This year we give thanks for an historically influential set of celestial bodies, the moons of Jupiter. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, the error bar, gauge symmetry, Landauer’s Principle, the Fourier Transform, Riemannian Geometry, the speed of light, and the Jarzynski equality.)
For a change of pace this year, I went to Twitter and asked for suggestions for what to give thanks for in this annual post. There were a number of good suggestions, but two stood out above the rest: @etandel suggested Noether’s Theorem, and @OscarDelDiablo suggested the moons of Jupiter. Noether’s Theorem, according to which symmetries imply conserved quantities, would be a great choice, but in order to actually explain it I should probably first explain the principle of least action. Maybe some other year.
And to be precise, I’m not going to bother to give thanks for all of Jupiter’s moons. 78 Jovian satellites have been discovered thus far, and most of them are just lucky pieces of space debris that wandered into Jupiter’s gravity well and never escaped. It’s the heavy hitters — the four Galilean satellites — that we’ll be concerned with here. They deserve our thanks, for at least three different reasons!
Reason One: Displacing Earth from the center of the Solar System
Galileo discovered the four largest moons of Jupiter — Io, Europa, Ganymede, and Callisto — back in 1610, and wrote about his findings in Sidereus Nuncius (The Starry Messenger). They were the first celestial bodies to be discovered using that new technological advance, the telescope. But more importantly for our present purposes, it was immediately obvious that these new objects were orbiting around Jupiter, not around the Earth.
All this was happening not long after Copernicus had published his heliocentric model of the Solar System in 1543, offering an alternative to the prevailing Ptolemaic geocentric model. Both models were pretty good at fitting the known observations of planetary motions, and both required an elaborate system of circular orbits and epicycles — the realization that planetary orbits should be thought of as ellipses didn’t come along until Kepler published Astronomia Nova in 1609. As everyone knows, the debate over whether the Earth or the Sun should be thought of as the center of the universe was a heated one, with the Roman Catholic Church prohibiting Copernicus’s book in 1616, and the Inquisition putting Galileo on trial in 1633.
Strictly speaking, the existence of moons orbiting Jupiter is equally compatible with a heliocentric or geocentric model. After all, there’s nothing wrong with thinking that the Earth is the center of the Solar System, but that other objects can have satellites. However, the discovery brought about an important psychological shift. Sure, you can put the Earth at the center and still allow for satellites around other planets. But a big part of the motivation for putting Earth at the center was that the Earth wasn’t “just another planet.” It was supposed to be the thing around which everything else moved. (Remember that we didn’t have Newtonian mechanics at the time; physics was still largely an Aristotelian story of natures and purposes, not a bunch of objects obeying mindless differential equations.)
The Galilean moons changed that. If other objects have satellites, then Earth isn’t that special. And if it’s not that special, why have it at the center of the universe? Galileo offered up other arguments against the prevailing picture, from the phases of Venus to mountains on the Moon, and of course once Kepler’s ellipses came along the whole thing made much more mathematical sense than Ptolemy’s epicycles. Thus began one of the great revolutions in our understanding of our place in the cosmos.
Reason Two: Measuring the speed of light
Time is what clocks measure. And a clock, when you come right down to it, is something that does the same thing over and over again in a predictable fashion with respect to other clocks. That sounds circular, but it’s a nontrivial fact about our universe that it is filled with clocks. And some of the best natural clocks are the motions of heavenly bodies. As soon as we knew about the moons of Jupiter, scientists realized that they had a new clock to play with: by accurately observing the positions of all four moons, you could work out what time it must be. Galileo himself proposed that such observations could be used by sailors to determine their longitude, a notoriously difficult problem.
Danish astronomer Ole Rømer noted a puzzle when trying to use eclipses of Io to measure time: despite the fact that the orbit should be an accurate clock, the actual timings seemed to change with the time of year. Being a careful observational scientist, he deduced that the period between eclipses was longer when the Earth was moving away from Jupiter, and shorter when the two planets were drawing closer together. An obvious explanation presented itself: the light wasn’t traveling instantaneously from Jupiter and Io to us here on Earth, but rather took some time. By figuring out exactly how the period between eclipses varied, we could then deduce what the speed of light must be.
Rømer’s answer was that light traveled at about 220,000 kilometers per second. That’s pretty good! The right answer is 299,792 km/sec, about 36% greater than Rømer’s value. For comparison purposes, when Edwin Hubble first calculated the Hubble constant, he derived a value of about 500 km/sec/Mpc, whereas now we know the right answer is about 70 km/sec/Mpc. Using astronomical observations to determine fundamental parameters of the universe isn’t easy, especially if you’re the first one to to it.
Reason Three: Looking for life
Here in the present day, Jupiter’s moons have not lost their fascination or importance. As we’ve been able to study them in greater detail, we’ve learned a lot about the history and nature of the Solar System more generally. And one of the most exciting prospects is that one or more of these moons might harbor life.
It used to be common to think about the possibilities for life outside Earth in terms of a “habitable zone,” the region around a star where temperatures allowed planets to have liquid water. (Many scientists think that liquid water is a necessity for life to exist — but maybe we’re just being parochial about that.) In our Solar System, Earth is smack-dab in the middle of the habitable zone, and Mars just sneaks in. Both Venus and Jupiter are outside, on opposite ends.
But there’s more than one way to have liquid water. It turns out that both Europa and Ganymede, as well as Saturn’s moons Titan and Enceladus, are plausible homes for large liquid oceans. Europa, in particular, is thought to possess a considerable volume of liquid water underneath an icy crust — approximately two or three times as much water as in all the oceans on Earth. The point is that solar radiation isn’t the only way to heat up water and keep it at liquid temperatures. On Europa, it’s likely that heat is generated by the tidal pull from Jupiter, which stretches and distorts the moon’s crust as it rotates.
Does that mean there could be life there? Maybe! Nobody really knows. Smart money says that we’re more likely to find life on a wet environment like Europa than a dry one like Mars. And we’re going to look — the Europa Clipper mission is scheduled for launch by 2025.
If you can’t wait for then, go back and watch the movie Europa Report. And while you do, give thanks to Galileo and his discovery of these fascinating celestial bodies.
And thanks to you Professor Carroll for always taking the time to teach us!
Found a typo:
“Using astronomical observations to determine fundamental parameters of the universe isn’t easy, especially if you’re the first one to do it.”
Bit weak this year, a discussion of discoveries from all the moons of the Solar System would have been more interesting. Noether’s theorem is a theorem in pure math, a good one, but you’d surely need to thank algebra, calculus, group theory etc before giving thanks to just one specialised theorem in pure math.
These days I give thanks for science in general. It has made the world (in its most general sense) more real, more fascinating and much more worthy of awe and inspiration than the old world of Gods and Spirits fighting each other for supremacy and using mankind as a tool. Unfortunately there are still too many people who need the Gods and Spirits to make their world real. So I also give thanks for scientists, like yourself, who devote precious time too keeping us thinking honestly about what we see and experience everyday.
Piffle! I am giving thanks for you! I just finished The Big Picture and it is about as brilliant a piece of science and philosophical writing as I have encountered in the last 60 years. I used to argue that Steven Pinker was the best science writer bar none. (After all, he once quoted Miss Piggy and Noam Chomsky on the same page.) Now, I will have to say that it is Dr. Pinker and you at the top of the list.
I have restarted reading The Big Picture because I am sure I missed some of the points made but by and large it is a tour de force. Well, done … and Thank You!
Thanks for an interesting article. It’s good to be reminded of how far we have come in our knowledge. When it comes to Galileo and the Inquisition, I believe he was not without fault. Implying that the current pope was a fool didn’t help, and he was treated fairly kindly by the standards of the day! House arrest is surely preferable to Giordano Bruno’s fate.
Evidently Roemer did not calculate a speed of light in km/sec, but only in AU/hour. No one at that time had a reliable estimate of the size of the earth’s orbit. But this is a minor point – the fact that light travels at a finite speed was about as important an ideological shift as the earth-to-sun-centered universe.
Thank you for a very insightful reminder not just of some major conceptual realizations by Galileo and Romer, but also an indicator of what we have yet to discover about Jupiter’s and other distant solar system moons. I well recall as a teenager the excitement of watching Io in transit using my modest backyard telescope, just one factor that convinced me to become a scientist. Today the possibility of life in the subsurface oceans of some of those moons has become a thrust in our eternal quest to answer the ultimate question of whether life is rare in the cosmos or ubiquitous.
Sean,
Fantastic post. I really, really urge you to read an incredible book by David Wootton called “The Invention of Science: A New History of the Scientific Revolution” that discusses cases like this and explains it in the context of a cluster of different concepts (mathematization of science, new technology, the rise of experiments, new linguistic concepts of “laws of physics” and evidence, the awareness that Aristotelian physics, along with an epistemic outlook that countenanced anything new being “discoverable,” was rapidly falling apart in the face of the Old World coming face to face with the existence of the New World, along with Tycho’s Nova and, as you mention, mountains and the general topography of the moon). I think you’d find it very interesting, and Wootton clearly does his homework with a huge number of endnotes and a tome of references.
The only minor qualm I had was when he mentioned, along with hating the excesses of those that would say science is 100% a social construct,that he also opposes the opposite philosophy of Scientific Realism (he doesn’t believe that scientific theories approach “truth” in any sort of fundamental way, instead adopting a more middle-of-the-road/pragmatic notion of convergence towards more usefulness/greater ability to exert our wills over the world around us and manipulate it).
What I find remarkable about the moons of Jupiter are their great variety within the same local environment. Whatever physical processes make planets and their moons must be complex and dynamic.
I think that the Scientific Revolution really started in the fourteenth century with Nicole Oresme. As noted here Nicole Oresme :
In his Livre du ciel et du monde Oresme discussed a range of evidence for and against the daily rotation of the Earth on its axis.[10] From astronomical considerations, he maintained that if the Earth were moving and not the celestial spheres, all the movements that we see in the heavens that are computed by the astronomers would appear exactly the same as if the spheres were rotating around the Earth. He rejected the physical argument that if the Earth were moving the air would be left behind causing a great wind from east to west. In his view the Earth, Water, and Air would all share the same motion.[11] As to the scriptural passage that speaks of the motion of the Sun, he concludes that “this passage conforms to the customary usage of popular speech” and is not to be taken literally.[12] He also noted that it would be more economical for the small Earth to rotate on its axis than the immense sphere of the stars.[13] Nonetheless, he concluded that none of these arguments were conclusive and “everyone maintains, and I think myself, that the heavens do move and not the Earth.”[14]
For me, science points to God, not away from him. In the words of Einstein: “The mathematical precision of the universe reveals the mathematical mind of God.” Certainly, science points away from superstition (astrology, etc., as well as biblical literalism), but towards intelligence and purpose over accident and chance.
I can’t leave alone the idea that Aristotlean physics involved natures and purposes and not mechanistic laws. This, for example, from Physics seems to me to be a pretty good first approximation of Newton’s second law:
“Now since wherever there is a movent, its motion always acts upon something, is always in something, and always extends to something (by ‘is always in something’ I mean that it occupies a time: and by ‘extends to something’ I mean that it involves the traversing of a certain amount of distance: for at any moment when a thing is causing motion, it also has caused motion, so that there must always be a certain amount of distance that has been traversed and a certain amount of time that has been occupied). then, A the movent have moved B a distance G in a time D, then in the same time the same force A will move 1/2B twice the distance G, and in 1/2D it will move 1/2B the whole distance for G: thus the rules of proportion will be observed.”
Elsewhere Aristotle has suggested that, even though he did not believe there was ever a void, that if there was a void then a moving object would continue to move indefinitely. And in A History of Animals he talks about motion requiring a pushing action and something to push against.
Carlo Rovelli has published a paper suggesting that Aristotle’s physics were in fact an excellent account of physics at a certain local level. As he says:
“The bad reputation of Aristotle’s physics is undeserved, and leads to widespread ignorance: think for a moment, do you really believe that bodies of different weight fall at the same speed? Why don’t you just try: take a coin and piece of paper and let them fall. Do they fall at the same speed? Aristotle never claimed that bodies fall at different speed “if we take away the air”. He was interested in the speed of real bodies falling in our real world, where air or water is present. It is curious to read everywhere “Why didn’t Aristotle do the actual experiment?”. I would retort: “Those writing this, why don’t they do the actual experiment?”. They would find Aristotle right.”
He is correct. In “On the Heavens” Aristotle makes it clear that the factors involved in falling speed are the force required to divide the medium the body is passing through (provided by the mass of the falling object), the resistance of the medium to the falling body and the shape the falling body presents to the medium.
If anyone tried that experiment they would find that Aristotle was correct.
Part of the problem is that Hawking and Mlodinow in “The Grand Design” repeated the myth that Aristotle claimed that falling objects experienced “jubilation”. I would like to know where Aristotle is supposed to have said this, or to have said anything like this.
This idea has probably arisen from a mistranslation of one of Aristotle’s Categories “paschein” as “experiencing emotion” rather than “being acted upon” as Aristotle intended.
I would be interested to see your take on a new paper claiming a negative mass fluid can replace both dark matter and dark energy: https://arxiv.org/abs/1712.07962
Thanks for the article. Thanks, also, for the past Thanksgiving articles, looking forward to a number of those. In this article, Reason 3 recalls a twist of an old popular song title: Looking for life in all the wrong places.
Wonderful post, as usual
Nice simple summary. The first graphic nicely represents the best visual result achievable with a good earthbound Newtonian of 12-18 inch primary. Mine’s a Zygo interferometer -tested and re-figured 12 inch.
I just took pictures of the doors of the original Cavendish labs in Cambridge, replete with Psalm 111v2, often said to have been done at the request of Maxwell, the lab’s conceptual architect. Like that great man, I thank God for the revelations of both Holy Writ and the natural world. We are the centre of the universe in terms of breadth of granted conscious experience and environmental provision, if not in terms of physical dynamic simplicity.
‘You may fly to the ends of the world and find no God but the Author of Salvation. You may search the Scriptures and not find a text to stop you in your explorations. …‘
Maxwell
‘The works of the LORD are great, sought out of all them that have pleasure therein’.
Psalm 111v2
Sean
I think you will be interested in this paper on quantum gravity https://www.hindawi.com/journals/ahep/2018/4537058/