Quantum mechanics is weird
Quantum mechanics is easily one of the most misrepresented and misunderstood domains of science. I say that out of personal experience because for a long time my understanding of the subject was based on over-simplified explanations and inaccurate analogies which I am only now starting to realize were almost completely wrong.
The uncertainty principle
One of the most fundamental ideas in quantum mechanics is Heisenberg’s uncertainty principle, which famously states that it is impossible to measure both the momentum and the position of a particle at the same time with complete accuracy. If you try to measure either the position or the momentum with high accuracy, the uncertainty in the measurement of the other will increase.
A common explanation given for the uncertainty principle is as follows:
In order to measure the position of a particle, we must shine light on it, which is reflected back and detected (this is also how we see with our eyes). However, the light transfers its energy to the particle when it hits it, changing its momentum. In order to measure the position more accurately, we must shine light of a smaller wavelength, but the shorter the wavelength, the higher the amount of energy the light carries, which means a bigger change in the momentum of the particle. So trying to measure the position more accurately leads to a much higher uncertainty in measuring the momentum. To measure the momentum accurately, you need to use low-energy, long-wavelength light so as to not disturb the particle too much, but this means that you won’t be able to measure the position accurately.
This explanation is extremely misleading, because it fails to talk about one of the crucial differences between classical and quantum mechanics, the way they define particles.
What is a particle?
Classical mechanics often depicts a particle, such as an electron or a quark, as a tiny, tiny sphere which has certain physical properties. Intuitively, we expect the particle to have a fixed position (unless it’s moving, in which case we expect it to have a certain speed and thus a certain momentum) and a fixed size or radius, just like everything else around us. However, when we go down to the subatomic scale, these properties no longer have definite values, because our very idea of a ‘particle’ breaks down.
The way quantum mechanics defines a particle is a bit more nuanced and somewhat incomplete, which is probably why most high school classes completely skip it (at least ours did) and move on. Here’s the QM definition of a particle: ‘A particle is a quantum excitation of a field’
Quantum Field Theory
In order to understand the above definition, first we need to take a look at quantum field theory, or QFT as it’s often abbreviated. QFT states that there are various different fields that occupy all of space, even the empty space. There are fields for all elementary particles, so there is an electron field, a quark field, and so on. A field can be described as a physical quantity that has a certain value for each point in space and time. They are often represented by grid lines, like graphs. If a field has a value of zero (actually close to zero because the field can never have a value of absolute zero due to quantum fluctuations) at a particular point, we cannot perceive the field, but it is still there. Such a state is called the ground state of the field.
A field can be excited, shifting it away from its ground state. This leads to the field having a non-zero value, and the region in which this happens, we call a particle. If the particle is moving, we can represent it as a wave of disturbance in the field. Notice how I said ‘region’ earlier and not ‘point’. This is an important distinction.
Quantum mechanics, unlike classical mechanics, does not consider a particle to be a point object, but instead as an excitation of a quantum field, which can be interpreted as a wave of disturbance in a field. As such, asking the exact position of a particle doesn’t make much sense since it doesn’t have a single position but is spread out over a region, as a wave.
I would like to point out that this is not an explanation for the uncertainty principle, that is indeed much more complex and needs a rigorous mathematical proof. However, this is the best way that I’ve found to make sense of the superposition of a particle.
The observer problem
As if all of this wasn’t already convoluted enough, whatever I have said about the position of a particle being uncertain is only true when it is unobserved. As soon as we observe a quantum particle, it collapses into a single position or state.
This might imply that particles are in some way sentient, as if they are aware of what we do and behave accordingly, but that’s just one interpretation. First of all, what exactly qualifies as an observation?
There is some debate surrounding what exactly the role of an observer is on a philosophical level, but as far as science is concerned, an observation can be any interaction between that particle and another particle. For example, if a light ray is shone at an electron, the photons making up the ray of light will collide with the electron, which counts as an observation. We only call this event an observation because essentially the only way we can observe things is by interacting with them. There is no way to observe something without disturbing it at all.
The reason an interaction causes a particle to collapse into a single position is because the interaction must have a definite outcome. Consider an example in which an electron in superposition does not collapse when a light ray collides with it. Now, the direction in which the light is reflected after the collision depends on the position of the electron, but since the electron doesn’t have a single position, where does the light ray go? For the reflection to happen and for the interaction to be complete, the electron must collapse into a single position.
However, the collapse only happens when a conscious observer is present. If a light ray hits an electron in superposition, the light ray itself goes into superposition. The second this light ray is observed by us however, the entire system, the light ray and the electron collapses into a single state.
The need for a conscious observer has puzzled physicists for decades. There are many different interpretations of quantum mechanics that all deal with it differently. The generally accepted interpretation, the Copenhagen interpretation is criticized for dodging this problem entirely.
It is also worth noting that we cannot predict the exact position in which the electron will end up collapsing in, which means this interaction is not deterministic even though it may seem like it at first.
I think it’s this counter-intuitiveness that makes quantum mechanics so notoriously difficult to understand. The math behind it is of course incredibly complex, but that is not unlike most modern physics theories. Even general relativity is only described as unintuitive. Quantum mechanics is not only unlike anything we have seen in the past, it goes against some of our very fundamental beliefs about the world around us. Quantum mechanics is.…. weird.
P.S.
Here’s a video by an actual quantum physicist talking about the uncertainty principle and why the commonly given explanation is misleading:
A few concise explanations for QFT:
A particularly informative thread about wave-function collapse and the observer problem:
