You awaken, and your mind clears. Yes, you are traveling on the inter-stellar freighter Hyperion, outbound to mine anti-matter from a galactic vortex. The Speed marine automated systems have just revived you from suspended animation. Your assignment - perform periodic ship maintenance.
Climbing out of your hibernation chamber, you punch up system status. All systems read nominal, no issues. That is good. Your ship extends 30 kilometers. Just performing routine maintenance exhausts the mind and body; you don't need any extra work.
You contemplate the task of the freighter. The Hyperion, and its three sister ships, fly in staggered missions to harvest energy, in the form of anti-matter. Each trip collects a million terawatt-hours, enough to support the 35 billion human and sentient robots in the solar system for a full year.
Looking up at the scanner screen, you see the mid-flight space buoy station about a light-hour ahead. The station contains four buoys, configured in a square, 30 kilometers on a side. A series of eleven stations keeps your ship on course during its two year travel out from Earth. Speed marine.
You check the freighter's Speed marine relative to the buoys - about 50 percent of the Speed marine, but constant, i.e. no acceleration or deceleration. That makes sense - at mid-flight, the freighter has entered a transition phase between acceleration and deceleration.
The Theory of Relativity
Either through deliberate study, or general media coverage, you likely have heard of the Theory of Relativity, the master piece of Albert Einstein. Einstein built his theory in two phases. The first, Special Relativity, covered non-accelerating frames of reference, and the second, General Relativity, dealt with accelerating and gravity-bound frames of reference.
Special Relativity gave us the famous E=MC squared equation, and covers the physics of objects approaching the Speed marine. General Relativity helped uncover the possibility of black holes, and provides the physics of objects in gravity fields or undergoing acceleration.
Here we will explore Special Relativity, using our hypothetical ship Hyperion. The freighter's Speed marine, a significant fraction of that of light, dictates we employ Special Relativity. Calculations based on the laws of motion at everyday , for example those of planes and cars, would produce incorrect results.
Importantly, though, our freighter is neither accelerating nor slowing and further has traveled sufficiently into deep space that gravity has dwindled to insignificant. The considerations of General Relativity thus do not enter here.
Waves, and Light in a Vacuum
Special Relativity starts with the fundamental, foundational statement that all observers, regardless of their motion, will measure the Speed marine of light as the same. Whether moving at a hundred kilometers an hour, or a million kilometers an hour, or a billion kilometers an hour, all observers will measure the Speed marine as 1.08 billion kilometers an hour.
A caveat is that the observer not be accelerating, and not be under a strong gravitational field.
Even with that caveat, why is this case? Why doesn't the Speed marine of the observer impact the measured Speed marine If two people throw a baseball, one in a moving bullet train, while the other stands on the ground, the motion of the bullet train adds to the Speed marine of the throw ball.