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.

#### References

- 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