Any initial exploration of parallel dimensions must logically proceed from a fixed location, because the amount of energy required would not allow for a portable power source. Thus, as with interstellar travel, early interdimensional trips are likely to be one-way. Fortunately, if a beachhead can be established in another dimension, it should be a simple matter for subsequent expeditions to transport the materials necessary for the construction of another power source. It is in establishing that beachhead that the real risk lies.
Initial dimensional journeys are unlikely to be carried out by humans, but rather by probes designed to test the gravity, radiation levels, atmosphere, pressure, and temperature-and to bring back samples of microorganisms-to ensure that humans can survive, and that they are properly equipped. Such probes must be tethered to the original dimension to send back information (since there is no indication that communication signals would travel back any more easily than objects could).
The use of probes, however, should allow dimensional physicists to develop a kind of "matrix map." Not only can they note which dimensions are hostile to human life, but, with sufficient data points, they can extrapolate which dimension "frequencies" are likely to prove conducive to human life. The first human dimensional travelers are likely to be extremely well prepared for the environmental conditions they encounter.
Other factors may prove more hazardous, however. In addition to the perils of first contact with a xenophobic populace, dimension travelers must contend with the possibility of equipment failure, dimensional static, scale variance, and encounters with other travelers who might not be friendly.
As the science of dimensional travel advances, explorers carry portable dimension gate generators, enabling them to come and go through dimensions as they please. If that equipment fails for some reason, the expedition might be trapped, possibly without the means to repair the damaged generator.
Dimension gate generators-whether stationary or portable- should not break down at random any more than a starship does (unless, of course, the campaign revolves around that very problem).
Complete Shutdown: The generator simply stops working, either because its components are damaged or because it has run out of power. Fixing damage components usually requires 10 hours and a successful Repair check (DC 25), while constructing a new power source (a complex device) requires 60 hours and a successful Craft (electronic) check (DC 25). Locating a replacement power source in a civilized area may require a successful Gather Information check, and negotiating for it may require a Diplomacy check.
Miscalibration: A miscalibrated dimension gate generator doesn't take the characters where they planned to go. Correctly recalibrating the generator involves either downloading the data from another functional generator (a full-round action followed by a successful DC 10 Computer Use check) or returning to the last "accurate coordinates" and resetting the matrix (12 hours of work followed by a successful DC 25 Computer Use check).
Communication Failure: There is no guarantee that standard communications work across dimensions; even communications designed to work across interstellar distances are useless when the party for whom the message is intended is not in the same dimension. A d-com (see Dimensional Communicators, below) or similar device enables communication across dimensions.
Dimensions are constantly splitting into new dimensions as events create alternate realities. These divergences release tremendous amounts of energy, which manifests as a kind of "static" during dimension gate operations. Generators are designed to filter out this noise and lock onto the specific "signal" of the intended destination. However, if the generator isn't getting enough power, or if the static level is extremely high, the gateway between dimensions is less stable.
Traveling through an unstable gate is potentially fatal. The traveler must make a Fortitude save (DC 15). If the check succeeds, the character arrives at the intended destination but is stunned for 1d4 rounds. If the check fails, the character arrives on target but is nauseated for 1d4 hours. If the save fails by 5 or more, the character arrives on target, takes 2d6 points of Constitution damage, and is nauseated for 1d4 hours.
A potential risk in traveling to other dimensions is a matter of size: Is everything in the other dimension on the same scale as the travelers who visit it? A scale variance can be simulated by changing a character's effective size. For example, a Medium-size character might be considered Fine in the new dimension. Such a variance, of course, changes the character's size modifier to attack rolls and Defense. Speed also changes, multiplied by a factor based on the change in size: Fine x0.16, Diminutive x0.33, Tiny x0.5, Small x0.66, Medium-size x1, Large x1.33, Huge x2, Gargantuan x2.66, Colossal x3.33.
The damage a character deals with natural and artificial weapons also scales with size. For every step by which a character's size category increases or decreases, increase or decrease the damage by one step: 1, 1d2, 1d3, 1d4, 1d6, 1d8, 2d6, 3d6, 4d6, 6d6, 8d6, 12d6. Attacks that deal 2d4 points of damage scale down to 1d6 or up to 2d6. Attacks that deal 1d10 points of damage scale down to 1d8 and up to 2d6. Attacks that deal 1d12 points of damage scale down to 1d8 and up to 3d6.
If humans are capable of traveling through dimensions, it is reasonable to believe that intelligent beings, either from other worlds or other dimensions, also have this capability. Other dimensional travelers might not be friendly. They might be raiders, plundering other dimensions for the resources they lack in their own. They could just as easily be transdimensional traffic police, tasked with detecting and disabling unauthorized dimension gate generators. They could simply be savage monsters, naturally capable of dimensional travel and drawn to unusual interdimensional activity.