Quantum physics is a branch of science that deals with the intricate details of matter and energy at tiny scales, accurately describing the behavior of atoms and light. In this realm, the predictable laws of classical physics we encounter daily become obsolete. In their place, a fresh set of rules emerged during the early 20th century. These rules introduce baffling concepts like wave-particle duality, entanglement, parallel universes, and superposition. Yet, to describe quantum phenomena in terms of tangible real-world entities might be misleading, as these quantum entities don’t conform to our conventional understanding.
Despite seemingly at odds with our intuition, these quantum principles have withstood the test of time and countless experiments. The impacts of quantum physics are ubiquitous, influencing every major scientific breakthrough and technological advancement since its birth. It’s thanks to quantum physics that we grasp the essence of our composition and the makeup of celestial bodies, as well as the intricacies of chemistry, biology, and all modern technology. From lasers and transistors to atomic clocks and medical imaging, quantum physics is the backbone of our high-tech world.
While it’s often quipped that “no one truly grasps quantum physics,” the reality is that many do. The challenge lies in articulating this comprehension. Often, physicists lean on simplified metaphors or analogies that might be misleading outside the contexts of laboratories and notebooks. It’s not a lack of understanding but rather a limitation in our language that makes discussing quantum realities a challenge. But since you’ve made it this far, I can see that you are up for it.
In quantum physics, energy is discrete (chunky).
In classical physics, conforming to our intuitions, energy is continuous, meaning it can change smoothly. Imagine the differences between classical and quantum energy as a slide versus a staircase. Classically, changes in energy can be arbitrarily small. Whereas quantumly, energy only comes in discrete amounts — called quanta, which are also small but ultimately finite.
This realization traces back to the late 19th and early 20th centuries. At that time, physicists encountered a series of puzzling phenomena that classical physics couldn’t explain. For example, the blackbody radiation problem, where classical predictions (dubbed the “ultraviolet catastrophe”) did not match observed results, and the photoelectric effect, where light shining on metals emitted electrons in a manner that didn’t fit the wave theory of light.
It was Max Planck who, in 1900, first introduced the concept of quantization to solve the blackbody radiation problem. He proposed that energy was emitted or absorbed in discrete units. Now appropriately called Planck’s constant, a fundamental constant of nature was introduced to describe the size of these energy chunks. It has the symbol ℏ and — when not simply called Planck’s constant — is called “h bar.” Following Planck’s lead, in 1905, Albert Einstein applied the idea of quantization to explain the photoelectric effect. He suggested that light itself was made up of discrete packets of energy, which came to be called photons.
The basic picture was completed by Niels Bohr in 1913 when he created the planetary model of atoms (now called the Bohr model), which is still the go-to visual representation of atomic structure and its charged particles. An atom contains a dense nucleus of protons (positive) and neutrons (neutral), which is orbited by electrons (negative). Crucially — or we can say quantumly — the electrons can only occupy a discrete set of fixed orbits. When electrons change orbits, they give or take a single photon with an amount of energy exactly matching the difference in orbital energy.
These foundational ideas set the stage for the first quantum revolution and had immediate and long-lasting implications.
Stability of Atoms: Quantum theory, with its quantized energy levels, explains why electrons don’t just spiral into the nucleus, which would be expected based on classical electromagnetic theory. This led to further ideas about the stability of nuclei and the discovery of entirely new fundamental forces.
Atomic Spectra: One of the most fruitful early implications was the explanation of atomic spectra. Atoms emit or absorb light at specific frequencies, leading to line spectra — something that can be precisely explained by the quantized energy levels of electrons in atoms. This allowed us to fingerprint elements and hunt for new ones.
Material Science: The understanding of discrete energy states became crucial in the development of semiconductors and transistors. Modern electronics, from computers to smartphones, owe their existence to our understanding of energy quantization.
In essence, the acknowledgment of energy’s “chunkiness” in quantum physics reshaped our understanding of the universe, paving the way for a myriad of technological advancements and deep philosophical contemplations about the nature of reality.
In quantum physics, everything displays both wave-like and particle-like behavior.
In classical physics, there are waves and particles — but they are separate things. Wave equations model familiar phenomena like the surface of the ocean and sound, while Newton’s laws of motion describe the trajectories of rigid bodies, like baseballs and planets. The Schrödinger equation in quantum physics is a wave equation that does not describe a physical wave or the trajectory of a physical object but shares similarities with both.
Waves spread out and interfere with one another, creating patterns of constructive and destructive interference. They also diffract, bending around obstacles. Particles, on the other hand, have definite positions and can be tracked as they move. They interact with other particles but do so through collisions that have predictable consequences.
Recall Einstein proposed that light, which was previously considered an electromagnetic wave, must also have particle-like qualities. In 1924, Louis de Broglie suggested that this dual wave-particle nature should also apply to matter. He proposed that everything — not just light, but all matter — has both wave-like and particle-like properties. Note this means that even everyday objects, like you and me, have an associated wavelength. However, for macroscopic objects, this wavelength is so unimaginably small that it becomes irrelevant.
The evidence supporting de Broglie’s hypothesis came shortly after, with experiments on electrons. When fired at a crystal, these subatomic particles display interference patterns just like light waves do when they diffract. A few years later, Erwin Schrödinger developed an equation to describe these waves. The only trouble was that the equation described the change in complex-valued variables, usually written ψ (Greek letter psi) and called the “psi function” or “wave function.” Like negative apples, complex numbers don’t seem to have a direct correspondence in the world. Max Born showed that the wave function could be used to obtain real numbers that accurately predict the probabilities for the outcomes of experiments.
Some important implications of this wave-particle duality are as follows.
Tunneling: Due to their wave-like nature, particles can “tunnel” through barriers even if they classically shouldn’t have the energy to pass. This is because waves are not localized but spread out in space. This effect has real-world implications in devices like tunnel diodes, which you can find as LEDs in most artificial sources of light these days.
Microscopy: Microscopes work because the wavelength of light is comparable to the size of the object we wish to resolve. Matter waves have much smaller wavelengths than light, so can resolve smaller things. Technologies like electron microscopy, where the wave nature of electrons is used to obtain detailed images at the atomic scale, are a perfect example of this.
Antimatter: Schrödinger’s equation was the inspiration for Paul Dirac to develop a similar one that respected the new rules of Einstein’s theory of special relativity. This relativistic equation implied the existence of a new form of matter — called antimatter — which annihilates when it comes in contact with ordinary matter.
The wave-particle duality is an elegant demonstration of how our classical intuitions can be completely upended when probing the universe at the quantum level. It reinforces the idea that at fundamental scales, our everyday logic needs to be replaced by a more encompassing, albeit counterintuitive, quantum view.
In quantum physics, some pairs of properties cannot simultaneously be defined, and attempting to measure one forces less precision in the other.
In classical physics, everything is predetermined, and the precision in actual measurement is only limited by imperfect engineering. In quantum physics, not everything we’d like to know about a system can be known in advance.
The principle of determinism in classical physics suggests that if one knows the present state of a system, then one can predict its future state with absolute certainty. In other words, if we knew the position and speed of every particle in a classical system, we could theoretically predict — using Newton’s laws — its future evolution indefinitely. However, quantum physics introduced a profound shift in this perspective.
In 1927, Werner Heisenberg introduced the uncertainty principle, which states that certain pairs of properties (known as complementary variables) cannot be precisely defined at the same time. The most famous example is the position x and momentum p of a particle. The more precisely we try to determine the position of a particle, the less precisely we can know its momentum, and vice versa. Mathematically, this relationship can be written as Δx Δp ≥ ℏ/2, where Δx is the uncertainty in position, and Δp is the uncertainty in momentum, and ℏ is, of course, Planck’s constant from before. When scientists talk about the uncertainty principle, they are usually referring to this specific equation.
The uncertainty principle has applications in understanding the mechanics of how quantum systems work, but it was also the first big philosophical departure from classical thinking in physics. In addition to being the basis of science jokes, is also the inspiration for Einstein’s famous quote, “God does not play dice!” There are, of course, more serious implications.
Fundamental Limit to Knowledge: Unlike the classical view, where the precision of our knowledge is limited only by our instruments, quantum physics imposes a fundamental limit to what can be known about certain pairs of properties. In essence, the universe seems to have randomness built into it. This could be useful in some technical applications, which is why things like this live stream of random quantum numbers exist.
Orbitals: Because of the uncertainty principle, we know that electrons in atoms don’t have definite orbits like planets in a solar system. Instead, contrary to the early visualizations of atoms, they exist in “probability clouds,” or what is now called orbitals. If you’ve taken high school chemistry, you’d recognize names like 1s, 2s, 2p, and so on.
Cosmology (to be read in the deep, sultry voice of your favorite science celebrity): The theory of inflationary cosmology posits that the universe underwent an exponential expansion in the first fractions of a second after the Big Bang. Quantum fluctuations, which are intrinsically tied to the uncertainty principle, were stretched to cosmological scales, forming the seeds for the large-scale structures we observe in the universe today, such as galaxies and clusters of galaxies.
In the realm of quantum physics, the very act of observation influences the state of what’s being observed. The uncertainty principle isn’t just a limitation of our current technology or methods — it’s a fundamental feature of the universe, highlighting the probabilistic and non-deterministic nature of the world. Far from being merely a philosophical curiosity, the uncertainty principle plays a vital role in understanding and designing advanced quantum technologies as well as our grandest theories of the universe.
In quantum physics, objects are mathematically described by adding fractions of classical states together.
In classical physics, objects have clear and well-defined states. For instance, if you flip a coin, it is either heads or tails, but not both simultaneously. Similarly, if a switch is on, it’s on, and if it’s off, it’s off. There’s no in-between. Quantum physics assigns states like these to things, but also other states that don’t have analogs in our everyday experience…but come ever-so-close to seeming like they do.
This concept of superposition was vividly caricatured in the famous thought experiment proposed by Schrödinger, now known as “Schrödinger’s cat.” In this hypothetical scenario, a cat is placed in a box with a radioactive atom, a Geiger counter, a vial of poison, and a hammer. If the Geiger counter detects radiation, the hammer smashes the poison, killing the cat. According to the principles of quantum physics, until the box is opened and an observation is made, the atom is in a superposition state of “decayed” and “not decayed,” which suggests the cat is both dead and alive simultaneously. Once observed, the superposed states collapse to a single outcome. Or, so the story goes. Luckily, no one has ever carried out the experiment.
Schrödinger’s point with his thought experiment was not to show that quantum physics predicts zombie cats but to challenge the prevailing wisdom that quantum effects like superposition are only things to worry about at atomic scales. Unfortunately, popular media didn’t get the point, and now we are stuck with the idea that superposition means things being in two places at once. Quantum physics does not directly say or imply this.
States are mathematical objects we assign to phenomena that we use in our theories to make predictions. Sometimes, the states can faithfully be interpreted through analogies to everyday experiences. We make copious use of these analogies as they provide an enormous conceptual economy. However, many states have no direct connection to our everyday experience. Using the typical analogies then quickly results in nonsensical statements and confusion. In essence, the mystery comes down to the fact that we must use everyday language to describe what we are doing as quantum physicists, even though the things we are talking about have no correspondence in the everyday world. In other words, don’t worry — there are no zombie cats. Superposition does have some pretty important consequences, though.
Interference: Superposition leads to interference effects. When you combine two quantum states, they can reinforce each other (constructive interference) or cancel each other out (destructive interference). This phenomenon underlies the results of the famous “double-slit experiment,” where particles like electrons or photons display interference patterns, a direct manifestation of their wave-like nature and superposition principle.
Molecular Bonding: The ability of particles to be in superposition states simultaneously is fundamental to the behavior of molecules. Atoms in molecules share electrons, which must exist in states that are superpositions of individual atomic orbitals.
Information: One of the most promising applications of quantum superposition is in the field of quantum computing. Classical bits in a computer are either 0 or 1. Quantum bits, or qubits, however, can be represented as superpositions of both 0 and 1. This capability could revolutionize computing, making certain problems tractable that are currently impossible for classical computers to solve in a reasonable timeframe.
In essence, the concept of superposition underscores the departure of quantum mechanics from classical intuitions. It encapsulates the probabilistic nature of quantum events and serves as a cornerstone for many quantum technologies on the horizon.
In quantum physics, correlated events have no common cause that can determine their outcome.
In classical physics, correlated events cause one another or can be traced back to a common cause
that could have determined the outcome. Everything has a “reason” in the classical world in terms of properties that existed before observation. Such a thing is not possible with quantum things.
The standard story goes like this. When two particles become entangled, the state of one particle becomes inexorably linked to the state of the other, no matter the distance between them. If you were to measure the state of one particle, you instantly affect the state of the other. This “spooky action at a distance,” as Einstein once termed it, doesn’t fit comfortably within our classical intuition. Luckily, for Einstein and us, that’s not really what’s happening with entangled particles. (It’s only what you are forced to conclude if you demand stringent objectivity in the world.)
Classical correlations are relationships between bits of information. The easiest high-level description of entanglement is as correlation between quantum bits (qubits) of information. At this level, we can already make use of the concept in technology and information theory — we don’t need to bring “physics” into it. This can be a good thing since “physics” is often seen as synonymous with “objective,” or what is real in the world independent of any observers. But that’s a very narrow view influenced by the success of classical physics. Quantum physics, with things like entanglement, throws a wrench into this neat classical picture of the world.
Most of what you hear and read about quantum entanglement is the shooting down of attempts to force it into a classical worldview, but framed with headlines like “quantum physicists just proved nature is spooky.” Technically, we call the results no-go theorems because they rule out theories that would restore classical objectivity to quantum physics. The 2022 Nobel Prize in Physics was awarded for the experiments which confirmed that quantum physics is incompatible with classical physics.
While entanglement is not as mystical as the internet would have us believe, that doesn’t mean it’s not useful.
Teleportation: Entanglement is necessary to “teleport” quantum information between parties using only classical information. While this doesn’t mean teleportation in the sci-fi sense of instantly transporting matter, it does involve the transfer of quantum information in a way that’s not possible without quantum entanglement.
Computing: Quantum computers utilize entanglement to perform calculations in far fewer steps than classical computers. While the computer doesn’t use entanglement as some physical fuel, we can show that without it, the computations it performs can be easily simulated with classical digital computers.
Cryptography: Quantum entanglement has implications for secure communication. Quantum key distribution uses entanglement to ensure that any attempt to eavesdrop or tamper with the communication protocol would be revealed.
Entanglement challenges our deepest desires for a universe of rigid cause-and-effect relationships. It’s a phenomenon that, even as it’s applied to cutting-edge technologies, continues to puzzle and inspire researchers.
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Dr. Chris Ferrie