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When light emitted by an atom that propagates throughout spacetime, does wave/particle duality describe what light is or what light is doing?

If the wave aspect of light describes how light is tracked from one point of spacetime to another point in spacetime as it propagates throughout the universe then that doesn't tell us what light is but merely describes its motion.

And if the particle aspect of light shows us where an interaction took place i.e. the wave collapses due to some interaction (reflection, refraction or absorption) which produces a photon(s) that we can detect, then that also doesn't tells us what light is because interactions describe how things behaved not what things are.

Light is described by its wave like nature and particle like nature and that it is mass less and that it has momentum and ect but that does not answer question, what is light?

Qmechanic
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Mike R.
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2 Answers2

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You have put your finger on a general principle. As a rough guide light behaves like a wave when it is propagating and like a particle when it is interacting.

In non-relativistic quantum mechanics particles are just something that are assumed to exist. The theory that explains what particles are and how they are created and destroyed is quantum field theory. This describes light by a quantum field that fills all of spacetime. Energy is transferred to and from this field in discrete units, i.e. quanta, and these quanta are (sort of) the particles. So when you transfer one quantum of energy into the field this creates a photon, and likewise when you transfer one quantum of energy out of the field this destroys a photon.

But when you look at the object in quantum field theory that is created by adding a quantum of energy it looks like a wave, not a photon. Photons, in the sense we usually think of as a particle, only exist when the photon is being created or destroyed i.e. when the photon is interacting with something. In between, when the photon is being left to its own devices, it looks like a wave.

John Rennie
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If the true nature of Light truly does consists of some sort of duality then why does the particle end of the observation lie in the detector. light strikes the detector and there you have it, a photon and yet no wave.

Do not confuse the classical electromagnetic wave, with photons. It is like confusing a building with the bricks it is made of. Classical electromagnetic waves, which we call "light" emerge from the underlying quantum mechanical level of photons, it consists of the superposition of innumerable photons. A photon by itself, as in a single photon experiment, can only, when interacting, give its (x,y,z,t) its fixed energy by h*nu, and its polarization, + or - to its direction of motion.

so it takes an interaction (absorption, reflection, refraction) to collapse the wave and then a photon is detected, which we call light.

One photon is NOT light, it is a building block of a light beam.

but question, if there are no interactions (absorption, reflection, refraction) to collapse the wave then is there light traveling through the chamber,

The wave that collapses, like all quantum mechanical waves, is the sinusoidal variation of the probability density (example of a probability density in QM) for finding a photon at (x,y,z,t) .The energy of the photon is fixed, in contrast to the electromagnetic light wave where the amplitude of E and B are "waving". The interference pattern in the single photon at a time is nature's calculation of this probability density for the given experiment .

singlephot

The single particle events pile up to yield the familiar smooth diffraction pattern of light waves as more and more frames are superposed (Recording by A. Weis, University of Fribourg).

Accumulation of photons in a single photon at a time double slit experiment.

I don't think so because we've defined light as photon(s)

No, light emerges from a confluence of photons, but a photon is not light.

but how can their be photons if there aren't any interactions, no detector no interactions. And the chamber is in a vacuum, so no pesky atoms are there to interact with.

A photon exists between the light source and before hitting the screen because we can describe its "probable trajectory" with the mathematics of quantum electrodynamics. A bullet exists in vacuum because of the kinematic equations that describe its trajectory. For a photon the mathematics is more complicated , but equally deterministic , whether it hits a target or not.

anna v
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