Four Forces

It bothers me that in popular science discourse, gravity is so frequently emphasized while other forces are overlooked. Nobody even discusses the strong and weak forces anymore! OK. Maybe they do sometimes and I’m just exaggerating. Furthermore, gravity is the weakest force! However, it does affect things on an infinite scale. Behold, a list of the four physical forces:

• Strong interaction — This is the strongest force—the force that holds the nuclei of atoms together, binding protons and electrons to nuclei
• Electromagnetism — Another force stronger than gravity—electromagnetism is the force that acts on charged particles. (e.g. light, radio waves, etc.)
• Weak interaction — A force weaker than electromagnetism, involved in subatomic interactions like radioactive decay or the decay of unstable particles (e.g. like muons or nuclear reactions in the the Sun)
• Gravity — The weakest force, but with range that inevitably affects large-scale things, like objects, planets, asteroids, and so on.

Radioactive Dating

Matter is composed of chemical elements. Every chemical element has its own arrangement of protons, neutrons, and electrons. As a consequence, each element also has its own atomic number, which indicates the number of protons in its nucleus.

Every element also has varying isotopes—differing versions of itself that possess a non-standard number of neutrons in its nuclei. Some of those isotopes are unstable (radioactive), experience decay, and turn into different elements over time.

The process of tracing these radioactive impurities in materials is known as radiometric dating. For example, thanks to meteorite samples, we know that the Earth is around 4.5 billion years old. But how, exactly, do we know this?

There are various types of radiometrics, and the process can involve different elements—from carbon, rubidium, potassium, samarium, uranium, to thorium.

The elements uranium and thorium both decay into lead over billions of years. Thus, it is possible to determine the age of materials like rocks and meteorites by measuring various isotopes of lead and retroactively inferring their age: Pb-206, Pb-207, Pb-208, and Pb-204. All of these, except for Pb-204, are considered radiogenic isotopes.

This trick relies on the fact that uranium and thorium decay in a constant, predictable way over time. For example, uranium-238 has a half-life of about 4.5 billion years, and it decays into lead-206. Uranium-235 has a half-life of approximately 700 million years and decays into lead-207.¹

Parent Isotope	Stable Daughter Product	Currently Accepted Half-Life Values
Uranium-238	Lead-206	4.5 billion years
Uranium-235	Lead-207	704 million years
Thorium-232	Lead-208	14.0 billion years
Rubidium-87	Strontium-87	48.8 billion years
Potassium-40	Argon-40	1.25 billion years
Samarium-147	Neodymium-143	106 billion years

Lead-lead dating does not directly involve uranium. Instead, it involves analyzing the ratios between specific amounts and isotopes of lead, the decay products of uranium and thorium.

Uranium-lead dating, on the other hand, relies on measuring the ratios via the decay routes of uranium and thorium. This method frequently involves sampling the mineral zircon. But it can also involve other minerals, such as monazite.

Why minerals? Why not just measure rocks? When formations like rocks develop, there’s a chance they may contain some preexisting amount of lead. This can derail measurements and produce unwieldy results. Additionally, the Earth is dynamic—magma and rocks are constantly undergoing change and having their geological clocks reset and tampered with.

Zircon, a crystal mineral, unlike rocks, essentially offers a clean starting point for the task of radiometric dating—because any lead found inside zircon almost certainly originated from decayed uranium and wasn’t there beforehand.

Due to zircon’s crystal lattice structure, it’s picky about its elemental friends. The structures like those found in zircon are useful for radiometric dating because they tend to reject lead during their formation while letting uranium in.

Theoretically, lead in an unusual oxidative state, like Pb+4, could potentially make its way into zircon.² But the most common compounds of lead found are in a +2 oxidative state, not +4—this is due to the inert pair effect.³

Despite the advent of “uranium-lead” dating, “lead-lead” dating is still useful. One of the earliest measurements to determine the age of Earth was a lead-lead measurement. As it turns out, meteorites are quite useful in offering a bigger than earth point-of-view.

Meteorites, which mostly come from the asteroid belt between Mars and Jupiter, are remnants from the formation of the early solar system. As such, they largely remain unchanged. They serve as a useful reference point and cosmic timestamp hinting at when our solar system began.

Lead dating was used to determine the age of the Canyon Diablo meteorite from the Barringer Crater. The result suggested the meteorite was roughly 4.5 billion years old—a value that has been replicated hundreds of times by other tests.

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Footnotes

1. Mathematical Treasure: James A. Garfield’s Proof of the Pythagorean Theorem ↩︎
2. Oxidation state and coordination environment of Pb in U-bearing minerals
3. Periodicity and the s- and p-block elements

Origins of Life

Today I learned the abiotic origin of organic compounds was established in the early 1800s, but the experiment wasn't actually intended to put forth a hypothesis for "abiogenesis"—or how life began on Earth.

The question of abiogenesis is the following one: how does so-called inanimate, non-living matter become animate, living matter? 

Friedrich Wöhler's so-called seminal contributions to organic chemistry would eventually lead to further hypothesis exploration about abiogenesis. Wöhler took two inorganic compounds—silver cyanate and ammonium chloride—and synthesized them to create urea, an organic compound that was previously believed to only be produced by living things carrying a "life force."

After Wohler's experiment, a large number of similar organic chemistry experiments would follow throughout the 19th century—and later those experiments would be followed by the Miller-Urey experiment.

The Miller experiment explored an origin of life scenario—simulating possible early conditions on Earth. By combining the gases methane (CH₄), ammonia (NH3), and hydrogen (H₂) with water—and exposing this amalgamation to electricity—various amino acids were produced, which are the building blocks of proteins. The related hypothesis is known as the prebiotic or primordial soup hypothesis.

But is the “prebiotic soup” theory a reasonable explanation for the emergence of life? Contemporary geoscientists tend to doubt that the primitive atmosphere had the highly reducing composition used by Miller in 1953. Many have suggested that the organic compounds needed for the origin of life may have originated from extraterrestrial sources such as meteorites. However, there is evidence that amino acids and other biochemical monomers found in meteorites were synthesized in parent bodies by reactions similar to those in the Miller experiment. Localized reducing environments may have existed on primitive Earth, especially near volcanic plumes, where electric discharges may have driven prebiotic synthesis. In the early 1950s, several groups were attempting organic synthesis under primitive conditions. But it was the Miller experiment, placed in the Darwinian perspective provided by Oparin’s ideas and deeply rooted in the 19th-century tradition of synthetic organic chemistry, that almost overnight transformed the study of the origin of life into a respectable field of inquiry. (via Prebiotic Soup—Revisiting the Miller Experiment)

The question of whether Earth's early atmospheric conditions were different from those in the Miller experiment is up for debate. The synthesis, however, continues to be a pioneering experiment in the study of abiogenesis—since it has further demonstrated that inorganic compounds can result in the formation of simple-to-complex organic compounds under circumstances potentially like those following asteroid impacts on Earth during the prebiotic atmosphere.

The hypotheses involving volcanic plumes and hydrothermal vents aren't the only abiogenesis hypotheses, of course. But they are particularly compelling ones, since one of the earliest forms of life on Earth was discovered in a ~3.42-billion-year-old subseafloor hydrothermal environment.

However, our last universal common ancestor is thought to have lived 4.2 billion years ago.

Einstein's Principles of Research

Einstein is perhaps most well known for his theory of relativity. But he is also known for being a scientist who broke new ground by virtue of holding close to known fundamentals and by engaging in basic research.