3115: Unsolved Physics Problems

2025-07-16T04:27:04Z

2603:8000:5E00:2913:8A9D:5D79:4D05:B356: /* Transcript */ phy

{{comic
| number = 3115
| date = July 14, 2025
| title = Unsolved Physics Problems
| image = unsolved_physics_problems_2x.png
| imagesize = 699x422px
| noexpand = true
| titletext = 'Tin pest' makes more sense to me. Tin just doesn't want to be locked down in a shape like that. I get it. But why would any metal want to grow hair??
}}

==Explanation==
{{incomplete|This page was created by A CURSED METAL FIGURE OF HAIRY. Don't remove this notice too soon.}}

This comic is a sequel to comic [[2529: Unsolved Math Problems]], which follows the same format. Unlike with that comic, however, all the examples given here are real phenomena.

{{w|Quantum gravity}} is the problem of determining how {{w|gravity}} interacts with the physics of {{w|quantum mechanics}}. There are {{w|General relativity#Relationship with quantum theory|contradictions}} between the two theories as currently understood. Gravity is described by the theory of {{w|general relativity}} and is well-characterized at large scales, while quantum mechanics is also well-understood and well-characterized but is easiest to observe at very small scales. This scale discrepancy makes it hard to conduct an experiment that includes both phenomena, leading to a wide range of unifying theories that are difficult to experimentally confirm or refute. In this comic, the question is specifically what part time plays in quantum gravity, but this is left vague and open-ended by considering several different options that may or may not be mutually contradictory.

The second issue, the {{w|Soviet–American Gallium Experiment#Gallium anomaly|gallium anomaly}}, is an example of a precise experiment to understand a specific phenomenon, namely a deficit of germanium (71Ge) when gallium (71Ga) is bombarded with neutrinos (''ν''e). The difference between the expectation and reality is small but statistically significant, and indicates our models of physics are inaccurate.

The third problem concerns {{w|Whisker (metallurgy)|zinc whiskers}} — a phenomenon that at first sounds extremely strange, in which a piece of metal (in this case zinc) can 'grow' hair-like filaments on its surface. Hair is usually thought of as an organic structure, and spontaneous change in a block of metal is not a problem most people would expect. For an inorganic object to grow hair seems both magical and grotesque, so [[Randall]] considers this phenomenon [[:Category:Comics with cursed items|cursed]]. Metal whiskers can cause problems in a lot of electronics, where metal hairs create unwanted electrical connections or act as antennae. In particular, they can degrade performance of rechargeable batteries. Since electronic devices are ubiquitous, the prevention of metal whiskers is a challenge that affects us widely, while it is hard to see how the other two issues affect most people. Lead has been widely used as an additive to solder for whisker prevention, but lead is toxic and has in fact been {{w|Solder#Lead-free_solder|banned in the E.U.}} for most electronics.

The title text mentions {{w|tin pest}} — a transformation which causes deterioration of formed tin objects into a powder in a chilly environment. Randall considers this an understandable rebellion of the tin against being forced into shapes that we want it to take, whereas he cannot understand the motivation of zinc to grow hair. In reality, metallic elements do not have [https://groups.google.com/g/sci.physics/c/C2CfIKWFduU/ motivations and intentions]. The actual cause of tin pest is that solid tin has two forms with different crystal structures. β-tin is white, metallic, and malleable, and the more stable at temperatures above 13.2°C; α-tin is grey, non-metallic, and brittle, and is stable at temperatures below 13.2°C. When an object is made from tin, it's generally done at higher temperatures, where it's easily worked, resulting in the β crystal structure. If such an object is then left at lower temperatures, it eventually spontaneously changes to the α crystal structure, but the transition causes it to disintegrate. Impurities in the tin can lower the transition temperature, or prevent the disintegration from occurring at all. Tin pest was famously responsible for the destruction of the tin pipes of pipe organs in churches, when those churches were allowed to get cold below the transition temperature at times they weren't being heated.

==Transcript==

{{incomplete transcript|Don't remove this notice too soon.}}
: The Three Types of Unsolved Physics Problem

: Vague
: [A Feynman diagram of two particles interacting via the electromagnetic force is to the right of Cueball. The diagram is drawn on a curved surface]
: Cueball: What is the nature of time in quantum gravity? Is it a background parameter, a dynamic aspect of spacetime, or an emergent phenomenon?

: Precise
: [A scientific instrument and a nuclear reaction equation of gallium-71 becoming germanium-71 are to Megan's left. The equation says: 71Ga + νe → 71Ge + e- .]
: Megan: Why does the ''S.A.G.E.'' Gallium Neutrino Capture Experiment produce only 75% as much germanium as predicted?

: Cursed
: [A rectangular block of zinc, with whiskers growing out of it, is visible to the right of White Hat]
: White Hat: Why does some metal randomly grow hairs?
: White Hat: It keeps causing short circuits and we have no idea what's going on.
: Voice from off-panel: Is this a joke?
: White Hat: No! Please help!

{{comic discussion}}<noinclude>

[[Category:Comics with cursed items]]
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Megan]]
[[Category:Comics featuring White Hat]]
[[Category:Physics]]

explain xkcd - User contributions [en]

3115: Unsolved Physics Problems