In a short answer: light is not in fact pure energy. It is, however, true that light has no mass. But the fact that light has no mass does not mean that light is pure energy. Light is actually made up of objects called photons. This can be compared to other fundamental quantum objects since they are similar to electrons and neutrinos. But electrons are negatively charged, and photons are not.
Below is a list of properties that photons, electrons and neutrinos have that determine how it will behave. To say that light is pure energy would also suggest that light only carries the property of energy, but that is not true.
A singular photon will carry these properties:
- Speed: we often talk about the speed of light. This is the rate that the photon will travel through any given space. This speed is always 299,792,458m/s, since this is the speed of light.
- Wavelength: since light is a wave, the wavelength is the distance between each peak on each photon’s wave.
- Frequency: the frequency of a wave is the number of times that the wave reaches a point or a peak in a certain time at a certain location in space. The frequency of light is what actually determines how we see it. The frequency is what the color depends on.
- Wavevector: the photon’s direction of propagation. It is also the number of peaks in the wave that will be present in any given length.
- Period: the time between two of the peaks on the wave of the photon, at a specific location.
- Wave phase: this is where two photons can interact… the wave phase is the relative location of one wave peak of one photon compared to another wave peak of another photon. This is important for if you want to describe interference.
- Spin: although it has a plain name, this is still a quantum property. The spin of a photon is also called its polarization state. Photons foe not obey the Pauli exclusion principle, which means that they have integer spin and also bosons. So, they can exist as laser beams do, in the same state.
- Momentum: this means that light has the ability to interact and collide with other objects and actually get them to move.
- Kinetic energy: the energy of the light due to its motion. Remember that a photon has no mass, because of this, its kinetic energy is actually equal to its total energy. And according to general relativity, light’s energy allows it to create a gravitational field.
- A quantized electromagnetic field: this is the most confusing sounding property, but it’s simply the fact that a photon contains electromagnetic fields. In fact, a photon is a quantized ripple within an electromagnetic field. Particles can produce photons and destroy them if they themselves have an electric charge. Because of this, photons follow the principles of quantum field theory.
Looking at this list, it is now clear that it is very reductionist (and just incorrect) to say that light only carries energy. We have listed all of the properties that light also carries. So, this means that photons are far more than ‘pure’ energy, since they carry more properties than just energy alone.
The reason that photons can exist without any mass is because they carry all of these other properties instead. This means they are still physically real, despite not having any mass as a property.
Interestingly, all of the properties on the list above, although appearing independent, are actually all dependent on each other or simply different ways of describing each other. If we take a look at some equations to do with light, we will see that these properties are all related.
E = hf
This equation sees E as the energy of a photon, and that the energy of a photon is equal to the frequency f times the constant h.
p = ℏk
This equation shows that p is the momentum of a photon, and that it is equal to the wavevector k times the constant.
T = 1/f
This equation shows that the period T is equal to the inverse of the linear frequency.
λ = 2π/k
This equation shows that the wavelength λ is equal to the inverse of the wavevector magnitude k multiplied by 2π.
c = fλ
And finally, this equation shows that the speed of the photon c is equal to the frequency times the wavelength fλ.
Even though some of the properties did not make it into these equations, it still goes to show that 1) all of the properties almost rely on one another and are important to the photon, and 2) that all of the properties exist in a photon even with the absence of mass, and 3) that photons do not just carry pure energy.
It follows then, to list the properties that photons and light do not exhibit, since we have detailed the properties that they do carry.
The following list shows the properties that are not present in photons:
- Electric charge
- Flavor quantum numbers
- Baryon number
- Lepton number
- Magnetic moment
This list, although showing the properties that photons do not have, also shows us that mass is not the only absent property in a photon. This means that the presence of mass in a particle does not offer any extra physicality, since photons are very much in existence and don’t carry mass.
Conversely, the absence of mass in a particle does not make the particle any purer, as the question is suggesting.
We are conditioned in science to think that an object with no mass cannot exist. When in fact that is not true. Mass is not the only fundamental property, so it is not the deciding property when it comes to a particle existing. The total energy of a fundamental object is always its mass energy and its kinetic energy added together.
So, photons still have energy since they have kinetic energy, they just do not have any mass to add to that number.
When you combine multiple photons together to form a beam of light, it is possible to encode images or patterns in the form of information.
This is the way that the photons are arranged. The really interesting thing about all of the properties listed above that light does have, is that each of them can actually be used to display information in different ways. The human eye extracts information from the frequency of a photon, or the color that it exhibits.
Radio antennas can vary the frequency (shown as FM) or even the photon count (shown as AM) along the length of radio waves. We can even get information from space using photon measurements. Interferometers that can be used in some telescopes will actually measure the phase properties of photons in order to find the source that created the beam of light.
The object that you are probably most used to using light to record information is a camera. Specifically, a light field camera. These will extract the photon positions, frequencies and even the wavevectors, and they will capture three-dimensional photos that we are used to seeing today.
Therefore, if light was just pure energy, we would not be able to use it to do all of these things, since a lot of them rely on other properties. If light was pure energy, we would not be able to take pictures because the frequency would not be able to be captured, nor the wavevector or the position of the photons.
To summarise, light is not pure energy, to answer the question. Energy is merely one of the factors that it carries, including frequency, wavelength and even spin.
If light were pure energy, we would not be able to see colors since the way we see color is with the difference in frequency of photon wavelength.