This was the year 1920, Werner Heisenberg had been an assistant to Niels Bohr at his institute in Copenhagen where they were busy formulating a new quantum mechanical theory.
Forward to 1929,Heisenberg gave a series of invited lectures at the University of Chicago explaining the new field of quantum mechanics. The lectures then served as the basis for his textbook, “The Physical Principles of the Quantum Theory”, published in the year 1930.
The concepts that emerged from this book and the combined work of Heisenberg & Bohr, are collectively known as the ‘Copenhagen Interpretations’, which was, and till now is the most generally taught theory in Quantum Mechanics. Now, the theory holds that quantum mechanics does not yield a description of an objective reality but deals only with probabilities of observing, or measuring, various aspects of energy quanta, entities that fit neither the general idea of particles nor the general idea of waves.
But this theory was not perfect, and soon in 1935, an Austrian physicist Erwin Schrödinger, observed a flaw in the ‘Copenhagen Interpretation’. He saw that, the nature of measurement, or observation, is not well-defined in this interpretation.
So, motivated by the EPR article—named after its authors Einstein, Podolsky, and Rosen, he devised a solution for the flaw. He conducted, what physicists call, a thought experiment. The experiment is popularly known as Schrödingers Cat. And here’s where our super cat comes into the picture. Since this experiment requires no fancy equipment, and will happen completely inside our brain with no hazardous outcomes, let’s perform the experiment to understand things better.
Assume a metal box. Inside the box we place a live cat (Our Super Cat) along with a small amount of radioactive material (with a 50% chance of one atom decaying over the period of 1 hour) placed inside a Geiger counter. Now if an atom of the radioactive substance decays, the counter tube will discharge and will release a hammer through a relay which will shatter a small flask of Hydrocyanic Acid. Now we have to make sure that the devices inside the box are secured against direct interference from the cat. once we have done that, we close the metal box and leave it alone for a complete hour.
After one hour, what happened inside the box is still unknown to us, but there can be two major possibilities:
- Atom doesn’t decay. Flask intact. Cat alive.
- Atom decayed. Flask shattered. Cat dead.
If the cat is live then it will not see the flask shatter, and if its dead it will see the flask shatter. There can be no case in which, the cat is dead and it doesn’t see the flask shatter or otherwise.
Now the classical Copenhagen interpretation would suggest that until and unless the metal box is opened, we cannot determine the state of the cat, weather dead or alive. But Schrödinger suggested otherwise. He said that, inside the box the whole system will remain in a superposition of states, and both the realities will exist simultaneously until the box is opened. On opening the box, the two states will collide into one to form the objective reality.
This experiment was a phenomenon in the quantum mechanics field and opened many closed doors.
One such door is the spin of an atom. We have all studied in high-school chemistry about the spin of an atom. An atom is not a still object, and to state it in lay-man’s language, its like a spinning top, it can rotate clockwise or anti-clockwise. But there as well is another state where the atom has not yet decided which direction to spin into, in that case its in a superposition of states and is rotating both ways at once. This concept was easily conceivable once Schrödinger’s theory was devised.
This theory can also be applied in quantum computing which is predicted to be the future of computing. Here the phrase “cat state” often refers to the special entanglement of qubits (quantum bits) wherein the qubits are in an equal superposition of :
- all being 0 and
- all being 1; e.g.,
There have been many posed extensions to the Schrödinger’s Cat theory.
Wigner’s friend is a variant on the experiment with two external observers: the first opens and inspects the box and then communicates his observations to a second observer. The issue here is, does the wave function “collapse” when the first observer opens the box, or only when the second observer is informed of the first observer’s observations?
In another extension, prominent physicists have gone so far as to suggest that astronomers observing dark energy in the universe in 1998 may have “reduced its life expectancy” through a pseudo-Schrödinger’s cat scenario, although this is a controversial viewpoint.
Concluding this article let us remember our cat, trapped inside that metal box with Hydrocyanic Acid, a cat both alive and dead. What could it possible be called except a ‘Super CAT’.