The enigma of photon behavior, photon spatial co-inhabitance

Question

The notion that multiple photons can occupy the same spatial coordinates seems perplexing. How is this experimentally validated, considering the intricate challenges and oddities it presents?

Imagine an entity that can emit two photons simultaneously, maintaining absolute consistency in their properties (e.g., phase, wavelength, direction). To our observational tools, would this not appear as a singular photon, given their identical properties, masking the presence of two distinct photons? How do we bypass this observational challenge to verify the shared occupancy of space by the photons?

Conversely, pondering a "single photon" that bifurcates into two along its vacuum path, a divergence would insinuate a minuscule yet inherent difference in their initial directional vectors from the onset and reveal that it was in fact not a single photon, but two.

Additionally, pondering photons emitted from different spatial origins, it seems fair to deem them as non-coincident for the most part, given that perfect overlap necessitates impeccable synchronization of initial states. Perhaps momentary intersection and co-inhabitance is possible, but it currently strikes me as equally plausible that they're merely bypassing each other without perfect overlap.

Photon–photon collisions are theorized to be a fundamental mechanism through which matter is generated in the universe. Would not the colliding of photons contradict the concept of co-inhabitance?

Answer 1

Some of your questions seem to come from thinking of photons as classical particles. This kind of particle can't overlap each other, so what gives?

Light is neither a classical particle nor a classical wave. It is something like both. This leads people to think in terms of the particle-wave duality. This is sometimes helpful and sometimes misleading. For example, see How can a red light photon be different from a blue light photon?

In this case, photons can overlap and do not turn into a single photon when they do.

The device you are thinking of is a laser. A laser is a cavity with two mirrors that reflects light back and forth over the same path. One mirror lets a small amount of light out, which forms the beam. Inside the cavity, a medium is pumped with energy to excite atoms. When the atoms decay, they emit light. In an ordinary neon light, this can happen spontaneously and you get ordinary light out. In a HeNe laser, a photon passing by stimulates a Ne atom to emit another photon. The two photons have identical direction, phase, wavelength.

You can pass a beam through filters to reduce its intensity. Some photons are absorbed and others pass through undisturbed. With enough filters, you can reduce the beam so much that it contains just one photon at any given time. If you light up a florescent screen, one atom at a time lights up.

With fewer filters, there can be multiple photons but few enough that you can see multiple atoms light up.

You might think the photons are side by side and not overlapping. You can add two slits to the experiment to disprove this.

With a full power beam, you see a diffraction patter with bright and dark stripes. Light has wave properties. Light traveling through two different slits travels different distances to reach the florescent screen. Where waves arrive out of phase, destructive interference leads to a dark stripe. Where they are in phase, constructive interference leads to a bright stripe.

With a single photon at a time passing through two slits, you get one atom at a time lighting up. But they only light up in the bright stripe areas. That is, a single photon passes through both slits and interferes with itself.

This is confusing if you think of photons as some combination of a classical particle and a classical wave. A classical particle can hit one atom at a time, but it can't go through two slits at the same time. A classical wave can go through both slits, but it can't light up just one atom. The behavior of light is something like both, but is really something different.