During the 20th century, scientists discovered the genesis of the elements in the periodic table. Einstein's theory of gravity precipitated a revolution and renascence period in cosmology; it transformed our picture of the large-scale universe. We learned that, contrary to the ideas which has prevailed for centuries since Newton's time, the universe is actually expanding and that in some distant epoch—the earliest moments of the young universe—all of the matter and energy in the universe must have been on top of each other and concentrated into a single, very small amount of space no bigger than the size of a grape fruit. The temperatures and pressures were so extreme in this early universe that hydrogen and helium could be formed. Later, the universe cooled and vast aggregates of atoms condensed into galaxies and stars. In the latter-half of the 20th century, we learned that the heavier elements in the periodic table were created in the nuclear furnaces and death-roes of the stars. We really are made of star stuff!
What is work? Work is a measure of how much total energy is transferred into or out of an object. In this lesson we'll see that by calculating the work done on an object (how much energy is transferred into or out of it), we can predict the object's future state of motion.
What happens if the only effect of the work done on an object is to change its height without changing its speed? The answer is that the work done on the object only changes in potential energy. In this lesson, we'll consider the work done by Earth's gravity on an object whose height above the ground changes but whose speed stays the same.
Work is energy transfer. But what happens when work is done on an object such that its height above the ground doesn't change? The answer is that only kinetic energy is transferred into or out of the object. And since work is the total amount of energy transfered into or out of the object, it therefore follows that the work is just equal to the object's change in kinetic energy.
The Proterozoic Eon is a sweep of time beginning when the Earth was 2.5 billion years old and ending when the Earth was 542 million years. During the first 500 million years, cyanobacteria and photosynthesis were invented which oxygenated the world. The proceeding one billion years was a time of enigmatic calm as the Earth did not change much. But in the last roughly 350 million years, the Earth's systems were spun into a whirlwind. The Earth experienced one of the most dramatic series of ice ages in its history and turned into a giant, white, ice ball. But after these ice ages ended, a second great oxidization event occurred. The last step was taken in the march from the simple to the complex: there was finally enough oxygen to support large, multi-cellular creatures and the Ediacaran fauna emerged.
After ancient stars exploded, their remnants conglomerated through gravity to one day form a place called Earth. Primordial Earth was a giant, red ball of magma and smoldering rocks; it was hellish and ablaze with erupting volcanoes and fiery skies. But over time hails of comets and asteroids bombarded the Earth to form the oceans causing Earth's outer layer to cool and turn grey; those heavenly bodies also seeded the oceans with rich organic chemistry which, somehow, eventually turned into the first microbe. Nearly one billion years later, photosynthesis was invented—this oxygenated the world, a little, and was the first step in the march towards the emergence of large, complex, multi-cellular organisms.
The extraordinary Carl Sagan long ago envisioned in his book, Pale Blue Dot, humanity eventually terraforming other worlds and building settlements on the asteroids and comets in our solar system. He imagined that these little worlds could be perhaps redirected and manuevered—used as little rocky "space ships"—in order to set sail for the stars. In this article, we discuss some of the techniques which could be used towards this telos.
General Relativity is hailed by many as one of the greatest achievements of human thought of all time. Einstein's theory of space, time, and gravity threw out the old Newtonian stage of a fixed Euclidean space with a universal march of time; the new stage on which events play out is spacetime, a bendable and dynamic fabric which tells matter how to move. This theory perhaps holds the key to unlocking H. G. Wells time machine into the past; according to Kip Throne, it will pave the way towards the next generation of ultra-powerful telescopes which rely on gravitational waves; and it also perhaps holds the key to breaking the cosmic speed limit and colonizing the Milky Way galaxy and beyond in a comparatively short period of time.