Waves move through a medium, like water or air. So it seemed logical to search for a medium that light waves move through. The Michelson-Morley Experiment attempted to search for this medium, known as the “luminiferous aether”. The experiment gave a negative result, and helped set the stage for the theory of General Relativity.
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Female Speaker 1: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast episode 368, Searching for the Aether Wind, the Michelson-Morley Experiment. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos. We’ll help you understand not only what we know but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the director of CosmoQuest. Hey, Pamela. How are you doing?
Pamela Gay: I’m doing well. How are you doing, Fraser?
Fraser Cain: I am good, but I have some sad news to report, which I don’t know if you heard this. Tammy Plotner, who is one of the longest time writers for Universe Today, passed away last week –
Pamela Gay: Oh, no.
Fraser Cain: – from a long struggle with MS. Tammy joined Universe Today back in 2004. She was really the first freelance writer that I every hired. She did tons of – hundreds, thousands of articles over the course of her work with me, including a fantastic series on the Messier objects, all the constellations, and did – which she was trying to turn into books. Then she did a What’s Up 2006, what to see in the night sky every night for 2006, then again for 2007. Really infectious amateur astronomer. Taught me a ton of stuff. And it sucks that she passed away. She went offline for periods of time, and then we heard from her local astronomical society that she passed away. I just wanna thank Tammy for all the work that she did for both astronomy and for us personally at Universe Today. She’s really gonna be missed.
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Fraser Cain: Waves move through a medium like water or air. So, it seemed logical to search for a medium that light waves move through. The Michelson-Morley experiment attempted to search for this medium, known as the luminiferous aether. The experiment gave a negative result and helped set the stage for the theory of general relativity.
We are going to be starting a new series – I’m not sure how long this will last, where we are going to talk about pivotal experiments in space and astronomy that really helped change the way we look at the universe. This is one of the biggies, so set the stage. What – before this experiment was performed, how did astronomers think about the universe and the medium?
Pamela Gay: It wasn’t just astronomers, it was the entire field of natural philosophy, is what they called it back then. There was the understanding that sound requires a medium to move through, and as it goes from one medium to another, its behaviors change, its velocity changes. It was understood that light also passes through stuff, glass, things like that. We saw how it refracts through prisms, and this seemed like it was behaving a lot like sound. We also know that you can raise waves in liquids and semi-solids. I don’t know what you refer to as a string, but you can make strings and guitars vibrate. These are waves moving across the string.
As the wave goes from one medium to another, its properties change. It was figured we have waves moving through bodies which now we call sound waves. We have actual sound waves that our ears hear that move through the air. It was assumed that there had to be something that also allowed light waves to move through space and that was, in part, responsible for how light changes the same way sound does.
Fraser Cain: But how did astronomers, or how did scientists, physicists know that light acted like a wave? What kinds of behavior had they seen that told them that it was a wave?
Pamela Gay: The first thing that was noticed was things like interference. You can take light and you can, by putting it through slits, by causing it to interfere in different ways, you get these patterns that resemble the same sorts of patterns you get when water waves pass through little slits in a breaker wall. We saw this wave-like behavior with diffraction, interference, and this seemed to be a very wave-like thing to do.
It made sense, but light, unlike water waves, also showed particle behaviors with the way it reflects off of things. There was this wave/particle argument that passed through history, that I believe we’ve done episodes on way back in the first 100 episodes. This wave/particle duality led to a lot of confusion and a whole lot of “I’m just gonna ignore the fact that this is two things at once and only deal with one part at a time.” It’s that only dealing with one part of the problem at a time, only dealing with it as a wave or only dealing with it as a particle which is, admittedly, part of you can only observer it as one or the other, so pick one. But this thinking of it as only one or the other is part of what led to the making up of the concept of aether.
Fraser Cain: Okay. What did they think this aether was?
Pamela Gay: It largely goes back to, actually, in some ways, how Newton was thinking about it. You have water – when you look at the way it interferes with things, the waves are generally – the waves are bigger than the particles of water. When you look at how sound waves move through the air, the sound waves are bigger than the particles of air. Well, light waves are pretty darn tiny. As we got better and better understandings of light waves, it was supposed that the aether had to be something that was smaller than the light waves.
We knew it was gonna be hard to detect, and I have to admit that in studying for this episode, it actually made me go, “How the heck did we figure out there isn’t air in outer space?” This is one of those questions that never previously bothered me and now deeply bothers me. Because while we understood from mountain climbing, first off, and then from the gas laws that were developed in the 1800s, that the pressure of air decreases with altitude, I have to wonder why they didn’t just suppose that the ever so diffuse nature of air might allow waves to continue to move through space from stars and such. More things to look into, evidence that we don’t know everything, or I don’t know everything, or both, probably both.
In order to try and understand light, they knew air wasn’t enough. The particles of air are way too big. Therefore, there has to be, they thought, something else, something smaller than air: the luminous aether, that allowed the luminous light to travel through space.
Fraser Cain: So, some kind of particle, some kind of dark matter that they didn’t recognize –
Pamela Gay: No, no.
Fraser Cain: I know. I know. Completely different. Completely different. Some sort of dark energy –
Pamela Gay: No!
Fraser Cain: – that was – no.
Pamela Gay: Stop with the misconceptions.
Fraser Cain: All right. I’m just – because the people are gonna go there already, so I just wanna go there before them. So, there were some things – some substance, it was water, it was air, it was the aether, and it was everywhere you looked. Not ether like the drug, but aether with an ‘a.’ This is great. You’ve got this thing. Then, I guess, what was the hypothesis, what was the experimental – that they set up to say, “If this aether exists, we should detect it by doing X?”
Pamela Gay: If you start looking at the equations for interference, you realize that the distance that light travels is one of the most important features for setting up how the diffraction pattern will appear. One of the things that really matters is when you’re combining light, you have to have light that’s what’s called collimated. This means all the wavelengths neatly line up. When you’re shining that collimated light at a slit, you have light that goes into the slit with all of the light waves lined up, and then based on the different distances that light travels from those two slits to wherever you’re focusing the light, you end up with your highs and lows in the amount of light observed.
Once that was figured out, and that’s a fairly straightforward experiment to do. In fact, it’s one you can do at home with the laser pointer you use to torture your cat. Take a couple of razor blades and make a very thin slit. You can create a fringe effect on the wall, and if you get fancy with a piece of aluminum foil and putting some slits in the aluminum foil, you can create that double slit and shine the laser beam on it, and again, see, in this case, your beautiful interference pattern on the wall.
We knew that. Michelson got to thinking, “If we start thinking about how to measure the speed of light. If we start thinking about how things are going to get affected by this motion. If we start thinking about what we observe with sound waves, which are easier to deal with.” We know that sound waves, you have to take into consideration the velocity of the source, the velocity of the observer, and the medium that they’re in.
When you’re dealing with light, you just look at the differential effect. When you look at light, you can deal with frequency and wavelength using standard equations. It’s not that hard. But having to deal with that medium suddenly makes everything much more difficult with sound, and they went down the much more difficult track originally, and came up with this awesome experiment.
This awesome experiment was if you take a partially sulfured mirror, one that reflects some of the light that hits it, but lets some of it transfer through it; essentially a one-way mirror like you have in police stations on television –
Fraser Cain: Like a beam-splitter glass.
Pamela Gay: Beam-splitter glass. This partially reflecting, partially transmitting glass will take the beam of collimated light that you send at it, send some of it to one mirror, send some of it to another mirror that is at an angle away. If you put these two things at right angles with that partially reflective, partially transmitting mirror at a 45 degree angle – basically, take a box. Put a mirror along the diagonal of the box. Put a light source on one side. Put a detector on the adjoining side and make the other two sides mirrors. This is a situation that allows you to start combining light that if there is any difference in the medium the physical identicalness of the distance the light travels going to both of those two reflecting mirrors will not be the only thing that matters.
That speed of the medium, in this case the aether wind, the lumiferious aether wind, will cause a path length difference.
Fraser Cain: So, let me see if I understand this correctly. They took the light. They beam-split the light into two separate, possibly equalish beams of light, but they came –
Pamela Gay: Physically equal.
Fraser Cain: – from the source, right?
Pamela Gay: Yes.
Fraser Cain: Then, you can then play with the two halves of the light which started out with the same source to do things like make them go down longer paths, make them go down – maybe make them go in different directions, one which is in the direction of the – and I’m putting some [inaudible] [00:13:57] here – the direction of the wind that you think it might be, and then the opposite direction of the wind, or perpendicular to the wind. But then you bring those light paths together and see if you get any change in the way that they interact with each other via the double-split experiment again. You’re looking for changes in interference patterns purely based on what you’ve done after you’ve split up your light. Is that right?
Pamela Gay: You made it a whole lot more complex. You were including the definition of a Michelson interferometer, which we use in astronomy to do imaging spectroscopy. What they did was a lot simpler. They literally had a big old stone slab, working out at Case Western Reserve University. They had two reflecting mirrors that were at right angles to one another, one of them in the direction of the Earth’s motion, one of them away from the direction of the Earth’s motion. They could rotate this entire thing, but the mirrors themselves were fixed on the stone slab. They had, in between this, at a 45 degree angle to both those mirrors, the transmitting reflecting mirror. The idea of having everything fixed was if everything was fixed, then the only difference in travel path would be if it was due to the aether.
One of the things they did, which when I read this made the parts of my brain that have done interference experiments go numb, was they were doing this with white light. Earlier I said, “Do this with your cat’s laser, or your dog’s laser,” whatever you use. I’ve heard you can actually get beta fish to chase laser pointers. That single-colored laser pointer you use, whether it be red or green or whatever color it is, that’s really easy to create an interference pattern with because it’s only one color of light.
With white light, you have to worry about the path length for all the colors being the same. You have to worry about there being nothing at all that varies across all the different wavelengths. This means that you have to start worrying about surface properties, a whole lot of other stuff.
Fraser Cain: Just the glass itself is gonna diffract and reflect the different colors of photons by tiny different amounts, right?
Pamela Gay: Mirrors are pretty good. They don’t actually do that, but you have to worry about is it truly a mirror where going through the mirror it’s hitting at an – it’s complicated, but chromatic aberration isn’t as – it isn’t the same sort of chromatic aberration that we’re used to dealing with with lenses that are bent and you have different path lengths through the lens. The path length through the glass, as long as the glass thickness is consistent, is gonna be the same. Anyways, it’s really stupid complex.
When they were doing this, they found that they could get that perfect alignment that comes from both paths being identical. They didn’t expect that. They didn’t want that. That meant that their result didn’t fit any of the theories, which is the most uncomfortable place you can be as a scientist. There’s really nothing worse than saying, “I just did an experiment that proved that all of these people who came before me were wrong.”
Fraser Cain: No hypothesis, right? They – no result. They ser – if everything had gone well, they should have figured out the direction the Earth was moving through the aether. They should have known – gotten a sense of what its particle size was, density. They should have uncovered – been able to verify all kinds of theoretical models for what this stuff was.
Pamela Gay: With their particular experiment, they were pretty much limited – particle size wasn’t something they were getting at back then. They were definitely going to start being able to say what is the rate of this aether that affects light that the Earth is moving through. How does it create a drag? What is its drag rate? What is the angle of it, its motion relative to the Earth? They were looking at all of these differential velocities.
Fraser Cain: You can get some deeper thoughts. Literally, what direction is the Earth moving through the universe?
Pamela Gay: Yes. When they did all of this, it came back with to one part in a lot, we don’t see a velocity, and can pretty much rule out the Earth’s motion as showing up in the data. There were things that they saw. This is something that I think we’re actually gonna go into in the next episode. The work that they did, they had to start taking into account the Earth’s velocity because things do contract along the direction of velocity. It wasn’t that they saw an absolute zero. It was that they saw something, but they saw something that didn’t match any of the prior aether theories.
It was Lorentz who came along and looked at the results, and started to figure out Lorentz contraction. This is what later went into the theory of relativity, where we talk about contraction along the direction of motion. This started to crop up in their results. It went to Lorentz –
Fraser Cain: Whoa! That’s –
Pamela Gay: Yes.
Fraser Cain: That’s crazy. I didn’t know that, that you were starting to see these relativistic effects of the Earth’s motion, and that showed up in the calculations that they were doing.
Pamela Gay: This is where, because they used white light, they were able to see very fine differences in path length that occurred between the two right angle path lengths that the light went down. Here’s where we start getting a test that would later, with better and bigger and finer versions of the Michelson-Morley experiment using what are now called Michelson interferometers, we’ve been able to start – while eliminating the fact that there might be aether – no, there is not aether! People who keep trying to blame dark energy on the aether – no, there is not an aether.
We’ve pretty much disproved aether, but while we’ve gone on to do so much more in terms of looking at Lorentz contractions and starting to find amazing new ways to do one wave length at a time, spectroscopy using interference filters and Michelson interferometers.
Fraser Cain: They were – they should have gotten the Nobel Prize. I don’t know if the Nobel Prize existed then. It should have just started up around that time.
Pamela Gay: It was 1887, so…
Fraser Cain: Yeah. So, they would have gotten their Nobel Prize. I don’t know if you get a Nobel Prize in null detection. Let’s talk about how this laid the ground work, then, for – you started to go into this, Lorentz contractions. They started to realize that there seemed to be contraction in the direction of motion that the Earth was moving in. How did that feed into Einstein?
Pamela Gay: I was probably going to get to that in a few more episodes, but we can give hints of the future here. First of all, this created the rather large problem of suddenly light didn’t have a medium to go through. Well, expletive! Light is a wave and a particle, and apparently doesn’t require a medium. What is this strange thing that we’re interacting with?
Their experiment, which they managed to get published, which is always hard to do when your results don’t match theory, their published results took our understanding of particle physics – not particle physics – rather, it took our understanding of the particle of light and the wave of light, and broke it, and forced us to rebuild it, not quite from scratch. We did have a lot of experimental data. But the theories that we used had to have new things placed into them. It made understanding of the Doppler effect of light way easier because suddenly it’s much simplified, how you deal with frequency and wavelength much less ugly.
Other than making Doppler equations when you’re looking at wavelengths instead of frequency much less ugly, it did create this whole “what do you do without a medium.” Lorentz got it, part of explaining the experiment. It would have to wait until Einstein came along and started trying to figure out the energy aspects, in many ways, of light that all of the answers would come out.
Fraser Cain: I’m assuming that this fancier and fancier versions of this experiment were then performed. People tried to get at it with more detail. We have a sort of similar situation, which was that people were searching for the Higgs boson, couldn’t find it with the scale of the particle accelerators that they had available, but it didn’t mean that the Higgs boson didn’t exist. It just meant that the particle accelerator wasn’t up to the task. Finally, when the large hadron collider was created, it had the – it was able to generate the kinds of energies required to turn up the Higgs boson according to the standard model.
It’s strange to me – how did the theory go? When they released their results, did everybody go, “Well, that’s it for the aether idea?”
Pamela Gay: No. Everyone tried to replicate the results to prove them wrong, as one does. That’s part of science. I totally get behind the idea of saying, “We’re not sure about this. Let’s replicate it.” There’s a lot of research out there that when you try and replicate it, you can’t. If you look at the amount of data on health studies and such that couldn’t be replicated, it’s really disturbing. But they had an experiment that could be replicated over and over and over again. People continue to replicate it, continuing to look for signs of the aether at smaller and smaller scales.
By 1955, they were using quartz clocks and cavity resonators, and were able to eliminate our motion through an aether down to the scale of three kilometers per second. This has, today, continuing to use – now they’re using Fabry-Perot resonators. They’re down to less than one part in ten to the minus 17th of the Earth’s motion. We can’t detect it.
Fraser Cain: That’s great, that this experiment continues to be done. It’s like every time someone comes up with a much better method of splitting light or calculating the wavelength of light or – with more precision, they run that experiment again at the next order of magnitude, just to be sure. I think it’s just great.
Pamela Gay: One of the reasons I came down so firmly when you were “Dark matter! Dark energy!” is because there are people using the word aether to try and explain dark energy. This is causing people to go, “Huh!” I wonder if just saying a word triggers ideas. There is this, “I wonder. I wonder.”
Fraser Cain: One of the terms that’s been used to explain what dark energy might be is this idea of quintessence, which sounds similar to aether when you roll these around in your brain. It’s, again, quintessence is both a nonsense theory from hundreds of years ago, but then a legit term that certain theoretical physicists are using to try and wrap their heads around dark energy. It’s a perfectly legitimate model for how dark energy works. The problem is you pull one of these terms up that already has problems from ancient times, you’re taking your – you gotta be careful.
Pamela Gay: This is definitely a very troubling situation where it’s so easy because of the flexibility of the English language to take words brandished because they sounded cool by scientists and make inappropriate connections. Dark energy has nothing to do with the electricity running through your walls. It is not going to save any energy crisis here on Earth. Dark energy in quiescence has nothing to do with the poetry of the past. These are modern ideas that we lack a new language to articulate. Once upon a time, black stars was a word; now we use brown dwarf and cold white dwarf, depending on the language. Language changes. Old ideas get lost. Unfortunately, language doesn’t necessarily eradicate the old ideas.
Fraser Cain: Did you know what you’re gonna talk about next week?
Pamela Gay: I think I’m gonna talk about the experiments around Lorentz coming up with his ideas. Poor Lorentz. Everyone talks about the Lorentz equations and forgets that his equations were the basis of relativity, and re-invented by Einstein in a lot of ways.
Fraser Cain: That sounds great.
Pamela Gay: Not entirely. They worked together.
Fraser Cain: There’s a bunch of these, so that all sounds great. Thanks a lot, Pamela.
Pamela Gay: My pleasure.
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Duration: 30 minutes