things are usually straightforward and logical. A car is either at rest or moving, the stereo is either on or off, a cat is either dead or alive—one state or the other. There is no middle ground or a combination of states. However, the same can't be said about the world on the most fundamental level—on the scale of the most basic particle we’re made of. Onthat scale, a particle doesn't exist in one state or the other, but in a number of states all at once. Like Schrodinger explained, a cat (of the fundamental world) would not be either dead or alive. It would exist between the two states, both dead and alive. This, he called superposition, where a particle exists in a combination of states. Strange, right? But this is the strange realm called the quantum world, ruled by quantum physics. It has become quite a popular subject in recent years and the bulk of credit goes to science fiction. The theories of alternate universes and time travel are certainly keeping us entertained, but what exactly is this subject that is feeding the world of sci-fi with so much great content? What is quantum physics? 'Quantum' is a Latin word that literally means 'how much,' but it is used for talking about the minimum amount of some physical entity. Aptly, we use it to represent the most elementary particles that are the basics or 'the minimum' of this very big world. So, quantum physics deals with the foundation of our world – the electrons in an atom, the protons inside the nucleus, the quarks that build those protons, and the photons that we perceive as light. These constitute everything that we are made of, both matter and energy. That sounds pretty simple, right? So how is it any different from regular physics? That 'regular' physics is the physics that deals with Newton’s laws of motion and its related mechanics. This is more formally called Classical Physics. This is the physics of our day-to-day lives—the rolling of balls, the rotation of the earth, and the mechanics of engines. However, if classical physics is so wide-ranging, why was it insufficient? We must go back in time to the 1900s to witness the birth of Quantum Physics. This was a time when scientists were still unable to understand the observations of the photoelectric effect. Simply put, the photoelectric effect is a phenomenon in which light hits metal, resulting in the knocking off of electrons. According to 'normal' physics, the electrons should start escaping the metal when the brightness of the light exceeds a sufficient limit. Yet, in reality, it was the color of light that determined whether electrons would be emitted or not. This was when Albert Einstein turned to Max Planck’s equation. Planck's equation stated that light waves carry energy in small packets called ‘quanta’. This explained how each color of light had a specific amount of energy associated with its wavelength. This energy, in turn, was responsible for the electrons getting knocked off. Einstein used Planck's equation to propose that light not only acts as a wave, but also a particle, a particle later named photon. Since then, physicists realized that regular physics simply wasn't sufficient to understand the microscopic world. They termed this 'regular' physics Classical Physics, a physics of the past, and another branch of physics dealing with the fundamental world as 'Quantum Physics'. This world is therefore now called the 'quantum world'. Let's be honest, our brains are made for this macroscopic world, where they can understand the running of a car on a road or the mechanics of a pen, but they are unable to initially understand the quantum world. For instance, imagine you are standing in front of a massive wall and want to reach the other side. This wall is surrounded by deadly alligators, so your only options are either climbing it or making a hole through it. However, if you don’t have enough energy to do either, you won’t be able to reach the other side, right? Well, that is how the macroscopic world works. But in the quantum world, a particle can still go through such an energy barrier without sufficient energy. This ability is called quantum tunneling. This phenomenon is responsible for the nuclear fusion that fuels the energy production of stars. As complicated as quantum physics may seem to some, it gets special love from popular culture. You may often see the dramatization of unusual quantum phenomena depicted in our macroscopic world through movies and TV shows. Take for instance, the Marvel universe. Marvel movies are famous for their heavy use of “quantum physics”, at least the words, if not the actual physics! Even though most of it is fiction being labeled as quantum physics, there are some scenes that are based on theoretical concepts. One of these is the 'multiple realities' of Doctor Strange. First, let's understand the quantum physics on which it is based. This subject deals with particles in terms of probabilities. For instance, while talking about the position of an electron, a physicist might say, “The electron is most likely to exist in this range of length, say between 1 to 3 units.” However, the same thing can be interpreted as electrons existing in different realities at different positions in the possible range. So, it can exist at 2 units in one reality, while at 3 in another reality or any other possible number between its most probable range. In short, there are many realities covering each possible position of the particle. See, mathematics and its implications again! A massive magnification of this concept is what we see in Doctor Strange’s multiple realities. Each possibility of a reality is a separate universe in itself there, allowing there to be multiple universes. Another very famous use of this concept is in the beloved movie, 'Endgame,' specifically its idea of time travel. The movie understands that if we change the past, the future will also be changed. So, if they go back in time and kill Thanos, who will bring about the apocalypse and why would someone travel back in time to kill him then? This issue is called the Grandfather paradox. A solution to this paradox was given by David Deutsch. Yes, that 'David Deutsch' mentioned by Tony Stark in the movie. Deutsch said that the only way to remove this paradox is if we talk about the events in terms of probabilities in the same way we talk about particles. In other words, changing the past only has a certain probability of happening. That means that they might not even be able to kill Thanos in the past, probabilistically speaking! To sidestep this paradox, the clever thing they did was go back and bring all the Infinity Stones before they were destroyed. Returning them to the same time would avoid any change due to the missing Stones. One must admit that it was indeed a clever fix! Quantum physics has certainly fed a lot of fiction to the general masses, but it has also become a necessary area of knowledge. It has assimilated itself into other domains of science and is still expanding. For instance, quantum computers are aimed at doing tasks that are too complex for a classical computer. There is heavy focus on building a better communication system using quantum physics for better safety and privacy. Energy production via a nuclear reactor was only possible due to the study of nuclear fission via quantum physics. It is also deeply ingrained into the subject of astrophysics. After all, the best way to answer the questions of the universe is if we understand its most basic building blocks. Regardless of whether we fully understand this subject or not, it is certainly growing in importance. With more talk of quantum physics to come, it is safe to say that it will soon become an integral part of our lives.