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Is there such thing as a gravitational wave, or what are
gravitational waves ?
Imagine for a moment, that a Mythical God reached into our solar system and swiped our Sun clean away from us, and then after a brief second, put it exactly back. When our Sun was missing, the centripetal and centrifugal forces which keep our Earth in constant orbit automatically become zero, and the Earth then shoots out in a straight-line at a tangent. But since the pull of gravity - or the sudden absence of gravity – cannot be experienced by the Earth any sooner than fastest speed at which things travel – which is the speed of light – it will take at least eight minutes for the Earth to experience the absence of the Sun’s gravity - before it shoots out in a straight line. 1
The Sun is swiped away by a Mythical God. The Earth will not experience the absence of the
Sun’s gravity until 8 minutes later, at which time it will head straight-out at a tangent from its orbit.
Now, since the Sun was put back in place after one second, this is most likely enough time to restore the Earth back to its orbit, with perhaps a slightly altered orbital path (sorry!). The sudden absence and reappearance of our massive Sun could be considered a “gravitational disturbance”.
Because it took eight minutes for the Earth to experience the gravitational disturbance of the Sun’s absence then sudden reappearance, it is said that the pull of gravity must have “travelled to the object” which it is trying to attract (or unattract) and so like other types of wave phenomena, it is also thought to travel in “some type of wave”. But the attraction or release of gravity is unlike other wave action that is contained in the electromagnetic spectrum (i.e., light, radio, x-rays, etc) - gravity is considered more physical in nature, much more like a sound wave. 2
This is how an instrument such as LIGO detects a gravitational wave. It is designed to measure physical wave action, and since it is set up to measure the physical displacement of mirrors, for instance, it will not get “mixed-up” trying to detect some other form of radiation coming from the electromagnetic spectrum. It’s most likely rival is a seismic ground wave followed by an acoustical sound wave, but LIGO is ultimately designed to be isolated from the slightest vibration and is nominally located in a quiet zone. If LIGO reads a signal potentially from a gravitational wave, the seismometer and acoustical sensoring in the surrounding local area should indicate “all clear” to render the reading as valid. What’s more, a LIGO observatory is located in two places, far apart from each other such that possible random local activity (e.g., seismic) should only set off one LIGO instrument. If a gravitational wave comes through, however, it will trigger both LIGO observatories simultaneously. If that’s the case, then the signal must have come from outer space, and since LIGO can’t be tripped by anything on the electromagnetic spectrum – it must be a gravitational wave.
An actual case of a gravitational wave disturbance considered to be like our hypothetical example of a Mythical God, is the sudden displacement of two Black Holes which are whirling around each other about to collide, or merge. It’s the activity right before the merging of the two Black Holes which causes a “gravitational disturbance.” A Black Hole is known to be a massive entity thought mostly to remain stationary at the center of a galaxy. However under a rare circumstance, two massive wandering Black Holes can approach one another and displace each other at great accelerations (the same as suddenly displacing the Sun in our Mythical God example) and cause a gravitational wave disturbance – a series of “sudden jerks” on all the forms of matter around it, continuing to ripple through the universe eventually reaching Earth as a very faint “pull-and-release” wave, and in-turn physically displacing the very sensitive LIGO observatory.
Two Black Holes swirl around each other about to merge, causing rapid displacement of the two largemasses at astronomical distances. This occurrence accounts for the detection of gravitational waves.
Detecting gravitational waves is considered a rare occurrence. If the creation of the universe was indeed derived from a “Big Bang” and all matter came from a “dense kernel” exploding outward to create the galaxies, stars, and planets - then gravity has a hold on everything since the beginning of time – and doesn’t need to travel anywhere. It can be stated that all “gravity fields were set” since the beginning of time, and the pull of gravity has no further place to go, and hence there are no propagating gravity waves to observe. Since the time of the Big Bang, the only observable gravity wave that can occur is from anomalies such as a progressing Black Hole collision. 3, 4
The above explains why you do not hear about gravitational waves very often, not nearly as much as you would about other waves such as sound, light, infrared and all the remaining waves of the electromagnetic spectrum.
Also, there is no need to worry about a gravitational wave causing a disturbance to the environment. When it reaches Earth, the size of the wave is about 1/1000 the size of proton. The 2-1/2 mile long beam comparators of the LIGO observatory can measure displacements as small as 1/10,000th the size of a proton, or 1/1,000,000,000,000,000 the width of a human hair.
1. The Sun’s light rays travel at a speed of 186,000 miles per second and takes 8 minutes and 20 seconds to reach the Earth. If you
intend to awake in the morning at the crack-of-dawn and arise when you first see the Sun, from the viewpoint of the universe
you are already 8 minutes late !
2. Gravitational waves, like sound waves, are not included in the electromagnetic spectrum. Sound waves physically compress the
air to transmit sound. Think of the opera singer who can resonate a wine glass with their voice (sometimes breaking the glass). A
radio wave or light wave cannot do such thing.
3. The actual lack of observation of gravitational waves is in support of The Big Bang Theory.
4. During a discussion of Black Holes, reference to gravity implies the application of the relation:
F = Gm1m2 / r2
which is the Universal Law of Gravity formulated by Sir Isaac Newton. It is a bit overwhelming that so much of the study of the
universe involves the universal concept of gravity. Black Hole studies are literally based on it. All the motion in Space, the orbiting
planets and moons, the forming of new stars and planets, and the collapsing of retired stars – all involve gravity – and refer to
the known universal law of gravity as stated by Newton.