Evolution in evolution

The story of how we came to understand evolution is a fascinating example of the real character of scientific revolutions.

Lee este artículo en español

The first edition of On the Origin of Species, published by Charles Darwin in 1859.
(Credit: Scott Thomas Photography.)

IT IS NATURAL to imagine scientific revolutions as events that unfold in the blink of an eye, like thunderbolts of blinding truth. We tend to depict the likes of Galileo, Newton, Darwin and Einstein as bold figures who utterly and single-handedly transformed our understanding of the world, to everyone’s shock and awe. It is rather hard to find, however, quite so many examples of such breathtaking shifts of paradigm developing nowadays. This is not, of course, because scientific research has in any way stalled; in fact, its progress is now faster and more impressive than ever before. The true reason that we cannot recall too many contemporary scientific revolutions is that these are not actually revolutions as such — sudden, dramatic transformations — but rather long processes which normally evolve and develop over decades. While popular narratives like to depict key scientific discoveries in an overly dramatic atmosphere of climax, enlightenment and triumph, the reality is that, because scientists are sceptical by nature, every great change of paradigm has taken many years to become accepted. All ideas in science, even the most transformative ones, are subjected to a considerably slow-paced process of academic scrutiny, and, if possible, experimental validation, until they gradually become part of the established scientific canon. The double-helical structure of DNA, for instance, despite being so highly praised today, was regarded as little more than a possibility for years after it was first proposed by Watson and Crick. Even Newton, the archetype of scientific genius and achievement, had to go through decades of bitter intellectual competition before his law of universal gravitation became widely recognised outside of England.

Among the many possible narratives of gradually unfolding scientific revolutions, one is arguably as fascinating as it is obscure: this is the story of how Charles Darwin’s famed theory of evolution by natural selection became the supreme dogma of biology (a story brilliantly recounted by Ernst Mayr in the 1980 book The Evolutionary Synthesis). Contrary to popular belief, this occurred not as a swift change of paradigm, but as a protracted process of fierce scholarly debate which would not come to an end until the mid-1940s, almost a century after Darwin first published his theory in 1859. During this time, the forking of biology into new fields of knowledge prompted an irreconcilable divide between classically trained naturalists and experimental biologists, leading to a pervasive chasm in evolutionary thought that would only be bridged with the forging of a new unified theory of evolution. This theory, which we know today as the modern evolutionary synthesis, states that the gradual evolution of species can be explained in terms of the accumulation of small genetic alterations, recombination (the shuffling of the genetic material as it passes from parents to offspring), and the action of natural selection on this genetic diversity. The key feature of the modern synthesis is that it explains how these low-level mechanisms give rise to higher-level evolutionary processes, such as speciation and macroevolution.

In his lifetime, Darwin saw his theory gain acceptance and regard among some naturalists, but he never witnessed its ultimate development into the unquestionable pillar of biology which it is today. In fact, it might be difficult for us to conceive the extreme opposition which the Darwinian theory of evolution faced throughout the late nineteenth century, and until as late as the 1930s. At the time, Darwinism was only one of a number of different theories which attempted to explain the processes whereby biological species originate. Some of these, known as essentialist theories, were based on the notion that species were uniform ‘pure lines’ composed of virtually identical individuals, and thus claimed that no significant natural variation existed within each species. In contrast, populationist theories interpreted species as populations composed of distinct, unique individuals, and therefore harbouring a considerable amount of biological variation. Furthermore, some theories accepted the existence of soft inheritance, characterised by the notion that the genetic material can be modified to some extent by the interaction of the organism with its environment; a particularly notorious example of this current was Lamarck’s theory of the inheritance of acquired characters, which posited that the physiological changes acquired by an individual during its lifetime are inherited by its descendants. Other theories recognised only hard inheritance, meaning that the hereditary material cannot be altered by the action of the environment. Today, we consider soft inheritance to be false, and the genetic material to be immutable by means of interaction with the environment (although recent discoveries of heritable epigenetic variation in some species may potentially pose a challenge to this idea). It might therefore appear surprising that nearly all of the early theories of evolution, including Darwinism, recognised some degree of soft inheritance. In particular, Darwin’s theory assumed a certain ‘plasticity’ of the genetic material, such that it could be modified to some extent through the use or disuse of biological traits. Neo-Darwinism, an elaboration of the theory developed by some of Darwin’s supporters, among them the naturalist Alfred Russel Wallace, later rejected this possibility for an inheritance of acquired characters, adopting a hard-inheritance standpoint.

Based on these and other principles, a wide array of evolutionary theories were developed between the 1860s and the 1940s, of which Darwinism was seldom among the favourites. The main factor which compelled authors to support one theory or another was their field of expertise, and the number of these was growing as never before. Over the second half of the nineteenth century, the broad science of biology, theretofore split into the disciplines of zoology and botany, rapidly differentiated into several new fields, including embryology, cytology and ecology. From the viewpoint of evolutionary theory, however, the most influential of the new disciplines was arguably genetics, the study of genes and heredity, which was born from the rediscovery of Gregor Mendel’s laws of genetic inheritance in 1900. From this year onwards, geneticists would develop an increasing understanding of the principles of mutation and inheritance; this fresh knowledge, however, rather than sparking advances in the study of evolution, would ignite a long and vicious conflict between the different biological disciplines.

From the outset, the founding fathers of genetics were firm opposers of Darwinism and natural selection. The very first geneticists, together with the palaeontologists, thought that the emergence of new species happened by means of discontinuous changes; in their view, an isolated and disruptive modification of the genetic material (which they termed a mutation) produced a radical physiological change in the organism, resulting in the instantaneous transformation of one species into another, without any intermediates. This theory, based on essentialist principles, was known as saltationism, because of its belief in speciation by ‘saltation’, or huge evolutionary leaps leading from one species to the next. Bizarre as it may sound today, this explanation fitted the initial observations of geneticists, as well as previous palaeontological evidence, remarkably well. In their experiments, geneticists relied on uniform ‘stocks’ of nearly identical individuals (normally fruit flies, which were relatively easy to breed and study), as a means of avoiding experimental interference. In their stocks of flies, the early geneticists observed that isolated genetic mutations led to drastic, heritable modifications of traits such as eye colour or wing shape. It seemed reasonable, then, to suppose that evolution proceeded in the same manner: mutations were infrequent and highly disruptive events, causing the instantaneous transformation of one species into another. Gradual evolution by means of natural selection acting on existing natural variation within a species appeared to be in complete contradiction with these early results, and some geneticists went as far as to declare that Darwinian evolution had been positively disproved by genetics. Notwithstanding these misconceptions, genetics made some significant contributions to evolutionary theory during this period, most notably the refutation of the existence of soft inheritance.

On the other hand, those biologists who had been trained as naturalists, such as zoologists and botanists, were used to deriving their conclusions directly from the study of natural populations, and insisted that all their observations supported Darwin’s theory of gradual evolution, rather than saltationism. The root of the disagreement, however, was certainly the lack of communication between both fields: naturalists and geneticists did not only defend different theories, but also had distinct approaches to science, pursued divergent biological interests, attended different meetings, published in different scientific journals, and even used distinct vocabulary (including incompatible definitions for essential concepts such as ‘species’ and ‘mutation’). In addition, geneticists tended to regard naturalists as speculative scholars who could never test their ideas in the laboratory, and therefore possessed no objectivity; naturalists, in turn, viewed geneticists as myopic experimentalists who had no real knowledge about natural populations, and were insensitive to the crucial difference between heredity and evolution. All this inevitably led to mounting misunderstanding and resentment, and perhaps more importantly, to an immense communication lag between both disciplines. An astonishing proof of this circumstance is given by the fact that, when a younger generation of geneticists — including names such as Hermann Muller, J.B.S. Haldane and Ronald Fisher — began to obtain, from the late 1910s onwards, new evidence against saltationism and in favour of neo-Darwinism and natural selection, this did not help to bridge the gap between geneticists and naturalists. Instead, because of the utter alienation brought about by the endless disagreements between both fields, communication was damaged to such an extent that naturalists would spend decades persevering to refute the already obsolete ideas of the earlier geneticists. It was mainly because of this abysmal academic segregation that the arrival of the modern evolutionary synthesis was deferred until the 1940s.

In this way, naturalists and geneticists progressed along isolated paths for the first three decades of the twentieth century, each dragging their own conceptual burdens: the former held wrong and obsolete views about the nature of genetic mutation and inheritance, while the latter were dominated by the belief that the evolution of species and higher taxonomic levels could be understood by simple extrapolation from knowledge about how single genes evolve in isolated, ideal populations. Up until the 1920s, when crucial experiments on artificial selection, together with the work of the first mathematical geneticists, contributed to establish a firm belief in natural selection, specialist textbooks still presented up to six theories of evolution as being potentially valid.

This bleak scientific panorama was completely transformed in the 1940s, thanks to the insight of one palaeontologist, George Gaylord Simpson, and two zoologists, Julian Huxley and Bernhard Rensch. Perhaps the only scientists of their generation who had amassed a detailed knowledge of all the latest advances in each of the relevant disciplines, they published three independent books in which they demonstrated how the findings of zoologists, palaeontologists, geneticists and others could be integrated in order to explain all of evolution, from the emergence of changes in individual genes to the origin of species, genera and higher levels, within a single consistent framework. In his book, Huxley christened this new theory with the name by which it is known today — the modern synthesis.

The forging of the modern evolutionary synthesis was not in itself a scientific revolution, but rather the completion of a shift of paradigm initiated by Darwin nearly a century earlier. Moreover, the synthesis did not imply the victory of one scientific tradition over another, but rather the fusion of two radically different conceptual frameworks — naturalism and experimentalism — into a new harmonious whole. For this fusion to arrive, it was first necessary to remove conceptual misunderstanding and communication barriers between the opposing camps, something that could only be achieved by those who, rather than focusing on narrow specialisation, were curious enough to learn about the advances made outside their own respective fields, and open-minded enough to appreciate commonality rather than disagreement. The real impact of the modern synthesis was the unification of evolutionary biology into a single field; after its arrival, the complete discord and hostility which had reigned over the three previous decades was replaced by widespread agreement. Bridges had been built which would remain solidly in place until the present day; although discussion is still ongoing regarding some aspects of the theory (such as the role of epigenetic inheritance and horizontal gene transfer), the basic framework of the synthesis has remained essentially untouched since it was first outlined in the 1940s.

The history of the modern synthesis, our current framework for studying evolution, is of value to scientists and historians alike. The long series of discoveries and conceptual advances that led from Darwin’s original theory to the arrival of a unified interpretation of evolution are a particularly informative illustration of phenomena which have manifested time and again throughout the history of science: resistance to new ideas, exceeding specialisation, terminological barriers, communication lags, sentiments of superiority and hostility between disciplines, and the critical importance of collaboration and mutual understanding for scientific advance. The story of the modern synthesis thus reflects the true method of scientific progress, which is of course harder, messier and more gradual than we like to imagine. It also constitutes a telling example of how exploring the history of scientific ideas provides us with a much deeper understanding than the mere study of their definitions; for while the latter pretend to be static and set in stone, the former conveys the truth that science is alive and restless, and that the search for knowledge is fundamentally arduous, incremental, collaborative, and eternal.

References
Mayr, E. (1980). ‘Some Thoughts on the History of the Evolutionary Synthesis’, in The Evolutionary Synthesis: Perspectives on the Unification of Biology (Harvard University Press).
Huxley, J. (1942). Evolution: The Modern Synthesis (Allen and Unwin).
Simpson, G. G. (1944). Tempo and Mode in Evolution (Columbia University Press).
Rensch, B. (1947). Neuere Probleme der Abstammungslehre (Enke).

Evolución en evolución

La historia de cómo llegamos a comprender la evolución es un fascinante ejemplo del verdadero carácter de las revoluciones científicas.

Read this article in English

La primera edición de El Origen de las Especies, publicado por Charles Darwin en 1859. (Imagen: Scott Thomas Photography.)

ES NATURAL IMAGINAR las revoluciones científicas como eventos que acontecen en un instante, cual relámpagos de razón deslumbradora. Concebimos a Galileo, Newton, Darwin, Einstein y tantos otros como figuras grandiosas y solitarias que transformaron por completo nuestro entendimiento del mundo, para conmoción y sorpresa de todos. Cuesta encontrar, sin embargo, ejemplos de revoluciones científicas de tal magnitud en la actualidad. Esto no se debe, por supuesto, a una menor intensidad de la empresa científica; de hecho, el progreso de esta última es hoy más rápido e impresionante que nunca antes. El verdadero motivo de que no podamos señalar demasiadas revoluciones científicas hoy en día es que éstas no son realmente revoluciones —transformaciones súbitas y dramáticas—, sino procesos muy prolongados, que normalmente requieren décadas de desarrollo. La narrativa popular tiende a representar descubrimientos científicos en un tono excesivamente dramático de culminación, iluminación y triunfo; en realidad, dado que los científicos son escépticos por naturaleza, cada gran cambio de paradigma ha necesitado muchos años para ser aceptado. En ciencia, cualquier idea, por muy transformativa, debe ser sujeta a un largo proceso de escrutinio académico y, de ser posible, confirmación experimental, hasta establecerse gradualmente como canon. La tan celebrada estructura de la doble hélice del ADN, tras ser inicialmente propuesta por Watson y Crick, fue considerada durante años como poco más que una mera posibilidad. Hasta Newton, arquetipo del genio científico, tuvo que soportar décadas de amarga competición intelectual antes de que su ley de la gravitación universal fuera finalmente aceptada fuera de Inglaterra.

Entre las muchas narrativas de revoluciones científicas, una es fascinante y desconocida por igual: la historia de cómo la famosa teoría de la evolución mediante selección natural de Charles Darwin se convirtió en el dogma supremo de la biología (esta historia fue brillantemente narrada por Ernst Mayr en el libro The Evolutionary Synthesis). Al contrario de lo que se cree, esto no implicó un cambio instantáneo de paradigma, sino más bien un prolongado proceso de feroz debate académico que no llegaría a su fin hasta mediados de la década de 1940, casi un siglo después de que Darwin publicara su teoría en 1859. Durante este periodo, la disgregación de la biología en nuevos campos de estudio propició el desarrollo de una brecha infranqueable entre los naturalistas clásicos y los biólogos experimentales, llevando a una segregación intelectual que solamente sería resuelta con la forja de una nueva teoría unificada de la evolución, conocida hoy como la síntesis moderna. En breve, esta teoría declara que la evolución gradual de las especies puede explicarse en términos de la acumulación de minúsculos cambios genéticos, recombinación (la reorganización del material genético entre progenitores y descendientes), y la acción de la selección natural sobre esta diversidad genética. La característica fundamental de la síntesis moderna es que explica cómo estos mecanismos genéticos dan lugar a procesos evolutivos de más alto nivel, tales como la especiación y la macroevolución.

En vida, Darwin vio cómo su teoría recibía la consideración y estima de algunos naturalistas, pero nunca presenció su desarrollo último como pilar incuestionable de la biología. De hecho, es difícil concebir hoy en día la intensidad de la oposición a la que el darwinismo tuvo que hacer frente desde finales del siglo XIX, y hasta tan tarde como 1930. En aquel entonces, el darwinismo era solamente una de muchas teorías que intentaban explicar el proceso por el que las especies biológicas se originan. Algunas de estas teorías evolutivas, conocidas como teorías esencialistas, se basaban en la noción de que las especies eran ‘líneas’ puras y uniformes de individuos prácticamente idénticos, declarando así la ausencia de variación natural dentro de cada especie. Por el contrario, las teorías populacionistas interpretaban las especies como poblaciones compuestas de organismos únicos y distintos, y por lo tanto poseedoras de una cantidad considerable de variación natural. Además, algunas teorías reconocían la existencia de herencia blanda, caracterizada por la modificación del material genético a través de la interacción de un organismo con su entorno; un ejemplo particularmente notorio de esta idea es la teoría lamarckiana de la herencia de caracteres adquiridos, la cual proponía que los cambios biológicos adquiridos por un individuo durante el curso de su vida son heredados por sus descendientes. Otras teorías reconocían solamente la herencia dura, la cual se distingue por la noción de que el material hereditario no puede ser alterado por acción del entorno. Hoy consideramos el concepto de herencia blanda como falso, y el material genético como inmutable mediante la interacción del individuo con su entorno (aunque descubrimientos recientes de variación epigenética heredable en ciertas especies podrían suponer un desafío a esta idea). Puede resultar sorprendente, por tanto, el hecho de que prácticamente todas las primeras teorías evolutivas, incluyendo la versión original del darwinismo, admitían un cierto grado de herencia blanda. En particular, el darwinismo asumía una cierta ‘plasticidad’ del material genético, el cual podría ser alterado hasta cierto punto mediante el uso o desuso de caracteres biológicos. No mucho más adelante, el neodarwinismo, una variación de la teoría de Darwin desarrollada por algunos de sus seguidores, entre ellos el naturalista Alfred Russel Wallace, rechazaría por completo la posibilidad de la herencia de caracteres adquiridos, adoptando una perspectiva de herencia dura.

En base a éstos y otros principios, una gran variedad de teorías evolutivas fueron desarrolladas entre los años 1860 y 1940, de las cuales el darwinismo era raramente la favorita. El principal factor que movía a cada autor a defender una u otra teoría era su campo de experiencia, y el número de tales campos se hallaba en una expansión sin precedentes. Durante la segunda mitad del siglo XIX, la ciencia de la biología, hasta entonces dividida en las disciplinas de zoología y botánica, experimentó una diferenciación en múltiples nuevos campos, incluyendo la embriología, la citología y la ecología. Desde el punto de vista del pensamiento evolutivo, sin embargo, la más influyente de estas nuevas disciplinas sería sin duda la genética —el estudio de los genes y la herencia—, la cual nació tras el redescubrimiento de las leyes de herencia genética de Gregor Mendel en 1900. A partir de este año, los genetistas acumularían un creciente entendimiento de los principios de mutación genética y herencia molecular; no obstante, este conocimiento, en lugar de motivar nuevos avances en el estudio de la evolución, daría pie a una larga y feroz confrontación entre las distintas disciplinas biológicas.

Desde un comienzo, los padres fundadores de la genética fueron firmes opositores del darwinismo y la idea de selección natural. Estos primeros genetistas, junto con los paleontólogos, defendían que el origen de nuevas especies tenía lugar por medio de cambios discontinuos, de modo que una modificación aislada y drástica del material genético (la cual denominaron una mutación), desencadenaba un cambio radical en la fisiología del organismo, resultando así en la transformación instantánea de una especie en otra, sin ninguna forma intermedia. Esta teoría, basada en principios esencialistas, era conocida como saltacionismo, debido a su defensa de la especiación por medio de grandes saltos evolutivos entre especies. Aunque pueda parecer absurda hoy en día, esta idea encajaba notablemente bien con las primeras observaciones de los genetistas, así como con anteriores descubrimientos paleontológicos. Con objeto de evitar cualquier forma de interferencia experimental, los genetistas hacían uso de ‘stocks’, o reservas de individuos casi idénticos (normalmente moscas de la fruta, por ser relativamente fáciles de criar y estudiar). En estos ‘stocks’ de moscas, los primeros genetistas descubrieron que mutaciones genéticas puntuales resultaban en modificaciones dramáticas de ciertos caracteres heredables, tales como el color de los ojos o la forma de las alas. Parecía razonable, por tanto, concluir que la evolución procedía del mismo modo, mediante mutaciones muy infrecuentes pero de gran efecto, las cuales ocasionaban la transformación instantánea de una especie en otra. La evolución gradual por medio de la acción de la selección natural sobre la variación genética existente en una especie parecía estar en contradicción total con estos resultados, llevando a algunos genetistas a afirmar que la evolución darwiniana había sido irrevocablemente refutada por la genética. A pesar de estos errores, la genética produjo algunas contribuciones significativas al pensamiento evolutivo durante este periodo, la más notable de las cuales fue quizá la refutación de la existencia de la herencia blanda.

Por otra parte, aquellos biólogos que habían recibido una formación de naturalistas, tales como los zoólogos y botánicos, estaban acostumbrados a obtener conclusiones directamente a partir del estudio de poblaciones naturales, e insistían en que todas sus observaciones apoyaban la teoría de evolución gradual de Darwin, y no el saltacionismo. La raíz de esta confusión, sin embargo, yacía sin duda en la falta de comunicación entre ambos campos: los naturalistas y los genetistas no sólo defendían distintas teorías, sino que tenían diferentes enfoques científicos, perseguían distintos intereses biológicos, asistían a diferentes reuniones y congresos, publicaban en diferentes revistas, e incluso empleaban distinto vocabulario (incluyendo el uso de acepciones incompatibles para términos tan esenciales como ‘especie’ y ‘mutación’). Es más, los genetistas tendían a considerar a los naturalistas como biólogos especulativos que eran incapaces de confirmar sus ideas en el laboratorio, y por tanto carecían de objetividad; a su vez, los naturalistas veían a los genetistas como miopes experimentalistas sin conocimiento real de las poblaciones naturales, e insensibles a la importante distinción entre herencia y evolución. Todo esto condujo a una interminable confusión y un creciente rencor entre las dos disciplinas, y, quizá más importante, a un tremendo vacío comunicativo. Una prueba extraordinaria de este fenómeno es el hecho de que, cuando una nueva generación de jóvenes genetistas —incluyendo nombres como Hermann Muller, J.B.S. Haldane y Ronald Fisher— comenzaron a obtener, desde finales de los años 1910 en adelante, nuevos resultados que refutaban la teoría saltacionista y apoyaban el neodarwinismo y la selección natural, esto no contribuyó a reparar la brecha entre genetistas y naturalistas. En su lugar, debido a la alienación creada por las perpetuas discrepancias entre ambos campos, la falta de comunicación era tan intensa que los naturalistas pasarían décadas esforzándose por seguir refutando las ideas ya obsoletas de los anteriores genetistas. Fue mayormente a causa de esta extrema segregación académica que la llegada de la síntesis moderna se atrasaría hasta la década de 1940.

De este modo, durante las tres primeras décadas del siglo XX, los naturalistas y los genetistas avanzaron por caminos aislados, cada cual cargando con sus propios lastres conceptuales: los primeros albergaban ideas erróneas acerca de la naturaleza de las mutaciones genéticas y la herencia; los últimos estaban dominados por la creencia de que la evolución de las especies y otros niveles taxonómicos podía entenderse mediante simple extrapolación del conocimiento acerca de cómo genes individuales evolucionan en poblaciones aisladas e ideales. Hasta la década de 1920, cuando cruciales experimentos de selección artificial, junto con el trabajo de los primeros genetistas matemáticos, contribuyeron a establecer la creencia en la selección natural, los textos especializados aún presentaban hasta seis teorías evolutivas diferentes como potencialmente correctas.

Este sombrío panorama sería transformado por completo a partir de 1940, gracias a la perspicacia de un paleontólogo, George Gaylord Simpson, y dos zoólogos, Julian Huxley y Bernhard Rensch. Siendo quizá los únicos científicos de su generación que habían adquirido un conocimiento detallado de los últimos avances en cada una de las disciplinas relevantes, Simpson, Huxley y Rensch publicaron independientemente tres libros, en los que demostraron cómo los hallazgos de los zoólogos, paleontólogos, genetistas y otros biólogos podían combinarse para explicar todos los niveles de la evolución, desde la aparición de alteraciones en genes individuales hasta el origen de las especies, géneros y niveles superiores, en un marco único y consistente. En su libro, Huxley bautizó esta nueva teoría con el nombre por el que es conocida hoy: la síntesis moderna.

La forja de la síntesis moderna no fue en sí misma una revolución científica, sino más bien la compleción de un cambio de paradigma iniciado por Darwin casi un siglo antes. Es más, la síntesis no implicó la victoria de una tradición científica sobre otra, sino la integración de dos marcos conceptuales radicalmente diferentes —naturalismo y experimentalismo— en un nuevo conjunto armonioso. Para hacer posible tal fusión, fue necesario, en primer lugar, eliminar confusiones conceptuales y barreras comunicativas entre los campos enfrentados, algo que sólo estaba al alcance de aquéllos que, en lugar de centrarse en la especialización académica, fueron lo suficientemente curiosos como para aprender acerca de los avances externos a sus respectivas disciplinas, y lo suficientemente abiertos de mente como para observar similitudes en lugar de diferencias. El verdadero impacto de la síntesis moderna fue la unificación de la biología evolutiva en un único campo; tras la llegada de la nueva teoría, la discordia y hostilidad total que habían reinado durante las tres décadas previas fueron reemplazadas por un acuerdo casi absoluto. Nuevos puentes habían sido alzados, los cuales permanecerían sólidamente en pie hasta el día de hoy; aunque aún existe discusión sobre ciertos aspectos de la teoría (tales como el papel de la herencia epigenética y la transferencia horizontal de genes), el marco fundamental de la síntesis ha permanecido esencialmente intacto desde que fuera trazado por primera vez en los años cuarenta.

La historia de la síntesis moderna, nuestro actual paradigma para el estudio de la evolución, posee valor para historiadores y científicos por igual. La larga serie de descubrimientos y avances conceptuales que condujo a una interpretación unificada de la evolución, a partir de la teoría original de Darwin, constituye una ilustración particularmente informativa de fenómenos que han tenido lugar una y otra vez a lo largo de la historia de la ciencia: resistencia a nuevas ideas, especialización excesiva, barreras terminológicas, brechas de comunicación, sentimientos de superioridad y hostilidad entre disciplinas, y la crucial importancia de la colaboración y el entendimiento para el avance científico. La historia de la síntesis moderna revela así el verdadero método del progreso científico, el cual es, por supuesto, más difícil, desordenado y gradual de lo que querríamos imaginar. Es también un ejemplo revelador de cómo la exploración de la historia de las ideas científicas ofrece una comprensión mucho más profunda que el mero estudio de sus definiciones; pues mientras que estas últimas pretenden ser estáticas y permanentes, la historia nos demuestra que la ciencia es algo vivo y en constante cambio, y que la búsqueda del conocimiento es fundamentalmente laboriosa, progresiva, colaborativa y eterna.

Referencias
Mayr, E. (1980). ‘Some Thoughts on the History of the Evolutionary Synthesis’, in The Evolutionary Synthesis: Perspectives on the Unification of Biology (Harvard University Press).
Huxley, J. (1942). Evolution: The Modern Synthesis (Allen and Unwin).
Simpson, G. G. (1944). Tempo and Mode in Evolution (Columbia University Press).
Rensch, B. (1947). Neuere Probleme der Abstammungslehre (Enke).

The story of a space odyssey

In its three decades of life, the Cassini–Huygens mission to Saturn transformed our view of the outer Solar System.

Lee este artículo en español

Natural-colour view of Saturn, taken by the Cassini spacecraft on January 2010.
(Credit: NASA/JPL-Caltech/Space Science Institute.)

ON 15 SEPTEMBER 2017, after thirteen years orbiting Saturn, a robotic spacecraft by the name of Cassini plunged into the atmosphere of the gas giant, where it quickly burned up. More than an hour later, a faint radio signal delivered the news of the space probe’s demise to Earth, thus marking the end of the Cassini–Huygens space mission. Over the previous two decades, this colossal scientific venture had completely revolutionised our understanding of one of the most remarkable regions of the Solar System — and the way we think about the possibility of extra-terrestrial life.

Despite Cassini’s destruction having been exhaustively planned, for many people at NASA, ESA and ASI — the American, European and Italian space agencies — the sudden radio silence that followed the probe’s destruction felt like the death of an old and dear friend. The long-lived space mission had occupied the best part of their careers; while each of its stages slowly unfolded billions of kilometres away, many of those involved in the project went on to start families, grow old and, in some cases, even die. One of those lucky enough to witness the entire life of Cassini–Huygens from up close was David Southwood, a distinguished planetary scientist who is also a former Director of Science at ESA, and a former President of the Royal Astronomical Society. In a recent talk hosted by the Cambridge University Astronomical Society, Southwood offered his gripping account of the story and legacy of Cassini–Huygens, and of the huge international effort that allowed the mission to take off and remain operative for so long, despite the considerable political and social changes that took place during its course. Most of what follows is based on the contents of Southwood’s talk.

Cassini and Huygens, the two robotic spacecraft that would carry out the grand mission, were launched into space in 1997, twenty years before Cassini’s death in 2017; the mission’s origins, however, date back as far as the early 1980s. This was a time when Europe was notably reluctant to invest in space exploration: after the breakneck space race between the United States and the Soviet Union, which culminated in the landing of Apollo 11 on the Moon in 1969, space had come to be regarded, as Southwood put it, ‘as either Terra Americana or Terra Sovietica: either American or Soviet territory’. Southwood not only witnessed the transition of space science from science-fiction into a tangible reality, but he played an active part in it. After his childhood in England, where he grew up devouring colourful science-fiction comics in the 1950s, he later moved to the United States in search of a better environment for space science, before returning to England in the early 1970s. It was in 1982 when Europe and the United States finally became unlikely partners in an ambitious mission proposal aimed at Saturn — a planet that had captured the public imagination ever since Galileo Galilei first trained his telescope at it in 1610. The ESA committed to the construction of Huygens, the space lander that was to descend on Saturn’s largest moon, Titan, and bore the name of its discoverer, Christiaan Huygens. NASA and ASI would build the larger Cassini probe, which would remain in orbit around Saturn to study the planet’s rings and moons, and was named after Giovanni Cassini, the discoverer of Saturn’s ring divisions and several of its moons.

Diagram of the Cassini spacecraft. The Huygens probe is hidden under the circular shield on the left side. (Credit: NASA Jet Propulsion Laboratory.)

Working at Imperial College, London, Southwood led the development of one of Cassini’s scientific instruments: its space magnetometer, a device designed to measure changes in magnetic fields. He also found himself playing an unexpected but critical part in defending the project against worrying shifts in the scientific priorities of NASA, most likely underlain by the altered political climate that followed the end of the Cold War. Safeguarding the mission from technical and political failure was of the foremost importance for those involved in it; with a height of about seven metres, a mass of over two tonnes, and a development cost of nearly one and a half billion dollars, Cassini was an ambitious spacecraft that supported the weight of hefty expectations.

After leaving Earth from Cape Canaveral in 1997, Cassini — carrying the Huygens lander with it — embarked on a grand tour of the Solar System, with the object of gaining sufficient impulse towards its final destination through a series of ‘gravitational slingshot’ manoeuvres. By flying very close to a planet, the probe could exploit its gravitational field to accelerate and fly away (‘slingshot’) without spending energy. In a succession of incredibly precise manoeuvres, Cassini performed two such fly-by passes of Venus, another of Earth itself (in 1999), and a final one of Jupiter, which sent the probe on its way to Saturn. After seven years of journey through empty space, Cassini finally entered orbit around the ringed giant in 2004. Shortly afterwards, the Huygens lander detached and headed towards Titan, where it would land in 2005.

Animation of Cassini's trajectory through the Solar System, from October 1997 to May 2008. (Purple: Cassini. Cyan: Venus. Blue: Earth. Yellow: Jupiter. Green: Saturn.) (Credit: Phoenix7777/Wikipedia, under Creative Commons Attribution-Share Alike 4.0 International licence.)

Titan was already known to possess a dense, orange-hued atmosphere which, like that of Earth, is chiefly composed of nitrogen. However, little was known about the surface below, apart from the fact that there was an abundance of methane. More interestingly, Titan’s average temperature of –180 ºC should allow methane to exist in liquid, solid and gaseous forms; this meant that, upon its arrival, Huygens might find itself in a world marked by methane clouds and rain, methane lakes and rivers, and even methane ice. And indeed, photographs and data from Huygens showed the existence of all those features, as predicted many years before — with some lakes reaching lengths of about a hundred kilometres. As Southwood rightly remarked, ‘one of the best things of being a scientist is when wild predictions turn out to be true’. Huygens transmitted to Earth a series of photographs that told the story of its violent descent onto a landscape of arid mountains and valleys, river beds and deep canyons, all slowly sculpted by the flow of liquid methane. After travelling over a billion kilometres, these images were received by watchful radio telescopes in India, China, Australia and the United States, and reconstructed into a breathtaking movie of the most distant landing ever accomplished by a human-made object.

While Huygens’s scientific task was completed shortly after its spectacular landing on Titan, Cassini would carry on with its study of the Saturn system for another twelve years — much longer than initially planned. The impressive array of discoveries made by Cassini is not easily summarised; to start with, it found seven new moons around Saturn, some of which are no more than small bodies formed by the accretion of rocky material within the planet’s rings. The probe also performed detailed measurements of Saturn’s rotational period, atmospheric composition and magnetic field — the latter of which is responsible for the planet’s sensational auroras — and mapped the structure of its iconic rings, as well as the largest storm ever recorded in any planet — surpassing even Jupiter’s celebrated Great Red Spot.

One of the most exciting discoveries made by Cassini started precisely with the space magnetometer built by Southwood’s team in London. While orbiting near the icy moon Enceladus, the instrument detected a change in the local magnetic field, which signalled the existence of an atmosphere around the moon. When this atmosphere was closely inspected, it was found to be composed of ionised water vapour, which was leaking from Enceladus’s interior through a series of huge geysers located at its south pole. The geysers were of such dimension that they could be photographed from space, resulting in images that inspired awe and fascination in equal measure. Amazingly, the geysers were also found to be the source of the so-called ‘E ring’, a faint loop of ice particles that tightly follows Enceladus’s orbit around Saturn. After further analyses, NASA finally reported the existence of an ocean of salty water under the moon’s icy crust. The discovery of this hidden ocean transformed our view of the places where life might lurk in our Solar System; scientists have proposed that tides in this ocean, produced by Saturn’s potent gravitational field, might provide a source of energy for organisms living within Enceladus, placing it among the prime candidates in the quest for extra-terrestrial life.

Image, taken by Cassini, of Saturn's moon Enceladus backlit by the sun, showing the geysers in its south polar region. (Credit: NASA/JPL/Space Science Institute.)

The obliteration of Cassini in Saturn’s upper atmosphere was, in fact, designed to prevent the spacecraft’s remains from contaminating the potentially life-supporting environment of some of Saturn’s moons. Although Cassini, like every spacecraft, had been built in an utterly sterile facility, there was always the possibility that microorganisms from Earth had managed to hitchhike a ride on the probe. Before burning up above the gas giant, Cassini squeezed its scientific output until the last minute, carrying out a series of daring passes through Saturn’s inner rings, which would have been deemed too unsafe had the probe not been already sentenced to destruction.

The Cassini–Huygens mission stands as an eloquent example of the sort of mind-boggling enterprises whereby the broad field of space science moves forward. Unlike projects in most other sciences, which are usually constrained by a slim budget and a strict five-year deadline, gargantuan space missions like this one demonstrate that, at least in some cases, thinking big yields its rewards. After the initial mission proposal, it took fifteen years for Cassini and Huygens to be designed, built and launched into space; and another seven years — and some clever manoeuvring — for them to reach their destination. The extreme degree of precision and anticipation involved in the design of a scientific project that is to rely on painstakingly accurate management over decades, while under permanent risk of complete failure — with the consequent loss of millions of human-hours’ worth of work and money — is something far beyond the experience of more ‘earthly’ scientists and engineers. Yet Cassini–Huygens turned out a success beyond anyone’s expectations, and is proof that institutions and governments, if firmly united around a common goal, can triumph even in endeavours of the most ambitious nature. Although Cassini and Huygens’s journey finally came to an end last year, their legacy is far from over: the trove of scientific data gathered through them remains the object of intense study, and the answers it yields will continue to add up to our understanding of this minute corner of the cosmos we call home.

Historia de una odisea espacial

En sus tres décadas de vida, la misión Cassini–Huygens a Saturno transformó nuestra visión del Sistema Solar.

Read this article in English

Vista de Saturno en color natural, tomada por la astronave Cassini en enero de 2010.
(Imagen: NASA/JPL-Caltech/Space Science Institute.)

EL 15 DE SEPTIEMBRE DE 2017, tras trece años orbitando Saturno, una astronave robótica llamada Cassini se zambulló en la atmósfera del gigante gaseoso, donde ardió rápidamente. Más de una hora después, una tenue señal de radio traía a la Tierra la noticia de la muerte de la sonda espacial, marcando así el fin de la misión Cassini–Huygens. A lo largo de las últimas dos décadas, esta colosal empresa científica había revolucionado por completo nuestro entendimiento de una de las regiones más extraordinarias del Sistema Solar, así como nuestra forma de pensar en la posibilidad de la existencia de vida extraterrestre.

Pese a que la destrucción de Cassini había sido exhaustivamente planificada, para muchos miembros de la NASA, la ESA y la ASI —las respectivas agencias espaciales de Estados Unidos, Europa e Italia— el súbito silencio de la sonda se sintió como la muerte de un viejo y querido amigo. La larga misión espacial había ocupado la mayor parte de sus carreras; mientras las distintas etapas del proyecto se desarrollaban lentamente a miles de millones de kilómetros de la Tierra, muchas de las personas involucradas envejecieron, formaron familias y, en algunos casos, incluso murieron. David Southwood, un distinguido científico planetario que ha ocupado los puestos de Director Científico de la ESA y Presidente de la Real Sociedad Astronómica del Reino Unido, se cuenta entre los pocos afortunados que han sido testigos directos de la totalidad de la vida de Cassini–Huygens. En una reciente charla organizada por la Sociedad Astronómica de la Universidad de Cambridge, Southwood ofreció su fascinante relato de la historia de Cassini–Huygens, así como del inmenso esfuerzo internacional que permitió a la misión despegar y permanecer activa durante tanto tiempo. La mayoría de lo expuesto a continuación está basado en el contenido de dicha charla.

Cassini y Huygens, las dos astronaves robóticas que llevarían a cabo la misión, fueron lanzadas al espacio en 1997, veinte años antes de la muerte de Cassini en 2017; los inicios de la misión, no obstante, se remontan a principios de los años ochenta. Por aquel entonces, Europa permanecía reticente a invertir en exploración espacial: tras la salvaje carrera espacial entre Estados Unidos y la Unión Soviética, que desembocaría en el aterrizaje del Apolo 11 en la Luna en 1969, el espacio había pasado a ser, en palabras de Southwood, ‘Terra Americana o Terra Sovietica: o bien territorio americano, o bien soviético’. Southwood no sólo fue testigo de cómo la ciencia espacial pasaba de mera ciencia ficción a ser una realidad tangible, sino que desempeñó un papel activo en la transición. Tras su infancia en Inglaterra, donde creció devorando cómics de ciencia ficción en la década de los cincuenta, Southwood se mudaría más tarde a los Estados Unidos en busca de un mejor entorno para la ciencia espacial, antes de regresar a Inglaterra a principios de los años setenta. No sería hasta 1982 cuando Europa y Estados Unidos se volverían socios en la propuesta para una ambiciosa misión espacial a Saturno, un planeta que ha capturado la imaginación del público desde que Galileo Galilei apuntó su telescopio hacia él por primera vez en 1610. La ESA se comprometió a la construcción de Huygens, la sonda que habría de aterrizar en la mayor luna de Saturno, Titán, y que portaba el nombre de Christiaan Huygens, descubridor de dicha luna. La NASA y la ASI, por su parte, construirían la sonda Cassini, de dimensiones mucho mayores y bautizada en honor a Giovanni Cassini, el descubridor de las divisiones de los anillos de Saturno y varias de sus lunas. Cassini permanecería en órbita alrededor del planeta con objeto de estudiar sus anillos y lunas.

Diagrama de la astronave Cassini. La sonda Huygens se encuentra bajo el escudo circular situado en el lado izquierdo. (Imagen: NASA Jet Propulsion Laboratory.)

Trabajando desde el Imperial College de Londres, Southwood encabezó el desarrollo de uno de los instrumentos científicos de Cassini: el magnetómetro espacial, un dispositivo diseñado para medir variaciones en campos magnéticos. Southwood también acabaría desempeñando un papel inesperado pero esencial en la defensa del proyecto frente a preocupantes cambios en las prioridades científicas de la NASA, probablemente debidos al alterado ambiente político tras el final de la Guerra Fría. Salvaguardar el proyecto frente a todo fallo técnico o político era de la mayor importancia para los miembros del mismo; con una altura de alrededor de siete metros, una masa de más de dos toneladas, y un coste de desarrollo de casi mil quinientos millones de dólares, Cassini era una astronave de diseño extraordinariamente ambicioso, sobre la que descansaba el peso de grandes expectativas.

Tras abandonar la Tierra desde Cabo Cañaveral en 1997, Cassini —portando con ella la sonda Huygens— se embarcó en un largo tour del Sistema Solar, con el propósito de obtener suficiente impulso para alcanzar su destino mediante una serie de maniobras de ‘asistencia gravitatoria’: al pasar cerca de un planeta, la sonda podía aprovechar el campo gravitatorio del mismo para acelerar y salir catapultada hacia el espacio sin necesidad de gastar energía. En una sucesión de maniobras increíblemente precisas, Cassini realizó dos pasadas sobre Venus, otra más sobre la Tierra (en 1999), y una última sobre Júpiter, la cual puso finalmente a la sonda camino de Saturno. Finalmente, tras siete años de viaje por el vacío, Cassini entró en órbita alrededor del gigante anillado en 2004. Poco después, la sonda Huygens se desprendería de Cassini para dirigirse hacia Titán, donde tocaría tierra en 2005.

Animación de la trayectoria de Cassini a través del Sistema Solar, entre octubre de 1997 y mayo de 2008. (Violeta: Cassini. Cian: Venus. Azul: Tierra. Amarillo: Júpiter. Verde: Saturno.) (Imagen: Phoenix7777/Wikipedia, bajo licencia Creative Commons Attribution-Share Alike 4.0.)

Estudios anteriores de Titán habían revelado que esta luna posee una densa atmósfera anaranjada que, al igual que la de la Tierra, está compuesta mayormente de nitrógeno. No obstante, poco se conocía de la superficie bajo esta atmósfera, aparte de que en ella había metano en abundancia. Es más, se sabía que la temperatura media de Titán es de –180 ºC, lo que debería permitir la existencia de metano en formas líquida, sólida y gaseosa —como es el caso del agua en nuestro planeta—. Es decir, a su llegada a Titán, Huygens podría encontrarse con un mundo surcado por nubes y lluvias de metano, ríos y lagos de metano, e incluso hielo de metano. Y en efecto, fotografías y datos recogidos por Huygens probaron la existencia de todos estos fenómenos, predichos muchos años antes —con algunos lagos de metano alcanzando los cien kilómetros de longitud—. Tal como Southwood señaló en su charla, ‘una de las mejores cosas de ser científico es cuando las predicciones más arriesgadas resultan ser ciertas’. Huygens transmitió a la Tierra una serie de fotografías que relatan la historia de su violento descenso hacia un paisaje de áridas montañas y valles, lechos fluviales y hondos cañones, cada uno lentamente esculpido por el flujo de metano líquido. Tras recorrer más de mil millones de kilómetros, las imágenes fueron recibidas por vigilantes radiotelescopios en la India, China, Australia y Estados Unidos, y usadas para reconstruir una imponente película del aterrizaje más remoto jamás realizado por un objeto de fabricación humana.

Mientras que la labor científica de Huygens concluyó poco después de su espectacular aterrizaje en Titán, Cassini proseguiría con su estudio del sistema de Saturno por otros doce años, mucho más de lo inicialmente planeado. El impresionante surtido de descubrimientos realizados por Cassini no se resume con facilidad; para empezar, la sonda descubrió siete nuevas lunas de Saturno, algunas de las cuales no son más que pequeños cuerpos formados por la acreción de material rocoso en los anillos del planeta. Cassini también realizó mediciones detalladas del periodo rotacional, la composición atmosférica y el campo magnético de Saturno —el último de los cuales es responsable de las sensacionales auroras del planeta—, y cartografió la estructura de sus icónicos anillos, así como la mayor tormenta jamás registrada en ningún planeta —que superó incluso a la famosa Gran Mancha Roja de Júpiter—.

Uno de los descubrimientos más emocionantes realizados por Cassini comenzó precisamente con el magnetómetro espacial construido por el equipo de Southwood en Londres. Mientras la sonda pasaba cerca de la luna helada Encélado, el instrumento detectó un cambio en el campo magnético local, señalando la existencia de una atmósfera alrededor de dicha luna. Un examen más exhaustivo de esta atmósfera reveló que estaba compuesta de vapor de agua ionizado, el cual manaba del interior de Encélado a través de una serie de gigantescos géiseres situados en su polo sur. Estos géiseres eran de un tamaño tal que podían ser observados desde el espacio, dando lugar a fotografías tan sobrecogedoras como científicamente fascinantes. Increíblemente, los géiseres también resultaron ser la fuente del llamado ‘anillo E’, un difuso anillo de partículas de hielo que sigue precisamente la órbita de Encélado alrededor de Saturno. Tras un análisis más detallado, la NASA anunció la existencia de un océano de agua salada bajo la corteza helada de Encélado, un descubrimiento que transformó por completo nuestra visión de los lugares en los que la vida podría ocultarse en el Sistema Solar. Algunos científicos han propuesto que las mareas producidas por el potente campo gravitatorio de Saturno en este océano podrían constituir una fuente de energía para organismos que habitaran el interior de Encélado, posicionando a esta luna como uno de los mejores candidatos en la búsqueda de vida extraterrestre.

Imagen, tomada por Cassini, de la luna Encélado iluminada por el sol, mostrando los géiseres en su polo sur. (Imagen: NASA/JPL/Space Science Institute.)

La destrucción de Cassini en la atmósfera superior de Saturno fue, de hecho, diseñada para prevenir que los restos de la sonda contaminasen el entorno potencialmente apto para la vida de algunas de las lunas del planeta. Aunque Cassini, como toda astronave, había sido construida en instalaciones extremadamente esterilizadas, siempre cabía la posibilidad de que microorganismos terrestres hubieran conseguido infiltrarse como polizones en la sonda. Antes de arder finalmente sobre el gigante gaseoso, Cassini exprimió su producción científica hasta el último minuto, realizando una serie de arriesgadas pasadas a través de los anillos de Saturno, las cuales se habrían considerado demasiado peligrosas si la sonda no hubiera estado ya sentenciada a ser destruida.

La misión Cassini–Huygens se yergue como un ejemplo revelador de la clase de proyectos inverosímiles que caracterizan al campo de la ciencia espacial. A diferencia de los proyectos típicos de la mayoría de ciencias, que se hallan sujetos a ajustados presupuestos y estrictos plazos de cinco años, las colosales misiones espaciales como ésta demuestran que, al menos en ciertos casos, pensar a lo grande tiene su recompensa. Tras la propuesta inicial de la misión, hicieron falta quince años para el diseño, construcción y lanzamiento al espacio de Cassini y Huygens, y otros siete años para que ambos alcanzasen su destino. El grado extremo de precisión y anticipación implicado en el diseño de un proyecto científico con una gestión tan meticulosamente detallada durante décadas, y permanentemente expuesto a la posibilidad de fracaso total —que conllevaría la pérdida de miles de millones de dólares y millones de horas de trabajo— está más allá de la experiencia de científicos e ingenieros más ‘terrenales’. Aun así, Cassini–Huygens fue un éxito superior a toda expectativa, y demostró que instituciones y gobiernos, si se encuentran firmemente unidos en torno a un objetivo común, son capaces de triunfar incluso en empeños del carácter más ambicioso imaginable. Aunque el viaje de Cassini y Huygens finalmente tocó a su fin el año pasado, su legado está lejos de terminar: el patrimonio de datos científicos recopilado gracias a ellos sigue siendo objeto de intenso estudio, y las respuestas que se deriven del mismo continuarán enriqueciendo nuestra comprensión de este minúsculo rincón del cosmos al que llamamos hogar.

Frontiers of gravity

The most familiar force of nature may be the key to a fuller understanding of the universe.

Lee este artículo en español

In his famous cannonball thought experiment, Isaac Newton demonstrated that gravity gives rise to orbital motions. (Source: Newton 1729/1962, vol. 2.)

OF THE FOUR KNOWN fundamental forces which are responsible for the physics of our universe—the electromagnetic force, the weak and strong nuclear forces and the gravitational force—the latter stands out in many senses. The gravitational force, commonly known as gravity, was the first of these forces to be described mathematically. Gravity is also, by far, the least powerful of the fundamental forces: the second weakest force—the aptly named weak force that gives rise to radioactivity—is over a trillion trillion times stronger than the gravitational force. But the reason why gravity, a phenomenon that has been familiar to us for so long, can still be so special, is its connection to some of the biggest questions in modern physics.

Everyone on Earth continuously experiences our planet’s gravitational pull, and as a result, the physics of gravity are intrinsically embedded in our mental model of how the world works. Indeed, one of the first things that babies learn is how objects fall to the floor when thrown or dropped—a phenomenon from which they seem to derive some delight. This made it quite hard for us to realise that gravity actually is a physical force, and especially, that the force which makes objects fall here on Earth is the same one that keeps the skies above us in perpetual motion.

The name that inevitably comes to mind when talking of gravity is that of Isaac Newton. In the late seventeenth century, the legendary natural scientist came up with a tremendously ingenious thought experiment. He imagined a cannon standing on top of an extremely tall mountain. If a projectile were fired from this cannon towards the horizon, it would travel horizontally while also falling by effect of gravity, before eventually landing. If the cannon’s power were increased, the projectile would fly horizontally further away, travelling greater distance than before. And, if the canon’s power were to be increased infinitely, the projectile would be fired with such force that it would never stop falling—always travelling horizontally around the Earth without ever hitting the ground. From this idea, Newton rightly concluded that the gravitational force which attracts objects towards the Earth is what keeps the Moon in orbit around us: the Moon is just forever falling towards the Earth.

By extending his logic, Newton declared the gravitational force to be also responsible for the orbits of the planets around the Sun. In his law of universal gravitation, he described gravity as a universal force between every two bodies (or particles) in the universe, whose intensity increases in proportion to the bodies’ masses and decreases in proportion to the square of the distance between them—that is, if the distance between two objects is doubled, then the gravitational force between them becomes four times weaker. Newton considered gravity an instantaneous and boundless force; according to him, the pull exerted by an object travels instantly and does not stop at a given distance, but it becomes weaker and weaker until being impossible to detect.

Newton’s relatively simple mathematical formula could explain such mysteries as the elliptical shape of the planets’ orbits and the tides of Earth’s oceans. His theory, however, failed to explain the nature of gravitation itself—why it exists. Furthermore, Newtonian gravity appeared as a mystical force capable of working instantly across empty space, and this was not happily accepted by contemporary scientists. Most European scholars at the time subscribed to René Descartes’s theory of planetary motion, which presented the planets’ orbits as the product of massive vortices swirling within an invisible fluid that filled the universe. But Newton’s theory would eventually prove more accurate than Descartes’s, forcing scientists to face the reality of an empty universe.

A successful explanation of the nature of gravity arrived in the early twentieth century, thanks to Albert Einstein. The iconic physicist’s celebrated general theory of relativity was truly ground-breaking, completely transforming the way we understand both the largest bodies and the smallest particles in the universe. Einstein’s theory built on the now popular concept of space-time: a mathematical model where the three dimensions of space and the one dimension of time are intertwined into a single four-dimensional continuum, hence defying our false intuition of space and time as separate aspects of reality. This model allowed Einstein to present gravity not as an attractive force exerted by particles or objects, but as a property of space-time itself, which arises from its distortion. We can think of this as if bodies with a very large mass, such as a planet or a star, were ‘bending’ the space-time fabric of the universe around them, just like a stone would deform an elastic rubber band by means of its weight. Gravity as we perceive it is a consequence of this deformation of space-time, and not a force that ‘pulls’ us towards the Earth; rather, it is the ‘warped’ space-time around the Earth that is ‘pushing’ us against it.

General relativity was transformative in many ways. It not only provided a long-sought explanation for the mysterious force of gravity and a better understanding of its properties—such as its actual speed, which is exactly that of light—but it also predicted novel physical phenomena, like black holes and gravitational waves, whose existence physicists have only been able to confirm many decades later. (In fact, gravitational waves were first detected in 2016, a hundred years since general relativity had predicted them.) Einstein’s theory also solved some puzzling astronomical mysteries, such as that of the orbital motion of Mercury. Astronomers had long identified a discrepancy between the observed motion of this planet and the one predicted by Newtonian law; as a consequence, the nineteenth-century mathematician Urbain Le Verrier posited the existence of Vulcan, a new planet between the Sun and Mercury whose gravitational pull altered Mercury’s orbit. Vulcan’s existence seemed the only explanation for the inconsistencies in Mercury’s motion; moreover, a similar reasoning had previously led Le Verrier to successfully predict the position of the planet Neptune, precipitating its discovery. However, the arrival of general relativity showed that Mercury’s behaviour could be accounted for without the need of an additional planet, thus nullifying Le Verrier’s hypothesis.

If twentieth-century physics was witness to revolutionary breakthroughs sparked by minds such as Einstein’s, today’s physics is grappling with a myriad of daunting mysteries—the greatest of which seem inextricably linked to gravity. In particular, astronomical observations over the last century have confronted physicists with two extremely puzzling phenomena. The first harks back to Edwin Hubble’s discovery that galaxies lying further away from our own are moving away from us at a greater speed. This led Hubble to conclude that the universe is expanding, and later research has showed that the speed of this expansion is actually increasing. Such accelerated inflation of the universe implies the existence of a force that acts to counteract gravity, as otherwise the gravitational pull of galaxies would restrain the expansion. This mysterious anti-gravitational force was given the name of dark energy, and apart from the rate of expansion of the universe, scientist have so far failed to obtain any direct evidence of its existence.

The other great astronomical mystery of our time, together with dark energy, is suitably named dark matter—although ‘invisible matter’ would be more accurate. The notion of dark matter stems from the observation that gravity seems to be persistently stronger on the outskirts of galaxies than dictated by general relativity. In other words, the stars in almost every galaxy are rotating around the galactic centre much faster than they should, which suggests that galaxies have more mass than the one in the matter we can see. To account for this missing mass, researchers proposed the existence of dark matter, a kind of matter which does not interact with conventional matter electromagnetically—and therefore is invisible to our eyes and telescopes, which can only detect electromagnetic radiation. Dark matter does, however, display gravitational interactions with visible matter, which is the reason why galaxies seem to have more mass than we can see.

Dark matter is thought to surround galaxies in a diffuse halo, which is shown in black in this artist’s view. (Credit: A. Evans, adapted by J. Freundlich & F. Ducouret.)

When physicists calculated how much dark matter would be needed to account for the excess mass, they were shocked to find that dark matter would outweigh all the visible matter in the universe by a factor of five to one; in other words, if dark matter does exist, then five-sixths of the universe are invisible to us. And more intriguingly, although dark matter is indirectly supported by plenty of astronomical observations, it resembles dark energy in its dogged resistance to direct detection.

If decades of research have yielded no direct evidence of dark energy and dark matter, should we believe in them? Many scientists argue that there is no reason why we should expect visible matter to be the only kind of matter there is. Our eyes have evolved to detect the electromagnetic interactions of matter—that is, the way in which matter produces or reflects light. If there existed a kind of matter incapable of electromagnetic interaction, then such matter would be invisible to us—but that could hardly be an argument against its existence.

Some physicists, on the other hand, regard dark matter as a modern-day analogue of Vulcan: it could be a false solution to fundamental flaws in our understanding of how gravity operates. Just as it happened to Vulcan, an improved theory of gravity may be able to explain all our astronomical observations without being contingent on elusive particles; and indeed, several alternative theories of gravitation have seen the light over the last decades, each aspiring to supersede general relativity by successfully explaining every existing observation. The most popular of these is modified Newtonian dynamics, or MOND. This theory proposes that, over relatively small distances, gravity behaves according to Newtonian law, decreasing in proportion to the square of the distance; however, over vast cosmic distances—like that between the centre of a galaxy and its edge—gravity ‘switches’ to a different formula, weakening much more slowly than Newton and Einstein predicted. This reformed law of gravitation would explain why galactic outskirts present stronger gravity than expected, without the need for massive amounts of dark matter to supply additional mass.

However, while dark matter and dark energy continue to dodge detection, Einstein’s theory of gravity is proving extremely robust. In fact, the latest measurements of gravitational waves have already invalidated some alternative gravity theories. Although MOND has yet to be proven right or wrong, its immediate future seems in jeopardy: recent discoveries of galaxies composed almost exclusively of dark matter, as well as galaxies with practically no dark matter in them, both constitute a serious challenge for theories like MOND. If gravity is just behaving differently than we believe, as these theories propose, then this modified gravity should still be universal, implying that the proportion of dark matter estimated in each galaxy should be roughly the same, and extremely ‘dark’ or ‘luminous’ galaxies such as the ones discovered should not exist. On the other hand, if dark matter does exist, these exotic galaxies will allow a more detailed study of its gravitational properties.

Our understanding of gravity has been transformed and transformative over the last three centuries. Notwithstanding, physics now seems to be at the crossroads between a redefinition of gravity and a belief in mysterious particles and forces. But we should not expect this uncertainty to linger for much longer. At this very moment, researchers are applying the most advanced technology on Earth and in space in unrelenting efforts to detect even the smallest signal confirming the existence of dark matter and dark energy, and searching for any minute crack in general relativity’s predictions, which could provide the missing piece in the puzzle. But so far, Einstein’s equations remain unbowed. And if Einstein was not mistaken, then the only explanation for what we see in the universe must be the existence of forces and particles which, thus far, we are largely unable to measure or comprehend. Whichever the case, the uncertainty will likely be resolved sooner than later, marking the dawn of the next revolution in our understanding of the cosmos.

References:
Castelvecchi, D. How gravitational waves could solve some of the Universe’s deepest mysteries. Nature (2018).
Randall, L. What is dark matter? Nature (2018).
Wolchover, N. The case against dark matter. Quanta Magazine (2016).
Sokol, J. A victory for dark matter in a galaxy without any. Quanta Magazine (2018).
Moskvitch, K. Troubled times for alternatives to Einstein’s theory of gravity. Quanta Magazine (2018).