Quanta has started publishing its yearly STEM reviews and I thought I’d make this post to showcase one highlight from each.
On memory formation:
Neuroscientists have long understood a lot about how memories form — in principle. They’ve known that as the brain perceives, feels and thinks, the neural activity that gives rise to those experiences strengthens the synaptic connections between the neurons involved. Those lasting changes in our neural circuitry become the physical records of our memories, making it possible to re-evoke the electrical patterns of our experiences when they are needed. The exact details of that process have nevertheless been cryptic. Early this year, that changed when researchers at the University of Southern California described a technique for visualizing those changes as they occur in a living brain, which they used to watch a fish learn to associate unpleasant heat with a light cue. To their surprise, while this process strengthened some synapses, it deleted others.
The information content of a memory is only part of what the brain stores. Memories are also encoded with an emotional “valence” that categorizes them as a positive or negative experience. Last summer, researchers reported that levels of a single molecule released by neurons, called neurotensin, seem to act as flags for that labeling.
On breaking down cryptography:
The safety of online communications is based on the difficulty of various math problems — the harder a problem is to solve, the harder a hacker must work to break it. And because today’s cryptography protocols would be easy work for a quantum computer, researchers have sought new problems to withstand them. But in July, one of the most promising leads fell after just an hour of computation on a laptop. “It’s a bit of a bummer,” said Christopher Peikert, a cryptographer at the University of Michigan.
The failure highlights the difficulty of finding suitable questions. Researchers have shown that it’s only possible to create a provably secure code — one which could never fall — if you can prove the existence of “one-way functions,” problems that are easy to do but hard to reverse. We still don’t know if they exist (a finding that would help tell us what kind of cryptographic universe we live in), but a pair of researchers discovered that the question is equivalent to another problem called Kolmogorov complexity, which involves analyzing strings of numbers: One-way functions and real cryptography are possible only if a certain version of Kolmogorov complexity is hard to compute.
On the W boson:
The Tevatron collider in Illinois smashed its last protons a decade ago, but its handlers have continued to analyze its detections of W bosons — particles that mediate the weak force. They announced in April that, by painstakingly tracking down and eliminating sources of error in the data, they’d measured the mass of the W boson more precisely than ever before and found the particle significantly heavier than predicted by the Standard Model of particle physics.
A true discrepancy with the Standard Model would be a monumental discovery, pointing to new particles or effects beyond the theory’s purview. But hold the applause. Other experiments weighing the W — most notably the ATLAS experiment at Europe’s Large Hadron Collider — measured a mass much closer to the Standard Model prediction. The new Tevatron measurement purports to be more precise, but one or both groups might have missed some subtle source of error.
The ATLAS experiment aims to resolve the matter. As Guillaume Unal, a member of ATLAS, said, “The W boson has to be the same on both sides of the Atlantic.”
On new number theory proofs:
It was a bumper year for number theorists of all ages, following a productive 2021. A high school student, Daniel Larsen, found a bound on the gaps between pseudoprimes called Carmichael numbers, like 561, which resemble primes in a certain mathematical sense but can be factored (in this case 561 = 3 × 11 × 17).
Jared Lichtman, a graduate student at the University of Oxford, showed that actual primes are, according to a certain measure, the largest example of something called a primitive set.
Two mathematicians at the California Institute of Technology proved a 1978 conjecture predicting that cubic Gauss sums, which sum numbers of the form e2iπn3p for some prime number p, always add up to about p5/6. Their proof assumed the truth of something called the generalized Riemann hypothesis, which mathematicians widely believe to be true but have not yet proved. Meanwhile a simpler analogue of the Riemann hypothesis called the subconvexity problem was solved.