Entangled particles will always have opposite spins, no matter what, no matter how far apart they are; even if they are at opposite ends of the Universe. That’s what makes quantum entanglement so difficult to understand. Entangled particles seem to share information faster than the speed of light. And not just a little bit faster in a margin-of-error kind of way. We’re talking quadrillions of times faster. Infinitely fast. Instantaneously, in fact. No matter how far apart entangled particles are, if you change one, the other does the opposite. Always. 100 percent of the time.

This “spooky action at a distance” as Albert Einstein called it has baffled physicists for over a century. Einstein believed that information between particles can never travel faster than light, so somehow, observing a particle and inferring its partner’s spin is merely recognizing information that was already there. Neils Bohr, that other early 20th century genius who gave us the model for the atom, however, disagreed. He believed that the act of observation determined the spin of one particle, thus changing the spin of the other. In other words, the spin of a particle is NOT predetermined; meaning a particle won’t spin up, down, left, right, etc until it’s observed.

Well, in 1964 Irish physicist John Bell performed an experiment that proved that Bohr was right, but only about half-way. It turns out, a particle’s spin is not only determined by observation, but that the spin is apparently chosen, at random, by the particle. Which means that its partner, which could be at the opposite end of the Universe, somehow knows which way to spin even though it has no idea which way its partner will spin in the first place. And again, this information is somehow shared instantaneously.

Let’s break this down into an everyday kinda situation. Pretend you’re sitting in Central Park and with you is a marble in a closed box, whose color is uncertain. Could be red, could be green. You best friend is in Trafalgar Square in London, also with a marble with two potential colors in a closed box. Neither of you have any idea, nor any way of predicting, which color your marble will be when you open the box. And of course, the marbles are entangled for the sake of this thought experiment.

The Central Park marble’s box is opened and the marble appears red. This means that the marble in London will absolutely, positively be green. And vice versa. But there is no way to know which color either will be, until one of them is observed, at which point the color of the other is determined before the box is even opened.

...and this was the simplified version.

Yeah. Weird. Why? Physicists are still struggling with this. We just know that this is what it is. There are many theories as to why particles behave this way, from connections via wormholes, to there being an infinite number of parallel Universes, to the Universe being a computer simulation where time and space are mere illusions (which totally solves this problem, incidentally).

What’s really cool, though, is that quantum entanglement has practical applications, primarily in quantum computers, cyber-security and communication encryption. Using quantum entanglement for security means that system is unhackable, accessible only with a special quantum key, specifically programmed for a particular pair of particles.

Spooky action indeed.

Number 6: Black holes.

Black holes are the remains of giant stars that die in supernova explosions that have so much mass, their cores collapse into an infinitely dense point known as a singularity. Nothing can escape the pull of a black hole, not even light. Black holes are where the known laws of physics break down.

Black holes are everywhere and dominate the Universe. Most black holes have masses from 3 to a few dozen Suns. These are known as stellar mass black holes. There are millions of them in our own galaxy. But the really big ones, known as supermassive black holes, are what holds galaxies together and what may be responsible for the existence of galaxies in the first place.

Supermassive black holes of millions to billions of times the mass of the Sun lurk at the center of every large galaxy. The Milky Way’s central supermassive black hole, Sagittarius A*, has a mass of 4 million Suns. And that’s small compared to some of the real monsters out there. In 2012, the biggest black hole yet discovered weighs in at a gargantuan 17 billion solar masses. Ironically, it sits at the center of a small galaxy, roughly 250 million light years away.

Supermassive black holes at the centers of galaxies that are actively feeding on matter are known as Quasars, and they are the most powerful objects in the Universe. They emit as much energy as a trillion Suns and outshine whole galaxies, and are visible across the entire visible Universe. Their defining characteristics are massive, concentrated jets of energy and charged particles reaching tens of thousands of light years into space.You don’t want to be in the pathway of one of these things.

Since gravity is infinite in the center of a black hole, the closer you get to its edge, known as the event horizon (the point past which nothing can escape), the more time slows down, relative to the observer. So if you and a friend were in space and your friend decided he wanted to jump in, time for him would pass normally as gravity pulls him apart atom by atom in a process known as spaghettification. YOU, on the other hand, would see him approaching the black hole, slower, and slower, and slower, until eventually, it would appear as though he stopped falling in as he approached the event horizon. Time and space are warped so much that it would take an infinite amount of time for you to see him actually fall into the black hole.

Pretty cool right? This time dilation has some serious implications for potential time travel. In theory, like in the movie Interstellar, a spacecraft could use the gravity and angular momentum around a black hole to shoot forward into time, since time would pass slower the closer a traveler is to a black hole (relative to the outside world, of course). Going backwards in time, however, is probably impossible, since you’d have to travel faster than light, and it opens up all sorts of nasty paradoxes that no one would really enjoy dealing with.

Anyway, black holes pose some problems for classical physics, if not solely for the mystery of where stuff goes once it falls inside. Matter, energy, nor information can be destroyed, so they must take some other form, or go somewhere else entirely.

There are a number of theories surrounding what happens to material that falls into a black hole. The holographic principle suggests that the information is encoded on the 2-dimensional surface of the event horizon. Some have theorized that matter that goes down the cosmic drain buds off into a new Universe, creating a new Big Bang. This theory, however, hints at the possibility that our Universe itself exists inside a black hole, which means that all of everything is just an infinitely deep rabbit hole of Universes inside of black holes. I… yeah. But hey, why not.

Since black holes absorb light, we cannot see them directly, but we can infer their existence by what orbits them and the glowing clouds of gas and dust swirling around their event horizons, known as accretion discs. This matter is moving at millions of miles per hour and can reach temperatures of billions of degrees. But like the stars that collapsed to form them, black holes too will one day evaporate due to Hawking radiation (more on this in the final section).

Number 7: Time.

Everyone's an expert on time even though no one knows what the hell it is. Seriously, think about it. What is it? Is it the passage of events? Is it you passing through predetermined sets of events? Is it a countdown to the end of the Universe? Let’s assess.

Time began, we think, at the moment of the Big Bang. We perceive ourselves to be moving through time in a forward direction, known as the arrow of time. But time is a lot stranger that we ever imagined, thanks to Einstein’s theory of special relativity. And, the laws of physics operate independently of time, meaning they can run forwards or backwards. E=MC^2 and the other equations of classical physics do not involve time at all.

First, time ticks at different rates everywhere in the Universe, relative to the observer. The closer you are to massive objects with strong gravitational fields, the slower time appears to pass to the observer. So, technically speaking, your time passes more slowly for your feet than for your head, since they are closer to the Earth. Anything with mass has gravity; galaxies, stars, planets, people, cats, even atoms have gravity.

Second, the faster you travel, the slower time appears to pass to you, rather than an outside observer. Light, having the maximum possible speed in the Universe, experiences no time at all. Time stops when you reach the speed of light.

These are all pretty well-known principles, but what if it’s all wrong? What if time is, in fact, counting down rather than moving forward? This is what the Second Law of Thermodynamics, known as entropy, may suggest. This fundamental law of nature, represented by the equation

S = k. Log W, is immortalized on its discoverer Ludwig Bolzmann’s tombstone.