Light, a fundamental aspect of our universe, plays a critical role in various phenomena, from enabling us to see to influencing our understanding of space and time. The speed at which light travels is a constant that has fascinated scientists and thinkers for centuries. But What Does Light Travel Fastest Through? Understanding this question requires exploring the nature of light, the media it traverses, and the historical experiments that have shaped our knowledge.
Abstract, futuristic image of blue light streaks radiating outward, giving the impression of rapid movement or traveling at high speed, inspired by the concept of faster-than-light travel
The Speed of Light: A Universal Constant
The speed of light in a vacuum is exactly 299,792,458 meters per second (approximately 186,282 miles per second). Represented by the symbol “c,” it is a universal constant, a cornerstone of modern physics thanks to Albert Einstein’s theory of special relativity. This theory postulates that nothing in the universe can surpass this speed, establishing it as a cosmic speed limit. As an object approaches the speed of light, its mass increases infinitely, requiring an infinite amount of energy to accelerate further, hence making it unattainable.
Defining Measurements
The immutability of light speed is so significant that it serves as a basis for defining international standard measurements. According to the National Institute of Standards and Technology (NIST), the meter (and consequently, the mile, foot, and inch) are defined using the speed of light. It even plays a role in defining the kilogram and the Kelvin temperature unit through complex equations.
Light-Year: Measuring Cosmic Distances
To comprehend the vast distances in our universe, astronomers use the term “light-year.” A light-year is the distance light travels in one year, roughly 6 trillion miles (10 trillion kilometers). Light from the moon reaches our eyes in about 1 second, meaning the moon is approximately 1 light-second away. Sunlight takes about 8 minutes to reach us, placing the sun at 8 light-minutes distance. The nearest star system, Alpha Centauri, is 4.3 light-years away, implying its light takes 4.3 years to reach Earth.
NASA’s Glenn Research Center illustrates the immensity of a light-year by comparing it to laying the Earth’s circumference in a straight line 31.6 million times.
Interstellar Travel Times
Traveling one light-year is an immense undertaking. An airplane traveling at 600 mph would take a million years to cover that distance. Even with a spacecraft like the Apollo lunar module, it would still take about 27,000 years to travel a single light-year.
Astronomers study distant objects by observing the light they emitted long ago, providing a glimpse into the universe’s past. Light from objects 10 billion light-years away shows them as they existed 10 billion years ago, shortly after the Big Bang.
Light’s Speed in Different Media
While the speed of light in a vacuum is constant, it changes when light travels through different materials. Light slows down when it interacts with the particles in a medium, a phenomenon described by the refractive index of the material.
A sparkling diamond amongst dark coal-like rock.
Examples of Light Slowing Down
- Air: Light travels almost as fast in Earth’s atmosphere as in a vacuum, slowing down by only about 0.03%.
- Water: Light slows to approximately 225,000 kilometers per second (140,000 miles per second).
- Glass: Light decelerates to roughly 200,000 kilometers per second (124,000 miles per second).
- Diamond: Light slows down to less than half its speed in a vacuum, around 124,000 km/s (277 million mph).
Light can even be trapped or stopped in ultra-cold clouds of atoms. Researchers have explored methods to halt light at “exceptional points” where separate light emissions intersect and merge. In 2015, scientists managed to slow down a single photon even in a vacuum, demonstrating the potential to manipulate light speed under controlled conditions.
Historical Attempts to Measure Light Speed
The quest to measure the speed of light spans centuries, with contributions from numerous scientists and philosophers.
Galileo Galilei is credited with discovering the first four moons of Jupiter.
Early Ideas
- Ancient Greece: Empedocles (5th century BC) suggested that light travels at a finite speed, while Aristotle believed it was instantaneous.
- Galileo Galilei (1600s): Galileo attempted to measure light speed using lanterns on hills, but his experiment only concluded that light travels at least 10 times faster than sound.
Key Experiments
- Ole Rømer (1670s): Danish astronomer Ole Rømer estimated light speed by observing eclipses of Jupiter’s moon Io. He noticed variations in eclipse timings based on Earth’s distance from Jupiter, concluding that light takes time to travel between the planets. His calculation was around 200,000 km/s (124,000 miles per second).
- James Bradley (1728): English physicist James Bradley based his calculations on the change in the apparent position of stars caused by Earth’s movement around the sun. He estimated the speed of light at 301,000 km/s (185,000 miles per second), accurate to within 1%.
- Hippolyte Fizeau (mid-1800s): Fizeau used a rotating toothed wheel to measure the time it took for light to travel to a mirror 5 miles away and back, allowing him to calculate its speed.
- Leon Foucault (mid-1800s): Foucault used a rotating mirror to perform a similar experiment, achieving results close to Fizeau’s findings.
- Albert A. Michelson (late 1800s – early 1900s): Michelson refined Foucault’s method, increasing the distance between mirrors and using high-quality optics. His initial result was 299,910 km/s (186,355 miles per second). Later, he built a mile-long vacuum tube to minimize air effects, obtaining an even more precise measurement.
Dr. Albert A. Michelson stands next to a large tube supported by wooden beams.
The Michelson-Morley Experiment
Michelson, along with Edward Morley, conducted an experiment to detect the “luminiferous aether,” a hypothetical medium thought to carry light waves. The experiment failed to detect the aether, proving that light can travel through a vacuum. This “non-discovery” earned Michelson a Nobel Prize and revolutionized our understanding of light.
Faster Than Light? Expansion of the Universe
While the speed of light is a local speed limit, the universe itself expands faster. The expansion rate is about 68 kilometers per second for each megaparsec (3.26 million light-years) of distance. This means that galaxies at vast distances recede from us at speeds exceeding the speed of light due to the expansion of space itself.
General relativity allows this behavior because it applies to large-scale phenomena, whereas special relativity governs local interactions.
Can We Travel Faster Than Light?
The concept of faster-than-light (FTL) travel is a staple of science fiction. While special relativity prohibits objects with mass from reaching or exceeding light speed, theoretical physicists explore possibilities to circumvent this limitation. One idea involves manipulating spacetime, such as creating a warp drive that folds space around a spacecraft.
Without FTL travel, interstellar journeys would take immense amounts of time, making many science fiction scenarios impossible.
Conclusion
So, what does light travel fastest through? The answer is a vacuum. Understanding the speed of light, the media it traverses, and its role in the universe is crucial for our exploration of space and our comprehension of the fundamental laws of physics. From historical experiments to the theoretical possibilities of faster-than-light travel, the study of light continues to inspire and challenge scientists and science fiction enthusiasts alike.
Additional Resources
- Visualize light speed with this tool from Academo.
- Learn about universal constants at the National Institute of Standards and Technology.
- Read “Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light” (Oxford, 2019) by John C. H. Spence for more history.
Bibliography
Aristotle. “On Sense and the Sensible.” The Internet Classics Archive, 350AD. http://classics.mit.edu/Aristotle/sense.2.2.html.
D’Alto, Nick. “The Pipeline That Measured the Speed of Light.” Smithsonian Magazine, January 2017. https://www.smithsonianmag.com/air-space-magazine/18_fm2017-oo-180961669/.
Fowler, Michael. “Speed of Light.” Modern Physics. University of Virginia. Accessed January 13, 2022. https://galileo.phys.virginia.edu/classes/252/spedlite.html#Albert%20Abraham%20Michelson.
Giovannini, Daniel, Jacquiline Romero, Václav Potoček, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, and Miles J. Padgett. “Spatially Structured Photons That Travel in Free Space Slower than the Speed of Light.” Science, February 20, 2015. https://www.science.org/doi/abs/10.1126/science.aaa3035.
Goldzak, Tamar, Alexei A. Mailybaev, and Nimrod Moiseyev. “Light Stops at Exceptional Points.” Physical Review Letters 120, no. 1 (January 3, 2018): 013901. https://doi.org/10.1103/PhysRevLett.120.013901.
Hazen, Robert. “What Makes Diamond Sparkle?” PBS NOVA, January 31, 2000. https://www.pbs.org/wgbh/nova/article/diamond-science/.
“How Long Is a Light-Year?” Glenn Learning Technologies Project, May 13, 2021. https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/how_long_is_a_light_year.htm.
American Physical Society News. “July 1849: Fizeau Publishes Results of Speed of Light Experiment,” July 2010. http://www.aps.org/publications/apsnews/201007/physicshistory.cfm.
Liu, Chien, Zachary Dutton, Cyrus H. Behroozi, and Lene Vestergaard Hau. “Observation of Coherent Optical Information Storage in an Atomic Medium Using Halted Light Pulses.” Nature 409, no. 6819 (January 2001): 490–93. https://doi.org/10.1038/35054017.
NIST. “Meet the Constants.” October 12, 2018. https://www.nist.gov/si-redefinition/meet-constants.
Ouellette, Jennifer. “A Brief History of the Speed of Light.” PBS NOVA, February 27, 2015. https://www.pbs.org/wgbh/nova/article/brief-history-speed-light/.
Shea, James H. “Ole Ro/Mer, the Speed of Light, the Apparent Period of Io, the Doppler Effect, and the Dynamics of Earth and Jupiter.” American Journal of Physics 66, no. 7 (July 1, 1998): 561–69. https://doi.org/10.1119/1.19020.
Siegel, Ethan. “The Failed Experiment That Changed The World.” Forbes, April 21, 2017. https://www.forbes.com/sites/startswithabang/2017/04/21/the-failed-experiment-that-changed-the-world/.
Stern, David. “Rømer and the Speed of Light,” October 17, 2016. https://pwg.gsfc.nasa.gov/stargaze/Sun4Adop1.htm.
