Categories: Guide to SpacePhysics

How Does Light Travel?

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century’s BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern era, the 20th century led to breakthroughs that showed us that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein’s Theory of Relativity. However, there remains many unanswered questions about light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light to the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle’s theory of light, which viewed it as being a disturbance in the air (one of his four “elements” that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or “corpuscles”). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton.

The first edition of Newton’s Opticks: or, a treatise of the reflexions, refractions, inflexions and colours of light (1704). Credit: Public Domain.

Newton’s corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise “Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light“. According to Newton, the principles of light could be summed as follows:

  • Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
  • These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to “wave theory”, which had been advocated by 17th century Dutch astronomer Christiaan Huygens. . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his Traité de la lumière (“Treatise on Light“). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment, which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young’s version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation, would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter’s moon Io to show that light travels at a finite speed (rather than instantaneously).

Prof. Albert Einstein delivering the 11th Josiah Willard Gibbs lecture at the meeting of the American Association for the Advancement of Science on Dec. 28th, 1934. Credit: AP Photo

By the late 19th century, James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell’s equations) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c).

In 1905, Albert Einstein published “On the Electrodynamics of Moving Bodies”, in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity, which reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including 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. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect, Einstein based his idea on Planck’s earlier work with “black bodies” – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein’s photoelectric effect was attempt to explain the “black body problem”, in which a black body emits electromagnetic radiation due to the object’s heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein’s explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named “photons”. For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg’s “uncertainty principle” (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger’s paradox that claimed that all particles have a “wave function”.

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly “collapse”, or rather “decohere”, to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the “Schrödinger Cat” paradox).

Artist’s impression of two photons travelling at different wavelengths, resulting in different- colored light. Credit: NASA/Sonoma State University/Aurore Simonnet

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly divert it, or arrest it, is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

We have written many articles about light here at Universe Today. For example, here’s How Fast is the Speed of Light?, How Far is a Light Year?, What is Einstein’s Theory of Relativity?

If you’d like more info on light, check out these articles from The Physics Hypertextbook and NASA’s Mission Science page.

We’ve also recorded an entire episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel.

Matt Williams

Matt Williams is the Curator of Universe Today's Guide to Space. He is also a freelance writer, a science fiction author and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful British Columbia.

View Comments

  • Light doesn't exist. This is an observation from light's point of view and not ours. Traveling at the speed of (wait for it) light, absolutely no time passes between leaving it's source and reaching it's destination for the photon. This means, to the photon hitting your retina, it is also still on that star you are observing 10 light years away. How is this possible? Maybe John Wheeler was right when he told Richard Feynman that there is only one electron in the universe and it travels forward in time as an electron, then back in time as a positron and every electron we see is the same electron.

    • MY QUESTION IS: Whether light is a wave , particle or both.. where does it get the energy to move through space/time. In other words is the energy of light infinite? Does it continue on without lose of energy.....forever.......

      • I believe that Special Relativity says that the energy of light is infinite due to the very fact it has no mass. E=MC^2

        In reverse, this is also why something with mass to begin with. If accelerated toward the speed of light, will see their mass and gravity increase to infinite points as they near relativistic speed (it actually starts around 95% with a steep upward curve from there), with a relative slowing to a stop of time.

    • Light and the universe are only illusions that are formed in our minds via technology that sends information from the simulation program we're living in. That information comes in the form of invisible wavelengths that includes wavelengths that we perceive as light. The visible retinas in our eyes are like tiny video screens where these particles are arranged into patterns that form into all the various objects we think are real objects. This information is also converted into thoughts within our minds which are like computer processors that process that information.

      We are living in a computer simulation that is much more advanced than anything the characters in the program have built according to the information called the Beast.

      • Brad,...So You're suggesting that "life" as we know and call it "is some kind of retro-virus" or "bio-intelligent format" heaped upon a perceived "set of accepted data sets" that are not in sync with each other in most cases with exception to Math 94% of the time....Even then it can vary which suggests Your idea would mean we all live in a fairy tale. That is what you suggest,...right?......

  • Correction: Even gravity doesn't slow light down.
    Light (EM radiation of any wavelength) always travels at speed c, relative to any local inertial (Lorentz) frame.
    It could also be noted that the wavelength of an EM wave is not a characteristic of that wave alone; it also depends on the state of motion of the observer.
    You might even say, "One man's radio wave is another man's gamma ray."

    • Light actually "slows down" every time it has to travel through anything but a vacuum. Look up Cherenkov radiation to see what happens when light initially travels faster than it can through a particular substance, like water. Light speed is not constant when traveling through any medium except pure vacuum. In fact that is why your pencil looks bent when you drop it in a glass of water. Light bends to find it's fastest path through any medium, and it slows down in that medium.

  • if all you scientist could ever get it in your pie brain that there is no time, no light speed, no warping space, no black holes for the purpose of moving through space quickly, no smallest no biggest when it comes to space and that all of everything has always been in existence but not necessarily as it is now. you will never find the smallest because if it exist it has an inside, and you will never find the end of space because it is infinite.

  • The article started out nicely, but I lost interest as mistakes began to appear. First Einstein did not "propose" the photoelectric effect. The photoelectric effect was first observed by Heinrich Hertz in 1887. Einstein used the idea of photons to explain the photoelectric effect and derive the photoelectric equation. Also, Max Plank had already derived the blackbody distribution, by assuming that electromagnetic energy of frequency f could only be emitted in multiples of energy E=hf, by 1900. Einstein's paper on the photoelectric effect was published in his "miracle" year of 1905. The photoelectric effect has nothing to do with black body radiation.

    Einstein did not coin the name "photons" for light quanta, as stated in this article. This term was first used by Arthur Compton in 1928.

    I have to say that I do not know what the author of the article means when he says " calculating the wavelength at which light functioned" in reference to Louis-Victor de Broglie. Louis de Broglie used the dual nature of light to suggest that electrons, previously thought of as particles, also had wave characteristics and used this notion to explain the Bohr orbits in the hydrogen atom.

    I gave up on the article after seeing these errors. I'm afraid I have a low tolerance for sloppy writing.

  • Oh, it's BCE now, "Before the Common Era" BC has worked for 2000 years but now the PC police have stepped in so as not to offend who? Some Muslims?

    • mecheng1, you must be very young. BCE has been in used in academia for decades. It's nothing "new", just out of your circle of knowledge.

        • Only in Euro-centric texts have your assertions been true, McCowen. The rest of the world not influenced by Christianity have used their own calendars and a "0" year or a "year 1" from which to reckon the passage of time, largely based on their own religions or celestial observations.

          Over the last century or so, through commerce, most of the world has generally accepted the use of a Western calendar (or use it along with their own for domestic purposes, like we here in the US still use Imperial units of measure that have to be converted to metric for international commerce). So, we are in a "common era" insofar as non-Christian societies are incorporating the Gregorian Calendar and the generally-accepted "year 1" established by that calendar (which is supposed to be the year of Jesus's birth, but it probably isn't according to current scholarship). Besides, the Gregorian calendar is an improved derivative of the Roman calendar - even the names of the months come from the Romans.

          In short, it is more accurate, as well as respectful, to go with BCE in these global times.

  • Where is the information carried on a photon hitting my eye(s), or cluster/group/pack of photons hitting my eyes(s), that I see as other distant galaxies and planets going around stars?

    • That's the mystery, isn't it? Even in scattering, light remains coherent enough to convey an enormous amount of information.

  • Since the miniscule equal masses with opposite charges, that make up the photon structure, interact at 90 degrees, this induces a spin (a finding from the 80's by the LANL plasma physics program) which creates a centrifugal force that counterbalances the charge attraction of the opposite charges. This establishes a stable structure for energies less than 1.0216 MeV, the pair-formation threshold, separating these "neutrino" sub-components by a specific distance providing wavelengths varying with photon energy. This composite photon propagates transversely at c/n, the speed of light divided by the index of refraction of the material traversed. In spite of the mass being defined as zero, for convenience in calculating atomic masses, there is actually an infinitesimal but non-zero mass for the photon that is required for calculations that describe its properties.

    • Tim, you poor guy! You have a discombobulated brain! Everything you wrote is just gibberish.

  • i would like to know the temperature in a black hole...maybe absolute zero? is absolute zero the moment that time stop?

    • I think the temp inside a black hole would be extremely high since temperature seems to increase with mass. Comparing absolute zero to time stopping is very interesting though. To the observer they would appear the same.

    • Theoretically there is no temperature in a black hole from any observer POV because time is stopped. Although JALNIN does bring up that point, and he also brings up the point of increasing mass corresponding to increasing energy. Everything in Hawking and Einstein's equations though, suggest that any energy would be absorbed back by the singularity, so there wouldn't be any heat. In fact it should be infinitely cold. But time is no more, so technically no heat or energy is emitted anyway from any observers POV. Yet recent images of black holes from Chandra show that they emit powerful Gamma Jets along their spin axis just like Neutron stars, and Pulsars. BTW edison. The accretion disk can reach temperatures of 20MN Kelvin on a feeding SM black hole (quasar). NASA just published an article on it through the Chandra feed a while back.

  • Light doesn't travel, it just IS. It is we, the condensed matter, that travels, through time.

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