Measuring the Speed of Light

The measurement of the speed of light, a fundamental constant in physics, began with Ole Rømer in the 17th century, who used observations of Jupiter’s moons. Later, in the 19th century, Fizeau and Michelson devised more refined techniques involving mirrors and rapidly rotating gears. Eventually, in 1973, the definition of the meter was based on the speed of light, leading to the highly accurate value we use today: approximately 299,792,458 meters per second.

APA 7: ChatGPT. (2023, July 23). Measuring the Speed of Light. PerEXP Teamworks. [Article Link]

The speed of light, a fundamental constant in the universe, has captivated the minds of scientists and philosophers for centuries. The quest to determine the speed of light has led to groundbreaking discoveries and innovations in the field of physics. This article explores the concept of the speed of light, delving into the historical efforts made by scientists to measure this elusive cosmic constant. Specifically, we focus on Rømer’s determination of the speed of light, a milestone achievement that laid the foundation for modern methods of measurement.

What is the speed of light?

The speed of light is the velocity at which light travels through a vacuum. Designated by the symbol “c,” this constant plays a foundational role in numerous scientific disciplines, from optics and electromagnetism to cosmology and relativistic physics. Its value, approximately 299,792,458 meters per second (or about 186,282 miles per second), represents the ultimate speed limit in the universe.

How was the speed of light first measured?

In the 17th century, Ole Rømer, a Danish astronomer, made a groundbreaking discovery that forever changed our perception of the speed of light. Rømer was tasked with observing the moons of Jupiter, particularly Io, as it orbited the gas giant. He noticed that the time intervals between successive eclipses of Io by Jupiter varied significantly throughout the year.

Rømer astutely reasoned that these variations in eclipse timings were due to the changing distance between Earth and Jupiter as they orbited the Sun. He hypothesized that light took a finite amount of time to travel from Jupiter to Earth, causing the observed time lag in eclipse occurrences.

Rømer’s determination of the speed of light

By carefully analyzing his observations and considering the geometry of the Earth-Jupiter system, Rømer estimated that light took approximately 22 minutes to traverse the distance between the two planets. This measurement marked the first successful attempt to determine the speed of light, providing a value of approximately 220,000 kilometers per second, which is remarkably close to the currently accepted value of about 299,792 kilometers per second.

Rømer’s pioneering work revolutionized the field of astronomy and established the concept of the finite speed of light, paving the way for future scientific investigations and advancements in the understanding of the universe.

Roemer made the first good estimate of the speed of light by measuring the speed of light using timing eclipses of Jupiter’s moon Io. He timed eclipses of Jupiter’s moon Io and measured the speed of light. In this figure, S is the Sun, E1 is the Earth when closest to Jupiter (J1), and E2 is the Earth about six months later, on the opposite side of the Sun from Jupiter (J2). When the Earth is at E2, the light from the timing system has to travel an extra distance, represented by the orbit of the Earth, delay represented by the diameter of the Earth, approximately the diameter of the Earth’s orbit. (American Museum of Natural History)

The measurement of the speed of light stands as a remarkable milestone in the history of science. From the classical belief in instantaneous light propagation to the groundbreaking work of Ole Rømer, our understanding of the cosmic constant has evolved significantly. Rømer’s determination of the speed of light laid the foundation for modern methods of measurement, inspiring generations of scientists to explore the properties of light and delve into the mysteries of the cosmos. The quest to measure the speed of light continues to drive scientific curiosity, fueling cutting-edge research and discoveries that shape our understanding of the universe.

History of measuring the speed of light

  • Ancient Philosophical Debates: The concept of light and its speed has been a topic of fascination for ancient civilizations, such as the Greeks and the Egyptians. Early thinkers, including Empedocles and Alhazen, proposed speculative ideas about the nature of light and its motion, paving the way for later investigations.
  • Ole Rømer’s Landmark Discovery: In the late 17th century, Danish astronomer Ole Rømer made a groundbreaking contribution to the measurement of light’s speed. Through meticulous observations of Jupiter’s moon Io, Rømer noted discrepancies in its eclipses, which he attributed to the finite speed of light. His calculations provided the first estimation of light’s speed at around 220,000 kilometers per second. Vector image redrawing of the diagram of Ole Roemer’s method to determine the speed of light by observing the eclipse (D) and reappearance (C) of (B) Jupiter’s moon Io from different locations (E, F, G, H, L, K) in Earth orbit around the Sun (A). (Wikipedia Turkish)
  • Fizeau and the First Laboratory Measurement: In the 19th century, French physicist Armand Fizeau conducted the first laboratory-based measurement of the speed of light. Utilizing a rapidly rotating toothed wheel and a distant mirror, Fizeau measured light’s speed as approximately 315,000 kilometers per second, a significant advancement in precision.
Apparatus used in the Fizeau experiment. In 1849 Armand Fizeau sent light pulses through a rotating toothed wheel. A distant mirror on the other side reflected the pulses back through gaps in the wheel. By rotating the wheel at a certain speed, each light pulse that went through a gap on the way out was blocked by the next tooth as it came around. Knowing the distance to the mirror and the speed of rotation of the wheel enabled Fizeau to obtain one of the earliest measurements of the speed of light. (Wikipedia English)
  • The Michelson Method and Electromagnetic Theory: In the late 19th century, American physicist Albert A. Michelson and American chemist Edward Morley refined the measurement technique using a rotating octagonal mirror and light beams. His experiments, in combination with the emergence of electromagnetic theory, led to an improved estimation of light’s speed at around “299,796 ± 4” kilometers per second.
Michelson and Morley’s interferometric setup, mounted on a stone slab that floats in an annular trough of mercury. On a basic level, the Michelson interferometer comprises a half-transparent mirror positioned at a 45° angle to a light beam. This arrangement divides the light into two equal parts, A and B. One part goes to a fixed mirror, while the other part is reflected to a movable mirror. The returning beams are also split into two by the half-transparent mirror. Consequently, two reduced light beams reach the screen, allowing for the observation of interference patterns by adjusting the position of the movable mirror. (Wikipedia English)
  • Modern Era: Einstein’s Relativity and Beyond: In the early 20th century, Albert Einstein’s theory of special relativity provided a new framework for understanding light’s behavior. According to Einstein’s theory, the speed of light is a constant, invariant for all observers, regardless of their relative motion. This profound concept revolutionized physics and deepened our understanding of the universe.
  • Contemporary Advances and Laser Interferometry: With advancements in technology, modern methods have further refined the measurement of light’s speed. Laser interferometry, which uses the interference of laser beams to measure tiny time intervals, has enabled unprecedented precision in determining the speed of light, yielding results consistent with Einstein’s theory. [299,792.458 (Exact)]


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