The Speed of Light, or Light Speed, is the maximum possible speed in the N-space universe, and is a constant speed of 299,792.458 km/s (often expressed as 3×108 m/s) in vacuum at which any particle of zero rest-mass (such as the photon) will automatically propagate.
- This value is commonly abbreviated as "c".
- Starships commonly travel at effective velocities measured in multiples of 170.0c (±20.0c jump variance).
- Exceeding Light Speed (FTL) requires a way to circumvent or modify the conventional physical laws of the universe.
Background: Special Relativity
The Special Theory of Relativity, is a subset of the more comprehensive General Theory of Relativity attributed to the ancient Solomani physicist Albert Einstein that deals strictly with the special case of the motion of objects in a flat spacetime environment independent of the influence of gravitational fields. The two principles that underlie the theory from which its consequences derive are:
- The laws of physics are invariant in all frames of reference.
- The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.
The latter principle is a consequence of the fact that electromagnetic radiation (i.e. "light") is a sinusoidal oscillation of the magnitude and direction of electric and magnetic fields in vacuum. It is not an oscillating wave within a physical material medium against which the velocity of the wave can be gauged (i.e. there is no absolute reference frame relative to which all independent observers can measure the speed of light - it is always measured relative to the observer because one cannot measure velocity relative to the "emptiness" of the vacuum).
Lorentz equations: Length-contraction & Time-dilation
In classical Newtonian physics, length/distance and time are independent invariant quantities that are the same relative to all observers. The basic relationship between distance, motion, and time is given by the equation:
- d = vt
- d = distance
- v = velocity
- t = time
In classical mechanics when an object is in motion, the distance (d) between two points and the time (t) experienced are the same for all observers, regardless of their state of motion. The velocity (v), however, is different depending on the state of motion for any given observer. Consider two air/rafts moving at 30 kph and 50 kph, respectively, relative to an observer on a world surface. The distance that the air/rafts traverse and the time it takes them to do so are the same for someone on the ground or on either of the air/rafts. However, the velocities observed by these observers are relative. If the two air/rafts are moving in the same direction, the person on the ground will say they are moving at 30 kph and 50 kph, respectively, but to the person on the first air/raft moving at 30 kph relative to the ground, the other air/raft is only moving at 20 kph relative to him, and the ground is moving at 30 kph in the opposite direction of the other air/raft relative to him. In classical physics this is known as the Galilean transformation.
However, as noted above, the speed of light is the same for all observers, independent of any relative motion. The implication of this simple observation has far-reaching consequences. In order for light to have both a constant velocity of propagation and exhibit that same constant velocity relative to any observer regardless of the observer's relative state of motion (i.e. you are not catching up to or receding from the light as you move), then for any two given observer's velocities the spatial distance through which the light is moving and the time it takes to do so must change in order for the laws of physics to remain constant in both reference frames. Put mathematically, if the speed of light (c) is observed to be the same for either of two observers moving at different velocities relative to one another, then in order for c to have the same value for either observer, d and t must change based on the relative velocities of the observers. By logical extension, therefore, when any two objects are moving relative to one another, the spatial distance through which they are moving and the time they take to do so are different depending on the relative velocities of one observer relative to another.
The degree of change of length/distance or dilation of the passage of time experienced by any given observer is given by the Lorentz transformation equations:
- Time dilation: t = to / (1 - R2)½, where R = v/c
- Length-contraction: l = lo × (1 - R2)½, where R = v/c
Note that at the speed of light when v=c, the term (1 - R2)½ = 0, meaning that time for the moving reference frame relative to an independent observer has dilated to infinity (i.e. it has ground to a halt) and distance/length has shrunk to zero along the direction of motion.
The effects of time dilation and length/distance contraction become a meaningful consideration for an alternate reference frame moving at speeds greater than ~ 0.10c relative to a given observer, whereas the effects of time dilation and length/distance contraction become significant for an alternate reference frame moving at speeds greater than ~ 0.90c relative to a given observer.
Simultaneity & Local Time
Another consequence of the invariance of the velocity of light in all reference frames is that the simultaneity of two or more events occurring within a given inertial reference frame is not preserved from the perspective of an alternate inertial reference frame. In other words, simultaneity of events is an entirely local phenomenon, not an absolute one.
By way of illustration, suppose a G-Carrier is moving with respect to an observer on the ground. If a passenger in the center of the G-Carrier payload compartment fires a dual laser, each beam propagating in opposite directions toward the fore and aft of the compartment, respectively, then from the perspective of the observer within the G-Carrier, both beams strike the fore and aft walls at the same time. However, to an external observer on the ground, he will also see the beams from the two ends of the laser-device travel at he same speed (c) in opposite directions, but the G-Carrier will be moving in the same direction as the forward-propagating beam and the light will be "catching-up" to the front compartment wall, and at the same time the light from the aft-propagating beam will be receding from the laser and propagating into the forward-approaching rear-wall of the compartment . The net result is that from the perspective of the ground observer, the aft-propagating beam of the laser will strike the rear of the compartment before the forward propagating beam strikes the forward wall of the G-Carrier compartment. What happens simultaneously in one reference frame does not occur simultaneously from the perspective of the other reference frame.
This has important consequences for relativistic vessels traveling between star systems at sublight velocities that are significant fractions of lightspeed. If a vessel is traveling between two star systems separated by 5.0 light-years at a velocity just under lightspeed, an observer on the origin or destination world would say it took them almost 5.0 years to complete the journey, but that the vessel was "compressed" in length along its direction of motion and that the space-farers aboard were moving very slowly, and experienced only 3.5 years (for example) of subjective time aboard the ship. On the other hand, aboard the ship time would be experienced normally, 3.5 years passing from the beginning to the end of the journey, but as they moved between the origin and destination world, the distance between their start and endpoint will have contracted, so that the distance they needed to travel was shorter, requiring only 3.5 years of flight time instead of 5.0 years. But at the same time, the origin and destination worlds were moving relative to the ship at just under lightspeed, meaning that though 3.5 years passed for them aboard ship, only about 2.45 years has passed on the destination or origin worlds by the time they arrive.
The end result is that from the perspective of the originating world, the journey took about 5.0 years, but the space-farers were only about 3.5 years older. From the perspective of the space-farers, the journey only took 3.5 years, but on the originating and destination worlds, only 2.45 years had passed. Thus, the vessel has moved from one location in spacetime, to another location in spacetime, with temporal coordinates out of synchronization with one another. Thus, time and simultaneity are local phenomena, not absolute or universal ones.
Mass-Energy Equivalency Principle
- E = mc2
- m = mass
- c = speed of light
Thus, any particle has an intrinsic "rest-energy" (E0) as defined by its mass while motionless (its "rest-mass", m0), independent of any internal thermal properties or kinetic energy it may acquire due to motion, as described by the equation:
- E0 = m0c2.
By extension, an object has a "kinetic mass" (mk) due to its kinetic energy (Ek) while in motion relative to an independent observer that is in addition to its rest mass (m0):
- Ek = mkc2
- mk = Ek / c2
Thus, as an object accelerates relative to an independent observer, its total mass (mtot), also known as "relativistic mass", relative to that observer increases:
- mtot = m0 + mk = (E0 / c2) + (Ek / c2)
The "total energy" (Etot) of a particle is described as the sum of its rest energy (E0) and its kinetic energy (Ek) by the following equation:
- Etot = E0 + Ek = moc2 / (1 - R2)½, where R = v/c
And is dependent upon its velocity relative to the one observing.
As a consequence, a constant force applied to a particle will produce a continually decreasing acceleration as its mass increases as a function of velocity, implying that in fact F ≠ m0a, but rather F ≈ m0a for small enough values of velocity. Thus, due to the equivalency principle of mass and energy, as m → ∞ with increasing velocity, a → 0, asymptotically approaching a maximum velocity. This limiting velocity is the maximum speed of the universe, and is also the constant speed at which zero-rest mass particles (such as photons and gravitons) propagate, and is colloquially known as the speed of light as a result. It is thus impossible to accelerate an object with non-zero rest mass to the speed of light in a finite amount of time (or alternatively, it would require an infinite force (and/or an infinite amount of energy) to produce the acceleration necessary to accelerate a non-zero rest mass particle to the speed of light).
Lightspeed as a distance metric
Due to the vast distances involved in interstellar measurements, it is often convenient to use the very large constant value of the speed of light in vacuum as the basis for a distance metric. As light travels at a constant speed of ~ 3×105 km/s in vacuum, the distance that light will travel in one year can be calculated to be approximately 9.47×1012 km. This distance is used as a common metric for interstellar distances, and is known as a lightyear. Shorter distances are also often defined in a similar manner, and include such distance metrics as the light-week, light-day, light-hour, light-minute, and light-second.
A related metric for measuring interstellar distances is the parsec, an abbreviation of "parallax second". A parsec is defined as the distance at which a shift in the position of an observer of 1.0 astronomical unit, or AU, will produce a shift in the observed position of a distant object of one second of arc. A parsec is calculated to be roughly 3.26 lightyears in distance, multiples of which also happen to be close to the maximum Jumpspace traverse distances of the standard Jump Drive.
Interstellar STL Exploration
The vast majority of known sophonts both current and extinct have historically relied on various methods of slower-than-light (STL) propulsion to explore and colonize on an interstellar scale, utilizing technologies such as "sleeper ships", generation ships, and high-velocity relativistic time-dilation to bridge the gap between the stars.   Such propulsion systems have included various types of fusion rocket (such as the Bussard ramjet or Nuclear pulse propulsion similar to the proposed ancient Solomani projects named Project Daedalus or Project Longshot, or possibly an Enzmann starship). However, many interstellar STL drives both modern and ancient have relied on the principles of the gravitic-based NAFAL drive. 
Interstellar FTL Exploration
A very few sophont-species have discovered a method of circumventing the normal restrictions of classical Einsteinian spacetime over the history of Charted Space, each having independently discovered the drive system known as the jump drive. The jump drive opens a bridge into an alternate dimension of spacetime that is related to normal Einsteinian space (or "N-space") in a non-linear fashion. As such, both velocity and positional information take on imaginary values during the operation of the drive, resulting in a transition from point-to-point in N-space that takes about 168 (±10%) hours, the N-space distance of the jump limited only by the performance characteristics of the drive. While in jumpspace (or "J-space") the positional and velocity values of objects in the N-space universe have corresponding imaginary values relative to the vessel. Since t = d/v, the imaginary components of position and velocity cancel each other, yielding a common real-valued time coordinate in both the N-space and J-space environments. Thus, since gravity is uniquely a curvature of both temporal as well as spatial coordinates in Einsteinian-spacetime, the only interaction a vessel in jump has with the N-space universe is the complex-valued jumpspace "shadow" of gravitational sources in N-space that project into jumpspace via a complex non-linear (and not well understood) interaction.
Major Races & FTL
Those races known to have independently discovered the jump drive are known in Imperial culture as the "Major Races", and number only seven throughout the known history of Charted Space (though only six are extant today): Humaniti (specifically the Vilani, Solomani, and Zhodani branches), the Vargr, the Aslan, the K'kree, the Hivers, the Droyne, and the extinct Ancients.
- Speed of Travel
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