Schrödinger’s Cat is a thought experiment devised by the Austrian physicist Erwin Schrödinger, which he designed to illustrate a paradox of quantum superposition wherein a hypothetical cat may be considered both alive and dead simultaneously because its fate is linked to a random event that may (or may not) occur. 

Schrodinger's cat illustration showing two cats side-by-side. One cat is alive and standing up and the other is laying down, supposedly dead
Illustration of Schrödinger’s Cat. | Image: Sara A. Metwalli

What Is Schrödinger’s Cat in Simple Terms?

Schrödinger’s Cat, as a thought experiment, states that if you seal a cat in a box with something that can eventually kill it, you won’t know if the cat is alive or dead until you open the box. So, until you open the box and observe the cat, the cat is simultaneously dead and alive. 


How Does Schrödinger’s Cat Work?

We often use Schrödinger’s thought experiment to explain the concept of superposition. The experiment states that a hypothetical cat is locked in a box with some radioactive substance controlling a vial of poison. When the substance decays, it triggers a Geiger counter that causes the poison to be released, thereby killing the cat. 

Since the box is locked, and we on the outside don’t know whether or not the radioactive substance has decayed and released the poison, we can’t tell if the cat is dead or alive. So, until we open the box to know for sure, the cat is both dead and alive. Mathematically speaking, there’s a 50 percent chance the cat is dead and a 50 percent chance the cat is alive. 

More From Built In’s Tech DictionaryWhat Is Superposition?


How Is Schrödinger’s Cat Both Alive and Dead?

In quantum mechanics terms, the cat’s ability to be in an ambiguous state of both alive and dead until it’s observed (i.e. when someone opens the box) is referred to as quantum indeterminacy or the observer’s paradox. The paradox states that an event or an experiment’s observer affects its outcome. In this case, whomever is performing this hypothetical experiment can affect whether the cat remains in an unknown state or they can open the box and know if the cat is dead or alive with 100 percent certainty.

The experiment also points out when the resolution of possibilities occurs. The experiment is intended to make people ask themselves if it was logical for the observation to trigger the answer. After all, wouldn’t the cat be either dead or alive even if we never open the box? 

Schrödinger's cat: A Thought Experiment in Quantum Mechanics — Chad Orzel. | Video: TED-Ed
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Schrödinger’s Cat and the Role of the Observer

In quantum mechanics, the observer (the person conducting the experiment) has a role in the results of the experiment. In this case, we are unaware of the cat’s state until the observer opens the box. Until the observer opens the box, the cat exists in a superposition state; that is, the cat is both alive and dead. Only by opening the box and looking at what’s inside (i.e., observing it) is the cat’s state confirmed to be one of the two states. This is called The Copenhagen interpretation of quantum mechanics, which basically explains that a quantum system exists in all of its possible states at the same time. Only when we make an observation can we confirm the true state of the system.

More Quantum Reading From Built In ExpertsWhy Do Quantum Objects Keep Getting Weirder?


Why Do We Use Schrödinger’s Cat?

We still use this thought experiment today to explain quantum physics concepts in an easy-to-understand way. Some people also use Schrödinger’s Cat to talk more philosophically about how the thought experiment can be extended to other situations in life. For example, let’s say you meet your friend for a night out and you’re both unsure about what to have for dinner; until you reach an agreement or one of you decides for the group, the possible food option is “every option that can exist where you and your friend are.” Looking at things from this perspective has led many people to think of everything in life as “quantum” because until the future is here, it technically (according to Schrödinger) exists in a state of superposition of all possible scenarios.

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