Einstein's Special Theory of Relativity states that the speed of light is measured to be 2.998 x 10^8 m/s in any reference frame. The significance of this is when you measure the speed of laser beams inside spaceships or satellites traveling in opposite directions relative to each other. (GPS satellite time correction)
Unlike bullets on trains, Special Relativity correctly predicts that observers on each spaceship will measure the speed of light on their spaceship and the opposite spaceship to be traveling at the same speed. To make this happen, the observers will disagree about the time, distance, and even sequence of events (simultaneity). This is the relative part of relativity. However, while the observers disagree on details of space and time, what they will agree on is causality or the interaction or non-interaction of objects.
I am not an astrophysicist but I was thinking about how these ideas may or may not apply to God (the Great Cause), the idea of covenants and the Atonement of Jesus Christ. First, God has revealed that He is timeless. God dwells in eternity or the "Eternal Now" where past, present, and future all present before Him. Joseph Smith taught:
“The great Jehovah contemplated the whole of the events connected with the earth, pertaining to the plan of salvation, before it rolled into existence, ... the past, the present, and the future were and are, with Him, one eternal ‘now'" (Teachings of the prophet Joseph Smith, p.220; History of the Church, 4:597)
Second, we know that that covenants like the Atonement of Christ are timeless and because they involve God, have effects even before they have happened yet. The Bible says that Enoch walked with God and that Elijah was taken up into heaven. These 2 prophets were still subject to the Fall of Adam and could not dwell with God unless they also had faith in and applied the atoning blood of Jesus Christ. But how could they accept and apply the atonement of Christ before it happened? Because, with God, it had already happened. The New Testament teaches that Christ was the "lamb slain from the foundation of the world" (Rev 13:8). The Book of Mormon teaches the same principle:
"And the Lord God hath sent his holy prophets among all the children of men, to declare these things to every kindred, nation, and tongue, that thereby whosoever should believe that Christ should come, the same might receive remission of their sins, and rejoice with exceedingly great joy, even as though he had already come among them." (Book of Mormon, Mosiah 3:13)
Lastly, the significance of the timeless nature of covenants and the Atonement of Christ is that we can begin to be blessed by the atonement of Christ and the covenants of the gospel by living up to them even before we have made them. What I mean is that my children can commit now to dress modestly and live a clean and virtuous life and be blessed now knowing they will enter into these covenants to live clean in the future. My daughters should not have to change their wardrobe when it comes time to go to the temple. And, marriages are blessed by individuals who keep themselves virtuous and chaste even before they commit to do so at covenant marriage.