What is the Speed of Light?

Since ancient times, philosophers and scholars have sought to understand light. In addition to trying to discern its basic properties (i.e. what is it made of – particle or wave, etc.) they have also sought to make finite measurements of how fast it travels. Since the late-17th century, scientists have been doing just that, and with increasing accuracy.

In so doing, they have gained a better understanding of light’s mechanics and the important role it plays in physics, astronomy and cosmology. Put simply, light moves at incredible speeds and is the fastest moving thing in the Universe. Its speed is considered a constant and an unbreakable barrier, and is used as a means of measuring distance. But just how fast does it travel?

Speed of Light (c):

Light travels at a constant speed of 1,079,252,848.8 (1.07 billion) km per hour. That works out to 299,792,458 m/s, or about 670,616,629 mph (miles per hour). To put that in perspective, if you could travel at the speed of light, you would be able to circumnavigate the globe approximately seven and a half times in one second. Meanwhile, a person flying at an average speed of about 800 km/h (500 mph), would take over 50 hours to circle the planet just once.

Illustration showing the distance between Earth and the Sun. Credit: LucasVB/Public Domain
Illustration showing the distance light travels between the Earth and the Sun. Credit: LucasVB/Public Domain

To put that into an astronomical perspective, the average distance from the Earth to the Moon is 384,398.25 km (238,854 miles ). So light crosses that distance in about a second. Meanwhile, the average distance from the Sun to the Earth is ~149,597,886 km (92,955,817 miles), which means that light only takes about 8 minutes to make that journey.

Little wonder then why the speed of light is the metric used to determine astronomical distances. When we say a star like Proxima Centauri is 4.25 light years away, we are saying that it would take – traveling at a constant speed of 1.07 billion km per hour (670,616,629 mph) – about 4 years and 3 months to get there. But just how did we arrive at this highly specific measurement for “light-speed”?

History of Study:

Until the 17th century, scholars were unsure whether light traveled at a finite speed or instantaneously. From the days of the ancient Greeks to medieval Islamic scholars and scientists of the early modern period, the debate went back and forth. It was not until the work of Danish astronomer Øle Rømer (1644-1710) that the first quantitative measurement was made.

In 1676, Rømer observed that the periods of Jupiter’s innermost moon Io appeared to be shorter when the Earth was approaching Jupiter than when it was receding from it. From this, he concluded that light travels at a finite speed, and estimated that it takes about 22 minutes to cross the diameter of Earth’s orbit.

Prof. Albert Einstein uses the blackboard as he delivers the 11th Josiah Willard Gibbs lecture at the meeting of the American Association for the Advancement of Science in the auditorium of the Carnegie Institue of Technology Little Theater at Pittsburgh, Pa., on Dec. 28, 1934. Using three symbols, for matter, energy and the speed of light respectively, Einstein offers additional proof of a theorem propounded by him in 1905 that matter and energy are the same thing in different forms. (AP Photo)
Prof. Albert Einstein delivering the 11th Josiah Willard Gibbs lecture at the Carnegie Institute of Technology on Dec. 28th, 1934, where he expounded on his theory of how matter and energy are the same thing in different forms. Credit: AP Photo

Christiaan Huygens used this estimate and combined it with an estimate of the diameter of the Earth’s orbit to obtain an estimate of 220,000 km/s. Isaac Newton also spoke about Rømer’s calculations in his seminal work Opticks (1706). Adjusting for the distance between the Earth and the Sun, he calculated that it would take light seven or eight minutes to travel from one to the other. In both cases, they were off by a relatively small margin.

Later measurements made by French physicists Hippolyte Fizeau (1819 – 1896) and Léon Foucault (1819 – 1868) refined these measurements further – resulting in a value of 315,000 km/s (192,625 mi/s). And by the latter half of the 19th century, scientists became aware of the connection between light and electromagnetism.

This was accomplished by physicists measuring electromagnetic and electrostatic charges, who then found that the numerical value was very close to the speed of light (as measured by Fizeau). Based on his own work, which showed that electromagnetic waves propagate in empty space, German physicist Wilhelm Eduard Weber proposed that light was an electromagnetic wave.

The next great breakthrough came during the early 20th century/ In his 1905 paper, titled “On the Electrodynamics of Moving Bodies”, Albert Einstein asserted that the speed of light in a vacuum, measured by a non-accelerating observer, is the same in all inertial reference frames and independent of the motion of the source or observer.

A laser shining through a glass of water demonstrates how many changes in speed it undergoes - from 186,222 mph in air to 124,275 mph through the glass. It speeds up again to 140,430 mph in water, slows again through glass and then speeds up again when leaving the glass and continuing through the air. Credit: Bob King
A laser shining through a glass of water demonstrates how many changes in speed (in mph) it undergoes as it passes from air, to glass, to water, and back again. Credit: Bob King

Using this and Galileo’s principle of relativity as a basis, Einstein derived the Theory of Special Relativity, in which the speed of light in vacuum (c) was a fundamental constant. Prior to this, the working consensus among scientists held that space was filled with a “luminiferous aether” that was responsible for its propagation – i.e. that light traveling through a moving medium would be dragged along by the medium.

This in turn meant that the measured speed of the light would be a simple sum of its speed through the medium plus the speed of that medium. However, Einstein’s theory effectively  made the concept of the stationary aether useless and revolutionized the concepts of space and time.

Not only did it advance the idea that the speed of light is the same in all inertial reference frames, it also introduced the idea that major changes occur when things move close the speed of light. These include the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer (i.e. time dilation, where time slows as the speed of light approaches).

His observations also reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations by doing away with extraneous explanations used by other scientists, and accorded with the directly observed speed of light.

During the second half of the 20th century, increasingly accurate measurements using laser inferometers and cavity resonance techniques would further refine estimates of the speed of light. By 1972, a group at the US National Bureau of Standards in Boulder, Colorado, used the laser inferometer technique to get the currently-recognized value of 299,792,458 m/s.

Role in Modern Astrophysics:

Einstein’s theory that the speed of light in vacuum is independent of the motion of the source and the inertial reference frame of the observer has since been consistently confirmed by many experiments. It also sets an upper limit on the speeds at which all massless particles and waves (which includes light) can travel in a vacuum.

One of the outgrowths of this is that cosmologists now treat space and time as a single, unified structure known as spacetime – in which the speed of light can be used to define values for both (i.e. “lightyears”, “light minutes”, and “light seconds”). The measurement of the speed of light has also become a major factor when determining the rate of cosmic expansion.

Beginning in the 1920’s with observations of Lemaitre and Hubble, scientists and astronomers became aware that the Universe is expanding from a point of origin. Hubble also observed that the farther away a galaxy is, the faster it appears to be moving. In what is now referred to as the Hubble Parameter, the speed at which the Universe is expanding is calculated to 68 km/s per megaparsec.

This phenomena, which has been theorized to mean that some galaxies could actually be moving faster than the speed of light, may place a limit on what is observable in our Universe. Essentially, galaxies traveling faster than the speed of light would cross a “cosmological event horizon”, where they are no longer visible to us.

Also, by the 1990’s, redshift measurements of distant galaxies showed that the expansion of the Universe has been accelerating for the past few billion years. This has led to theories like “Dark Energy“, where an unseen force is driving the expansion of space itself instead of objects moving through it (thus not placing constraints on the speed of light or violating relativity).

Along with special and general relativity, the modern value of the speed of light in a vacuum has gone on to inform cosmology, quantum physics, and the Standard Model of particle physics. It remains a constant when talking about the upper limit at which massless particles can travel, and remains an unachievable barrier for particles that have mass.

Perhaps, someday, we will find a way to exceed the speed of light. While we have no practical ideas for how this might happen, the smart money seems to be on technologies that will allow us to circumvent the laws of spacetime, either by creating warp bubbles (aka. the Alcubierre Warp Drive), or tunneling through it (aka. wormholes).

Until that time, we will just have to be satisfied with the Universe we can see, and to stick to exploring the part of it that is reachable using conventional methods.

We have written many articles about the speed of light for Universe Today. Here’s How Fast is the Speed of Light?, How are Galaxies Moving Away Faster than Light?, How Can Space Travel Faster than the Speed of Light?, and Breaking the Speed of Light.

Here’s a cool calculator that lets you convert many different units for the speed of light, and here’s a relativity calculator, in case you wanted to travel nearly the speed of light.

Astronomy Cast also has an episode that addresses questions about the speed of light – Questions Show: Relativity, Relativity, and more Relativity.


28 Replies to “What is the Speed of Light?”

  1. This was a really good article, Frasier. Nice review of the history, and broken down in a way regular people can understand. Great job.

  2. Travel Light (October, 2012 – Pat Lueck)

    They are born travelers…
    these little photons,
    Zipping away from their creation.

    Weighing nothing, it takes nothing
    to speed them on their way

    They don’t dally, they don’t dither.
    Choosing the most likely path,
    guided by quantum probability.

    Straight across the universe,
    pausing only for an observation,
    they carry past and future within.

    Oh, little photons…
    If only we could follow you…
    see your trajectory.

    To you, your universe wide trip
    is just a “timeless blip,”
    no time, no wait, just zip!

    The great mystery.

    The photon begs the question:

    What creates your cosmic speed limit?
    What universal traffic cop
    waves you through at such
    a constant rate?

    Newly dedicated to Neil Degrasse Tyson

  3. Thanks for this article Frasier, and for helping my uneducated and average intelligence brain / mind obtain at least the tiniest inkling of understanding regarding light and the concept of spacetime.

    I am now left thinking about the source of light and where it originates from, so will have to do some homework I guess. 🙂

  4. If you understand light speed, you will understand why this makes sense:
    Why can’t you travel faster than the speed of light?
    Because you can’t travel slower than the speed of light.

    1. Fascinated by all the comments here. Am I right in thinking that there may be space in the expanding universe that is without light…

  5. It’s kind of funny. I have watched any number of programs and read numerous books on Einstein’s theories, as well as taking in similar material on the universe, light, space travel, etc. I ‘get’ the limitations of the energy involved trying to move mass closer and closer to the speed of light, but one thing I’ve always asked myself, and yet to see explained, is what actually limits the speed of light. I mean, photons actually have no mass, right, so what is the speed limiter, the breaking mechanism, that says to a photon “hey buddy, that’s it for you, 186,000 miles per second is your limit.” In a vacuum, with no mass, shouldn’t the speed of light in theory be infinite, even though I obviously know that’s not the case? What am I missing? This article is a case in point. It talks about the science involved in nailing down the exact speed, and goes to great lengths to describe Einstein’s theories on how light always remains a constant, contrary to former thoughts on the matter. But nowhere does it state WHY it’s set at that particular constant. Why not 90,000 miles per second, or 300,000 miles?

    1. I, too have wondered about this for a long, long time. See my little poem prior to this post. For many years I believed that the presence of an “ether” was firmly dismissed. However, my amateur studies of “space..” have led me to believe that “space” HAS an ether-like property. Fields? Quantum Nature of space? Interaction with dark energy? Very confusing. I have read that “fields” (one for each particle.. !) exist in space, that the wave-like nature of light requires interaction with ‘something,’ and that the energy ‘field’ at every point in space acts like an ether, etc, etc.

      I plan to read a lot this winter and take some serious notes. I am NOT a mathematician, but I have a keen mind for analogy creation. Teacher.

    2. Basically, c is what it is because the electric permittivity and magnetic permeability of free space have the specific values that they do.

    3. @timbo59
      Your question boils down to: Why does 1 = 1?

      The best answer I can give is found in “Beyond Einstein: non-local physics” by Brian Fraser (2015) which states: “And so we discover that the Universe not only has built in mathematics (enough of a mystery), it also has built in unit quantities.” Apparently, all fundamental equations in physics reduce to 1 = 1. The examples given in the article include Newton’s gravitation, Einstein’s mass-energy relation, and the gamma correction factor.

      The free 22 page paper can be downloaded from: http://scripturalphysics.org/4v4a/BeyondEinstein.html The .html file gives a link to the .pdf file but the former has additional information, and many more links and insights.

  6. First let me start by saying this article was a great read. Thank you for writing and posting it. I’m no scientist, but in my mind it seems as though if we could somehow capture light we could use it to propel an object. Does light exert a force on an object it runs into? That is a real question, to which I have no answer. If it does then if a large enough object that had sufficiently low mass was placed in front of a light in a vacuum it would move it right? Again, I’m not a scientist, I don’t profess to understand any of this, they are just questions from a simple mind. Thanks.

    1. Light absolutely does exert a force on objects. Though they have zero invariant mass, photons do carry momentum, and can transfer it in interactions with other objects. This is called ‘radiation pressure’, and is the entire basis for the design of solar sails.

  7. Minor details to fix: in picture – mps and not mph.

    This is a good article but thee is no smart money on circumventing the speed of light. That is the most immutable law of this universe. Or perhaps you could suggest that dark energy and dark matter are those moving through our universe at supra-C?

  8. What do “Black Holes” and “Dark Matter” have in common? Both of them are indicative that light changes into a form which is neither matter or light and/or evolves into either matter or light in gradations based upon the physical environment in which it is found. Thus, light might not escape “Black Holes” because of the intense gravity in the same but because light, itself, has been altered into another physical form because of the physical environment of the “black hole”, itself. Thus, the preponderance of “dark matter” in the Universe might be explained in this same context. The speed of light, the expansion of the universe, and the “big bang” might, also, be explained in light of the aforementioned.

  9. Good article. There is one aspect of the speed of light that is rarely if ever discussed and that is why can’t rest matter be accelerated, which isn’t the equivalent of space expansion, beyond the speed of light. It turns out that its due to the time dilation effect that Einstein described with the famous Lorentz Transformation. You see momentum is conveyed through force carriers which are field photons which move at the speed of light. As a body is accelerated the path of the force carriers to convey momentum are lengthen which lowers the rate of increased velocity per unit of time(seconds) from acceleration. As the body is accelerated closer to the speed of light the force carrier paths are so long that there comes a point where the rate of velocity increase is very very small. Ultimately the force carrier paths get stretched so long that a body never reaches the speed of light but can only get close to it.

  10. Although this was a good synopsis of the conventional view of light, there are a few points that need to be made in understanding the actual physical mechanism responsible for its velocity. Actually, photons do have a finite, though miniscule, mass depending on their specific energy. The definition of zero mass was given when atomic electron orbital transitions became too cumbersome in calculating atomic mass . The “negligible” photon mass could be eliminated, simplifying the atomic mass calculations Also, antenna reception of EM waves should clarify how the dual charges are inherent in photons. These rotating charges average to zero charge per cycle, but the dynamics of the photon depend on the interaction of these charges. A finding at the LANL plasma research facility was announced at the Los Alamos International Atomic Physics Summerschool class in 1989 that opposite charges interact at right angles, inducing a spin which keeps the charges at a specific distance when balanced against the attraction of opposite charges. Equal charges (with equal finite mass) would perfectly balance, resulting in a specific wavelength depending on the photon energy.

    1. Photons have zero invariant mass, at least as far as theory and experiment can tell (well, experiment can only really set an upper limit, of course, but is consistent with it being vanishingly small).

  11. Tach One is the speed of light
    Extra Terrestrials create the star systems and planets similar to how Humans create housing developments.
    Extra Terrestrials place the humans on Earth and live among them as Humanoid Extra Terrestrials.
    Humanoid Extra Terrestrials have lived here the whole time.
    Millions of the most famous, powerful, wealthy etc.. people living on the planet are Humanoid Extra Terrestrials and most Earthlings aren’t even aware this is a possibility.
    Notice how this is not even given any consideration by all the so called experts of the “ET” search.
    The even funnier part is when all the Earthlings wake up and realize those vehicles transiting our star system aren’t even FLYING and we call them Unidentified Flying Objects.
    The Earthlings are in for a rude awakening one day.

  12. Sound waves, their speed is relative to the air. Light “waves”, their speed is relative to the aether. Really. weinsteinsletter.weebly.com
    Or do you think that light travels at “c” relative to a moving car and its stationary tracks? (Have to check for insanity.) 🙂 🙂

    1. Light travels at c in all (vacuum) inertial frames of reference. This is kind of fundamental to special relativity.

  13. Also, the Speed of Light is a local phenomenon as opposed to the Quantum Effects, which are non-local, e.g. Wave-Particle Duality, Superposition, Entanglement, etc.

  14. May I respectfully point out a glaring error? In the graphic showing the speed changes as the laser light passes through various media, the speeds are shown as “mph”. I believe you intended it to read miles/sec. Please correct me if I’m wrong.

  15. Thanks for the article Matt. I disagree with ” light moves at incredible speeds and is the fastest moving thing in the Universe”. We know that the influence of gravity moves at the same speed. Really it’s the relationship between space and time that determines the fastest possible speed in the Universe. It just happens that light moves at that fastest speed. So, it’s better to say ‘the speed that light also moves at’ – rather than ‘the speed of light’.

  16. The speed of light is constant, if the photons moving in the universe, only where the matter is formed. This Einstein’s claim that the speed of light is the same irrespective of whether the light source is moving or stationary, relatively, there is no rationale.
    Imagine a stationary light source and another object that is moving away from him at lightning speed. If this object emits light toward a stationary object, it is not one of them can not see the light. What will happen if both objects are going the speed of light in the same direction and the back light is emitted, whether the first one to see the light.? Sure it will. According to Einstein this would not have happened, because the first object escapes the speed of light. Whether it is worth the Doppler effect for light? According to Einstein, no, but what is the red and blue shift?

  17. Great article. Again. Hmm. Correct me if I’m wrong but doesn’t it take light 0 sec to reach the nearest star? It is for the observer on earth it takes 4,2 years to watch the light get to its destination. If you actually travel at the speed of light, you can go anywhere in the universe in no time. Haven’t done the math lately but I remember the figure 75 years to get to the other side of the universe with a rocket accelerating with 1g.

  18. One thing that I’ve always wondered about the speed of light…
    As a computer guy, the analogy that comes to mind is networks, and the information propagation latency of packets travelling through a network. Even there, there is an absolute top limit of lightspeed, but before that, there’s the latency of the switching fabric itself.
    So, my question is this: In the crudest terms, is the speed of light representing the latency limits of information propagation (a photon) through the ‘switching fabric’ of whatever quantum elements constitute space-time?

    It’s always been easier for me to think about this stuff in terms of a 4th spatial dimension that measures the distance between quantum bits, which I imagine increasing as gravity and/or speed increases. Thus, the macro-level structures (like atoms and people) perceive time as slowing down, due ot the propagation of information becoming slower as the density of the quantum bits decreases (or distance between the bits increases), as measured by this 4th spatial dimension.

    By that logic, Gravity becomes a measure of the density of space-time in this 4th spatial dimension, which I’ve never heard mentioned anywhere, so I assume my analogy breaks down at this point 🙂

    1. read back up the comments.. (to ‘Qev” on Sep 2). He basically answers your questions…though I have not had the time to study “…electric permittivity and magnetic permeability of free space…” Let me know if you do the research.. : ) — : )

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