Ask Ethan: What does it mean to live in a quantum universe?

In all the Universe, with all we’ve learned about the underlying properties of reality, perhaps nothing mystifies our intuition more than the notion that reality is fundamentally quantum in nature. It isn’t puzzling so much for the fact that matter and energy can be broken down into fundamental, indivisible units known as quanta; that intuitive idea goes all the way back to ancient times, traceable to Democritus of Abdera. Instead, the troublesome aspect comes from the fact that, when we examine it closely, reality appears to be fundamentally indeterminate in nature. Moreover, the better we try and determine some aspect of it, the greater the fundamental uncertainty that arises in other places.

Is there a way to make intuitive sense of this? And how does the quantum nature of our very existence show up in our macroscopic, everyday lives? That’s what several people have recently written in to inquire, with the most succinct version of that question coming from Pat Connolly, who asks:

“Can you explain what it means to live in a quantum universe? Specifically, how does it impact and effect our “normal” day to day human activities?”

It sounds trite to say it, but our Universe as we know it, as well as life on Earth, could not possibly exist without the quantum nature of reality. Here’s how to make sense of that.

In a traditional Schrodinger’s cat experiment, you do not know whether the outcome of a quantum decay has occurred, leading to the cat’s demise or not. Inside the box, the cat will be either alive or dead, depending on whether a radioactive particle decayed or not. Although it’s rarely discussed, the validity of a Schrodinger’s cat experiment depends on the system being isolated from its environment; if the isolation isn’t perfect, the quantum nature of the superposition-of-states will be disrupted.

At the heart of quantum physics are a few simple realizations. The first one is obvious: that if you break the “stuff” that the Universe is made of down into its smallest, indivisible components, you will arrive at what know as fundamental particles. These particles come in two main classes:

• Fermions, which are half-integer spin particles, including quarks (like the up and down quarks that make up protons and neutrons), charged leptons (like the electron), and neutrinos, as well as their antiparticles.
• And bosons, which are integer spin particles, including the gluons (which hold quarks together into bound states, like protons and neutrons), the photon (i.e., the quantum of light), the weak force bosons, and the Higgs boson.

These particles all have their own intrinsic properties: masses, electric charges, color charges, weak hypercharge and weak isospin, and in the case of a great many of these particles, a finite lifetime, after which they will decay into more energetically favorable (i.e., lower rest mass) particles.

The second aspect at the heart of quantum physics, however, is far less obvious: these particles, each and every one of them, doesn’t come along with intrinsic properties that can be exactly known to arbitrary precision. Instead, there are several sets of physical properties — properties that we think about as being “inherent to a physical system” in the macroscopic world — that are inherently uncertain, with many of those sharing a mutual uncertainty with other, equally important physical properties that can never be reduced below a certain limit.

This diagram illustrates the inherent uncertainty relation between position and momentum. When one is known more accurately, the other is inherently less able to be known accurately. Both position and momentum are better described by a probabilistic wavefunction than by a single value. Other pairs of conjugate variables, including energy and time, spin in two perpendicular directions, or angular position and angular momentum, also exhibit this same uncertainty relation.

This intrinsic uncertainty is commonly known as Heisenberg uncertainty, and it exists between a variety of properties that we call conjugate variables in physics. The most common one that we encounter, illustrated above, is the uncertainty between a particle’s position (Δx) and its momentum (Δp). In practice as well as in principle, it’s never possible to know — even with an ideal measurement — the exact value of a particle’s position (i.e., where it is at any moment in time) or its momentum (i.e., how quickly it’s moving in a particular direction at a particular moment in time). This isn’t due to what we might think of as a measurement uncertainty, but rather it’s an intrinsic, quantum property of the Universe itself.

Moreover, the more precisely you measure one of them (Δx, for example), the less precisely it’s possible to simultaneously know the other (Δp, in this example). This isn’t just true for position and momentum, but for all the conjugate variables we know of, including:

• orientation and angular momentum,
• energy and time,
• a particle’s spin in mutually perpendicular directions,
• electric potential and free electric charge,
• magnetic potential and free electric current,

as well as several others that we haven’t specifically mentioned here. It means more than just, “oh, I can’t know what the ‘true’ value of these two quantities are at once.” It means that when you measure one of them, the more precisely you measure it, the more you actually destroy any pre-existing information you may have had about the other one.

A beam of particles fired through a magnet could yield quantum-and-discrete (5) results for the particles’ spin angular momentum, or, alternatively, classical-and-continuous (4) values. This experiment, known as the Stern-Gerlach experiment, demonstrated a number of important quantum phenomena.

The most illustrative experiment that I know of that shows this directly is the Stern-Gerlach experiment. The original experiment used silver atoms, but you can do it with any fermion, such as an electron: a spin-½ particle. All you do is pass this particle through a magnetic field, and because that magnetic field points in a specific direction, a beam of particles will split into two when it encounters the magnetic field. Specifically:

• the particles that happen to be oriented with a spin of +½ in the direction of the magnetic field will deflect positively, aligned with that field,
• while the particles that are oriented with a spin of -½, opposite to the direction of the magnetic field, will deflect negatively, or anti-aligned with that field.

Big deal, you say?

Well it’s a giant deal, because of this: now let’s say you want to know what this particle’s spin is in a different, perpendicular direction? In other words, let’s say your first magnetic field was oriented in the z-direction, and the deflected particles split — half up and half down — in that z-direction. Now, you go ahead and set up a second magnet that’s oriented perpendicular to the first: one whose field points along the x-direction. What’s going to happen?

Sure, half the particles will deflect in the +x-direction, while the other half will deflect in the –x-direction: just as you’d expect. But the messed up thing is this: you’ve now destroyed any knowledge of their orientation in the previous (z-)direction. If you put a third magnetic field in place, back in the z-direction, the particles will split again, as though they have no memory of the first “splitting” because they experienced a later interaction that wiped out that prior information.

When a particle with quantum spin is passed through a directional magnet, it will split in at least 2 directions, dependent on spin orientation. If another magnet is set up in the same direction, no further split will ensue. However, if a third magnet is inserted between the two in a perpendicular direction, not only will the particles split in the new direction, but the information you had obtained about the original direction gets destroyed, leaving the particles to split again when they pass through the final magnet.

There are other bizarre quantum experiments that reveal these unintuitive properties of our reality, such as the double slit experiment, the quantum eraser experiment, the Mössbauer effect, and many others. But at the heart of it all is a fundamental truth of quantum physics: one that makes people very uncomfortable. Matter, although we visualize it as being made of particles, does not exhibit particle-like behavior except under one set of circumstances: whenever there’s an interaction with another quantum particle. An interaction involves two quanta:

• exchanging another quantum with each other,
• colliding with one another,
• or absorbing one another,

with some specific amount of energy involved in that interaction.

Now, here’s where it gets weird. There’s an equation you’ve likely heard of: E = mc². That equation tells you that for any quantum with a rest mass of m, it has an amount of energy to it that cannot be removed, its rest-mass energy, that’s given by that equation. If the particle is in motion, then it turns out that the simple E = mc² is only half of the equation, where the full equation is given by E = √(m²c⁴ + p²c²), where p is the particle’s momentum. When two quanta collide or otherwise interact, we need to move to the center-of-momentum of that collision, and then add the total energy, E = √(m²c⁴ + p²c²), for each quantum together, to arrive at the total energy of the system.

This image shows a hole that was made in the panel of NASA’s Solar Max satellite by a micrometeoroid impact. Although this hole likely arose from simply a piece of dust, the “v²” term in the equation for non-relativistic kinetic energy (½mv²) can become very large, very quickly. For particles that move close to the speed of light, the effects of kinetic energy, or energy-of-motion, become even more severe when relativistic effects are taken into account.

This might not seem “weird” in a quantum sense, and indeed it isn’t: it’s only weird in a relativity sense. This equation combines the rest-mass energy and the energy-of-motion (what physicists call kinetic energy) together, giving us a measure of the particle’s (or system of particles’) total energy.

But there’s another part to the story. These quanta, or particles if you prefer/insist, aren’t “localized” in one location in space, as you might commonly think of them. These quanta aren’t point-like, but rather are wave-like in a sense: they have a frequency and a wavelength to them, which is also determined by their energy. The equations that govern this are simply:

• E = hf, where h is Planck’s constant and f is the frequency of the wave describing the particle,
• c = fλ, where c is the speed of light, f is the wave’s frequency, and λ is the wave’s wavelength,
• and so E = hc/λ.

We can then go a step further and set the E from E = √(m²c⁴ + p²c²) equal to either E = hf or E = hc/λ, and see that the wave-like properties inherent to all quanta — including both massive (where they have a positive rest mass) and massless (where the rest mass equals 0) quanta alike — which allows us to determine what the frequency (f) and the wavelength (λ) is for any quantum that has either mass or momentum or both.

Although we now know that light, as well as all quanta, can be described as both a wave and a particle under specific physical circumstances, the debate over whether light was wave-like or corpuscle-like goes all the way back to the 1600s. In many ways, both sides of that ancient argument can lay claim to being correct today, as all quanta propagate as waves, but interact in a particle-like fashion.

This, right here, is the key insight of quantum physics, and the most complicated aspect of it that anyone attempting to make sense of it has to reckon with. Quanta interact like particles, with specific properties that can be measured down to the quantum limit of fundamental uncertainty, when they collide with or absorb one another, or even when they simply deflect one another by exchanging other quanta between them.

However, when a quantum propagates, or simply doesn’t interact with any other quantum, it behaves like a wave. This means if you do something like put a single quantum particle in a box, and you ask the seemingly mundane question of, “where is the particle?” you won’t ever get a single location within the box as the answer.

Instead, you can only get a probability distribution, based on the properties of the particle and the environmental conditions set by the boundaries of the box, for where the particle is at any moment in time. It’s important to note that these insights about the nature of our quantum reality are agnostic about the interpretation one chooses: in all interpretations of quantum mechanics, these limitations for what we can know, what we can determine, and how quanta behave in the absence of an interaction, are identical.

Trajectories of a particle in a box (also called an infinite square well) in classical mechanics (A) and quantum mechanics (B-F). In (A), the particle moves at constant velocity, bouncing back and forth. In (B-F), wavefunction solutions to the Time-Dependent Schrodinger Equation are shown for the same geometry and potential. The horizontal axis is position, the vertical axis is the real part (blue) or imaginary part (red) of the wavefunction. These stationary (B, C, D) and non-stationary (E, F) states only yield probabilities for the particle, rather than definitive answers for where it will be at a particular time.

Think about this two-sided nature of reality: when quantum particles propagate freely, they behave as waves, and when they interact with other quanta, their interactions force them to behave like particles.

This plays a role in the macroscopic world as well: for example, in the double slit experiment. It’s well-known that if you take a wave — such as a water wave in a tank — and allow it to propagate until it reaches a barrier with two slits in it, the waves will:

• strike the barrier,
• propagating through the slits,
• where they’ll make circular waveform patterns that propagate outward,
• interfering with one another,
• and producing that very same interference pattern on a screen places on the opposite side of the tank.

This works for quanta too: light, electrons, atomic nuclei, etc. Fire them at a barrier with two narrowly spaced slits in them, and that interference pattern is what we’ll see. This remains true even if you don’t fire multiple quanta at once, but rather individual quanta one-at-a-time.

However, if you try and observe “which slit did each quantum go through?” by causing an interaction with another quantum at each of the slits, the very act of forcing that interaction will destroy the wave-like pattern, and will cause your experiment to reveal particle-like behavior instead: just two “piles” on the opposite side of the barrier, corresponding to quanta that went through slit 1 and slit 2, respectively. In other words, we can experimentally confirm this picture: quanta propagate like waves, unless and until they interact with another one, in which case they behave as particles.

By setting up a movable mask, you can choose to either block one or both slits for the double slit experiment, seeing what the outcomes are and how they change with the motion of the mask. So long as both slits are unobscured and you don’t measure which “slit” particles go through, you’ll see the interference pattern.

However, perhaps the most profound place where quantum physics plays a role in our everyday lives lies in the greatest source of energy planet Earth has ever encountered: the Sun. Inside the Sun, the various atomic nuclei and electrons inside zip around at tremendous speeds, as determined by their kinetic energy at the temperatures present inside the core, between four million and fifteen million K, from the outer edges of the core to the very center of the Sun. At these temperatures, we can calculate a probability distribution for each of the particles inside the Sun, including for the most important particle present there of all: bare protons.

You see, the Sun gets its energy from nuclear fusion, and specifically nuclear fusion through a process known as the proton-proton chain. But even with the incredible kinetic energy of the hottest protons present in the innermost core of the Sun, there isn’t enough of it to get two protons, each with a radius of just under 1 femtometer (10^-15 meters), to collide with one another.

And yet, the Sun produces an incredible 4 × 10²⁶ Watts of power, continuously, meaning that somewhere around ~10³⁸ protons fuse together in the Sun’s core every second. How does that occur?

When two protons meet each other in the Sun, their wavefunctions overlap, allowing the temporary creation of helium-2: a diproton. Almost always, it simply splits back into two protons, but on very rare occasions, a stable deuteron (hydrogen-2) is produced, due to both quantum tunneling and the weak interaction.

Because, believe it or not, there are somewhere around ~10⁶⁶ proton-proton interactions that occur inside the Sun each second: because the wavefunctions of the individual particles very, very slightly overlap. Even though most of these overlapping wavefunctions — about 99.99999999999999999999999999% of them — go immediately back to their initial states of simply being two independent protons, about 1-in-10²⁸ of those interactions result in a net fusion reaction, producing a deuterium nucleus, plus a positron, a photon, and a neutrino. This is the process by which the Sun shines: an inherently quantum one, and one that would literally be impossible without these wave-like propagation features of quantum physics.

Sure, there are plenty of other everyday effects that are inherently quantum: the LED lights on your smartphone or computer screen that require electrons to fall into specific energy states, the photoelectric effect that is the core technology lying inside every solar panel, the functioning of MRI machines relies on the quantum effect of nuclear magnetic resonance, and even the basic biological process of photosynthesis, through chlorophyll molecules, is quantum in nature. If you can remember just one aspect of quantum physics however, take away this: all the various quanta in the Universe may interact like particles, but they propagate like waves, including by overlapping, by superimposing atop one another, and by interfering. Once you understand that everything that exists has wave-like properties, you’ve begun to accept the most important — and arguably, the weirdest — aspect of our quantum Universe.

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