[RPG] Is it possible to move between the worlds of the Material Plane by travelling through the Border Ethereal

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The 'Creating a Multiverse' chapter of the DMG tells us surprisingly little about the Material Plane. There is a list of Known Worlds of the Material Plane (p. 68), and the introduction to The Planes (p. 43) specifies that 'the worlds of D&D exist within the Material Plane'.

Since:

  • All the worlds are housed within the one Material Plane1
  • The 'shores' of the Ethereal Plane 'overlap the Material Plane' such that 'every location … has a corresponding location'

Would it therefore be possible to enter the Border Ethereal in one world, traverse the Ethereal Plane, and leave the Border Ethereal in another world within the Material Plane?

I'm not asking about the practicalities or feasibility of such travel, just whether or not it is strictly possible according RAW and existing canon.

I'm aware that the design intent behind the 'worlds' is primarily that they can function as distinct campaign settings, which might make this question a little silly/technical.

1 Jeremy Crawford has made this more explicit in a Sage Advice video, which a user has summarized in this GitP forum post. The worlds are all within a single Material Plane, and (perhaps related/helpful) it's technically possible to use Teleport to move between them.

JC references 'Crystal Spheres' surrounding the worlds in the above SA. This Forgotten Realms wiki page provides an overview of these. I'm not sure whether or not one can escape these spheres through spatial/isometric travel in the Border Ethereal.

Best Answer

The Material-Ethereal Isometry

The Dungeon Master's Guide's description of the Ethereal Plane states (p. 48):

Its “shores,” called the Border Ethereal, overlap the Material Plane and the Inner Planes, so that every location on those planes has a corresponding location on the Ethereal Plane.

So, every point on the Material Plane corresponds to a point on the Ethereal Plane; we have a bijective map between the set of points of each plane. The question then, is whether or not this map is isometric - that is, distance-preserving.

Take two points in the Material Plane that are 60 feet apart. Are their corresponding points on the Ethereal Plane also 60 feet apart? Yes.
The spell etherealness assumes that we have an isometry between the two planes:

You can see and hear the plane you originated from, but everything there looks gray, and you can't see anything more than 60 feet away.

The rules (and lore) give no indication that the 60 feet we can see corresponds to anything other than 60 feet on the Material Plane; the spell assumes the two planes are isometric.

So what? This means that the distance between Toril and Eberron on the Ethereal Plane is the same as it is on the Material Plane. So moving through the Ethereal Plane does not solve the greatest obstacle to interplanetary travel, which is the great distance.

If we assume it is theoretically possible to travel between worlds of the Material Plane by mundane means, it is also theoretically possible to do so on the Ethereal Plane. But this is not "rules". The rules (and documented lore) don't address travelling between worlds this way, though some Illithids have been known to be space-faring people.

We could mention the possibility of travelling through the Deep Ethereal and popping out at some other point of the Ethereal Plane, but the rules about that are (intentionally) ambiguous. Planar travel is left largely up to the DM as the builder of worlds, and so from an actual gameplay perspective, this is only happening if the DM wants it to happen in the first place.

The only "rules" that permit travel between worlds is the spell Dream of the Blue Veil.

Dream of the Blue Veil is a spell added in Tasha's Cauldron of Everything (p. 106):

You and up to eight willing creatures within range fall unconscious for the spell’s duration and experience visions of another world on the Material Plane, such as Oerth, Toril, Krynn, or Eberron. If the spell reaches its full duration, the visions conclude with each of you encountering and pulling back a mysterious blue curtain. The spell then ends with you mentally and physically transported to the world that was in the visions.

This is the only instance where travel between these worlds is given any substantial mechanical support.

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