MICHELLE THALLER: So, Tom, you asked the question, "How does mass increase as you go faster?" And this is really a wonderful part of Einstein's theories. It actually is also relatively slippery and kind of complicated because to even answer this question at all, we have to ask the rather strange question: "What do you mean by mass? What is your definition of mass?" You may have heard that nothing with mass can possibly go at the speed of light. The only things that travel at the speed of light are photons pure energy, light, the speed of light. Nothing with any mass at all can travel at the speed of light because as it gets closer and closer to the speed of light, its mass increases. And if it were actually traveling at the speed of light, it would have an infinite mass.

So think about that. Even if you had a tiny little particle that was, say, billions of times less massive than an electron just a tiny, tiny little piece of mass if for some reason, that tiny thing accelerated to the speed of light, it would have an infinite mass. And that's a bit of a problem. So let's talk about this. One of the things that you really have to realize is the speed of light is very, very special. It's not just simply a speed of something moving through space. As you go faster and faster and closer to the speed of light, time itself begins to slow down. And space begins to contract. As you go close to the speed of light, the entire universe becomes smaller and smaller until it basically just becomes a single point when you're going at the speed of light. And time, as you go closer to the speed of light, gets slower and slower until basically time is a single point at the speed of light.

Light does not experience space or time. It's not just a speed going through something. All of the universe shifts around this constant, the speed of light. Time and space itself stop when you go that speed. So the reason you can't accelerate to the speed of light, and the reason we say you have an infinite mass is a little complicated because the idea that mass actually is a measurement of energy. You may remember Einstein's famous equation, E equals MC squared. Energy equals mass times the speed of light squared. Energy and mass are equivalent. Mass is basically a measurement of how much energy there is in an object. When you're moving, you have the energy of your motion, too. That's called kinetic energy, energy of motion. So E equals MC squared, now your mass has not just the stuff that's in you but also the energy of your motion.

And that's why mass seems to increase as you go faster, and faster, and closer to the speed of light. It's not that you are actually getting any heavier. The increase in mass is something that's only observed by people that are watching you go by. If you were on a spaceship going very fast at the speed of light, you don't notice anything getting heavier. You are on your spaceship. You could jump up and down. You can skip rope. You can do whatever you want. You don't notice any change at all. But if people try to measure your mass as you go by, they not only are measuring your rest mass — your mass when you were still — but this added energy of this huge speed that you have through space. And that's called a relativistic mass. And it's a complex idea to think that mass itself is a measurement of energy, so that changes depending on how fast you're going. If you were to slow down on your spaceship, you would not keep that mass.

You would go back to being the same mass you were before you started moving that quickly. So as you can see, it's a complicated answer depending on how you define mass. Because as you're going very close to the speed of light and you have a huge speed, you need to take into account that energy because of the equation E equals MC squared.