###### By Dan Hooper, Ph.D., University of Chicago

## A consensus had formed around the Copenhagen interpretation of quantum mechanics between the late-1920s and mid-1930s. Most prominent physicists of the time, except Albert Einstein, had accepted this consensus. Einstein had been actively trying to discredit the Copenhagen interpretation, and in Austrian physicist Erwin Schrödinger he found someone sympathetic to his position.

### Einstein Finds an Ally in Schrödinger

Schrödinger didn’t come out nearly as forcefully against the Copenhagen interpretation as Einstein did. However, he did find the new consensus view to be problematic from a philosophical and conceptual standpoint.

Over the years, Schrödinger and Einstein exchanged several letters in which they expressed their mutual discomfort with the consensus version of quantum theory. As they saw it, the theory they both did so much to create had lingering problems. And although Schrödinger didn’t feel as strongly as Einstein did about this, he too thought that the theory was likely to be incomplete in some important and fundamental way.

Learn more about what other great scientists got wrong.

### Schrödinger’s Famous Thought Experiment

In November 1935, just a few months after the EPR paper had been published, Schrödinger wrote and published an essay in which he introduced the thought experiment that is now known as Schrödinger’s Cat. It’s probably the single most famous illustration of just how weird quantum mechanics can be.

In his essay, Schrödinger described a hypothetical apparatus, designed to illustrate and even exaggerate some of the stranger consequences of the Copenhagen view of quantum theory.

This apparatus starts with a radioactive substance, which has a half-life of an hour. Due to the probabilistic nature of quantum mechanics, you can’t predict when or whether the radioactive substance will decay. It can only be said that there is a 50/50 chance it will decay sometime within an hour’s time.

This unstable atomic substance is then attached to a Geiger counter. If the substance decays, it will be detected by the Geiger counter, which will cause a hammer to swing, shattering a flask of potent acid. This apparatus is placed along with a healthy cat inside an impenetrable chamber. Finally, the door of the chamber is sealed, and you wait.

This apparatus might seem complicated, but all that really matters is: if the radioactive substance decays, the acid will kill the cat, and if the radioactive substance does not decay, the cat will remain alive.

Learn more about Einstein’s quest to overturn the standard view of quantum mechanics.

### What Does the Copenhagen Interpretation Have to Say About This?

Let’s consider how one would view Schrödinger’s apparatus and the cat from the perspective of the Copenhagen interpretation of quantum mechanics. Once you seal the door of the chamber, you can’t tell whether the radioactive substance has, or has not, decayed, and therefore you can’t tell whether the cat is dead or alive.

However, according to the Copenhagen interpretation, this is not merely a matter of our ignorance. The radioactive substance is in a state that is a combination of both decayed and not decayed. Therefore, the cat is simultaneously both dead and alive.

Pause for a moment and think about the popular example used to illustrate the theory of superposition. There’s an electron described by a wave function that extends across locations A and B. Until we measure its location, this electron is in both location A and location B, at the same time. A physicist would say that the wave function is a superposition of the electron being at location A and being at location B.

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What Einstein Got Wrong. Watch it now, Wondrium.

In the case of Schrödinger’s apparatus, the wave function of the radioactive substance is in superposition of decayed and not decayed states. Therefore, the wave function of the cat is also in a superposition of dead and alive states. Only when you open the box and observe the cat does its wave function collapse, causing it to take on a uniquely alive or uniquely dead state.

What’s the central message of this thought experiment? Until the door of the chamber is opened, the cat is not ‘only dead’ or ‘only alive’. It’s not just a matter of us not knowing whether it is alive or dead, the cat is simultaneously both dead and alive.

### Does the World Follow Scientific Realism?

What the Schrödinger’s Cat thought experiment manages to accomplish is that it takes the quantum weirdness of the subatomic world, and shows how these effects could potentially be manifest in something more macroscopic and tangible.

We might be able to accept that electrons, photons, or other quantum particles could exist in multiple states at once, but it’s much more difficult to think that this could be true of something like a cat. For most of us, our intuition tells us to insist that the cat must be either alive or dead. at any given moment in time.

If we strictly follow the Copenhagen interpretation of quantum theory or quantum mechanics, we have to conclude that until it is observed, the cat is in a superposition of alive and dead states. It is not alive or dead. It is simultaneously both alive and dead. However, our intuition tells us to insist that the cat must be uniquely alive or uniquely dead at a given time. What’s happening is that our intuition is just insisting on a form of scientific realism, similar to what Einstein would have advocated for.

In the scientific realist view, there is a real and well-defined world out there. In such a world, cats are either alive or dead, and cats are never both alive and dead at the same time. Regardless of what our intuitions might say, there is no real evidence that the world has to work this way. It is entirely possible that the world does not choose to adhere to the tenets of scientific realism.

Learn more about Einstein’s quest for a new theory of relativity.

### What Is an Observation?

This is essentially what prominent physicists of the period such as Niels Bohr, Max Born, and Werner Heisenberg were all arguing. It’s difficult to argue that either Einstein or Schrödinger showed them to be wrong in this respect.

However, Schrödinger’s Cat does raise a valid objection to the Copenhagen interpretation of quantum mechanics. In the Copenhagen view, the wave function of an object collapses when it is observed, which raises the question of what exactly constitutes an observer? What exactly constitutes an observation?

In the example of Schrödinger’s Cat, we say that the wave function collapses when we open the door to the chamber and observe the cat. But what if there is a flea on the cat? Could it also cause the wave function to collapse? And what is so special about an observer that causes a physical system to transform into a collapsed state? It can’t be just any kind of interaction, as experiments have repeatedly shown that quantum particles can interact with each other without their wave functions collapsing.

In the Copenhagen view, there is something strangely special about the act of observation. To many physicists this seems to be a deeply problematic issue, and it leaves many of us skeptical about the Copenhagen interpretation of quantum mechanics.

### Common Questions About Schrödinger’s Cat

**Q: Did Schrödinger’s Cat survive?**

In the thought experiment known as Schrödinger’s Cat, the cat is a quantum object or quantum matter and is said to exist in a superposed state of both alive and dead, at the same time. The radioactive substance inside the apparatus is also in a superposed state of both decay and not decay, at the same time. Hence, till the cat is observed at the end of an hour, it is both alive and dead, at the same time.

**Q: Did Schrödinger actually put a cat in a box?**

Schrödinger’s Cat is a thought experiment and not an actual experiment. The cat in this thought experiment is an imagined cat, and no real cat was ever put in any locked chamber containing radioactive substances by Schrödinger.

**Q: How does quantum superposition work?**

The principle of quantum superposition essentially says that a system comprising quantum particles exists as a combination of all possible configurations of the system, at the same time.

**Q: What is Schrödinger’s Cat trying to prove?**

Erwin Schrödinger created this thought experiment called Schrödinger’s Cat to demonstrate that quantum theory can throw up extremely absurd or weird results, due to simple misrepresentations of the principles of the theory.