Schrödinger's Cat: The Quantum Paradox of Life and Death

 There is a dialogue by actor Mohanlal in the movie Pakal Nakshatrangal (Day Stars).

“Does this world belong to the living or the dead? The number of the dead is so many times more than ours. Aren’t we, the currently living, merely earthworms before their collective strength? Let’s put that aside… What is the guarantee that we are truly alive? Couldn’t the real world… life… be theirs? What if we die and step into a new life…?” 

Hearing the idea that we might step into another world after death may sound a bit philosophical, but there is a secret the universe has hidden within it. A hapless cat, teetering between life and death. What if the state of a radioactive particle determines the cat's life? Today, we are going to talk about Schrödinger's Cat, the simplest explanation of Quantum Theory—the scientific revolution of the twentieth century.

'Schrödinger’s Cat' is a Gedankenexperiment (Thought Experiment), meaning an experiment that can only be performed through thought. This is because there are some practical difficulties in performing it in reality. The scientist Erwin Schrödinger proposed this experiment in 1935. A cat is placed inside a moderately large steel box. In another corner of the same box, placed so as not to disturb the cat, is a radioactive substance. Radioactivity is a property where a nucleus of an atom is unstable and emits radiations. Generally, heavy atoms tend to exhibit radioactivity. I have placed a special type of radioactive material in one corner of this box. The peculiarity of this radioactive substance is that the probability of one atom decaying or not decaying in approximately one hour is 50-50. This means the probability is equal, just like the chance of getting a 'head' or a 'tail' when we toss a coin. Similarly, the chances of it decaying and not decaying are equal.

Next to this radioactive substance, I have placed a Geiger counter. A Geiger counter is a device that senses or detects radioactivity. Connected to this Geiger counter, I have placed a hammer and a vial of poison positioned such that the hammer will break it if it strikes. 

If an atom in the radioactive substance ever decays, the radiation from that decay will strike the Geiger counter in this device. The Geiger counter will activate, the hammer will move, smash the vial, and the poison will be released. Once the poison is released, the cat nearby will inhale the poisonous gas and die. This is how I have set up the box.

Erwin Schrödinger asks us some questions regarding that one hour of waiting. The question is simple: Is the cat alive right now, or is it dead? Here, the cat's life is decided by that single radioactive particle. The cat is saved only if that substance does not decay. If it decays, the cat must be dead. We first mentioned that the radioactive substance has a 50% chance of emitting one particle in one hour. Until we open the box and see the cat directly, the cat is both alive and dead in our minds simultaneously. Can an object simultaneously assume two different states? 

Through this experiment, Erwin Schrödinger intended to point out a fundamental problem with quantum theory. Before talking about what that problem is, we need to know a few other things. One of them is the difference between Classical Mechanics and Quantum Mechanics.

Classical Mechanics, or Newtonian Mechanics, is a completely Deterministic branch of science. Classical Mechanics offers us precise calculations regarding various properties of objects. For example, the position of an object, or its velocity, or its momentum, acceleration, the force applied on an object, the object's energy—we can precisely calculate all such measurements pertaining to an object. This is what is called the deterministic property. All measurements or all theories we talk about in Classical Mechanics are primarily theories with this deterministic character. We require precise measurements for everything.

Now, the characteristic of Quantum Mechanics, or mechanical theories concerning particles, is that they are Probabilistic. That is, we can only ever provide probable values in Quantum Mechanics.

It was the scientist Louis de Broglie who first stated that all objects around us exhibit two natures. One is Particle Nature; all objects around us are made up of particles. The second is that all these objects also exhibit a Wave Nature. That is, not just electrons, protons, and atoms as we mentioned earlier, but all objects show both particle and wave nature. In fact, as we go to smaller and smaller scales, this wave nature becomes more prominent. At the same time, a ball in our hand will never feel like a wave because its size is very large, so its wave nature has not really come out. The truth is that when moving to the atomic scale, every atom that makes up this ball exhibits wave nature quite well. The branch of science called Quantum Mechanics grew out of this theory presented by de Broglie. Many scientists have strived to develop it into its current form.

In 1927, Davisson and Germer conducted an experiment. Electrons were continuously fired into a crystal called nickel chloride. Their aim was to observe the pattern of the electrons bouncing back after hitting the nickel chloride. Electrons are particles. So, at the spot where the electrons hit, we should get a precise point. Using that, we can trace the path of the electrons. However, when this experiment was finished, they accidentally got a different result. The marks created on the screen were not the same as the marks created by a particle hitting the screen. Instead, the pattern they got when firing electrons was the same pattern they got when the same experiment was done using X-rays. We know that X-rays are electromagnetic waves. Through this experiment, they confirmed that the electron, which is a particle, behaves in the same way an electromagnetic wave behaves. If an electron is a wave, we can be sure that all objects, referred to as particles, have a strong wave nature. Thus, the branch of science called Quantum Mechanics, which only existed theoretically, has been verified experimentally.

Today's Quantum Mechanics has been built from the theories of countless scientists like Louis de Broglie, Erwin Schrödinger, Werner Heisenberg, Max Born, Max Planck, Albert Einstein, Paul Dirac etc. Although everyone contributed to this theory, they had minor differences of opinion among themselves.

Max Born was the scientist who first implemented the term Probability into Quantum Mechanics. If particles show wave nature, it will be difficult to measure many of their ordinary quantities. Instead, Born says that we can only measure the probabilities of these physical quantities. The wave nature of these particles can only be shown as a Probability Wave.

Here, I am turning an electron, which we see as a particle, into a probability wave. When it was previously considered a particle, we could think of the electron as a particle that could always be found at a precise point. But when we turn it into a probability wave, or to indicate the wave nature mentioned in Quantum Mechanics, we change it into a wave of probability. The special feature of the probability wave is that the possibility of the electron being found is what we indicate through this wave. The possibility of finding the electron is very high only at the peak of this 'Probability Wave'. However, as we move to other sides of the wave, the possibility of spotting the electron becomes very low. But understand that the possibility is not zero. We can spot the electron sometime. But the highest probability of the electron being found is at the center. Max Born said that all particles can be represented as a probability wave like this.

Let me also give a funny real-life example. Imagine an exhibition hall in a museum. I have displayed a crown in the middle of that exhibition hall. It is the crown of an old king. A thief comes to steal this crown. Our thief has entered the hall to steal the crown. The police have received information that the thief is inside. Where in this hall is it most likely the police will catch him? Certainly, the thief will be caught somewhere near the crown. If it's a thief who was determined to take the crown, he will definitely be found near the crown. There is a possibility that the thief is in other parts of the hall, but it is very low. But is there a possibility that the thief is outside this hall? No. Understand that the same applies to the case of particles. Max Born said that we can explain particles using probability waves.

Keeping this observation in mind, let's try to measure several properties of an electron. We can measure many properties like position, velocity, momentum, angular momentum, and spin. But one thing to keep in mind when measuring is that the value we get each time we measure is not necessarily the same. That is, if we can see an electron here now, when we measure it later, we will find the electron in a different place.

In a situation where we are not measuring, or in a situation where we are not observing, what would the electron be doing? The reason we get the electron from different places every time we measure its position is that the electron was situated in multiple places simultaneously until just before we measured it. That is, the electron's presence was in all places. It is only at the time we measure, or the time we observe, that all these possibilities combine and merge into a single point. We call this a ‘Wave function collapse’. If the electron is a wave, we change it into a mathematical model called the Wave function. The probability or possibility of this wave function, which was spread widely, shrinks to a single value at the time I measure or observe, and I only get that single value.

It is based on this theory that Niels Bohr, Werner Heisenberg, and some other scientists at their institute gave an explanation for our Quantum Mechanics. That is, the framework developed to explain the working of Quantum Mechanics is called the Copenhagen Interpretation. The Copenhagen Interpretation aims to provide an explanation or interpretation for many things that we can only see at the quantum level. Although the Copenhagen Interpretation states many things, the main thing we are going to discuss here is Quantum Superposition. Superposition means a state that shows more than one state simultaneously. It is said that all quantities associated with a quantum object—be it position, energy, momentum, or spin—any quantum property, is actually always in a superposition state. But this superposition breaks only at one time. The situation arises where all these states in superposition merge into a single value, in the manner of Wave Function Collapse, as we mentioned earlier. When a person from outside, or an observer, looks at or measures this system, the state of superposition ceases to exist and merges, or collapses, into one value. This is what the Copenhagen Interpretation says.

This is where the importance of a phenomenon called the Observer Effect comes in. That is, I am trying to observe from outside a quantum system that is running very smoothly. Or I am trying to take a measurement. In the second that this measurement takes place, this quantum system collapses. All the possibilities of that quantum system will shrink into one value, which is what we will get. That is, we have lost quantum information.

Erwin Schrödinger was not at all fond of this Quantum Superposition mentioned in the Copenhagen Interpretation. It is important to remember that Schrödinger is the person who discovered the wave equation which is the most basic foundation of Quantum Mechanics: Schrodinger’s wave equation. Such a person is emphatically saying that Quantum Superposition is impossible. The Schrödinger’s Cat Experiment was actually put forward by Schrödinger to criticize this Copenhagen Interpretation. The decay or non-decay of the radioactive atom placed inside the box is what decides the life and death of this cat. That is, according to Quantum Superposition, what we see inside the box is an atom that is simultaneously decaying and not decaying.

The question is, if you talk about superposition for microscopic atoms, why don't you talk about superposition for a large, macroscopic object like a cat? If your atom is in a superposition state, then my cat is also in a superposition state. Along with this, he added one more thing. There is a property called the Wave Function associated with a quantum object. That is, the wave function is a mathematical function capable of explaining all the characteristics of that quantum object. Schrödinger said that the wave function of an object has no connection with the reality of the object. Therefore, the various possibilities caused by the wave function cannot be simultaneously attributed to an object.

When Schrödinger presented this paper, Albert Einstein also came forward to support it. Albert Einstein once ridiculed the use of probability in Quantum Mechanics by saying, “God doesn’t play dice”. The truth is that even as we discuss whether the Copenhagen Interpretation is correct or what Erwin Schrödinger and Albert Einstein said is correct, we have not been able to reach a precise answer even today. However, based on the evidence we have received so far, the most effective interpretation of Quantum Mechanics that is accepted today is the Copenhagen Interpretation.

So, one may ask if the Schrödinger's Cat Experiment is wrong. In fact, we must say it is not wrong. This is because the Schrödinger's Cat Experiment tells us a few other things. If we make a specific change at the microscopic or quantum scale, it is difficult to bring that change to a macroscopic effect. That is, there will always be a boundary between the quantum scale and the macroscopic scale. Understand that Quantum Mechanics has a limitation in explaining the states of large objects. The experiment that talks about this limitation is the Schrödinger's Cat Experiment.

Another thing it tells us is that reality is never based on our observation. Reality is not something that only comes into existence when we perform an observation on a system. Reality is already there, and we are later observing that reality. This experiment says that there is no cat inside the box that is simultaneously dead and alive. The cat is either not dead, or the cat is dead. Anyway, the only way for us to know is to open the box and look. This becomes a little clearer when we move it from the case of the cat to the case of the radioactive particle. The radioactive particle has a probability of decaying and a probability of not decaying. But if we change the meaning from 'has a probability' to 'it must have either decayed or it must not have decayed', then we will definitely get one result in a later observation. By setting aside the state of superposition, we can later observe, assuming that a reality already exists there.

The truth is that even after a long time, Schrödinger was not ready to accept this quantum superposition. He said, “I do not like it, and I am sorry that I had any part in it”.

Now, what if we approach this experiment in a different way? Our character is the cat that is simultaneously dying and living. I slowly open the box. The cat is dead. But what if in another parallel universe, another 'me' opens another box just like this, and sees a cat that is alive inside? Here, reality has split into two. A sight where both possibilities simultaneously become reality. Does this experiment suggest the possibility of parallel universes? We can look at that another time.