字幕表 動画を再生する 英語字幕をプリント Relativity - the idea that everything is relative, right? Relative to your perspective, your upbringing, your age, your place and orientation in space and time? Except that plenty of things - in fact, perhaps most things - aren't relative. For example, George Washington was the first president of the United States, World War I happened before the movie Star Wars was made, this picture shows three apples, and so on. It is true that certain things ARE relative to one's perspective - is the apple on your left, or my right? - is 50°F hot or cold? - is a car fast or slow? big or small? - and that's precisely what makes these concepts less interesting to scientists. In physics (and in most science), anything that changes if you change perspective can't be a fundamental property of the universe - only things that are absolute are considered 'physical' or 'real'. And for a long time, physicists thought that distances in space and intervals of time were absolute, fundamental properties in the universe. The special theory of relativity first described by Albert Einstein was merely a statement of the realization that we were wrong: distances in space and time are actually relative - they change depending on how fast you're moving. But more importantly, Einstein also described several quantities related to space and time which *are* absolute: the distance between two events in spacetime, the energy-momentum of an object, and of course, the speed of light. Similarly, the general theory of relativity was essentially the recognition that in fact neither the acceleration nor the gravitational force experienced by an object are absolute quantities. Accelerations can transform into gravitational fields (and vice versa), depending on your perspective and the path you take through spacetime. The more fundamental absolute quantity is the *curvature* of spacetime, which you can think of as a kind of "underlying" or "absolute" gravity. Special and General relativity, are, at their core, not about what's relative - they're about what's real irrespective of perspective. If everything were relative, then there could be no science, no laws, no justice - just opinion. Science exists because it turns out there *are* absolutes in the universe - truths which are the same regardless of your perspective. You might even say that science is simply about finding the truths that will still be true if you remove the scientist. So... goodbye!