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| Crystal Gazing |
 Title text: 'Beyond that lies a vale of fire through which my vision cannot penetrate' is the kind of fun thing geologists, heliophysicists, and early universe cosmologists have a lot of opportunities to say. |
Explanation
A geologist dressed up as a wizard (assuming that isn't how they normally dress) announces — in archaic language befitting his costume — that he has determined the age of the Earth by analyzing zircon. Zircon is a crystal with the formula ZrSiO4. During its formation it can incorporate uranium instead of zirconium in its crystal lattice, but cannot incorporate lead. The uranium then decays (via several intermediates) into lead. Thus a sufficiently old zircon crystal will contain some lead, allowing geologists to calculate its age. This method is especially reliable since uranium-238 decays into lead-206 with a half-life of 4.5 billion years, while uranium-235 decays into lead-207 with a half-life of 0.7 billion years, allowing geologists to determine the age even if some lead was lost from the crystal. The geochronologist says he predicted the age of the earth by gazing into the crystal, similar to how fortune tellers traditionally make predictions be gazing into crystal balls.
A prophet of doom is someone who predicts impending disasters, especially the end of the world. (and there are lots of ridiculous ways that could happen.)These prophets and predictions are especially common in fantasy media. The joke in the caption is that someone who can determine when the world began is just doing this in reverse, and that's what geochronologists are doing when they calculate the age of the Earth. His last line, the beginning was nigh, is a past tense variation of what people who fear for the end of the world traditionally say: "the end is nigh". This is in keeping with the idea represented in the comic.
The title text points out, in similar flowery language (in keeping with the character being a Tolkienesque portrayal of a wizard, where vales are part of the geographical language), that many areas of physical sciences are limited in how far back or how far into/away they can describe their part of the world or universe. Geologists are limited by the early molten history of the Earth, that most of the crust has been recycled back into the molten mantle at some point, and the mantle acting as a physical 'vale of fire' for the core β each limiting what can be studied. Similarly, heliophysicists have difficulty knowing what happens within the Sun due to the physical 'vale of fire' that is the Sun's surface. Finally, cosmologists have difficulty studying beyond the 'recombination' era, past which the universe is shielded by a 'vale of fire' (cosmic microwave background) from electromagnetic observation β both in time (investigating the early universe) or in space (investigating beyond the event horizon of the visible universe).
Transcript
- [A character in wizard garb with long beard and hair talks to Cueball. To the left of the wizard is a poster with illegible text over a graph with two lines intersecting.]
- Wizard: By gazing into my crystals of zircon, I have divined the date of the hour of fire marking the limit of this world's existence.
- Wizard: 4.54 billion years ago, the beginning was nigh!
- [Caption below comic:]
- Geochronologists are just reverse prophets of doom.
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