The Gygaxian vision of the astral and ethereal planes is largely functional: they are a way to get to the other planes. I think this is slightly misleading, causing a great deal of confusion, and does a disservice to the potential of the astral realm. [To put this in context, this whole thing comes about from my considerations of how to incorporate the Githyanki and Githzerai into the AZ Adventures campaign, as both of these races are said to actually live on the Astral Plane. This led me to think about exactly how we can interact with that plane, and forced me to confront what I see as some weaknesses in the Gygaxian and neo-Gygaxian/4e worldviews.]
The most important thing to realize about the Astral Plane is that it is not really separate from the material plane. Rather, it overlaps with the material plane. It is the material plane, but on a slightly different wavelength. Meaning, the physical geography of the material plane is the same physical geography of the astral plane. But the wavelength of "life energy" is different. That is, different forms of life inhabit the astral plane (although certain creatures can interact with both, as discussed below).
Astral = Spirit Plane
The astral plane is the location for "spirit beings". Because humans are "spirits in the material world", certain people can project their spirits out of their bodies and interact directly with the beings in the astral plane. This is the psionic discipline of Astral Projection, and the cleric spell Astral Travel. Some people contact the astral plane in their dreams, as their spirit leaves their body and travels around. The psionic disciplines of Clairaudience and Clairvoyance are essentially astral projections into nearby locations, allowing one to hear or see what is going on there.
A "spirit being", i.e., one on the astral plane, can be in the very same room as you, and you normally wouldn't know it. Your entire city might have ghostly astral beings all throughout it, but only psychics would be aware of it. In the 1e MM, certain monsters have vision which extends into the astral plane, especially ones with gaze attacks. Thus, if you attempted to spy on someone with Astral Projection, Clairaudience, or Clairvoyance, you could be setting yourself up for danger if their awareness extends into the spirit realm.
What it Looks Like
In some depictions, the spirit world is visually indistinct from the material world, and spirits might not even realize they are spirits. However, in other more "spooky" depictions, the astral plane is like a "dark, misty-moisty" version of the material world. It is a world of swirling vapors, and home to what Gygax calls the "psychic wind". This "dark, misty-moisty" feel is the version appropriate in the gaming milieu, in my opinion.
This is why dark, foggy nights are so primally terrifying. They remind us of the spirit realm, which most people only vaguely remember from their nightmares. This is also why people need a silver cord when astrally traveling, as the unguided spirit in the dark fogginess of the astral realm would get permanently lost.
Etherealness - the Astral and Material Come Together
At certain times, the two planes come fully together. Think of Samhain, the last night of the year, when the spirits of the dead walk the earth. This actually gave us Halloween, as the living are forced to dress as the dead to avoid attack, and to leave out sacrificial gifts to placate the hungry spirit beings.
Ethereal bodies do not exist in their own plane, i.e, there is no ethereal plane. Rather, being in an ethereal state means that a material body is "phase shifting" between the astral and material planes. This allows the ethereal body to pass through physical objects, the way astral bodies can pass through material objects on the astral plane.
Its Relation to the Undead
The astral plane is where common (non-monster) ghosts reside. A ghost haunting happens when someone's body is gone, but their spirit seeks to linger on the material plane, refusing to go on. Thus, they inhabit the world, but only in spirit form, and are thus invisible and normally unable to affect material objects. These common ghosts are low-powered astral beings, and can occasionally summon enough energy to perform only some small actions in the material world, blowing out a small flame, knocking a picture down, whispering in a soft voice, that kind of thing.
Astral beings can also attempt to inhabit a material body, leading to possessions (a hostile takeover), as well as channeling by psychic mediums (a friendly takeover). This is the common-sense and popular definition of the astral plane, one that we see depicted in such movies as Ghost, and Sixth Sense, as well as numerous horror movies.
A high-powered astral being can rise to dangerous monster status, taking the form, in game terms, of a Ghost, a Banshee, or a Specter. What separates a "low-powered" astral being from a "high-powered" astral being is the among of spirit energy it has consumed. The astral plane is the spirit plane, so beings there must survive on spirit energy, especially terror, rage, and so on.
Undead monsters are manifestations out of the spirit plane, and so, to a greater or lesser degree, they are basically ethereal in nature This is clearly seen in the case of Vampires, who can dissolve their bodies into gaseousness, which is the material manifestation of astral substance. It also explains the blood drinking habit, which is their way of consuming spirit energy, which, as spirit beings, they require to survive.
The level draining power of the undead is also explained by seeing them as ethereal manifestations of astral/spirit beings. The touch of the undead has such a devastating effect on the living because the undead are attacking the astral-spirit body of the material creature. They are directly affecting the life-force of the living creature on the astral plane, separating it from their material shell, thus causing the horrible aging and level draining effects.
This helps explain why most undead creatures (as well as demonic and angelic beings) can be hit only by magical weapons. Being ethereal, phase shifting from the astral to the material, normal weapons pass through them without harm. Magical weapons, being imbued with "spirit energy", can affect them, just as the magical gaze of creatures like the Basilisk can affect astral and ethereal bodies.
Application to the Githyanki and Githzerai
Etherealness being the "phase shift" of material bodies into an astral state (and vice versa) is the key to understanding the odd nature of Githyanki and Githzerai, who are said to live on the astral plane. The Githyanki were originally material beings, but through their psionic powers, attained the ability to shift their bodies to an extreme ethereal state, and thereby live unseen on the astral plane. They operate most of the time in the safety of the astral realm, but because they are still material beings, they need to travel back to the material world as well.
The emaciated bodies of the Githyanki are because of their prolonged separation from the material world. Their bodies, lacking food, water, air, sunshine and so on, are perpetually wasting away. This leads directly to their "almost skeletal" appearance.
The penchant for savage, gratuitous violence and general evil is also explained by their prolonged residence on the astral plane. Like other spirit beings on the astral plane, they gain sustenance from spirit energy. They have learned from the undead how to feed upon negative human emotions, and like the undead, they will victimize people in order to create this horrible feast. The Githyanki are, literally, a half-undead race.
The Githzerai are both less evil and less skeletal, exhibiting the fact that they spend less of their time in an ethereal state on the astral plane, and more of their time in the material world.
In a subsequent post I will provide the setting and hooks for these amazing psionic creatures in the AZ Adventures campaign.