particles were bouncing off one another, even when they were fired one at a time. But if physicists observed the photons as they went through the slits, they went back to form two straight lines, as one would predict from a particle.


Quantum physicists have theorized that this happens because the human observer plays a role in determining the fate of the system it’s observing. Until an observer steps in, according to what’s known as the Copenhagen interpretation of quantum mechanics, a quantum system exists in many states all at once, like a wave of possibilities. But once someone measures it, it collapses into one state. The ability for a quantum system to be in multiple states at once is known as quantum superposition


The physicist Erwin Schrödinger was deeply disturbed by the implications of the Copenhagen interpretation, and in 1935, he devised the Schrödinger’s cat experiment to expose what he considered the ridiculousness of quantum superposition. Because the decay of the radioactive material is a subatomic event, and thus follows the principles of quantum mechanics, it both decays and does not decay, and the cat would technically be alive and dead at once until someone opens the box. The impossibility of this suggestion, to Schrödinger, proved that the Copenhagen interpretation was erroneous. 


However, since then, the experiment has been treated not as proof that quantum superposition is impossible, but as an invitation to ask how it could be possible. Toward that aim, in 1957, physicist Hugh Everett proposed a competing interpretation of quantum physics: the many-worlds interpretation, which says there are parallel universes representing each possible state of every atom. When someone observes the system, they’re not determining what happens to it; they’re simply entering into one of these universes. 


Another interpretation, proposed by physicist Roger Penrose in 1989, says the superpositioned possibilities do collapse into one, but it’s not the conscious observer who causes the collapse. Rather, it’s the other way around: The collapse — which occurs spontaneously — causes consciousness. To account for how particles can be in multiple places simultaneously, Penrose referred to Einstein’s theory of general relativity, in which matter correlates with curvature in underlying space-time geometry — the fine-scale structure of the universe, explains Stuart Hameroff, professor of psychology and anesthesiology at the University of Arizona, who worked with Penrose on developing a quantum theory of consciousness. A particle in two locations would represent opposing, alternate curvatures — a separation in space-time, the fabric of reality. 

Erwin Schrodinger (1887-1961) founded wave mechanics, creating Schrodinger's equation.
Image by SSPL/Getty Images

Were the separation to continue, each possibility could evolve its own universe, as suggested by the many-worlds interpretation of quantum mechanics. But Penrose proposed that the separations were unstable and would collapse or reduce spontaneously to definite curvatures and particle locations, resulting in a moment of consciousness, says Hameroff. 


“The ineffable nature of feelings, emotions, awareness, and what philosophers call ‘qualia’ are thus tied to the structure of the universe,” he explains. 


Hameroff and Penrose have proposed that quantum computations in tiny structures inside brain cells, called microtubules, undergo collapse by the Penrose mechanism described above, producing sequences of discrete moments of awareness, which regulate neuronal functions and produce our familiar “stream of consciousness.” 


“Experiments seem to show that consciousness depends on quantum vibration in microtubules, which are dampened by anesthetics, producing loss of consciousness,” Hameroff explains. 


In recent years, the investigation of theories around Schrödinger’s cat has moved from hypothetical situations to real experiments. While no actual cats have been involved, thankfully, physicists have devised several methods to test the hypotheses Schrödinger’s cat sparked. 


Some of these experiments have attempted to figure out how small a system needs to be in order to exhibit quantum superposition, given that it’s been observed with particles but not with cats. In 2013, a team of physicists, led by Alexander Lvovsky at the University of Calgary, fired a photon at a mirror so that it was superimposed in two states: It passed through the mirror and reflected off of it. They then used a laser to amplify one of the states to around 160 million photons, finding that even a system that big was able to exist in superposition. 


More recently, in 2018, Daniela Frauchiger and Renato Renner of the Swiss Federal Institute of Technology published a paper in Nature Communications that presents a new version of the experiment to expose contradictions in the original one. In their thought experiment, two physicists are each in their own box. One tosses a coin and sends a message to the other about the results of the coin toss using their knowledge of quantum physics. Each box has its own observer, and when they open the boxes, they try to guess the results of the coin toss. The paper’s authors found that the observers would sometimes come to contradictory conclusions. 


“If a physical theory describes the world well, the theory should also describe those people who are using the theory and how they are using it,” says Renner. “If I have quantum mechanics and in this particular case, we want to see if this is a reasonable theory of the whole world, a universal theory, then it should also describe physicists who are themselves using quantum mechanics … If it describes everything, it should describe us.” The fact that it doesn’t suggests that the Copenhagen interpretation may not hold true. 

visitor takes a phone photograph of a large back lit image of the Large Hadron Collider (LHC) at the Science Museum's 'Collider' exhibition
Image by Peter Macdiarmid/Getty Images

This June, a paper in Nature suggested that the fate of the cat could actually be predicted before the cat has faced the consequences of opening the box. Zlatko Minev, a Yale graduate student at the time, came up with a new observation method to monitor an atom for signs of quantum jumps, or changes in state, and was able to detect signals indicating that a jump was about to take place. This means that the outcome of the Schrödinger’s cat experiment could be predicted before the cat dies or survives and therefore is not as random as previously believed. 


“While in the long run, the occurrence remains unpredictable and, loosely speaking, we can’t say ‘If I open the box on Sunday at 2 p.m., the cat will be dead or alive,’ in the short run, in the process of opening the box, there’s a certain tell-tale signal you can detect that allows you to glimpse into the dynamics of what’s going on and to anticipate the occurrence of the observation,” says Minev, now a researcher at IBM Thomas J. Watson Research Center. “The process by which the cat goes from a state of superposition to one of these definite states is not an abrupt, discrete process; rather, it’s a continuous, smooth, and even deterministic transition — deterministic in the sense that every single time the atom in our experiment jumped to the excited state, it did so by the exact same trajectory.” What determines which state the atom (or the cat) goes into remains an open question.


As physicists debate how to interpret the Schrödinger’s cat experiment, neuroscientists have been thinking about its implications for our understanding of the brain. One area of research asks if — and at what levels — the brain behaves like a quantum system, explains James Giordano, professor of neurology and biochemistry at Georgetown University Medical Center. Giordano believes that quantum operations may occur at the very small scale, perhaps at the subatomic or atomic level, yet they combine at larger scales to create brain processes that adhere to the laws of classical Newtonian physics, similar to the cat’s life being determined by a tiny radioactive atom. 


“We use the Schrödinger’s cat model as an analogy for the way that scalar processes may operate to establish the hierarchical functions in a complex nervous system,” says Giordano, whose research examines the ways small quantum systems could both receive signals from and transmit signals to larger neural systems.


Despite all these new ideas that have cropped up over the past few years, none have solved the mystery of Schrödinger’s cat — if anything, they’ve added even more mysteries. So, don’t worry if the whole thing is still baffling to you. “I hope it’s not too frustrating for a non-physicist to hear about this cat, but I can assure you, physicists also don’t know what it means that the cat is dead and alive at the same time,” says Renner. “That’s in some way our motivation for the whole work: to make this more concrete.”