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Why is light pure energy?
Category: Physics
Published: January 12, 2015
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University
Light is not pure energy. While it is true that light has no mass, this fact does not imply that light is pure energy. Light is composed of fundamental quantum objects called photons which we list alongside other fundamental quantum objects such as electrons and neutrinos. Each object on this list contains several different properties which determine how the object behaves. Mass and kinetic energy are only two of several properties that a fundamental quantum object can carry. Saying that light is "pure energy" would imply that light only carries the property of energy and no other properties, which is simply not true. A single photon, which is the smallest bit of light possible, carries the following properties:
- Wavelength – This is the spatial distance between the peaks of the photon's wave.
- Frequency – This is the number of times that the wave reaches a peak in a unit time at a fixed location. The human perception of the color of light is very closely related to the light's frequency. Therefore, the word "frequency" can loosely be used interchangeably with the word "color".
- Wavevector – This is the photon's direction of propagation, as well as the number of wave peaks that exist in a unit length.
- Period – This is the time between two peaks of the photon's wave at a fixed location.
- Speed – This is the rate at which the photon travels through space, which is always 299,792, 458 meters per second.
- Position – This is the physical location of the photon in space. Although the position of an individual photon is not well defined and contains intrinsic uncertainty while it exists, a photon does carry some degree of location information, thus enabling us to record images in a digital camera based on where the photons hit the sensor.
- Wave Phase – This is the relative location of the wave peaks of two different photons, and is important in properly describing interference effects.
- Momentum – This is a motional property that describes light's ability to collide with other objects and get them moving.
- Spin – This is a quantum property that loosely resembles the type of spinning we see in everyday life. The spin of a photon is also called its polarization state and represents an intrinsic angular momentum. Photons have integer spin, are therefore bosons, and thus do not obey the Pauli exclusion principle. This means that photons can exist in the same state, such as in laser beams.
- A Quantized Electromagnetic Field – A photon contains electromagnetic fields. More accurately, a photon is a quantized ripple in the overall electromagnetic field. As such, photons are able to interact with electric charge. Particles with electric charge can create photons, destroy photons, and scatter photons. Also, photons can exert forces on charged particles. Furthermore, photons obey the principles and equations of quantum field theory.
- Kinetic Energy – This is the energy of the light due to its motion. Note that because a photon has no mass, its kinetic energy equals its total energy. The energy of light allows it to create a gravitational field according to General Relativity.
As should be obvious, energy is just one of many properties that photons carry. Photons are much more than "pure energy". Photons can exist just fine without having mass since they carry many other properties to make them physically real. Note that many of the properties listed above are very closely related to each other. You could even argue that many of the properties listed above are not independent properties, but are simply slightly different ways of defining the other properties. For instance, the energy E of a photon equals its frequency f times a constant, E = hf. Similarly, the momentum p of a photon equals its wavevector k times a constant, p = ℏk. Also, the period T is just the inverse of the linear frequency f, T = 1/f, the wavelength λ is just the inverse of the wavevector magnitude k times 2π, λ = 2π/k, and the speed c is just the frequency times the wavelength, c = fλ. Despite the possibility that some of the properties listed above can be seen as redundant, this does not change the fact that photons exhibit many more properties than just their energy.
There are also some properties that photons do not exhibit, simply by their nature of being photons. The following list denotes properties that photons do not have:
- electric charge
- lepton number
- baryon number
- flavor quantum numbers
- magnetic moment (although a photon may indirectly have a magnetic moment through pair creation effects)
- mass
As we see, mass is just one of many properties that a fundamental object may or may not have. As such, the presence of mass does not confer on an object any extra degree of physical reality, even though mass is the property that we are most familiar with in everyday life. Furthermore, the absence of mass does not make an object any more "pure". We are so familiar with mass in everyday life that we may be tempted to say, "an object with no mass does not really exist." But this statement is false. The more accurate statement would be, "an object with no physically observable properties does not really exist." Since there are so many fundamental properties besides mass, we see that objects can exist just fine without it. Again, the lack of mass does not automatically imply that the object is pure energy, since there are so many other properties involved. Note that mass is actually just another form of energy. The total energy of a fundamental object is its mass energy plus its kinetic energy (note that potential energy is held by systems of objects and not by single objects).
Interestingly, if we combine many photons into a beam of light, we can encode information such as images in the pattern of the photons. Each of the photon's properties listed above can be exploited to carry information. For instance, human eyes, conventional cameras, and traditional space telescopes extract photon position and frequency (color) information from a group of photons in order to form images. Radio antennas vary the frequency (FM) or the photon count (AM) along the length of the radio waves that they create in order to encode information. Interferometers such as used in some space telescopes measure the phase properties of the photons in a beam to extract information about the source that created the beam. A light field camera extracts photon position, frequency, and wavevector directionality from a group of photons in order to capture three-dimensional photographs. If light was just "pure energy", then human eyes, cameras, radio antennas, and space telescopes would not function.