One Hundred Years of General Relativity

22:06 minutes

Albert Einstein, by Orren Jack Turner. Library of Congress Prints and Photographs Division Washington
Albert Einstein, by Orren Jack Turner. From the Library of Congress Prints and Photographs Division, Washington, D.C.

Albert Einstein published his theory of general relativity 100 years ago. The theory has shaped the idea of black holes, pulsars, and modern cosmology. Science historian David Kaiser guides us through the history of Einstein’s insight, and physicists Michael Turner and Alex Filippenko discuss where the theory might take us in the future.

Segment Guests

David Kaiser

David Kaiser is the Germeshausen Professor of the History of Science and a Professor of Physics at the Massachusetts Institute of Technology, in Cambridge, Massachusetts.

Michael Turner

Michael Turner is director and Rauner Distinguished Service Professor at the Kavli Institute for Cosmological Physics at The University of Chicago in Chicago, Illinois.

Alex Filippenko

Alex Filippenko is an astronomy professor at the University Of California, Berkeley in Berkeley, California.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. A bit later in the program, we’ll check in on NASA’s Dawn Mission, which arrived that the dwarf planet Ceres this morning. 

But first, a hundred years ago, the carnage of that great war that chewed up a generation of young soldiers did not halt young scientists from formulating big ideas. In 1915, Albert Einstein published his theory of general relativity. The four-page paper upended the idea of gravity and Isaac Newton’s 200-year stranglehold on that school of thought. 

General relativity is the basis for modern cosmology and astrophysics. How did the theory percolate in Einstein’s head? How did it change our view of the world, and will it become obsolete in this quantum multiverse view of the world? David Kaiser is a science historian and professor of physics at MIT. Welcome to Science Friday, David. 

DAVID KAISER: Thanks so much, Ira. Very glad to be here. 

IRA FLATOW: You’re welcome. Let’s talk about the history. How did Einstein come up with this special relativity– he came up with general relativity 10 years after special. What was going on in the meantime in those years? 

DAVID KAISER: It was really a remarkable intellectual journey. i think it was not a solo journey. Einstein, it turns out, had to draw on the help of several friends along the way. But it really was a remarkable kind of intellectual voyage. 

And the main thing, the place he wound up, where we know he got to, though it wasn’t so clear to him when he set out in 1905, but where he got to by 1915 was, as you just mentioned, a really radically different view of how the universe should work, or how it could behave. 

And the biggest change, what he really got to at the end, was this notion that space and time, or their union, what he came to call spacetime, could be as wobbly as a trampoline. And that really was a big, big shift from what had come before. 

As you mentioned, Isaac Newton’s physics or even an Einstein own special theory of relativity from 1905, in those conceptions, space and time provided a kind of unchanging, rigid platform, a kind of solid stage on which all the other activity of physics could unfold. So you could have apples falling, and planets revolving, and ultimately, galaxies jostling. 

But the idea in those older sets of theories of physics was that none of that could impact space and time themselves. They remained this kind of rigid stage, a platform for all the laws of physics to play out. 

And what Einstein finally got to– not right away, but along this sort of almost 10-year journey– was a very different notion, that spacetime itself was a player in that action, that spacetime could warp, or distended, or bend in response to where the stuff is, in response to the distribution of matter and energy. 

And then in turn, the warping of space and time could affect the behavior of all that other stuff so that planets would move slightly differently if they were moving in a region of space and time that had already been warped by some other large mass. So he gets this notion there’s a dynamical, a changing nature to spacetime. 

IRA FLATOW: So he actually gives a geometric interpretation of it. In other words, it’s something– the geometry of space, so space has a geometrical value to it. It bends, it warps, things like that. 

DAVID KAISER: That’s exactly right. And again, where he got to by the end of that journey in 1915 was that gravitation was only geometry, and that’s a really big shift. 

So in the physics of Newton, as our high school students still learn, and we still use all the time to do helpful things like build bridges and even spacecraft, in Newton’s physics, we talk about forces. One body, one object of a certain mass will exert an attraction, will tug on some other body with mass. It will exert a force. 

And that language, that framework had worked remarkably well for centuries, as you’d said. But instead, as Einstein gets to his surprising conclusion in 1915, that gravity, the phenomena we associate with gravity, whether it’s the fall of an apple or the orbit of the moon, that has nothing to do with forces. It really is all about geometry. 

And that’s when he– along the way, part of what took so long was that unfortunately, he had not paid much attention to his geometry classes as a student, so it really came back to bite him that he cut a lot of classes as a kid. So he actually had to work pretty hard just to learned some pretty sophisticated geometry from friends of his along the way. 

IRA FLATOW: How readily accepted was it? I mean, we’re still testing it out now a hundred years later, aren’t we? 

DAVID KAISER: Well, with hindsight, we can say with extraordinary confidence that it’s extremely well. It stood the test of time exquisitely well. In fact, every effort we’ve made to try to find a weakness in it has been stymied. 

Ingenious, clever, clever efforts to find some deviation, some change from what Einstein’s equations predict, none of those has borne out in a hundred years, which is a great testament to this theory, probably why we’re so excited for this centennial year. That’s, of course, our view from today. 

Einstein did not light up the world right away with this new set of ideas. It was couched in very abstract and difficult mathematics, at least for common physicists at the time, what would’ve been standard for them to know. It relied on an unfamiliar language of a kind of geometry, and it did have all these very strange-sounding concepts embedded in it. So it was a pretty difficult and abstruse kind of theory in its early days. 

IRA FLATOW: It took a famous eclipse of 1919, though, to make Einstein famous about that, right? 

DAVID KAISER: That’s right. Now, that changed things almost overnight. And we should say that Einstein had made predictions that this extreme warping or bending of space and time around an object like the sun, or even now we know, even more massive objects, like neutron stars, the big hulking masses, that the notion that gravity could bend the path of starlight, he actually figured that out on his own even before 1915. 

And he was very busy writing letters to astronomers in the United States throughout Germany saying, will you please test this hypothesis? And Einstein realized it could be tested during a total eclipse, a time when the moon would block out almost all the brightness of the sun from our point of view here on Earth, so that one could see very dim stars whose light had to travel around this warping space warp near the sun. 

And so Einstein figured that out, and he actually helped to try to get an expedition mounted to test that in 1914. And it turns out he helped raise funds within the German government. He got a colleague of his from Berlin, Erwin Freundlich, to get some assistance and head out. 

And they had the misfortune of landing in the Crimea, which was right where they expected to see this eclipse, in August of 1914. So suddenly, there were Germans in Russia right as war broke out. They were immediately locked up, their equipment confiscated. Their experiment was scuttled. 

So the fact that we get this dramatic, all the more dramatic announcement in 1919, under really different circumstances, a year after war had ended, that really just galvanized Einstein’s own celebrity and interest in this theory. 

IRA FLATOW: David, thank you for taking the time to join us, and happy hundredth anniversary to you. 

DAVID KAISER: My pleasure. Thanks, and happy anniversary to you, too. 

IRA FLATOW: David Kaiser is a science historian and professor of physics at MIT. General relativity has generated the idea of black holes, quasars, and even given us the GPS. But Einstein never meant for that theory to be the final answer. What questions are still left? Has general relativity exhausted its usefulness? Will we move beyond the theory in the next hundred years? 

My next guests are here to walk us through this. Michael Turner is the director of the Kavli Institute for Cosmological Physics at the University of Chicago. Good to see you, Michael. 

MICHAEL TURNER: Good to see you again, Ira. 

IRA FLATOW: Nice to have you here in New York. Alex Filippenko is a professor of astronomy at the University of California Berkeley. Welcome to Science Friday. 

ALEX FILIPPENKO: Thanks so much, Ira. 

IRA FLATOW: Michael, why do we still need to know about general relativity? 

MICHAEL TURNER: Well, we’ve pretty much after that hundred years, we’ve pretty much digested. I want to take off on where David was talking about, that it was really boutique science, boutique theory for the first 50 years, that it was beautiful mathematics, and understood by few, but not really necessary for the universe. 

And in 1963, about 50 years ago, all hell broke loose with the discovery that we live in a universe where general relativity is very important. Black holes, the quasars were discovered, spinning neutron stars, the cosmic microwave background. And it went from boutique science to the essential mathematical tool of any astrophysicist today. 

And so I guess the big untested piece of general relativity is gravitational waves. So these are the ripples in spacetime that are generated when, say, one black hole coalesces with another black hole. And these ripples are very, very tiny, less than the size of an atom. 

And the US LIGO observatories, the gravitational wave observatories in Louisiana and in Washington state, hope to detect gravitational waves. And that would pretty much close out the standard predictions. 

Of course, people are probably interested in wormholes after interstellar, and some of the more esoteric pieces. But I think we would be able to say that the primary features of general relativity have been tested and verified. 

IRA FLATOW: Alex, you’re working on measuring a supernova to capture something called the Einstein cross, and there was good news this week about that. 

ALEX FILIPPENKO: Well, that’s right. Yes, Ira. It’s very, very exciting. 50 years ago, it was predicted that if a supernova goes off behind a massive galaxy or a cluster of galaxies, the spacetime around that galaxy or cluster could be bent so much that it would actually produce several distinct images of the supernova. 

And the light that forms these distinct images would travel different amounts of time through space, and so you would see the supernova brighten and fade at slightly different times. And that would actually allow us to, for example, measure the distribution not only of the visible matter in that galaxy, but also of the pervasive dark matter that we think fills the universe. And we found such an object for the first time in 50 years. 

IRA FLATOW: There were pictures of a supernova, of actually, the lensing effect. 


IRA FLATOW: You got four instead of one. 

ALEX FILIPPENKO: Yeah, four instead of one. And in fact, we can say that there used to be one. There was one 20 years ago, and another one about 50 years ago. And so we missed those. But we’ve made a prediction based on the models that there should be one that is another image of the same supernova sometime in roughly the next five years. 

And so we’re going to be taking pictures of that same part of the sky to try to verify that prediction, and then use the new observed position to further refine the models of how much dark matter there is and how it’s distributed in this galaxy and in the cluster of galaxies in which that galaxy resides. 

IRA FLATOW: [INAUDIBLE] that dark matter is that is one of the new frontiers on this. 

ALEX FILIPPENKO: Well, that’s right. And in fact, this is one of the great, exciting things in astrophysics now, this dark matter, and also the dark energy that really is the dominant stuff of the universe. 

IRA FLATOW: All right. We’re going to take a break. We’ll come back and talk lots more about Einstein and of dark matter, dark energy, with Michael Turner, director of the Kavlia Institute for Cosmological Physics at the University of Chicago. Alex Filippenko, professor of astronomy at UC Berkeley. Our number if you’d like to join us, 844-724-8255. You can also tweet us @scifri. Stay with us. We’ll be right back after this break. 

This is Science Friday. I’m Ira Flatow, in case you just joined us. We’re talking about a hundred years of Albert Einstein’s general theory of relativity– gosh, time flies, a hundred years ago– with my guest Michael Turner, director of the Kavli Institute for cosmological physics at the University of Chicago, Alex Filippenko, professor of astronomy at UC Berkeley. 

And when we went to the break, we were talking about Einstein never foresaw the use– right, Michael– of the lensing effect for anything. You said it was sort of boutique stuff. 

MICHAEL TURNER: Yeah. And well, first of all, that Einstein crossed with the supernova is so cool. Congratulations to Alex and his student. 

ALEX FILIPPENKO: Thanks, Michael. Isn’t it fantastic? 

MICHAEL TURNER: It really is. 

ALEX FILIPPENKO: Boy, that image is so beautiful. 

MICHAEL TURNER: And Einstein, of course, looked at gravitational lensing, and made some calculations, and said, this will never work. It’s way too small. And in 1979, the first gravitational lenses were discovered, twin quasars. And of course, to prove it, you had to show these two images were identical. They weren’t just two similar quasars, but they must have been the same object lensed by gravity. 

And now here we are today, where gravitational lensing is so commonplace, any of your listeners can go look at the Hubble Deep Field or any of those images from the Hubble. You won’t see multiple images, but you’ll see the little arcs that are caused by gravitational lensing. Just stare at the picture long enough. 

And so this going from boutique to being a tool, and now, we’re going to use it to study the really esoteric things, dark matter, and my favorite, dark energy. Because by studying the time delays in these systems, you can figure out how the universe was expanding way back when. 

IRA FLATOW: If it’s such a good theory, and it’s proven itself so well, why are we trying to change it to get it to unite with the world of the quantum? Because quantum is a whole different world, isn’t it? 

MICHAEL TURNER: Well, partly because of the fun. We don’t want anyone to have the last word on gravity. We don’t want Einstein to have the last word on gravity. But what David was saying about how Einstein came to general relativity, he realized Newton and special relativity, those two theories were inconsistent. And that’s what led him during that 10-year period to general relativity. 

And we’re in the same dilemma today, that we have general relativity, one of the great intellectual accomplishments of the 20th century, and quantum mechanics, and they’re inconsistent. And Einstein realized this, and others have realized it, and so you need a theory that combines the two. 

The great hope of some people is string theory that brings these two together. And then, of course, what’s exciting about it is not displacing Einstein, although that’s pretty exciting– is all the surprises it will bring. Because as David said, general relativity told us that spacetime is dynamical, but it didn’t explain where they came from. And string theory might actually explain where space and time came from. 

IRA FLATOW: But it’s getting old, string theory is. It’s 30 years old. 

MICHAEL TURNER: Well, there was another birthday party this year, 30 years of string theory. And so well, let’s go back and look at general relativity. The first 50 years were pretty quiet. And then all of a sudden, there was a breakout. It took the discovery of quasars and other objects that needed general relativity to describe them, so you never know when that’s going to happen. 

IRA FLATOW: Yeah. In fact, we can’t talk about Einstein without actually hearing a little bit from Einstein. So for many decades, and you talk about, we’ve been trying to unite general relativity and quantum mechanics. And Einstein himself said that when you have a goal like that, that the big scientific method is the tool to get you there. 

ALBERT EINSTEIN: Whatever this tool in the hand of man will produce depends entirely on the nature of the goals alive in this mankind. Once these goals exist, the scientific method furnishes means to realize them. 

IRA FLATOW: Alex, we have that the goal. We have the tools. 

ALEX FILIPPENKO: Well, that’s right. Yeah. Well, I think an important point is that the nature of science is that we’re trying to come up with a progressively better description of what we see, a model for what we call reality. 

When general relativity was developed, it didn’t mean that planes would’ve started falling out of the sky had there been a lot of planes up there at the time. Newtonian physics is fine for most everyday phenomena, but general relativity provides a more complete description. 

And presumably, at some point, we’ll have a quantum theory of gravity that will also provide an ever more complete, ever more all-encompassing description. But it’s always a model for the universe. It’s not necessarily what is truth or reality with a capital T or an upper case R. 

IRA FLATOW: But would a quantum theory of gravity have the same geometric design that– 

ALEX FILIPPENKO: No, it probably would be that the exchange of particles, what we would call gravitons, those are what in quantum theory would, in a sense, tell objects that gravity exists, just like in electromagnetism, we have the exchange of photons. 

And if you just have a stationary proton and an electron on a table, they exchange what are called virtual photons. They’re not even real, in a sense, but they do mediate this electromagnetic force. And in a similar way, gravitons are the quantum theory equivalent of photons. 

IRA FLATOW: Now, I see why it’s so hard to unite the two, because you have a conceptual model of what Einstein said. This is actually warping space. It’s changing the fabric, and you’re talking about particles, which– 

ALEX FILIPPENKO: But that’s because if you look at the warping of space on ever more microscopic scales, you find that it becomes chaotic and extreme, and that’s something that general relativity is not equipped to deal with. 

And yet it’s a clear prediction of quantum theory that on very small scales over very small times, you have these gigantic fluctuations in the energy of space. Indeed, it leads to an infinite or a nearly infinite energy for the vacuum, and we clearly don’t live in such a vacuum. So something is going wrong in our understanding of the theory. 

MICHAEL TURNER: And, of course, the progress– Einstein wanted to unite them geometrically, and others want to unite them with forces and particles. And if history is any guide, if you look at the change from Newtonian gravity to general relativity, it doesn’t look anything like that. And so that’s the most interesting part. There are likely to be surprises. 

And string theory gives hints of that, where dimensions come and go. So the number of dimensions, four is not the total number of dimensions, and dimensions can come and go. And so the new theory is likely to be very different from either the theory that describes the particles at the fundamental level or geometric. And it might allow us to answer big questions like what happened before the Big Bang. 

IRA FLATOW: Is it going to require new physics, new things that we just don’t even know about the way things work yet? 

MICHAEL TURNER: I think that’s probably the safest bet, that at any given point in time, we think we know a lot. But as the area of our understanding expands, the perimeter of our ignorance gets equally larger. 

So there are likely to be new things, and it would be nice to predict them beforehand and then go out and discover them. And that’s really been the trouble with string theory is that the predictions that can be tested are few and far between. 

IRA FLATOW: But who would have predicted dark energy, right? It’s popped out probably just a few years ago. 

ALEX FILIPPENKO: Well, there were hints. In fact, even Michael Turner in his career said a number of times that certain aspects of the observations would be fixed, in a sense, would be understood if there were something like dark energy. But the evidence before the discovery of the expansion through these observations of distant supernovae, the evidence was always indirect. But the idea was out there. 


ALEX FILIPPENKO: Yeah. Well, even Einstein had the idea that there might be, in a sense, this weird energy pervading the universe. 

IRA FLATOW: And that brings us back to how science progresses. So Einstein went from special relativity to general relativity by pure thought and Gedankenexperiment. 

ALEX FILIPPENKO: He thought in his head through pictures. 

MICHAEL TURNER: He thought in his head. He did experiments in his head. And often, it’s the other way around, where startling discoveries and the acceleration of the universe, even though we theorists like to think we anticipated it, that could be the loose thread in general relativity, the thing that doesn’t quite fit that as we understand it better, points us to the next theory of gravity. 

IRA FLATOW: So we need somebody doing a little thoughtful experiment again. 

MICHAEL TURNER: Well, we never know if it’s going to be a theoretical idea. So string theory purely comes from theory. That’s the pure thought again. And on the other hand, every once in a while, people like Alex make discoveries that the theories should pay attention to that really gives us the clue as how to go forward. 

IRA FLATOW: Years ago when I used to hang out at the Institute for Advanced Study and Einstein’s students were still there– they’re all gone now– I said, how did Einstein think? And he’d say, you’d ask him a question, and he’d say, I’m going to take a walk and I give a little think. 


And that’s how he would do it. Gentlemen, we’ve run out of time. I want to thank you both for taking time to be with us. The clock is– well, we know there’s no time anymore, so that’s gone. 

Michael Turner is director of the Kavli Institute for Cosmological Physics, University of Chicago. Alex Filippenko is professor of astronomy at the University of California Berkeley. Thank you, gentlemen, and happy anniversary to both of you. 

ALEX FILIPPENKO: Thank you, Ira. 



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