I’m slowly coming around to the idea that trying to demystify quantum mechanics might be the one intellectual pursuit that I struggle with until the day I die. I fell in love with the question when I was 12. My dad took me to Hastings a local book store / video rental store (that’s right crowd under 30, people used to rent physical videos) and told me to pick out any book in the store. I brought back two books: one thick book titled ‘What is Quantum Mechanics’ and another very small book titled ‘Special Relativity’ – Einstein’s original outline of the ideas of special relativity including the original german and along with an english translation. I couldn’t decide. Dad, bless him, shook his head and bought me both. Today, at the ripe old age of 36, I spend a non-trivial portion of my time pondering deeper explanations of the realities outlined in those two books. Quantum mechanics – which describes our world in terms so startlingly unfamiliar to our everyday experiences – has had particular staying power in terms of my mind-share.
This pondering always takes a familiar form. I think about some mysterious quantum result (there are many). I think about what prevailing interpretations tell us about what may be going on beneath the surface of this result – what is the comprehensible mechanism that explains the incomprehensible result? I wonder if there are other possibilities. I wonder if it’s all so simple as Many Worlds austere Quantum Mechanics. I go cross-eyed and fall asleep. The next night, I repeat the process.
This pattern is great for sleep regularity. Unfortunately it doesn’t represent any sort of progress. It’s time for that to change. Instead of randomly selecting one of the many mysterious quantum results, I want to outline here the key results that a new quantum interpretation must explain. I’ll list them from most to least fundamental in terms of their implications about reality, as I see it.
- Bells inequality and non-locality
Bell demonstrated that the predictions of quantum theory disagreed in principal with the predictions of a theory in which local hidden variables explained the behavior of entangled particles. Any local hidden variable theory logically implies a certain amount of correlation between entangled outcomes. Calculations using quantum theory, however, predict a higher degree of correlation than would be possible using only local hidden variables. Bell’s inequality amounts to a prescription for testing whether reality is fundamentally local in nature. To date, every experiment testing Bell’s inequality has corroborated the quantum mechanical predictions. The implication here is that the universe is fundamentally non-local.
In practice, entangled particles separated in space resolve to correlated properties instantaneously upon the measurement of either particle – this simultaneous resolution has been described as ‘sending information faster than light’ between the particles, violating a key principle of General Relativity (although it is thought that the information couldn’t possibly be used for anything – so I’m not totally sure about this claim). This all stems from the famous EPR paper, but that’s not really central to the mystery.
More information on this mystery can be found at the following links:
https://en.wikipedia.org/wiki/Bell%27s_theorem
https://www.youtube.com/watch?v=qd-tKr0LJTM
And my personal favorite:
https://www.youtube.com/watch?v=sAXxSKifgtU&t=576s - Quantum Erasure
I couldn’t find great resources on the quantum erasure experiments. This is probably the best resource, but it gets a little mystical at times for my liking:
https://www.youtube.com/watch?v=H6HLjpj4Nt4
It’s easier to watch the video than it is for me to describe the experimental setup, but it’s probably most helpful if you ignore any of the mystical or exaggerated language. The quantum erasure does not necessarily demonstrate that consciousness / conscious knowledge plays any role in quantum mechanics – though that is one tempting interpretation.
What the experiment shows is that unambiguous which-path information is equally disruptive to the quantum wave function as is direct measurement (say, at the slit), even if that information is acquired “after the fact” from the experimenters perspective. Any deeper understanding of quantum mechanics needs to explain how such seemingly retro-active (or forward looking?) behavioral choices can take place in quantum systems in a quantum erasure setup. - Double Slit Experiments
What??! How can the most famous quantum experiment of all time be the 3rd most relevant or important mystery in quantum mechanics? Well, because frankly speaking the double-slit experiment isn’t really that mysterious once you accept that matter can act as a wave or a particle. The explanation neither defies our intuitions about space and time like the Quantum Eraser nor does it violate some fundamental assumptions (like locality) of Einstein’s General Relativity as Bell’s Inequality apparently does. In terms of incomprehensibility, the double slit experiment is the least of our concerns.
That being said – for those who are just becoming interested in quantum mechanics as an idea and field of study, the double slit experiments are the most natural place to start. And, though this isn’t the point of the experiment, by forcing us to accept the wave-nature of reality the double-slit experiment eventually forces us wonder: why do we ever see particles at all? This question opens a massive can of worms… crazy worms… like existentially crazy worms, if you’re someone who wants reality to makes sense.
There are tons of awesome resources on the double-slit experiment, just google Quantum Mechanics Double Slit and away you go. Welcome to the mad house. - Gravity
I wanted to list this here because, while gravity is not part of any quantum theory, that’s sort of a problem. Because gravity is super real. Ask Einstein. Developing a theory of Quantum Gravity – explaining how gravity could somehow ‘fall out’ of quantum mechanics – is a Nobel Prize worthy intellectual accomplishment. One of my great hopes is that I live to see this happen and am capable of understanding how it was accomplished. - Mystery of a universe with ‘Missing Stuff’
I intentionally don’t use the terms of art Dark Matter and Dark Energy here because these terms imply that we have a handle on what exactly can explain the deviation between predicted and expected measurements of galaxy rotational momentum. We don’t. - Black holes and information
The question here is simple: do black holes destroy information? General relativity would say yes. But quantum mechanics insists that information is preserved. What gives? https://en.wikipedia.org/wiki/Black_hole_information_paradox
That’s the list for now. As additional mysteries emerge or current mysteries are resolved I’ll update this list accordingly to make sure we are wracking our brains over the most relevant possible issues and not pondering things that have already been resolved.
It is my strong suspicion that solving one of these problems would solve or provide in-roads into all of them. This is very exciting from the point of view of having some grand intellectual challenge to pursue. It’s also terribly daunting. Hundreds of thousands of physicists in the last 30 or 40 years, all at least as intelligent and much more qualified than myself, have pondered these problems and failed to make progress. But honestly… what else do we have to do with our time?
ThinkThread 