Schrödinger's cat

In quantum mechanics, Schrödinger's cat is a thought experiment about quantum superposition. It was created by physicist Erwin Schrödinger in 1935 to show some problems with how people understand the tiny particles that make up the world. In this experiment, we imagine a cat in a closed box with a flask of poison and a radioactive source. If a tiny part of the radioactive material breaks apart, it could break the flask and kill the cat. If not, the cat stays alive. While we don’t look inside the box, the cat might be thought of as both alive and dead at the same time because we don’t know what will happen until we look.

This idea helps us understand how very small things, like atoms, can be in many states at once. But when we finally look, we always see the cat as either alive or dead, not both. This makes people wonder when the tiny world of atoms changes into the world we see every day.

Even though it was meant to point out problems, Schrödinger’s cat has become very famous. It is used in many talks and discussions about how quantum mechanics works. Real experiments have shown that very small objects can be in many states at once, but doing this with something as big as a cat would be very hard. The big question this experiment asks is how long these special states last and when they change into just one real situation.

Origin and motivation

Schrödinger created his famous thought experiment to discuss a paper written by Einstein, Podolsky, and Rosen in 1935. This paper talked about how strange quantum superpositions can be, where two particles stay connected even when far apart. Schrödinger wanted to show that some ideas about quantum mechanics didn’t make sense.

He imagined a cat in a closed box, where the cat’s fate depended on a tiny radioactive atom. If the atom decayed, the cat would die; if not, the cat would live. Schrödinger suggested that, according to some ideas at the time, the cat could be thought of as both alive and dead until someone looked inside the box. He didn’t believe this was true; instead, he used this example to show how strange those ideas seemed.

Thought experiment

In a famous idea, a scientist named Schrödinger asked us to imagine a cat placed in a sealed box with a special machine. Inside the box, there is a tiny bit of a special material that might, or might not, release something harmful to the cat during an hour. If it does, the cat would be harmed; if not, the cat would stay safe.

Schrödinger used this idea to show how some science ideas can be confusing. He wanted to explain that, in science, things can sometimes seem to be in two states at once until we look and find out what really happened. This helps us understand why scientists think carefully about what is real and what we can see.

Analysis

In simple terms, Schrödinger's cat shows a big question in quantum physics: when does something change from being in many places at once to being in just one place? The story asks, "when does a tiny part of nature stop being in two states at the same time and become just one state?"

Quantum physics talks about many possible results for tiny things, but we only ever see one result happen. This story helps us understand that strange idea. It makes us wonder why we think a cat can only be alive or not alive, not both at the same time.

Interpretations

Since Schrödinger's time, many ways to understand quantum mechanics have been suggested. These ideas try to answer questions about how long mixtures of states last and when (or if) they settle into one state.

Copenhagen interpretation

Main article: Copenhagen interpretation

A common way to think about quantum mechanics is the Copenhagen interpretation. In this view, when we measure something, we only see one result. This idea does not explain what happens to the cat while the box is closed. The description of the system includes both a decayed nucleus and a dead cat, and an undecayed nucleus and a living cat. Only when we open the box and look can we say for sure whether the cat is alive or dead.

Role of consciousness

Main article: Consciousness causes collapse

In 1932, John von Neumann described a pattern where a radioactive source is watched by a device, which is watched by another device, and so on. It does not matter where in this chain the mixture of states settles into one state. This chain could end if the last device is watched by a conscious observer. This idea suggests that a person's consciousness cannot be in multiple states at once. Eugene Wigner said that an observer is needed for the mixture to settle into one state, like either a live cat or a dead cat. Wigner talked about this in an experiment called Wigner's friend. Wigner imagined that a friend opened the box and saw the cat without telling anyone. From Wigner's point of view, his friend is now part of the mixture and has seen both a live cat and a dead cat. To someone else, Wigner himself becomes part of the mixture once he learns the result from his friend. This could go on forever.

Bohr's interpretation

Analysis of the work of Niels Bohr, a key scientist with the Copenhagen interpretation, suggests he thought the cat's state before the box is opened was unclear. The mixture itself did not mean anything physical to Bohr: the cat would be either dead or alive long before the box is opened, but the cat and box are one combined system. Bohr did not think a human observer was needed. Bohr stressed the classic nature of measurement results. An "irreversible" process gives the classic behavior of "observation" or "measurement".

Many-worlds interpretation

Main article: Many-worlds interpretation

In 1957, Hugh Everett created the many-worlds interpretation of quantum mechanics, which does not treat observation as special. In this view, both the alive and dead states of the cat continue after the box is opened, but they are separated from each other. When the box is opened, the observer and the possibly-dead cat split into one observer seeing a box with a dead cat and another observer seeing a box with a live cat. Because the dead and alive states are separated, there is no communication or interaction between them.

When the box is opened, the observer becomes linked with the cat, so "observer states" for the cat being alive and dead are formed; each observer state is linked with the cat so that the observation of the cat's state and the cat's state match. Quantum separation makes sure the different results do not interact with each other.

Ensemble interpretation

In Ensemble interpretations, mixtures are parts of a larger group of similar experiments. The description would not apply to one cat experiment, but only to the statistics of many similar cat experiments. Supporters of these ideas say this makes the Schrödinger's cat problem simple or not a problem. When the scientist opens the box, they simply find out which group that specific cat belonged to.

Relational interpretation

Main article: Relational quantum mechanics

The relational interpretation does not separate the human experimenter, the cat, and the apparatus; all are quantum systems following the same rules, and all can be thought of as "observers". But the relational interpretation allows that different observers can describe the same events differently, depending on what they know about the system. The cat can be thought of as an observer of the apparatus; meanwhile, the experimenter can be another observer of the system in the box (the cat plus the apparatus). Before the box is opened, the cat, by being alive or dead, has information about the state of the apparatus (the atom has either decayed or not decayed); but the experimenter does not have information about what is in the box. In this way, the two observers have different descriptions of the situation: To the cat, the wave description of the apparatus seems to have settled; to the experimenter, the contents of the box seem to be in a mixture. Only when the box is opened, and both observers have the same information about what happened, do both system states seem to settle into the same result, a cat that is either alive or dead.

Transactional interpretation

In the transactional interpretation the apparatus sends a wave backward in time, which combines with the wave the source sends forward in time, forming a standing wave. The waves are seen as real, and the apparatus is considered an "observer". In this interpretation, the wave mixture settles "at the same time" and happens along the whole interaction between the source and the apparatus. The cat is never in a mixture. Instead the cat is only in one state at any time, no matter when the human experimenter looks in the box. The transactional interpretation solves this quantum puzzle.

Objective collapse theories

According to objective collapse theories, mixtures are destroyed on their own (no matter what is observed) when some physical limit (of time, mass, temperature, irreversibility, etc.) is reached. So, the cat would be expected to settle into one state long before the box is opened. This could be loosely said as "the cat observes itself" or "the environment observes the cat".

Objective collapse theories need a change to standard quantum mechanics to allow mixtures to be destroyed over time. These theories could ideally be tested by creating middle-sized mixture states in the experiment. For example, energy cat states have been suggested as a precise way to test models related to quantum gravity energy separation.

Applications and tests

The Schrödinger's cat experiment is just a idea and has never been built. But scientists have done experiments with similar ideas. They have shown that bigger things than before can be in two places at once, though not as big as a cat. These states don’t last long, even when things are very cold.

Some examples include:

• A "cat state" has been made with light.
• A tiny particle called a beryllium ion has been in two places at once.
• A special device called a SQUID shows how many tiny parts can move both ways at the same time.
• A tiny tuning fork can vibrate and not vibrate at the same time. It has about 10 trillion atoms.
• Ideas have been suggested to test this with a tiny virus or a tiny living thing with a special tool.

In quantum computing, a “cat state” can mean several tiny parts being in many states together at once.

In popular culture

Schrödinger's thought experiment became well-known in the 1970s. Writer Ursula K. Le Guin helped make it popular with her short story titled "Schrödinger's Cat" in 1974. After that, many science fiction writers began using the idea in funny and creative ways. The story has been used in many books, movies, poems, plays, TV shows, cartoons, music, and online comics. It has even been the title of several works, showing how interesting and useful the idea is in telling stories.