Gravitational waves: Vibrations in the fabric of space-time.

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6 May 2024

Gravitational waves are radically different from any other type of radiation. Electromagnetic waves, for example, are the result of the acceleration of electric charges and propagate in space and time. The gravitational waves, however, are generated by the acceleration of massive bodies and are undulations in the very space-time fabric.

According to the theory of gravity stated by Isaac Newton, the gravitational interaction between two bodies is instantaneous. However, Albert Einstein's theory of special relativity says that nothing can travel faster than light. If one body changed shape under the attraction of another, it would produce a disturbance in its gravitational field that would propagate at the speed of light.

In 1805 Laplace stated that, if the gravitational interaction propagated at a finite speed, the forces of attraction in a binary system would not point along the line that joins the two stars, and the system would lose angular momentum with the pass of the time.

Currently, scientists think that a binary system loses energy and angular momentum through the emission of 'gravitational waves'. In the late 1970s, the first indirect evidence of the existence of this type of waves was found when observing the binary pulsar PSR 1913+16. However, they have not yet been able to be detected directly.

Forty years after the publication of Einstein's theories, relativity theorists such as H. Bondi demonstrated that gravitational radiation could be observed, that gravitational waves transport energy, and that a system that emits this type of waves would be losing energy .

The theory of general relativity states that time and space do not exist separately, but rather form an entity inseparable from physical nature. Massive objects 'deform' the space-time fabric, curving it as if it were an elastic medium. Any other body will move in this deformed space following the shortest path, which is no longer a straight line.

Stretchy but firm! When an object is asymmetrically deformed, the resulting disturbance propagates as a space-time ripple: a 'gravitational wave'. Gravitational phenomena with spherical symmetry do not emit gravitational radiation. The perfectly symmetrical collapse of a supernova would not emit these types of waves, but it would if it were irregular. For this reason, a binary system would always emit gravitational radiation.

Gravitational waves alter space-time as they pass, modifying the position of large masses. A gravitational wave traveling through our Solar System would generate a variable tension that would make all the planets oscillate in the direction perpendicular to the propagation of said wave.

However, the displacements caused by the passage of a gravitational wave are extremely small. For example, a white dwarf binary system located 160 light years from our planet would move about 10-10 meters, which illustrates the difficulty of detecting such small oscillations at such a distance.

Although a supernova explosion in a distant galaxy would bathe Earth in gravitational radiation of several kilowatts per square meter, the resulting displacement would still be tiny. Space-time is an incredibly firm elastic medium.

The gravitational waves that a mission like LISA could detect would come from two types of sources: galactic binaries and massive black holes (MBH) that are assumed to exist at the center of most galaxies.

Binary systems would have to be in our own galaxy. Some of them, such as the X-ray binary 4U1820-30, are so well studied that they would be very reliable sources to begin this search.

If a mission like LISA did not detect gravitational waves coming from these binary systems, with the intensity and polarization predicted by the theory of general relativity, the foundations of gravitational physics would be shaken.

It is also very important to better understand the environment and density of massive black holes, as well as their formation and growth processes. Scientists suspect that black holes with a mass between 1 and 100 million times that of the Sun exist in the centers of most galaxies, starting with our own. Detecting gravitational waves emitted by the merger of massive black holes in remote galaxies would demonstrate many aspects of general relativity and black hole theory in an unprecedented level of detail.