A Worldhopper is any character from within the cosmere books who is capable of 'traveling' from locale to locale in the Physical Realm by the use of some form of Investiture. Likely, this is achieved through access to Shadesmar (i.e., the Cognitive Realm).
Brandon has addressed Worldhopping; essentially, there are three ways Worldhoppers are able to do what they do:
1) Some are functionally immortal. They do not age.
2) Some age very, very slowly.
3) Some benefit from relativistic effects.
The only time manipulation for which there is solid evidence is in Allomancy, bendalloy and cadmium (within The Alloy of Law, Shadows of Self and The Bands of Mourning), and that's effectively a relativistic effect. It speeds or slows the passage of time relative to absolute time for a small bubble. This could be used to travel forward in time, but cannot be used to go back.
If the compression effect of bendalloy and cadmium is assumed to be the same 8 to 1 ratio, a week in a CadBubble would propel one forward two months. Not a huge leap really, but if one is functionally immortal from Atium burning, breaths, or some other Investiture, it's reasonable.
Travel across the cosmere is difficult and dangerous. The Cognitive Realm may be a home to dreams and ideas of men, but some of those dreams are nightmares and come with some serious teeth.
The philosophers of the various worlds often discuss the dangers and natures of ideas among a population. Naturally, they were making references to the fickle hearts of men in times of transition and stress. Some individuals know that the dangers of ideas are real in the Cognitive Realm. There is nothing more terrifying than the realization that one slip, one misstep or encounter with a powerful Cognitive being and one's life can be snuffed out like a candle in the wind.
Although one may understand the traversing of the Cognitive Realm in an academic sense, one may not truly understand the strange and fascinating perils that reside there. Roshar is of itself particularly dangerous, for the spren there are far more powerful than their diminished counterparts in the Physical Realm. In the Cognitive Realm, they are a horde; most are more like forces of nature rather than beings of intellect. They move and act, bound by their natures and intents. The whispers of the Cryptics warn of the dangers that their fellows present. One should not travel the Cognitive Realm lightly. On the other hand, it is fascinating to converse with the spren. The Cryptics are hard to ply for information, but if one listens carefully, one can decipher meaning in their circling words.
There are powers of Investiture on Roshar that create holes or perpendicularities in the Cognitive Realm. This is most fascinating because Shardpools are points on the various worlds where the raw power and Investiture of a Shard collects and condenses. There are such pools on each of the cosmere worlds, though not all have as yet been revealed. Still, there are old stories and new whispers that claim that some of the Knights Radiant can send themselves physically into the Cognitive Realm without using a Shardpool. Also, the Oathgates also seem to operate by traversing the Cognitive Realm, but just how this works remains a mystery.
Perhaps, someday, everyone will have access to the Cognitive Realm. The issue is, can just anyone survive this process? What would happen if the concentrated power would harm or kill? The concentration of pure Investiture is so great that it effectively dissolves the boundaries between the Physical and Cognitive Realms. Perhaps like so many things in the Cosmere, using the Shardpools in this way is more about intent, rather than physical limitation.
There are those who are adept at using Shardpools for traversing various worlds. Hoid is such a master. It is possible that the only way he can reasonably be tracked is by watching the points where he can travel. However, he is far too crafty to make it that easy to catch him.
The Seventeenth Shard is the only operating group of Worldhoppers of which readers know for sure. They seem to see themselves as a policing force, attempting to keep some kind of balance and observing the actions that are happening across the cosmere. They seem to feel that the peoples of Roshar are far too easy for an outsider to manipulate. However, subtlety is often a more powerful tool in making things happen.
There are likely others out there that traverse the worlds beyond their home. The only possible exception would be travelers from Sel. Sel is nearly impossible to visit, and very dangerous at that. All the Cognitive Realm is dangerous for the physical bodies that walk the dark paths, but Sel is far more so even than Roshar.
At the moment, only a small number of people know that Worldhopping is possible. But as the peoples of these worlds grow, it becomes more and more likely that others will eventually learn of the secrets of the Shardpools. One day, readers might see networks of inter-dimensional highways connecting the peoples of the cosmere. Worlds may ever push to expand their borders, seeing opportunity in the lands of beyond.
The future of Worldhopping is as unpredictable as the people who practice the art.
- Paraphrased from transcript.
There are ramifications beyond leaving one's world behind when one Worldhops to other worlds (i.e., physical ailment, aging, time travel lag, etc.).
There is an article by Chris Lough on Worldhoppers here.
There is an article by Drew McCaffrey on Worldhoppers here.
Q&A with Brandon
Q. Can anyone with access to Shadesmar learn how to Worldhop?
A. In theory; getting to Sel is really tough.
Q. How do Worldhoppers communicate on all the cosmere worlds? Is there one universal language? Do they just learn a new language for each world?
A. There is not a universal language.
Q: Obviously the world covers different planets, but most people on individual planets don't know about the other planets. Is there a planet within the cosmere where Worldhopping is common knowledge?
A: Is there a planet in the cosmere where Worldhopping is common knowledge. Um ... it's not a planet.
Someone: It's a space station? [Audience laughs] That’s no moon!
A: That's ... not as far off as people laughing think that it is. It's not a space station; it's not that futuristic, but there is a place in the cosmere where a lot of Worldhoppers have settled, it's where Iyatil is from, even though her ethnicity is not from there.
- JordanCon 2016
- The Way of Kings, 54. Gibletish
- The Way of Kings, Inter1.
- Words of Radiance, Inter6.
- Words of Radiance, 43. The Ghostbloods
- Words of Radiance, 63. A Burning World
- Words of Radiance, 88. The Man Who Owned The Winds
- Oathbringer, 61. Nightmare Made Manifest
- Oathbringer, 106. Law Is Light
- Getting to Sel
- JordanCon 2016