PeterDonis said:I was using the word "linear" in a more restricted sense. The "bare" QED Lagrangian is linear in the sense that its only interaction term ##\bar{\psi} \gamma^\mu A_\mu \psi## just has one power of the EM field ##A_\mu##. In other words, there are no first order two-photon processes, i.e., no vertex where multiple photon lines meet.
But there are, as you note, nonzero contributions to the photon-photon scattering amplitude from higher-order processes; the diagrams for these processes involve multiple vertices, at each of which there is just one photon line. (@vanhees71 made a similar point in post #3; as he notes, the actual amplitude for photon-photon scattering, taking into account all of the higher order processes, is very small.) I agree, in the light of both of your posts, that QED is nonlinear in this sense, and that this sense of "linear" vs. "nonlinear" is more appropriate for this thread.
jtbell said:Photons can in fact scatter off each other "indirectly", via intermediate virtual electron-positron pairs. See e.g. page 4 (problem 3) of this document:
http://faculty.ucmerced.edu/dkiley/Physics161Fall2011HW2solns.pdf
The cross-section (i.e. the probability) is very small because the Feynman diagram has four vertices as opposed to only two in electron-electron scattering.
As I suspectedDrClaude said:(@vanhees71 made a similar point in post #3; as he notes, the actual amplitude for photon-photon scattering, taking into account all of the higher order processes, is very small.)
When light travels, it behaves like a wave and can exhibit interference, which is when two or more waves combine and either amplify or cancel each other out. However, in certain situations, light rays can travel in the same medium without interfering with each other, meaning they do not combine or affect each other's path.
This phenomenon is known as "coherent light." It occurs when light rays have the same frequency, wavelength, and phase. This means that the peaks and troughs of the light waves align perfectly, allowing them to travel without interacting or interfering with each other.
One common example is laser light, which is highly coherent because it is produced by a single source. Laser beams can travel long distances without spreading out or interfering with each other. Another example is when light passes through a narrow opening or slit, the rays that pass through do not interfere with each other due to their coherence.
When light rays interfere with each other, they can create patterns of light and dark areas known as interference patterns. This phenomenon is commonly observed in experiments with two or more sources of light, such as the famous double-slit experiment.
Yes, light rays can interfere with each other under certain conditions. For example, when light passes through a medium with varying densities, such as a soap bubble or a prism, the different wavelengths of light can interfere with each other, causing them to split and create a spectrum of colors.