The following is the second part of an interview with Jeffrey Bennett, author of What Is Relativity?: An Intuitive Introduction to Einstein’s Ideas, and Why They Matter. You can read part one of the interview here.
Jeffrey Bennett: The conceptual ideas of relativity are somewhat counterintuitive, but they are not difficult to understand. All you need is an open mind and a willingness to follow some simple “thought experiments” through to their logical conclusions, and then to consider the evidence that shows these conclusions to be correct. As to why relativity has a reputation for being difficult: For the most part, it’s an undeserved reputation coming from the fact that it seems weird when you first study it. However, if you want to go beyond understanding the concepts and actually use relativity to test scientific ideas or design new technologies, then you need to work with the mathematics of relativity as well as with the concepts. The mathematics can become quite involved, especially for general relativity, and I certainly hope that some of my younger readers will be inspired to learn this mathematics — but don’t worry, you won’t find any of this mathematics in my book, which focuses only on the conceptual ideas.
Q: Following up on that, you say that relativity can seem counterintuitive, but in the book you say it does not violate “common sense.” What do you mean?
JB: By definition, we can only have “common sense” about things that we commonly experience, and the surprising effects of relativity are not noticeable under the conditions of our everyday lives. Instead, they become noticeable only at speeds much faster than we ever travel, or in gravity far stronger than Earth’s. In the book I use an analogy to “up” and “down.” In our daily lives, common sense tells us that “up” is over our heads and “down” is below our feet, and this common sense works fine for things like basketball games. But if that was all there was to it, then people on the other side of Earth would fall off. The fact that they don’t fall of therefore tells us that our common sense isn’t telling us the whole story. In a similar way, the fact that relativity tells us that we’d measure space and time differently at high speeds means that our common sense about motion must not be the whole story either, even though it works fine for most things in our daily lives.
Q: You start the book with a chapter in which you take readers on an imaginary future voyage to a black hole, and in the process you say that “black holes don’t suck.” What do you mean by that?
Jeffrey Bennett: For some reason, it’s commonly assumed that if you went anywhere near a black hole, you’d be sucked in, or that if the Sun turned into a black hole then Earth would get sucked in. But it’s not true. At a distance, the gravity of a black hole is no different than the gravity of a more ordinary star, and you’d have to get extremely close to the black hole before you noticed any difference. Because black holes are so well known in popular culture, I decided that an imaginary journey in which we learned what would really happen on a voyage to a black hole would be a good way to introduce Einstein’s amazing ideas.
Q: You said that relativity deals with gravity, but I thought Newton discovered the law of gravity. Did Einstein tell us something different?
JB: Newton discovered the law of gravity, and it works so well — for example, we use it to calculate exactly how to send spacecraft to distant worlds — that we consider it to be part of a scientific theory of gravity. (Remember that in science, “theory” means something that has been repeatedly tested and verified.) But even Newton recognized that while his theory of gravity worked, it didn’t really explain why gravity worked that way. Einstein’s general theory of relativity gives us the explanation that Newton’s theory of gravity didn’t, and even more important, it shows that Newton’s theory was not the complete story. Newton’s theory works really well in most cases, but it breaks down in conditions of very strong gravity or when we try to apply it to the universe as a whole. In that sense, Einstein’s theory is an expansion of Newton’s, because it applies to a wider range of situations than Newton’s theory. Still, it’s important remember that Newton’s theory was not “wrong,” because in most circumstances both theories give essentially the same answers.
Q: So what does Einstein’s theory tell us that gravity actually is?
JB: The short answer is that gravity is curvature of spacetime, but of course this short answer won’t mean much until you first learn what we mean by spacetime and how it can be curved. In fact, that’s a major reason why I wrote the book, so that you’d have a chance to understand what this short answer really means. As I’ve said, relativity is not difficult to understand, but I won’t pretend that I can make it clear to you in just a few minutes. However, if you’re willing to spend enough time to read my short book, I’m confident that it will get you started on your way to a real understanding of space, time, and gravity as Einstein showed them to be.
Q: Why can’t we travel faster than the speed of light?
JB: Again, I won’t pretend I can answer this question to your satisfaction in just a few minutes, but the basic answer, explained in detail in the book, is this: As we discussed earlier, relativity is built upon the fact that the speed of light is the same for everyone; that is, no matter how things are moving, everyone will always say that light is going the same speed of light — 300,000 kilometers per second (186,000 miles per second) — in space. The implication is that if you are in a spaceship, you’ll always see the light from your own headlights traveling away from you at this speed. This means that anyone looking at you will see that the light from your headlights is going faster than you are, and since everyone always measures the same speed of light, they’ll conclude that you are going slower than the speed of light. There’s no way around it.
Q: Which science fiction movies & TV shows would you say most accurately portray space travel?
JB: I like to point out that one reason you know that you really can’t go faster than the speed of light is that not even science fiction writers try to do it. Instead, because the speed of light is too slow for action movies about travel between the stars, science fiction writers look for loopholes in the laws of physics that might allow us to get from here to there faster than light would permit. They generally do this by inventing ways of bending or warping space (e.g., Star Trek’s warp drive) or simply leaving space (e.g., Star Wars’ hyperspace) so that you are no longer subject to the rules that apply in ordinary space. These ideas are interesting because they don’t break any known laws of physics, but there’s also no reason to think they are actually possible. One idea that has a deeper grounding in real science is the idea of worm holes that “tunnel” through hyperspace to connect distant parts of the universe. The movie Contact, based on the novel by Carl Sagan, did a nice job of using these.
Q: E = mc2 is the most famous equation in history. Why is it so important, and what does it have to do with relativity?
JB: E = mc2 is actually one of the discoveries that Einstein made as part of the theory of relativity; that is, it is one of the equations of relativity. In words, the equation states that energy (the E) is equal to mass (the m) times the speed of light (the c) squared. What this means is that under certain circumstances, mass can be turned into energy, or vice versa. This is important for many reasons, not least of which is that it explains how the Sun shines and how nuclear power and nuclear bombs work.
Q: I understand that you have written five science books for children, and that these are currently orbiting Earth aboard the International Space Station for the new Story Time From Space program. Does relativity have any effect on your orbiting books?
JB: Yes! The International Space Station orbits Earth at a speed of about 28,000 kilometers per hour (17,000 miles per hour), and special relativity tells us that we should observe time to be running slower on the station than on Earth. However, time runs only very slightly slower, because while the station’s speed is high by human standards, it’s still only about 1/40,000 of the speed of light. At this speed, the difference in the passage of time is small enough that during a 6-month stay, the books — and the astronauts —age less than objects and people on Earth by only about a hundredth of a second.