What would happen if you fell into a black hole?
If you were to take a step into a small black hole, your body would most closely resemble toothpaste being extruded out of the tube. Tidal forces are so strong at a black hole's threshold (called the "event horizon") that they would stretch your body into a string of atoms as you fell into the abyss. But if you fell into a somewhat larger black hole with less extreme tides, you could maintain your internal structure. Einstein's theory of time dilation suggests that if you looked forward toward the black hole's center, you'd see every object that has fallen into it in the past. Behind you, you'd see everything that will ever fall into it in the future. [Learn more about life at the edge of a black hole]
What if there were another technologically advanced species?
If Earth had another species as tech-savvy as us, we would probably be locked in a constant battle for supremacy. If, after hundreds of thousands of years, no one won, we would start to adapt to require separate resources, and would come to ignore one another. [Find out what they'd look like.]
What if Earth were twice as big?
If Earth's diameter were doubled to about 16,000 miles, the planet's mass would increase eight times, and the force of gravity on the planet would be twice as strong. All the plants and animals that currently exist would collapse under their own doubled weight, and new, stockier species would arise. [Get the full explanation]
What if a giant asteroid hadn't wiped out the dinosaurs?
If that asteroid hadn't struck, the dinosaurs would in all likelihood still rule the Earth. After all, dinos dominated for 160 million years prior to the Earth-shattering event. Researchers have speculated that intelligent "dinosauroids" might have evolved in humanity's place, based on the relatively large brain size of late-emerging trodontid species, which were bird-like predators. [Get the full explanation]
What if everyone on Earth jumped at once?
If all 7 billion of us crowded close together and jumped, we would make the Earth move just one-hundredth of the radius of a single atom per second. When we landed, Earth would move back in place, like a spring. [Get the full explanation]
What if the moon had never formed?
Huge tides generated by the moon – which orbited much closer to Earth when it formed – washed the chemical building blocks for life from land into the oceans and helped stir up the primordial soup. Without it, life may never have arisen, or living things would have very different behavioral patterns to cope with the six-hour day and extreme climate changes that would exist on a moonless Earth. [Get the full explanation]
What if humans were twice as intelligent?
If humans were twice as intelligent as we are now, experts think we would be more fulfilled on an individual scale, as well as healthier, better looking and less religious. But people would still have a diverse range of personalities, and so society as a whole might be just as conflicted as it is today. [Get the full explanation]
What if all the cats in the world suddenly died?
Cats may seem ineffectual, but they're in fact vital members of the global ecosystem. From studies that have looked at the effects of removing cats from small islands, we know that, without them, Earth would quickly become overrun by rodents. Mice and rats would probably destroy grain supplies, spread disease, and wipe out ground-nesting birds. [Get the full explanation]
What if our solar system formed closer to the Milky Way's edge?
Though there is just one-third the quantity of metallic elements at the galaxy's edge compared to where we are, life could still have arisen and evolved in much the same way. However, gas giants like Jupiter and Saturn would not be able to exist, and the lack of such planets might spell doom for Earth by allowing more frequent asteroid impacts. [Get the full explanation]
What if the first animals to crawl out of the ocean had six limbs instead of four?
The four-limbed body plan that dominates higher-order animals dates back to the time when lobe-finned fish made the transition to terra firma. They could have just as easily had six fins, and if they had, scientists think life would have stayed low to the ground, and large, intelligent animals might never have evolved. [Get the full explanation]
What if the Neanderthals had not gone extinct?
Neanderthals could have persisted in pockets in Europe even until modern times, and it's possible they would have the capacity to think, speak and act much like us. But experts think it's far more likely they would have been assimilated, interbreeding with humans to create a hybrid species. [Get the full explanation]
What if there were no gravity?
If the universe had formed with no force of gravity, it would be completely flat and featureless. If it had developed normally up to the present moment, and then gravity suddenly shut off, we'd all go flying into space. [Get the full explanation]
What if Earth's magnetic poles flip?
It's not a question of if, but when. Every once in a while during Earth's history, north and south have switched. It happens when iron atoms in the liquid outer core gradually reverse their orientation over a few thousand years. The in-between phase, when the geomagnetic field is weak, is roughest on Earthlings. [Find out what might happen to you when the field flips.]
What if the forces that form molecules were stronger (or weaker)?
Molecules form when the protons in adjacent atoms "share" electrons. If the electromagnetic force that drives this relationship had a different strength, the universe would probably be devoid of life, and even stars and planets. The attraction between positively charged protons and negatively charged electrons is perfectly tuned at a value that allows atoms, and larger aggregates of atoms like those in biomolecules, to form. [Get the full explanation]
What if you only ate one type of food?
There is no one food that has it all. Choosing to eat only one fruit, or vegetable or grain would lead to organ failure. Consuming only meat would eventually force your body to start munching on your own muscles. And if you stuck solely to almost any single food besides fruit, you would develop a serious case of scurvy. All single-food roads lead to death. [Find out the best thing for a single-food diet.]
What if the sun were half as big?
At that size, the sun would be much cooler and redder, a stellar variety known as a "red dwarf." The band around it that would be suitable for life, known as the "habitable zone," would be much smaller, and Earth would not be in it because our water would freeze solid. Mercury, on the other hand, would be sitting pretty. [Get the full explanation]
What would happen if you shot a gun in space?
Guns can shoot in space, and this allows for all kinds of absurd scenarios. If you're in the vacuum between galaxies, pulling the trigger will send you and your bullet careening through space literally forever. If you shoot a gun in the solar system, your bullet will get sucked toward the sun or one of the giant planets. And if you shoot a gun toward the horizon while standing on a mountain on the moon, you could theoretically shoot yourself in the back. [See the crazy gun Russian cosmonauts take into space.]
What if all of America's toilets were flushed simultaneously?
If we coordinated the simultaneous flushing of all 350 million toilets in the United States, there would likely be pipe explosions in some places caused by a shortage of water for refilling all the bowls (and thus, air entering the pipes, causing pressure changes). But for the most part, crisis would be averted by a technicality: Each toilet is located a different distance away from the main sewage line, so simultaneous flushes are actually staggered when it comes to delivering their water to the main. Rather than a peak in sewage flow, the system would receive a smooth curve of commode contributions. [Get the full explanation]
What if there were no seasons?
If Earth's axis weren't tilted, there would be no seasonal variation, and humans would be confined to the continuous warmth of the planet's mid-section. We most likely would never have developed advanced agriculture, as most staple crops require cold winters, and would constantly be plagued by horrific insect-borne diseases. Forget the Industrial Revolution and the modern conveniences that sprung from it: Much of our technology has its roots in the existence of winter, because it is a by-product of inventions of new and better ways to keep warm. [Read more about life in a season-less world]
What if humans had eagle vision?
If you swapped your eyes for an eagle's, you could see an ant crawling on the ground from the roof of a 10-story building. You could make out the expressions on basketball players' faces from the worst seats in the arena. Objects directly in your line of sight would appear magnified, and everything would be brilliantly colored, rendered in an inconceivable array of shades. [Read more about life with eagle eyes]
What would we do if we discovered aliens?
It is theoretically possible (if unlikely) that we could find alien life on Mars or in the underground lakes of Europa, a moon of Jupiter (pictured). Samples of simple, non-sentient life forms would be collected for careful laboratory analysis, while more advanced beings would be approached with extreme caution. We would take a series of steps to ensure that human-alien contact would be beneficial for both civilizations before proceeding. [Read about the seven steps to contact]
What would happen if you fell into a volcano?
Lava's high density and low viscosity means you'd smack onto the surface of a volcano's lava pit rather than sinking in. Because of the tremendous heat, you would burst into flames, and the gases given off my your burning flesh would react with the lava, creating mini eruptions called fountaining. [Find out more about death by volcano]