Science Forums 2

[CraigD]

Originally Posted by KickAssClown

… if you ask the question of if the barrier is massed, and mass is 90% empty space, then why doesn't the photon merely pass through the barrier?
…
in the case of a closed barrier, why must I accept that the photon is still contained within the experiment area?

For the first question, “why doesn’t the photon pass through the barrier”, we should start by noting both that, in a classical sense, any barrier is much more than 90% empty – if you consider an atom’s nucleus (classical diameter about m) and electrons ( m) to be “solid”, and the atom ( m) to be “hollow”, a typical carbon atom (6 electrons) is about = 99.9999999999 % empty. Even densely packed carbon atoms – diamond (3513 kg/m^3) or graphite (2267 kg/m^3) add about another division by 2 to its density, so a typical dense solid is about 99.99999999995 % empty. Using this figure, we can calculate the “classical” probability of a small, uncharged particle passing through a 1.5 m slab of graphite ( packet atoms), and get a surprising (and strongly contradicted by observation) 99.95 %.

From this, we should conclude what early 20th century physicists did: there’s something very wrong with viewing photons and sub-atomic particles as tiny little classical objects.

A detailed explanation would recapitulate most of modern physics, so jumping to the end, we explain why even a thin (1 mm, about 3000000 atoms) layer of something like graphite appears to allow effectively 0% of photons of visible light to pass through it as follows:
The electrons in the carbon in graphite are free to assume many different energies. For every frequency of visible light an electron is available to change energy, absorbing the photon. The electron then emits one or more photons, most of them either virtual photons of magnetic force that carry the energy to it’s atom’s nucleus and electrons in surround atoms in a phenomenon known macroscopically as heat, or if the electron’s energy is great enough to allow it to reduce its energy by a great enough amount, as one or more photons of visible or invisible light, in a phenomenon known macroscopically as glowing. If the electron emits a photon of the same energy as the one absorbed, the phenomenon is known as reflection. Graphite doesn’t reflect much.

On the scale of photons and electrons, and other fundamental particles, the idea of classical size and collisions isn’t useful. Instead, we must consider their interaction to be a summation of the wave functions of the photon and a superposition of the electron at various energies. Mathematically, this is very complex, but despite the difficulty we humans have calculating it, appears to be what huge numbers of photons, electrons, and other fundamental particles do to produce macroscopic phenomena, such as opacity to visible light

For the second question, “why must I accept that the photon is still contained within the experiment area?”, referring to the preceding explanation, we can see that the photon is not contained by the opaque barrier. It is absorbed, ceasing to exist, by an electron, which then emits photons of other than visible light.

Note that none of these explanations require consideration of gravity. The whole of observed macroscopic optics is explained by quantum mechanics, and would work about the same if gravity did not exist.

(All of the data used in this post is available in numerous references, such as wikipedia)