Schrodinger's Cat (Simplified): What Is It & Why Is It Important?
In 1935 – two years after winning the Nobel Prize for his contributions to quantum physics – Austrian Physicist Erwin Schrödinger proposed the famous thought experiment known as Schrödinger's cat paradox.
What Is Schrödinger’s Cat Paradox?
The paradox is one of the most well-known things about quantum mechanics in popular culture, but it isn't merely a surreal and funny way to describe how the quantum world behaves, it actually strikes at a key criticism of the dominant interpretation of quantum mechanics.
It endures because it proposes the absurd idea of a simultaneously alive and dead cat, but it has some philosophical weight because, in a sense, this really is something that quantum mechanics might suggest is possible.
Schrödinger came up with the thought experiment for precisely this reason. Like many other physicists, he wasn't completely satisfied by the Copenhagen interpretation of quantum mechanics, and he was looking for a way to convey what he saw as the central flaw in it as a way of describing reality.
The Copenhagen Interpretation of Quantum Mechanics
The Copenhagen interpretation of quantum mechanics is still the most widely-accepted attempt to make sense of what quantum physics actually means in a physical sense.
It essentially says that the wave function (which describes a particle's state) and the Schrödinger equation (which you use to determine the wave function) tell you everything you can know about a quantum state. This might sound reasonable at first, but this implies a lot of things about the nature of reality that don't sit well with many people.
For example, a particle's wave function spreads across space, and so the Copenhagen interpretation states that a particle doesn't have a definitive location until a measurement is made.
When you make a measurement, you cause wavefunction collapse, and the particle falls into one of several possible states instantly, and this can only be predicted in terms of a probability.
The interpretation says that quantum particles actually don't have values of observables such as position, momentum or spin until an observation is made . They exist in a range of potential states, in what is called a "superposition" and can essentially be thought of as all of them at once, although weighted to acknowledge that some states are more likely than others.
Some take this interpretation more strictly than others – for example, the wave function could simply be viewed as a theoretical construct that allows scientists to predict the results of experiments – but this is broadly how the interpretation views quantum theory.
Schrödinger’s Cat
In the thought experiment, Schrödinger proposed placing a cat in a box, so it was hidden from observers (you can imagine this to be a sound-proof box, too) along with a vial of poison. The vial of poison is rigged to break and kill the cat if a certain quantum event takes place, which Schrödinger took to be the decay of a radioactive atom which is detectable with a Geiger counter.
As a quantum process, the timing of radioactive decay can't be predicted in any specific case, only as an average over many measurements. So with no way to actually detect the decay and the vial of poison breaking, there is literally no way to know whether it has happened in the experiment.
In the same way as particles are not considered to be in a particular location prior to measurement in quantum theory, but a quantum superposition of possible states, the radioactive atom can be considered to be in a superposition of "decayed" and "not decayed."
The probability of each could be predicted to a level that would be accurate over many measurements but not for a specific case. So if the radioactive atom is in a superposition, and the life of the cat depends entirely on this state, does this mean the cat's state is also in superposition of states? In other words, is the cat in a quantum superposition of alive and dead?
Does the superposition of states only happen at the quantum level, or does the thought experiment show that it should logically apply to macroscopic objects too? If it can't apply to macroscopic objects, why not? And most of all: Isn't this all a bit ridiculous?
Why Is It Important?
The thought experiment gets to the philosophical heart of quantum mechanics. In one easy-to-understand scenario, the potential issues with the Copenhagen interpretation are laid bare and proponents of the explanation are left with some explaining to do. One of the reasons it's endured in popular culture is undoubtedly that it vividly shows the difference between how quantum mechanics describes the state of quantum particles, and the way you describe macroscopic objects.
However, it also tackles the notion of what you mean by "measurement" in quantum mechanics. This is an important concept, because the process of wave function collapse depends fundamentally on whether something has been observed.
Do people need to physically observe the outcome of a quantum event (for example, reading the Geiger counter), or does it simply need to interact with something macroscopic? In other words, is the cat a "measuring device" in this scenario – is that how the paradox is resolved?
There isn't really an answer to these questions that's widely-accepted. The paradox perfectly captures what it is about quantum mechanics that is hard to stomach for humans accustomed to experiencing the macroscopic world, and indeed, whose brains ultimately evolved to understand the world in which you live and not the world of subatomic particles.
The EPR Paradox
The EPR paradox is another thought experiment intended to show issues with quantum mechanics, and it was named after Albert Einstein, Boris Podolsky and Nathan Rosen, who devised the paradox. This relates to quantum entanglement , which Einstein famously referred to as "spooky action at a distance."
In quantum mechanics, two particles can be "entangled," so that any one of the pair cannot be described without reference to the other – their quantum states are described by a shared wave function that cannot be separated into one for one particle and one for another.
For example, two particles in a specific entangled state can have their "spin" measured, and if one is measured as having spin "up," the other must have spin "down," and vice-versa, although this isn't determined beforehand.
This is a little difficult to accept anyway, but what if, the EPR paradox proposes, the two particles were separated by a huge distance. The first measurement is made and reveals "spin down," but then very shortly afterward (so fast that even a light signal couldn't have traveled from one location to the other in time) a measurement is made on the second particle.
How does the second particle "know" the result of the first measurement if it's impossible for a signal to have traveled between the two?
Einstein believed this was proof that quantum mechanics was "incomplete," and that there were "hidden variables" at play that would explain seemingly illogical results like these. However, in 1964, John Bell found a way to test for the presence of the hidden variables Einstein proposed and found an inequality that, if broken, would prove that the result couldn't be obtained with a hidden variable theory.
Experiments performed on the basis of this have found that Bell's inequality is broken, and so the paradox is just another aspect of quantum mechanics that seems strange but is simply the way quantum mechanics works.
- University of California, Riverside: Does Bell's Inequality Rule out Local Theories of Quantum Mechanics?
- APS Physics: Einstein and the EPR Paradox
- CERN: On the Einstein Podolsky Rosen Paradox
- IFL Science: Schrödinger's Cat: Explained
- National Geographic: The Physics Behind Schrödinger's Cat Paradox
- University of Oregon: Copenhagen Interpretation
Cite This Article
Johnson, Lee. "Schrodinger's Cat (Simplified): What Is It & Why Is It Important?" sciencing.com , https://www.sciencing.com/schrodingers-cat-simplified-what-is-it-why-is-important-13722577/. 5 December 2019.
Johnson, Lee. (2019, December 5). Schrodinger's Cat (Simplified): What Is It & Why Is It Important?. sciencing.com . Retrieved from https://www.sciencing.com/schrodingers-cat-simplified-what-is-it-why-is-important-13722577/
Johnson, Lee. Schrodinger's Cat (Simplified): What Is It & Why Is It Important? last modified August 30, 2022. https://www.sciencing.com/schrodingers-cat-simplified-what-is-it-why-is-important-13722577/
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Erwin Rudolf Josef Alexander Schrödinger (born on August 12, 1887 in Vienna, Austria) was a physicist who conducted groundbreaking work in quantum mechanics , a field which studies how energy and matter behave at very small length scales. In 1926, Schrödinger developed an equation that predicted where an electron would be located in an atom. In 1933, he received a Nobel Prize for this work, along with physicist Paul Dirac .
Fast Facts: Erwin Schrödinger
- Full Name: Erwin Rudolf Josef Alexander Schrödinger
- Known For: Physicist who developed the Schrödinger equation, which signified a great stride for quantum mechanics. Also developed the thought experiment known as “Schrödinger’s Cat.”
- Born: August 12, 1887 in Vienna, Austria
- Died: January 4, 1961 in Vienna, Austria
- Parents: Rudolf and Georgine Schrödinger
- Spouse: Annemarie Bertel
- Child : Ruth Georgie Erica (b. 1934)
- Education : University of Vienna
- Awards : with quantum theorist, Paul A.M. Dirac awarded 1933 Nobel Prize in Physics.
- Publications : What Is Life? (1944), Nature and the Greeks (1954), and My View of the World (1961).
Schrödinger may be more popularly known for “ Schrödinger’s Cat ,” a thought experiment he devised in 1935 to illustrate problems with a common interpretation of quantum mechanics.
Early Years and Education
Schrödinger was the only child of Rudolf Schrödinger – a linoleum and oilcloth factory worker who had inherited the business from his father – and Georgine, the daughter of a chemistry professor of Rudolf’s. Schrödinger’s upbringing emphasized cultural appreciation and advancement in both science and art.
Schrödinger was educated by a tutor and by his father at home. At the age of 11, he entered the Akademische Gymnasium in Vienna, a school focused on classical education and training in physics and mathematics. There, he enjoyed learning classical languages, foreign poetry, physics, and mathematics, but hated memorizing what he termed “incidental” dates and facts.
Schrödinger continued his studies at the University of Vienna, which he entered in 1906. He earned his PhD in physics in 1910 under the guidance of Friedrich Hasenöhrl, whom Schrödinger considered to be one of his greatest intellectual influences. Hasenöhrl was a student of physicist Ludwig Boltzmann, a renowned scientist known for his work in statistical mechanics .
After Schrödinger received his PhD, he worked as an assistant to Franz Exner, another student of Boltzmann’s, until being drafted at the beginning of World War I .
Career Beginnings
In 1920, Schrödinger married Annemarie Bertel and moved with her to Jena, Germany to work as the assistant of physicist Max Wien. From there, he became faculty at a number of universities over a short period of time, first becoming a junior professor in Stuttgart, then a full professor at Breslau, before joining the University of Zurich as a professor in 1921. Schrödinger’s subsequent six years at Zurich were some of the most important in his professional career.
At the University of Zurich, Schrödinger developed a theory that significantly advanced the understanding of quantum physics. He published a series of papers – about one per month – on wave mechanics. In particular, the first paper, “ Quantization as an Eigenvalue Problem ," introduced what would become known as the Schrödinger equation , now a central part of quantum mechanics. Schrödinger was awarded the Nobel Prize for this discovery in 1933.
Schrödinger’s Equation
Schrödinger's equation mathematically described the "wavelike" nature of systems governed by quantum mechanics. With this equation, Schrödinger provided a way to not only study the behaviors of these systems, but also to predict how they behave. Though there was much initial debate about what Schrödinger’s equation meant, scientists eventually interpreted it as the probability of finding an electron somewhere in space.
Schrödinger’s Cat
Schrödinger formulated this thought experiment in response to the Copenhagen interpretation of quantum mechanics, which states that a particle described by quantum mechanics exists in all possible states at the same time, until it is observed and is forced to choose one state. Here's an example: consider a light that can light up either red or green. When we are not looking at the light, we assume that it is both red and green. However, when we look at it, the light must force itself to be either red or green, and that is the color we see.
Schrödinger did not agree with this interpretation. He created a different thought experiment, called Schrödinger's Cat, to illustrate his concerns. In the Schrödinger's Cat experiment, a cat is placed inside a sealed box with a radioactive substance and a poisonous gas. If the radioactive substance decayed, it would release the gas and kill the cat. If not, the cat would be alive.
Because we do not know whether the cat is alive or dead, it is considered both alive and dead until someone opens the box and sees for themselves what the state of the cat is. Thus, simply by looking into the box, someone has magically made the cat alive or dead even though that is impossible.
Influences on Schrödinger’s Work
Schrödinger did not leave much information about the scientists and theories that influenced his own work. However, historians have pieced together some of those influences, which include:
- Louis de Broglie , a physicist, introduced the concept of “ matter waves." Schrödinger had read de Broglie’s thesis as well as a footnote written by Albert Einstein , which spoke positively about de Broglie’s work. Schrödinger was also asked to discuss de Broglie’s work at a seminar hosted by both the University of Zurich and another university, ETH Zurich.
- Boltzmann. Schrödinger considered Boltzmann’s statistical approach to physics his “first love in science,” and much of his scientific education followed in the tradition of Boltzmann.
- Schrödinger’s previous work on the quantum theory of gases, which studied gases from the perspective of quantum mechanics. In one of his papers on the quantum theory of gases, “On Einstein’s Gas Theory,” Schrödinger applied de Broglie’s theory on matter waves to help explain the behavior of gases.
Later Career and Death
In 1933, the same year he won the Nobel Prize, Schrödinger resigned his professorship at the University of Berlin, which he had joined in 1927, in response to the Nazi takeover of Germany and the dismissal of Jewish scientists. He subsequently moved to England, and later to Austria. However, in 1938, Hitler invaded Austria, forcing Schrödinger, now an established anti-Nazi, to flee to Rome.
In 1939, Schrödinger moved to Dublin, Ireland, where he remained until his return to Vienna in 1956. Schrödinger died of tuberculosis on January 4, 1961 in Vienna, the city where he was born. He was 73 years old.
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- Heitler W. “ Erwin Schrödinger, 1887-1961. ” Biogr Mem Fellows Royal Soc , 1961; 7 : 221-228.
- Masters B. “ Erwin Schrödinger’s path to wave mechanics. ” Opt Photonics News , 2014; 25 (2): 32-39.
- Moore W. Schrödinger: Life and thought. Cambridge University Press; 1989.
- Schrödinger: Centenary celebration of a polymath. Ed. Clive Kilmister, Cambridge University Press; 1987.
- Schrödinger E. “ Quantisierung als Eigenwertproblem, erste Mitteilung. ” Ann. Phys. , 1926; 79 : 361-376.
- Teresi D. The lone ranger of quantum mechanics. The New York Times website. https://www.nytimes.com/1990/01/07/books/the-lone-ranger-of-quantum-mechanics.html . 1990.
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