Why You Should Give Thanks For These Three Quantum Physics Phenomena – Forbes

This Thursday will be Thanksgiving in the US, which we celebrate by engaging in the most American of activities: eating way too much and watching football. The more serious reason for the holiday, though, is to serve as a day of reflection on the good things in our lives that were grateful for.

For me, like many physicists, that list includes quantum mechanicsand yours should, too. Dont believe me? You think quantum physics is too strange and disturbing to count as a thing to look on with gratitude? Let me try to prove you wrong with this list of quantum phenomena without which life as we know it would be impossible.

A gingerbread couple in their bathing suits working on the perfect tan.

You should be thankful for the particle nature of light. This is the place where quantum mechanics gets its start, with a desperate trick on the part of Max Planck as he tried to find a way to derive the black-body spectrum of light emitted by a hot object like the heating element in the toaster oven you use to heat side dishes for the Thanksgiving feast. Theres a (conceptually) simple way to approach this problem, obvious to physicists circa 1900: you count up all the frequencies of light the object might emit, allocate each one an equal share of the energy available due to the heat of the object, and then youre done.

The problem is, this method fails miserably, because there are infinitely more ways to emit short wavelengths (high frequencies) than long ones, and they suck up all the energy. The simple and obvious method predicts that any hot object should spew out a vast amount of ultraviolet and x-ray light. Thats not a great feature to have in a toaster oven.

Planck fixed this ultraviolet catastrophe by assigning each frequency of light a characteristic energy, a quantum, that depends on the frequency, and saying that light can only be emitted in integer multiples of that frequency one, two, or three quanta, but never half-a-quantum, or pi quanta. For really high frequencies, the characteristic energy is greater than the share of thermal energy that would be allocated to that frequency, so no emission is possible. The short-wavelength radiation is suppressed, the ultraviolet catastrophe is avoided, and your dinner rolls brown nicely rather than being incinerated by a storm of high-frequency radiation.

bstract scientific background

You should be thankful for the wave nature of electrons. The other half of the wave-particle duality, this is one of the features that people find most disturbing about quantum physics. Its easy to think of material objects as particles with a definite position in space and time, but just plain weird to imagine material objects as wavelike disturbances spread over some region of space.

But without this wave nature, atoms as we know them would be impossible. This was first realized back in 1913 when Ernest Marsden and Hans Geiger observed alpha particles bouncing straight backward from collisions with gold atoms. Their boss, Ernest Rutherford realized that this meant that most of the mass of the gold must be concentrated in a tiny nucleus at the center of the atom, and introduced the solar-system model of an atom that kids learn in grade school these days, with negatively charged electrons orbiting a positively charged nucleus.

The problem with this model, as many people quickly pointed out, is that an electron orbiting an atom would constantly be accelerating, and accelerating charges emit radiation. An electron in Rutherfords atom, according to classical physics, should spray out an enormous blast of x-ray radiation (something of a theme, here...), losing energy in the process and spiraling in to crash into the nucleus. Thats not a recipe for the existence of stable matter.

As a solution to this problem, Niels Bohr introduced the idea of the quantum atom in 1915, with electrons existing happily in certain special orbits without emitting any radiation. He didnt have a great justification for this, but the model was undeniably an empirical success. Justification of the idea came in 1923 when Louis de Broglie suggested the idea of electrons having wave nature. A wave-like electron orbiting a nucleus would pick out a special set of states, in which an integer number of waves fit around the circumference. This idea put Erwin Schrdinger on the hunt for a wave equation for the electron, which led to his eponymous equation and one of the first complete formulations of quantum mechanics.

As a physics professor, I am obliged to note that the electron is not, in fact, a wave orbiting in a nice circular path with a little wave-like overlay the real picture is more like a fuzzy ball of probability surrounding the nucleus in one of a limited number of possible states of well-defined energy. On a conceptual level, though, you can understand the whole idea in terms of the wave nature of electrons, so when you look around and see stable atoms that arent furiously emitting x-rays, you have quantum physics to thank.

Wolfgang Pauli (1900 - 1958), winner of the 1945 Nobel Prize for Physics, receives a chocolate ... [+] cockchafer from the Lindau Casino in Bavaria, 27th June 1956. (Photo by Keystone/Hulton Archive/Getty Images)

You should be thankful for electron spin. Bohrs quantum atom, as justified by de Broglie and Schrdinger, is a great thing, but the huge variety of atoms that we see, and the chemical bonds that make complex molecules and give Thanksgiving dinner its wonderful flavor require one more quantum element. This is maybe the strangest of the quantum properties, intrinsic spin, but without it, the everyday world would be impossible.

The fundamental problem is that Bohrs idea gives us a set of allowed energy states within an atom, and works great to explain cases where you only need to worry about a single electron, but it doesnt explain how to distribute multiple electrons among these states in a more complex atom. The simplest way to do this, in fact, would be just to pile all the electrons into the lowest-energy allowed state, which would not allow for the enormous variety of elements we see in the periodic table or the complex chemistry that makes life possible.

The justification for all of that comes from two things introduced by Wolfgang Pauli: one additional quantum property, and one new rule. The new property is now called the spin of the electron, a property that can take on one of two values (typically called up and down, because physicists always default to really boring names). The new rule is that no two particles with spin can share the exact same quantum state.

This Pauli Exclusion Principle limits any of the allowed states in a quantum atom to at most two electrons (one spin-up, one spin-down). This is exactly whats required to explain the pattern of chemical properties seen in the periodic table every time you add an electron, you fill up a state, forcing later electrons to go into higher-energy states. This gives us the rich variety of chemical compounds that make life possible, and big fancy dinners enjoyable.

As if that werent enough, electron spin is also essential for the very existence of macroscopic amounts of matter. In the late 1960s, Freeman Dyson showed that a collection of arbitrary numbers of electrons and nuclei can find a stable configuration only if theyre fermions that is, particles with the right sort of spin to be subject to the Pauli Exclusion Principle. Without that property, it would always be possible for a collection of electrons and nuclei to lower their energy by packing more tightly together, imploding and releasing an enormous blast of x-rays (you knew that was coming...).

(When I was researching and writing Breakfast with Einstein (from which most of these are taken), the most striking thing from the whole process was how essential electron spin is to basically everything. Its the weirdest quantum property in a lot of ways, but absolutely essential for the existence of all the things that make us think its weird.)

Thanksgiving or Friendsgiving holiday celebration party. Flat-lay of friends feasting at ... [+] Thanksgiving Day table with turkey, pumpkin pie, roasted vegetables, fruit and rose wine, top view

So, whether you celebrate the holiday or not, when you sit down to dinner on Thursday, you have quantum physics to thank above all else. Without the particle nature of light, the wave nature of electrons, and the Pauli Exclusion Principle, the cooking, eating, and mere existence of your dinner would be impossible.

Happy Thanksgiving, everyone!

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Why You Should Give Thanks For These Three Quantum Physics Phenomena - Forbes