Today Google honors Heinrich Rudolf Hertz, the German physicist who, in his all-too-short career, taught the world invaluable lessons about optics, electromagnetism, and, in a contribution that is often overlooked, the science of nothingness.
"Horror vacui," goes the phrase, usually attributed to Aristotle's fourth book of Physics. Nature abhors a vacuum. True or not, it's certainly the case that those studying nature have long struggled with the concept of empty space. Aristotle thought that, because space empty of all matter offers no resistance, objects moving within it would move infinitely fast. Thus the objects surrounding any void would instantly fill it before it could form. Emptiness, he concluded, was therefore impossible. Every part of the universe must be filled with something, even if we can't detect it.
Aristotle's arguments persuaded scholars for a good 1,500 years or so. Medieval Christians were enjoined from entertaining the possibility of a vacuum, until the Catholic Church's Condemnations of 1277 broke Aristotle's monopoly on the natural sciences by admitting that, at the very least, a vacuum would not be beyond the powers of an omnipotent God.
But even though contemplating empty spaces became theologically permissible, the idea of nothingness still proved troubling to early modern thinkers, even as others were setting about constructing pumps and siphons. In the seventeenth century, when Irish chemist Robert Boyle demonstrated his "Pneumatical Engine" and when French physicist Blaise Pascal developed a barometer, they were attacked by Thomas Hobbes and René Descartes, who each embraced a philosophy known as plenism, which left no space for emptiness.
The plenists arguments were persuasive. Sure, they argued, you might be able to remove all the air from a glass tube, but how is it that, say, two magnets inside the tube will still attract one another, if there really is nothing at all between them? How is it that electric fields can pass through the tube?
In the 19th century, after scientists firmly established that light travels in waves, scientists wondered how waves of light from the stars could ever reach the earth after traversing millions of miles of allegedly empty space. A wave, after all, needs something to ripple through, right?
Hertz initially complicated the picture even further, but his work also foretold a way out. While attempting to demonstrate the theories of Scottish physicist James Clerk Maxwell he conclusively demonstrated the existence of electromagnetic waves, and then caught a glimpse of how these waves act in very un-wavelike ways.
The Monitor's Chris Gaylord describes Hertz's famous experiment:
In his lab, the German scientist rigged up two tiny brass spheres, placed very close to one another. When he electrified them, sparks leaped from one ball to the other. If Maxwell was correct, these sparks should send invisible waves radiating through the air. To test the theory, he needed to build a receiver. This second instrument consisted of a curved wire that almost made a full circle, except for a tiny gap at the top. He placed the transmitter and the receiver several yards apart and made sure that nothing connected the two. Sure enough, when sparks shot through the transmitter, invisible waves traveled through the air, lighting up new sparks on the receiver.
Later on, Hertz measured the speed of electromagnetic radiation, confirming Maxwell's calculations that it was the same as that of light.
To Maxwell, this was more than a coincidence. "We can scarcely avoid the conclusion," wrote Maxwell, "that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena."
But what medium, exactly, is doing the undulating?
To answer this, scientists borrowed an idea from the ancient Greeks. Empty space, they reasoned, must be completely filled with a transparent, non-dispersive substance. This substance had to be fluid enough so that the Earth could travel through it without slowing, but rigid enough to vibrate at high enough frequencies to carry light waves. Maxwell dubbed this mysterious stuff the "luminiferous aether."
But just after Hertz was using the luminiferous aether to link together the seemingly disparate phenomena of light, electricity, and magnetism, others were busy undermining it. Working in the 1880s at what is now Case Western Reserve University the American scientists Albert Michelson and Edward Morley reasoned that, if the Earth was moving through an aethereal substance, we should be able to detect an "aether wind," which would cause light waves to travel at slightly different speeds, depending on the time, season and the direction of the light waves. But, after a set of careful measurements, the pair found that the speed of light was unaffected by these factors.
But if there was no aether, then how did electromagnetic waves propagate?
A satisfactory answer wasn't put forth until 1905, the year that Albert Einstein upended classical physics with a series of groundbreaking papers. First, Einstein's theory of special relativity removed the need for a static, absolute reference frame through which objects and waves could move. Special relativity does away with the twin Newtonian absolutes of space and time, replacing it with a single absolute: the speed of light.
Second was Einstein's photoelectric effect. Hertz was actually among the first people to notice that sparks jumped across the gap in his receiver more readily when it was exposed to ultraviolet light. Exposing it to more ultraviolet light made it even easier for the sparks to fly.
That light could electrify metal was not, by itself, surprising. But what was odd was that the color of the light, not its brightness. Shine a bright red lamp on a brick of potassium, and you won't get a current. But a dim blue light will do the trick. This doesn't fit in with the notion that light is a wave. Despite studying the phenomenon intensely for six months, Hertz never figured it out.
But Einstein did. By imagining light not as a wave, but as a particle carrying discrete packets of energy, which he called "quanta," Einstein found that he could predict how certain frequencies of light would electrify certain metals. Einstein's explanation of the photoelectric effect won him the Nobel Prize in physic in 1921, and helped usher in the era of quantum physics.
So now we understand light, and all electromagnetic radiation, as having a dual role of both wave and particle. Electromagnetic radiation, including light, travels as a wave, but arrives as a particle, and there's no need to invoke any mysterious aethers.
Or is there? Einstein himself continued to use the word, particularly when attempting to describe how gravity acted on distant objects. And the quantum mechanical conception of vacuums are anything but empty: they contain ephemeral particles that pop in and out of existence, and even fleeting electromagnetic waves. Once you get to a very small scale, the universe starts too look a little more like Aristotle and the other plenists imagined it.
Take a look at this Scale of the Universe animation, created by a pair of extremely precocious 14-year-old twins named Cary and Michael Huang earlier this month. Zoom in, past the penny, past the matchstick, past the paramecium and the DNA molecule. Keep zooming. Go past the gamma ray and the proton and the neutron. Go past the quarks and the neutrinos. Eventually, you'll get to a whole lot of nothing.
In fact, most of what we take to be solid matter actually consists of empty space. If you imagine an atom the size of a cathedral, its nucleus would be roughly the size of a fly. Thanks to electromagnetism, in this case the tendency for electrons to repel each other, everything doesn't collapse in on itself. You may think that you are sitting in a chair right now, but you are actually hovering above it at a distance of one angstrom, about 250 millionths of an inch. Neither your electric field nor that of the chair wants to get any closer.
Anyway, keep zooming in. Eventually, you'll get to the Planck Length, which is what physicists say is the smallest unit of measurement in the universe. At anything smaller than this distance, it would be impossible to tell the difference between two locations.
At this scale, physics is really weird. "Virtual" particles are flashing in and out of existence at extremely high energies, warping space and time into a quantum "foam," or so one theory goes. One-dimensional strings, according to another theory, vibrate in eleven dimensions, forging and maintaining the very fabric of our reality.
Now zoom all the way out. All the way, past the planets, galaxies, and nebulae, until you get to our entire, expanding, universe. What is the universe expanding into? Nothing at all, according to the best current cosmological models. What was there before the universe? Was that nothing too?
The ancient Greeks were fond of another phrase about nothingness: It comes to us via the Latin expression "Ex nihilo nihil fit," or "nothing comes from nothing." They believed that the gods fashioned the universe out of a primeval matter, which they called "chaos."
Today, as cosmologists try to explain how our universe sprang from nothing, it's worth remembering that, in science, nothing is not what it seems.