Quantum Mechanics is the youngest of all theories of physics we have today, but its predictive power is mind blowing. In a span of less than 100 years, physicists have developed quantum mechanics to further incorporate other, existing theories of nature, resulting in quantum field theory. QFT is one of the most complete theories that explains how the cosmos works on a microscopic level.
A brainchild of Max Planck and Albert Einstein, quantum mechanics started off as a way to save electricity. Planck was studying the light emitted by a bulb at different temperatures in order to determine how to get maximum output from it. He soon realised, however, that the existing wave theories of light didn’t help at all. In fact, they were contradictory to the observations he was getting.
In an act of desperation, he threw away the existing theories, and established that Light could only travel in packets of Energy, called Quanta. Every quanta would carry only a fixed amount of energy. Planck essentially re-established the particle nature of light, one that was discarded by scientists ever since the time of Newton. While initially met with scepticism, it gained approval when Albert Einstein used this theory to successfully explain the photoelectric effect, winning a Nobel Prize in the process.
Today, quantum field theory, along with general theory of relativity, remains the most tried and accurately tested theories of nature that we have. But the theoretical predictions of quantum mechanics are somewhat bizarre and counter-intuitive to our everyday notions of the world.
Quantum mechanics, however accurate, makes predictions that puzzle even the smartest minds in the room. So it is natural to assume a non-physics person to be equally bewildered by the fact these predictions have actually been found to be true.
Principles of quantum mechanics, such as the Heisenberg Uncertainty Principle, give us an insight into the bizarre world of quantum mechanics. There are many such weird occurrences that we can reproduce in the lab, such as:
- A particle has no fixed position: When a thing is there, it is there. Your mobile has a fixed position, so does your T shirt. But photons and electrons? Not so much. An idea conceived by De-Broglie states that just like light has a wave and particle nature simultaneously, so does ordinary matter. Matter also obeys the wave particle duality, and every particle has an associated “Matter wave” with a wavelength
Since there is no fixed location of where a wave is, there is no fixed location for a particle as well. The particle location can be anywhere across the de Broglie wavelength. While this effect is ridiculously miniscule on larger scales, this effect is visible at atomic levels. Electrons orbiting the nucleus do not do so in a fixed orbit, like Bohr proposed. Instead, there is an “electron cloud” around the nucleus for every orbit, and the electron can be at any location in the cloud.
- Uncertainty: Physics evolved as man’s curiosity to find out the truth about the nature. However, in our quest for knowledge of everything, we have hit a dead end: we can never know everything. Quantum particles like to keep secrets, and do not share everything. Heisenberg Uncertainty Principle tells us that, in a broad sense, you cannot know all the variables and observables of a physical system. For instance, the position and momentum of a particle cannot be determined simultaneously; there will always remain some inaccuracy in measurement of one.
Heisenberg’s principle puts a limit on how much knowledge we can extract from a system, and applies to many other physical observables. If you know that angular momentum of a particle, you can only determine one of the three components of the spin, not more.
So Heisenberg tells us that it is impossible to determine everything with 100% certainty, except one thing: he is the one who knocks.
- Virtual particles: Oh yes, they exist. Energy and time also follow Heisenberg’s formula. The physical implication of this is that particles can appear out of nowhere and interact and disappear by pair annihilation, as long as it occurs in a miniscule time period. Such particles do not have any physical existence for longer duration but are theoretically possible, and hence are called Virtual particles. At high energy scattering, such particles keep coming into existence and disappearing, seemingly violating the principle of conservation of mass-energy. But quantum mechanics tells us that this violation is allowed as long as the time scale is short.
- Quantum Entanglement: You know something is crazy when even Albert Einstein refuses to believe it. We can have a pair of particles, such as photons, and have their physical observable properties linked to each other. Such particles are called Entangled Particles. It can be any observable, such as the particle spin.
Let us take two electrons and entangle them such as they always have opposite spins. Now, you take one electron in Japan and keep the other in London. If the electron in Japan has spin up state, you can straightaway conclude that the second electron is in a spin down state, without even measuring it. Now that seems kind of believable, but it messes with the Holy Grail of Physics – Einstein’s Theory of Relativity.
You can take one particle to a galaxy 100 million light years away, and then measure its’ spin to be, let us say, spin up. At the very same instant, you measure the second electron’s spin state here on Earth. It will still, always, be spin down. Information about the measurement travels instantly, at infinite speed, from one quantum entangled particle to other, in violation of Einstein’s principle that nothing can travel faster than light.
- Quantum Tunneling: Classical mechanics has a simple principle, if there is a potential, you need to have more energy than the potential barrier to cross it. If you don’t have enough energy, you will never be able to cross the barrier. Quantum Mechanics, on the other hand, is more chilled out. If you don’t have enough energy to cross a potential barrier, there is still a possibility that somehow, you will. In real world, that would be like throwing a ball on a wall, and somehow the ball goes through the wall without having enough energy to break it. While it seems funny and improbable, it can be experimentally verified, and also successfully explains alpha decay of heavy nuclei.
Quantum mechanics seems weird, not only to the common people, but also to people who study it. Nevertheless, it is the most accurate description of the world at small scales that we have, and someday, it might be expanded enough to explain the working of the entire cosmos.