ARI SHAPIRO, HOST:
It was the discovery that changed the universe - or rather our understanding of the universe. One hundred years ago today, Albert Einstein presented his theory of general relativity. So for the next few minutes, this is going to be a safe space for everyone who might think they have a vague understanding that maybe the theory of relativity is a really big deal, but maybe you don't really know exactly why or what that means. Consider this a physics amnesty. Astrophysicists and NPR blogger Adam Frank promises not to judge as I ask some really ignorant questions right now. Hey, Adam.
ADAM FRANK: Hey, how's it going, Ari?
SHAPIRO: Let's start with the basics on this 100th anniversary. What exactly is the theory of general relativity?
FRANK: So I think the first thing to understand is what Einstein gave us with the general theory of relativity. And what it was was a insight into the fundamental nature of space and time. You know, and that's kind of a big deal.
SHAPIRO: And that insight was that they're not separate things, but they're actually connected to each other, right?
FRANK: Right, exactly. I mean, when you think about space and time, they're kind of weird, right? They rule our life. We move through space our entire lives, time is always ticking away and yet, we don't see them, right? They're invisible. And what Einstein was able to do for physicists was to show us a way of understanding them as a unified whole. And the vision that he gave us was so radical from anything else that anybody had ever conceived.
SHAPIRO: And what was that vision? What was that insight about the relationship that space and time have to one another?
FRANK: Well, the first thing he understood was that space and time are not separate things, right? So from Newton, people had - sort of had the idea that, you know, time was just this river that flowed at the same rate, you know, one hour per hour, everywhere in space. And space was just an empty stage on which the stuff of life happened. And what Einstein showed was first of all, that you could never separate them. Space and time were one thing. It was this whole four-dimensional reality we call space-time. And more than that, the stage was dynamic - that space-time was a thing. It could bend and it could stretch and it could mold. And that idea that space-time was not just this emptiness, but that it was actually a dynamic player in the drama of the universe.
SHAPIRO: But this is where I start to get lost. Einstein's theory tells us that if we send someone out into space, they're going to age at a different rate than someone on earth? Do I even have that right?
FRANK: Yeah, it's true. So the general theory of relativity showed us that the rate at which time flows depends on how close you are to a massive body.
SHAPIRO: That is so weird. I'm sorry, that is so weird.
FRANK: No, it's weird for me too. It never stops - that's the great thing about relativity. It never stops being weird, right? No matter how - I've been working with it for 30 years, and I'm still like, oh, my - you're kidding me. So what this means is when you're - if you're downstairs and your kids are upstairs, time is flowing slower for you than it is for the kids up there - relatively.
SHAPIRO: Are there real-world technologies that we use today that wouldn't work if Einstein had not had this insight?
FRANK: You know, it's an interesting question because if you'd asked me that question 20 years ago or 30 years ago, I'd have been like, no, this is - you know, this is super important for physicists and their understanding of the universe, but there's really nothing around that we use. But now everything is changed because literally without general relativity, we would all be lost. And I say that because it's essential to GPS.
FRANK: All of us - yeah. All of us are using GPS technologies, right? It's in our phone. And the only reason it's accurate is because we have to take into account that time difference. You know, GPS works by having these satellites orbiting, and they're sending signals back and forth. And if we didn't take into account the fact that the time is flowing at a different rate for the satellites as it is for us on the ground, it would start to lose time in some sense. It would lose accuracy - about 45 microseconds per day. And you think, oh, that's not very much. But, you know, after a week, for example, if you were to step out on your porch in Columbus, Ohio, the GPS would tell you that you were 5,000 meters above Detroit. So GPS is so accurate, the small differences matter a lot.
SHAPIRO: That's Adam Frank who teaches astrophysics at the University of Rochester. Thanks for the explainer.
FRANK: Oh, it was great. Thank you.
NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.