I'm willing to converse with anyone. I extricated myself from the previous thread, though not before making a few remarks I am not proud of, because it was no longer discussing the topic. I tried to keep it on track but we all saw what it became. I am not pointing fingers at any person nor assigning blame for why it became so, perhaps it was even my own fault. However I think we can agree by page 6 or so very little productive thought was enmeshed within the shouting and anger. I'm very glad Scott is cleaning it up.
I must admit I have become a bit prickly on this forum due to the many insults directed at me in my short time here. I noticed your thread on why people are insulted and, though I abstained knowing myself too well to delve into anything so tendentious at this time, the discussion seemed interesting and I noted a few strong and interesting points by a few posters. Though you may have never intended insult in your first post to me, as it seems increasing clear you did not, I have been faced with extremely open and blatant attacks on me by name from others. This has likely increased my sensitivity to such matters. If it was I who caused the devolution of the previous thread I am quite sorry. That aside, I'll try to do my best to explain what I can.
You ask
Spiral Out wrote:hat is the current scientific assessment of what light is, a particle or a wave, or is it up in the air at the moment?
The current assessment is not simple enough to be fully explained in a one word answer. The short version is that light is an electromagnetic wave which has discrete characteristics which cause it to interact with matter much like we think of traditional particle doing. What does this mean? I'll do my best to explain in detail, and apologize in advance for the length.
Maxwell discovered in the late 1800s that a changing electric field induced a magnetic field and that a time varying magnetic field induced an electric one. This is the principle used to "step up" and "step down" current and voltage in transformers. Current in one coil is used to induce current in another unconnected coil with a different number of loopings, this loop count difference determines the raising or lowing of voltage and current in the traveling line. What Maxwell realized was that in the right conditions a time varying electric field (E) could set up a magnetic field (H) which would be perpendicular to the E field and vis versa. As the E field decayed it would drive the H field, which would decay after the E field and thus drive it since the time variance of the H field gives rise to an E field. Thus the two fields could propagate as a transverse wave, each field's normal vector pointing perpendicular to the direction of the waveform. If you think of a Cartesian coordinate axis it would be like having the E field point along the X axis, the H field point along the Y axis and the wave would be traveling along the Z axis.
This research was done completely independently of any theory of light. Due to the nature of E and M fields it is possible to calculate the speed at which such a wave would move. By calculating the electric permittivity (epsilon) and the magnetic permeability (mu) it is possible to determine the speed of the wave. What Maxwell was shocked to find was that the speed of this wave is exactly the measured speed of light. This is when EM wave first came to be considered in optics. Before then many experiments had shown light to behave in a wavelike manner but no one could answer the question "what is light." Since then many more experiments and advances in physics have shown that light is an Electromagnetic wave of the type Maxwell discovered. However, that is not the end of the story.
The discovery of the Photoelectric effect caused serious reconsideration of the nature of light. I'll explain the effect in case you don't know about it (I've been accused of being pedantic or pedagogical, I hope I do not appear overly so, I'm just trying to be as thorough as possible since I do not know the backgrounds of anyone on these forms a priori). The photoelectric effect is the name given the displacement of electrons from the "electron sea" on the surface of a metal by incident electromagnetic radiation. What scientists found was that if a light is shined on the metal no electrons are displaced until the light reaches a certain wavelength (equivalently a certain frequency). Before this frequency no electrons are dislodged, after it a constant number are dislodged.
Say the required frequency is at the wavelength 500nm (the smaller the wavelength the greater the energy it contains) above 500nm (less energy) no electrons are dislodged from the metal, at 500nm say 1 billion a second are (the metal gets new ones from the increasingly ionized air around it so the effect over time is that a constant voltage is created on the surface, that is the metal reaches an equilibrium with the environment so that electrons displaced over time do not "fully deplete" its resources). What was found was that increasing the energy in the wave, say by dropping the wavelength down to 200nm did not increase the number of electrons dislodged. You could take your EM radiation all the way down into the X rays and still not get a single electron more off of the metal each second than you could with 500nm. However, increasing the overall amount of light incident on the metal
would increase the electrons being kicked off.
What this means is that, if you had a 40w bulb pointing at the plate you get say 1 billion electrons a second, bump it up to 100w bulb and you still get 1 billion a second, but get a second 40w bulb and put it with the first and you now get 2 billion electrons a second. This lead scientists to realize that electromagnetic radiation comes in "quanta" or later they renamed the "quanta" as "photons." Basically the electromagnetic waves are not a continuous waveform incident on the plate like we think of ocean waves as being, rather each wave contains a "quantized" amount of energy. What happens is each time an EM wave reaches the surface of the metal is that it is "absorbed" by an electron there. The electron is able to use the energy in this wave to "leap" up to a higher quantum state if the energy in the photon is enough for it to do so, if not the photon is re-emitted and the electron goes back to business as usual. If the energy in the wave is not high enough to raise an electron to an excited state on its own the odds are astronomical that any electron will absorb two such waves fast enough to add their energy together and thus make it out.
There is an analogy ( albeit a rather cruel one) which illustrates the effect. Imagine a pit guarded by a spiteful demon and full of tens of billions of children. The children can leave at any time, but they must pay 1 dollar to the demon to do so, when they leave the demon takes all the money they have. The air in the pit is caustic to metal and so any money in the pit decays rather quickly. Adults outside the pit can throw money down to the children to get them out. If the adults throw down a rain of quarters no child will ever be able to leave the pit since probability dictates that the quarters are spread out too evenly among them for anyone to accumulate a dollar before the money decays. But if the adults switch and start throwing down silver dollars every coin that reaches the bottom frees 1 child who can use it immediately to escape. If they change to throwing 5 dollar coins no extra children get out because each child who gets a coin escapes but cannot pay the fare of any other. If instead of throwing 5 dollar coins they simply increase the number of 1 dollar coins they throw by 500% five times as many children escape because 5 times as many receive the 1 dollar coins. This lead to the realization that the incident energy from the light source was not arriving in a constant stream (like money continuously ticking up in a bank account over time) but in discrete intervals like the rain of quarters or dollars.
Numerous physical effects are a result of this quantized interaction including the spectral lines seen in the sun, neon signs, and other highly monophysical light sources when viewed through a prism. In this way we often say that light "interacts like a particle" since it does not come in one continuous wave (like an ocean wave) from source to target but rather in billions of "wave packets" each originating from a single atom at the source and incident on another single atom with a definite quantization of energy at the target. So again the short version is that light is a transverse Electromagnetic wave as described by Maxwell, but due to the fact that each wave packet of light carries a specific quantization of energy dependant on its wavelength, it interacts with matter in a way more like what we expect a particle to behave like because like a particle the energy is not a continuous feeding (like from a hose) but rather a "kick" like from a pool ball.
I love talking about physics and I hope this answers your question.
- By Bethel Saint Bright
- By Sy Borg