### JK Nelson

Shared publicly -**Einstein's Succession, Part 7**

(Other parts: https://plus.google.com/collection/AP7WX)

**Besides black holes and gravitational lensing the maybe most exciting consequence of Einstein’s theory is the postulation of gravitational waves [33].**

The Einstein field equations predict thfat accelerated matter (or energy in general) emits gravitational quadrupole radiation as illustrated in the figure. These waves stretch and compress spacetime perpendicular to the direction of travel and cause directly observable distance fluctuations between freely falling objects. Let’s assume we have a ring of cubes freely floating in the xy-plane and a gravitational wave propagates along the z-direction. As illustrated in figure, the distance between the masses oscillates with time. The direction of this oscillation depends on the polarization of the gravitational wave. The usual basic set of polarization states are plus (+) and cross (×) polarization, others can be formed by linear combinations of these two.

While the strength of the gravitational field falls o with the square of the distance, this effect, an amplitude, falls off linearly proportional to the distance

**[34]**and even sources located at the other end of the observable universe can produce relative distance fluctuations on the order of 10^-20 or more, depending on the frequency of the signal.

**Example:**On a distance of 4 kilometers, relative distance fluctuations of 10^-20 correspond to 40 attometers. This is much smaller than an atomic nucleus. For a distance of 1 million kilometers, the same fluctuations correspond to 10 picometers, which is a factor of ten below the diameter of a hydrogen atom.

This effect, as tiny as it might seem, can tell us about electromagnetically invisible objects and has a huge discovery potential for new physics. Gravitational radiation travels unaffected throughout the entire universe; in contrast to electromagnetic radiation that interacts strongly with matter and hence can be distorted or blocked. Gravitational waves were able to propagate unimpeded even in the young hot universe prior to 380,000 years after the big bang. Thus gravitational waves are a superior messenger that holds complimentary or even otherwise completely unobtainable information about processes in the universe. Gravitational wave observatories are expected to bring the next big revelations in astronomy, cosmology, and fundamental physics alike.

Although general relativity passed all tests with flying colors, and despite indirect yet irrefutable proof of the existence of gravitational waves

**[35]**, gravitational waves have never been detected directly. Currently, research teams look into indirect evidence for gravitational waves produced during cosmic inflation, now red-shifted to a static polarization pattern imprint in the cosmic microwave background radiation

**[36, 37]**. The clear detection or non-detection of a primordial gravitational wave background from the inflationary epoch would rule out one of the two leading theories about the origin of our universe. This—for me—is the most fascinating prospect of gravitational wave astronomy: we might not only determine the shape and structure of our own universe, but also learn about the nature of the global universe it is embedded in.

**Learn about the many different sources of gravitational waves in the next parts.**Subscribe to https://plus.google.com/collection/AP7WX

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**[33]**Albert Einstein and Nathan Rosen. On gravitational waves. Journal of the Franklin Institute, 223(1):43–54, 1937.

**[34]**E.F. Taylor, J.A. Wheeler, and E.W. Bertschinger. Exploring Black Holes: Introduction to General Relativity. Addison-Wesley, 2010.

**[35]**Joseph H Taylor and Joel M Weisberg. Further experimental tests of relativistic gravity using the binary pulsar PSR 1913+ 16. The Astrophysical Journal, 345:434–450, 1989.

**[36]**PA Ade, RW Aikin, D Barkats, SJ Benton, CA Bischo, JJ Bock, JA Brevik, I Buder, E Bullock, CD Dowell, et al. Detection of B-Mode Polarization at Degree Angular Scales by BICEP2. Phys. Rev. Lett., 112:241101, Jun 2014.

**[37]**Michael J. Mortonson and Uroš Seljak. A joint analysis of Planck and BICEP2 B modes including dust polarization uncertainty. Journal of Cosmology and Astroparticle Physics, 2014(10):035, 2014.

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