The Speed of Light: Constancy and Cosmos

8. Ambient Light

87 Before going further, we need to remind ourselves that the speed of light as it normally figures into physics is the speed of light in a vacuum. Light slows down as it moves through physical media like glass and water, and so its speed is not constant under all circumstances . When, however, light travels in vacuo, all inertial observers measure its speed at the same rate, and it is this speed—denoted by c in science—that is deemed a constant of nature. Light’s relation to a vacuum is emblematic of light’s nature, emblematic of something that flies beneath the radar of experience until striking other things. Then it flashes into experience, and, it seems, into existence. Empirically speaking, that flash is fully coincidental with light’s reality. We get no hint of light’s existence upon looking into a vacuum, even after shining light therein. But Light in a Vacuum Yet light, it seems, is always ready with another surprise. Ralph Baierlein, Newton to Einstein: The Trail of Light 88 · The Speed of Light add a material object to the vacuum and light immediately informs us of its own existence by visually announcing the existence of the object. To all appearances, light coexists as comfortably with the nothingness of the vacuum as it does with the somethingness of material entities. Consistent with its expansive nature and what we have learned about light in other contexts, light straddles another divide: in this case, the divide between somethingness and nothingness. Modern science has learned to shy away from sharp demarcations . Once waves and particles were starkly dichotomized, while space, time, matter, and energy all fell into different categories. Now these boundaries have been erased, and the Cartesian mind-matter divide is under assault. Not only that, but the once clean demarcation between being and non-being no longer commands assent. This is a point to which we shall return. First, however, let us assume that being is distinct from non-being and see where that lands us. Let us ponder the relation of something—light—to nothing—the classical (pre-quantum) vacuum of nineteenth-century physics. I propose that even before modern physics began to blur this divide there was, at least from an empirical standpoint, something quietly puzzling about Einstein’s dictum that the speed of light in a vacuum is constant in all reference frames. What would it mean to witness light moving in a vacuum? Trying to answer this question is another pathway into the realization that light is infinitely more than we know it to be. A Basic Incongruity For Einstein, the vacuum entailed the absence of the luminiferous ether or any other material entity. Nothing like the ether could support the propagation of light, for that would turn the speed of light over to the properties of a material medium, and this, in turn, would imply different relative speeds for different observers. Light would once again be ensnared by physical matter. But what does it mean for light to be completely free of physical matter, to exist in a vacuum? The light we see about us—reflected, scattered, refracted light—has been stopped, altered, and slowed by Light in a Vacuum · 89 material bodies and media. Like many scientific laws, light speed constancy seems an idealization not easily achieved amid everyday circumstances: no one has ever seen or made a perfect vacuum. Near-perfect vacuums do, however, exist in outer space, and so confirmation of Einstein’s second postulate is not difficult. An astronaut need only send a light beam toward a mirror and wait for its reflection . If she knows the distance to the mirror and can measure how long the beam takes to make its round trip, she can calculate its speed. And by varying her inertial motion from one experiment to the next, she can further establish that her own motion does not affect the speed of light—that speed is constant across all measurements . This is exactly what Einstein proposed. All this is straightforward, but let us note that the astronaut never actually sees light moving in a vacuum. She works from known facts to calculate its rate of motion, but she does not see it moving in intermediate space—the vacuum itself—as we might see a baseball moving through space. This limitation is not her fault or the fault of the experiment. It is inherent...