*Nine leading thinkers interpret the meaning of Schrödinger’s famous thought experiment. Amanda Gefter; Sheldon Goldstein; Jenann Ismael; Chiara Marletto; Tim Maudlin; Alyssa Ney; Tim Palmer; Carlo Rovelli; Lev Vaidman. *

**Introduction**

Contemporary versions of Erwin Schrödinger’s famous cat thought experiment often prefer to use sleeping gas instead of cyanide. But for a cat in a box to be both asleep and awake - as opposed to the original cat which was both dead and alive - is, if decidedly less cruel, just as strange.

Writing to Einstein in 1935, Schrödinger’s imaginary experimental set-up was designed to expose the critical flaws of the Copenhagen interpretation of quantum mechanics, which holds that quantum systems stay in a superposition of two or more states until the system interacts with an external observer [1]].

We might be able to dismiss this effect as a peculiarity of the microscopic world of atoms, but what happens when that world has a direct consequence on the macroscopic, everyday world of tables, chairs, and cats? That’s what Schrödinger’s thought experiment sought to illuminate, and in the process expose the Copenhagen interpretation of quantum mechanics as absurd. It’s one thing having particles be in a state of superposition. But cats? Cats are either one thing or another, dead or alive, they can’t be both, surely...

SUGGESTED READING Reality is just a quantum wave function By AlyssaNey “One can even set up quite ridiculous cases”, began Schrödinger as he outlined the equipment: “A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid.

If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function [wave-function] of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.”

*Schrödinger's cat. Source: Wikimedia by Dhatfield*

Schrödinger didn’t believe that a cat in a smeared-out state of being dead and alive was a serious possibility. He wanted to show the absurd conclusions of the ways in which his ideas had been misunderstood.

Nearly 90 years later, the story of Schrödinger’s Cat still divides philosophers and physicists and gets to the heart of the big philosophical issues with interpreting quantum mechanics.

There are many contemporary versions and readings of the thought-experiment and its lasting implications. Some seem to restore order to a world made topsy turvy by quantum mechanics. Others, which see the creation of multiple cats in multiple universes, might be said to make the original ‘ridiculous case’ look rather mundane.

Here is a selection of the many meanings of Schrödinger’s Cat from some of our favourite thinkers.

*Scroll down to read the answers or click to jump to:*

___

Amanda Gefter on Schrödinger's QBist cat

___

The tale of Schrödinger’s cat, told with a QBist twist, is not a story about a cat at all. In the usual telling, the wavefunction describes the boxed feline. In QBism, it describes an agent’s beliefs about what will happen if she opens the box.

Say she’s a gambler. She wants to bet on whether she’ll find the cat dead or alive, and she knows a quantum wavefunction will give her the most accurate probabilities. But the world doesn’t come labelled with wavefunctions. She’s got to write one down herself. All she has at her disposal are her own past actions and their consequences, so her resulting wavefunction doesn’t reflect an independent reality. It doesn’t reflect an independent cat. It’s a personal history of how the world responds to her touch.

Now she opens the box. She experiences a dead cat—or a live one. Either way, she updates her beliefs, informs her expectations for future encounters. What others refer to as the mysterious “collapse of the wavefunction” is, for the QBist, an agent tweaking her bets.

While it’s the agent’s beliefs that form the superposition—not the cat’s vitals—the structure of those beliefs does tell us something about the cat. That’s because the wavefunction encodes the agent’s beliefs about *all *the actions she could take on the box—even mutually exclusive ones—and the only way for her beliefs to be consistent with one another is if the unmeasured cat doesn’t have an intrinsic state at all.

The moral of the QBist story, in the words of John Wheeler, is that this is a participatory universe. And Schrödinger’s is a participatory cat.

*Watch Amanda Gefter debate quantum mechanics and consciousness in Planck and the Conciousness Puzzle*

___

Sheldon Goldstein on The Bohmian view

___

According to the Copenhagen interpretation of quantum mechanics, a quantum particle such as an electron does not have a position until one looks---until one performs an appropriate "measurement." Schrödinger showed that if the Copenhagen interpretation is to be believed, what is true here for electrons is also true for larger objects, and in particular for cats: situations could be created in which a cat is neither dead nor alive until one looks at it.

This raises some questions:

• Why should "looking" be so important?

• Is it necessary to be so extreme?

The answer to the latter question is: Not at all! There is a simple and rather obvious version of quantum mechanics, Bohmian mechanics, in which quantum particles always have positions; when we look, we find the particles where they are: a measurement reveals the positions (just as the word measurement suggests) rather than creates them. And similarly for cats and their states.

Why should physicists have insisted on anything as strange and implausible as Schrödinger's cat? The reason is that they assumed that the quantum description of any system, as provided by its wave function, must be the complete description of that system. That this should be so should have seemed implausible from the very beginning. Surely, one should have thought, the complete description of a system of particles must include the positions of those particles. If one insists on that, then one arrives rather quickly at Bohmian mechanics. And if one insists on the implausible completeness assumption one arrives instead at Schrödinger's paradoxical cat. Garbage in, garbage out!

___

Jenann Ismael on the possibility of knowledge

___

I’ve come around to thinking that the real meaning of Schrödinger’s cat has nothing to do with realism. It has to do with the possibility of knowledge. The problem is not that the quantum world unreal; it is that we cannot stabilize quantum systems as objects of knowledge.

The ordinary logic of knowledge presupposes that there is an object *there to be known*, independently of the questions we put to it. In the quantum case, this presupposition fails. The questions that we put to a quantum system in the form of measurements interfere with the answers that we get.

The situation that we are in is like that of the boy who asks his mother every day from July to December about his Christmas present: is it an X? is it a Y? She may be sworn to answer truthfully, but she is waiting until Christmas eve to make the purchase and bent on surprise.

*Read Jenann Ismael's article Physics forgets we are part of reality.*

___

Chiara Marletto on the counterfactual essence of Schrödinger’s cat

___

Schrödinger’s experiment has three fundamental implications.

First, quantum theory *can* be applied to everything, even macroscopic systems. No matter how counterintuitive, states like the cat’s are *possible* according to quantum theory.

Second, the poison mechanism must lead to entanglement, as it is a measurement acting on a superposed particle. That’s because of Heisenberg’s uncertainty principle: it is *impossible* to distinguish perfectly between a superposition of properties and a state where those properties are definite — which surprisingly leads to entanglement being possible.

Finally, quantum theory is* reversible*. It is possible to reverse the interaction that entangles the cat and the particle, even though that’s very difficult to achieve in practice because the cat is highly complex.

These essential features are 'counterfactuals': they are about what's possible or impossible, rather than about what is or is not (the actual). In fact, the whole of quantum theory rests on counterfactuals. A far-reaching revelation, because counterfactual properties are more general than quantum theory's laws of motion, revealing a deeper structure.

Quantum theory's successor may have radically different laws of motion but will display the counterfactual properties that permit superpositions, entanglement and perhaps even new, more exciting phenomena. Long live Schrödinger's cat!

What point was Schrödinger making with his hypothetical cat experiment? It is commonly said nowadays that he was arguing that quantum theory implies the physical possibility of cats in some suspended state, neither dead nor alive. But the opposite is true. Schrödinger thought that such a thing was manifestly absurd, and any attempt to understand quantum theory yielding such a result should be rejected.

Schrödinger was reacting to the Einstein-Podolosky-Rosen paper, which argued that the quantum-mechanical wavefunction cannot provide a complete physical description of an individual system. EPR focused on correlations between distant experimental outcomes and “spooky-action-at-a-distance” to come to that conclusion.

Schrödinger arrives at a similar conclusion, but from two premises and independently of action-at-a distance. He shows that if 1) the wavefunction provides a complete physical description and 2) it always evolves via his own (Schrödinger) equation until a “measurement” is made, then cats could end up in such a state, which is manifestly absurd. Therefore, in the words of John Bell, “Either the wavefunction, as given by the Schrödinger equation, is not everything or it is not right”.

If that wavefunction is not everything, then we must postulate so-called “hidden variables” (although they better not be hidden). If it is not right, then there is Objective Collapse of the wavefunction. These are the two approaches to understanding the quantum formalism that Schrödinger recognized. The so-called “Many Worlds” interpretation tries to get by without rejecting either 1 or 2 and ends up faced with the conclusion Schrödinger thought ridiculous. But both “hidden variables” and Objective Collapse have nothing to fear from Schrödinger’s cat.

*Watch Tim Maudlin's in-depth interview for IAI TV Quantum Theory and where it might take us*

Schrödinger’s example demonstrated that one cannot confine the indeterminacy of quantum systems to the microscopic realm. Since one can conceivably entangle indeterminate microscopic systems with macroscopic systems like a cat, quantum mechanics implies indeterminacy for macroscopic systems as much as it does for microscopic.

The question is whether this indeterminacy should be interpreted as metaphysical (in the world) or merely epistemic (in what we know). “There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog,” Schrödinger noted, finding both alternative interpretations of quantum indeterminacy problematic. Quantum entanglement thus presents us with an antinomy.

In 1935, before Bell and the experimental tests of his theorem, before the development of quantum technologies that exploit the reality of entangled states, and techniques for creating macroscopic entangled systems, it was reasonable to take the metaphysical cloud option off the table. But if entanglement is real, then we need a metaphysical interpretation of it.

Wave function realism is an approach to the interpretation of quantum systems that takes them to be wave functions, fields that may evolve to have amplitudes in regions corresponding to a dead cat and regions corresponding to a live cat. As Schrödinger knew, if we take this approach seriously, the background space over which these fields are spread is the very high-dimensional space capable of accommodating the degrees of freedom of the quantum wave function. So the result is no ordinary cloud. But it is a reasonable starting point for resolving Schrödinger’s antinomy given what we know today.

*Read Alyssa Ney's argue that Reality is just a quantum wave function.*

**A Superdeterministic Perspective based on Invariant Set Theory**

Invariant Set Theory (IST) is a model of quantum physics derived by re-applying Planck’s insight about the discretised nature of energy, this time to the state space of quantum mechanics. Hence, in IST, the continuum Hilbert Space of quantum mechanics is replaced with a particular type of discrete lattice. The inevitable gaps in this lattice correspond to counterfactual worlds where an experimenter might have performed a measurement on a quantum system, but didn’t: such counterfactual worlds are inconsistent with the structure of the lattice. As such, IST is formally a “superdeterministic” theory: the measurements that experimenters make are not independent of the particles that they measure.

In IST, the states that lie on IST’s lattice correspond to ensembles of worlds, each of which is a deterministic system evolving on a special subset of state space. Motivated by nonlinear dynamical systems theory, this subset is called the “invariant set”. Counterfactual worlds associated with gaps in the lattice do not lie on the invariant set.

In this way, Einstein’s vision of the quantum wavefunction as describing an ensemble of worlds, with neither spooky action-at-a-distance nor indeterminism, can be realised. In particular, Schrödinger’s cat is either dead or alive, but not both.

References:

The mistake that generates the confusion in the Schrödinger’s cat fable is the wrong metaphysical assumption that physical systems have non-relational properties. If all properties are relational, the apparent paradox dissolves.

With respect to the cat, the poison is either released or not; the cat itself is either alive or dead (either awake or asleep in the version I prefer). But this occurrence has no bearing with respect to a physical system outside the box.

With respect to a physical system outside the box, the cat is neither awake nor asleep, as, lacking an interaction with the cat, these properties are not realized; future interactions between the box and an external system may, in principle, include interference effects that would have been impossible had the cat been definitely awake or definitely asleep with respect to that system.

In other words, the ‘collapse of the wave function’ describes the realization of some properties relative to the cat as this interacts with the poison, while the ‘unitary evolution’ describes the evolution of the probabilities for the realization of properties relative to the external system. This is the solution of the apparent paradox, in the relational interpretation of quantum theory.

*Watch Carlo Rovelli in conversation with Jim Al-Khalili On fundamental physics and the nature of reality*

___

Lev Vaidman on Many Worlds and many cats

___

At the beginning of the twentieth century, physicists realized that classical physics fails to explain observed phenomena and the phenomenological rules of quantum theory were discovered. But it is only when Schrödinger invented his equation that quantum mechanics became an accepted scientific theory.

Schrödinger realized that his equation, when applied to the analysis of quantum measurements such as detecting radioactive decay, leads to the existence of multiple outcomes in parallel, such as the existence of both a live and dead cat. In fact, this situation corresponds not to the parallel existence of just two (as frequently assumed), but many different cats: one alive cat and many dead cats which died at different times.

This understanding was viewed by Schrödinger as a serious problem for his equation, and he reluctantly accepted the failure of its universal validity for describing the evolution of the quantum system due to the collapse of the quantum state during quantum measurements. In my view, this was a mistake. Collapse, with its randomness and action at a distance should not be accepted. Instead, Schrödinger’s Cat demonstrates the existence of parallel worlds. This is the only possibility to avoid nonlocal action and to save determinism in Nature.

*Watch Lev Vaidman debate parallel universes from our archive: Are there really many worlds?*