Quantum physics folk wisdom

There is so much 'maths magic' attributed to quantum physics that I thought I'd give my own take on myth vs reality. The takeaway for me is that the biggest hurdle is the Many Worlds Interpretation, so let's start with that, then work through the others. I might add more over time, there's so many.

Myth: The universe splitting into billions every nanosecond is ridiculous

Reality: This is the wrong picture. In the Many Worlds Interpretation there is a huge universal wave function that represents the entire multiverse. This wave function acts perfectly deterministically according to the known laws of Quantum Field Theory, such as the Dirac equation.

"worlds" (meaning independent child universes) are the apparent effect when one part of the wave function becomes decoherent from the rest. Imagine for example an ocean wave (which can carry information in its undulations) hitting and defraction around a rock to cause two orthogonal waves diagonally inwards on the far side of the rock. These two 'child universes' are now independent and act as superpositions, each carrying and evolving that same information differently over time.

In this scenario the ocean does not get bigger at any point, we just have a splitting of information from one to two now-independent wave fronts. 

Myth: Quantum physics is stochastic

Reality: each scientist in each child "world" sees their observed particle at a different definite location. They are each seeing a small section of a wave function and a section of a wave function looks like a wave packet: a particle. This gives the appearance of stochasticity because all the other options have split off and are inaccessible to each version of the scientist. 

So in the Many Worlds Interpretation stochasticity is phenomenological, not fundamental. 

Myth: Quantum physics is about discrete things

Reality: it is often about bounded fields and bounded fields produce standing waves. If you pluck a guitar string it only has a few discrete vibration patterns called harmonics, you can create the first couple by softly placing your finger on the string halfway or a third of the way down the string before plucking. 

This is the only source of discreteness in quantum physics. All the equations are continuous equations, as are the equations of guitar strings or waves in a pond.

Myth: Space and time must be discrete due to the Planck scale

Reality: Nothing in the Standard Model of particle physics claims or requires this. We do expect things to be different at the Planck scale, but it doesn't prevent waves from being smaller wavelength, it is only a problem to *sense* those details due to the energy required causing your sensor to turn into a black hole!

Anyone who claims quantum physics has a minimum scale needs to remember the fundamental role of Lorentz invariance in both General Relativity and Quantum Field Theory: any length is an arbitrarily smaller length to some other observer going at a different speed, due to length contraction. 

Myth: Quantum physics is inherently weird because it uses imaginary numbers

Reality: Imaginary and Complex numbers represent rotations in a plane. They can equally be used in classical physics. A wave on an ocean can be described by its height and vertical velocity as a Complex number wave equation.  

Myth: Quantum physics is weird because you square a complex number to get a probability

Reality: While there isn't a single killer explanation for the Born rule in any interpretation of quantum physics, there are several proposed reasons within Many Worlds, they just aren't bullet proof. Whatever the final proof, it seems very sensible that the number (or weight) of child "worlds" would be in proportion to the wave intensity or energy density of the wave at each point. This energy integrates the "effort" (against a linear reaction) in reaching a particular height or velocity from 0, which is the square norm of the complex number. The weight for each point on the wave is the size of the multiverse state space following the sensor interaction. And we divide by the total weight (state space size) in the Born rule because the total size does not affect our perception.

Each "world" sees a different measurement outcome, and repeated experiments appear to be give stochastic results due to the inability to predict or control which branch of the branching worlds will be aware in.

Myth: Quantum physics uses spinors, which are square roots of vectors, a mysterious space

Reality: spinors are not the square root of vectors, they are closer to the 'square root' of sections of vector bundles. So they represent transformations in a connected space.

Myth: particles have to turn twice to return to the same orientation, it is mysterious

Reality: this spin 1/2 property (represented in spinors) can be seen in real life whenever the object remains connected, such as the belt and plate trick, and most clearly seen on anti-twister videos.

so spin 1/2 most likely means the particle is connected to the space around it. 

Myth: Spinors are only ever observed as up or down no matter the angle

Reality: Such particles are a superposition of both in any continuous mixture. The 'up' and 'down' refer to the two components of the spinor, which represent two types of rotation in the above animation, or the two types of rotation on the Clifford torus (using a stereographic projection of the spinor's 4 components)

Those two types of rotation are independent parts of the wave fucntion, so can be physically split by a magnet leading to the binary results of the Stern–Gerlach experiment.  

Myth: None of the maths is real until you make an observation

Reality: If the probability of observation results from that maths then that is as real as any other mathematical law of nature. The wave function a very real and accurate description of what is happening, and in the Many Worlds Interpretation it is real since there is no distinction between the quantum and the classical.

Myth: The uncertainty principle says that some states of particles cannot be known together

Reality: This is the wrong way to look at it. Particles are really waves and a fact about all waves (including waves on a beach) is that a sharp wave is built from almost all wavelengths (making its momentum poorly determined) and a long sine wave's momentum is well determined but doesn't have a well determined location. That is all the uncertainty principle is telling us. It is not that position and momentum are the fundamental quantities that we can't access, it is that they are a poor chocie of quantities to represent a wave. It is an inadequacy of our choice of summarising parameters, not mysteriously inaccessible knowledge.

Myth: The uncertainty principle is because you affect the object by measuring it

Reality: This is an old attempt at explaining the uncertainty principle, but is not the cause of it.

Myth: Quantum physics isn't just beyond what we know, it is beyond what we *can* know

Reality: There's no indication that this is true, or that the physicist who said it was qualified to make a claim on what we can understand.