How Portal Stones Work
Author: Taimi Vandene
"From Stone to Stone run the lines of "if", between the worlds that might be."
Loial in TGH, p.217
To understand how Portal Stones work, we must first understand the structure of multiverse. I will apply to this essay David Lewis's theory called "modal realism". According to modal realism, the world we are part of is only one of the plurality of worlds. The rest of worlds are possible worlds. Our world is a spatiotemporal continuum: according to Lewis, we are worldmates to things that are distant in space like stars and distant galaxies, but also to things that are distant in time, like ancient Romans or flowers in year 6700. Lewis claims also that we are world-bound, meaning that we cannot travel to other worlds. Likewise, inhabitants of other worlds cannot come to this world.
However, in the WoT universe, we must assume that Lewis is mistaken, because Portal Stones do just what he claims impossible: people who use them travel to other worlds. Lewis's mistake is quite forgivable, as in the when and where he lives, channeling is not known. It is therefore logical to assume that connecting to the True Source is essential for successful operating of Portal Stones. It's also quite expected that it requires a strong channeler to operate a Portal Stone: a bond between an individual and his actuality isn't easily changed, and to do that, one needs to draw a fair amount of One Power.
Portal Stones work by operating actuality. People who use them do not travel anywhere in the traditional sense of the word. Instead, they manipulate actuality and modality. To understand what this means, we need to divide actuality into two concepts: personal actuality (actuality of given person, the Traveler) and objective actuality (actuality of the whole world of which the Traveler is a part). In normal circumstances, these are identical. But, when the Traveller activates the Portal Stone by channeling One Power, these get separated: the Travelers' personal actuality changes while objective actuality remains unchanged (it must be assumed that use of Portal Stone also prevents the causal effects that would normally follow from person being violently removed from objective actuality - these effects would probably resemble effects of balefiring). The separation of actualities is observed as the Traveler disappearing from Objective actuality.
However, it appears that the person's tie with objective actuality isn't completely severed: the worlds appear more or less "solid" depending on how far the Traveler is from his actual world. That is most logically explained by stating that personal actuality is in part inseparable from objective actuality and this manifests itself as distortion of Traveler's perception. From the "DO's point of view" - from the point of view of someone who is completely separated from multiverse - all worlds are equally "real". The only thing that makes one world real for the Traveler is the fact that there is only one world where he belongs - in other worlds, the beings that resemble him and make modality work are just his counterparts.
Sometimes, however, people appear to travel via Portal Stones in the sense of covering a spatial distance. Also, it's said that Portal Stones could be used for traveling inside a world (TGH p.525). This traveling can be explained by different worlds differing in size or time. If different worlds differ in size or in time, a day's ride in one world could be just a half a day's ride or two weeks' ride in some other world.
Traveling between two Portal Stones in this world appears to be a bit more complicated, if we assume that it isn't the same thing as normal Traveling without using the Stone. The only logical explanation is that as the Portal Stone's operating mechanism is always manipulating actuality, it must take the Traveler to his actual world destination via another world - namely such a world where the distance between two points of actual world is zero. The Traveler would perceive such Traveling as instant transportation from one Stone to another. This doesn't seem to be entirely without danger - it is quite likely that such a world would be a reasonably distant one - the more so depending on how far off from each other the two points of the actual world are. If something went wrong, the Traveler could find himself on a world that would be dangerously different from ours.
How likely it is then for our Traveler to get lost when he is moving between the worlds? First of all, he can't get lost in between worlds - there is no space in between the worlds. Moreover, existence itself is always "existing in a world" so, if the Traveler isn't in one world, he is always in another world. But, in more traditional ways it is possible to get lost.
For example, not knowing the symbol for his own world when he wants to return, or not being able to find a Portal Stone that has a counterpart in his world, or Traveling to a world where he didn't mean to Travel can result in the Traveler deciding that he's gotten lost. But even if he got lost in some distant world where there wouldn't be a Portal Stone that connected the Traveler directly back to his homeworld, it is possible that he could find his way back home. Portal Stones don't just drop people to random places - there is always another Stone in the end that is receiving the Traveler, and that stone could always help him get somewhere. As the tie with his homeworld can never be completely severed, he might be able to reason his distance from the world he is in and might be able to navigate home via heading towards closer worlds - or at least to a world that would resemble his homeworld so much that the difference would be insignificant.
The last interesting question is the location of Finnland. We have read that time and space act weirdly in Finnland, so it is clearly another possible world - not just a distant place in our world. It is therefore at least logically possible that somewhere there is a Portal Stone that is connected to Finnland. As the ter'angreal that led to Finnland have been destroyed, finding a Portal Stone connected to Finnland and knowing the right symbol could be alternative to entering the dangerous Tower of Ghenjei.
References: Lewis, David 1986: On the Plurality of Worlds. Blackwell Publishing, Plymouth.