The Earth’s gravity attracts us with the force we feel as acceleration while falling. This acceleration keeps us on the Earth’s surface. At this acceleration, we feel “normal”. In a spaceship accelerating by one G, corresponding to the Earth’s gravitational pull, we would feel quite ordinary. With a constant acceleration of 1G, after one year spent on a journey through the cosmos, we would be moving at speeds close to the speed of light. For the passengers on the ship to feel “natural” when slowing down at 1G, the ship would have to make a turn. Because of the movement at speeds close to the speed of light, the flow of time slows down dramatically. Measured by the clock on the spaceship, in this manner, we would reach a destination in the universe located six light years away from us in about eight years. The center of the galaxy would be reached in 21 years. Instead of those 21 years, clocks on Earth would measure 30,000 years. It would take the passengers about 28 years to reach the first galaxy. Traveling this way, it would be possible to tour the known cosmos in 56 years measured by the clock located on the ship. The passengers would return alive to Earth, where by then tens of billions of years would have passed (more than 20,000,000,000 years). The Sun would have burned out and the Earth would be scorched. Common sense tells us that if traveling at the speed of light the shortest distance measured in time to the imagined destination is the time it takes light to travel that distance. It is both true and false. It is true for an observer on Earth who is “being still”, but false for a passenger who is traveling fast. Time changes its speed for objects in relative motion.
When discussing phenomena related to light we must replace the classical law of velocity addition (Galilean transformations) with equations that keep the speed of light unchanged.