divulgatum está dedicado a la difusión de conocimiento científico en español e inglés, mediante artículos que tratan en detalle toda clase de temas fascinantes pero poco conocidos.
divulgatum is devoted to the dissemination of scientific knowledge in Spanish and English, through articles that go into the details of all sorts of fascinating, if not
widely known, subjects.

Tuesday, June 5, 2018

Frontiers of gravity

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

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.

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).