What are gravitational waves?

I wrote this before the recent detection evidence, but I include that new information at the end of this.

The technical definition for gravitational waves is that they are ripples in the curvature of spacetime that propagate as waves, traveling outward from a source. -Wikipedia.

First of all, in order to understand how a ripple in spacetime is possible, you must go back to Einstein's theory of general relativity in which he predicted that spacetime is like a fabric that can be compressed, stretched, bent and folded. Einstein's math is solid for this. Based on this idea, a gravitational ripple wave is possible. The problem is that they are very weak because they originate from crazy stuff way far away from us.

Think of it this way. If you toss a pebble into a calm pond of water, the pebble creates concentric circle ripples that radiate away from the pebble's entrance into the pond. As the ripples travel further away they weaken and then fade. That's the same way that gravitational waves work on spacetime. Of course, you can't see them in the spacetime like you can on the pond water, but they're still there.

Now, that we understand what a gravitational wave is, the next step is to figure out what causes them. This is where we get into theory, because no one is sure about the origin of these waves. The sources that are usually cited are: white dwarfs, neutron stars, and black holes, objects that are massive and relatively small. When these small but massive objects cause acceleration, they can generate gravitational waves as long as the motion is not perfectly spherically symmetric. For example, a spinning white dwarf star would not form a gravitational wave. But, two close orbiting stars, one being a white dwarf or pulsar and the other a massive star will create a wave. A 1a type (white dwarf orbiting close to a large star) supernova will also do it because most of these massive explosions are chaotic and not symmetrical. Basically, you have to have two objects of different sizes orbiting each other to form a gravitational wave. That's the reason black holes can do it if smaller stars are orbiting them or if the two black holes are orbiting, especially if they collide. Although the Earth-Sun system can generate gravitational waves, they are miniscule. What you need is small massive objects orbiting or crashing into one another to create sizable waves.

Now that we know how they're created, the next step is to detect them. That's a difficult task. It's not something that you could see or measure with telescopes. What you need is an interferometer, a huge interferometer.

What does an interferometer do? An interferometer measures variation in coherent electromagnetic waves (laser light) after it passes through air at right angles. What you're looking for is if the waves interfere or are out of phase. If they interfere and dim then the waves are in phase and thus one could conclude that the separate spacetime regions they passed through are the same. If they are slightly out of phase then it means that the regions are different in size. The laser light is passed through a half silvered mirror and then reflected from two mirrors at exactly the same distance away before falling back on a detector. That's sounds confusing.

Here is a better explanation of how it works: the laser light passes through a half-silvered mirror turned at 45 degrees. This results in two beams. One beam goes straight through while the other beam is reflected at a right angle. Both the straight beam and the right angle beam pass through separate spacetimes (at right angles to each other) and are then reflected (from silvered mirrors) back to the half-silvered mirror. The right angle beam passes straight through the half-silvered mirror and goes to the detector. The straight through beam is reflected at right angles into the detector. The detector measures the strength of the two beams when combined. If the two beans are not in sync then there is evidence for a distortion in spacetime, a distortion that changes the length of one of the paths. The longer the two paths (both straight and right angle) are the better the chance of detecting something. The paths have to be through vacuum (they use long tubes with air pumped out). A gravitational wave will shorten one path and lengthen the other, but only by a very small amount. The detection electronics sees the waves as interference patterns, and these are converted to sound because these laser beams contain waves at the frequency of the gravitational waves themselves. The bottom line is that you get a chirp.

What makes the detection of gravitational waves difficult is that they are very weak. Even with the paths at 2 to 4 kilometers, the distance change is only 10 to -18 power meters with a strong gravitational wave. There is also the problem of noise in the detectors from various sources like seismic activity, cosmic background radiation, random photon generation in the laser, vibration of mechanical parts, electrical disturbances and spurious vibrations. Since the effect of gravitational waves is so weak these noise problems make it difficult.

Gravitational waves were created by the Inflation period after the Big Bang. Detecting these gravitational waves is the holy grail of cosmology, which would constitute direct evidence for this Inflation theory. Alas, it hasn't been done yet, despite some preliminary reports that were not verifiable.

The truth be told, there have been no verifiable detections of gravitational waves. One reason for this could be that dark matter is interfering with the waves. There is indirect evidence for gravitational wave existence in the microwave background measurements of the universe. Other evidence is in slight variations or perturbations in the frequencies of Pulsars. The math that describes this concept is wild.

Update! Gravitational waves have been detected on Feb 11, 2016 by the LIGO (Laser Interferometer Gravitational-Wave Observatory). The waves came from the collision of two black holes 1.3 billion years ago and the signal was detected on September 14, 2015. The power of this collision was 50 times that of entire output of all the stars in the universe, but by the time the gravitational waves from this event got here they were weak. The LIGO interferometers (they have two of them) have paths of 4km, but it's a problem when one uses these long paths because they're more sensitive to vibrations. They solved this by using what is known as 'Fabry Perot cavities' These cavities allow the laser beams to reflect back and forth 400 times (effective length 1600 km) before they merge to enter the detector. This extra effective length allows the operators to detect the spurious vibrations and cancel them. They also use a mirror trick to boost the laser power from 200 W to 750 kW. That's kilowatts. A higher power laser beam helps reduce noise. Another trick is to enhance the output signal that the detector creates from the interference pattern of the two beams. This helps to subdue the random vibrations and signal noise and measure differences of less than the size of a proton. That's mighty small!

The LIGO interferometer has the highest quality mirrors, the highest obtainable vacuum in the path tubes, and active and passive dampening that senses ground movement and cancels it. Supercomputers process the data (1000s of gigabytes per day) and special programs process the data to find gravitational waves.

I'm sure that this discovery will undergo peer review when it's published. Hopefully, it will be verified because it would be a great milestone in astronomical observational science.

Thanks for reading.