The first blog in a three part series on the modern marvel of the century: quantum mechanics.
But what exactly is quantum mechanics, and why has it fascinated scientists for over a century? From the strange behaviour of particles such as photons and quarks to the possibility of revolutionary technologies like quantum computers, quantum mechanics lies at the centre of some of the most exciting ideas in modern science. Understanding it means stepping into a world where the rules of everyday life no longer seem to apply. Albert Einstein himself had a love-hate relationship with the idea. It challenges you to forget everything you think you know and embrace the unthinkable.
Quantum mechanics is probably the second most sought-after field in physics, following astrophysics. It is where ground breaking theories emerge, strange ideas challenge our understanding of reality, and physicists dream of having a discovery or theory named after them. Recently, quantum computing has pushed quantum mechanics further into the spotlight, with new breakthroughs and mysteries appearing almost every year.
Quantum mechanics is the description of the behaviour of subatomic particles such as photons and electrons, how they work and how they interact with light. Richard Feynman, a physicist in the mid-1900s said that particles are particles (not waves) and they can hop from place to place with a particular probability. To calculate the probability that the particle will be at a different place later is as follows: assign a probability to every point in the room and then add them up. This is called the path integral formulation and can be used to calculate the probability of a particle travelling from point A to B. If a particle travels from one corner of a room to another, the path integral formula can be used to calculate what the probability of that particle moving to another point in the room is.
Before the turn of the 20th century, scientists believed light was a transverse wave and particles could never be waves. According to Newtonian physics a hot object should emit infinite energy at short wavelengths. Red light gives out some energy; blue light gives out more energy and tiny wavelengths like ultraviolet gives out huge amounts of energy. Their math stated that a hot object should pour out endless energy in tiny wavelengths. However, there are some clear problems with this theory as it would mean some objects would have infinite energy which means, that energy would come from nowhere. We now know that this would go against the theory of conservation of energy and evidently made no sense. Their mistake became known as the Ultraviolet Catastrophe.
Max Plank in 1900 had a bold idea. He said that light energy does not come in smooth endless amount but instead it comes in tiny packets, called quanta. For example, you cannot use half a coin, you must use a whole coin. This idea meant that very tiny wavelengths need bigger energy packets (quantum), which meant that shorter wavelengths emitted more energy.
Einstein expanded on this further. He thought, what if light itself was made up of quanta. The colour of the light tells you how ‘energetic’ it is. The shorter the wavelength, or the more purple the colour, the stronger the packets. So, in theory a blue hot iron would actually emit more energy than a red-hot iron. Einstein used this to explain the photoelectric effect. This is when shining dim blue light on metals could knock electrons out, but very bright red light sometimes could not. This led Einstein to believe that the energy was not about the total brightness, rather about how strong each tiny packet is. The blue light had a small number of large packets, and the red light had a large number of small packets. He called these energy packets, photons.
Photons are a massless particle which means they do not feel a gravitational attraction. They move at the speed of light and have no rest mass; this means that a photon is always moving at the speed of light. These photons sometimes acted like waves and sometimes acted like particles. This meant that tiny particles could be in many possible states at once and its state only becomes definite once measured.
This arises from Schrödinger’s theory that a cat can be both dead and alive at the same time. He imagined that a cat is placed in a box with radioactive material that has a 50% chance of killing the cat in the next hour. At the moment just before you open the box, the cat is both alive and dead at the same time. It is only once you open the box that the cat’s single state is visible. This quantum theory perturbed Schrödinger so much that he gave up physics and moved to biology.
Quantum mechanics began as an attempt to explain strange mysteries about light and atoms which normal Newtonian physics simply could not explain, but it ended up completely changing our understanding of reality itself. From photons and wave-particle duality to uncertainty and superposition, quantum theory reveals answers to problems that had bizarre solutions earlier. Although many of its ideas seemed confusing at first, quantum mechanics has helped scientists build technologies like lasers, computers, and MRI machines, while also opening the door to future innovations such as quantum computing. Most importantly, it reminds us that not all questions are answered and certainly not all questions have arose.
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