Ep. 369: The Fizeau Experiment

Light is tricky stuff, and it took scientists hundreds of years to puzzle out what this stuff is. But they poked and prodded at it with many clever experiments to try to measure its speed, motion and interaction with the rest of the Universe. For example, the Fizeau Experiment, which ran light through moving water to see if that caused a difference.

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This episode is sponsored by: Swinburne Astronomy Online, 8th Light

Show Notes

Fizeau Experiment explained by Adam Savage in a TED talk
Speed of light through various media
Experimental setup of Fizeau Experiment
Online archives of original Fizeau papers here and here


Transcription services provided by: GMR Transcription

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 for more information.

Fraser Cain: Astronomy Cast episode 369, The Fizeau Experiment. Welcome to Astronomy Cast, your 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, Pam. How are you doing?

Pamela Gay: I’m doing well. How are you doing, Fraser?

Fraser Cain: Doing great. We’ve got a couple of announcements to make today, so why don’t you go first.

Pamela Gay: I think the biggest one is we have coming on April 25/26, the 36-hour CosmoQuest hangoutathon. So far, the only reason we’re still going in the face of NASA cuts is because of awesome people out there donating to keep our programs going. We are in the process of writing a giant NASA grant, and if we get it, we will have the money in September. If we don’t get it, I will personally cry a lot because this is a once in every five year opportunity. But we have to still be going in September, and we need your help to do it.

This year, unlike past years, we’re going to ask people to show up and not just give money but also donate their time to clicking on pictures of worlds and helping us map them out because if we’re not producing science, we might as well not exist. So, come, do science. Come hang out. Watch me lose my mind over 36 straight hours –

Fraser Cain: Always entertaining.

Pamela Gay: Fraser will be joining me, and let’s map other worlds and raise money to keep the science going in the face of politically-driven budget cuts.

Fraser Cain: All right. The second thing, I just – just to let you know, if you didn’t know, we record Astronomy Cast as a live Google+ hangout. We take an hour to do the whole show. The first half an hour, we record the show that you’re listening to right now, and then we stick around for another half hour and answer questions live from the audience about space and astronomy. In addition, I will be asking for you the questions that we get by email. So, if you have a question about space and astronomy, go ahead and email us, but you won’t hear the answer unless you come and watch the full length video. Sorry.

We’re gonna want to experiment a bit and maybe make some of these other, the audio, available in other methods. We’ll probably either release full audio in a different feed or release the question show separate in the feed. We’ll figure this out, but let us know how you consume Astronomy Cast so that we can make this stuff available to you.

All right. Let’s get rolling.

Female Speaker 1: This episode of Astronomy Cast is brought to you by 8th Light, Inc. 8th Light is an agile software development company. They craft beautiful applications that are durable and reliable. 8th Light provides disciplined software leadership on demand and shares its expertise to make your project better. For more information, visit them online at Just remember, that’s www. the digit 8 Drop them a note. 8th Light. Software is their craft.

Fraser Cain: Light is tricky stuff, and it took scientists hundreds of years to puzzle out what it is. But they poked and prodded at it with many clever experiments trying to –

Pamela Gay: They didn’t poke it.

Fraser Cain: They poked it and prodded it with many clever experiments to try and measure its speed, motion, and interaction with the rest of the universe. For example, the Fizeau experiment, which ran light through moving water to see if that caused a difference. All right, Pamela. Set the scene for us. When did this experiment happen?

Pamela Gay: This was in 1851. This fell just a couple of years after Fizeau had successfully become the first person to accurately measure the speed of light and was part of the process of trying to figure out how is it that light is affected, altered, slowed, as it goes from one medium to another. In particular, how does the effect of that medium moving and our own motion through the, at the time, supposed ether affect everything as well.

Fraser Cain: Fizeau had fairly accurately determined the speed of light. I guess that was a shock because people weren’t even sure that light had a speed, right? That’s kind of a weird thing to think about. But I was thinking that sound had a speed, and so…

Pamela Gay: And we had some early kind of bad estimates of the speed of light that were based on things as simple as using orbital mechanics equations. Folks had been able to figure out when you should be able to see various moons emerging from behind Jupiter. It was found that if you were nearest to Jupiter on one side of the sun, the amount of time that it would take to see Io emerge from the far side of Jupiter was a different amount of time than if you were on the other side of the sun, further away. This difference in when you finally got to see the information arrive at Earth gave a first initial understanding of what order of magnitude the speed of light might be. But that’s really all we had at that point, was a “Wow! That’s a big number. You can’t do this reasonably.” But then Fizeau came along.

Fraser Cain: That’s a pretty elegant experiment, to think about it. It’s the kind of thing that as soon as you had a telescope, you could probably start working – you could probably come up with that experiment and do your timings once you knew the positions of the moons. So, Fizeau did his experiment. What was his experiment for actually measure the speed of light?

Pamela Gay: He took a cog wheel that had 720 very precisely made notches up and slots down that were the exact same size going all the way around it, and he set up an optical experiment that was actually eight kilometers across. Outside of Paris, on two different hills, he set up systems where on one side he took light, focused it through a lens so that it came down to a point. That point where it came into focus was just as it was sliding through where the cogs would be. Then it came through to another lens that collimated it to send a beam of light across Paris. Then, eight kilometers away, that beam of light got intercepted, focused back down onto a mirror, reflected off of that mirror, and started its way back across the city.

Now you’re looking at a significant difference. But they knew what that distance was. The neat thing was, if you’re looking at the light and you start out with the cog not moving, the light will clearly go through, reflect, come back, you see light. If you start the cog spinning, you will see light, not light, light, not light, as the cogs block out the light that’s coming towards you. As you spin it faster, you eventually reach a point where the light takes off, comes back, and instead of coming back through that same slot in the cog, instead hits the tab.

If you know how fast the cog is turning, then you know how wide – and you know how wide each of the slots are, you can calculate the amount of time it takes for that last bit of light to leave and come back, and this gives you a, within 2 percent, measurement of the speed of light.

Fraser Cain: That is really cool. I can imagine that it’s all in the timing, right? That if the slots have moved enough, that the light, instead of being able to come back through that slot, now bonks into the block, and then you don’t get the light. I guess you spin up the wheel to the point that you no longer see that light, and then, boom. You’ve got – you’ve measured your speed of light. It’s such an amazing experiment and they did that. It’s so brilliant.

Pamela Gay: It’s so simple at a certain point. Not spinning, constant light. Slowly spinning, light, not light, light, not light. Spinning fast enough, the light never gets back.

Fraser Cain: If you had a green laser –

Pamela Gay: You can replicate this.

Fraser Cain: You can replicate this experiment –

Pamela Gay: And with awesome – they did this in 1849! No green lasers were harmed.

Fraser Cain: How did they do this without a laser? How did they make a light that could be focused to go eight – was it eight miles, right?

Pamela Gay: It was eight kilometers, fiveish miles.

Fraser Cain: Each way.

Pamela Gay: Yeah. It was all about using a bright enough light collimated so that it, just like a laser is collimated, so you can do that with lenses. They took regular old light, collimated it with hand-ground lenses. No machines were harmed in the making of this experiment. Everything was mechanical. That’s the amazingness of this. This was in the days of hand-wound clocks, hand-wound gears, springs, levers. They weren’t hand-winding the gear that they were using for this experiment. Clearly, that would have been super imprecise. But it was still kind of – like I said, no machines were harmed in the making of this experiment.

Fraser Cain: As you said, the number that they got was within 2 percent of our modern understanding of the speed of light.

Pamela Gay: And if you give this experiment to a group of students today, they’re gonna be like, “There’s too much light pollution. This is too hard. How do we measure the distances?” The thing is, he didn’t have as much light pollution but he sure had a lot of coal dust. He figured this out. He did it accurately across a major metropolis.

Fraser Cain: This is how, I guess, the first way, back to how we know what we know. Wanna measure the speed of light? Measure the time it takes for Jupiter’s moons to appear behind the planet. Boom. You’ve got an experiment you can do. The second one was developed by Fizeau, but he continued on with his experiments, right? This is – now, it’s a matter of what affect is it gonna have moving through a medium? What was the thinking at the time about what would – what might happen with light as it moved through these various mediums?

Pamela Gay: It was messy. We knew, going back to Newton and even earlier, when people started making glasses, that when light hits different medium, its speed changes. This is what causes the bending that we see at a certain level. As early as 1807, Thomas Young started talking about what – he coined the term ‘the refractive index’ of a medium. Earlier folks like Newton had used ratios and it was much more cumbersome to deal with. He’s like, “No. We’re gonna talk about this in terms of refractive index, and this compares everything to vacuum.” What is the amount that the speed of light gets changed when an object enters a medium?

If you have your normal speed of light, you take the phase velocity and multiply it by this index of refraction, and that new number is equal to the speed of light.

Fraser Cain: Okay. Back to my question, what were they expecting was gonna happen? What did they not understand about light?

Pamela Gay: A given medium – if you have water that’s not moving, you know that when you send the light beam into that non-moving water, the speed of light is going to slow as the light enters that water. You can actually measure this through different interference experiments. Send a beam of light through water. Send a beam of light through vacuum. Recombine them. Look at the interference fringes. This tells you the difference in the distance the two different beams traveled, which is another way of saying the different rate at which they traveled.

What happens if that medium is moving, though? The idea was, let’s say, that you’re trying to walk forward in an airport and for whatever reason you step on the moving walkway that’s going in the wrong direction. You’re essentially going to double, triple, the distance you have to walk because you’re going against the movement of the moving walkway. Your velocity relative to people not on the moving walkway is the rate at which you’re walking relative to the walkway plus the walkway’s motion. So far are you with me?

Fraser Cain: Yeah. I’m imagining another model of that, which is imagine you’re hearing a sound from – while you’re in the middle of a hurricane.

Pamela Gay: Ignore sound.


Fraser Cain: If you’re hearing sound and you’re in a hurricane, and the wind is blowing towards you, is that gonna have any impact on the wavelengths of the sound?

Pamela Gay: We’re not talking about wavelengths right now.

Fraser Cain: I understand.

Pamela Gay: We’re only talking about velocities.

Fraser Cain: Yes.

Pamela Gay: They were doing all of this with white light. They weren’t paying attention to the Doppler shifting, either. We’re simply looking at the velocity, not the change in color right now.

Fraser Cain: Right. I guess the point, though, is that if the medium is moving, if the medium is moving towards you, then in theory the velocity of the light should speed up.

Pamela Gay: Yes.

Fraser Cain: And if the medium is moving away from you, then the velocity – your – you should see the velocity slow down. If the medium is moving sideways, then you should see no real change in the velocity.

Pamela Gay: Exactly. Your moving walkways in the airport –

Fraser Cain: That’s a perfectly rational thing to expect.

Pamela Gay: Yeah. They went and they did the experiment. You have Frize, and this is what’s called either The (capital T), The Frize experiment, and he did 100s of experiments, so that’s kind of unfair to the poor guy. He did a lot of work.

Fraser Cain: Frizeau?

Pamela Gay: Yes. Again, can’t pronounce his name. Or it’s called his water experiment. That’s how Einstein often referred to it. What he did was he took a beam of light and a beam splitter, so he took the one beam, split it into two beams. One of those two beams he sent through a tube of water where the water was rushing in the direction the light was moving, reflected it off the mirror, off of a mirror on the other side, rather, and it came back through another tube that had the water rushing toward the observer. Now, with another beam of light, he went the exact opposite path. The opposite path, you’re constantly going the opposite direction of the water.

This was an experiment that had to be done extraordinarily quickly because he essentially had a water reserve on both sides, let gravity move the water through the system, and once he was out of water, he was out of water. In order to keep constant velocity of the water, you have to have a very shallow, very large surface area container that you’re draining out of so that you don’t have to deal with the height of the water changing the velocity it’s coming out very much. It’s basically a pain to do this. Today we have things like water pumps that he just didn’t have. He had gravity.

Did the experiment. Did it very quickly. What you would expect is you have light going with the current and against the current through these two different paths. It gets recombined. You look at the interference fringes. You look for the difference in how long the light was traveling. What he found was the rate of the water absolutely had nothing to do with the experiment.

Fraser Cain: What?

Pamela Gay: Yeah!

Fraser Cain: This is unexpected. This is not what you were expecting. We need to know what the number is.

Pamela Gay: What’s worse is there was a change. It just had nothing to do with the rate of the water.

Fraser Cain: So, he actually did detect a minor dragging effect by the water, right?

Pamela Gay: Yeah. It’s unfair to say that it had nothing to do with the rate of the water. It just wasn’t a linearly proportional to the rate of the water. If the water’s not moving, the measured velocity is equal to the speed of light divided by the index of refraction. If you work through the maths of what he was expecting, he was expecting it to be speed of light divided by index of refraction plus the velocity of the water. It was not plus the velocity of the water. It was plus the velocity of water times this little tiny, tiny number that was 1 minus 1 over the index of refraction squared. Really tiny, tiny effect that was annoyingly directly related to a physical quantity.

This is one of those times where you get a result and you’re like, “What the hell? I got a result. It has nothing to do with what I was expecting. What equation actually fits my data?” This actually had some backing in it. There had been other experiments that found that things were proportional to this 1 minus 1 over the index of refraction squared. This was considered to be an incomplete linking of what was going on to the ether.

You’re in this situation kind of like trying to understand the motion of the planets if you’re Kepler. You know they’re moving. You’re not sure why they’re moving. You have ellipses. Ellipses work. You’re not sure why ellipses work, but they work, so you have three laws named after you.

In this case, we have what has gone on, because of the interference work that was done to be called the Fresnel drag coefficient, which is the 1 minus 1 over index of refraction squared. It is exactly what our fair French-named fellow found in his water forcing experiment.

Fraser Cain: So, explain this, Einstein.

Pamela Gay: Sure. He did. Up through 1904, people kept doing experiments, going “What the – insert expletives in appropriate foreign language –.” Kept doing this over, replicating with this with different media, increasing the accuracy. Michaelson-Morley got involved. They had systems that were able to pump the water for longer durations and apparatus that were much less likely to flex. Increased accuracy was found. Increased – we know this is what’s happening was found and increased confusion as to why. We had, in 1895, Hendrik Lorentz, who is really the person doing the moral equivalent of Kepler in this particular area of science, figured out how to explain all of this using his contraction direction of motion equations. Here we’re looking at Lorentz contraction, accurately explains what’s going on.

Fraser Cain: You just glossed over Lorentz contractions. Can you give a quick explanation of what that is? The fact that things change in shape the faster they’re moving.

Pamela Gay: They don’t change in shape as much as –

Fraser Cain: From your perspective.

Pamela Gay: – they get distorted.

Fraser Cain: They get distorted.

Pamela Gay: Along the direction of motion, things are found to contract. This means that a very fast running pole vaulter can – will shrink the length of his pole if he runs fast enough, which might make it hard to get over the jump, but makes it easier to fit into a barn if that’s what you’re trying to do which is what you’re always trying to do in your physics homework sets.

Fraser Cain: How fast must the pole vaulter be running to be able to fit the pole vault through the barn?

Pamela Gay: Usually it’s some fraction of the speed of light depending on the size of the barn.

Fraser Cain: Right. But a spaceship – let’s just go back to the happy spaceship that we all understand. As the spaceship is moving towards you at the speed of light or closing in on the speed of light its length contracts from your perspective.

Pamela Gay: Yes. Only along the distance of travel. This means that if you want to be very skinny, stand very tall and upright perpendicular to the direction of motion. If you want to be very short, lie down on your side and your girth won’t change, but you will suddenly be a shorter human.

Fraser Cain: Right. Okay. This is not – you as the spaceship traveling –

Pamela Gay: Have no awareness.

Fraser Cain: No, but you see the Earth, for example, changing in its shape, right? Because it’s all relative.

Pamela Gay: It’s all relative. It’s just messy.

Fraser Cain: Then, of course, you increase in mass, and can you turn into a black hole? We don’t know. Anyway –

Pamela Gay: You can’t. I got – I finally got the answer to that, but that’s a different show. We’ll come back to that as a different show.

Fraser Cain: Awesome. The point being that Fresnel thought that it was the interfere – Fresnel thought it was the movement through the ether that was explaining this, and Lorentz came along and said, no, it turns out it’s this –

Pamela Gay: No, Lorentz didn’t get rid of the ether. He just added an extra term.

Fraser Cain: Right, but he was starting to set the groundwork for what Einstein would then later be able to use.

Pamela Gay: We’re at the point where people are like, “Okay. If there’s ether, we’re not really attached to it all that well. It’s not something that’s creating a lot of dragging. Perhaps it builds up with larger amounts inside the prism than outside of the prism.” All sorts of things were factored in. Hendrik Lorentz started trying to explain what was being seen, and saying that instead of being attached to the ether, perhaps what we’re seeing is contraction due to the direction of motion. He didn’t get rid of the ether; he simply found new mathematical explanations for what was going on that didn’t require the ether.

All of this sort of set up the stage for Einstein to come along and, working completely autonomous of Lorentz, this is the awesome thing. Einstein didn’t actually know about the Lorentz contraction, according to some of the histories. He saw things like the experiment we’re discussing and went, “Oh! This can be described by looking at how time is seen to actually change such that the speed of light is constant for everyone.” This change in the perceived speed of light is actually causing this contraction that, of course, was the exact same equation Lorentz had already come up with in 189 – in 1895.

In 1905, when Einstein published, suddenly we had a new context that was explaining from first principles, not from we have this experiment and these numbers seem to line up with this equation, but rather from first principles, what was going on.

Fraser Cain: That’s one of the big – this was the thing that Einstein was looking to figure out. He saw the experiment, saw how everyone was trying to wrap their heads around it, and he thought in a completely unique way to explain what was going on. But he clearly was standing on the shoulders of giants. It’s back to that statement, that Newton always makes, that made. It’s not like Einstein just sat there and thought about the universe and then came up with his relativity. He saw and knew the challenges that the other physicists and experimenters were struggling with, saw what the outcomes of their experiments were, but then made this creative leap to figure out what might be the underlying physics behind what we see.

Pamela Gay: And I love how this shows that scientists will completely throw out everything they thought they know. We went from this idea that waves require a medium to travel through therefore lets invent the ether, to, in 1810, Argot realized it actually doesn’t couple completely. We have this weird factor of the index of refraction, so let’s call it partial ether dragging. But that didn’t seem to work, so Lorentz tried to find ways to explain it with math based on what was going on, came up with Lorentz contraction.

You have our fair French fellow, whose name I can’t pronounce, who measured the speed of light and then looked to see how it was altered by moving mediums now that he had an accurate measurement, and found we’re back to the partial frame dragging. This still isn’t working correctly, to Einstein stepping forward and saying, “Wait. I’ve got some physics that will back all of this up. Now we have a more complete picture.” And it’s always more complete. It’s never complete. It’s just more complete.

Fraser Cain: I always think about these kind of experiments, and you see – if you read the history of this experiment, and you see all of the – you just gave the synopsis of it – all of the experiments and all of the theories and all the math that everyone had done, they flipped back and forth, and they’re narrowing down the options. We can now look back with history and go, “Well, obviously, it was relativity, duh.” But we’re living through a bunch of these right now, right? What is the nature of dark matter? What is the nature of dark energy? We don’t know. We can only detect it. As they ramp up experiments in the large hadron collider and discount various theories and give further evidence to other theories, we start to move toward a more thorough understanding of what is the underlying process here.

It is still this – it is discovery, at the very edge of understanding.

Pamela Gay: It’s really amazing how similar dark matter feels to the tracking down of the ether, just going in the opposite direction. You see the press releases coming out from various theorists along the lines of we’ve figured out the excess heat in the earth may be due to asymmetries, and how dark matter and anti-dark matter particles interact. We are starting to figure out discrepancies in solar radiation that we hadn’t been able to figure out before, as potentially explained by densities of dark matter inside the sun. Still not sure how I feel about that one.

But there’s all of these different things where now every place we have a discrepancy between theory and observation, people are like, “Wait. We have new tools.” Let’s take these new things that we know exist, we know dark matter is a thing, we see it through its gravity. Let’s take these new things that we know about, and try and sort out what role they play.

Fraser Cain: It’s got to be tough when you’re an experimenter and you’ve got a – maybe it’s not a completely no result, but you’ve got a tiny – a much different result than you were expecting, but you can’t explain it. Then you can really see how useful it is and how amazing it is to have these various researchers talking to each other. They were sending each other letters. They were visiting one another, bringing each other up to date on their experiments. Kind of going, “I got this thing. I don’t know what it means. I have been working on this – these kinds of experiments over here, and maybe what I’m detecting over here has something to do with what you’re doing over there.”

There’s so much of that cross-pollination. Now, the Internet has even made that faster.

Pamela Gay: That’s actually something that fascinates me because you have all of these amazingly thoughtful, long letters sent between scientists that would sometimes take weeks to get from one place to another. Now we’re dashing off half-hearted emails at every minor consideration. You have to wonder if we might all benefit from sitting down and being thoughtful with a two-week delay now and then. Science is done so differently today. It’s interesting to contemplate, and that’s social sciences for you, how we’ve changed, how science is thought about by changing how we communicate with one another.

Fraser Cain: Very cool. We’ve got another [inaudible] [00:30:09] next week, so stay tuned. Thanks, Pamela.

Pamela Gay: Thank you.

Male Speaker 2: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astro Sphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at You can email us at info@, Tweet us @AstronomyCast, like us on Facebook, or circle us on Google+. We record our show live on Google+ every Monday at 12 p.m. Pacific, 3 p.m. Eastern, or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at

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[End of Audio]

Duration: 32 minutes

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