For centuries, scientists studied light to comprehend the visible world. Why are things colored? What is a rainbow? How do our eyes work? And what is light itself? These are questions that preoccupied scientists and philosophers since the time of Aristotle, including Roger Bacon, Isaac Newton, Michael Faraday, Thomas Young, and James Clerk Maxwell.

But in the late 19th century all that changed, and it was largely Maxwell’s doing. This was the period in which the whole focus of physics—then still emerging as a distinct scientific discipline—shifted from the visible to the invisible. Light itself was instrumental to that change. Not only were the components of light invisible “fields,” but light was revealed as merely a small slice of a rainbow extending far into the unseen.

Physics has never looked back. Today its theories and concepts are concerned largely with invisible entities: not only unseen force fields and insensible rays but particles too small to see even with the most advanced microscopes. We now know that our everyday perception grants us access to only a tiny fraction of reality. Telescopes responding to radio waves, infrared radiation, and X-rays have vastly expanded our view of the universe, while electron microscopes, X-ray beams, and other fine probes of nature’s granularity have unveiled the microworld hidden beyond our visual acuity. Theories at the speculative forefront of physics flesh out this unseen universe with parallel worlds and with mysterious entities named for their very invisibility: dark matter and dark energy.

This move beyond the visible has become a fundamental part of science’s narrative. But it’s a more complicated shift than we often appreciate. Making sense of what is unseen—of what lies “beyond the light”—has a much longer history in human experience. Before science had the means to explore that realm, we had to make do with stories that became enshrined in myth and folklore. Those stories aren’t banished as science advances; they are simply reinvented. Scientists working at the forefront of the invisible will always be confronted with gaps in knowledge, understanding, and experimental capability. In the face of those limits, they draw unconsciously on the imagery of the old stories. This is a necessary part of science, and these stories can sometimes suggest genuinely productive scientific ideas. But the danger is that we will start to believe them at face value, mistaking them for theories.

A backward glance at the history of the invisible shows how the narratives and tropes of myth and folklore can stimulate science, while showing that the truth will probably turn out to be far stranger and more unexpected than these old stories can accommodate.

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Occult light

Who now will stand up for the British physicist Edmund Fournier d’Albe, who in 1908 put forward the theory that the human soul is composed of invisible particles called “psychomeres” possessing a rudimentary kind of intelligence? He estimated the mass of these particles in a single soul (about 50 milligrams) and suggested that his hypothesis could account for paranormal phenomena such as ghosts and fairies, perhaps even Moses’ burning bush. Sure, it sounds wild, but Fournier d’Albe was a respected researcher in radio telecommunications, and he insisted that, in the light of recent discoveries such as X-rays and the electron, we’d be unwise to discount something just because we can’t see it directly.

“We must resolutely combat the tendency to look for the unseen beyond the seen,” Fournier d’Albe wrote. “The unseen is all about us… a single octave in the gamut of light-waves impresses our retina, revealing a very small proportion of what would be visible to a more completely equipped intelligence.”

In other words, he and his contemporaries insisted, there’s more to heaven and earth.

Things that we can’t see or touch once belonged to the realm of the occult. This simply meant that they were hidden, not necessarily that they were supernatural. But the occult became the hiding place for all kinds of imaginary paranormal phenomena: ghosts, spirits and demons, telepathy, and other “psychic forces.” These things seem now to be the antithesis of science, but when science first began to fixate on invisible entities, many leading scientists saw no clear distinction between such occult concepts and hard science. They invented wonderful stories to link and explain them. Victorian physicists were particularly prone. Some conjectured that there exist intelligent, unseeable beings on the subatomic or the cosmic scale. Others speculated that high-frequency waves outside the visible range could transmit thoughts between minds, or that immortal souls were consistent with the laws of thermodynamics. Anything seemed possible, as it often does when we awaken to our ignorance.

This swerve toward the invisible started with efforts to comprehend the ancient question: What is light? In the early 19th century Faraday introduced the idea of a field—an invisible, pervasive influence—to explain the nature of electricity and magnetism. In the 1860s, Maxwell wrote down a set of equations showing how electricity and magnetism are related. Maxwell’s equations implied that oscillations in these coupled fields—electromagnetic waves—would move through space at the speed of light. It was quickly apparent that these waves in fact are light.

But where visible light has wavelengths of between about 400 and 800 millionths of a millimeter, Maxwell’s equations showed that there was no obvious limit to the wavelength that electromagnetic waves can have. They may exist beyond both the upper and lower limits of the visible range.

These predictions were soon confirmed. In 1887, the German scientist Heinrich Hertz showed that fluctuations of electrical current could give rise to long-wavelength radiation, which became known as radio waves. It took less than a decade for the Italian Guglielmo Marconi to show that radio waves could be used to transmit messages across vast distances.

It’s hard to appreciate now how revolutionary this was, not just practically but conceptually. Previously, messages beyond shouting range had to be sent either by a physical letter or as pulses of electricity down telegraph wires. The telegraph was already extraordinary enough, but still it required a physical link between sender and receiver. With radio, one could communicate wirelessly through “empty space.”

Many leading scientists saw no clear distinction between the occult and hard science.

It is no coincidence that these discoveries happened at the height of the Victorian enthusiasm for spiritualism, in which mediums claimed to be able to contact the souls of the dead. The two trends supported each other. The new physics hinted at explanations for thought transference, whether from other people or from spirits; and a widespread belief in invisible influences and intelligences created a receptive environment for ideas in physics that seemed scarcely less incredible. If radio waves could transmit invisibly between a broadcasting device and a receiver, it did not seem so hard to imagine that human brains—which are after all quickened by electrical nerve signals—could act as receivers.



But what, then, of the senders? Scientists, familiarized now to the concept of invisible fields, had begun to speculate about non-material beings that inhabit unseen planes of existence. Maxwell’s friends Peter Guthrie Tait and Balfour Stewart, both professors of physics, published The Unseen Universe (1875), in which they presented the ether—supposedly the rarefied fluid that carries Maxwell’s waves—as a bridge between the physical and spiritual worlds, both of which they considered to be populated with intelligences. Some of the pioneers of the telegraph had already drawn parallels with spiritualism, which they called “celestial telegraphy.” Now the wireless spawned a vision of empty space as being alive with the traces of unseen beings. All you had to do was tune in, just as radio enthusiasts scanned the airwaves for crackly, half-heard snatches of messages from Helsinki or Munich. Rudyard Kipling’s short story “Wireless” (1902) described a man who, feverish from tuberculosis, becomes a receiver for fragments of a poem by Keats, while elsewhere in the house a group of amateur radio operators picks up broadcasts from a nearby ship. While the spiritual “telegraph line” channeled by the medium in a séance offered the comfort of words from departed loved ones, the wireless seemed instead to make the spirit world a source of impersonal, often meaningless missives cast adrift in an unheeding universe.

The discovery of X-rays by Wilhelm Röntgen in 1895 further stimulated these imaginings. X-rays, it soon became clear, were invisible rays at the other end of the spectrum from radio, with wavelengths much shorter than those of light. Their discovery was an offshoot of experiments on the “cathode rays” found to be emitted from hot metal electrodes, which were themselves found in 1898 to be streams of electrically charged subatomic particles that were named electrons.

What made X-rays so astonishing and evocative was that they not only were invisible but revealed the invisible—not least, the bones beneath our flesh, in an unnerving presentiment of death. In the 1890s, people flocked to public demonstrations at shows such as the Urania in Berlin or Thomas Edison’s stage spectacles in New York to watch their skeletons appear on fluorescent, X-ray-sensitive screens. X-ray photography seemed a straightforward extension of the “spirit photography” that had become popular in the 1870s and ’80s, which claimed to reveal ghosts by faked or genuinely inadvertent double exposures. It confirmed the photographic emulsion as a “sensitive medium” that could render the invisible visible. Others claimed to see evidence of new types of invisible rays recorded in photographs, and even to be able to photograph “thought forms” and souls.

At the fin-de-siècle, invisible “rays” were everywhere, and no claim seemed too extravagant. There were cathode rays and anode rays, wholly spurious radiations such as N-rays and “black light” (although ultraviolet light also acquired that name), and most famously, the “uranic rays” that Henri Becquerel discovered coming from uranium salts in 1896. These streamed in an unchecked and unquenchable flow, suggesting a tremendous hidden source of energy that, through the work of Pierre and Marie Curie, Ernest Rutherford, and others, was eventually traced to the nuclear heart of the atom. The Curies renamed these rays “radioactivity.”

There was an old cultural preconception that invisible “emanations” could have life-enhancing agency, whether these were the “virtues” ascribed to medicinal herbs in the Middle Ages or the “animal magnetism” or “mesmeric force” of the 18th-century German physician Franz Anton Mesmer. We shouldn’t be surprised, then, that radioactivity was widely believed to have miraculous healing powers. “Whatever your Ill, write us,” advertised the Nowata Radium Sanitarium Company in 1905. “Testimonials of Cases cured will be sent you.” “Therapeutic” radium was added to toothpastes and cosmetics, and spa towns proudly advertised the radioactivity (from naturally occurring radon) in their waters. It wasn’t until the 1920s that the health effects of radioactive elements were shown to be quite the opposite: too late to save Marie Curie herself, or some of the Radium Girls—factory workers who had for a decade been licking paintbrushes dipped in radioactive paint for the dials of watches.





Ghost factories

One of the key messages of these discoveries of the late 19th century was that the universe we perceive is only a small part of what is out there. There was a long tradition of “spirit worlds” going back at least to the Middle Ages, when it was a commonplace that invisible, malevolent demons lurked all around us. These beliefs provided the unconscious template for making sense of the new “invisible universe.” The respected physicist William Barrett, who co-founded the Society for Psychical Research in 1882, proposed in his book On the Threshold of the Unseen (1917) that there exist human-like invisible “elementals.”

That was an idea shared by one of the most prominent of the “psychical” scientists of the age, William Crookes. A chemist and entrepreneur who served as the President of the Royal Society between 1913 and 1915, Crookes became famous when he discovered the new chemical element thallium in 1861. Yet he seems to have been particularly credulous of spiritualists’ claims, perhaps even collusive with them. He was taken in by several mediums, including the renowned Florence Cook. Like many mediums, Cook was a striking young woman who found it easy to manipulate the judgment of Victorian gentlemen of more advanced years. Crookes was convinced that “there exist invisible intelligent beings, who profess to be spirits of deceased people.” (He evidently took this to be the skeptical view.) To investigate the “psychic force” that he thought mediums commanded, Crookes invented a device called the radiometer or “light mill,” in which delicate vanes attached to a pivot inside a sealed glass bulb would rotate when illuminated by light. Although the reason for the rotation was not, as at first thought, due to the “pressure” exerted by light itself—despite having no mass, light rays can deliver a little kick to surfaces they strike—that pressure is a real enough phenomenon, and the radiometer helped to establish it. Thus, an instrument motivated by a belief in the paranormal prompted useful scientific work on the properties of light.

© Nevit Dilmen





The same may be said of Crookes’ “radiant matter,” allegedly a “fourth form of matter” somewhere between ordinary material and pure light. Crookes discovered this stuff while conducting important work with gas discharge tubes, in which charged electrodes ionize a vapor so that an electrical current can pass through it, producing a glow. In 1879 he claimed that radiant matter existed in “the shadowy realm between known and unknown.” He suspected that it, like the ether, might be a bridge to the spirit world.

Radiant matter was another figment of Crookes’ overactive imagination. But this too bore fruit. He invoked radiant matter to explain a mysterious region inside the discharge tubes called the “dark space.” But it turned out that this dark region was instead caused by cathode rays, and Crookes’ research on this phenomenon led ultimately to the discovery of electrons and X-rays and, coupled with Marconi’s radio broadcasting, to the development of television. Indeed, several of the early pioneers of television were motivated by their paranormal sympathies, whether it was Crookes refining the cathode ray tube, Fournier d’Albe devising his own idiosyncratic televisual technology, or John Logie Baird, usually regarded as the device’s real inventor, who believed he was in spiritualistic contact with the departed spirit of Thomas Edison.

Both radio communication and television were representative of all modern media in that they are ghost factories, forever manufacturing what in 1886 the psychic researcher Frederic W. H. Myers called “phantasms of the living”: disembodied replicas of ourselves, ready to speak on our behalf. Radio could conjure the illusion that the prime minister, or a film star, had become manifest, though disembodied like a phantom, in your sitting room. How much more potent the illusion was, then, once you could see such electronic ghosts as well as hear them. It might have seemed natural and harmless enough to refer to the double images of early television sets, caused by poor reception or bad synchronization of the electron beam, as “ghosts”—but this terminology spoke to, and fed, a common suspicion that the figures you saw on the screen might not always correspond to real people. After all, they might already be dead. News reporters flocked to the home of Jerome E. Travers of Long Island in December 1953 to witness the face of an unknown woman who had appeared on the screen and wouldn’t vanish even when the set was unplugged. (The family had turned the screen toward the wall, as if in disgrace.)

Several of the early pioneers of television were motivated by paranormal sympathies.

By appearing to transmit our presence over impossible reaches of time and space, and preserving our image and voice beyond death, these media subvert the laws that for centuries constrained human interaction by requiring the physical transport of a letter or the presence of the persons themselves. We submit to the illusion that the voice of our beloved issues from the phone, that the Skyped image conjured on the screen (now by light-emitting diodes rather than electron-blasted phosphors) is the faraway relative in the flesh.

Who could possibly be surprised, then, that the Internet throngs with ghosts? As folklore historian Owen Davies says, “cyberspace has become part of the geography of haunting.” Here too the simulated traces of the dead may linger indefinitely; here too pseudonymous identities are said to speak from beyond the grave. More even than the telephone and television, the Internet, that invisible babble of voices, seems almost designed to house spirits, which after all are no more ethereal than our own cyber-presence.

Time and again we see that technology by itself doesn’t banish folk beliefs and superstitions, it merely creates new environments for them. Maybe that shouldn’t be any surprise, as technical advances don’t in themselves produce any general advance in people’s ability to reason or assess evidence. But surely science still pushes back the dark clouds of ignorance and superstition? Yes, it surely does. That is not to say, though, that scientists today are less prone to flights of imaginative fantasy than their predecessors. And thank goodness, for science needs imagination as much as cold logic. But as in Victorian times, sometimes it’s hard to distinguish fantasy from fact when both are couched in the jargon of the day.





Hidden worlds

Take, for example, the Many-Worlds Interpretation of quantum mechanics. There’s no agreement about quite how to interpret what quantum theory tells us about the nature of reality at tiny scales, but the Many-Worlds Interpretation has plenty of influential adherents. It supposes the existence of parallel universes that embody every possible outcome of the many possible solutions to the equations describing a quantum system. According to physicist Max Tegmark of the Massachusetts Institute of Technology, “it predicts that one classical reality gradually splits into superpositions of many such realities.” The idea is derived from the work of physicist Hugh Everett in the 1950s—but Everett himself never spoke of “many worlds.”

At that time, the prevailing view in quantum theory was that, when you make a measurement on a quantum system, this selects just one of the possible outcomes enumerated in the mathematical entity called the wavefunction—a process called “collapsing the wavefunction.” The problem was there was nothing in the theory to cause this collapse—you had to put it in “by hand.” Everett made the apparently innocuous suggestion that perhaps there is no collapse: that all the other possible outcomes also have a real physical existence. He never addressed the question of where those other states reside. But some of his successors build up around them an entire universe, identical to our own in every respect except for that one aspect. Every quantum event causes these parallel universes to proliferate, so that “the act of making a decision causes a person to split into multiple copies,” according to Tegmark. (More properly, these alternative universes have always existed, it’s just that things evolve differently in each of them.)

But this idea itself collapses into incoherence when you actually try to populate the Many-Worlds with sentient beings. It’s not (as Tegmark might be thought to imply above) that there are alternative versions of us in these many worlds—they are all in some sense us, but there’s no prescription for where to put our apparently unique consciousness. This conundrum arises not (as some adherents insist) as an inevitable result of “taking the math seriously,” but simply because of the impulse, motivated by neither experiment nor theory, to make each formal mathematical expression a complete world of its own, invisible from this one. That is done not for any scientific reason but because it is what, in the face of the unknown, we have always done. In the Middle Ages, the tangible evidence for invisible forces like magnetism made invisible beings like demons seem more plausible. The discovery of X-rays, radio waves and radioactivity likewise bolstered beliefs in a populous “unseen universe.” The apparent collapse of the wavefunction is indeed a profound mystery—should we then be surprised that one response is to build invisible universes to compensate?

The Internet, that invisible babble of voices, seems almost designed to house spirits.

Much the same consideration applies to the concept of “brane” (short for membrane) worlds. This arises from the most state-of-the-art variants of string theory, which attempt to explain all the known particles and forces in terms of ultra-tiny entities called strings, which can be envisioned as particles extended into little strands that vibrate. Most versions of the theory call for variables in the equations that seem to have the role of extra dimensions in space, so that string theory posits not four dimensions (of time and space) but 11. As physicist and writer Jim Baggott points out, “there is no experimental or observational basis for these assumptions”—the “extra dimensions” are just formal aspects of the equations. However, the latest versions of the theory suggest that these extra dimensions can be extremely large, constituting extra-dimensional branes that are potential repositories for alternative universes separated from our own like the stacked leaves of a book. Inevitably, there is an urge to imagine that these places too might be populated with sentient beings, although that’s optional. The point is that these brane worlds are nothing more than mathematical entities in speculative equations, incarnated, as it were, as invisible parallel universes.

Dark matter and dark energy are more directly motivated by observations of the real world. Dark matter is apparently needed to account for the gravitational effects that seem to come from parts of space where no ordinary matter is visible, or not enough to explain the tug. For example, rotating galaxies seem to have some additional source of gravitational attraction, beyond the visible stars and gas, that stops them from flying apart. The “lensing” effect where distant astrophysical objects get distorted by the gravitational warping of spacetime also seems to demand this invisible form of matter. But dark matter does not exist in the usual sense, in that it has not been seen and there are no theories that can convincingly explain or demand its existence. Dark energy too is a kind of “stuff” required to explain the acceleration of the universe’s expansion, discovered by astronomers observing far-away objects in the mid-1990s. But it is just a name for a puzzle, without any direct detection.

It seems quite possible that dark energy, and perhaps dark matter too, will turn out to be like Crookes’ “dark space” and “radiant energy”: not exactly stuff, but symptoms of some hitherto unknown physical principle. These connections were exquisitely intuited by Philip Pullman in the His Dark Materials trilogy, where (the title alone gives a clue) a mysterious substance called Dust is an amalgam of dark matter and Barrett’s quasi-sentient psychomeres, given a spiritual interpretation by the scientist-priests of Pullman’s alternative steampunk Oxford University who sense its presence using instruments evidently based on Crookes’ light mill.

Scientists, of course, are not just making things up, while leaning on the convenience of supposed invisibility. They are using dark matter and dark energy, and (if one is charitable) quantum many-worlds and branes, and other imperceptible and hypothetical realms, to perform an essential task: to plug gaps in their knowledge with notions they can grasp.

These makeshift repairs and inventions are needed if science is not to be derailed or demoralized by its lacunae. When this happens, it seems inevitable the inventions will take familiar forms—they will be drawn from old concepts and even myths, they will be “mysterious” particles or rays or even entire imagined worlds, replete with inhabitants. These might turn out to be entirely the wrong concepts, but they make our ignorance concrete and enable us to think about how to explore it.

When working at the frontiers of knowledge, pondering questions of time and space that we can hardly intuit or visualize, cold logic and strict adherence to experiment and falsifiability might not be enough. But then the trick is to invent and imagine without losing sight of the fact that this is what we’re doing. We’ll probably find that our inventions are wrong, precisely because they tend to come from an old canon. But hopefully they’ll be wrong in a good way, and illuminate the path to a deeper understanding of the universe.





Philip Ball is the author of many books on science and its impacts on the broader culture, including Life’s Matrix: A Biography of Water; Universe of Stone; The Music Instinct; Curiosity: How Science Became Interested in Everything; and Serving the Reich. His book Critical Mass was awarded the 2005 Aventis Prize for Science Books. His next book is Invisible: The Dangerous Allure of the Unseen.