5 Lesser Known (But Fascinating) Facts about Quantum Mechanics

Quantum physics is a branch of science so complicated that even many of those that dedicate their lives to studying it don’t claim to really understand it. Despite this, many principles of quantum mechanics have been heavily reported on and made their way into the public consciousness, such as quantum entanglement, superposition, and the uncertainty principle. However, the quantum physics rabbit hole goes much deeper than that, and today we’ll be looking at some of the lesser known facts about quantum mechanics.

Virtual Particles

One of the most modern approaches to quantum mechanics is Quantum Field Theory. According to this theory, the universe is covered with quantum fields, with each field representing a different type of particle. When these fields get excited they vibrate, and when they vibrate they create particles.

For example, when the electron quantum field vibrates in just the right way, it will create a particle with the appropriate mass and energy of an electron. And under quantum field theory, that’s what all particles in the universe are: vibrations in the various quantum fields giving rise to matter.

That’s certainly an interesting interpretation on the nature of matter, but it’s when we look deeper into these quantum fields that things get really weird. Imagine if you will the empty vacuum of space, an area so isolated as to be completely devoid of energy or matter. However, according to quantum field theory, that empty space isn’t truly empty. There may not be any matter or energy, but the quantum fields still exist there.

We’ve already stated that when the electron field vibrates in just the right way it will create a particle with the appropriate mass and energy to be an electron, but what happens when it vibrates in not quite the right way?

Theoretically, this is when it creates virtual electrons. The field will create a virtual electron and a virtual antimatter electron, the two combine, and they cease to exist. This is believed to be happening constantly all over the universe, with each quantum field creating its own particles and corresponding antimatter particles for fleeting moments.

So how can this be possible? According to the first law of thermodynamics, energy can neither be created nor destroyed. And since mass is energy, this means that no particles should be able to randomly come into existence or cease to exist. While that is true, the phenomenon of virtual particles is explained using Heisenberg’s uncertainty principle.

According to the relevant part of this principle, the uncertainty in energy multiplied by the uncertainty in time is greater than or equal to some very small, nonzero number. This means that, even in empty space, the amount of energy can change so long as the amount of time it changes for is very short. That allows for these virtual particles to exist, though the larger they are the shorter period of time they can exist for. Of course, long and short are extremely relative in this context, as these virtual particles have lifespans shorter than we can properly comprehend anyway.

As of now, virtual particles are still widely considered to be theoretical. We have no way to directly observe them since they are too small, short-lived, and unpredictable. However, there is some experimental data that is believed to have been influenced by the existence of virtual particles.

For example, certain measures taken when observing the Casimir effect (a phenomenon where uncharged conductive plates in a vacuum will attract or repel one another) are about 0.1% different from what the math predicts they should be. It’s possible that these tiny yet consistent discrepancies may be the result of virtual particles blinking in and out of existence.

No-Cloning Theorem

The no-cloning theorem of quantum mechanics states that it is impossible to create an independent and identical copy of an arbitrary unknown quantum state. So what exactly does that actually mean, and what are the implications of this?

Suppose you want to build a machine that can clone an arbitrary given quantum state. By “arbitrary given”, we mean that although we are being given a quantum state to copy, we don’t know what it will be ahead of time. As such, the machine would need to be able to clone any arbitrary state it was given rather than being designed to clone a specific state. Because at that point it wouldn’t be a machine that clones something, it would just be a machine that makes a bunch of the same identical thing.

Oh, and although we’re using the same term, this isn’t the same thing as cloning in biology where you make something with the same DNA that can go on to live a very different life. In quantum mechanics, cloning requires an identical copy, down to all of the various quantum elements such as spin and superpositions.

In order to clone such a state, you need the original state, material that will be used to create the clone, and some machine or process by which to create a clone. However, according to the theorem, this isn’t possible. And we don’t mean that it’s impossible with current technology or that we haven’t figured out how to do it yet, but cloning a quantum state is mathematically impossible within our universe.

There are multiple proofs that show this is impossible, and they rely on how quantum superpositions are added together or multiplied when various actions are performed. The proofs get a bit more math heavy than we really need to get into, but in simplified notation this form of quantum cloning would require (A+B)² to equal A² + B². But it doesn’t, it equals A² + 2AB + B². It is this broken inequality that proves quantum cloning is impossible.

Since a number of you are probably wondering, this does not mean that no two quantum states in the universe can be identical. This theorem does not preclude the existence of identical quantum states, it just says that we aren’t allowed to intentionally copy them.

This has a surprising number of practical applications, many of which relate to quantum computing. An easy one to understand is how this relates to error correction. In classical computing, a common technique for error correction during complicated calculations is to make backup copies of the current state of progress. If an error is detected, the computer can pull the copy from memory and restart the calculations from wherever it had left off.

But because quantum computers use quantum bits in a state of superposition, cloning the current state to create such a backup isn’t possible. There are other implications as well, but this is the simplest way to demonstrate the real world importance these obscure quantum rules can sometimes have.

Wave-Particle Duality

During the 17th century, the nature of light was a controversial topic. Isaac Newton believed that light was made of particles, while Christiaan Huygens argued that it was a wave. Seeing as you definitely have heard of Isaac Newton but may not have heard of Huygens (despite him being a key figure in the scientific revolution), it’s unsurprising that Newton’s corpuscular theory of light would have held a lot of sway, despite all the contradictory evidence.

For example, light could be observed bending around sharp edges, two beams of light could pass through each other in opposite directions without collision, and white light entering a prism refracts to create bands of different visible colours of light. All of these behaviours are indicative of light being a wave, and could not be properly explained by Newton’s theory.

However, it was Thomas Young’s interference experiment in 1801 that would finally settle the debate, albeit temporarily. Young’s interference experiment was the original double slit experiment, in which a light source was shone on a plate with two slits in it. Because light acted as a wave, it passed through the slits and spread out, with the two waves interfering with one another. When waves interact they essentially combine, with two peaks doubling in magnitude and a peak and a valley canceling each other out.

As would be expected from this type of interference, the resulting pattern on the surface behind the plate was a series of alternating bands of light and dark, with the brightest being closest to the slits. Had the waves acted like particles, the expected result would have just been two bands of light on the back surface, each the same size and shape as the slits the particles passed through.

That seemed to be the nail in the coffin for Newton’s theory, but a century later experiments by Max Planck, Albert Einstein, and Arthur Compton would demonstrate properties of light that classical physics could not properly explain if it was a wave rather than a particle. This led to the idea of the wave-particle duality in light, in which light was described as being made of particles that exhibited wave-like behaviour.

Around the same time as Compton’s experiments, a similar discovery was being made with electrons, but in reverse. Early experimentation had shown that electrons acted as particles, while later tests showed that they exhibited wave-like behaviour. For example, replicating the double slit experiment using a beam of electrons rather than a beam of light results in the same interference pattern on the back surface. If electrons were simply particles, this shouldn’t be the case.

However, the electrons appeared to be acting as waves that interfered with one another.

Interestingly, even when the electrons were fired through the slits one at a time, the same pattern emerged. This suggested that an individual electron was a wave capable of interfering with itself.

This principle has since been extended to all quantum entities, the behaviour of which depends on the experiment. In short, when something is being observed and measured it appears as a singular particle, but otherwise it acts as a wave.

Of course, the name “wave-particle duality” is also a bit of a misnomer, as quantum mechanics isn’t suggesting that these are literally waves and particles at the same time, nor that they switch from one state to the other. Instead, the term describes how quantum objects exhibit behaviours of both waves and particles, making them something that isn’t able to be fully described by classical physics.

Quantum Tunneling

An interesting thing that occurs thanks to the wave-particle duality of quantum objects is something known as quantum tunneling.

If you throw a tennis ball at a brick wall, the ball is going to bounce off the wall and back towards you. But what if you were to throw that ball a hundred times, or a million times? Assuming that you don’t have weird, superhuman abilities, that ball is always going to bounce off the wall because you can’t throw it with enough energy to break through to the other side.

However, that’s not how things work at the quantum scale. If you were to fire an electron at a barrier that it doesn’t have enough energy to pass through, sometimes it will still wind up on the other side. This is known as quantum tunneling, and it relies on the electron’s wavelike nature combined with the Heisenberg uncertainty principle.

According to the uncertainty principle, it is impossible to know both an electron’s momentum and location. The more accurately you know one, the less sure you can be of the other. Since, in this scenario, we know that the electron doesn’t have enough momentum to break through the wall, that means we can’t say with certainty exactly where the electron is. Instead, we have to rely on the wavelike nature of particles, with the wave showing us the potential locations the electron might be with various probabilities.

So we don’t know exactly where the electron is yet, but we’re still pretty sure it should be on the same side of the wall that it started on. After all, it doesn’t have enough momentum to break through, so the wave should completely reflect off of the barrier. And while that is true, the universe insists on being slightly more complicated than that.

Even when a wave is fully reflected by a surface, it can create an evanescent wave that passes through the barrier slightly. These evanescent waves are extremely short, only lasting a couple wavelengths, but technically a very tiny piece of the electron’s wavelength will be on the other side of the barrier. That may only result in a probability of 1 in several trillion that the electron will actually wind up on the other side, but there’s still a chance. And considering how innumerable all the quantum particles in the universe are, even something that only happens 1 in a trillion times is going to be surprisingly common.

In fact, not only does quantum tunneling happen on a regular basis, but without it life on Earth wouldn’t even exist. You’re probably aware that the Sun is powered by nuclear fusion, primarily fusing hydrogen atoms together into helium. However, hydrogen nuclei are positively charged, causing them to repel one another. Though it is possible for atoms moving fast enough to overcome this repulsive force and collide with one another, it requires extreme speeds and thus extreme temperatures.

Or, you can just cheat the system using quantum tunneling.

Without the effects of quantum tunneling causing atoms to fuse together without reaching the necessary speeds, the rate of fusion reactions within stars would be greatly reduced. It’s likely that without fusion facilitated by quantum tunneling that stars would not be capable of producing enough fusion reactions to remain stable. At the very least, their energy output and life cycles would be extremely different than what we know, meaning that without quantum tunneling life on Earth would be very different, if it even existed at all.

It’s All Probability

We tend to think of all science as being an exact science. We rely on science to explain the world around us and to make predictions based on firmly established principles and equations. If we want to launch a satellite into space, we can calculate the exact amount of fuel needed, the exact speed needed to achieve orbit, and so on. All of the answers are definite, and if the measured result is different from the predicted result, then either something was wrong with the measurement, some additional factor wasn’t properly accounted for, or the equations used to make the predictions were incorrect or incomplete.

And this is how most of science works. Theories are tested through experimentation, and those results allow scientists to refine theories as needed so that they can create more accurate predictions in the future. Though that fundamental process holds true for quantum mechanics as well, nothing is as exact as we see in other sciences.

Everything in quantum mechanics is probabilistic, which means that any predictions made by it are in the form of probabilities. Quantum physics can’t predict the exact outcome of an individual measurement, but that doesn’t mean that the theories of quantum mechanics can’t still be tested. Through rigorous testing and large data sets, all of the different results combine to reveal the probability of outcomes predicted by quantum mechanics.

Again using quantum tunneling as an example, it is impossible to look at two hydrogen nuclei and predict whether or not they will undergo fusion thanks to quantum tunneling. However, when we look at the sun as a whole and its roughly 1057 hydrogen atoms, quantum mechanics allows us to predict the rate at which fusion reactions caused by quantum tunneling will occur. It may be very different than how most sciences are applied, but nobody ever said that quantum physics isn’t really weird.

Key Takeaways

• Quantum Field Theory describes particles as vibrations in universal quantum fields, with virtual particles briefly appearing in empty space via Heisenberg’s uncertainty principle.
• The no-cloning theorem makes creating identical copies of unknown quantum states mathematically impossible, complicating error correction in quantum computing.
• Wave-particle duality means quantum entities behave as particles when observed and measured, but act as waves otherwise, defying classical physics descriptions.
• Quantum tunneling allows particles to pass through energy barriers they shouldn’t overcome, enabling stellar fusion that makes life on Earth possible.
• Unlike classical sciences with exact predictions, quantum mechanics operates entirely through probabilities that only become verifiable across large datasets and many measurements.

Sources

• Original Side Projects video: 5 Lesser Known (But Fascinating) Facts about Quantum Mechanics
• Hero image source by Greg A L / openverse, by-sa.

Related Coverage