Does light travel "quantised"? Or , is it perceived as quantised only when it interacts ,i.e, undergoes measurement?

Question

When light interacts (non self interaction) , it is perceived as photon (I am assuming this to be so) . So, is photon then a collapsed light wavefunction?

Answer 1

It is called photon because it has particle like characteristics. It carries for example momentum and spin and can only carry energy in portions. It is also a quantum mechanical vibration of a photon field which exists everywhere in the universe. The excitations of that field we call photons, light or an electromagnetic wave. It is a duality.

Answer 2

For photons in vacuum, the answer is quite simple:

The spacetime interval between the place of emission and the place of absorption is zero, that means that both places are adjacent, and there is no place for such thing as a traveling photon.

As an example, we may observe electromagnetic waves of photons which are traveling from Sun to Earth, taking 8 minutes for a space distance of 8 light minutes. The underlying spacetime interval is zero, that means that the particle properties of the photon - in particular the momentum - are transmitted directly from the electron which is emitting the photon to the electron which is absorbing the photon.

For answering your question: only electromagnetic waves of light are propagating, but there are no quantized particles because the quantized momentum is transmitted directly from one electron to the other, without need for a traveling particle.

Answer 3

Does light travel “quantised”?

Choose the one you like best from the many linked answers. Or in other words, blame yourself if you are interested in something that is not observable.

Or, is it ... quantized only when it interacts ,i.e, undergoes measurement?

Clear answer, no:

• In the photoelectric effect it is observed or better measured as a particle.
• In the theory of emission of photons from excited subatomic particles it is treated as a quanta, later called photon.
• In the case of deflection (slit and edge experiments, prism) it is treated as a wave.

In all cases an interaction with the measurement apparatus takes place. What happens between the emission and the deflection or absorption is pure speculation.