In cosmology it's frequently said that photons from the early universe carry information from that time. However, wouldn't they also carry data from later interactions? How do we differentiate between the data from various time periods? As an analogy, consider photons that strike your face and reach my eyes. We say that that photon carries information about your face which then helps me to identify you, but don't these photons collide midway with air molecules, gaining information from the air? By the same logic that I can see your face through the information gained with collisions with your face, why can't I see the photon's collisions with the air, or even something the photons hit before they collided with you, like fans, bridges, etc?
[Physics] How do photons carry information
informationopticsphotonsvisible-light
Related Solutions
Yes - we are surrounded by a "sea of photons".
An individual object that reflects light (let's assume a Lambertian reflector - something that reflects incident photons in all directions) sends some fraction of the incident photons in all directions. "Some fraction" because the surface will absorb some light (there is no such thing as 100% white).
The propagation of photons follows linear laws (at normal light intensities) so that two photons, like waves, can travel on intersecting paths and continue along their way without disturbing each other.
Finally it is worth calculating how many photons hit a unit area per unit time. If we assume sunlight, we know that the intensity of the light is about 1 kW / m$^2$. For the purpose of approximation, if we assume every photon had a wavelength of 500 nm, it would have an energy of $E = \frac{h}{\lambda} = 3.97 \cdot 10^{-19}\ J$. So one square meter is hit with approximately $2.5\cdot 10^{21}$ photons. Let's assume your grey column reflects just 20% of these and that the visible component of light is about 1/10th of the total light (for the sake of this argument I can be off by an order of magnitude... this is for illustration only).
At a distance of 200 m, these photons would have spread over a sphere with a surface of $4\pi R^2 \approx 500,000\ m^2$, or $10^{14}$ photons per square meter per second.
If your pupil has a diameter of 4 mm, an area of $12\ mm^2$, it will be hit by about $12\cdot 10^8$ photons per second from one square meter of grey surface illuminated by the sun from 200 m away.
At that distance, the angular size of that object is about 1/200 th of a radian. "Normal" vision is defined as the ability to resolve objects that are about 5 minutes of arc (there are 60 minutes to a degree and about 57 degrees to a radian). in other words, you should be able to resolve 1/(57*(60/5)) or about 1/600 of a radian. That's still lots of photons...
Finally you ask "how do we distinguish what photons are reflected from what"? For this we have to thank the lens in our eye. A photon has a particular direction, and thanks to the lens its energy ends up on a particular part of the retina (this is what we call "focusing"). Photons from different directions end up in a different place. Nerves on the back of the retina tell us where the photons landed - and even what color they were. The visual cortex (part of the brain) uses that information to make a picture of the surrounding world in our mind.
It's nothing short of miraculous.
You talk about light as if it were a person carrying a clip board writing down things on its way to you. It is a physical phenomenon that gets affected as it propagates.
Depending on the various processes that it goes through before it reaches your eye, its amplitude,polarisation, frequency (or wavelength), pulse time etc. get affected from which we can infer what it must have gone through and get to know of the object it must have reflected off or gone through or originated from.
If the frequency is changed, the photon is said to have a different energy from $E=h\nu$. Since light has both particle and wavelike properties, depending on the situation we are in, we can equally talk about $k=\frac{2 \pi \nu}{c}$
Consider these examples:
Say you have a pocket laser source. You shine it on two walls, one at 500 m and one at 1 km. The light travels for more time to get back to you from the second wall. Here, the light is unaffected but only the time is recorded. If you did not know the distance the walls were from you, now you can calculate the distance the walls are at. This is information
Leaves are green. This means that they reflect green light and absorb all the other colours that are present in the sunlight. When you go outside and can "see" a leaf, it is information. Now, the frequencies of a light have been partly affected.
You see stars at night. Light has travelled for many years and the photons have hit your eye. Now you know how the star looked some few years ago. (the light from the nearest star takes around 4.5 years to come to you). Thus, information on the star's position is being carried, along with it's temperature. The wavelength of the light reaching you is carrying information.
Light from objects are also "doppler shifted" : the police use this effect to get the information - the speed of the car that they shine the radar gun on. The frequency is actually changed in this process. This frequency change is carrying information.
Best Answer
You are confusing the individual photons with the electromagnetic wave that is light, which is composed out of a zillion photons. It is the superposition of the wavefunctions of individual photons that can carry information.
An individual photon can be absorbed or scattered out of the beam it is composing. A few photons scattered out of the beam that is carrying information make no difference as the number of photons is enormous.
If a photon is scattered coherently with the other zillion photons the beam itself carries information of the way all these photons are building it up, in the total wave function, which is a superposition. That is how images can be refracted and reflected.
At some point in opaque and non reflecting surfaces the coherence is lost and any information the beam has been carrying is lost.
The individual photon is distinguished by its energy and direction, +1 or-1 spin projection on its momentum. When you see an image, you see the collective superposed interaction of the beam of photons, light. When light strikes a "fan, a bridge etc." it disperses and carries information from the object. The part of the light reflected from the bridge when it hits a second object loses the previous coherence by striking and reflecting back, and coheres in a new image pattern.
Generally coherence is lost as electromagnetic beams interact with matter.
The experiments measuring cosmic microwave background radiation are very careful to look at clear regions of the sky. Some are on satellites for this reason so as not to have interference from backgrounds. The photons they are measuring have not interacted before interacting with the detectors of the experiment. If they had been absorbed they would not have arrived at the detector.
The individual photons carry information of their directions, which way the were coming, and their energy, which was absorbed to give the count. That is how the CMB maps are made.
The ensemble of CMB photons arriving at the detectors may have interacted with cosmic dust, for example, or been deflected by strong gravitational fields, and this will affect the polarization displayed by the "beam" of arriving photons. This polarization has been measured in the Planck experiment and with BICEP2 and it is a research question whether it is an original polarization from the inflationary period due to gravitational waves, or due to scatterings from dust. This information is a collective information carried by the beam the CMB photons build up.
So some information can be carried by the ensemble of photons, as described above.