Category Archives: The Heavens

Why Can't We Travel Faster Than Light?

James writes: I know light is the fastest thing in the universe, but why can’t anything else go that fast? Is it just that we haven’t built a ship fast enough yet?

James, I am no physicist. I believe I understand the reason for “lightspeed” being the cosmic speed limit, but I could be wrong. Here goes my best shot:

The hard part, to me, is why light travels at a constant velocity – and why that velocity is 299,792,458 meters per second (in a vacuum; going through air or other media slows it down). I have no idea why light travels at exactly that speed, instead of going faster or slower. But Albert Einstein’s famous relativity equation – E=mc² – explains why we cannot build a ship that goes that fast.

In the relativity equation, “E” stands for the energy of an object. The “m” on the right side of the equation is the object’s mass. And “c” means “constant”: the velocity of light. In math, a constant is a value that does not change. Variables are properties that can change. “E” is a variable, because the energy an object has can change. For example, if a karate master moves his fist very slowly towards a wooden board, he might push it out of the way, but the board will not break. But if the karate master throws a fast punch at the board, his fist will break through it easily. Why? Because his fist had much more energy when it was moving fast. Energy increases with speed; this is why cars (and passengers) are damaged a lot worse if they crash at high speeds than if they crash when moving slowly.

An equation is a statement that both sides are equal, like 3+2=5. The left side equals the right side. If we add to the left side, we must add the same value to the right side for the equation to be true: 3+2+1=5+1. Einstein’s relativity equation states that an object’s energy is equal to its mass multiplied by the square of a constant, “c”. The constant does not change, so if “E” changes, “m” must also change. In effect, an object moving very quickly becomes more massive.

More mass means that it takes more energy to accelerate. This is why a Ferrari can accelerate faster than a dump truck, even if the dump truck has more power. So if we build a starship and move it faster and faster, its mass will begin to increase. The more massive it is, the more energy it takes to make it go faster. As we approach the speed of light, the mass becomes so great that it would take an infinite amount of energy to make it go as fast as light. Since we do not have an infinite amount of energy, we cannot reach lightspeed.

That is as much as I understand about your question, James. I hope it helps. If there are any physicists in the audience who can explain it better, or correct any mistakes I have made, I will appreciate it.

Stay curious, my friends!

Were the Apollo Moon Landings Real, or Not?

This summer, I took my two sons and my nephew to the Frontiers of Flight Museum at Love Field Airport in Dallas.

If you like airplanes half as much as I do, you will want to visit Frontiers of Flight next time you are in the Dallas area. The exhibits cover the entire history of human flight, from Leonardo da Vinci’s sketches to the current space program. What sets this flight museum apart from most others is the sheer number of real planes on display; an aircraft enthusiast could easily spend all day there learning and exploring, and every time I visit, there seems to be something new to see.


The best piece of all is the Apollo capsule. This is not a replica. It is the actual command module from the Apollo 7 mission. That fact in itself is enough to make me stand silently for a few moments every time I visit, and reflect on the audacity of the human spirit. Three human beings sat in that very box for eleven days, the rude metal cone hurtling through the vacuum of space at 50,000 miles per hour, guided by a primitive calculating machine with far less power than any cellphone you can buy today. Their courage and skill paved the way for the triumphant moment a year later, when an air-breathing mammal from Earth set foot on the dusty, airless surface of the Moon with the unforgettable words “one small step for a man; one giant leap for mankind.”


The Apollo capsule represents something that makes me proud to be a man. That’s why it makes me sad when a student asks me if the moon landings were real – because they read some silly web page (written by someone too ordinary to capture anyone’s attention without capitalizing on fear, distrust, and ignorance) about how the whole space program was faked.


The Apollo program was a giant leap for humankind. It was a gigantic push, by a nation of dreamers, to go where no one had gone before, to do the impossible. It took a decade and cost $25 billion, which sounds like a lot of money until you compare it with the amount we spend on other kinds of hardware from stealth bombers to aircraft carriers (if you care to research this, make sure you look at operating costs, not just cost to build). And I have no doubt whatsoever that Apollo was a genuine program that delivered genuine results – among the most spectacular results ever achieved by any human enterprise. The reason for my lack of doubt is called Occam’s Razor.

Occam’s Razor is a general rule of logic, the idea being that when you have to choose between a number of explanations, the simplest one – the explanation that requires the fewest assumptions to support it – is the most reliable.

There are many websites devoted to the idea that the Apollo moon missions – if not the entire U.S. space program – were a hoax. I will not list all the arguments here; you can find them easily enough if you are interested (or more likely, if you are really bored). The most obvious weakness of these theories is that they fail the test of Occam’s Razor. They depend on many more assumptions without evidence to support them – let alone the fact that they fail to explain how GPS works if we never went into space. But the thing that annoys me the most about these people is the way they disrespect all the courageous astronauts who risked their lives – and a few who lost their lives – for the sake of lifting a nation’s eyes and spirits to the stars. I wonder if they would have the nerve to look Buzz Aldrin in the face and call him a liar. Somehow I doubt it.

If You Fell Through a Black Hole, Where Would You Go?

Black holes are definitely some of the most mysterious things in the universe. A hole that looks the same from all sides is quite a puzzle!

If I were to go up into the attic and saw a circular hole around me, I would fall down into the living room. Likewise, if there were a hole in my living room floor, I might keep falling down into the basement. This is easy to imagine. It is not too much harder to picture gravity reversing, so that I would fall up from the basement through the hole in the living room floor, ending up back on the couch where I started. In each of the three places I had been – the living room, the attic, and the basement – I could look through (or fall through) a hole to another space.

Before I start talking about black holes, I should say that everything we know about them is based on math. Until 1971, there was no observed evidence that they even existed. Today, there are many known black holes, but we will never see one of them directly, so all our knowledge of black holes is based on measurements of things happening in space that cannot be explained without the math model of a black hole. Most of my readers are not math professors, so you may be wondering what I mean by “math model”. If I tell you that I am thinking of an object that is 5 cm long, 5 cm wide, and 5 cm tall, you could guess that it is a cube. But what if it is a sphere? All you really know is how big it is. Now, if I tell you that the object has six square sides, you know it is a cube. You can picture it in your mind. There is no real cube, but your image of it is based on numbers I gave you. The cube in your mind is a math model.

Remember the holes in my ceiling and floor? Now imagine a hole in the middle of empty space. It is not a hole in a wall or anything else; it is a hole in space. You might be able to see the hole if you got close enough, but you would not be able to see through it. If you moved in a big circle around the hole, it would look the same from any angle! If you were to throw an object – like a marshmallow – through a hole in a wall, you could look over the wall (or through the hole) and see the marshmallow on the other side. But if you threw a marshmallow into a black hole, there is no other side. It is gone forever!

How can this be? About 250 years ago, a scientist named John Michell imagined a thing nobody had thought of before.

(Scientists of his time already knew that objects had to reach a certain speed to escape the gravity of any planet or star; this speed is called the “escape velocity”. The more massive a planet or star is, the faster an object has to move to escape out into space. This is why a rocket can get to the moon, but a bullet from a gun cannot. It is not fast enough. A rocket fast enough to escape the Earth’s gravity would still not be able to escape the Sun, because the Sun is so much more massive than the Earth. You would need a much faster rocket. On the other hand, if you have seen pictures of the Apollo missions to the Moon, the rocket they used to get off the Moon was not very fast at all. It didn’t have to be; the Moon is much less massive than the Earth, so its gravity is much weaker.)

John Michell imagined a star so massive that even light would not have enough speed to escape its gravity. If the light from the star could not escape out into space, then nobody would be able to see it! John Michell called his imaginary star a “dark star”. Later scientists, including Albert Einstein and Karl Schwarzschild, made math models of dark stars to describe the behavior of light and of objects that got close to them. It was a scary but fascinating idea! In 1964, a journalist named Ann Ewing wrote a report about these math models. The report was called “Black Holes In Space”. Since then, people have been calling them “black holes”.

Even though many scientists made lots of math models of black holes, nobody had ever seen one. They are, after all, invisible! Between 1971 and 1973, a team of astronomers watched a giant star far out in space. It behaved unlike any other star. By 1973, they knew from their measurements that the star had a black hole next to it, just like you knew (after getting enough information) that the object I was describing was a cube. The star system fit the math model: it could only be a black hole!

That first observed black hole is called Cygnus X-1. Since then, we have observed many other black holes. Although they are invisible, we know they are black holes from observing what happens around them. If it fits the math model, it must be a black hole.

So what is the answer to the question we started with? If you fell into a black hole, where would you go? By now, you know that black holes are not really holes at all. They are objects with such strong gravity that nothing, not even light, can escape them, which is why they are perfectly dark. Because its gravity is so strong, anything that falls into a black hole is crushed into zero volume. Since one of the properties of matter is that it takes up space, objects falling into a black hole would really not even be objects anymore. The center of a black hole is called a “singularity”, and the math model for a singularity seems to break the rules for what we know about the universe.

Black holes may always be one of the universe’s mysteries!

Do Things Really Look Different From "Down Under"?

I wish I could answer this question from experience. When I was ten years old, a neighbor lent me a book called The Complete Adventures of Blinky Bill – Blinky Bill was a koala – , and I fell in love with Australia. It would be truer to say I fell in love with the stories about Blinky Bill and his marsupial friends, because they were all I knew about Australia. It’s a good book. It made me want to go off and explore Australia immediately – a desire which, regretfully, I have never had the opportunity to indulge. Still, the Land Down Under has always had a special place in my imagination since then.

Most of us have heard that water goes down the drain one way (counterclockwise) in the Northern Hemisphere and the opposite way (clockwise) in the Southern Hemisphere. This is supposed to be due to the Coriolis Effect. To be very brief, the Coriolis Effect is the way the Earth’s rotation makes large bodies of fluids – like water and air in the oceans and the atmosphere – start to spin. The Coriolis Effect is what causes hurricanes to form. But it does not affect the way water goes down a drain in either hemisphere. The amount of water in a bathtub or toilet is too small to be affected by Coriolis forces. You can prove this yourself by filling the sink a few times and watching it drain. Sometimes the water will start to rotate, and sometimes it will just drain away with no spinning motion at all. You can run a finger around the drain clockwise to start a clockwise rotation, or make it drain counterclockwise if you like. It will not change due to the Coriolis Effect.

Tropical cyclone storms are another matter. These rotating storm systems (called “hurricanes” around the Americas and “Typhoons” around Asia) form because of the Coriolis Effect, and they turn counterclockwise in the Northern Hemisphere and clockwise south of the equator. The huge amounts of air and water involved are affected by Coriolis forces, unlike draining water in a sink or tub.

What else looks different in the Southern Hemisphere? If you watch the moon change phase throughout its cycle, you will notice that the change happens from right to left. (Click here for a really good animated model.) In the Southern Hemisphere, the pattern is reversed: the phases change from left to right! This would seem very strange to me. Maybe someday I will get to visit Australia and find out for myself!

Another thing that looks different from the Southern Hemisphere is the starry night sky. Since the Southern Hemisphere looks out on space in the opposite direction from the Nothern Hemisphere, the field of vision is completely different. Near the equator, the view from either hemisphere is most similar, but the difference increases the farther you go toward either pole. (Click here for a great map of the stars from both hemispheres.)

It will never be the same to see something online as it would be to see it with your own eyes. I have always enjoyed traveling and hope to keep discovering and exploring new places as long as I live. St Augustine once said, “The world is a book, and those who do not travel read only a page.” How true.

How Do We Know the Distance to Far-Away Galaxies?

If you read my post about Hubble’s Law, you may be asking “How did Hubble know the fastest-moving stars were farther away?”

That is a good question, and it means I am going to have to explain something called “parallax”. Parallax is the difference in the way an object looks from two different points of view. It is easy to observe parallax. Close one eye and look at something close to you, like the computer monitor or an object you can reach. Now quickly close that eye and open the other. Switch eyes like this a few times in rapid succession. The object you are looking at seems to jump back and forth. Of course it isn’t really moving – your eyes are a few centimeters apart, so each eye sees a slightly different view. Now look at something a little farther away, maybe across the room, and repeat the experiment. The object still seems to jump, but not as much. Next, go outside. Look at the farthest object you can see: trees on the horizon, mountains if you have any to look at, the moon if it is visible. Do the eye thing again. The object may not seem to move at all. You have just discovered parallax rangefinding.

With highly sensitive scientific instruments, we can detect parallax even with very distant objects. The difference in the appearance of a galaxy in spring and autumn (when Earth is at two different points separated by about 300,000,000 kilometers along its orbit) makes distant galaxies “jump” a little when the images taken are compared, just like the objects you experimented with. By analyzing the parallax, the galaxies’ distance can be calculated.

Together with Hubble’s Law, parallax allows us to describe the size and movement of our universe.

How Do We Know That the Universe Is Expanding?

The answer to this question is connected to Hubble’s Law: all the things we can observe in deep space – stars, nebulae, galaxies, and everything else – show a redshift, or Doppler shift, proportional to their distance from us (or from each other). Since you are reading this article, I am going to assume you do not know about Hubble’s Law, or the Doppler Effect, so I will do my best to explain those concepts.

The Doppler Effect is what you hear when a fast-moving, noisy object passes by. If a car went by at 160 kilometers (100 miles) per hour and the driver was leaning on the horn the whole time, you would notice that the horn would suddenly start to drop in tone as the car passed you. This is because sound waves do not travel that much faster than a speeding car – only about 1100 kilometers (720 miles) per hour. As the car speeds away from you, the sound waves get “stretched out”, and longer wavelengths make a lower tone. The first time this was explained scientifically was in 1842 by an Austrian scientist named Christian Doppler, which is why we call it the Doppler Effect.

The Doppler Effect works for light waves too, but we don’t notice it all around us on Earth because light travels so fast (300,000 kilometers / 186,000 miles per SECOND) that nothing on Earth is far enough or fast enough to make it change colors by the Doppler Effect. Distant stars and galaxies are a different matter. They are very far away, and moving very fast. The farther away they are, the more their light is “stretched” to a longer wavelength, which makes them appear redder (this is called “redshift). The American astronomer Edwin Hubble observed this by watching many stars through a telescope. Other scientists had predicted that the universe is expanding, but Hubble was the one who proved it. Before Hubble, we didn’t even know there was a universe beyond the Milky Way galaxy. Even astronomers thought that the blurry objects in the telescope were nebulae, clouds of space gas. After Hubble’s discovery, they started to realize that there were many other galaxies quite like the Milky Way, deep out into a universe that was far bigger than anyone had imagined.

Maybe the best model for the expanding universe is to take a light-colored balloon and make a few small dots on it with a Sharpie. Ask someone to blow it up, and watch what happens to the dots as the balloon inflates. The dots that are farthest apart will move away from each other faster than the ones that are closest together.

Our country honored Edwin Hubble by naming the first orbiting telescope after him. Since 1990, the Hubble Space Telescope has given us the clearest and most beautiful pictures of distant galaxies and other objects in deep space.