Mass Relay

Mass relays are mass transit devices scattered throughout the galaxy, usually located within star systems. They form an enormous network allowing interstellar travel. Hailed as one of the greatest achievements of the extinct Protheans, a mass relay can transport starships instantaneously to another relay within the network, allowing journeys that would take years or even decades with only FTL drives. A Prothean data cache found on Mars led humans to the Charon Relay, a mass relay encased in ice and orbiting Pluto, previously thought to be a moon.

For information on launching trans-relay fleet assaults, see the Codex.

Overview
Mass relays are built with two long, curved metal "arms" with a set of revolving rings in between them, containing a massive, blue-glowing element zero core (that never seems to run out). The relays are made of an unknown but incredibly resilient material, the same material that the Citadel is built from. One of the relays even survived a supernova's wake without being damaged. They are 'cold' objects that don't emit heat or radiation, unlike starships, making them difficult to find if their position changes. Some relays, like the Charon Relay, are 'gravitationally anchored' to planets; others appear to be out in space and are carefully tracked.

Mass relays function by creating a virtually mass-free 'corridor' of space-time between each other. This can propel a starship across enormous distances that would take centuries to traverse, even at FTL speeds. Before a vessel can travel, the relay must be given the amount of mass to transit by the ship's pilot before it is moved into the approach corridor. In Mass Effect: Revelation, when a relay was activated, it moved to align itself with the corresponding relay before propelling the ship across space.

Many mass relays are currently dormant for unknown reasons, though they can be easily reactivated. Humanity's 'home' relay, the Charon Relay, had become encased in ice, but scientists were able to reactivate it using data recovered from the Prothean ruins on Mars.

There are two kinds of mass relay, primary and secondary. Primary relays can send a ship hundreds of light years but only link to one other relay, its 'partner'. Secondary relays can link to any other relay over shorter distances. After the Rachni Wars, space faring species won't open a primary relay without knowing where it links to, in case they run into another powerful and hostile species like the rachni. This caused a rift when the turians found human pioneers, ignorant of this Citadel Council prohibition, trying to open any mass relay they could find while exploring the relay network, eventually leading to the First Contact War.

A piece of Prothean artwork depicting a mass relay, the Relay Monument, can be found on the Citadel Presidium. It has been interpreted as either a symbol of Prothean vanity, expressing the relays as their means to build a galaxy-wide empire, or possibly as a symbol of galactic unity, which the relay network also embodies. Tali'Zorah nar Rayya claims that after their long voyage through the galaxy, the quarians have also come to appreciate the aesthetic value of the mass relays.

Notable mass relays

 * Charon Relay
 * Mu Relay
 * Omega 4 Relay
 * Relay 314
 * Shanxi-Theta
 * Arcturus Prime

Mass Effect
The mass relays, despite common belief, were not created by the Protheans but by the Reapers. As made clear by Sovereign, the Protheans were merely one of many alien races to find the relays and the Citadel and take advantage of them. When the Reapers wiped out the Protheans, the asari were the next race to find the relays, thousands of years later. According to Sovereign, using the relays means that organic civilizations evolve according to the Reapers' wishes, but it refused to explain further.

However, the Protheans did have a keen interest in the relays, and managed to crack the secret behind their operation. This enabled them to build the Conduit before being destroyed by the Reapers.

Commander Shepard also discovers that the Citadel itself is an enormous, inactive mass relay leading to dark space, as well as the control center for all the relays, enabling the Reapers to sever travel between clusters. Its reactivation is, fittingly, more complex than that of an ordinary mass relay, requiring either a coordinated effort by the keepers or manual intervention by a Reaper.

Trivia

 * The mass relays were originally known as 'phase gates'; a relic of this can be seen in the Codex under Systems Alliance: Military Doctrine.
 * The mass relays evolved from more monolithic designs to their final 'tuning fork' shape (as described in The Art of Mass Effect).
 * See the modeling process for the mass relays here.
 * The relays resemble the farcaster portals in the Hyperion Cantos by Dan Simmons. Those are devices used for instant travel (while the ships are relatively slow) given to the humanity by a race of AI's to serve their own unclear purposes (which turn out to ultimately include the death of humanity). They are impossible to replicate. Like the mass relays, the portals are virtually indestructible.