Transcription
Hi, you're on a rock. It's floating through an infinite void at 67,000 mph, spinning at 1,000 mph, orbiting a giant ball of nuclear fire, which is itself hurtling through a galaxy at 500,000 mph. And you're worried about your inbox?
This is space. It's everything beyond Earth's thin blue atmosphere. And despite what you might think, it's not empty. It's full of galaxies, radiation, black holes, dark matter, and questions we can't answer yet. The observable universe stretches 93 billion light-years across. We physically explored approximately 0.000001% of it. We've sent robots to a few nearby rocks and a couple of humans to our own moon. That's it. But we figured out a lot just by looking. Here's everything we actually know about space.
Your cosmic address. Let's establish where you are, like really where you are. You're on Earth, a planet 7,918 miles in diameter. Earth orbits the sun, one star among billions. The sun sits about 26,000 light-years from the center of the Milky Way galaxy. The Milky Way is part of the local group, a collection of about 80 galaxies held together by gravity. The local group is part of the Virgo Supercluster, which contains thousands of galaxies. The Virgo Supercluster is part of an even larger structure called Laniakea, which contains 100,000 galaxies. And Laniakea is just one of millions of superclusters in the observable universe. Your address, cosmically speaking, Earth, solar system, Orion arm, Milky Way galaxy, local group, Virgo Supercluster, Laniakea Supercluster, observable universe.
Carl Sagan called Earth a pale blue dot. From just past Saturn, that's all we look like, a pixel. Everything that has ever happened in human history happened on that pixel. Earth is about 4.5 billion years old. The universe is 13.8 billion years old. Humans have existed for about 300,000 years, which means we've been around for 0.002% of Earth's history and 0.00002% of cosmic history. We're extremely recent additions to a very old party.
Here's something that might keep you up at night. You are literally made of dead stars. Every atom in your body heavier than hydrogen was forged inside a star that exploded billions of years ago. The iron in your blood, the calcium in your bones, the oxygen you're breathing, all of it was cooked up in stellar furnaces and scattered across the cosmos when those stars died. You are recycled stardust that learned to think about itself.
The solar system. Let's start local. Our solar system is the sun plus everything gravitationally bound to it. The sun is the main character. It contains 99.86% of all the mass in the solar system. Everything else, all the planets, moons, asteroids, and comets combined is a rounding error. The sun is a giant ball of hydrogen and helium, 864,000 mi in diameter, with a core temperature of 27 million degrees Fahrenheit. It's powered by nuclear fusion. Hydrogen atoms smash together to form helium, releasing energy in the process. This has been happening for 4.6 billion years and will continue for another 5 billion years or so.
Orbiting the sun, we have eight planets divided into two categories. The inner rocky planets are small and solid. Mercury is the closest to the sun, scorched on one side and frozen on the other with no atmosphere to distribute heat. It's basically a crater-covered ball bearing. Venus is Earth's evil twin, similar size but with a runaway greenhouse effect that makes it hotter than Mercury despite being farther from the sun. Surface temperature, 900° Fahrenheit. Atmospheric pressure, 90 times Earth's. Sulfuric acid clouds. It's hell with better branding. Earth is the Goldilocks planet, not too hot, not too cold. Liquid water, magnetic field blocking radiation, just right for life. We got lucky. Mars is rusty and dead. It once had liquid water, a thicker atmosphere, maybe even life. Now it's a frozen desert with air so thin you'd die in seconds, but it's our best candidate for colonization, mostly because everything else is worse.
Between Mars and Jupiter sits the asteroid belt, millions of rocky objects left over from the solar system's formation. They never became a planet because Jupiter's gravity kept disrupting them. The outer gas giants are massive and weird. Jupiter is the king. 1,300 Earths could fit inside it. It's mostly hydrogen and helium with no solid surface to stand on. It's Great Red Spot is a storm larger than Earth that's been raging for at least 400 years. Jupiter also acts as a cosmic shield, its massive gravity capturing or deflecting asteroids that might otherwise hit Earth. We owe Jupiter some thank you cards. Saturn is the show-off with its famous rings made of billions of particles of ice and rock ranging from tiny grains to house-sized chunks. Fun fact, Saturn's density is so low that if you found a big enough bathtub, Saturn would float. Uranus and Neptune are the ice giants. Uranus rotates on its side, probably knocked over by a collision early in its history. Neptune has the fastest winds in the solar system reaching 1,200 mph.
Beyond Neptune is the Kuiper Belt, a region of frozen debris including dwarf planets like Pluto. Yes, Pluto got demoted and yes, people are still upset about it. Even farther out is the Oort Cloud, a theoretical shell of icy objects that might extend halfway to the nearest star. Comets come from here, chunks of ice knocked loose by gravitational disturbances that fall toward the Sun and grow tails as they evaporate. Our solar system also has over 200 moons, some more interesting than the planets they orbit. Europa, a moon of Jupiter, has a global ocean beneath its icy crust that might contain more water than all of Earth's oceans combined. Where there's water, there might be life. Titan, Saturn's largest moon, has a thick atmosphere and lakes of liquid methane. It's the only moon with a substantial atmosphere and the only place besides Earth with stable liquid on its surface.
Stars. Stars are giant balls of gas undergoing nuclear fusion. That's it. That's what they are. In the core of a star, temperatures and pressures are so extreme that hydrogen atoms fuse together to form helium. This releases enormous amounts of energy as light and heat. The outward pressure from fusion balances the inward pull of gravity and the star remains stable for millions or billions of years. Our Sun is classified as a G-type main sequence star, also called a yellow dwarf. It's medium-sized and medium-aged, pretty average as stars go. But stars come in a huge range of sizes. Red dwarfs are the smallest, only about 10 to 50% of the sun's mass. They burn their fuel slowly and can live for trillions of years. Most stars in the galaxy are red dwarfs, but they're so dim we can barely see them. At the other extreme are super giants like Betelgeuse, which is about 700 times the diameter of the sun. If you put Betelgeuse where the sun is, it would swallow Mercury, Venus, Earth, and Mars and reach nearly to Jupiter. These massive stars burn through their fuel quickly and live only a few million years.
The life cycle of a star depends on its mass. Average stars like the sun will eventually exhaust their hydrogen fuel, swell into red giants, shed their outer layers, and leave behind a dense core called a white dwarf. This is what will happen to our sun in about 5 billion years. It will expand to engulf Mercury and Venus, maybe Earth, then shrink into a glowing ember that will cool over trillions of years. Massive stars die more dramatically. When they run out of fuel, their cores collapse catastrophically in a supernova explosion, briefly outshining entire galaxies. What's left behind is either a neutron star, an incredibly dense object where a teaspoon would weigh a billion tons, or if the star was massive enough, a black hole.
Here's a number to consider. There are more stars in the observable universe than grains of sand on all the beaches on Earth, around 200 billion trillion stars, give or take. And most of them probably have planets. And remember, every atom in your body heavier than hydrogen, the carbon, oxygen, nitrogen, iron, calcium, all of it was created inside stars and scattered by their explosions. You are literally made of stellar nuclear waste. That's not an insult, that's just chemistry.
Galaxies, stars don't drift alone through space. They cluster together in massive structures called galaxies, held together by gravity. Our galaxy is the Milky Way. It contains somewhere between 200 and 400 billion stars spread across a disk about 100,000 light years in diameter. That means light traveling at 186,000 miles per second takes 100,000 years to cross from one side to the other. The Milky Way is so big that we've never seen it from the outside. Every image of a spiral galaxy you've seen is another galaxy. We can only photograph our own from the inside.
Galaxies come in different shapes. Spiral galaxies like ours have a central bulge surrounded by rotating arms of stars, gas, and dust. They're actively forming new stars. Elliptical galaxies are older and blob-shaped, containing mostly old stars with little gas for new star formation. Irregular galaxies are chaotic, often the result of collisions or gravitational disturbances. At the center of most large galaxies sits a supermassive black hole, millions to billions of times the mass of the sun. Our galaxy's central black hole is called Sagittarius A, and it's about 4 million solar masses. These monsters anchor their galaxies, and everything orbits around them.
The nearest large galaxy to ours is Andromeda, about 2.5 million light-years away. It's slightly larger than the Milky Way and contains about a trillion stars. Here's the thing. Andromeda is heading toward us at about 250,000 miles per hour. In roughly 4.5 billion years, we're going to collide. But don't panic. Galaxies are mostly empty space, so the chance of any two stars actually hitting each other is almost zero. The collision will be more like two clouds passing through each other. The gravitational chaos will scatter stars into new orbits, and eventually the two galaxies will merge into one giant elliptical galaxy. Astronomers have already named it No Camita, which is a terrible name, but we'll all be dead anyway.
The observable universe contains approximately 2 trillion galaxies. Some are small dwarf galaxies with only a few billion stars. Others are giants containing trillions, and most of space is the unimaginable emptiness between them, vast voids where almost nothing exists.
Black holes. When a massive star dies and its core collapses, gravity can win completely. If enough mass is crushed into a small enough space, even light can't escape. That's a black hole. At the center of a black hole is the singularity, a point of theoretically infinite density and zero volume. All the mass is concentrated there. The laws of physics as we understand them break down at the singularity. We literally don't know what happens there. Surrounding the singularity is the event horizon, the point of no return. Cross it and you're trapped forever. Nothing, not even light traveling at 186,000 mi per second, can escape. That's why it's black. Light goes in but doesn't come out.
Black holes come in different sizes. Stellar black holes form from collapsed stars and are typically five to 50 times the mass of the sun. Supermassive holes sit at the centers of galaxies and can be millions to billions of solar masses. We're not entirely sure how the supermassive ones form. They might grow by consuming matter over billions of years or they might form differently. Near a black hole, time slows down. This is Einstein's general relativity in action. Gravity warps space-time and strong gravity warps it severely. If you watch someone fall toward a black hole, you'd see them slow down and freeze at the event horizon, redshifting until they faded from view. From their perspective, they'd cross the horizon in normal time and be torn apart by tidal forces. Black holes don't suck like vacuums in movies. They're just gravity wells. If you replace the sun with a black hole of the same mass, Earth would continue orbiting normally. You'd freeze to death without sunlight, but you wouldn't get sucked in.
In 2019, we captured the first image of a black hole, specifically the supermassive black hole at the center of galaxy M87, 55 million light years away. It took a network of radio telescopes spanning the entire planet, collectively functioning as an Earth-size telescope, to do it. The image shows the black hole's shadow against a glowing ring of superheated matter swirling around it. Stephen Hawking discovered that black holes aren't completely black. They emit a tiny amount of radiation due to quantum effects near the event horizon. This is called Hawking radiation and it means black holes slowly evaporate over immense time scales. A stellar mass black hole would take something like 10^67 years to evaporate. A supermassive one would take 10^100 years. The universe is only 10^10 years old, so we won't be watching any black holes evaporate anytime soon.
Dark matter and dark energy. Here's where things get humbling. Everything we've talked about so far, stars, planets, galaxies, black holes, all of it is only about 5% of the universe. The remaining 95% is stuff we can't see, can't touch, and don't understand. Dark matter makes up about 27% of the universe. We can't detect it directly because it doesn't interact with light. It doesn't emit light, reflect light, or absorb light. It's invisible. But, we know it's there because of its gravitational effects. Here's the problem. Galaxies spin too fast. If you calculate how fast a galaxy should rotate based on the visible matter, the outer stars should fly off into space. They're moving too quickly for the visible gravity to hold them. But, they don't fly off, which means there must be extra invisible mass holding everything together. That invisible mass is dark matter. We don't know what dark matter is made of. It might be some undiscovered particle. It might be something weirder. Physicists have proposed candidates with names like WIMPs, weakly interacting massive particles, and axions. But, nobody has detected them yet.
Dark energy is even stranger, and it makes up about 68% of the universe. In 1998, astronomers discovered that the universe isn't just expanding. The expansion is accelerating. Something is pushing space apart faster and faster. That something is dark energy. We have even less understanding of dark energy than dark matter. It might be a property of space itself. It might be some kind of field permeating everything. It might be the cosmological constant Einstein proposed and then called his biggest blunder. We don't know. So, here's the humbling truth. 95% of the universe is made of stuff we can't see or explain. All of physics, all of chemistry, everything we think we understand applies to only 5% of what exists. We're not as smart as we think we are.
The Big Bang and cosmic history. 13.8 billion years ago, everything began. The universe started from a state of incredible density and temperature. This wasn't an explosion in space. It was the expansion of space itself. Space, time, matter, and energy all emerged together. Before the Big Bang, there was no before because time itself didn't exist yet. Asking what happened before the Big Bang is like asking what's north of the North Pole. The question doesn't quite make sense.
In the first fraction of a second, the fundamental forces separated. Particles and anti-particles formed and annihilated each other. A slight imbalance meant slightly more matter than anti-matter survived. Good thing, or we wouldn't be here. For the first 380,000 years, the universe was a plasma soup so hot that light couldn't travel through it. Photons kept scattering off particles. The universe was opaque. Then, as expansion cooled everything down, atoms formed. Electrons bound to protons to make hydrogen. Suddenly, light could travel freely. The universe became transparent. That first light is still out there, stretched by 13.8 billion years of expansion into microwave radiation. It's called the cosmic microwave background, and it's the oldest thing we can see. It's everywhere, filling the universe a faint glow leftover from the moment light broke free.
After about 100 million years, gravity pulled hydrogen clouds together until they ignited into the first stars. These stars were massive, lived fast, died young, and exploded, seeding the universe with heavier elements. New generations of stars formed from that enriched material, including smaller stars like our sun. Galaxies formed as gravity clumped stars together. The cosmic web took shape, filaments of galaxies connected by dark matter with vast voids between them. Our solar system formed about 4.6 billion years ago, roughly 9 billion years after the Big Bang. Earth formed shortly after, cooled down, developed oceans, and eventually spawned life. Humans showed up about 300,000 years ago, which is the last 0.000002% of cosmic history. We are late arrivals to a very old universe.
Exoplanets and alien life. For most of history, we didn't know if planets existed outside our solar system. They were too small and dim to see next to the glare of their stars. The first confirmed exoplanets were discovered in 1992, orbiting a pulsar. The first exoplanet around a sun-like star was found in 1995. Since then, we've discovered over 5,500 confirmed exoplanets with thousands more candidates awaiting confirmation. The Kepler space telescope alone found over 2,600. We find exoplanets mainly by two methods. The transit method watches for stars dimming slightly as planets pass in front of them. The radial velocity method detects the tiny wobble in a star's motion caused by an orbiting planet's gravity.
Exoplanets come in wild varieties. Hot Jupiters are gas giants orbiting closer to their stars than Mercury orbits the sun with surface temperatures hot enough to melt lead. Super-Earths are rocky planets larger than Earth but smaller than Neptune. Rogue planets drift through interstellar space not orbiting any star at all. The most exciting exoplanets are in the Goldilocks zone, the habitable zone where temperatures allow liquid water. Not too hot, not too cold, just right. Based on Kepler data, astronomers estimate there are billions of Earth-size planets in habitable zones within the Milky Way alone.
So, here's the big question. Where is everybody? If there are billions of potentially habitable planets in our galaxy and the galaxy is 13 billion years old, there should have been plenty of time for intelligent life to evolve and spread. Even at slow speeds, a civilization could colonize the entire galaxy in a few million years. Yet, we see no evidence of alien civilizations, no signals, no structures, no visitors. This is the Fermi paradox. If aliens exist, where are they? Possible explanations range from depressing to terrifying. Maybe intelligent life is extraordinarily rare. Maybe civilizations inevitably destroy themselves before becoming space-faring. Maybe there's a great filter, some barrier that prevents life from advancing and we either passed it already or it's ahead of us. Maybe aliens exist but are too far away or using communication methods we can't detect or deliberately hiding. Maybe we're the first intelligent species in the galaxy. We've been broadcasting radio signals into space for about 100 years, which means there's a bubble 100 light-years in radius where our signals have reached. In a galaxy 100,000 light-years across, that's a tiny patch. We've been listening through SETI for decades and heard nothing definitive. Maybe we're alone, maybe we're not. We don't know yet.
Space exploration, humans have been looking at the sky forever, but we've only been going there for about 70 years. 1957, the Soviet Union launched Sputnik, the first artificial satellite. It was a metal sphere the size of a beach ball that beeped from orbit. The space race began. 1961, Yuri Gagarin became the first human in space completing one orbit around Earth. He was 27 years old and spent 108 minutes in space. 1969, Apollo 11 landed on the moon. Neil Armstrong and Buzz Aldrin walked on another world while Michael Collins orbited above. One small step for man, one giant leap for mankind. We went back five more times then stopped. The last moonwalk was in 1972. No human has been beyond lower Earth orbit since.
The 1970s gave us Voyager 1 and Voyager 2, twin probes that flew past the outer planets and kept going. Voyager 1 is now over 15 billion miles from Earth, the most distant human-made object. It's technically in interstellar space beyond the sun's influence. It carries a golden record with sounds and images from Earth in case aliens find it. At its current speed, it would take about 70,000 years to reach the nearest star. The space shuttle program ran from 1981 to 2011 providing reusable spacecraft but at high cost and with two tragic losses, Challenger in 1986 and Columbia in 2003. 14 astronauts died. The International Space Station has been continuously occupied since 2000. Humans have maintained a permanent presence in space for over 25 years. The ISS orbits at about 250 miles up, which sounds far but is actually closer than Los Angeles is to San Francisco.
Mars has received numerous visitors. Spirit and Opportunity landed in 2004. Opportunity lasted until 2018. Curiosity arrived in 2012 and is still roving. Perseverance landed in 2021 with a helicopter named Ingenuity that achieved powered flight on another planet. The James Webb Space Telescope launched in 2021 and began sending images in 2022. It sees an infrared detecting light from the oldest and most distant objects in the universe. It's already looking at galaxies that formed less than 400 million years after the Big Bang.
Private space companies have changed the game. SpaceX developed reusable rockets, dramatically lowering launch costs. They land boosters like science fiction promised. Blue Origin and others are competing. Space tourism has begun. What's next? NASA's Artemis program aims to return humans to the moon, this time to stay. SpaceX is developing Starship for Mars missions. Elon Musk talks about Mars colonies. Whether that's realistic or delusional remains to be seen. We might see humans on Mars within our lifetimes. We might not.
How it all ends. Even the universe. In about 5 billion years, the sun will exhaust its hydrogen fuel. It will swell into a red giant, expanding past the orbits of Mercury and Venus, possibly reaching Earth. Even if Earth isn't swallowed, it will be scorched lifeless. Eventually, the sun will shed its outer layers and collapse into a white dwarf, slowly cooling over trillions of years. But that's just our local apocalypse. The universe has its own timeline.
Stars will continue forming for another 100 trillion years or so, as long as there's hydrogen to fuel them. Then star formation will cease. Existing stars will burn out. The universe will go dark. What remains are stellar remnants, white dwarfs, neutron stars, black holes. These will persist for an almost incomprehensible time. But even they aren't eternal. White dwarfs will cool until they're black dwarfs, invisible and cold. This takes longer than the current age of the universe. Protons might decay over 10^40 years, meaning even atoms will eventually fall apart. Black holes will be the last objects remaining. But even they evaporate through Hawking radiation. The largest supermassive black holes will take about 10^100 years to fully evaporate. That's one followed by 100 zeros years. After that, nothing. Maximum entropy, the heat death of the universe. No stars, no planets, no matter, no structure, just a cold, dark, diffuse sea of particles and radiation slowly spreading out forever. Nothing will ever happen again unless the universe has other plans. Maybe dark energy will accelerate expansion until space itself tears apart. The big rip. Maybe the universe will eventually collapse back on itself. The big crunch. Maybe it will bounce and start over. The big bounce. We don't know. Either way, enjoy while it lasts.
So, here's where we are. We're a tiny species on a tiny rock orbiting an average star in an ordinary galaxy in one corner of an incomprehensibly vast universe. We've existed for the blink of a cosmic eye. In the time scale of the universe, human civilization is a rounding error. And yet somehow we figured things out. We know where we came from. We know how stars are born and die. We know the universe is 13.8 billion years old and started from a hot dense state. We know about dark matter and dark energy even though we can't see them. We know our atoms were forged in stars. The universe doesn't care about us. It's not hostile. It's just indifferent. Space will continue existing whether or not humans are around to observe it. But that makes our existence kind of miraculous. Out of all the cold dead matter in the cosmos, some of it came together on this particular rock in a way that produced life, then consciousness, then curiosity. The universe made something that can wonder about itself. We're stardust that learned to ask questions. That's pretty remarkable even if nobody's listening to the answers. That's space. 93 billion light years of it and we've barely scratched the surface. Okay, I need to go sit quietly in existential contemplation for a while. If this video made you feel appropriately small and cosmically insignificant, hit the like button. That's how the algorithm works and the algorithm is the only god we can prove exists.