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

Question

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.

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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.

Answer 1

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.

Answer 2

Sure, though I'm not aware of much specific information about doing so

We can work this out sequentially from what little information is given on the Ethereal Plane in the DMG. First, let's look at some information on the Border Ethereal (p. 48):

The Ethereal Plane is a misty, fog-bound dimension. 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.

The main item of note in that quotation is that every location in the Material and Inner Planes has a corresponding point in the Border Ethereal. So if it exists on the Material Plane, it has a corresponding location (or "location") in the Border Ethereal.

Whether or not you can travel to another world through the Border Ethereal alone is not totally clear, though it may not be advisable. The Border Ethereal may or may not include regions which don't correspond to physical regions on the Material Plane, but if it is a 1:1 correspondence with physical Material Plane locations then travel might be particularly arduous.

You also don't get any special benefits to covering distances quickly, as that feature is associated with the Deep Ethereal region. This makes travel via the Etherealness spell pretty uncertain, as you only get eight hours to travel. You can travel while ignoring physical obstructions on the Material Plane, but will still be limited by your travel speed. You're probably not going to get to another world in eight hours, even with a fast travel pace.

The Deep Ethereal

The Deep Ethereal region of the Ethereal Plane offers more potential, but is harder to reach. Its main advantage here is that you get to travel without worrying about the distance you have to cover (p. 48):

Traveling through the Deep Ethereal to journey from one plane to another is unlike physical travel. Distance is meaningless, so although travelers feel as if they can move by a simple act of will, it’s impossible to measure speed and hard to track the passage of time. A trip between planes through the Deep Ethereal takes 1d10 × 10 hours, regardless of the origin and destination.

This would allow you to reach a distant physical location by transiting through a region where physical distance is meaningless. If you find the right Ethereal Curtain (you'll want a turquoise one to get back to a region of the Border Ethereal connected to the Material Plane), you can end up in a physical location arbitrarily distant from where you started.

So if the difference between worlds on the Material Plane is one of physical distance, you can reach any location via the Deep Ethereal. All you need to get started is some preparation to cast the appropriate Gate or Plane Shift spells. Whether or not physical distance is the only thing separating the worlds on the Material Plane is... less clear.

No Shortcuts

The final element of this answer is that this mode of planar travel, even if it works perfectly, is not a shortcut. It may make traveling the necessary physical distances possible, but it is not intended to make the planar travel easy (p. 44-45):

When adventurers travel to other planes of existence, they undertake a legendary journey that might force them to face supernatural guardians and undergo various ordeals. [...]

[...]

Passing through a planar portal can be the simplest way to travel from the Material Plane to a desired location on another plane. Most of the time, though, a portal presents an adventure in itself.

First, the adventurers must find a portal that leads where they want to go. Most portals exist in distant locations, and a portal’s location often has thematic similarities to the plane it leads to. For example, a portal to the heavenly mountain of Celestia might be located on a mountain peak.

Second, portals often have guardians charged with ensuring that undesirable people don’t pass through. Depending on the portal’s destination, “undesirable people” might include evil characters, good characters, cowards, thieves, anyone wearing a robe, or any mortal creature. A portal’s guardian is typically a powerful magical creature, such as a genie, sphinx, titan, or native of the portal’s destination plane.

Finally, most portals don’t stand open all the time, but open only in particular situations or when a certain requirement is met. [...]

Answer 3

No, you can't

There's a difference between the Border Ethereal and the Deep Ethereal. Every world has its own Border Ethereal Plane that coexists like a distorted mirror of that world's reality. You would have to leave the border into the Deep Ethereal, travel the distance between the worlds, enter the Border Ethereal of the new world, and leave the Ethereal Plane into the new Material Plane.

It's not really a rules question, but a question of how the cosmology works in D&D. That's why I was very confused about the RAW part of your question. You can picture the Border Ethereal like when Frodo puts on the One Ring in The Lord of the Rings. It's more like an in-between... a threshold dimension between the Deep Ethereal and the Material Plane. You can travel through it in all three dimensions, but that's just not enough to change worlds... because in that threshold dimension, you're still bound to your "planet" (so to speak).

Though it's theoretically possible

But because the Ethereal Plane is pretty much the glue that holds the multiverse together, you can travel to other worlds through the Deep Ethereal. But that's not a walk in the park - more of a whole odyssey/adventure/campaign. Just imagine it this way: You want to get to Mars. Etherealness is an airplane, but what you need is a spaceship.

D&D used to have several options to get to the Deep Ethereal, but they're almost all gone in 5e. So it's more or less up to the DM to get the party through the Deep Ethereal to another world before they can cast spells like Gate or Plane Shift.

But...

... there's a catch about all that. It's 3.5e lore.

The 5e Dungeon Master's Guide is not very clear about the Deep Ethereal. It just tells you how to get there and that you're somehow able to traverse between planes (DMG, p. 48-49):

To reach the Deep Ethereal, one needs a plane shift spell or arrive by means of a gate spell or magical portal. Visitors to the Deep Ethereal are engulfed by roiling mist. Scattered throughout the plane are curtains of vaporous color, and passing through a curtain leads a traveler to a region of the Border Ethereal connected to a specific Inner Plane, the Material Plane, the Feywild, or the Shadowfell. [...]

Traveling through the Deep Ethereal to journey from one plane to another is unlike physical travel. Distance is meaningless, so although travelers feel as if they can move by a simple act of will, it’s impossible to measure speed and hard to track the passage of time. A trip between planes through the Deep Ethereal takes 1d10 × 10 hours, regardless of the origin and destination. In combat, however, creatures are considered to move at their normal speeds.

It doesn't tell anything about different worlds on the Material plane. But the Deep Ethereal apparently still connects the Material Plane, the Shadowfell and the Feywild. But it's not very clear how worlds on the Material Plane are (if so) interconnected through the Deep Ethereal.

The Border Ethereal stayed pretty much the same, though (DMG, p. 48):

From the Border Ethereal, a traveler can see into whatever plane it overlaps, but that plane appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy. Ethereal denizens watch the plane as though peering through distorted and frosted glass, and can't see anything beyond 30 feet into the other plane. Conversely, the Ethereal Plane is usually invisible to those on the overlapped planes, except with the aid of magic.

[...] The Ethereal Plane also disobeys the laws of gravity; a creature there can move up and down as easily as walking.

So what we know about the Ethereal Plane... Toril, Shadowfell and Feywild are embedded within it like islands in the ocean, where the Border Ethereal is the coastlines and the Deep Ethereal is the deep/high seas.

If we take JC's confirmation into account, it seems that every Material Plane world is embedded into the Ethereal as well. But we don't have anything about how they're connected exactly. I suspect that that's intentional, so that a DM can decide whether (and how) they limit the universe for their table to their likings. It's theoretically possible to teleport from Faerun to Eberron, but that doesn't mean, that you can reach it through the Border Ethereal. The nature of the ethereal plane is very much untouched from JC's statement.