100 Mysteries of Science Explained Read online

  The Ebbinghaus illusion describes this perceived effect. Two circles of identical size are placed near each other. One circle is surrounded by smaller circles, and the other circle is surrounded by larger circles. Although we know the original circles are identical, we perceive the circle surrounded by smaller circles as larger than the neighboring circle surrounded by larger circles. We also view the Moon as we do other objects, like clouds and birds, that recede into the skyline. We expect them to look smaller as they get farther away. In what is known as the Ponzo illusion, our brain tricks us into thinking the Moon is getting smaller as it rises in the sky, and in our minds only, farther away from Earth. But the Moon is still mostly the same distance away in its orbit. Nothing has changed in its size or its distance from our planet. To practice this, draw two identical parallel lines horizontally across a photo of a receding railroad track. The line closest will appear smaller than the line farther away, because as the tracks recede into the horizon, they become smaller, and your brain expects the line to do the same.

  Still not convinced? Try taking a picture of the large Moon at the horizon. The camera doesn’t suffer from the same visual cues that make the Moon appear as massive as in real life. This illusion is not unique to the Moon—the Sun and stars show the same properties. And while interesting to consider, the Moon illusion offers little insight into astronomy and the atmosphere. Instead, it proves an example of optical illusion.

  An artist’s rendition of a black hole drawing matter from the blue star beside it.

  What’s at the Bottom of a Black Hole

  Black holes are already among the most mysterious objects in the universe, even before we begin to contemplate what might be at the “bottom” of one. The concept of a tiny star whose gravitational field is so strong that neither light nor matter can escape was so foreign to those who first theorized their existence that even Albert Einstein himself, whose math confirmed their possibility, dismissed the likelihood of their existence. As to the question of what’s at the bottom, the answer—depending on the physicist—may be just about anything, or nothing, or even another universe.

  At the outer edge of a black hole is the event horizon, the boundary where velocity required to escape its gravity exceeds the speed of light. Past this point, all energy and matter that enter the black hole will proceed infinitely toward the singularity, a point of infinite density that, according to Einstein’s theory of general relativity, represents a bottomless pit of space-time. If the hole is truly infinite and nothing can escape past the event horizon, then the bottom of a black hole could theoretically hold an infinite amount of matter and energy.

  However, while that interpretation may square with general relativity, the laws of thermodynamics maintain that a system cannot infinitely increase its mass while maintaining a similar temperature and level of disorder. Other theories that account for black hole thermodynamics suggest that anything falling toward the event horizon never really reaches the singularity, eventually evaporating back into space. According to astrophysicist Stephen Hawking, this is because black holes aren’t truly black: They emit a minute amount of radiation, far less than the background radiation of space, but enough to eventually return the mass of the black hole back to the rest of the universe.

  Other more exotic theories posit that at the bottom of a black hole lies an entire universe. How can this be? The combination of the insanely high temperatures, densities, and rotational velocity at the center of a black hole is so powerful that it could produce a massive expansion in space-time that might give rise to a new universe—a process not unlike that of the Big Bang that gave rise to our own universe. The logical extension of this theory implies that even our universe may lie at the bottom of a black hole.

  The mystery has only deepened of late as prominent astrophysicists (including Hawking) change their minds on whether black holes even exist. According to Hawking and others, the laws of quantum mechanics may prevent a neutron star from collapsing beyond a small enough radius to fit within its event horizon. This would mean that no black hole is ever small enough for its escape velocity to exceed the speed of light, and thus there is no black hole.

  What Does Space Smell Like?

  The final frontier smells a lot like a NASCAR race— a bouquet of hot metal, diesel fumes, and barbecue. The source? Dying stars.

  The by-products of all this combustion are smelly compounds called polycyclic aromatic hydrocarbons. These molecules “seem to be all over the universe,” says Louis Allamandola, the founder and director of the Astrophysics and Astrochemistry Laboratory at NASA Ames Research Center. “And they float around forever,” appearing in comets, meteors, and space dust. These hydrocarbons have even been short-listed as the basis of the earliest forms of life on Earth. Not surprisingly, polycyclic aromatic hydrocarbons can be found in coal, oil, and even food.

  Though a pure, unadulter- ated whiff of outer space is impossible for humans (space is a vacuum, after all; we would die if we tried), we can get an indirect sense of the scent: When astronauts work outside the International Space Station, spaceborne compounds adhere to their suits and hitch a ride back into the station. Astronauts have reported smelling “burned” or “fried” steak after a space walk, and they aren’t just dreaming of a home-cooked meal.

  The smell of space is so memorable and distinct that, three years ago, NASA asked Steven Pearce of the fragrance maker Omega Ingredients to re-create the odor for use in its training simulations. “Recently we did the smell of the Moon,” Pearce says. “Astronauts compared it to spent gunpowder.”

  Allamandola explains that our solar system is particularly pungent because it is rich in carbon and low in oxygen, and “just like a car, if you starve it of oxygen, you start to see black soot and get a foul smell.” Oxygen-rich stars, however, have aromas reminiscent of a charcoal grill.

  Once you leave our galaxy, the smells could get really, really interesting. In dark pockets of the universe, molecular clouds full of tiny dust particles may host a veritable smorgasbord of odors, from wafts of sweet sugar to the rotten-egg stench of sulfur.

  How Long Would It Take to Walk a Light-Year

  If you had started just before the first dinosaurs appeared, you’d probably be finishing your hike just about now.

  Here’s how it breaks down. One light-year—the distance light travels in one year, used as the yardstick for interstellar distances—is about 5.9 trillion miles (9.5 trillion km). If you hoofed it at 20 minutes a mile, it would take 225 million years to complete your journey (not including stops for meals or the restroom). Even if you hitched a ride on NASA’s Mach 9.8 X-43A hypersonic scramjet, it would take more than 90,000 years to cover the distance.

  You’d need to bring a big backpack, too: Walking such a distance requires substantial supplies. The average adult burns about 80 calories per mile walked, so you’d need about six trillion granola bars to fuel your trip. You’d also produce a heap of worn-out shoes. The typical pair of sneakers will last you 500 miles (800 km), so you’d burn through some 11.8 billion pairs. And all that effort wouldn’t get you anywhere, astronomically speaking: The closest star to the Sun, Proxima Centauri, is 4.22 light-years away.

  Jupiter’s great red spot is a high-pressure storm that has battered the planet consistently for 400 years. Its size is so great that you can see it with a backyard telescope.

  What Causes Jupiter’s Red Storm?

  At one time, the storm was at least 20,000 miles (32,000 km) in diameter and big enough to envelop three Earths. It is similar to a hurricane on Earth, rotating counterclockwise with a maximum wind speed of 268 miles per hour (430 km/h), almost twice as fast as the worst hurricanes on Earth. Historic observations date as far back as the 1600s. Since then, the spot has changed, fluctuating between a deep red and a pale salmon color. Laboratory experiments suggest that complex organic molecules, red phosphorus, and other sulfur compounds cause the vibrant color. But since the 1930s, the storm has shrunk to half its largest diameter
. Even though it may be dwindling in size, the longevity and enormity of our solar system’s biggest storm is full of mystery.

  The reason for the persistence of the Great Red Spot is unknown, but presumably comes from the fact that it never moves over land, unlike hurricanes on Earth. Jupiter is composed of hydrogen and a small amount of helium and has no “land” in its form. Jupiter’s internal heat source is a driving force, and the spot tends to absorb nearby weaker storms. However, based on computer models, the spot should have disappeared after several decades. Waves and turbulence in and around the storm sap it of energy. The powerful jet streams that surround the spot should slow its spinning. And even though the storm absorbs smaller ones, researchers say that doesn’t happen enough to explain the storm’s longevity. Some scientists think vertical flows in the storm are just as important as the more-studied horizontal flows. When the storm loses energy, vertical flows move hot and cold gases in and out of the storm, restoring energy.

  Understanding Jupiter’s red storm could reveal more clues about the vortices in Earth’s oceans and also the nurseries of stars and planets. Philip Marcus, a fluid dynamicist and planetary scientist at the University of California at Berkeley, explains the importance of understanding the Great Red Spot: “Vortices with physics very similar to the GRS are believed to contribute to star and planet formation processes, which would require them to last for several million years”—even as the Great Red Spot shrinks, it retains enormous significance for Earth and the very beginnings of the solar system.

  Relative Sizes of Habitable Zone Planets: Illustrated representations of exoplanets that have potential to support life as we know it. They are ranked here closest to farthest from Earth.

  Are There Habitable Planets Beyond Our Solar System

  Ever since people first tilted their gaze up toward the heavens, they have wondered about the possibility of other worlds like ours orbiting distant suns. Until very recently, such questions were left to the realm of speculation. Today, thanks to telescopes like the Kepler space observatory and increasingly advanced surveys from ground-based technology, we know that the galaxy is swarming with planets. But are any of them habitable? Do any of them resemble our own?

  The question of habitability is a tricky one, and the odds of any individual planet possessing Earth-like properties are rather low. That said, the numbers are in our favor. Kepler recently confirmed the discovery of its 1,000th exoplanet. Some astronomers now estimate that there is one exoplanet for every star, on average. That means there are billions and billions of planets in our universe! Many of these planets, though, are nothing close to habitable. The first exoplanets that astronomers found orbited impossibly close to their suns, tidally locked, exposing one side to scorching heat and radiation and the other side to permanent night. In contrast, Earth orbits the Sun in the so-called Goldilocks Zone: not so close that all liquid water boils away, but not so far that it is perpetually frozen in ice.

  What’s water got to do with the existence of other planets? The capacity to harbor liquid water is the key characteristic that astronomers look for in the search for habitable alien worlds, due to water’s paramount importance to life on our own planet. But liquid water and a planet’s average orbital distance are but two of several key factors. For instance, the class of star that serves as the sun is important: Habitability requires a sun that emits the right type of radiation and is likely to live long enough to allow life to evolve. A stable orbit is also important, ensuring that the planet’s climate doesn’t fluctuate wildly. The mass of the planet—massive enough so that it’s capable of generating and holding onto an atmosphere, but not so massive that the atmosphere is oppressively dense—is also critical.

  While astronomers have not yet confirmed the presence of habitable exoplanets, all signs currently point to the affirmative. Scientists reviewing data from the Kepler observatory recently discovered eight planets, roughly the size of Earth, in their respective sun’s Goldilocks Zone. Other candidates exist, from as nearby as 40 light-years to thousands of light-years distant, some orbiting superclose to colder suns, and some much larger than Earth; so-called super-Earths range in size from two to 10 Earth masses. We seem on the verge of discovering a planet that might not only be capable of supporting life, but could hypothetically support life.

  Whether these habitable planets already support life-forms and whether those life-forms are intelligent—well, that’s a whole other mystery.

  In this illustration, dark energy, a force found throughout the universe and theorized to stimulate its expansion, is represented by the purple color. The green grid represents gravity. For now, the mystery of dark energy continues to confound scientists. The NASA website concludes, “The thing that is needed to decide between dark energy possibilities—a property of space, a new dynamic fluid, or a new theory of gravity—is more data, better data.”

  What Is Dark Energy?

  In 1929, American astronomer Edwin Hubble studied a number of exploding stars, or supernova, and determined that the universe was expanding. The notion that distant galaxies were moving away from ours was a radical idea.

  It seemed obvious to astronomers that gravity—the mutual attraction between all matter—would affect the expansion process. But how? Would the pull of gravity completely halt the expansion of the universe? Could the universe stop expanding and then reverse itself back toward us? Or would the universe eventually escape the gravitational effect and continue to expand? The universe may be expanding, reasoned the scientific community, but its expansion was surely slowed by the forceful effects of gravity.

  Fast forward nearly 70 years to a time when two teams of astrophysicists—one led by Saul Perlmutter at the Lawrence Berkeley National Laboratory and the other by Brian Schmidt at Australian National University—began studying supernovas to calculate the assumed deceleration of expansion. To their astonishment, they discovered that supernovas as far as 7 billion light-years away were not brighter than expected but rather dimmer, meaning they were more distant than the teams had calculated them to be. The universe isn’t slowing down, they concluded. It’s speeding up.

  The discovery turned the scientific world on its head: If gravity isn’t the most dominant force in the universe, what is? In 1998, American theoretical cosmologist Michael S. Turner dubbed the mysterious new something “dark energy.” Yet even with a name, we know little about dark energy.

  Theorists have come up with several explanations for dark energy. The leading theory claims that dark energy is a property of space. Albert Einstein claimed it is possible for more space to come into existence and that “empty space” can have its own energy. “As more space comes into existence,” reports NASA, “more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster.”

  NASA reports that scientists have been able to theorize how much dark energy there is out there because we know how it affects the expansion of the universe. Roughly 69 percent of the universe is dark energy. Dark matter accounts for about 27 percent, leaving the rest—all normal matter, everywhere—adding up to less than 5 percent of the universe.

  Another explanation posits that dark energy is a new type of energy field or energy fluid that fills space but affects the expansion of the universe differently than matter and normal energy. Scientists have labeled this energy “quintessence,” but we still don’t know what it interacts with or why it even exists.

  An image of a small area of space in the constellation Fornax, created using Hubble Space Telescope data from 2003 and 2004. By collecting faint light over many hours of observation, the data revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time.

  How Will the Universe End?

  In 1929, Edwin Hubble discovered that the universe is not in fact static, but expanding. In the years following his discovery, cosmologists took up the implications of the discovery, asking how long t
he universe had been expanding, what forces caused the expansion, and whether it will ever cease.

  Cosmologists are pretty confident about the first question: just shy of 14 billion years. A great deal of evidence supports the predominant answer to the second question: The universe rapidly emerged from a singularity in an event that cosmologists call the Big Bang. The third question is a bit more mysterious, and the answer relies on an obscure, confounding phenomenon known as dark energy. The density of dark energy in the universe determines its ultimate fate. In one scenario, the universe does not possess enough dark energy to forever counteract its own gravity and thus ends in a “Big Crunch.” Under this scenario, the universe’s gravity will overcome its expansion and the cosmos will collapse in on itself, resulting in a singularity that may precipitate another Big Bang. However, the evidence cosmologists have gathered over the last few decades leads us away from this scenario.

  For the Big Crunch to occur, we’d see signs that gravity was winning out over dark energy, slowing its expansion. However, measurements of distant galaxies indicate that cosmic expansion is not slowing down—it’s speeding up! Apparently, the density of dark energy in the vacuum of space is simply too high to permit a Big Crunch.

  Some say the world will end in fire

  Some say in ice.

  — ROBERT FROST

  This is the way the world ends

  Not with a bang but a whimper.

  — T.S. ELIOT

  That leaves two possible fates for the cosmos: 1) a Big Freeze, in which the acceleration eventually halts but the universe keeps expanding, creating a system where heat becomes evenly distributed, allowing no room for usable energy to exist and thus, “heat death,” or 2) a Big Rip, in which the expansion of the universe continues to accelerate forever. In the former scenario, the universe will progressively become darker and colder until the end of time. In the latter, all matter down to the most fundamental particles will be torn asunder.