Realm Travel and Inter-Realm Travel need to be separated here.
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Inter-Realm Travel is done via Bifrost Crystals, objects that can capture and hold the Light of the Realm Álfheim. This Crystal is integrated into a LANTERN and thus travel between Realms is possible. Realms are connected via Branches of Yggdrasil and a LANTERN allows a vessel to move through the branches via the Light. Realm travel is protected via Realm Gates, which puts a throttle on the Realm entry/exit points to control the flow of traffic.
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In the event that a Realm is severed from Yggdrasil's branches, a LANTERN cannot get you there. At this point, you will need to implement a Void Space Engine. The Void is the embodiment of Ginnungagap, the space between space, and by appeasing Ginnungagap one can reach otherwise unreachable locations. A VSE does just this.
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Realm Travel, typically referring to Realms containing planets and stars and galaxies, is done via Solar Rails. SR are massive structures placed at junctions where the fabric of space is a little weaker. The owning party will make a connection with other Solar Rails, tax all Star-Ships, and allow interstellar travel.
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Before there were Solar Rails, explorers relied on Jump Drives to chart the stars. As the name implies, a Jump Drive could make up to 30 - 35 small distance space jumps before it needed to recharge. This technology is often affixed on modern starfighters and escape pods to make quick escapes.
