The concept of wave-particle duality in quantum mechanics allows us to describe electromagnetic radiation both as waves and as discrete particles called photons. While it may seem contradictory to think of something as both a wave and a particle, this is a fundamental property of the quantum world.
When we say that electromagnetic radiation travels as waves, we are referring to the wave-like behavior of the electric and magnetic fields that make up the radiation. These waves can be described by properties such as wavelength, frequency, and amplitude. They exhibit characteristics like interference and diffraction, similar to other types of waves.
On the other hand, when we say that electromagnetic radiation interacts in a quantized way, we are referring to the fact that its energy is transferred in discrete amounts called quanta or photons. Photons are indivisible units of electromagnetic energy, and each photon carries a specific amount of energy determined by its frequency. When an interaction occurs between electromagnetic radiation and matter, such as absorption or emission, it takes place in terms of these discrete quanta.
So, while electromagnetic radiation can be described as waves, its interaction with matter occurs in a quantized manner through the exchange of photons. This quantization is a fundamental aspect of quantum mechanics and is supported by extensive experimental evidence.
Regarding your statement about photons from Andromeda not existing for the entire distance traveled, it is essential to clarify that photons do not "age" or experience time in the same way that macroscopic objects do. From the perspective of a photon, which travels at the speed of light, the entire journey is instantaneous. However, from our perspective as observers, we can measure the time it takes for the photons to travel from Andromeda to us based on the speed of light. It's worth noting that photons emitted by celestial objects like Andromeda take an incredibly long time to reach us due to the vast distances involved. But once they are emitted, they exist as discrete entities carrying energy across space until they interact with something, such as being detected by a telescope or absorbed by matter.