Perspectives on Gravitational Waves

By Isac Artzi, PhD
Faculty, College of Science, Engineering and Technology

globe and lines

Much has been written recently about the discovery of evidence that points towards the existence of gravitational waves. Rather than repeating what has already been widely reported, let us examine another perspective on this discovery.

Back in 1916, Albert Einstein proposed the existence of gravitational waves as a “side effect” of his general theory of relativity. Einstein noted that since mass and energy are interchangeable, large bodies of mass or large amounts of energy could affect (i.e. distort) space-time, which can be viewed as a fabric. What is commonly referred to as gravitation is nothing other than a bend in the fabric of space-time. Distortions could in turn cause ripples throughout the fabric.

An analogy most can relate to, is the collision between two giant glaciers. Let us assume one regularly observes the ocean waves, tides and winds. A collision of two massive glaciers is known to generate massive ripples (waves), sometimes even resulting in tsunamis.

If space-time is a continuum throughout the universe, then the ripples should be noticeable throughout the universe as well. If Einstein assessment is correct, we should eventually be able to devise the necessary techniques to measure the ripples, otherwise known as gravitational waves, just like we can measure ocean waves.

Laser Interferometer Gravitational Waves Observatory (LIGO) has created a bank of templates of simulations of waves created by every known type of possible massive celestial collision such as binary black holes and neutron stars. These simulations attempt to model the creation of ripples in the space-time fabric, aka gravitational waves between 30 Hz and 150 Hz.

LIGO continuously scans for signals within these frequencies and calculates the correlation between detected signals and the ones in the templates. When a statistically significant correlation between a signal and a template is found, a positive identification of a gravitational wave is tentatively assumed. The recent discovery announced by LIGO is based on a signal that lasted 0.2 seconds.

One may think that LIGO is listening to waves in the same way one would listen to a radio: scanning radio frequencies until a desired station is identified. In fact, LIGO is doing just the opposite: It is trying to detect anomalies in the regularly detected signals. Not just any detectable anomaly, but a statistically significant one. Thus the analysis refers to an anomaly being correlated to an anomaly predefined in a template. Such anomalies are interpreted to be the evidence for the existence of gravitational waves. Continuing with the analogy to glaciers, if unusually large and fast moving waves are observed, one might conclude that an unusual event must have happened. Possibly an earthquake or a massive collision.

After four months of signal analysis repeated statistical tests, the 0.2’s long signal emanating from a system labeled GW150914 has been officially identified as a gravitational wave. The question is how does an anomaly, essentially noise, become a waveform? Since LIGO did not actually detect a gravitational wave, but the noise, the anomaly, created from a distortion of a gravitational wave, the natural question to ask next is what caused the distortion?

One possible cause is assumed to be the spin of a black hole. Physicists at the University of Michigan were able to collect X-ray photons and thus measure the spin of a black hole. Physicists refer to such streams of X-ray photons as gravitational lens due the fact that the technique is used to gain insight into distant celestial collisions that cause gravitational ripples throughout the space-time continuum.

In a fortunate turn of celestial events, the alignment of a supermassive black hole and a massive elliptical galaxy has a created a unique opportunity for collecting X-ray photons. The search for gravitational waves lead to ensuing calculations that measured the spin of the black hole to be approximately half the speed of light. The ability to measure X-ray photons stems, in turn, from earlier development of techniques for measuring X-ray photons emanating from pulsars.

Looking back at decades of observations of pulsars, we can today assess that these observations have been instrumental in developing many techniques, instruments and analytical processes used to detect gravitational waves. From our limited human perspective, our limited ability to express celestial phenomena and the continuous development of signal detection and processing, more questions are in order, such as:

  • If the universe is finite, what happens if the waves hit a “wall”? Will they bounce back?
  • If the waves do hit a “wall” are they the cause for the continuous expansion of the universe by “stretching” the wall? Or, if they “bounce back” are they aiding (or causing) its collapse?
  • If there are multi-verses do/can gravitational waves transcend universes? What does this even mean?

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