What Heinrich Rudolf Hertz taught us about nothingness
Heinrich Rudolf Hertz, who was honored Wednesday on his 155th birthday, helped explain how even nothing at all can be something.
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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.Skip to next paragraph
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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.