The following is part one of an interview with Jeffrey Bennett, author of What Is Relativity?: An Intuitive Introduction to Einstein’s Ideas, and Why They Matter
Jeffrey Bennett: Nearly everyone has heard of Einstein’s theory of relativity, perhaps because it is so prevalent in popular culture. For example, relativity lies behind real science ideas like black holes and the expanding universe, and also behind science fiction ideas of things like warp drive, hyperspace, and worm holes. The reason it comes up in these contexts is that the theory of relativity represents our current understanding of the nature of space, time, and gravity. As such, it provides the foundation of almost all of modern physics and astronomy, which means it also plays a critical role in modern technology. To sum up, relativity tells us how the universe actually works, and through technology it comes up in nearly everything we do in our daily lives.
Q: How does it gets its name? That is, what is “relative” about relativity?
JB: Let’s start by dispelling a common misconception: Einstein’s theory does not say that “everything is relative.” Rather, the theory refers specifically to the relativity of motion. You can think of it like running on a treadmill: If the display says you are running 6 miles per hour, it means that is your speed relative to the rubber mat on the treadmill. Your speed is different if you measure relative to something else. For example, your speed relative to the exercise room is zero, because you’re running in place; your speed relative to the Moon is nearly 1000 miles per hour, because that is the speed at which Earth’s rotation carries you in a circle around Earth’s axis each day; and your speed relative to the Sun is about 60,000 miles per hour, because that’s how fast Earth moves in its orbit. Einstein’s theory gets its name because it describes how measurements of space and time differ for observers moving relative to one another.
Q: You also say that “relativity” is in some sense a misnomer for Einstein’s theory, because the theory rests on foundations built from two absolutes. What are these absolutes?
JB: The theory gets its name from the relativity of motion, but the fact that motion is relative had already been known for centuries. So the real foundations of Einstein’s theory lie in his assertion that two particular things in nature are absolute: (1) The laws of nature are the same for everyone; and (2) the speed of light is the same for everyone. All the astonishing consequences of relativity can be derived from these two absolutes, both of which have been verified by countless observations and experiments.
Q: What are these “astonishing” consequences of relativity?
JB: They are the things that relativity tells us that we would not have expected before Einstein’s work. I won’t list them all right now, but for example: Relativity tells us that if you observed people in a spaceship traveling by you at high speed, you’d find that their time was running slower than your time (meaning that they age slower than you), their spaceship is shorter than it would be if they were at rest relative to you, and their mass is greater than it would be at rest.
Q: You explain that Einstein actually published relativity in two parts: the “special” theory in 1905 and the “general” theory in 1915. What’s different about them?
JB: The “special” theory gets its name because it applies to the special case in which we consider space and time without considering the effects of gravity. The general theory gets its name because it also includes gravity, which means it applies to a wider set of circumstances (which scientists call a more “general” set) than the special theory.
Q: Your subtitle is “An intuitive introduction to Einstein’s ideas, and why they matter,” and next year is the 100th anniversary of Einstein’s 1915 publication of his general theory of relativity. Why should ordinary people care about Einstein’s ideas now, 100 years after they were discovered?
JB: Einstein’s ideas have been important to everyone for a long time, but unfortunately very few people have ever had a chance to learn about them. So their 100th anniversary seems as good a time as any to try to bring more people up to speed. As to why people should care, I like to focus on four particular reasons:
(1) Einstein’s theories underlie nearly all of modern science and technology; for example, modern electronics, nuclear power, and even your GPS navigation all rely on them. This means that unless you know something about relativity, then you are using technology without understanding it — and while you are free to do this, I think it’s always better if you understand the tools that you use.
(2) As mentioned earlier, relativity provides our current understanding of the nature of space, time, and gravity, and this nature turns out be somewhat different than we would expect from everyday life. So if you really want to understand what it means to be a human being, living on a planet that orbits a star in a vast universe, then you have to first understand the ideas of relativity.
(3) Relativity is astounding in both its implications and its basic simplicity, which I think makes Einstein’s work a shining example of what human beings can do when we put our minds to work for positive things rather than negative things.
(4) Relativity shows that there’s a permanence to what we call “spacetime” that, in my opinion, ought to affect they way we all behave toward one another; in fact, I think if everyone understood the ideas of relativity, we’d live in a better and more peaceful world.