Faster-Than-Light: Antigravity and Exotic Matter

It is almost universally believed among experts that traveling faster-than-light (FTL) is impossible. But if it weren’t, how would we do it?

Relativity and the Speed of Light

The speed of light is special. The reason it is special has nothing to do with light; it is the fixed feature of the universe against which we measure space and time. Our human brains naturally think of space and time as absolute. It makes sense to us to believe that if two things happen at the same place but different times, or two things happen at the same time but different places, then that’s the way it is no matter how you look at them.

But space and time are not absolute, they are relative. Space is easier to comprehend. Suppose you are standing by a train track. As I pass by in a train car, you snap your fingers twice. To you, in the reference frame of the surface of the Earth, the two snaps happen at the same place. But to me, in the reference frame of the train car, the snaps happen at different places.

In the station's frame the snaps happen at the same place, but in the train's frame the snaps happen in different places.

There is no objective reference frame by which to determine whether something is stationary or moving. Everything is stationary from its own viewpoint.

What about time? The relativity of time is harder to wrap our heads around, because it only appears when observers are moving significantly close to the speed of light relative to each other.

Let’s go back to the train example. You hold your arms out to their full length, and snap both your hands at once as I pass by. In my reference frame, however, you snap the fingers of the hand that points forward along the track first, and the hand that points backward second.

We aren’t talking about the difference in how long it takes for sound and light to get to our eyes and ears. The discrepancy is still there after we account for that. In your reference frame, you snap the fingers of both of your hands at once, but in my reference frame, you snap them at different times.

In the station's frame the snaps happen at the same time, but in the train's frame the snaps happen at different times. This is not because of light lag; it is found to be true after we account for light lag.

It is a fact baked into the fabric of the universe that there is no objective “here,” and there is no objective “now.” The speed of light is the absolute which ties together all perspectives of space and time. It does not matter where you are, nor what speed or acceleration you are going, the speed of light is the same for you as it is for everyone else.

Chasing Light Beams

Suppose you turn a flashlight on and off, sending a pulse of light out into space, and then chase after it. You accelerate and accelerate and accelerate, but no matter how fast you go, that light pulse is still retreating away from you at the speed of light. You can never catch it.

What is your trip like in the reference frame of Earth? It is an objective fact that you never catch the flashlight’s pulse. Therefore, in the reference frame of Earth, you speed up and speed up, but the closer you get to the speed of light, the less you speed up. No matter how much you accelerate, you never reach light speed.

This is the trajectory of an object with constant proper acceleration under Special Relativity. The shape is a hyperbola, and it never becomes parallel with the light beam it chases. The dashed line is the asymptote, shown as a visual aid.

However, you can, in fact, travel across the universe. In Earth’s reference frame, you are experiencing time dilation. Time is running slower on your ship than it is on Earth. In your reference frame, the universe is undergoing space contraction. The distance to the stars in front of you is shrinking—not just that you’re getting closer, but there is less total space for you to go through. The distance from Earth to your destination is shorter in the ship’s reference frame than it is in Earth’s reference frame.

In a sense, this can almost feel like traveling faster than light, even though light always races ahead of you. But because of the way the time dilation and space contraction work out, it still takes years to travel light-years according to your departure and destination reference frames. We don’t just want to visit Alpha Centauri, we want to get back in time for work the next week. How do we do that?

Both science fiction writers and scientists have pondered this conundrum. I myself wrote my undergraduate thesis on the topic. There are three families of FTL methods: space warping, wormholes, and hyperspace. Space warping involves shortening the distance between the beginning and end of the journey. A wormhole is a shortcut between points in space. And hyperspace is a hypothetical more-than-three-dimensional space, in which our 3D universe is just a slice. All of these deserve their own discussions, so today we will focus on an essential ingredient for making and using these things, exotic matter.*

Negative Energy and Imaginary Mass

Exotic matter sounds exciting. What is it? If we put the space-time curvature for a warp drive or a traversable wormhole into the Einstein field equations, we find we need to generate antigravity. In order to get antigravity, we need negative energy. You might think negative energy means negative mass, because E = mc², but that equation is just a special case of the real equation,

As you can see, the mass in this equation is squared, so in order to end up with a negative energy, the mass must be complex. I don’t mean difficult to explain, we’re talking imaginary numbers, the square roots of negative numbers. Yeah, now you know why it’s called exotic matter.

Dark Energy

All right, so how do we make exotic matter? Do we have any clues to suggest it exists? Yes, in fact. Just a hint, but it’s more than nothing (or should I say, less than nothing?). The universe is expanding, and not only that, it is speeding up. Under attractive gravity, it should be slowing down. But because it is speeding up, we know there is a repulsive antigravity, some kind of “dark energy,” pushing everything apart.

We don’t know what this dark energy is. It is one of the biggest unsolved mysteries in physics. It could turn out to be completely useless. Or, it may be that it can be used to create exotic matter, or harnessed directly as negative energy.

Zero-Point Energy

There is one phenomenon people often point to as negative energy in the lab: the Casimir effect. So let’s talk about it, and see if it really is the magic ticket that will get us to the stars.

To begin, a little background knowledge. What we think of as empty space is not, strictly speaking, nothing. It has a number of quantum fields overlapping one another. For a deeper discussion of quantum fields, check out part 1 of our quantum physics series.

As it turns out, even when there are no particles, the quantum fields aren’t at 0 energy. An energy of absolute 0 leads to a contradiction in the math. Thus, the fields always have a buzz of vibrations far smaller than any particle. This is zero-point energy, also known as quantum foam.

The quantum foam is influenced by objects. If, for instance, there are two metal plates with empty space between them, these plates act as boundary conditions for the zero-point waves in the quantum fields. It’s like the harmonics of a guitar string, except instead of a string, it’s empty space. In empty space without the plates, there are no limits on the frequencies allowed in the quantum foam, but between the plates, only the harmonics are allowed.

If that was confusing, here is the bottom line: between the plates, the energy of the empty space is less than the energy of the empty space surrounding them. This is the Casimir effect.

The argument goes, if there is less energy between the plates than in normal empty space, that’s negative energy, so it should be usable for space warping and stuff like that. Here’s the problem, though. When it comes to gravity, it’s not the relative energy that matters, it’s the absolute energy. The zero-point energy in the Casimir effect may be less than the normal zero-point energy, but it is still more than absolute zero. Thus, although the Casimir effect looks like it creates negative energy, it is really still positive energy, and cannot be used for FTL technology.


As it turns out, getting negative energy is really, really hard. When it comes to FTL travel, we’re pretty much dead in the water. Of course, we don’t have any proof that exotic matter can’t exist, so there’s the possibility it’s hiding out somewhere in what’s left of the unknown. Maybe dark energy can provide a thread to follow, but that’s a big maybe. As things stand, it looks pretty certain that we’ll be stuck taking the slow route. Of course, that doesn’t mean we can’t use our imagination, and you can bet we will return to the topic of FTL in the future.

*We currently have no reason to believe hyperspace exists, nor any idea how to use it if it does. Exotic matter may be necessary, or may not be of any use at all.

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.

Why Faster-Than-Light Travel Allows Time Travel

In our everyday experience, things can always move faster. Give a car a little more gas, and it will speed up from 100 miles per hour to 110 miles per hour. Give a rocket a little more thrust, and it will speed up from 1000 miles per hour to 1050. But weird stuff starts to happen when things get close to the speed of light, and to understand it, we have to talk about space and time.

Our brains automatically think of space and time as absolutes. A yard in a straight line is a yard, no matter who is measuring or calculating it. The present is a special moment in time, and it exists right now all across the universe. A minute is a minute, and it is the same for everyone everywhere. It’s intuitive and obvious. And none of it is true.

To understand why, we have to look at a little theory called Special Relativity. It is one of Einstein’s most famous insights, and one of the reasons he is known worldwide as the face of genius, because he questioned our natural understanding of space and time and found a deeper truth.

The simplest place to start is to draw a coordinate plane, with position going horizontally and time going vertically. As an object moves through time, it moves upward on this graph. We ignore the other two dimensions of space, both because a 4-dimensional diagram is hard to draw and to look at, and because they aren’t necessary for the concepts we’re interested in.

If something is staying still according to its coordinates, its path goes straight up. If something is moving in these coordinates, its path goes up at an angle. If it is accelerating, its path goes up along a curved path. Light travels at 45-degree angles.

Now that we’ve set up our coordinates, we define a term called a reference frame, a set of coordinates where zero is set to a specific location and speed. It might be tailored to an object, or just a point in space. From the origin (0,0), we draw four lines at 45-degree angles. These lines below the x-axis are the paths light from the past takes to reach the object at point 0, and the lines above the x-axis are the paths light takes coming from the object at point 0. These are called light cones.

Let’s look at everyday relativity we all know well. Suppose you’re standing still, your friend is driving by at 50 miles per hour, and a truck is driving in the same direction at 100 miles per hour. If we switch into your friend’s reference frame, they are the ones sitting still in their car, the truck is moving forward at 50 miles per hour, and you are moving backward at 50 miles per hour. In the reference frame of the truck, your friend is moving backward at 50 miles per hour, and you are moving backward at 100 miles per hour.

Now you might think, “What’s the big deal? Just speed up until you catch up to light, and then you’ll be going faster than it.” Well here’s where the craziness comes in. You see, light always travels at 45-degree angles on the space-time diagram in every reference frame, no matter how fast you or any other person or object is moving. If you are standing still on the Earth, and your friend takes a bullet train past you at 1000 miles per hour, the speed of light for you is the same in all directions as the speed of light for your friend: c. Your friend does not calculate light in front of them traveling any slower than the light behind them. They calculate both light beams traveling at the same speed, c.

The light cone must always be at 45 degrees, not skewed as it is in the middle diagram.

What does this mean? Well the math is complicated, but in order to get light to travel at c in all reference frames, space and time get messed up. In your reference frame, the time axis points straight forward in time, not any direction in space. Your friend’s time axis points in the direction through space-time as if they are not moving. On our graph, this means your time axis points straight up, but your friend’s time axis points along the path they are going to take though space-time at their current speed. Because the speed of light stays constant, this needs to be counterbalanced by your friend’s space axis changing too. In the transformation between your reference frame and your friend’s reference frame, space and time get rotated toward each other, and vise versa.

This is why nothing can go faster than the speed of light. No matter how much you speed up, light will always be traveling at c ahead of you, and you can never catch up to it no matter how much you accelerate. In the reference frame of someone standing still and watching you, you go closer and closer to the speed of light, but never reach it. This causes your time to slow down, your mass to increase, and your shape to flatten. You of course don’t notice any of this stuff, because it’s not happening in your reference frame.

Make a note of the fact that time passes slower for someone traveling close to the speed of light. This is going to be important later on.

If we look carefully at the way the axes change when we transform between coordinate systems, we’ll see that this means a universal “now” doesn’t exist! To show this, let’s take the x-axis, which is the slice of space-time where t=0, and what we think of as “now” across the universe. But your friend’s x-axis point in a different direction through space-time, meaning their “now” slice is different from yours! Mind blown, right? To drive the point home, let’s suppose there is a firecracker set to go off at a certain time 1000 miles away. According to your “now” slice, the firecracker is about to go off. But according to your friend’s “now” slice, the firecracker has already gone off!

There is no universal “now.” Each point in space-time is its own “now,” both in time and space. “Now” for you is only now for you, everyone and everything else has their own “now.”

There is nothing special about the t-axis in our diagrams. You can move in one direction, and then you can stop and turn around and go the other direction. In the same way, there is nothing special about the x-axis. If something were somehow able to travel faster than light, there would be nothing stopping it from going in a slightly future direction, then turning around and going in a slightly past direction. The fundamental boundaries are the light cones, not the t- or x-axes.

This means, if you were somehow able to make a ship go faster than light, all of the space-time between your future and past light cones would be open to you. This means you could set off going slightly backward in time, then turn around and go slightly backward in time the other direction, and get back to where you started before you began. It doesn’t matter what the method is, if you can travel faster than light, you can go back in time.

If you could go faster than light, all of the space-time outside of your light-cones would be available to you, but you would be locked out of your past light cone, unless you take a roundabout path. However, a past light cone is the same kind of barrier as a future light cone, so if you have the technology to cross the future light cone, that same technology can probably let you cross the past light cone, and you won’t have to go faster than light to time-travel; you’ll be able to do it while staying in the same place.

Warp drive, hyperspace, whatever your method is, the math doesn’t lie; according to Special Relativity, if you can travel faster than light, you can also go backward in time. But what about taking shortcuts? What if you don’t have to travel the vast distance between stars, but can get there in a single step? What if we could use a wormhole?

A wormhole is a theoretical object that comes out of Einstein’s other theory, General Relativity. Wormholes are interesting enough that we might give them their own discussion, but all we need to know today is that a wormhole is a shortcut between points in space-time. You can think of it like a doorway, but instead of leading to another room, it leads to a different planet.

You might think a wormhole is something that picks you up and teleports you away. This is a misconception. As you walk through the wormhole, nothing is happening to you that doesn’t happen when you take a walk down the street. The two ends of the wormhole might be light years apart the normal way, but on a path through the wormhole, they are only separated by a few feet. This isn’t just a metaphor, it’s literally true.

Wormholes, it turns out, also allow for time travel. To demonstrate this, let’s start with a simple setup, a wormhole where the two mouths are five feet away from each other and synchronized in time. If you look through the wormhole, you can see your own back five feet in front of you.

We put one of the wormhole mouths on a spaceship, and fly it around near the speed of light. Remember from earlier, if something is moving near the speed of light, time is slowed down for it. This means time is passing slower for the mouth of the wormhole on the ship than it is for the mouth on Earth—but only on the path through space from Earth to the ship. On the path through the wormhole, time is passing at the same rate on both sides. You could step through, have tea for half an hour with the astronauts, and when you step back, half an hour would have passed on Earth.

Think about this. On a path through space, time is passing slower on the ship than it is on Earth. But on a path through the wormhole, time is passing at the same rate on both sides. What is going on here? It’s a paradox! Two different ways of calculating the same problem give us two different answers. Which is right? Is time flowing at different rates, or isn’t it?

The answer is, both calculations are correct. Time is flowing at different rates on a path outside the wormhole, but it is flowing at the same rate on a path through the wormhole. This means the wormhole is not only connecting two points in space, but also two points in time.

Suppose the astronauts decide to return to Earth. When they land, less time has passed for them than on Earth. Mission control says, “Hey, you’ve finally arrived.” An astronaut says, “What do you mean, ‘finally’?” The astronaut looks through the wormhole, and on the other end, the same mission control member says, “You’ve landed? But we still see you flying around up there!” Outside the wormhole, the mission control member chuckles and says, “I remember having this conversation a week ago.”

By putting one mouth of a wormhole on a spaceship and flying it around near the speed of light, and then landing, the team has created a gateway through time. In our example, the difference between wormhole ends is one week; step through the end that traveled on the ship, and you’ll find yourself a week in the past. Step through the end that stayed home, and you’ll find yourself a week in the future.

What about quantum entanglement? Can’t we send messages instantaneously by measuring one particle and instantly affecting another one light years away? Wouldn’t this achieve faster-than-light communication without time travel? The answer is no, because in order to send information by quantum entanglement, the two parties must compare notes via traditional channels. Also, it’s incorrect to say whose measurement affected whose, because in some reference frames person A measured their particle first, and in other reference frames person B measured their particle first. When measuring entangled particles, there is no causation, only correlation.

In science fiction, we see faster-than-light travel all the time, but time travel usually takes some special magic sauce. This isn’t because of science, but because easy time travel would ruin the plot. Another reminder of the difference between narratives and reality. We may try hard to come up with a loophole that doesn’t allow time travel, but the fact that there is no universal “now,” and all spacelike trajectories are open to a faster-than-light traveler, nails the box shut. I am all for imagination, of course, but when it comes to reality, despite how strange it may be, we should allow ourselves to follow the evidence where it leads.

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.

Types of Stars in the Universe

On a clear, dark night, stars fill the expanse of the sky. These tiny dots of light twinkle and shine, as if the canopy between Heaven and Earth were pricked by a million needles and the holy light of God were shining through. Ever since our distant ancestors separated from the chimpanzees, we have gazed at the Milky Way with awe, imbuing it with images and meaning and stories. The stars are among the most wondrous things in existence.

With the dawn of science, we learned that the stars are other suns, each with its own set of planets. Stars come in many sizes and colors, depending on what they are made of and how far along they are in their life cycle. A star’s light comes from its surface temperature, as it radiates its heat energy away. The hotter the star, the brighter it shines, and the higher the peak frequency of its light. It is the same as why metals glow when they are heated. In order of increasing temperature, we get red, orange, yellow, white, and then blue. Because of the distribution of the light radiated, and the way our eyes work, we will never find a green or purple star.

A star forms when the gas (mostly hydrogen) in a region of space becomes dense enough that its gravity causes it to collapse together. As it shrinks, the tiny bits of angular momentum here and there build up, causing it to swirl around and form a protoplanetary disk. Most of the gas clumps in the center, forming the star, and the rest eventually becomes planets, moons, and asteroids. While this is happening, the star is called a protostar.

Once all of the dust has settled, the star officially begins its life, and is called a main sequence star. The mass of the star determines how hot it is, and therefore its color. From low to high, we have red dwarfs, orange dwarfs, yellow dwarfs, and blue . . . giants. A star massive enough to shine blue at birth is too big to be called a dwarf.

Our sun is a yellow dwarf, although it’s actually white, not yellow. The reason it looks yellow, orange, or red when it is low in the sky is because Earth’s atmosphere scatters the shorter wavelengths. this is also why the daytime sky is blue.

There are also brown dwarfs, but they are a little different. Brown dwarfs are objects that ride the fuzzy line between stars and gas giant planets, only hot enough to glow a faint dark red. They are a little over ten times the mass of Jupiter, and a hudredth the mass of the sun.

Stars don’t stay as they are forever. As they fuse up their hydrogen, they expand. A lot. As in, orders of magnitude. Once their hydrogen is spent, they contract until the helium in their cores begins to fuse. As the helium runs low, they expand again. The cycle goes on a few more times. When stars are in their expansion phase, they are called giants, supergiants, or hypergiants depending on their masses, and their color shifts toward the red end of the spectrum. Some stars, however, are so blue that they remain blue even at their largest.

These are all the types of stars that we know about that are around today. However, despite the nearly 14 billion years the universe has been around, it is very young compared to how old it will get. The smaller a star, the slower it fuses its fuel, so the longer every stage of its life lasts. In fact, red dwarf stars are so slow that none of them have used up all of their hydrogen yet. Here’s where things get interesting. It is predicted that red dwarfs don’t expand like other stars. Instead, they get hotter and brighter, turning into blue dwarfs. I find it just amazing that some trillion years in the future, a star type the universe has never seen before will start to appear.

But wait, you say. We can’t be done with stars yet! We haven’t talked about white dwarf stars or neutron stars. And you are correct. The reason we haven’t talked about them today is because they are dead stars, and dead stars are interesting enough that I wanted to give them their own discussion.

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.

Types of Planets in the Universe

Our world is a planet. This place, where we live our lives and strive for meaning and love, is one large stone among many flying around the sun. But our sun is not alone; there are an uncountable number of stars, and breakthroughs in the past few years have shown us that most if not all stars have their own planets. As of May 5, 2018, over 3700 planets have been found, and the number is growing exponentially. As we learn more about these extrasolar planets, we find that there are many different kinds of planets, some of which are not found in our solar system. So let’s take a look at all of the different and interesting types of planets that can be found around the universe.

For this discussion, I am not going to make any distinction between planets, dwarf planets, and moons. What counts as a moon or dwarf planet in our solar system could easily count as a planet if it were in the right orbit around the right kind of star. In our own solar system, Jupiter’s moon Ganymede and Saturn’s moon Titan are bigger than the planet Mercury, and they would have been planets themselves if they had not been caught by their parents.

Rocky

Starting off with the kind of planet everyone knows about, rocky planets have solid surfaces. Earth is one, as are Mercury, Venus, and Mars. Because Earth is the only planet we know of that has life, we make a guess that the best place to look for life elsewhere in the galaxy is on rocky planets. We have just recently achieved the technology to find them in their host stars’ habitable zones, but we do not yet have powerful enough telescopes to look for life’s signatures in their atmospheres.

Gas giant

These planets are thought to have no surface. They are so big that the pressure from their gravity causes a smooth transition between their gaseous atmospheres and liquid cores. Jupiter and Saturn are our solar system’s representatives of this group. Because they are so massive, they are the easiest type of planet to observe around other stars.

Ice giant

When a planet that would be a gas giant is formed far enough away from its host star, it is formed from the water ice that collects in the outer parts of the stellar system, rather than the hydrogen and metals that collect in the inner regions. Like gas giants, they have no surface, but smoothly transition between gaseous atmosphere and liquid core. In our solar system, Uranus and Neptune are ice giants.

Ice ball

Everyone's favorite.

Like ice giants, planets that are small enough to be rocky but are formed in the outer regions of the solar system where water ice is abundant have a high concentration of ice. Such planets are like Antarctica all the time. Ceres in the asteroid belt, and Pluto and its friends in the Kuiper belt are our neighborhood ice ball planets.

Icy Ocean

There is a special world type that, as far as we know, only exists as moons around gas or ice giants. These moons are far enough from their parent star that their surfaces are covered in thick ice, but the gravity difference across them from the giant planet they orbit causes internal friction, which heats them up inside. These worlds have vast oceans beneath their icy shells, like the Earth’s molten mantle beneath its rocky crust. It is thought that the conditions at the bottom of these oceans might be right for organic molecules to form life, and so they are of interest to exobiologists. Jupiter’s moon Europa and Saturn’s Enceladus are icy ocean worlds.

Tidally locked

Full, half, or crescent, we always see the Man in the Moon.

These are technically not a different kind of planet, but they are interesting enough to have their own mention. When rocky planets are too close to their host stars, the same side always faces the star. They are close enough that the difference between the star’s gravity on one side of the planet and the other is great enough that the heavier density clumps of the planet get stuck pointing toward the star. It is kind of like how balloons float with the tie pointing downward. Our own Moon is tidally locked with Earth, which is why the same face always points toward us. Around stars that are small and dim, like red dwarfs, there can be Earth-sized tidally locked planets in the habitable zone. In fact, the closest star to the sun, Proxima Centauri, has just such a planet. Tidally locked habitable planets have been a hot topic of the astrobiology community, with debates about whether the climate and weather of such places could allow life to exist.

Our sun is not the only star to have planets. In fact, from our various methods of detecting exoplanets in our stellar neighborhood, we have learned that it is extremely rare for a star not to have a few. As with all endeavors to peer into the unknown, the exoplanet search has been a treasure trove of new, exciting knowledge, and we have discovered several exotic planet types that are not found in our solar system.

Hot Jupiter

The first planets we discovered around other stars were larger than Jupiter and had closer orbits than Mercury. This makes sense, because the bigger and closer a planet is to its host star, the more it pulls on that star, and the easier it is to see the signs that it’s there. We used to think gas giants had to be far from their stars, like our solar system’s outer planets, but now we know that inner gas giants are just as common.

Magma

Solar systems are made from collapsing clouds of gas and dust called protoplanetary disks. When this happens, all of the gravitational energy turns into heat, so small enough planets that are close enough to their suns are formed as magma balls. As eons pass, their outsides cool into rocky crusts, becoming the rocky planets with molten cores we are familiar with. Some planets around other stars, however, orbit so close that the heat from the sun keeps their surfaces molten after the formation phase. Magma planets can also be created when gas giants migrate too close to their host stars, and enough of their atmospheres get blown off that the remaining matter forms a molten rock ball.

Ocean

No land.

Our solar system jumps from Earth’s mass to Uranus’s, a 15-fold increase. But there is no reason planets couldn’t form in this mass gap. Indeed, we have found plenty of planets around other stars with multiples of Earth’s mass in the single digits, sometimes called super-Earths. Based on solar system models, we suspect such planets to attract and hold onto more water than Earth, covering their entire surfaces. It is suspected that ocean planets might be far more common than Earth-like worlds with both ocean and land. If this is true, it offers an optimistic solution to the Fermi Paradox: that life is common in the universe, but it almost exclusively arises on ocean planets, where space programs and radio broadcasting are extremely difficult to invent.

Brown dwarf

There is no limit to how big a planet can be; or rather, if it is big enough, it is a star instead of a planet. Because of this, there is a gray area a few times heavier than Jupiter where we are not sure whether to call it a star or a planet. It may have a tiny bit of nuclear fusion in its core, but it is not hot enough to shine. These are called brown dwarfs, following the trend in stars from yellow dwarf, orange dwarf, and red dwarf.

Diamond

Not actually what it looks like.

55 Cancri e is a strange planet. Made almost entirely of carbon, it is eight times the mass of the Earth, but only twice as wide. And what is another name for super-dense carbon? Diamond! That’s right, though extremely rare, there are planets in the universe that are made of diamond, though unfortunately their surfaces are probably covered in graphite. So I guess we could call them pencil planets? Still, 55 Cancri e is a creative spark for writers and artists envisioning a romantic future of humanity in space.

These exotic planets break open the possibility of many more rare planet types waiting to be discovered. Doubtless, this list will be much longer in the future. Heck, our descendants may advance enough someday that they will be able to create custom planets of their own. They might have planets with multiple surfaces, the matter in between carved out like matryoshka dolls. They might have planets covered in computronium, where life has advanced beyond biology and covered the surface with network circuitry, running their own virtual reality universe. For a galactic civilization, such feats of engineering would be child’s play. The universe is strange and wonderful and full of mystery waiting to be uncovered, and possibility waiting to be realized.