Tag Archives: relativity

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!

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.