# Why don't photons interfere/interact with one another, as common mechanical waves would?

After all, they are a (self-sustaining) perturbation of the same field, like sound waves or water waves are "energy flow" (except these ones experience dissipation). And how can our eyes be so clever to perfectly sort and recognize objects if the air is "polluted" with all kinds of photons bouncing all around?

• hint: if you need glasses, then your eyes can't sort it out. – JEB Oct 26 '20 at 17:17
• I'm referring to dr. Feynman lecture "Seeing things" – ric.san Nov 10 '20 at 14:16

You just raised a question about a very important topic, the distinction between interference and interaction. A lot of answers on this site mention interference in connection to the double slit experiment. And you see other phrases like "photons do not interact with each other". I think this needs a little clarification:

1. Interference, you can see this from the double slit experiment, done by shooting single photons at a time. Emphasis on single photons. What interferes with what? You just shot a single photon. The pattern arises only if you repeat the experiment, and shoot many photons after each other. The boundary conditions are all the same, and each photon that is shot from the same setup laser, the interference will show up, showing an interference between the photons that were actually shot after each other.

https://en.wikipedia.org/wiki/Double-slit_experiment

1. interaction, this is about the vision question in your example. the photons bouncing off objects do not interfere with each other (visible wavelength and energy level in your example), to the first order. Photon can and do interact, but you need much higher energy levels, and that is called nonlinear optics. We are lucky that at the visible wavelength energy levels there is linear optics, and no photon-photon interaction, because otherwise we would not be able to see.

https://en.wikipedia.org/wiki/Nonlinear_optics

The four electromagnetic vertices make the contribution so small , it can be ignored for visible light frequencies. The electromagnetic spectrum has higher energy photons though, up to gamma rays, and the probability of photons scattering goes up with energy

Is Light intangible to other Light? And how does all the intersecting light exist in space?

So the answer to your question is, that photons do interact, but that becomes an apparent phenomenon only at high energy levels, much higher then the energy of visible photons, thus we are able to see.

• +1 This answer makes clear the distinction between interference and interaction, and also that the energy of the photons is important in measurable interactioneffects. – anna v Oct 26 '20 at 21:21
• @annav thank you so much! – Árpád Szendrei Oct 26 '20 at 21:33

Photons do interfere, there are places where you can see the classical interference patterns like in the double slit experiment (or every interferometer) and some places you can see quantum interference (e.g. Hong Ou Mandel experiment).

The "sorting" of photons is an outcome of the lens in our eye, sorting photons coming from different directions to different places on our retina. The sorting by color is due to the different wavelength sensitivity of the detectors in each "pixel" on our retina (read more about the RGB cones)

• Thanks for your answer but.. like at this moment, I'm seeing a variety of distinct objects in front of me (the computer, the pencil case et cetera) without any problem, but if I were at the corner of a swimming pool, I wouldn't know ANYTHING solely from analyzing the waves, and that's because are all intertwined. Why doesn't this happen ordinarily with photons? – ric.san Oct 26 '20 at 13:58
• In the ocean, you can learn where the storms are that generated the wave from analyzing them in a small area. You can mathematically determine which components of the waves are coming from which directions. This is much the same thing as the lens in your eye does – mmesser314 Oct 26 '20 at 14:26
• @mmesser314 what do you mean by "small"? I guess you won't be able to tell much from analysis of a 1m×1m area. – Ruslan Oct 26 '20 at 22:23
• Photons do not interfere. – my2cts Oct 26 '20 at 23:11
• @Ruslan - Several meters across is enough. An upward looking Doppler sonar with 4 breams on the bottom can measure current speeds generated by waves. Direction vs wavelength can be calculated. From that you can separate different wave sources. See teledynemarine.com/workhorse-waves-array?ProductLineID=12 and click on Workhorse Waves Array Datasheet to get a screenshot of some results. You can measure waves over time and watch frequencies change because of dispersion. From this you can get distances to wave sources. – mmesser314 Oct 27 '20 at 0:24

Photons of different energy have different wavelengths. When they interfere with each other it isn't done in a linear fashion. Our sensors in our eyes can understand only a few frequencies of light.And the information of each wave is not lost in the collection of waves "polluting your eye".

Your question is correct, photons do not really interfere. The DSE taught at the high school level is a convenient theory and it also works well mathematically but 2 photons cancelling is a violation of conservation of energy. In university in quantum optics courses deeper explanations are provided.

Think of 2 tsunamis one from Japan and the other from USA, starting with opposite phase .... when they meet (at say Hawaii) they cancel and Hawaii is saved ... but a second later the waves emerge again and continue on their way to Japan and USA, the energy was only stored temporarily in the elasticity of the water! The energy will only be absorbed when the wave crashes on the land. For photons we can never really observe the field directly ... we can only see a photon when our eye or camera absorbs it. We assume the photons are interfering in the EM field .... it makes sense .... but every photon is created by an atom and eventually absorbed by an atom.

• „it works mathematically, but 2 photon cancellin is a violation...“: Nice. Could you read my answer to: Wave and particle nature of light during detection in a single-photons double slit experiment? And a question about Experiment carried out to measure the Huygenian wave behind an opening all over 180° – HolgerFiedler Oct 27 '20 at 4:58
• For answer 2 yes there is a probability of seeing a photon at +/- 90 from the single slit, The photon has considered many paths the most probable are the ones that are integer wavelength ones. The photon considers the whole EM field including the state of the EM field around all the possible absorbing atoms, the result is all probability also called Quantum Mechanics. – PhysicsDave Oct 27 '20 at 19:07
• For q1, attempt at measurement will upset the electron trajectory and no pattern will be observed. The theory of transiting the edge is interesting, an interaction of the slit material with the photon field is required, this explains the spreading of the photon(s) path (diiffraction). We need to also explain the banding seen, I do not like to use the word interference, it is misleading but very historical. Feynman provides a good explanation for the banding. Electrons in the edge may be bound or in a conduction band and this can cause effects, like polarizers. – PhysicsDave Oct 27 '20 at 19:15
• Dear PhysicsDave. It is usually frown upon to directly copy-paste identical answers. (The problem is if everybody start to copy-paste identical answers en mass.) – Qmechanic Dec 22 '20 at 20:37

The waves associated with a single photon can interfere with each other (and contribute to producing an interference pattern). Different photons in a laser beam (all of which have the same wavelength and phase) can also interfere with each other (making holograms possible). Photons from an ordinary light source may have many different wavelengths and no fixed phase relationship. Any interference effects would be fleeting and vary from point to point.

• So interference can be seen macroscopically only with single photons (like in double-slit experiments), or whenever there is a "collision" with different coherent streams of photons all polarized in the same way, right? – ric.san Oct 26 '20 at 15:48
• All photons are single photons. The term single photons refers to experiments where one photon is created at any one time, this is done by carefully exciting a quantum dot or similar structure in a laboratory. Scientists still observe the "interference" pattern after they count like one photon every day for a year! Interference is a very old historical interpretation (still taught today). In the double slit exp dark areas have no photons, bright areas all the photons. – PhysicsDave Oct 26 '20 at 16:43
• RW Bird mentions waves associated with a single photon, although technically a single photon is a single wave. What scientists say (Feynman) is that a photon looks at all possible paths ... and chooses the one that is the shortest that has a path length that is a multiple of the wavelength. What is also mention is that this initial process is done with virtual (or force carrier) photons. I.E. an excited electron in an atom is able to exert forces and find a high probability path. So we have virtual photons first and then the real photon that transfers energy to another atom like in your eye. – PhysicsDave Oct 26 '20 at 16:48
• @PhysicsDave you can also make single photons (or a small number of them) with a gas Cherenkov detector. – JEB Oct 26 '20 at 17:16
• The interference pattern produced by the wave associated with a single photon cannot be observed. It only determines the probability that the photon will be detected at a particular point. It takes many photons (each with an identical pattern) being detected at many different points before the pattern emerges. – R.W. Bird Oct 27 '20 at 13:31

Photons are particles and not to be confused with electromagnetic waves or wave packages. They do not interfere. EM waves do interfere. The EM interference pattern, more precisely$$^*$$ the value of $$E^2$$ at a position, gives the probability to detect a photon at that position.

$$^*$$ This assumes the photon is detected by an electric dipole transition. For a magnetic dipole transition $$B^2$$ is the relevant quantity.