Artificial Gravity

In most space ships in sci-fi movies, people stand on the ground or sit in chairs. This makes sense intuitively on two levels. For one, we all naturally spend almost all of our lives standing, sitting, and lying down. Space travel seems like just another method of transportation, so it feels like it should be similar to what it is like to travel in a car, bus, train, or plane. Secondly, it’s historically been really hard to depict weightlessness on a movie budget.

The Millennium Falcon is designed for walking.

But we know there isn’t a universal “down,” and we aren’t magically stuck to the ground. Instead, mass attracts other mass with a force called gravity, and we are stuck to the ground on Earth because the planet’s giant mass is pulling our relatively tiny masses toward its center. In space, we don’t have that. Instead, small objects orbit large masses like planets, moons, and stars, being pulled in the direction of all the nearest gravitational forces added together. This leads to “free fall,” also called “microgravity,” (or incorrectly, “zero gravity”) in which occupants of a spacecraft float in the air, pulled along the same currents of gravity as the ship they are inside. There is no up or down, no floor nor ceiling.

This leads us to the question, can we create artificial gravity like in the movies? The answer, which might surprise you, is yes. There are in fact several ways, one of which we might see in ten years or less.

The first is to use acceleration to mimic the effect of gravity on the surface of the Earth. The Earth pulls downward on us with an acceleration of 9.8 m/s², which means if we were freely falling, we would speed up another 9.8 meters per second every second. It is also called 1 g. But we are stopped from falling by the ground, which pushes us upward with the exact force needed to counter gravity, because the Earth is packed full of solid and liquid matter. In space, without this balance of gravity and ground, we could get the same effect by having the ship accelerate at 1 g. The back walls of the ship push on us with the same force as the surface of the Earth, becoming the floor. And there we have it, artificial gravity.

behold my awesome MS Paint skills

Accelerating a space ship at 1 g is really hard. Yes, we do several g’s to get rockets into space, but a rocket’s mass has to be over 90% fuel just to make it to low Earth orbit, so keeping up 1 g even for an hour is beyond us right now, much less the weeks it would take to visit anywhere farther than the moon. A more feasible approach is spin gravity.

Newton’s first law says an object in motion will keep its speed and direction unless acted upon by an outside force. If a force is applied parallel to the direction of motion, the speed will change. If the force is applied perpendicular to the motion, its direction will change. In order for something to move in a circle, a constant force must be pulling it toward the center. Thus, our second method of giving space ships artificial gravity is to make them spin.

The direction of gravity, “down,” is the direction opposite the direction our surroundings push us. The ground pushes us up, therefore on Earth, down is down. A rocket pushes us forward, therefore down is toward its tail. A rotating space station pulls us inward, therefore down is outward. This illusory sense of being pushed away from the center of something spinning is called centrifugal force. So if we set a space station spinning, we get artificial gravity pointing away from the axis of rotation, and can walk around on the rim of the wheel or curved side of the cylinder.

The first problem we’ll have to face is making a ship big enough that standing up won’t make you dizzy. If you’re in a ship with spin gravity, your head experiences less gravity than your feet, because it is closer to the center. The significance of the difference depends on what percentage of the radius your body takes up. The smaller the circle, the greater the difference in gravity between your head and your feet. I don’t know how big it would have to be not to make you sick, but I expect we wouldn’t want a diameter smaller than a 6-story building.

There is one more option, which is much more futuristic. It is, drum roll please, straight up old fashioned gravity. Just cram enough mass into the center of our ship that it has its own gravity. These types of space ships occur naturally, and we call them planets.

Which leads us to the downside of using this type of gravity: it’s really freakin’ hard to get your ship to move. According to Newton’s second law, the more mass something has, the more force is needed to accelerate it. We’re not going to sail Earth around the solar system by pointing a rocket engine at the ground and firing its exhaust into space. We can get around that problem by building our ship around a black hole. A black hole can give an Earth’s worth of gravity to a normal-sized ship, using a whole lot less mass.

Suppose you had a black hole the mass of the Earth, 6*10²⁴ kilograms. How far away from it do you think you would have to be in order to experience 1 g of gravity? The answer: exactly the radius of the Earth. Whether the mass of the Earth is the size of a planet or a marble, the strength of its gravity is the same at the same distance from its center. If we went inside the Earth, gravity would get weaker, because some of Earth’s mass would be above us. But if we got closer to an Earth-mass black hole, the gravity would get stronger, because all of that mass would still be below us. Thus, to have a small space ship with a black hole providing 1 g, much less mass is required.

How much mass? To answer that, we need to know how big our ship is going to be. The tidal force (head-to-foot difference) for a black hole scales differently from spin gravity, so our black hole ship will have to be bigger than 6 stories. Let’s say 10 stories, or 30 meters. Using Newton’s law of gravity (which doesn’t have a number), a radius of 15 meters, and 1 g of gravity, we calculate the mass of the black hole to be 30 trillion kilograms. That seems like a lot, but it is just the mass of a small mountain, 100 billion times less massive than the Earth.

This black hole would be the size of a proton, and give off 400 kilowatts of power in Hawking radiation, which you could use to power your ship’s life support, and have about the same acceleration as an ion thruster on a satellite of normal mass of about .00001 g. So a properly-sized black hole could supply a ship’s artificial gravity its power, and a small amount of thrust.

If we had the technology to make micro black holes, options would become available. The smaller a black hole, the more power it gives off. We could opt for a slow-accelerating ship with a black hole providing gravity, or we could use a much smaller black hole and accelerate at 1 g. Heck, we could even have two black holes in the same ship, one for gravity, and one for thrust. Or more, if we wanted to build a larger ship.

Artificial gravity may seem like pure science fiction, but as we have seen, there are ways to do it in real life, one of which, spin gravity, isn’t even that hard. Someday, perhaps even soon, we will have space stations we can walk around in, almost just like we do on Earth. They won’t look like the space planes or battleships we see in science fiction. Rather, they’ll be something new and unique, wheels and cannisters speeding through the solar system.

From 2001: A Space Odyssey

Black Holes are a Thousand Times Weirder than You Think

In the depths of space, there are objects that undermine everything we feel we know about space and time. These objects are black holes.

A realistic depiction of what a black hole would look like up close, using the universe simulator Space Engine.

A black hole forms when an amount of matter is squeezed smaller than its Schwarzschild radius, the size at which its gravity is so strong that nothing, including light, can escape. When this happens, an event horizon forms at the S-radius, and the matter inside collapses to form a singularity. We’ll get a feel for what these mean as this post goes on.

The incredible amount an object must be compressed to form a black hole is staggering. In order to turn the sun into a black hole, we would have to crush it down to 6 kilometers across. That’s the total distance I walked every day to and from my university when I was in graduate school. If you were to compress the Earth into a black hole, it would be the size of a marble. If you didn’t have to worry about it tearing you apart, you could hold it in the palm of your hand.

Why do black holes look the way they do? Let’s start with a lone black hole in the cold depths of space. Since light can’t escape, it looks like a circle of blackness, and because the strength of their gravity is so great that they bend light around them, they distort the stars behind them like a lens.

From the YouTube video, “Colonizing Black Holes,” by Isaac Arthur

When a black hole is feeding on material around it, perhaps from a companion star or an object that came too close to it and was torn apart, it gets an accretion disk. Why does a disk form, rather than all the matter just falling in? There are two reasons working together. First, we have angular momentum. The matter isn’t falling straight into the black hole, but orbiting around it at insanely high speeds. Because of this, any stray matter that collides with the disk becomes part of the disk. Secondly, as the matter gets closer to the black hole, it crams together and can only go in so much at a time, like sand trickling through an hourglass.

An artist's impression of an accreting black hole. From Wikipedia

We can see an example of angular momentum creating a disk in the above artist’s rendition of a black hole accreting matter from a companion star. The star and black hole are revolving around each other, so the stream going from the star to the accretion disk curves in the forward direction, partially because of the star’s inertia, and partially because the black hole moves out of the way and the stream has to catch up.

With all that matter spiraling inward, feeding black holes build up quite the magnetic field. This magnetic field points outward from the poles, so instead of falling into the event horizon, some of the matter gets shot away at extreme speeds by these magnetic fields in two jets, as can be seen in the above picture.

As the matter in the disk crams together, it heats up due to friction, which causes the accretion disk to glow. The closer to the center, the hotter it gets, making it go from red on the outside to white on the inside. Because of how fast it is revolving, the light gets Doppler shifted, brighter and whiter on the side moving toward us, and dimmer and redder on the side moving away from us. This is heightened by the fact that the disk is spinning close to the speed of light.

A realistic depiction of a black hole up close, from the movie, Interstellar

You might notice that there is a halo around the event horizon. That’s an illusion, an image of the accretion disk on the back side of the black hole, which we can see both above and below it because of the light bending around the event horizon.

Up until 2015, we didn’t have direct observations of black holes. Now we do, first by gravitational waves with LIGO, and then a few years later by the Event Horizon Telescope, a composite telescope that uses advanced techniques to put together data from observatories all over the world, giving us an effective dish size as big as the entire Earth. Here is what that image, given artificial color, looks like.

The supermassive black hole at the center of galaxy M87.

The black circle in the center is the shadow, a blown-up view of the entire event horizon, front and back, because of the way light bends around it. If you could see it without any lensing effects, the event horizon would be a fraction of its size. There is a great YouTube video by Veritasium explaining this in more detail.

Black holes look cool, and a lot of interesting science goes into why the look like they do. But there’s more, oh so much more. So now it’s time to get to the good stuff: what black holes do to space and time. In order to discuss this, we have to be comfortable with space-time relativity, the principle that space and time are parts of the same thing, and this space-time can be curved and stretched. We talked about this in our discussion of faster-than-light travel and time travel, but here’s a little refresher.

When drawing space-time diagrams, we draw one dimension of space on the horizontal axis, and time on the vertical axis. An object sitting still will move straight up on the diagram, parallel to the time axis. An object moving will travel at a diagonal angle. We scale the plot so that light beams travel at 45-degree angles.

We choose a point to be the origin of the plot, which we call “here and now.” This could be you, it could be a space ship, a rock, or just a random point in space-time. From this point, we draw the light beams that go out into the future and come in from the past. If you imagine a third axis pointing out of the screen, these light beams circle around the time axis, making cones, which we call the past and future light cones. In reality, there is also a third dimension of space, making it a hypercone, but it is very hard to visualize four dimensions, so we talk about light cones.

From Wikipedia

No matter where you are or what the space-time curvature around you looks like, your light cone looks like this diagram. But someone else’s light cone at a different point in space could be distorted compared to your coordinate system. For instance, if we are in normal space far away from a black hole, then the light cone of something falling into a black hole curves to point toward the event horizon. And after an object has passed the event horizon, its entire light cone points toward the singularity. There is no direction it could go that would not end up at the singularity.

This is confusing, and not good for calculating. A much better and more intuitive way to understand black holes is with a Penrose diagram. To make a Penrose diagram, we use a math trick to change our coordinates so that we compress all of space-time into a diamond. The two bottom edges of the diamond are infinitely far back in time, and the top right is where time reaches future infinity. The top left is the event horizon of the black hole. On the other side of the event horizon, we join another half-diamond, representing the inside of the black hole.

In this diagram, lines of constant position and constant time are curved. Yet light always moves at 45-degree angles, making it easy to tell what observers in each region would see. Imagine two people, person A staying outside, person B falling in, sending signals to each other. To A outside, it would take an infinite amount of time for B to reach the event horizon, and A could keep receiving signals from B forever. However, in B’s reference frame, B passes the event horizon in a finite amount of time no problem. Then, a short time later, B reaches the singularity. Contrary to popular myth, B does not see the entire future of the universe, as we can see from the following diagram.

The paths of A and B exchanging light beams as they go. Everything moves upward, because that is the direction of time. As we can see, once B passes the event horizon, their light beams end at the singularity. B can receive signals from A, but there comes a point when A’s signals reach the singularity without reaching B first.

With a Penrose diagram, we can see a startling conclusion: inside the black hole, space and time have switched places! The singularity is no longer a point in space, but a moment in time, which all things inside the event horizon move inexorably toward. In addition, according to an observer outside the event horizon, everything that happens inside takes place after an infinite amount of time has passed!

Have a moment’s rest before we continue on. What, did you think that was as crazy as it gets? We haven’t even gotten to wormholes yet.

To get there, we first have to talk about spin. Everything floating in free space has a little bit of angular momentum, its outer parts rotating around its center of mass. Black holes are no different; the matter that forms and feeds them spirals in, contributing to their angular momentum. As a result, black holes have a property called spin. This spin prevents the matter from falling all the way to a single point, as the closer it gets the more centrifugal inertia it has to counteract the gravity. What this means is that the singularity is not a point, but a ring of zero thickness. If the spin is so high that the radius of the ring is larger than its Schwarzschild radius, then there is no event horizon, and it is called a naked singularity. Currently, no naked singularities are known to exist.

A simulation of what a naked singularity would look like from above.

If mass distorts space-time, do black holes pull space around them as they spin? Yes, they do. It’s called “frame dragging,” a name as cool as the phenomenon it describes. If we were to drop something directly toward a black hole near its plane of rotation, it would spiral around the black hole as it fell, due to the motion of space itself. There is a region near the event horizon called the ergosphere, where space is pulled so fast that an object has to travel faster than light in the opposite direction just to stay in the same place. The ergosphere is outside the event horizon, so in principle it is possible to send a ship or a probe into it and back out again. Now that would be a trip!

From Wikipedia

All right, now we’re ready to get to wormholes. Since black hole singularities are rings, it stands to reason that it’s possible to go through it. What happens? Well, you might think you just end up on the other side of the ring. After all, that’s what happens when you pass through any other kind of ring. But something weird happens. When you were a child, did you ever imagine you could go around a lamp post or a tree and find yourself somewhere else? That’s what happens when you pass through a ring singularity. Let’s look at the Penrose diagram:

With a path through the singularity, we can see that there is another side. When we get there, instead of the other side of the black hole, we find something called a white hole, an object with repulsive gravity and an event horizon nothing can get into. No one knows if white holes exist, and we don’t know of anything that has repulsive gravity. Thus, we have a one-way wormhole, with a black hole on one side nothing can get out of, and a white hole on the other side nothing can get into. What is on the other side? It could be a different place in the universe, maybe at a different time. Or, it could be another universe altogether, a space-time continuum completely separate from ours except for this one bridge.

We have never seen a white hole. This does not bode well for the possibility of their existence, since they would probably be extremely bright, launching out everything that fell into their black hole partner at relativistic speeds. Except, well, maybe we have. There is one thing that had a singularity and looks kind of what we’d expect a white hole to look like: the big bang. It’s common to think of the big bang as the beginning of everything, including time, but it might not be. Maybe it was the result of a black hole in another universe. This may seem like nothing more than sci-fi fancy, but there are a few high-profile physicists who take the possibility seriously.

So there we go, a glimpse into the depths of the weirdness of black holes. There is one more thing I’d like to say: we don’t actually know much about singularities. When we try to apply our best theories to them, General Relativity and Quantum Field Theory, we get unsolvable problems. In order to be able to model singularities, we need a theory of quantum gravity. Our two major contenders, String Theory and Quantum Loop Gravity, have both made predictions that turned out to be false. So the final tidbit of black hole weirdness I’ll leave you with is that there is still a feature about them, the singularity, that we know basically nothing about. I hope you sleep well tonight. See you next time.

Dead Stars

A month ago, we looked at the varieties of stars that can be found in the universe. But stars have a limited amount of fuel, and when it runs out, stuff happens. Exactly what happens depends on the mass of the star, and some of them are among the weirdest and most interesting things in the universe.

The most common stellar remnant by far is a white dwarf. White dwarfs are what we get when a star stops fusing its atoms and its matter settles down. In a white dwarf, the gravity is so strong and the pressure is so high that it runs into a physical limit called electron degeneracy. You may have learned in chemistry class that atoms have electron orbitals, sometimes called electron shells, which can only hold a certain number of electrons each. This is an example of electron degeneracy. In a white dwarf, the atoms' outer layers of electrons are unbound, moving freely around the material, and they are degenerate because they have the maximum density allowed by the laws of physics. Because of their variety of temperatures, white dwarfs are not necessarily white, but can also be yellow, orange, red, and brown.

Once our sun goes through all of its phases, it will become a white dwarf, slowly cooling down until the end of time.

There are ways to increase a white dwarf's pressure beyond the electron degeneracy limit. One of them is for it to be made of denser material. Some white dwarfs are made of helium, which has two protons and two electrons, neither of which are bound to it. Other white dwarfs are made of various mixtures of carbon, oxygen, neon, and magnesium, each of which is more dense and has more bound electrons not contributing to degeneracy.

The other way to increase the pressure is for gravity to be so strong that the electrons combine with the protons, making neutrons. When this happens, there is no more electrical repulsion, and all of the matter collapses to the density of an atomic nucleus, where neutron degeneracy once again makes things stable. You can probably guess what neutron degeneracy is; if electron degeneracy is what we get when we have the maximum density of electrons, neutron degeneracy is what we get when we have the maximum density of neutrons. A stellar remnant made of neutron degenerate matter is called a neutron star.

White dwarfs do not turn into neutron stars. Instead, we start out with a very massive star, which has enough pressure to fuse its atoms beyond the elements mentioned above. The higher the element number, the faster the fusion happens, until it reaches core collapse, which causes a supernova and leaves behind a neutron star.

When neutron stars are young, they shoot high-energy gets of light and other radiation from their magnetic poles. They are also spinning, and their rotational poles are not lined up with their magnetic poles. This means their jets spiral around in a pair of cones. If Earth is in the path of one of these jets, the neutron star appears to pulsate in the night sky. Because of this, we call this kind of neutron stars pulsars. Most pulsars rotate once every few seconds, but some are as fast as a few milliseconds.

By Kevin Gill on Flickr

Some neutron stars have the strongest known magnetic fields in the uinverse, strong enough to deform atoms. These neutron stars are called magnetars.

If a relatively small supernova makes a neutron star, what happens if we turn up the mass? If we explode progressively heavier supergiant stars, we get heavier and heavier neutron stars, until suddenly . . . there is nothing. The star explodes, leaving only empty space behind. According to supernova theory and observational data, there is a mass gap between neutron stars around three times the mass of the sun and our next type of stellar remnant at five times the mass of the sun. This heavier next type is what I’m sure you have been waiting for this whole blog post: black holes.

An ancient philosophical question goes like this: what happens when an immovable object encounters and unstoppable force? Well it turns out that there is no such thing as an immovable object, but gravity can get strong enough to become an unstoppable force. If enough mass gets crammed into a small enough space, not even neutron degeneracy can prevent it from collapsing down to an infinitesimal point called a singularity. A certain distance away from the singularity, called the event horizon, gravity switches between weak enough to escape from and too strong for anything to resist. The event horizon is the black ball we picture when we think about black holes. Black holes are so mind-bendingly fascinating that they deserve a whole discussion to themselves.

These are all of the stellar remnant types we have evidence for (and as of last month, I might add with pride, we have pictures to back all of them up). However, there are still more which are theorized to exist, either presently unobserved or far in the future.

It may be that between neutron stars and black holes, there is another stopping-off point. Neutrons are made of quarks, so perhaps quark degeneracy can stop the formation of a black hole when neutron degeneracy is not enough. Such an object would, unsurprisingly, be called a quark star. It is unknown whether this is possible; the only hint we have is the small amounts of quark-gluon plasma made in particle accelerators under completely different conditions than we would expect in quark stars.

Remember how we left white dwarfs cooling off indefinitely? One day, many eons in the future, white dwarf stars will have cooled so much that they no longer give off any visible light. Then, they will be called black dwarfs. The time it will take for white dwarfs to cool down this much is orders of magnitude longer than the current age of the universe.

Yet even black dwarfs are not the end state of stellar remnants. To find out why, we have to talk about nuclear fission and fusion. Radioactive materials break apart into lighter elements, each with its own half-life, the time it takes for roughly half of the atoms to decay. This is natural nuclear fission. Fusion happens when atoms fuse together into heavier elements, releasing energy. Now you might notice that I said both fission and fusion release energy. This is only true when the product has less mass per nucleon (less energy density) than what we started out with. Iron has the lightest mass per nucleon of all, so the elements lighter than iron fuse, and the elements heavier than iron fission.

When we talk about radioactivity, we say that some atoms heavier than iron are radioactive, and some are stable. When we talk about fusion, we imagine we need the pressures and temperatures at the core of a star. These are both not entirely true. High temperatures and pressures raise the probability of fusion, but that probability never goes to zero. Similarly, the “stable” heavier-than-iron elements have an extremely long, but not infinite, half-life. For us humans, it is true enough to say fusion requires enormous temperatures and pressures, and many heavy elements are stable.

But if we look ahead in the future, and I mean so far ahead that it might as well be infinite, we get a different story. Given an infinite amount of time, anything with a static non-zero probability is guaranteed to happen, no matter how small that probability is. On a large enough timescale, black dwarfs will fuse their atoms together into iron, and any elements heavier than iron will break apart into iron. Long after even the biggest black holes have evaporated, iron stars will be the last objects left in the universe.

The Event Horizon Telescope – Science and the Human Spirit

Black holes are extremely dense. A black hole the mass of the sun would be the size of a small village. They are also extremely dim, giving off no light of their own. The only light that comes from a black hole is from its accretion disk, a swirl of matter bunching together and heating up as it falls into the black hole. All known black holes are extremely far away, in the hearts of star clusters and galaxies. And on Wednesday, the Event Horizon Telescope collaboration released humanity's first photograph of a black hole.

Our first honest-to-God image of a real black hole.

The galaxy the black hole hides in.

The Event Horizon Telescope is one of several modern feats of staggering ingenuity. Eight radio telescopes in Hawaii, Arizona, Nevada, Mexico, Chile, and Antarctica, were synchronized and pointed at the center of the galaxy M87, where a black hole 6.5 billion times the mass of the sun lies. Working together, these telescopes used interferometry to act as a single telescope the size of the entire Earth. This gave them the resolution they needed to collect a long-exposure picture of a supermassive black hole in radio waves.

The universe is stranger and more amazing than we can imagine. From the plains of Africa, to agriculture, to metallurgy, to the industrial revolution, to computers, to supercomputers, we curious humans have explored our world and created new devices of exploration in a cycle that grows ever more impressive. We do things that are bigger than ever before, and then we start on new projects that are even bigger. Someday we will have particle accelerators that go around the sun. Telescopes the size of the solar system. We will resurrect species that have gone extinct. Build artificial minds as versatile as humans, or even more so. Though we cripple ourselves with wars, and greed, and ideological disputes, there is a part of us that sees mystery and just wants to explore. And this spirit of curiosity within us moves us to bridge the gaps, to harness the synergy that arises when many work together for a common goal. And that goal: to learn more about this wonderful, strange, mysterious universe we find ourselves in.

Awesome Energy Sources of the Near and Far Future

Energy. It is the magic ingredient that makes the difference between an empty, lifeless universe and one teeming with action. Whether it is by eating food to fuel our bodies, producing electricity to power our homes, or atoms fusing together in the cores of stars, energy is what makes everything possible.

In our ever-growing world, we use more and more energy, both as individuals and as societies. Energy production and technological progress go hand in hand. Sometimes, inventions of new methods of generating power, like the steam engine, lead to a boom in growth. Other times, a growing economy builds up the resources to transition to new kinds of energy. We are in the midst of one such transition right now, with solar, wind, and other types of renewable energy creeping up to take the place of fossil fuels. But what about the future? Are there other ways of generating power in large amounts that we have yet to invent?

The answer is a definite yes. Thanks to modern physics, we know of many ways to store and produce energy that are possible, but beyond our current technology or the resources.

Nuclear Fission
Before we go there, I want to nod to an underappreciated method of generating power that modern physics has gifted us, and which already exists: nuclear fission.

Now when many people hear “nuclear,” they think “bombs.” But it’s really referring to the energy stored in the bonds between the protons and neutrons in the nuclei of atoms. If you look at the periodic table of the elements, you will see that all of the elements are numbered from 1 to 118 (as of when this post was published). This number is the number of protons in the atom’s nucleus. There are also neutrons, but those aren’t counted on the periodic table. Since saying “protons and neutrons” can get tiring, when they are referred to together, they are called nucleons.

The energy in the masses of the nucleons and the bonds between them is different for each atom. From Hydrogen, the energy per nucleon goes down, until we reach iron. After iron, the energy per nucleon goes back up. This means that if elements heavier than iron break apart, they release energy. This is called nuclear fission, and we already harness it in nuclear power plants around the world.

Nuclear fission is not very popular, because people associate it with harmful radiation. However, nuclear reactors are actually among the safest ways to generate power. There are an enormous number of safety procedures, many of them redundant, to protect everyone inside and near the plant from radiation, and to prevent meltdowns. The number of people who have died from nuclear reactors in the 70 years they have been around is less than 100.

However, there is a downside. Nuclear power may kill basically nobody today, but the risk goes up over time. This is because when uranium is fissioned, it leaves behind nuclear waste, radioactive material that is dangerous to stand near. Some of this can be used or purified, but some of it just has to be stored away in special warehouses for ten thousand years until it becomes safe. In the meantime, we continue to use more energy, nuclear power plants keep getting built, and the waste keeps piling up. So nuclear fusion has many advantages, and much less risk than people think, but the risks it does have last a long time.

Nuclear Fusion
On the opposite side of the periodic table from fission, we get nuclear fusion. Fusion occurs when two atoms come together to create a larger atom. If the product is iron or anything lighter, energy is released. The sun and stars get their power by fusing hydrogen and some other elements in their cores.

Nuclear fusion requires extreme temperatures and extreme pressures. Our best reactor design is the tokamak, which uses strong magnetic fields to confine super hot hydrogen plasma in a donut shape. However, we have not yet built a tokamak that can generate more energy than is used to heat the plasma, so commercial fusion remains a technology of the future.

Fusion power would be an incredible prize. It is way more energy-dense than any other power source we have tapped into, including fission. It uses hydrogen, which is three times more common in the universe than all of the other elements combined. There is almost no waste; the product at the end is mostly helium, which is about the least harmful substance in the universe. And if something goes wrong and the containment is broken, there is no meltdown. The machine just shuts off.

There is a lot of cynicism in society about fusion power. After all, people have said it is only 20 years away for over 60 years. To the average person, that sounds like no progress has been made. But that is not true; progress has been made, and is still being made today. The problem is that it is hard to predict when research will be finished, because that would require knowing the results ahead of time.

Another reason people are cynical is because fusion seems too good to be true. The fuel for it is everywhere, and it would yield so much energy that compared to the amount we use today it might as well be infinite. It is immensely more friendly toward the environment than fossil fuels or fission. It is such a great positive with so few downsides that it feels too good to be true.

But that kind of thinking is based on human intuition, not on facts. Behind the scenes, fusion research continues to press forward and make progress, and right now there is a tokamak is in the works called ITER, which is designed to produce ten times more energy than it takes. The perception that it is no closer than it has ever been is just an illusion.

What about beyond fusion? Surely in the vast, wide universe there are other power generation options that might become available to us in the far distant future? Why yes, there are.

Solar Satellites
The sun is a giant gravity-powered fusion reactor, and it is putting out enough energy every second to power the entire current world economy for eight trillion (8,000,000,000,000) years.

You read that right. Eight trillion years of human civilization. Every second. Our planet is floating in a sea of practically limitless energy. If only there was a way to go and get it.

In the future, after much technological and economic growth, we might be able to. The idea is to launch satellites into Earth orbit, with giant curved mirrors that focus the sun’s light into a collector, which is hooked up to a laser that would beam the energy to power stations on the Earth’s surface. This will be far more efficient than land-based solar power, because there are no clouds and no night in space, and there is virtually no limit to the number of satellite collectors we could add to the grid.

Now when you think about giant lasers shooting at the Earth, you might worry that they would be used as weapons. However, you need not fear, because there are ways of designing the satellites such that weaponizing them would be impossible. If the output is limited and a sufficiently long laser wavelength chosen, then they won’t be able to hurt anyone or anything.

You can read more about solar satellites at the US Department of Energy website.

Antimatter
So far, we have talked about things that are more or less within the everyday person’s imagination. Now it’s time to take things to the wild parts of physics. In fission and fusion, the energy we get comes from a fraction of the mass getting converted into energy by Einstein’s equation E=mc². But most of the mass is still there after the reaction. Might there be a way for us to get all of it? The answer, is yes. And it might surprise you to learn that there are more ways than one.

90 years ago, the underappreciated physicist Paul Dirac was playing with one of his equations, when he discovered that there should be particles that are exactly like electrons, but with a positive electric charge instead of negative. Positrons. Since then, we have discovered in lab experiments that there are oppositely charged versions of all particles. These negative doppelgangers of matter are called antimatter. And when particles of matter collide with their antimatter counterparts, they are annihilated, and all of their mass turns into energy. In nuclear fusion, only 1% of the mass gets converted into energy. This means that a matter-antimatter power plant would yield a hundred times as much power!

Unfortunately, because antimatter gets destroyed when it touches matter, it is extremely rare. There isn’t enough around to collect and put into reactors. In fact, the vast majority of the antimatter on Earth is created in particle accelerators, taking the same amount of energy to make as we would get from it. So someday in the far future, rather than being an energy source, we might use antimatter as an ultra-high capacity battery.

A cartridge of antimatter the size of a D-cell would carry about 1/10 the energy of an atomic bomb. And it would very easily explode like one too. You can’t just put it in a bottle, because touching the sides of the bottle would make it go boom. The only way to contain antimatter is to use ultra-precise magnets. Needless to say, if we accumulated enough antimatter to run a power plant with, such a plant would be extremely dangerous. So although it is physically possible, I don’t see us or any future civilization using antimatter as a power source.

Micro Black Holes
As I mentioned earlier, there is another way of getting all of that sweet energy locked away in mass: black holes. Specifically, microscopic black holes. You have probably heard that anything that enters a black hole can never get out. That is true, mostly. However, black holes lose mass very slowly by giving off light in a process called Hawking radiation. Maybe I will try to explain it in a future post, but for now, the important thing is that it exists.

Hawking radiation is usually extremely slow. As in, the slowest process in the universe. But the smaller the black hole, the faster and more powerful the radiation. If you have a black hole, say, the size of a proton (the mass of a mountain), it will put out about the same amount of power as a fission plant, and will last for hundreds of billions of years.

I did my calculations at this handy dandy site.

Of course, there are some questions that arise when we talk about using black holes for anything. First of all is safety. It’s a black hole! Won’t it suck in everything around it and cause lots of destruction? Surprisingly, no. The point of no return is called the event horizon, which is what we mean by the “size” of the black hole. In our case, it is a lot smaller than an atom, making it difficult for anything to get trapped inside. However, anyone closer than 50 meters would feel a force from the black hole equivalent to Earth’s gravity, which means we want to give the black hole a lot of room.

Speaking of which, how are we going to hold the black hole? It is as heavy as a mountain and tinier than an atom, so what is stopping it from falling straight through the table, the floor, and to the center of the Earth? One option is to give it a small electric charge, and have it spiral around in a magnetic field. That magnetic field would require quite a lot more power than our mountain-mass black hole would give off, but we could solve this by making the black hole smaller, which would both reduce its mass and give it a higher energy output. However, there is a better option: build the power plant in space. There is no down in space, so the black hole would just float there, coasting along the gravitational landscape along with the ship that carries it.

Another important question is how we get a black hole of that size. The smallest black holes in nature that we know of are around 3 times the mass of the sun, when enough matter falls onto a neutron star to push it past its Schwarzschild radius. Unfortunately, the rest of the universe will have cooled off and died by the time these black holes’ hawking radiation is enough to use as an energy source.

However, only technology is stopping us from making artificial black holes. Einstein’s General Theory of Relativity showed us that energy has just as much gravity as mass, so if we shoot powerful enough lasers into a small enough point, a black hole will appear. Of course, we need to generate all of that power in the first place, so like antimatter, micro black holes would probably end up more as batteries than power sources. That is, unless life can figure out a way to survive the googols of years until the naturally-occurring black holes are small enough.

Black Hole Spin
There is another way to use black holes for energy generation. Big black holes, in fact, not microscopic ones. This comes from the fact that black holes have spin. What exactly it means for a black hole to have spin is a very complicated topic, so we’ll save it for another day. You can pretend it means the event horizon is revolving around the center, if that makes it easier. The important thing is that there is energy in the spin, and given the right technology and resources we could get to it.

It might surprise you to learn that gravity does not pull objects together. Instead, Einstein showed us in the General Theory of Relativity that space-time is pulled toward the gravitational source, and objects float along with it. It is like a stick floating on a pond with a drain pipe. The drain does not pull the stick toward it. Rather, the stick floats along with the water toward the drain. There is another effect of gravity predicted by Einstein’s equations, which is rare enough that we have never directly observed it. When an extremely massive object like a black hole spins, the rotation pulls space-time around with it. This is called frame-dragging. If regular gravity is like water flowing toward a drain, frame-dragging is like a whirlpool.

When a black hole spins, its frame-dragging is so strong that it creates a region of space outside the event horizon called the ergosphere, where in order to stay still as seen from far away, something has to travel against the black hole’s spin faster than the speed of light. This means that if something dips into the ergosphere without entering the event horizon, it gets a major boost in speed and kinetic energy. This energy comes from the black hole’s rotational kinetic energy, making the black hole’s rotation slow down ever so slightly. Since the object hasn’t passed the event horizon, it can get back out, and we can use its extra kinetic energy for electricity.

The most efficient way to do this would be to build a sphere of mirrors around the black hole, and let light bounce around inside. Every time the light goes through the ergosphere, it picks up some of the black hole’s rotational energy. If there is an opening in the mirror contraption for the light to get out, it becomes an energy fountain. For a visual and entertaining explanation about black hole spin power, check out Kurzgesagt’s video on it.

Unfortunately, extracting a black hole’s rotational energy requires letting some mass fall into the black hole. This is because of something called the Penrose process, which I wish I could explain, but I don’t understand it. The details don’t matter today, though, because it has a bad consequence; the black hole increases in mass, meaning we have to wait longer before it shrinks to sub-atomic size and we can use its Hawking radiation as a power source. It will be a kind of poetic tragedy that the last civilizations in the universe, by taking the energy they need in order to survive, seal away some of the precious energy into a future beyond their grasp. Not to worry, though. Before then, we have billions and trillions and higher-tier-illions of years to figure out how to stick out the long dark purgatory of the black hole era.

In our struggle today to replace fossil fuels with renewables, it's easy to forget that there will be more to the story in the future. As poor countries develop and developed countries get wealthier, our consumption of energy continues to increase exponentially. In another century, we may get vast amounts of energy from space lasers, or from mini-suns we create. Even after all of the stars die, there will still be enough energy to last for a long, long time.