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Science & Tech
Albert Einstein
Encyclopædia Britannica, Inc.

One hundred years ago this month, Albert Einstein published a series of four papers that introduced the theory of general relativity. After the publication of his theory of special relativity in 1905, Einstein realized that special relativity could not be applied to gravity or an object undergoing acceleration.

In 1907 Einstein came to a key realization. Imagine someone inside a closed room sitting on Earth. That person can feel the gravitational field. Now put that same room out in space, far from the gravitational influence of any object, and give it an acceleration of 9.8 meters per second. There would be no way for someone inside the room to distinguish between gravity and uniform acceleration.

Einstein then wondered how light would behave in the accelerating room. If one shines a flashlight across the room, the light would appear to bend downward since the floor of the room would catch up with the light. Since gravity and acceleration are equivalent, light would bend in a gravitational field.

Finding the correct mathematical expression of these ideas took Einstein several more years. In 1912, Einstein’s friend, mathematician Marcel Grossman, introduced him to the tensor analysis of Bernhard Riemann, Tullio Levi-Civita, and Gregorio Ricci-Curbastro. Three more years of wrong turns and hard work followed, but in November 1915 the work was complete.

In the four November 1915 papers, Einstein laid the foundation of the theory, and in the third he used general relativity to explain the precession of the perihelion of Mercury. The point at which Mercury has its closest approach to the Sun, its perihelion, moves. This movement could not be explained by the gravitational influence of the Sun and other planets, and so in the 19th century a new planet, Vulcan, orbiting close to the Sun, had even been proposed. No such planet was needed. Einstein could calculate the shift in Mercury’s perihelion from first principles.

However, the true test of any theory is if it can predict something that has not yet been observed. General relativity predicted that light would bend in a gravitational field. In 1919, British expeditions to Africa and South America observed a total solar eclipse to see if the position of stars near the Sun had changed. The observed effect was exactly what Einstein had predicted. Einstein instantly became world-famous.

When the eclipse results were announced, British physicist J.J. Thomson described general relativity not as an isolated result but as “a whole continent of scientific ideas.” And so it proved to be. Black holes and the expanding universe are two concepts that have their roots in general relativity. Even GPS satellites must account for general relativistic effects to deliver accurate position measurements to people on Earth.

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Science & Tech
Illustration for Demystified "How Dangerous is Quicksand"
Encyclopædia Britannica, Inc./Patrick O'Neill Riley

It used to be a standard trope in action movies, although you don’t see it much these days: a patch of apparently solid ground in the jungle that, when stepped on, turns out to have the consistency of cold oatmeal. The unlucky victim starts sinking down into the muck; struggling only makes it worse. Unless there’s a vine to grab a hold of, he or she disappears without a trace (except maybe a hat floating sadly on the surface). It was a bad way to go. Quicksand was probably the number-one hazard faced by silver-screen adventurers, followed by decaying rope bridges and giant clams that could hold a diver underwater.

Given how often quicksand deaths and near-deaths occur in film, you would think we would be seeing news about quicksand tragedies in real life. But an Internet search for deaths by quicksand won’t turn up much. Is quicksand actually as dangerous as advertised?

Nope. Quicksand—that is, sand that behaves as a liquid because it is saturated with water—can be a mucky nuisance, but it’s basically impossible to die in the way that is depicted in movies. That’s because quicksand is denser than the human body. People and animals can get stuck in it, but they don’t get sucked down to the bottom—they float on the surface. Our legs are pretty dense, so they may sink, but the torso contains the lungs, and thus is buoyant enough to stay out of trouble.

If you do find yourself stuck in quicksand, the best idea is to lean back so that the weight of your body is distributed over a wider area. Moving won’t cause you to sink. In fact, slow back-and-forth movements can actually let water into the cavity around a trapped limb, loosening the quicksand’s hold. Getting out will take a while, though. Physicists have calculated that the force required to extract your foot from quicksand at a rate of one centimeter per second is roughly equal to the force needed to lift a medium-sized car. One genuine danger is that a person who is immobilized in quicksand could be engulfed and drowned by an incoming tide—quicksands often occur in tidal areas—but even these types of accidents are very rare.