Photons are fundamental subatomic particles that carry the electromagnetic force — or, in simpler terms, they are light particles (and so much more). The photon is also the "quantum," or fundamental unit, of electromagnetic radiation. Everyone is surrounded by photons: The light coming from the screen you're viewing is composed of photons, the X-rays doctor use to see bones are made of photons, the radio in a car receives its signal from photons, and the magnets on a fridge use photons to hold themselves up.
Like all other subatomic particles, photons exhibit wave-particle duality, meaning that sometimes they behave as tiny particles and sometimes they act as waves. Photons are massless, allowing them to travel at the speed of light (opens in new tab) in a vacuum (299,792,458 meters per second) and can travel an infinite distance.
Although physicists have studied the nature of light for centuries, arguments went back and forth as to whether light was made of tiny particles or was wave-like in nature. In the late 1800s, however, the pioneering work of German physicist Max Planck changed the entire picture.
Planck was studying something called blackbody radiation, or light from a special device that emitted light at all frequencies as efficiently as possible. Until Planck, nobody could explain the spectrum of light coming from these devices, so Planck added a "fix" to the equations. By assuming that light could be emitted only in discrete chunks of energy, known as quanta, he was able to develop a formula that perfectly explained the blackbody spectra, according to HyperPhysics (opens in new tab).
Physicists weren't exactly sure what to make of Planck's result, but several years later, Albert Einstein took it one step further. To explain the photoelectric effect, which is the release of electrons from a metal when light shines on it, Einstein proposed that light itself is composed of discrete little chunks, according to the American Physical Society (opens in new tab). Over time, those little chunks became known as photons.
The work of Planck, Einstein and others to study the nature of light kick-started the development of quantum mechanics.
Are photons particles?
Strictly speaking, photons are neither particles nor waves; they are a combination of both. In some situations, their particle-like nature comes out more, and in others, their wave-like nature is more apparent.
For example, a detector can register the arrival of a single photon, which appears as a point-like particle. The process known as Compton scattering involves a photon striking an electron, and in that situation, the photon acts as a particle.
However, it's impossible to predict exactly where or when a photon will strike a detector. In quantum mechanics, one can only assign probabilities to events. Those events are modeled by equations for waves, with peaks in the waves corresponding to regions of high probability of receiving a photon and troughs corresponding to regions of low probability, according to AccessScience by McGraw Hill (opens in new tab).
This concept is best exemplified by the famous double-slit experiment, which solidified the dual wave-particle nature of light (and, eventually, other subatomic particles). When light passes through a screen with two slits cut into it, it forms an interference pattern on the detector on the other side of the screen, where the peaks of waves line up with each other in some places, and where the peaks and troughs cancel each other out in others. Even though only one photon passes through the screen at a time — with each individual photon acting like a particle — the interference pattern that emerges on the detector is the exact same pattern that would occur if waves were passing through the slits instead.
Do photons have mass and momentum?
Photons have zero mass, which allows them to travel at the fastest possible speed in the universe, the speed of light. However, they do have energy and momentum. The energy of a photon is given by Planck's constant times the frequency of the light, and the momentum of a photon is given by Planck's constant times the frequency of the light times the speed of light, according to the University of Calgary's Energy Education website (opens in new tab).
The fact that photons have momentum enables a broad array of applications. For example, solar sails are experimental propulsion devices that use sunlight to push a spacecraft. According to NASA (opens in new tab), the photons from the sun bounce off of the reflecting sail, thus imparting their momentum on the sail and moving the spacecraft.
Do photons experience time?
Our understanding of the rate of the passage of time comes from Einstein's theory of special relativity, which states that objects traveling closer and closer to the speed of light will experience slower and slower rates of the passage of time. In other words, moving clocks run slowly, according to John D. Horton of the University of Pittsburgh (opens in new tab).
However, the mathematics of special relativity apply only to objects traveling more slowly than the speed of light and don't apply directly to photons, which do travel at the speed of light. Thus, it's impossible to say what a photon "experiences" in terms of the flow of time, because scientists have no mathematical language to support it. Another way to put this is that the concept of the flow of time is meaningless to photons.
Are photons affected by gravity?
Because photons have both energy and momentum, they are influenced by gravity. Under Einstein's theory of general relativity, which is our modern understanding of gravity, anything with any form of energy (including mass, momentum and torsion) is influenced by gravity. Specifically, massless particles, such as photons, follow "geodesics," which are paths of minimum distance from one point to another, according to EarthSky (opens in new tab).
In general relativity, space-time is curved due to the influence of massive objects. This can make the "minimum distance" path a curved line, just as jets have to follow a curved path to go straight from one city to another, because Earth itself is curved.
The curvature of space-time affects photons in several ways. When photons are moving from a region of strong gravity to a region of weaker gravity, they will lose energy, which lowers their frequencies to the redder end of the spectrum. When photons pass near massive objects, their direction of motion will change.
- You can dig deeper into the relationship between light and time in this YouTube video (opens in new tab) hosted by the author of this article, astrophysicist Paul M. Sutter.
- For a fun exploration of the nature of quantum mechanics (which, of course, also discusses photons), check out "How to Teach Quantum Physics to Your Dog" (opens in new tab) (Scribner, 2010) by physicist Chad Orzel.
- The Physics Asylum also hosts a great video explainer on the nature of the photon, which you can watch here (opens in new tab).
Afework, B., Boechler, E., Campbell, A., Hanania, J., Heffernan, B., Jenden, J., Street, K., & Donev, J. (2021, October 22). Photon. Energy Education. https://energyeducation.ca/encyclopedia/Photon (opens in new tab)
American Physical Society. (2005, January). This month in physics history: Einstein and the photoelectric effect. APS News. https://www.aps.org/publications/apsnews/200501/history.cfm#:~:text=Light%2C%20Einstein%20said%2C%20is%20a,collision%20produces%20the%20photoelectric%20effect (opens in new tab)
Hall, L. (2021, October 6). Advanced composite solar sail system: Using sunlight to power deep space exploration. NASA. https://www.nasa.gov/directorates/spacetech/small_spacecraft/ACS3 (opens in new tab)
Kleppner, D. (2019). Photon. AccessScience. https://www.accessscience.com/content/511100 (opens in new tab)
Nave, R. (n.d.). Blackbody radiation. HyperPhysics. Retrieved March 8, 2022, from http://hyperphysics.phy-astr.gsu.edu/hbase/mod6.html (opens in new tab)
Norton, J. D. (2018, October 10). General relativity. Einstein for Everyone. https://sites.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/general_relativity/ (opens in new tab)
Whitt, K. K. (2021, September 8). What is gravitational lensing? EarthSky. https://earthsky.org/space/what-is-gravitational-lensing-einstein-ring/#:~:text=Gravitational%20lensing%20occurs%20when%20massive,bending%20and%20magnifying%20the%20light (opens in new tab)