It states very clearly,
"No communication is possible with a ship in jumpspace. It is effectively cut off from the universe for the week it is in jumpspace."
As you stated above and I will accept that they are part of the same universe. Therefore, no communication is possible because they are "effectively cut off" from the universe.
You also say that clearly this only applies to realspace, but this is you changing the meaning of the words used. You need to decide. If jumpspace is within our universe, then no communication is possible because, "The Mongoose Traveller Core Rulebook (2022 Update) states that a vessel in jumpspace is “completely isolated from the rest of the universe.” It says that the vessel is completely cut off from the rest of the universe. So maybe you can communicate from inside jumpspace, you just have to go outside of the ship to do it. lol. Rest of the universe is anywhere other than where you are at currently. In this instance, your current location would be in a jump bubble in jumpspace.
If I am swimming in the Atlantic Ocean off of the cost of Iceland and I use the phrase "rest of the planet" am I referring to everything on the planet where I am not? Or am I only referring to anything that is not the Atlantic Ocean, meaning I would not be counting the Atlantic Ocean just off the coast of Brazil as part of the "rest of the planet" since I consider them both to be part of the same location? Common usage would seem to indicate the former as opposed to the latter.
So, my previous reply was intended in all seriousness. I apologize if you felt it was flippant. That was not my intent. So, for the misunderstanding, I apologize.
As a scuba diver, I would take that to mean "the rest of the planet not covered by water". Surface swimming blurs that edge, because you are at the cusp of transition.
"No communication is possible with a person diving underwater. They are effectively cut off from the universe for the week they are underwater.". 50 years ago, that was true. We have eventually learned to use acoustic, photonics and other tech to allow for communications between divers, and even recreational divers might be found with beacons and wireless intercoms.
I can see your perspective, but the nuance of "effectively" should not be interpreted as "explicitly and immutably"
I think it is the fact that the rules don't mention ships in jump space communicating with each other by some mechanism is all the wiggle room needed for this, which also makes me wonder if two ships jumping along similar jump lines are in the same jump dimensions...
"JUMP THEORY
There are several differing theories of jump space*, and although jump technology has been used for star drives for thousands of years, a precise understanding of it is not necessarily a prerequisite; high quality data on jump space is difficult to obtain.
The basic concept of jump space is that of an alternate space**. Theoretically, jump spaces are alternate universes, each only dimly understood from the standpoint of our own universe***. Within jump space, different physical laws apply****, making energy costs for reactions and activity different and imposing a different scale on size and distance."
* this is in setting and metagame
** alternative space, different universe, alternative dimension is often substituted here
*** jump spaces, note the plural, are alternative universes
**** different physical laws that have a very sever interaction with matter and energy from our universe, yet somehow the jump field protects...
Lots of discussion about this in canon.
From the Starship Operators Manual (emphasis added by me):
JUMPSPACE
Many believe jumpspace comprises the ‘bulk’ in which our universe is embedded. A common, if trite, analogy is of viewing our universe as a 2D sheet in a 3D space, with jumpspace all that is above and below it. It is unclear whether jumpspace is part of our universe, another universe, a series of universes parallel to our own, or something external to our universe altogether. This uncertainty gives rise to different nomenclatures denoting the reality in which we live, with the most common being ‘realspace’. Critics regard this term as a misnomer, pointing out that jumpspace is very much as real as the space in which we live, irrespective of how alien it might be. This book uses the term because it is how most of our audience already refers to it.
When performing a jump, ships translate from the ‘surface’ of realspace to a certain ‘height’ in jumpspace, called the jumpspace level. Studies prove jumps of different distances correlate to different levels, with the longer the jump in realspace the deeper a depth travelled through jumpspace. No other correlation is found between realspace and jumpspace, and studies suggest that scientists will not ascertain any. When jumping, a ship is removed altogether from realspace, such that the concepts of its ‘position’ and ‘speed’ are meaningless for the duration, until reemergence. It is possible that the ship might have position and speed within jumpspace but these do not map to realspace equivalents and there are no perceptible external reference points within jumpspace, making determination of location and velocity impossible.
From Traveller 5, Book 2:
Multiple Jump Spaces
Jump Space is a continuous series of alternative spaces, each with a slightly different set of different physical laws.Each of the Jump Spaces is infinite, but smaller (often muchsmaller) than Real Space.Jump Space is classified by the distance it tolerates.Jump-1 allows a ship to move about a parsec in about aweek. Jump-2 tolerates a ship moving two parsecs in abouta week. The maximum Jump-9 tolerates nine parsecs.
Jump Space.
Jump Space is an alternative space in which the laws of physics from the true universe do not apply. JumpSpace itself is truly alien: inimical to life and even to matter; it is only the special protections of the jump field that protect a hull and its contents from immediate destruction. Jumpspace exists in multiple levels which are exploited by jump drives to produce different distances travelled. More specific names for portions of jumpspace include hopspace, skipspace, leapspace, boundspace, and vaultspace.
3 IN JUMPSPACE
A ship in Jump Space is totally isolated from Real Space.
It is Undetectable. A ship in Jump Space is undetectable. There is no known system available to detect the presence of ships in Jump Space.(Some psionicists claim to be able to detect ships injump, but test results are inconclusive.)
It Is Almost Unstoppable. A ship in Jump Space is theoretically unstoppable. It remains in Jump Space until it exits at its planned destination. A ship cannot voluntarily terminate jump early. If its drives are destroyed, it still exits Jump Space at the end of the Jump time period (rather than immediately). If the Jump Field is breached, much of the matter is destroyed by Jump Space. What matter that remains exits Jump Space at the end of the time period.
Movement Vector Can Be Changed
A ship can change its speed and direction while in jump space. Vector change requires non-gravity-based drives or devices; gravity-based drives (due to their need to interact with gravity sources) are generally ineffective.