Can humans ever directly see a photon?
Published: September 3, 2015
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Yes. In fact, photons are the only thing 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 fill 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 and passing through the lenses of your eye, these photons have been arranged in a shape on your retina that resembles the shape of the chair. When these photons strike your retina, your cone and rod cells detect these photons and send its information 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 striking your retina, arranged in the shape of the chair. Furthermore, the photons do not only carry shape information. The photons striking your retina are also arranged in the color pattern of the chair and the brightness pattern of the chair.
Your eyes can see bunches of photons, but can they see a single, isolated photon in otherwise total darkness? For decades scientists thought that the answer to this question was no. The eyes are not really designed to do this. The neural circuitry that passes on a single visual signal to the brain expects several photons to be detected in a short amount of time in order for it to count. This is actually a good thing because if your eyes were wired to visually experience single photons in an effective way, the everyday visual images that you experience would contain more noise (like the graininess of a low-quality photo). With that said, recent research has found that humans can indeed see a single photon in otherwise total darkness if the eyes are totally dark adapted and the conditions are just right. However, even if everything is set up just right, humans can still only sometimes successfully see a single photon in isolation; only if the photon hits a rod cell just right and efficiently transfers its energy. A human seeing a single photon in isolation is an interesting lab experiment but is otherwise not significant and does not happen in everyday life. Note that, even if a human manages to see a single photon in isolation, the visual sensation is nothing special or different. The human just sees a very dim, very brief, very small, single dot of white.
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 present (i.e. the 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 contain 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 that responds to red light better than the other types, another type responds better to green light than the other types, and the last type responds to blue light better than the other types. 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 significantly 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 only used in low lighting conditions. In such conditions, therefore, you see brightness patterns (i.e. grayscale images) instead of color and brightness patterns.
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, and therefore the shape of objects, is directly detected by the retina. Note that photons can come from many different directions and blur together. For this reason, the eye has lenses in the front which focus only the light to a certain cell in the retina which comes from a single point on the object being viewed. When this is done successfully, we say in optics that an image has been formed. In this way, the lenses play an essential role in extracting location information about the object being viewed from the location information of the photons striking the retina. If the lenses malfunction, a single location on the retina no longer corresponds exactly to a single point on the object being viewed and you end up seeing a blurry picture. However, this can often be corrected using eyeglasses or contact lenses. Note that the human optical system can only directly image two dimensions of the photon location information. In other words, the retina only captures two-dimensional images of the three-dimensional world. The human brain extracts information about the third dimension indirectly using a variety of clever techniques called "depth perception cues." The brain does this so well that the external world is visually experienced as convincingly three-dimensional despite the fact that we are really just detecting two-dimensional images on our retinas.
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. The brain also compares the detected relative brightness of different neighboring places in the image in order to decide how to visually experience brightness.
Direction information is only crudely detected by humans by having the brain keep track of which way the eyes are pointed. If you look directly down at the floor, your brain can deduce that the photons striking your retina were traveling in the upward direction, up away from the floor. Therefore a blob of paint seen in such a situation must be down on the floor or near the floor. Similarly, if you stand on the beach and look directly toward the mountains far inland, your brain knows that the photons striking your retina are coming from the direction of the mountains. Therefore, you will be unable to see ships out on the ocean unless you turn and change your gaze to be able to receive photons coming from the direction of the ocean. Because the brain just deduces direction information from the direction that the eyes are looking, it can be confused by mirrors and mirages which redirect the photons in complex ways. However, with experience, the brain can learn on an intellectual level what the mirrors and mirages are doing and therefore not be confused.
Since the human eye ultimately only sees photons, a photon-generating machine can make a physical object visually seem to be present, without the object actually being physically present, by recreating the correct patterns of photons. For instance, a computer screen can make it look like a chair is present, without a physical chair actually being present, by properly creating photons in the correct color, brightness, and shape patterns. The easiest way to know the correct photon patterns for a chair or other object is using a camera. A camera captures the color, brightness, and shape patterns of the photons coming from the chair and then stores this information as bits of electricity. A computer screen then uses this information to mostly recreate the same collection of photons and you see a picture of the chair. Rather than using a camera, sophisticated computer programs can solve the laws of physics and calculate the photon patterns coming from a geometric description of the chair in the computer, thereby creating a convincing visual sensation of a chair being present on a computer screen. The process is called computer animation, 3D rendering, ray tracing, or CAD rendering.
However, standard computer 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. However, the brain is still able to use many, but not all, of the depth perception cues in order to visually experience a three-dimensional object on a flat, two-dimensional computer screen. There are many tricks that can be used to enable even more depth perception cues and therefore make the image look even more convincingly three-dimensional, such as using polarization classes in "3D" movie theaters or lenticular lenses on top of specially printed pictures. However, such systems are still not entirely realistic because they do not actually recreate the full three-dimensional photon distribution. This means that such "3D" recreations of objects can only be viewed from a limited range of look angles and are still not entirely convincingly three-dimensional. Some people find that because such "3D" cinemas use visual tricks rather than a fully correct three-dimensional photon distribution, these cinemas give them headaches and nausea.
A true hologram is able to accurately reproduce the three-dimensional distribution of photons, and therefore give a fully three-dimensional visual experience of the chair or whatever object is represented. However, true holograms traditionally cannot convey true color information or motion information, and therefore can still not convey a fully convincingly three-dimensional visual experience. Generating true holograms that convey true color information and motion information is an ongoing area of research. Note that there are many images in popular culture that are misleadingly labeled "holograms" in order to boost their appeal that are not actually holograms.
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 and brightness 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. Because photon phase can carry detailed information about the distance that the photon has traveled, phase information can be used to detect small variations in the flatness of a reflecting surface, such as done in adaptive optics.
In summary, humans can indeed see photons, even individual isolated photons. Humans can see all of the properties of photons except for spin and wave phase. Since photons are arranged 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 of 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 photons coming from the dust particles along the path of the beam.