What is quantum physics? Put simply, it’s the physics that explains how everything works: the best description we have of the nature of the particles that make up matter and the forces with which they interact. Quantum physics underlies how atoms work, and so why chemistry and biology work as they do.
But how should we explain quantum physics to a person who doesn’t know anything about it? Simple, we tell him the basics which will help the person to easily understand quantum mechanics.
Everything Is Made Of Waves; Also, Particles
There’s lots of places to start this sort of discussion, and this is as good as any: everything in the universe has both particle and wave nature, at the same time.
Of course, describing real objects as both particles and waves is necessarily somewhat imprecise. Properly speaking, the objects described by quantum physics are neither particles nor waves, but a third category that shares some properties of waves (a characteristic frequency and wavelength, some spread over space) and some properties of particles (they’re generally countable and can be localized to some degree). This leads to some lively debate within the physics education community about whether it’s really appropriate to talk about light as a particle in intro physics courses; not because there’s any controversy about whether light has some particle nature, but because calling photons “particles” rather than “excitation's of a quantum field” might lead to some student misconcept
This “door number three” nature of quantum objects is reflected in the sometimes confusing language physicists use to talk about quantum phenomena. The Higgs boson was discovered at the Large Hadron Collider as a particle, but you will also hear physicists talk about the “Higgs field” as a delocalized thing filling all of space. This happens because in some circumstances, such as collider experiments, it’s more convenient to discuss excitation's of the Higgs field in a way that emphasizes the particle-like characteristics, while in other circumstances, like general discussion of why certain particles have mass, it’s more convenient to discuss the physics in terms of interactions with a universe-filling quantum field
Quantum Physics Is Discrete
It’s right there in the name — the word “quantum” comes from the Latin for “how much” and reflects the fact that quantum models always involve something coming in discrete amounts. The energy contained in a quantum field comes in integer multiples of some fundamental energy. For light, this is associated with the frequency and wavelength of the light — high-frequency, short-wavelength light has a large characteristic energy, which low-frequency, long-wavelength light has a small characteristic energy.
This property is also seen in the discrete energy levels of atoms, and the energy bands of solids — certain values of energy are allowed, others are not. Atomic clocks work because of the discreteness of quantum physics, using the frequency of light associated with a transition between two allowed states in cesium to keep time at a level.
Quantum Physics Is Probabilistic
One of the most surprising and (historically, at least) controversial aspects of quantum physics is that it’s impossible to predict with certainty the outcome of a single experiment on a quantum system. When physicists predict the outcome of some experiment, the prediction always takes the form of a probability for finding each of the particular possible outcomes, and comparisons between theory and experiment always involve inferring probability distributions from many repeated experiments.
This is also the aspect of the theory that leads to things like particles being in multiple states at the same time. All we can predict is probability, and prior to a measurement that determines a particular outcome, the system being measured is in an indeterminate state that mathematically maps to a superposition of all possibilities with different probabilities.
Quantum Physics Is Non-Local
Another of the remarkable features of the microscopic world prescribed by quantum theory is the idea of non locality, what ALBERT EINSTEIN rather dismissively called “spooky actions at a distance”. This was first described in the “EPR papers” of EINSTEIN, Boris Podolsky and Nathan Rosen in 1935, and it is sometimes referred to as the EPR (Einstein-Podolsky-Rosen) paradox. It was even more starkly illustrated by Bell’s Theorem, published by John Bell in 1964, and the subsequent practical experiments by John Clauser and Stuart Freedman in 1972 and by Alain Aspect in 1982.
Nonlocality describes the apparent ability of objects to instantaneously know about each other’s state, even when separated by large distances (potentially even billions of light years), almost as if the universe at large instantaneously arranges its particles in anticipation of future events.
Nonlocality suggests that universe is in fact profoundly different from our habitual understanding of it, and that the “separate” parts of the universe are actually potentially connected in an intimate and immediate way. In fact, EINSTEIN was so upset by the conclusions on non locality at one point that he declared that the whole of quantum theory must be wrong, and he never accepted the idea of non locality up till his dying day.
For example, if a pair of electrons are created together, one will have clockwise spin and the other will have anticlockwise spin (spin is a particular property of particles whose details need not concern us here, the salient point being that there are two possible states and that the total spin of a quantum system must always cancel out to zero). However, under quantum theory, a superposition is also possible, so that the two electrons can be considered to simultaneously have spins of clockwise-anticlockwise and anticlockwise-clockwise respectively. If the pair are then separated by any distance (without observing and thereby decohering them) and then later checked, the second particle can be seen to instantaneously take the opposite spin to the first, so that the pair maintains its zero total spin, no matter how far apart they may be, and in total violation of the speed of light law.
In theory, the concepts of entanglement and non locality may have applications in communications and even teleportation, although these ideas are still largely hypothetical at this stage. Due to the effects of the uncertainty principle, the mere act of observing the properties of particles at a quantum level (spin, charge, etc), disturbs the quantum system irrevocably, and this would appear to prevent us from using this system as a means of instantaneous communication. However, Anton Zeilinger’s work at two observatories in the Canary Islands has shown promising indications that entangled particles can indeed be reconstituted in a different place
Quantum Physics Is Very Small
Quantum physics has a reputation of being weird because its predictions are dramatically unlike our everyday experience
This happens because the effects involved get smaller as objects get larger — if you want to see unambiguously quantum behavior, you basically want to see particles behaving like waves, and the wavelength decreases as the momentum increases.
This means that, for the most part, quantum phenomena are confined to the scale of atoms and fundamental particles, where the masses and velocities are small enough for the wavelengths to get big enough to observe directly. There’s an active effort in a bunch of areas, though, to push the size of systems showing quantum effects up to larger sizes.
Quantum Physics Is Not Magical
Quantum physics is most emphatically not magic. The things it predicts are strange by the standards of everyday physics, but they are rigorously constrained by well-understood mathematical rules and principles.
So, if somebody comes up to you with a “quantum” idea that seems too good to be true — free energy, mystical healing powers, impossible space drives — it almost certainly is. That doesn’t mean we can’t use quantum physics to do amazing things.
SO, there’s it. I have explained the essential concepts of QUANTUM MECHANICS.It should help you to build up some understanding about quantum mechanics.
I have probably left a few things out, or made some statements that are insufficiently precise to please everyone, but this ought to at least serve as a useful starting point for people who don’t have knowledge about QUANTUM MECHANICS.
