Who can keep up? What is the speed of light, and why is it so fast?
- Mar 20
- 14 min read
Updated: Mar 21
We all know that the speed of light is ultimately the fastest thing in the universe. But why? Why is light so fast, can this speed be surpassed, or could we one day physically harness it? Perhaps. At least today we have enough information to answer questions like what is the speed of light? Buckle up, because we're about to explore what the speed of light is, why it's so fast, and much more!
Contents
What is the speed of light?
The speed of light is the highest speed limit that an object or particle can reach within the framework of the physical laws that govern our universe.
What is the speed of light in km/h?
In a perfect vacuum, the speed of light is 299,792,458 meters per second, or 300,000 km per second.
How many times faster is the speed of light than the speed of sound?
The speed of light is 874,635 times faster than the speed of sound. The speed of sound is 343 m/s, while the speed of light is 300,000,000 m/s. This is why when lightning strikes on a rainy day, we hear the light first, and then the thunder seconds later. Sound travels a little slower than light.
Why is the speed of light so fast?
Light both has and doesn't have mass; that's why it travels so fast. Light is composed of photons, the smallest particles ever observed and is electromagnetic radiation in the form of photons that functions like a wave. Photons actually have no mass, but their high speed gives them a kind of mass due to their kinetic energy. This is why black holes can swallow light!
Furthermore, everything in the universe moves within a field called the "Higgs field," and within this field are particles called "Higgs bosons." Every particle in the universe interacts with bosons in the Higgs field and, as a result, gains mass. Photons (light) do not interact with Higgs bosons and therefore do not gain any mass.
How does light move?
Light is a form of energy composed of both waves and particles. Photons travel along a narrow path and continue to do so until they strike an object.
Who discovered the movement of light first?
Christiaan Huygens developed the mathematical wave theory of light in 1678 and published it in his work Treatise on Light in 1690. He proposed that light propagates in all directions as a series of waves in a medium.
Who discovered the speed of light?
In 1676, Ole Roemer was the first to measure the speed of light and, through his calculations, determined that light does not travel instantaneously. Roemer was compiling observations of the orbit of Io, a moon of Jupiter. By timing Io's eclipses by Jupiter, Roemer hoped to determine a more accurate value for the moon's orbital period. However, he noticed that the time interval between successive eclipses shortened as the Earth moved closer to Jupiter and lengthened as the Earth moved further away from Jupiter. Roemer realized this was due to the speed of light and estimated that light needed 22 minutes to traverse the diameter of Earth's orbit. This allowed him to make the first quantitative estimate of the speed of light, which proved to be an accurate estimate.
The first person to perform the arithmetic calculation was the Dutch scientist Christiaan Huygens, who found a value of 210,824,064 meters per second for the speed of light. However, the correct value is 299,792,458 meters per second. The difference stems from errors in Roemer's estimate of the maximum time delay and inaccurate information regarding the diameter of Earth's orbit. However, the fact that Roemer's data provided the first quantitative estimate for the speed of light and proved correct opened a major door for us.
History and Results of Measurements of the Speed of Light
Year | Person | Technical | Speed (km/h) | Margin of Error |
Before 1638 | Galileo Galilei | Torch | Inconclusive | |
Before 1667 | Accademia del Cimento | Torch | Inconclusive | |
1675 | Rømer and Huygens | Satellite Observation | 220,000 | 27% less |
1729 | James Bradley | The Deflection of Light | 301,000 | 0.40% more |
1849 | Hippolyte Fizeau | Gear wheel | 315,500 | 5.1% more |
1862 | Léon Foucault | Rotating Mirrors | 298,000 | 0.60% less |
1907 | Rosa and Dorsey | Electromagnetic Constants | 299,710 | 0.0999% less |
1926 | Albert A. Michelson | Rotating Mirrors | 299,796 | 12 ppm excess |
1950 | Essen and Gordon-Smith | Microwave Cavity | 299,792.50 | 0.14 ppm excess |
1958 | Keith Davy Froome | Radio Interferometry | 299,792.50 | 0.14 ppm excess |
1972 | Kenneth Evenson | Laser Interferometry | 299,792.4562 | 0.006 ppm less |
1983 | 17th General Conference on Weights and Measures (CGPM) | Definition of a Meter | 299,792,458 | Full measurement |
PPM = Parts-per million (a unit of measurement used for parts per million)
Techniques Used to Measure the Speed of Light
Throughout history, scientists have used various techniques to measure the speed of light:
Lighthouse Observation
Satellite Observation
Light Aberration
Gear and Rotating Mirror Technique
Electromagnetic Constants
Microwave Cavity
Interferometer
Definition of a Meter
Now let's briefly detail how these techniques are applied.
Lighthouse Observation
In 1638, Galileo Galilei proposed an experiment he claimed to have performed several years earlier, aiming to measure the speed of light by observing the delay between the exposure of a lantern and its detection from a certain distance. He could not discern whether the journey of light was instantaneous, but concluded that if it was not, it must be extraordinarily fast.
In 1667, the Accademia del Cimento in Florence reported that Galileo had conducted his experiment with the lanterns approximately one mile apart, but no delay was observed. The actual delay in this experiment would have been approximately 11 microseconds.
Satellite Observation
The first quantitative estimate of the speed of light was made by Ole Rømer in 1676. Based on his observation that the periods of Jupiter's innermost moon, Io, are shorter when Earth approaches Jupiter than when it moves away from it, he concluded that light travels at a finite speed and estimated that it takes 22 minutes for light to traverse the diameter of Earth's orbit. Christiaan Huygens combined this estimate with an estimate of the diameter of Earth's orbit, arriving at a speed of light estimate of 220,000 km/s, 27% lower than the actual value.
Light Aberration
Light aberration was discovered by James Bradley in the 18th century. This effect arises from the vectorial addition of the speed of light from a distant source (such as a star) and the speed of the observer.
A moving observer would see the light coming from a slightly different direction, and consequently, would see the source in a position shifted from its original location. Because the direction of Earth's velocity constantly changes as the Earth revolves around the Sun, this effect causes the apparent positions of the stars to shift. From the angular difference in the positions of the stars, it is possible to express the speed of light in terms of Earth's velocity around the Sun; this can then be converted into the known length of a year, the time it takes to travel from the Sun to Earth.
Using this method in 1729, Bradley found that light travels 10,210 times faster in its orbit than it does from Earth (actually 10,066 times), or equivalently, that light takes 8 minutes and 12 seconds to travel from the Sun to Earth.
Gear and Rotating Mirror Technique
This technique is used to measure the time it takes for light to travel to and from a mirror at a known distance. This is the working principle behind the experiments of Hippolyte Fizeau and Léon Foucault.
The apparatus used by Fizeau consists of a beam of light directed at a mirror 8 kilometers away. As the beam travels from the source to the mirror, it passes through a rotating gear wheel. At a given rotational speed, the beam passes through a gap on its exit and another on its return, but at slightly higher or lower speeds, the beam strikes a tooth and does not pass through the wheel. The speed of light can be calculated by knowing the distance between the wheel and the mirror, the number of teeth on the wheel, and its rotational speed.
Foucault's method replaces the wheel with a rotating mirror. As light travels to and from the distant mirror, the mirror continues to rotate, causing the light to reflect off it at a different angle than when it exits the rotating mirror. From this angle difference, the known rotational speed, and the distance to the distant mirror, the speed of light can be calculated. Based on a suggestion by François Arago, Foucault used this device to measure the speed of light in air and water.
Electromagnetic Constants
One option for deriving c, which is not directly related to the measurement of electromagnetic wave propagation, is to use the relationship between c and the vacuum permeability ε0 and vacuum permeability μ0 determined by Maxwell's theory: c² = 1/(ε0μ0).
While vacuum permeability can be determined by measuring the capacitance and dimensions of a capacitor, the value of vacuum permeability is precisely fixed as 4π×10⁻⁷ H⋅m⁻¹ through the definition of amperage. Rosa and Dorsey used this method in 1907 to obtain a value of 299.710 ± 22 km/s. Their method relied on the "international ohm," a standard unit of electrical resistance, and therefore its accuracy was limited by how this standard was defined.
Microwave Cavity
Another way to measure the speed of light is to independently measure the frequency f and wavelength λ of an electromagnetic wave in a vacuum. The value of c can then be found using the relation c = fλ. Another option is to measure the resonance frequency of a cavity resonator. If the dimensions of the resonance cavity are also known, these can be used to determine the wavelength of the wave. In 1946, Louis Essen and AC Gordon-Smith determined the frequency of a microwave cavity with precisely known dimensions for various normal microwave modes. Since the wavelengths of the modes are known from the geometry of the cavity and electromagnetic theory, knowledge of the associated frequencies opened the door to calculating the speed of light.
The Essen-Gordon-Smith result of 299.792±9 km/s was significantly more precise than those obtained using optical techniques. By 1950, repeated measurements by Essen yielded a result of 299.792.5±3.0 km/s.
Interferometer
Interferometry is a method used to determine the wavelength of electromagnetic radiation in order to determine the speed of light. Before laser technology, coherent radio sources were used for interferometric measurements of the speed of light. Interferometric determination of wavelength becomes less sensitive with increasing wavelength, and therefore experiments were limited in sensitivity with the long wavelength (~4 mm) of radio waves. Sensitivity can be increased by using light with a shorter wavelength, but this makes it difficult to directly measure the frequency of the light.
One way to overcome this problem is to start with a low-frequency signal whose frequency can be precisely measured, and from this signal, synthesize higher-frequency signals whose frequency can then be coupled to the original signal. A laser can then be locked onto the frequency, and the wavelength can be determined using interferometry. This technique was later developed thanks to a group at the National Institute of Standards and Technology.
Definition of a Meter
At the 17th meeting of the General Conference on Weights and Measures (CGPM) in 1983, it was determined that wavelengths obtained from frequency measurements and a given value for the speed of light were more repeatable than the previous standard. They retained the 1967 definition of the second, so that the frequency of cesium hyperfin would now define both the second and the meter. To do this, they redefined the meter as "the length of the path traveled by light in a vacuum in a time interval of 1/299,792,458 of a second".
As a result of this definition, the speed of light in a vacuum became exactly 299,792,458 m/s and is defined as a constant in the SI (International System of Units) system of units. Furthermore, improved experimental techniques used to measure the speed of light before 1983 no longer affect the known value of the speed of light in SI units.
Formula for the Speed of Light
The speed of light is calculated using the equation c = λf. “c” stands for “celeritas” in Latin, meaning speed. λ is the 11th letter of the Greek alphabet, “Lambda,” and is used for wavelength. “f” is used for frequency. Simply put, the formula for the speed of light is defined as “Speed = Wavelength x Frequency.”
How is the speed of light measured?
The formula v = c/n is used to measure the speed of light per second, where “v” represents the speed of light in the medium, “c” represents the speed of light, and “n” represents the refractive index.
Knowing that the speed of light in a vacuum is 3 x 10^8 m/s, we need to know the refractive index, which indicates how much slower light or other electromagnetic waves travel through a vacuum compared to light. For example, the refractive index of glass is 1.6, so the speed of light as it passes through glass is:
We can express it as v = 3 x 10^8 / 1.6 = 1.88 x 10^8 m/s.
Why is the speed of light constant?
The fact that the speed of light is constant is a fundamental proposition of the theory of relativity. This can be divided into two parts: The speed of light is independent of the observer's motion. The speed of light does not change with time or location.
An example that shows the speed of light is constant.
Stay where you are right now and look around; there are photons everywhere illuminating the environment, which we perceive but cannot see.
Is Light Matter?
No, light is not matter, it is energy. Matter has atoms, and even if those atoms were only 0.00000001%, they would still give the object mass, making it impossible for it to reach the speed of light. Because light is a type of electromagnetic radiation, it is composed of quantum particles. If light were matter, it could not travel so fast.
Is it possible to reach the speed of light?
No. To reach the speed of light, any object would need to have zero mass. Furthermore, an object would require infinite energy to reach the speed of light.
The Speed Closest to the Speed of Light
There are two particles in the universe that come very close to the speed of light: the OMG particle and the protons in the Large Hadron Collider.
The Oh-My-God particle is a high-energy cosmic ray detected on October 15, 1991. This ray struck our atmosphere at precisely 0.999999999999999999999951 times the speed of light. Known to have originated from the direction of the Perseus constellation, this particle remains a mystery.
The Large Hafron Collider is the world's largest and highest-energy particle collider. It was opened in 2008 by the European Nuclear Research Center (CERN). Here, protons are accelerated to approximately 0.999999990 times the speed of light using electromagnetic fields.
Is it possible to surpass the speed of light?
No, according to the laws of our universe, nothing can travel faster than the speed of light. Light has infinite mass and no time. It is especially impossible for an object with mass to reach the speed of light.
What is faster than the speed of light?
Although inconsistent with the laws of physics for most physicists , tachyons are theoretically particles that travel faster than the speed of light. The slowest speed for tachyons is the speed of light, and they cannot reach that speed.
What happens if we exceed the speed of light?
According to special relativity, if something exceeds the speed of light, it will travel backward in time.
Why Can't the Speed of Light Be Exceeded?
Einstein's special theory of relativity, proposed in 1905, completely changed our view of the universe. The special theory of relativity describes physical phenomena occurring in space-time. Before this theory, it was thought that the interaction forces between two particles changed depending on the distance between them. That is, it was believed that the force changed when the distance between these two objects changed. However, according to the special theory of relativity, the change in force is not instantaneous, but is delayed with respect to distance and propagates through space with a certain speed. This propagation speed must be an insurmountable speed, because a particle must not exceed the propagation speed of the interaction. The reason why the speed of light cannot be exceeded is, according to the special theory of relativity, because the propagation speed of the interaction is the speed of light itself.
Why does time slow down as we approach the speed of light?
The concepts of speed and time are inversely proportional. This is due to Einstein's general theory of relativity: time flows at the speed of light, and as you approach the speed of light, the flow of time slows down for that object. This phenomenon is called time dilation, and one of the best examples is the event horizon of a black hole. One hour spent at the event horizon, from which even light cannot escape, is equivalent to 100,000,000 years for a person on Earth. This is because there is no light there, and light means that information cannot reach you.
Does time stop at the speed of light?
Yes, the concept of time disappears for an object reaching the speed of light because time flows at the speed of light. To make it easier to understand, let me give you the example of two cars moving in the same direction and at the same speed. Let one of those cars be you, and the other be time. If you both move at the same speed and in the same direction, you cannot tell if you are actually moving or not; you would be equal. In this case, time does not flow for you.
Is the speed of light the same everywhere?
The speed of light is always the same, but it can vary depending on the medium. When light enters a medium, it interacts with the atoms or molecules in that medium. These interactions cause the light to be absorbed and re-emitted by the atoms or molecules, which slows down the overall speed of light in the medium. The more interactions that occur, the slower the speed of light becomes in the medium.
How can we determine if the speed of light is slowing down or decreasing?
The extent to which the speed of light slows down in a medium is determined by the refractive index of that medium. The refractive index is a measure of how much the speed of light decreases as it passes through a medium compared to its speed in a vacuum. The refractive index is affected by the density and composition of the medium, as well as other factors such as temperature and pressure.
In which substance is the speed of light greater?
Light travels faster in air or a vacuum because its refractive index is very close to 1. Therefore, its interaction rate is low.
In which substance is the speed of light slower?
Light travels slower in water because water has a refractive index of 1.33. Therefore, light travels approximately 25% slower in water than in a vacuum.
How many seconds would it take for light to travel around the Earth?
Light travels steadily through space at 300,000 km/s and can circle the Earth's equator 7.5 times in one second.
Is the universe expanding faster than the speed of light?
The answer to that question depends entirely on where you're looking. The universe is steadily expanding by 1 megaparsec (3.26 light-years) per second relative to Earth, amounting to 73.3 ± 2.5 km. Now, imagine the universe as dough left to ferment, and the galaxies as pieces of chocolate in the dough. As time passes, the dough grows, and even though the chocolate pieces don't move, they move further apart. Galaxies billions of light-years away from Earth are technically moving away from us faster than the speed of light. This is because of our distance from other galaxies. In this context, did you know that 97% of the galaxies in the universe are moving away from us at the speed of light?
Is Traveling at the Speed of Light Possible?
No, it would take infinite energy to accelerate an object with mass to the speed of light. Therefore, traveling at the speed of light is impossible according to the laws of physics.
1. Travel Times at the Speed of Light
1. How far can you travel from Earth at the speed of light, and in what timeframe? All the answers are here:
To where? | Required Time |
From One End of the World to the Other | 0.13 seconds |
Month | 1.25 seconds |
Anthem | 3 minutes |
Sun | 8 minutes |
Pluto | 4 hours |
The Center of the Milky Way | 25,000 years |
Andromeda Galaxy | 2.5 million years |
The End of the Observable Universe | 46.5 billion years |
