Why are we concerned about the world at levels smaller than atoms? We care about this subatomic realm because we want to understand how the world works -- and because it is so wildly different from our everyday experience. This is the quantum world. You need only a few key concepts to view this domain – and you don’t need any math (unless you are or want to be a physicist). We do, though, need to put aside our experience of how the world works.
Quanta. For example, in our everyday, macroscopic world, energy is continuous. On a wintry day it is possible to have a temperature anywhere between, say, zero and 32 degrees. This is not always true in the quantum world. Electrons orbiting an atom can only have set energies – they can never have an energy between these points, even when moving to a higher or lower orbit. They jump instantaneously from one energy to another. This is the famous “quantum leap”.
It’s as if, when I measure the outdoor temperature, the reading can only be 15, 25 or 32 degrees – and nowhere in between. This is one way quantum physics (aka quantum mechanics) differs from the world we experience.
The quantum principle – things can only be in multiples of specific-sized chunks and nothing in between – is a recurring idea. For example, photons (particles of light or other electromagnetic energy) can have different energies but there can’t be half a photon. There is also some reason to believe space itself is not continuous but at an extraordinarily tiny level is divided into discrete minimum distances so nothing can move less than that amount.
Quanta were first proposed by Max Planck, a German physicist in 1900. Yes, it’s been 100 years or so, depending on how one counts it, since quantum physics was developed – and there is still much we do not understand. Planck devised quanta out of frustration in searching for a formula that correctly predicted the electromagnetic radiation emitted from a hot object that at room temperature would be black, a “black body”. His formula worked but Planck initially thought quanta were only a mathematical trick and did not exist. It turned out he was mistaken. More on that in a bit, but first some on electromagnetic radiation such as light.
Wave-Particle Duality. From “classical” physics, the physics that more or less conforms to how we experience the world, we tend to believe that electromagnetic energy (everything from radio waves to visible light to gamma radiation) comes in waves – imagine the ripples after you throw a rock in a pond – and matter comes in particles, traditionally atoms. (We now know atoms are made of electrons, protons and neutrons and the latter two are made of quarks.) But in the quantum realm, the notion that energy takes the form of waves and matter takes the form of particles is not necessarily true.
Albert Einstein, German physicist and anti-war activist, theorized (in a 1905 paper about the photoelectric effect) that light comes in discrete packets (quanta), now called photons. In essence he proposed that quanta were not just mathematical but real. There long had been debate over whether light was waves or particles, and waves were the dominant belief until then. But Einstein’s revelation was not the end of the story.
If you shine light (photons) through two parallel slits with the right size and distance, the light will form a nice interference pattern. That’s a classic indication of a wave. Where a crest in the wave of light coming from one slit runs into a crest in the wave of light coming from the other crest there will be a bright spot, and where a crest and a trough meet they cancel each other out. So far, so good.
If, though, you fire single photons at the slits one at a time, something very strange happens: If you mark where each photon strikes the screen behind the slits, you see a wave-interference patterns build up. Each individual photon is manifesting as a wave and interfering with itself.
If you get clever and decide to detect which of the two slits each photon goes through, you can determine the slit it entered all right – but then the wave pattern will not emerge even though you shoot the same number of single photons through as before.
So, is light a wave or a particle? It acts like a wave in some circumstances and a particle in others, though never both at the same time. Light contains both possibilities.
And it turns out that matter, for example an electron, is exactly the same: It will act like a particle in some situations and a wave in others. (An electron in orbit around an atom is better seen as a cloud than as a particle for this reason.) This is true even with matter at our level; our bodies, for example, have wave characteristics and particle characteristics. It’s just that at our size the wave aspect is mightily swamped by the particle characteristics so we don’t ever observe the wave. Still, the wave aspect really does exist.
The Heisenberg Uncertainty Principle. There are other ways our experience does not work in the quantum realm. In our world, we can measure both the position of a moving object and its velocity. A batter hits a ball and we can see where the ball is, where it’s going and how fast. Not so in the quantum world. In 1927 German physicist Werner Heisenberg – who loved hiking and later may or may not have hindered the Nazis in their attempts to construct an atomic bomb – set out the Heisenberg uncertainty principle. It says the more one learns about the position of a particle in a quantum state the less one can know about its velocity – and vice versa. (There are other pairs of variables, such as time and energy, where this applies as well.)
This is not a problem caused by our instruments being so large in comparison to a subatomic particle that any measurement forces a change in its velocity or its position. Instead, this indeterminacy is an essential feature of the quantum world; it is built in.
It is very odd in comparison to how we experience things. Returning to our batted ball, this is equivalent to saying that the more we know about the baseball’s position the less we can know about its velocity.
Superposition. Actually, the situation is even wilder: When a photon or electron is traveling before it interacts with something, it can be in what we call a quantum state. We don’t know exactly where it is until it hits (or is measured or influenced by) something. For us, objects have definite positions even when moving. When a batter hits a ball, we can see where it is. This is not the case at the quantum level. In 1926 Austrian physicist Erwin Schrödinger, while ensconced in a villa in the Swiss Alps with his mistress, developed an equation about the quantum world. It is a wave equation that, among other things, describes the probability of finding a photon or electron at a given position at a given time.
And that’s the rub: It can only describe probabilities and not certainties. Much effort has been spent trying to determine the location or path of a photon or electron in a quantum state but all evidence so far indicates we can never do so. One way to interpret this is that, until it interacts with something, the photon or electron exists across all possible positions at once, which is called superposition (although it’s probably better to think of this in terms of probabilities of where the particle might be found).
On our level, it’s as if, once the batter hits the ball, the ball disappears and does not reappear again until it lands – and it could land anywhere and nothing we can do (radar, computers, etc.) can predict with certainty where it will land. We would know only the probability of it landing in any particular place.
This randomness has other consequences as well. An electron classically may not be able to pass through an insulator, but because its position is a probability there is always a small chance it can be found on the other side. Although most electrons will be blocked by the insulator, a few will inevitably turn up on the other side of the insulator, in a process called electron tunneling.
Schrödinger’s Cat. Schrödinger always regretted discovering his equation; he did not like the fact it provided only probabilities. It also led to another issue: If items in a quantum state only have probabilities, how does this fit with the world we know where something can only be in a definite state?
To illustrate this dilemma, Schrödinger designed a thought experiment involving a cat. (This was not a real experiment and no cats have ever been subjected to this.) A cat is put in a box with a vial of poison and a mechanism that will break the bottle if a radioactive material emits a decay particle. Radioactive decay is governed by quantum principles and is therefore random within certain limits. The box is to be opened at exactly the time where there is a 50-50 chance radioactive decay has occurred. Is the cat alive or dead just before the box is opened or somehow both alive and dead?
The point is that for us a cat must be either alive or dead, it can’t be both. Yet when items are in a quantum state, they can be simultaneously in contradictory states. How, then, does the uncertain quantum world result in the certainties of the world at our level? One might think a particle traveling in a quantum state simply loses its quantum properties and is no longer governed by the Schrödinger equation immediately on smacking into something.
Instead, the particle’s quantum nature seeps into the environment (and vice versa) in a process called “decoherence”; the concept was introduced by German physicist H. Dieter Zeh in 1970. It’s similar to dripping hot wax into water: The heat of the wax dissipates into the water and the wax takes a hard shape. With quantum decoherence, though, what happens is the particle loses its quantum state.
Despite knowing this mechanism, we still do not understand why a particle in a coherent quantum state slams into our double-slit test screen at a particular location and not another one. There are a number of competing theories but the truth is we don’t know. The quantum equations are superb: they allow calculations to some of the best precision in all of science. Yet what the equations mean in terms of what is actually happening in the quantum world is the subject of great debate.
Einstein hated the randomness of the quantum world as well. “God does not play dice” was his expression (translated liberally from the German). It’s understandable why he felt that way. Sir Isaac Newton opened the era of modern science and technology by showing we could explain and predict the world with mathematics: If you know where the balls are on a billiards table and you know how hard and in what direction the cue ball will be struck, you can predict the resulting resting positions of the post-contact balls. In short, determinism. Replace that with mere probabilities and you may limit the usefulness of science itself, at least at the quantum level. Still, it is now more certain than ever that this is how the quantum world operates. Ironically, Einstein, could never accept this, despite being one of the founders of quantum physics (recall his theory that light really comes in quanta, discrete packets of energy called photons).
Quantum Entanglement, Locality and Realism. Further, in our world two objects can only affect each other if they come in contact or something carries a force from one to the other. This is called “locality” and is one reason why magic does not exist, much as we might like it to. I can’t make a balloon move by merely willing it to move, although I can move it with a fan or by swatting it. In quantum mechanics things are different. Two particles in a quantum state that are brought close together can share a mutual wave function. (Remember that matter has both a particle aspect and a wave aspect.) When that happens, we say the particles are “entangled”, that they share certain properties.
Realism is the belief that even a quantum particle must have objective values that exist even before it is measured or interacts with something. As Einstein said, the moon is still there whether I’m looking at it or not. That is certainly true in our world, but perhaps not at the quantum level.
This can be tested. One characteristic subatomic particles have is “spin”, a property that can have a magnetic effect pointing “up” or “down”. (“Spin” is a terrible name that came from an initially mistaken belief the particles were spinning and therefore creating magnetic fields like a generator would.)
One can entangle two particles and then separate them by a large distance and retain the entanglement. At this point both particles are in a shared quantum state and so neither has an “up” spin or a “down” spin; their spins are indeterminate. You can force one of the particles out of its quantum state, though, and it will immediately show either an up or down spin. What is amazing is that instantaneously the other particle will take exactly the opposite spin due to their just-ended entanglement.
Even more astounding, this happens faster than the speed of light, meaning faster than an electromagnetic wave (or any information) can travel from one particle to the other. And, as Einstein showed in his theory of special relativity, nothing can travel faster than the speed of light. Given this, can this entanglement effect really be the entire story by itself or is something else involved, something hidden?
The EPR Paradox. In a challenge, Einstein and some colleagues, Podolsky and Rosen, (“EPR”) argued in 1935 this presented a paradox: Either locality (or realism) was being violated --- resulting in what Einstein derisively called “spooky action at a distance” --- or there were hidden factors (variables) which, if we only could discover them, would explain what was happening.
But at the time physicists were busy discovering, explaining and using new subatomic particles and forces -- and there seemed no way to test what was occurring with the EPR challenge. Physicists were even told spending time on what quantum mechanics means could end their careers. The mindset later became known as “Shut up and calculate” -- and not worry about how entanglement works.
It was not until 1964 that Irish physicist John Stuart Bell discovered it was possible to test whether hidden variables existed. His Bell Inequalities said if forcing one entangled particle into a definite state really did result in the other particle instantaneously showing the opposite state without any hidden factors, then experimental results would show a higher correlation between the particles’ end states than they would according to chance. At the time, the technology did not exist to test this theory.
But beginning with Alain Aspect's work in 1972 – although perhaps not decisively resolved until 2015 by Ronald Hanson at Delft University – experiments show the correlation is substantially greater than chance, so either locality or realism (or both) is not true at the quantum level. That’s certainly not how the world works at our level.
Note that even though quantum entanglement is real (and there are no hidden factors), it does not allow communication to happen faster than the speed of light. While the first particle can be forced to exhibit either an up spin or a down spin, there is no way to control which it will exhibit – and so no way to send a message.
Conclusion. The subatomic world is wildly different from our own. In this realm, energies often are not continuous but occur in discrete chunks called quanta. Energy and matter can manifest either as particles or as waves. The more one can determine the position of a particle, the less one can know about its velocity, and vice versa. Particles and photons in a quantum state seem to have no specific position or velocity until we measure them or they interact with something else. How this indeterminacy leads to the certainties at our level is unknown. Entangled particles somehow affect each other instantaneously despite being separated by large distances and having no known way to influence each other.
Still, the quantum equations are superb: They allow calculations to some of the best precision in all of science and have never been found wrong. Yet what the equations mean in terms of what is actually happening in the quantum world is the subject of great debate. This is what is known as interpretations of quantum mechanics and includes such approaches as the many-worlds interpretation, objective collapse, consistent histories, and the (somewhat amorphous) Copenhagen Interpretation.
Research continues into quantum states, how they collapse, entanglement, locality and realism. Many books on quantum physics are available for non-physicists (and are light on the math). Just don’t get a physics textbook by mistake, and avoid the books invoking mysticism; quantum physics is already wild enough on its own. Welcome to the quantum world.
