IRA FLATOW: This is Science Friday. I’m Ira Flatow. And for the rest of the hour, we’re going to be diving into the vaults of science history, because the hosts of our podcast “Undiscovered” are working on a new series all about science history. And if you’ve read any of my books, you know how much I love science history. Co-host Annie Minoff is here to tell us about it. Hey, Annie, let’s– 

ANNIE MINOFF: Hey, Ira! Yeah, so, like you, I and my co-host Elah Feder– we’re huge science history buffs. And recently, we started thinking about all the scientific theories and ideas that we used to think were true. Like, over the course of history, these ideas were accepted science, and then all of a sudden they kind of weren’t anymore. 

So spontaneous generation might be an example, phrenology– and those ideas, they’re kind of punchlines today, but we thought, what if we take them seriously? What if we ask, well, why did we think these things? What convinced us that they were true? And then how did we figure out that maybe we weren’t? So that’s what the series is all about. And today, I thought we’d kick off with one– a theory that’s one of my favorites. I think it is brilliant it was useful. It made a ton of sense. It just wasn’t true. 

IRA FLATOW: Details! Details! 

ANNIE MINOFF: Right. And that is the theory of the luminiferous aether. So to be our guide through the luminiferous aether, we have with us today, David Kaiser. He’s a professor of Physics and History of Science at MIT in Cambridge, Massachusetts David, thank you so much for joining us. 

DAVID KAISER: Oh, it’s a great pleasure. Thanks for having me. 

ANNIE MINOFF: So I think I want to ask you first about this word, “luminiferous?” What does that mean? 

DAVID KAISER: Yeah, it’s a great word, isn’t it? It really just means “light carrying.” So luminous, like “lumos” for our Harry Potter fans– it means light. And “ferous” is like a Ferris wheel. The fairy something. So it’s a light-carrying or light-bearing aether. That’s where the word came from. 

ANNIE MINOFF: And does that tell us about what this theory was supposed to do to explain? I mean, where does “the luminiferous aether” come from? 

DAVID KAISER: Yeah, it’s a very descriptive term. So the idea which goes back a little over 200 years– I mean how early in the 1800s– a number of naturalists, of physicists of many stripes, were trying to understand the nature of light. Isaac Newton, even before them, had had very specific ideas that light was a stream of particles, of corpuscles coursing through the air. And that account had some sort of cracks in it, that people were less and less satisfied by the early 1800s. 

And what ultimately replaced it was a wave theory of light, that light was a wave phenomenon, and that would explain things like interference, or diffraction, or many, many very particular phenomena that people could actually see, could produce with light waves. And so the new idea was that light was a wave, and that immediately raised the question. A wave of what? An ocean wave is a wave of water on the ocean. Sound waves are where are disturbances in the air traveling through the air. 

So if a light is a wave, the sort of unavoidable next question that these people began to face in the early 1800s was, it’s a wave of what? What medium is sort of burying or ferrying that wave? And they figured there must be some new substance, some, as yet unexpected material that must pervade all of the universe, fill every nook and cranny, and it must be the light-bearing or light-carrying substance– the luminiferous aether. 

ANNIE MINOFF: And how many people believe this? Like, was it everybody? Most people? 

DAVID KAISER: It was pretty much everybody. Everybody who thought hard about optics, about the behavior of light. It wasn’t just sort of one idea among many. It was, once the wave theory of light really took hold– and that was pretty quick in the early 1800s, then it really seemed unavoidable to people thinking about the behavior of light, that there must be some medium, some light-baring medium that could account for all the really quite amazing things that people were learning about optics. 

ANNIE MINOFF: So I think we have to talk about one guy in particular, and that is Albert Michelson, correct? So he was one of these people who was super invested in investigating aether. Who was he and where did he come from? He had a quite eventful early life, I understand. 

DAVID KAISER: He really did. I mean, it’s just an amazing story. Albert Michelson was born in a tiny little rural village in Northern Europe on the border of what would later become the border between Germany and Poland. And when he was only two years old, he and his family moved to Northern California in the mid 1850s. This was the height of the gold rush. This was like the boom towns of the upper California, Northern California. 

And so he moved there as a two-year-old. His father became a merchant. He set up a dry goods store in this very kind of minimal mining town. And he literally just tried to sort of scrape by. A few years later, the family moved to Nevada. So it was really this sort of displaced immigrant family that found themselves in the far Western United States and not any of the big cities, really in some up-and-coming but kind of border-type areas. 

IRA FLATOW: Mm-hm, and tell us about how they believe the aether existed, but it was invisible, right? How are you going to measure it? And that’s what he set up an experiment to do. 

DAVID KAISER: He did. He was a really brilliant experimenter. In fact, he made his name as someone who was especially good at coaxing these very subtle, very difficult-to-measure effects, especially around the behavior of light. So their best bet to learn about the aether was to study the stuff that the aether seemed to support, meaning light, to do very careful optical experiments. 

One of the first that Michelson did when he was actually still a student was to try to measure the speed of light to better accuracy than anyone had done before. He was building on an experimental design that others had thought of a few decades before, but he really went at it with real ingenuity. He improved it so much that he started to get the attention even of the experts in Europe. 

And that was a big deal. This was a young person in the United States at a time when physics in the US was still really not even on the kind of– on the map for the great leaders in Europe. But this one kid, basically, began kind of getting attention for very careful, very clever experiments around optics. 

ANNIE MINOFF: He’s like a measurement freak. Like, this is his thing, to like, how exact can I get it? 

DAVID KAISER: That’s right. In fact, his measurement of the speed of light was within a few thousandths of a percent of the present modern-day best value. He didn’t have lasers or fancy electronics, and yet he got so, so close to our present value. And even what impressed his contemporaries was the precision of that measurement. The error bars that he could report were so, so minimal because he was indeed so gifted with his optical experiments. 

ANNIE MINOFF: So explain, what is the experiment that Michelson comes up with to try to detect the aether? How does that work? 

DAVID KAISER: He got a fellowship in 1880. So he’s able to leave. He’d been studying at the Naval Academy, actually in Annapolis, and then he got a fellowship to study in some of these great centers in Europe to continue to learn about optics and the more modern theories about physics more generally. 

And while he was there, he was doing a lot of reading of the works by the great James Clerk Maxwell, who had, not too long before– only a decade or two earlier, had really pieced together the sort of great synthesis of electricity, magnetism, and optics. And it was really Maxwell’s work that convinced generations to come, the light was nothing other than waves of electric and magnetic fields, propagating, moving through this light-bearing aether. 

So Michelson was reading all that he could from people like Maxwell, and he began to realize, Michelson did, that if the aether is everywhere and if we’re on the Earth– the Earth is not sitting still, the Earth moves around the sun, The sun seems to have local motions with respect to the galaxy, we’re moving through the aether. And so Michelson began to wonder, could we measure our own motion? Could we measure the Earth’s motion through this all-pervasive, mysterious medium of the aether. 

And he reasoned sort of as follows. He said, if you’re standing outside on a still day, then you don’t feel any particular breeze on your face. But once you get on a bicycle and start pedaling really quickly, you’ll feel a wind on your face because you are moving through the medium. 

In that case, the meeting would be the air. And so he said the same thing must be happening as the Earth, you know, whizzes through this medium of the aether. And if so, could we try to measure its effects on how light would behave? 

IRA FLATOW: In other words, can we measure the aether wind on our face? 

DAVID KAISER: Exactly. Can we measure the impact of the aether as made manifest as made clear, because we’re moving through the aether. So basically, study optics here on earth really carefully with great precision, and we should be able to measure the effect of the aether because we’re moving through it. 

IRA FLATOW: And so that experiment was a tremendous success, right? 

DAVID KAISER: It was. And he first dreamed it up in 1881. He actually got some funding for it from Alexander Graham Bell. This guy was really on the rise. He first built a small kind of prototype, where the device was about one meter, roughly three feet on each side– so not huge, manageable, to see if he could get the ideas literally to fit together. 

And he conducted the test with this sort of small-scale device, did not find any particular evidence for our motion through the aether. But he figured that’s because it’s a small device and he can keep going. A few years later, he tried a much more ambitious version with a colleague named Edward Morley. So it became known as a Michelson-Morley experiment. They were now working in Cleveland at what’s now Case Western University. 

And so they built a device where the arms were 11 meters long. This really filled a room. Think of the ambition. They had to shield against any kind of vibrations. They set this thing in a huge vat of mercury, which I don’t recommend, for those trying this at home today. But they really wanted to tamp down any vibrations from the outside or anything like that. 

ANNIE MINOFF: And David, just to give people an idea, like, this instrument that they’re building, I mean we still use this kind of instrument today for stuff. 

DAVID KAISER: For all over the place, that’s right. The instrument has outlived its original motivation many, many times over. And it’s central to many areas of science and technology. 

ANNIE MINOFF: The LIGO Project used it to detect gravity waves. 

DAVID KAISER: It’s at the heart of LIGO. It’s used for all kinds of industrial calibrations. It’s used throughout many, many fields of science. It’s an amazing tool that now we sort of take for granted. But it really was a great, great advance when Michelson was sort of following this dream back in the 1800s. 

IRA FLATOW: So he built this giant version of the original experiment, gets it going, and that’s a huge success, too, right? 

DAVID KAISER: Well, it depends on how you measure success. 

[LAUGHTER] 

I should say Michelson continued to impress the real elite scientists in Europe. He was the first physicist based in the United States to win the Nobel Prize when those began to be offered around 1900. 

ANNIE MINOFF: For not finding something? 

DAVID KAISER: For doing highly precise optical experiments. And not finding something, that has now– very smart people could say, well, maybe there’s something about how the instrument behaves that’s more subtle than we thought. Maybe the thing we’re trying to measure has more subtle properties than we thought. So it spurred much, much more research. 

IRA FLATOW: But he didn’t find the aether is what you’re saying? 

DAVID KAISER: He did not. In fact, I should say, he lived till 1931– so that’s 50 years after he built his first device– and to his dying day, he considered himself something of a failure, may all our Nobel laureates be easier on themselves and the rest of us, too. So he won the great prestige in the profession, but was convinced, really, literally, decades later, that the aether must be there, and it was his darn fault for not finding it. 

IRA FLATOW: So what finally killed the idea of the luminiferous aether? The popular story goes that Einstein used this experiment as his launching pad and killed the idea. Is that how it happened? 

DAVID KAISER: The short answer is no. And so whether Einstein even knew about this experiment is still really pretty hotly debated among the experts. If he knew about it, it would have been most likely second or third hand, reading about other people’s accounts of it. He was certainly not kind of obsessing over it, although many of his colleagues in Europe were at the time. 

And so Einstein was coming at the question of how light should behave or how we should measure the effects of light when either the sender or the receiver are in motion with respect to each other. He was following a very different line of thinking than pretty much everyone else on the topic. So Einstein was not driven to relativity because of his experiment. And in fact, most people didn’t buy relativity right away. So the aether lived a lot longer, even after the introduction of special relativity, really for, at least, a solid decade to decade and a half. 

IRA FLATOW: This is Science Friday from WNYC Studios. I’m Ira Flatow here with Annie Minoff, co-host of our “Undiscovered” podcast, working on a new history of science idea, talking about the aether, first with David Kaiser, professor of physics and of history of science at MIT in Cambridge. 

ANNIE MINOFF: I mean so David, is this a tragic story for you? You have Michelson, poor guy, working his whole career to try to measure this stuff that he’s convinced is there, and he just thinks he’s a failure because he can’t design the instrument that’s going to be fine enough to detect it? That seems to me like a pretty sad story. 

DAVID KAISER: You know, I think, on the individual level, it does have some sadness. Now again, let’s be clear. He had a brilliant career. 

ANNIE MINOFF: I mean, he had a Nobel. Surely, that cushions the blow. 

DAVID KAISER: That’s right. He did OK. The local boy did good. But nonetheless, he had considered himself, scientifically, that he’d never really achieved what he set out to. There’s an element of real sadness to that. 

On the other hand, the instrument has outlived its original motivation manyfold. He was able to do other things, even in his own lifetime with that instrument that really did trigger enormous progress. He used this instrument to be the first person to measure the diameter of a distant star. That’s pretty amazing. 

He also was able to measure effects in atomic physics that really helped jump start quantum theory. I mean, there were many things that he could– even in his own lifetime, could point to with real pride, even though he died thinking there must be an aether, and he failed to find it. 

ANNIE MINOFF: So did that aether do us any good or would we have been better never to have conceived of this idea? 

DAVID KAISER: You know, I think it did as worlds of good. I mean, I always joke that our students here at MIT and many places can still buy t-shirts with Maxwell’s equations on them. I love those t-shirts. Maxwell derived the laws that we still use, the governing laws for electricity, and magnetism, and therefore all of optics and everything else, because he was trying to understand the physics of the aether. 

His colleague Lord Kelvin said in the 1880s, the luminiferous aether is the only substance we are confident of in dynamics, the only substance. One thing we are sure of is that reality and substantiality of luminiferous aether, it drove these people’s work, and we still use their equations. We use their work in many ways as a guide, really, to this day. 

IRA FLATOW: Hm, is there any modern-day equivalent of the aether? 

DAVID KAISER: Well, that’s a good question. You know, Einstein himself toyed somewhat tongue in cheek later in his career, wondering if his own later work in relativity had sort of reintroduced something like an aether. His work on the general theory of relativity, the warping space time, maybe it wasn’t a material substance like a bowl of jelly, but maybe there’s some other substance that we should think of. And then in modern days, we have to think about the Higgs Boson pervading all of space, giving rise to observable properties. I mean, I think there are many ideas we can see, with some analogies at least. 

IRA FLATOW: David Kaiser, professor of physics at the History of Science, the History of Science at MIT in Cambridge. Thank you for joining us. 

DAVID KAISER: Thank you. It’s great fun. 

IRA FLATOW: Annie Minoff, up co-host of “Undiscovered” podcast, who’s hard at work on a new series all about failed ideas of science history. Thank you, Annie, we look forward to this first one. 

ANNIE MINOFF: Oh, thank you.