We have seen that every reference frame in four-dimensional spacetime is accelerating. We have witnessed, however, the detection of gravity signatures with the only reliable means at our disposal now, a method that could be denoted:
Laser Interferometry (LI) utilizing the LIGOs.
LI allows us to use photon propagation in a near-vacuum to monitor the “clicks” or registers by photon-counters as gravitational quanta pass by them. Why?
Because we know photons “seem” to travel exactly at the proportionality constant, “c”, of Space/Time = c ~ 3,000,000 metres/second. 
In every reference frame of spacetime!
Are these gravity waves? Well, we could model the “clicks” we hear as “waves”, but what happens if we do not model them – at all?
Let’s ask Mr. Stevie Wonder !!! His ears have been well-tuned to hear notes and perfect tone as a musician. He is also blind and cannot be deceived so easily.
What Mr. Wonder may express to us is:
“No, I cannot hear a click,”
Or oppositely, “Yes! That is a click.”
With no formal gravitational or mathematical training, he will perceive gravitational signatures. Why?
Mr. Wonder understands we are not measuring waves with a meter-stick, like Mr. Jacobson, in Part 1. He cannot see a meter stick. We are measuring clicks he can hear from a photon-counter. That is the beginning of our knowledge.
With no model whatsoever, he know the clicks do not get louder or softer, they only increase and decrease in number. Clicks only have one tone to Mr. Wonder.
He might say, ‘I only hear one tone. Waves and music have many tones and overtones. Of love, and insight. Of understanding. This is monotone, monotonous.’
So how has Mr. Stevie Wonder, helped us? By exposing a small amount of superstitition in our thinking.
 “I Really Appreciate Science,” – In a fifth installment of this website, I would summarize our simple view that all we observe in our 4 dimensional universe is driven by interactions that disperse energy. And more, that our 4 dimensional universe is the only universe made up of space and time that could survive with the right amount of stability and instability to allow evolution and the dispersal of energy towards chaos. Yet, there remains gaps and still more questions in our thinking. What is matter and mass, for example? And energy, what’s that? Is this all simple, too? In fact, they are simple. We know Einstein’s famous equation E= Mc^2 (this is how math and physics majors write “squared” with a ^2) showing that Energy is equal to Mass times the square of the speed of light, c.
Let’s start with the speed of light, the little “c”. The little c was picked when it became clear from experiments that the speed of light was a constant no matter how you measure it, whether jogging, or standing still, or traveling on a train. Always light moved at a constant speed. That’s unlike anything else we can measure. If you are on a train and measure anything else moving in relation to you, you will measure a different speed than if you measured it standing still. Think of a catastrophic head-on collision of two cars each moving at 50 mph. If you were unfortunate enough to be in one car, the other car would look like it was travelling 100 mph towards you! Not so with light. If you were in a car moving 50 mph and you were targeted by a state trooper and his laser speed gun, you would measure the speed of the laser light as “c”, the same speed that the trooper would measure the speed of laser light as “c”, standing still. As you move towards him, the time it takes light to reflect back to him is shorter and shorter. He can easily calculate your speed by measuring the time difference in reflections, because the speed of light is constant.
So, is there something “special” and “mystical” about the speed of light? I know when I started learning about stuff, a few teachers treated it as mystical. Or as something that could only be understood by a few great men, like Einstein (maybe, just maybe, because they didn’t understand it themselves?). It’s something, like Newton’s gravity. We just had to accept it to the point it numbed our brains. The speed of light is constant “c”. It’s constant “c”, because it’s the speed of light. Or the most mind-numbing mysticism of all, “You can’t travel ‘faster’ than the speed light. Why not? You just can’t.”
K.I.S.S. is a great concept for understanding our universe. It’s brought us this far. Look for the simplest explanation that disperses energy. We have a universe of spacetime made up of 3D space and 1D time dimensions that is tied together. It exists and evolves, because it can equate or transform dimensions of space (measure in feet, yards, meters, etc.) into the dimension of time (measures in seconds, hours, etc.). It needs some kind of proportionality constant that can do the conversions, and it has to be constant throughout space and time. And that constant is what we call the speed of light (measured in feet per second or meters per second, etc.) It equates a certain amount of space to a certain amount of time. That’s all it is! Nothing mystical; nothing a carpenter or mason, or assistant manager at KFC can’t understand. The proportionality constant of our universe happens to be about 299,792,458 meters per second or 186,000 miles per second, or 671 million miles per hour. Or about one foot per nanosecond. It just so happens that some particles like photons are constantly transforming space and time, moving as waves and particles. Lucky for us, because we can then measure them and begin to figure this stuff out.
 “For many years after Newton, partial reflection by two surfaces was happily explained by a ‘theory of waves’*, but when experiments were made with very weak light hitting photomultipliers, the wave theory collapsed: as the light got dimmer and dimmer, the photomultipliers kept making full sized clicks – there were just fewer of them. Light behaves as particles.” – (Richard Feynman, 1985)
“* This idea made use of the fact that waves can combine or cancel out, and the calculations based on this model matched the results of Newton’s experiments, as well as those done for hundreds of years afterwards. But when experiments were developed that were sensitive enough to detect a single photon, the wave theory predicted that the clicks of a photomultiplier would get softer and softer, whereas they stayed at full strength – they just occurred less and less often. No reasonable model could explain this fact. This state of confusion was called the wave – particle duality of light.”