Can humans ever directly see a photon?
Published: September 3, 2015
Yes. In fact, photons are the only things that humans can directly see. A photon is a bit of light. Human eyes are specifically designed to detect light. This happens when a photon enters the eye and is absorbed by one of the rod or cone cells that cover the retina on the inner back surface of the eye. When you look at a chair, you are not actually seeing a chair. You are seeing a bunch of photons that have reflected off of the chair. In the process of reflecting off of the chair, these photons have been arranged in a pattern that resembles the chair. When the photons strike your retina, your cone and rod cells detect this pattern and send it to your brain. In this way, your brain thinks it's looking at a chair when it's really looking at a bunch of photons arranged in a chair pattern.
Your eyes can see bunches of photons, but can they see a single, isolated photon? Each rod cell in your eye is indeed capable of detecting a single, isolated photon. However, the neural circuitry in your eye only passes a signal along to the brain if several photons are detected at about the same time in neighboring rod cells. Therefore, even though your eye is capable of detecting a single, isolated photon, your brain is not capable of perceiving it. If it could, an isolated photon would just look like a brief flash of brightness at a single point. We know this because a sensitive camera sensor is indeed able to detect and process an isolated photon, and the photon just looks like a brief flash of brightness at a single point.
A photon has several properties, and each of these properties carries information about the source that created the photon or the last object that interacted with the photon. The basic properties of a photon that carry information are color (i.e. frequency), spin (i.e. polarization), location, direction of propagation, and wave phase. There are also many other properties of a photon; such as energy, wavelength, momentum, and wavenumber; but these are all dependent on the frequency and therefore do not carry any extra information. Additionally, when many photons are present, information can be carried by the number of the photons (i.e. brightness). When a group of photons reflects off of a chair, the photons form patterns of color, spin, location, direction, wave phase, and brightness that contains information about the chair. With the proper tools, each of these patterns can be analyzed in order to gain information about the chair. The human eye is designed to detect the color, location, direction, and brightness patterns of a group of photons, but not the spin or wave phase.
Color information is detected in the eye by having three different types of cone cells that each have a different range of color sensitivity. One of the types has a sensitivity range centered on red, another type has a range centered on green, and another type has a range centered on blue. The eye can see almost all of the colors in the visible spectrum by comparing the relative activation of these three different types of cone cells. For instance, when you look at a yellow tulip, yellow photons stream into your eye and hit your red, green, and blue cone cells. Only the red and green cone cells are triggered by the yellow photons, and your brain interprets red plus green as yellow. In contrast to cone cells, there is only one type of rod cell, and so the rod cells can only detect brightness and not color. The rod cells are primarily used in low lighting conditions.
Location information is detected in the eye by having the cone and rod cells spread across different locations along the retina. Different photons existing at different locations will trigger different cells. In this way, the spatial pattern of photon location is directly detected by the retina. Note that photons can come from many different directions and blur together. For this reason, the eye has a stack of lens in the front which focuses only the light to a certain cell which comes from a single point on the object being viewed. The lens plays an essential role in extracting location information about the object being viewed from the location information of the photons on the retina. If the lens malfunctions, photon location on the retina no longer corresponds exactly to point locations on the object being viewed and the image ends up blurry. Note that the human optical system can only directly image two dimensions of the photon location information. Information about the third dimension is indirectly extracted by humans using a variety of visual tricks (called "depth cues"), the main trick being the use of two eyes that are slightly offset from each other.
Direction information is only crudely detected by humans by having the brain keep track of which way the eyes are pointed, and by having the eyes look at an object from many different angles. For instance, a room with one wall painted red and the opposite wall painted blue has red photons from the wall shooting in one direction and blue photons from the other wall shooting in the opposite direction. At a given spot in the room, the bunch of photons at that spot includes red photons and blue photons traveling in opposite directions. However, a human can only deduce that the red and blue photons are traveling in different directions (and therefore deduce that the red and blue walls are at different locations) by turning his head and analyzing two different views while his brain tracks the orientation of his head.
Brightness information is directly extracted by the retina by measuring how many photons strike a certain region of the retina in a certain time increment. Both the rod cells and the cone cells can collect brightness information.
Since the human eye ultimately only sees photons, a light-generating machine can make a physical object seem to be present by recreating the correct patterns of photons that would come off of the object if it were really present. For instance, we can make it look like a chair is present if we create a collection of photons with the same patterns as the collection of photons that is present when a chair really is there. This is what computer display screens do. A camera captures the patterns in the photons coming from a chair and stores the information as bits of electricity. A computer screen then uses this information to recreate the photon collection and you see a picture of the chair.
However, standard computers screens can only specify the color, brightness, and two-dimensional location of the photons they create. As a result, the image of a physical object on a computer screen is two-dimensional and not completely realistic. There are many tricks that are used to try to convey the third dimension of information to humans, including the polarization glasses used in 3D cinemas and the lenticular lenses used on some book covers. However, such systems are usually not entirely realistic because they do not actually recreate the full three-dimensional photon field. This means that such "3D" recreations of objects can only be viewed from one look angle and are not entirely convincing. Some people find that because such "3D" systems use visual tricks rather than a full three-dimensional photon field, these systems give them headaches and nausea.
In contrast, a holographic projector comes much closer to recreating the full three-dimensional photon field coming from an object. As a result, a hologram looks much more realistic and can be viewed from many different angles, just like a real object. However, true holograms are currently not able to effectively reproduce color information. Note that many color-accurate images that are claimed to be holograms are actually flat images with tricks added in to make them look somewhat three-dimensional. A fully-realistic photon recreation of a physical object will not be possible until holograms are able to accurately recreate color information.
The two properties of photons that human eyes cannot see are spin (i.e. polarization) and wave phase. Note that under the right conditions some people can detect the overall polarization state of an entire light beam; but no naked human eye can directly see the polarization pattern. By looking through rotatable polarization filters, which convert polarization information to color intensity information, a trained human can learn to indirectly see the polarization pattern of the photons coming from an object. An example of this is the photoelasticity method which allows people to see mechanical stresses in certain objects. In contrast to humans, some animals such as honeybees and octopuses can indeed directly see the polarization pattern of a collection of photons. For instance, honeybees can see the natural polarization pattern that exists in the daytime sky and use it for orientation purposes. Photon wave phase can also not be directly detected by humans but can be detected by machines called interferometers. Phase information is often used to determine the flatness of a reflecting surface.
In summary, humans can indeed see photons. Humans can see all of the properties of photons except for spin and wave phase. Since photons travel in patterns dictated by the source that created them or the last object that the photons interacted with, we usually don't realize we are looking at photons at all. Rather, we think we are looking at the physical objects that are creating and scattering the photons.
Now, perhaps you meant to ask, "Can humans ever see a photon in the same way we see a chair?" Again, we can see a chair because photons bounce off of it in a certain pattern representative of the chair and enter our eyes. In order to see a photon in the same way you see a chair, you would have to have a bunch of photons bounce off of the one photon you are trying to "see" and then have this bunch enter your eye. However, photons never directly bounce off of each other, so this could never work. Even if photons could bounce off of each other, you would not see anything special from this setup. You would still just see a flash light at one point when the small bunch of photons strikes your retina. When you think you see a light beam sitting out in space, such as coming from a flashlight, you are in reality seeing the dust particles along the path of the beam because of the photons bouncing off of the dust particles.