Gravitational radiation reminds science of its debts to the physics of waves
Physics fans are a lot like surfers. Both think waves are really fun.
For surfers, it’s all about having a good time. For physicists, it’s about understanding some of nature’s most important physical phenomena. Yet another detection of gravitational waves, announced June 1, further reinvigorates the world’s science fans’ excitement over waves.
Waves have naturally always been a topic of scientific and mathematical interest. They play a part in an enormous range of physical processes, from heat and light to radio and TV, sonograms and music, earthquakes and holograms. (Waves used to even be a common sight in baseball stadiums, but fans got tired of standing up and down and it was really annoying anyway.)
Many of science’s greatest achievements have been discoveries of new kinds of waves or new insights into wave motion. Identifying just the Top 10 such discoveries (or ideas) is therefore difficult and bound to elicit critical comments from cult members of particular secret wave societies. So remember, if your favorite wave isn’t on this list, it would have been No. 11.
10. Thomas Young: Light is a wave.
In the opening years of the 19th century, the English physician Young tackled a long-running controversy about the nature of light. A century earlier, Isaac Newton had argued forcibly for the view that light consisted of (very small) particles. Newton’s contemporary Christiaan Huygens strongly disagreed, insisting that light traveled through space as a wave.
Through a series of clever experiments, Young demonstrated strong evidence for waves. Poking two tiny holes in a thick sheet of paper, Young saw that light passing through created alternating bands of light and darkness on a surface placed on the other side of the paper. That was just as expected if light passing through the two holes interfered just as water waves do, canceling out when crest met trough or enhancing when crests met “in phase.” Young did not work out his wave theory with mathematical rigor and so Newton’s defenders resisted, attempting to explain away Young’s results.
But soon Augustin Jean Fresnel in France worked out the math of light waves in detail. And in 1850, when Jean-Bernard-Léon Foucault showed that light travels faster in air than water, the staunchest Newton fans had to capitulate. Newton himself would have acknowledged that light must therefore consist of waves. (Much later, though, Einstein found a way that light could in fact consist of particles, which came to be called photons.)
9. Michelson and Morley: Light waves don’t vibrate anything.
Waves are vibrations, implying the need for something to vibrate. Sound vibrated molecules in the air, for instance, and ocean waves vibrated molecules of water. Light, supposedly, vibrated an invisible substance called the ether.
In 1887, Albert A. Michelson and his collaborator Edward Morley devised an experiment to detect that ether. Earth’s motion through the ether should have meant that light’s velocity would depend on its direction. (Traveling with the Earth’s motion, light’s speed wouldn’t be the same as traveling at right angles to the direction of motion.) Michelson and Morley figured they could detect that difference by exploiting the interference phenomena discovered by Young. But their apparatus failed to find any ether effect. They thought their experiment was flawed. But later Einstein figured out there actually wasn’t any ether.
8. James Clerk Maxwell: Light is an electromagnetic wave.
Maxwell died in 1879, the year Einstein was born, and so did not know there wasn’t an ether. He did figure out, though, that both electricity and magnetism could be explained by stresses in some such medium.
Electric and magnetic charges in the ether ought to generate disturbances in the form of waves, Maxwell realized. Based on the strengths of those forces he calculated that the waves would travel at the fantastic speed of 310 million meters per second, suspiciously close to the best recent measurements of the speed of light (those measurements ranged from 298 million to 315 million meters per second). So Maxwell, without the benefit of ever having watched NCIS on TV, then invoked Gibbs’ Rule 39 (there’s no such thing as a coincidence) and concluded that light was an example of an electromagnetic wave.
“It seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field,” he wrote in 1864. His “other radiations, if any” turned out to be an entire spectrum of all sorts of cool waves, from gamma radiation to radio signals.
7. Heinrich Hertz: Radio waves.
Not very many people took Maxwell seriously at first. A few, though, known as the Maxwellians, promoted his ideas. One physicist who had faith in Maxwell, or at least in his equations, was Hertz, who performed experiments in his lab in Karlsruhe, Germany, that successfully produced and detected radio waves, eventually to be exploited by propagandists to spread a lot of illogical nonsense on talk radio.
His success inspired much more respect for the equations in Maxwell’s theory, which Hertz found almost magical: “It is impossible to study this wonderful theory without feeling as if the mathematical equations had an independent life and an intelligence of their own, as if they were wiser than ourselves,” Hertz said. His prime experimental success came in 1887, the same year that Michelson and Morley failed to detect the ether. Hertz died in 1894, long before his discovery was put to widespread use.
6. John Michell: Seismic waves.
Michell, an English geologist and astronomer, was motivated by the great Lisbon earthquake of 1755 to investigate the cause of earthquakes. In 1760 he concluded that “subterraneous fires” should be blamed, noting that volcanoes — “burning mountains” — commonly occur in the same neighborhood as frequent earthquakes.
Michell noted that “the motion of the earth in earthquakes is … partly propagated by waves, which succeed one another sometimes at larger and sometimes at smaller distances.” He cited witness accounts of quakes in which the ground rose “like the sea in a wave.” Much later seismologists developed a more precise understanding of the seismic waves that shake the Earth, using them as probes to infer the planet’s inner structure.
5. Wilhelm Röntgen: X-rays.
When Hertz discovered radio waves, he knew he was looking for the long-wavelength radiation foreshadowed in Maxwell’s equations. But a few years later, in 1895, Röntgen found the radio wave counterpart of the opposite end of the electromagnetic spectrum — by accident.
Mysterious short-wavelength rays of an unknown type (therefore designated X) emerged when Röntgen shot cathode rays (beams of electrons) through a glass tube. Röntgen suspected that his creation might be a new kind of wave among the many Maxwell had anticipated: “There seems to exist some kind of relationship between the new rays and light rays; at least this is indicated by the formation of shadows,” Röntgen wrote. Those shadows, of course, became the basis for a revolutionary medical technology.
Besides providing a major new tool for observing shattered bones and other structures inside the body, X-rays eventually became essential tools for scientific investigation in astronomy, biology and other fields. And they shattered the late 19th century complacency of physicists who thought they’d basically figured everything out about nature. Weirdly, though, X-rays later turned out to be particles sometimes, validating Einstein’s ideas that light had an alter ego particle identity. (By the way, it turned out that X-rays aren’t the electromagnetic waves with the shortest wavelengths — gamma rays can be even shorter. Maybe they would be No. 11.)
4. Epicurus: The swerve.
Not exactly a wave in the ordinary sense, the swerve was a deviation from straight line motion postulated by the Greek philosopher Epicurus around 300 B.C. Unlike Aristotle, Epicurus believed in atoms, and argued that reality was built entirely from the random collisions of an infinite number of those tiny particles. Supposedly, he thought, atoms would all just fall straight down to the center of the universe unless some unpredictable “swerve” occasionally caused them to deviate from their paths so they would bounce off each other and congregate into complex structures.
It has not escaped the attention of modern philosophers that the Epicurean unpredictable swerve is a bit like the uncertainty in particle motions introduced by quantum mechanics. Which has its own waves.
3. Louis de Broglie: Matter waves.
In the early 1920s, de Broglie noticed a peculiar connection between relativity and quantum physics. Max Planck’s famous quantum formula related energy to frequency of a wave motion. Einstein’s special relativity related energy to the mass of a particle. De Broglie thought it would make a fine doctoral dissertation to work out the implications of two seemingly separate things both related to energy. If energy equals mass (times the speed of light squared) and energy equals frequency (time Planck’s constant), then voilà, mass equals frequency (times some combination of the constants). Therefore, de Broglie reasoned, particles (of mass) ought to also exist as waves (with a frequency).
That might have seemed wacky, but Einstein read de Broglie’s thesis and thought it made sense. Soon Walter Elsasser in Germany reported experiments that supported de Broglie, and in America Clinton Davisson and coworkers demonstrated conclusively that electrons did in fact exhibit wave properties.
De Broglie won the physics Nobel Prize in 1929; Davisson shared the 1937 Nobel with George Thomson, who had conducted similar experiments showing electrons are waves. Which was ironic, because George’s father, J.J. Thomson, won the 1906 Nobel for the work that revealed the existence of the electron as a particle. Eight decades later Ernst Ruska won a Nobel for his design of a powerful microscope that exploited the electron’s wave behavior.
2. Max Born: Probability waves.
De Broglie’s idea ignited a flurry of activity among physicists trying to figure out how waves fit into quantum theory. Niels Bohr, for instance, spent considerable effort attempting to reconcile the dual wave-particle nature of both electrons and light. Erwin Schrödinger, meanwhile, developed a full-fledged “wave mechanics” to describe the behavior of electrons in atoms solely from the wave perspective. Schrödinger’s math incorporated a “wave function” that was great for calculating the expected results of experiments, even though some experiments clearly showed electrons to be particles.
Born, a German physicist and good friend of Einstein’s, deduced the key to clarifying the wave function: It was an indicator of the probability of finding the particle in a given location. Combined with Werner Heisenberg’s brand-new uncertainty principle, Born’s realization led to the modern view that an electron is wavelike in the sense that it does not possess a definite location until it is observed. That approach works fine for all practical purposes, but physicists and philosophers still engage in vigorous debates today about the true physical status of the wave function.
1. LIGO: Gravitational waves.
Soon after he completed his general theory of relativity, Einstein realized that it implied the possibility of gravitational radiation — vibrations of spacetime itself. He had no idea, though, that by spending a billion dollars, physicists a century later could actually detect those spacetime ripples. But thanks to lasers (which maybe would have been No. 11), the Laser Interferometer Gravitational-Wave Observatory — two huge labs in Louisiana and Washington state — captured the spacetime shudders emitted from a pair of colliding black holes in September 2015.
That detection is certainly one of the most phenomenal experimental achievements in the history of science. It signaled a new era in astronomy, providing astronomers a tool for probing the depths of the universe that are obscured from view with Maxwell’s “other radiations, if any.” For astronomy, gravitational radiation is the wave of the future.
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