Quantum Wonders

Dr. S. S. VERMA; Department of Physics, S.L.I.E.T., Longowal; Distt.-Sangrur (Punjab)-148 106

2019-05-24 04:41:55

Credit: pixabay.com

Credit: pixabay.com

Quantum world in general and Quantum Physics in particular is full of miracles/wonders or so called weirdness concepts which are being practically getting proved with time. Switching over from classical mechanics domain to quantum mechanics i.e., from macro world consequences to micro world consequences, students always get mesmerized as well as confused with the unexpected possibilities offered by the quantum world.  Quantum mechanics taking basis on Planck’s hypothesis, Heisenberg’s uncertainty principle and de-Broglie’s matter waves, could easily justify its need by successfully explaining the concepts of black body radiation spectrum photoelectric effect and intensity of spectral lines which were unexplained earlier. Based on the duality nature of light (particle and/or wave), quantum mechanics describes any system with a wave function having no physical meaning itself but giving all the required information about the system like expectation values, probability, energy and momentum etc. 

The wave function has to exhibit some characteristics in order to act as a well behaved function. Students are introduced with the formation of a Schrodinger wave equation (time dependent and time independent) for quantum state. Schrodinger wave equation for a given system with known initially states of energy and momentum, can be solved to find out the wave function followed by the extraction of other information about the system with time or without time. Using Schrodinger wave equation gives unique outcomes for energy and probability of a particle in a infinite box/well, finite box/well like: discrete (quantized) nature of energy, minimum energy value, eigenfunctions and eignvalues, probability of particle (paly hide and seek), transmission and reflection coefficients of a particle from a potential step with higher or lower energies. Most important concept of tunneling of a particle through a barrier with a much higher energy than the particle which has many applications like Tunneling Electron Microscope and Tunnel Diode can only be explained on the basis of quantum mechanics. Further, behavior of simple harmonic oscillators as well as energy spectrum of single, double, or many electron atoms can only be explained successfully on the basis of quantum mechanics.

This is only a tip of the iceberg based on quantum mechanics which is said to be a science of probabilities. Contradicting the science of probabilities, great scientist Albert Einstein was a strong opponent of quantum mechanics and used to say that “God never play dice” in running the nature and wanted to say that anything in the universe is not random as stated in quantum mechanics.  According to Nobel prize-winning physicist Richard Feynman “Nobody understands quantum mechanics.”  The explanations attempted here use the most widely accepted framework for thinking about quantum weirdness, called the Copenhagen interpretation after the city in which Niels Bohr and Werner Heisenberg thrashed out its ground rules in the early 20th century. With its uncertainty principles and measurement paradoxes, the Copenhagen interpretation amounts to an admission that, as classical beasts, we are ill-equipped to see underlying quantum reality. Any attempt we make to engage with it reduces it to a shallow classical projection of its full quantum richness. However, quantum mechanics is proving right or exceptionally good in explaining many happenings in the micro world and is becoming a tool for new experimental techniques in order to explore things at atomic level. Therefore, some of the quantum wonders are summarized here:

Particle or wave duality

The oldest and grandest of the quantum mysteries relates to a question that has exercised great minds at least since the time of the ancient Greek philosopher Euclid: what is light made of? Isaac Newton thought light was tiny particles – “corpuscles”. Then, Thomas Young showed how a beam of light diffracted, or spread out, as it passed through two narrow slits placed close together, producing an interference pattern on a screen behind just as if it were a wave. So which is it, particle or wave? Quantum theory provided an answer soon that light is both a particle and a wave – and so, for that matter, is everything else. A single moving particle such as an electron can diffract and interfere with itself as if it were a wave. Louis de Broglie in 1924 showed that by describing moving particles as waves, could explain why they had discrete, quantized energy levels rather than the continuum predicted by classical physics.

Schrödinger’s cat

Quantum watched pots do refuse to boil – sometimes and at other times, they boil faster. At yet other times, observation pitches them into an existential dilemma whether to boil or not. This madness is a logical consequence of the Schrödinger equation to describe how quantum objects evolve probabilistically over time. Imagine, for example, conducting an experiment with an initially undecayed radioactive atom in a box. According to the Schrödinger equation, at any point after start the experiment the atom exists in a mixture, or “superposition”, of decayed and undecayed states as each state has a probability.  Over time, the wave function evolves as the probability of the decayed state slowly increases. As soon as we do look, the atom chooses – in a manner in line with the wave function probabilities – which state it will reveal itself in, and the wave function “collapses” to a single determined state. Suppose the radioactive decay of an atom triggers a vial of poison gas to break, and a cat is in the box with the atom and the vial. Is the cat both dead and alive as long as we don’t know whether the decay has occurred?

Casimir effect

Classically it is well accepted that “Nothing will come of nothing”. However, in the quantum world, it’s different: there, something comes of nothing. Specifically, two uncharged metal plates placed side by side in a vacuum will move (may be very minuscule movement) towards each other, seemingly without reason. Two plates with an area of a square metre placed one-thousandth of a millimetre apart will feel a force equivalent to just over a tenth of a gram. The Dutch physicist Hendrik Casimir first noted this minuscule movement in 1948. The Casimir effect is a manifestation of the quantum weirdness of the microscopic world. It has to do with the quantum quirk known as Heisenberg’s uncertainty principle, which essentially says the more we know about some things in the quantum world, the less we know about others.

The Elitzur-Vaidman bomb-tester

A BOMB triggered by a single photon of light is a scary thought. If such a thing existed in the classical world, you would never even be aware of it. Any photon entering your eye to tell you about it would already have set off the bomb, blowing you to kingdom come. With quantum physics, you stand a better chance. According to a scheme proposed by the Israeli physicists Avshalom Elitzur and Lev Vaidman in 1993, you can use quantum trickery to detect a light-triggered bomb with light – and stay safe a guaranteed 25 per cent of the time. The secret is a device called an interferometer. It exploits the quantumly weird fact that, given two paths to go down, a photon will take both at once. We know this because, at the far end of the device, where the two paths cross once again, a wave-like interference pattern is produced.  To visualize what is going on, think of a photon entering the interferometer and taking one path while a ghostly copy of itself goes down the other. In Elitzur and Vaidman’s thought experiment, half the time there is a photon-triggered bomb blocking one path.  Only the real photon can trigger the bomb, so if it is the ghostly copy that gets blocked by the bomb, there is no explosion – and nor is there an interference pattern at the other end. In other words, we have “seen” the bomb without triggering it.

Spooky action at a distance

Entanglement is the idea that particles can be linked in such a way that changing the quantum state of one instantaneously affects the other, even if they are light years apart. This “spooky action at a distance”, in Einstein’s words, is a serious blow to our conception of how the world works. In 1964, physicist John Bell of the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, calculated a mathematical inequality that encapsulated the maximum correlation between the states of remote particles in experiments in which three “reasonable” conditions hold: that experimenters have free will in setting things up as they want; that the particle properties being measured are real and pre-existing, not just popping up at the time of measurement; and that no influence travels faster than the speed of light, the cosmic speed limit.

The field that isn't there

HERE’S a nice piece of quantum nonsense. Take a doughnut-shaped magnet and wrap a metal shield round its inside edge so that no magnetic field can leak into the hole. Then fire an electron through the hole. There is no field in the hole, so the electron will act as if there is no field. But, the wave associated with the electron’s movement suffers a jolt as if there were something there. Werner Ehrenberg and Raymond Siday were the first to note that this behaviour lurks in the Schrödinger equation. The Aharonov-Bohm effect is the proof that there is more to electric and magnetic fields than is generally supposed. You can’t calculate the size of the effect on a particle by considering just the properties of the electric and magnetic fields where the particle is. You also have to take into account the properties where it isn’t. Casting about for an explanation, physicists decided to take a look at a property of the magnetic field known as the vector potential. For a long time, vector potentials were considered just handy mathematical tools – a short hand for electrical and magnetic properties that didn’t have any real-world significance. As it turns out, they describe something that is very real indeed.

Superfluids and supersolids

In the real world, it’s quantum theory that gives superpowers. Take helium, for example and at room temperature, we can fill floaty balloons with it, or inhale it and talk in a squeaky voice. At temperatures below around 2 kelvin, though, it is a liquid and its atoms become ruled by their quantum properties. Close to absolute zero, it becomes superfluid helium and climbs up walls and flows uphill in defiance of gravity. It squeezes itself through impossibly small holes. The opposite might be said of superconductors. These solids conduct electricity with no resistance, making them valuable for transporting electrical energy, for creating enormously powerful magnetic fields – to steer protons around CERN’s Large Hadron Collider, for instance – and for levitating superfast trains. We don’t yet know how all superconductors work, but it seems the uncertainty principle plays a part. At very low temperatures, the momentum of individual atoms or electrons in these materials is tiny and very precisely known, so the position of each atom is highly uncertain.