Schrödinger’s cat & other physics tales

Sir Isaac Newton, in 1687 published Philosophiæ Naturalis Principia Mathematica (Latin for Mathematical Principles of Natural Philosophy), which is regarded as one of the most important works in the history of science. Through his seminal book, he put forward his Laws of Motion, Law of Universal Gravitation, & derived Kepler’s Law of Planetary Motion. Newton thus marked the epoch of a great revolution in physics and shaped our understanding of the physical world in the centuries to follow.

In the spirit of what is called “Newtonian Mechanics”, we have made tremendous progress in developing theories for Wave Motion, Heat & Thermodynamics, and various other fields of physics. The binding principle of all Newtonian theories, whether it was obvious or not, was that we live in a deterministic world. That is to say, a complete knowledge of the initial conditions (popularly known as the state at ‘t = 0’), and the complete statement of the physical laws that govern it together is sufficient to determine with certainty its physical state at any time in the future. As a simple example, if we know the height from which I drop an apple then I will be able to assert with certainty its position and speed after any given time interval. This is also known as Physical Determinism.

Scientists thought that Newton’s work and the theories that were built upon it was all that was to be known of the physical world. But that was not meant to be. With the advancement in engineering technology, experimentalists could perform exotic experiments, which dealt with the physics of the subatomic world. In the 19th century, physicists started observing some experimental results, which couldn’t be explained using any existing theory. New physics was the need of the hour.

Quantum shift

On 14th December 1900, German theoretical physicist Max Plank published his seminal paper on the Quantum Theory of Radiation, and quantum physics was born! In the early years of the 20th century, quantum physics was further developed and applied to explain several other experiments by Albert Einstein, Paul Dirac, Neils Bohr and many others.

Quantum theory is extremely non-intuitive, and came as a shock to the physics community, shaking the foundations of scientific knowledge. American physicist Richard Feynman once said, “I think I can safely say that nobody understands quantum mechanics.”

Though a century old now, its interpretations still puzzle scientists and students alike, making it a hot topic for research, with new ‘theories’ coming up all the time!

One of the ground-breaking postulates of quantum mechanics is is called the Wave-Particle Duality. If you look around on a beach, you’ll see waves in water and particles of rock, and they are nothing alike! Similarly, it was believed that light consisted of waves, while electrons are particles.  

But the wave-particle duality states that everything in the universe, from light to electrons to atoms, behaves like both a particle and a wave at the same time! Atoms could also behave like waves, while light could interact like particles. These have been experimentally observed and Albert Einstein won the Nobel Prize for his work on the particle behaviour of light, while French physicist Louis de Broglie is credited for postulating the wave nature of particles.

Indeterministic state of existence

Another important feature of the quantum theory is Superposition. As the name suggests, it states that an entity can exist in a ‘superposition’ i.e. in two or more states at the same time (state corresponds to any measurable quantity like position, speed, etc.), and it is only when we make a ‘measurement’ to learn its state, it collapses into one of the constituent states.

For example, suppose you set up an experiment to measure the speed of an electron in a superposition state of 1,000 km/hr and 10,000 km/hr. This means that the electron’s speed is in a superposition of 1,000 and 10,000, and only when you measure it you get one of the two values. But which of the two speeds will you obtain? It could be any one of them, one with a higher chance than the other, and when we perform this experiment, we can get completely different outcomes on repeating the experiment in identical conditions.

Back to the example, your friend has done the Math and tells you that each time you make the measurement to learn its speed, the chance that you will find its speed to be 1,000 is 60 per cent, and to be 10,000 is 40 per cent. This means that if you repeat this experiment a large number of times (say 1,000), then you may expect to measure 1,000 km/hr 600 times, and at 10,000 km/hr 400 times.

This is in stark contradiction to the principle of Physical Determinism and suggests that we live in an inherently random indeterministic world. This principle also has many philosophical implications. It is the “act of measurement” that gives the entity a definite state (appropriate to the measurement apparatus) and hence in a philosophical sense, it can be interpreted as “There is no event without an observer”.

Austrian physicist Erwin

Schrödinger, one of the founders of quantum mechanics, posed this famous question called Schrödinger’s cat, “If you put a cat in a sealed box with a device that has a 50% chance of killing the cat in the next hour, what will be the state of the cat when that time is up?” Common sense suggests that if we repeat this experiment with a lot of cats in lots of boxes, in half of them we expect it to be dead while in the other half it’s still alive.

According to quantum mechanics, though, when the time is up, the cat is in a ‘Superposition’ of being ‘dead and alive’ and it is our act of learning the outcome that causes it to manifest as one of the two possibilities. So, our curiosity kills the cat! As the American author, Neale Donald Walsch put it, quantum physics tells us that nothing that is observed is unaffected by the observer. That statement, from science, holds an enormous and powerful insight. It means that everyone sees a different truth because everyone is creating what they see.

(The author is pursuing electrical engineering at IIT Mumbai.)