Gravitational waves are ripples that happen within the curvature of spacetime and are generated in certain gravitational interactions and propagate as waves outward from their source at the unbelievable speed of light. Albert Einstein predicted the existence of gravitational waves in 1916 as part of the theory of general relativity. In Einstein’s theory, space and time are aspects of a single measurable reality called space-time. On the contrary Newton’s law of universal gravitation (part of classical mechanics) does not provide clues for their existence, since that law is based on the assumption that physical interactions propagate at infinite speed, showing that the statements of classical physics are unable to explain phenomena associated with relativity.
Matter and energy are two expressions of a single instance or material. As an analogy, we can imagine space-time as a fabric where the presence of large amounts of mass or energy distorts the space-time causing the fabric (or grid) to “warp”, therefore we observe this warpage as gravity. Any kind of freely falling objects such as golf balls, satellites or beams of light follow indistinctly a direct path in an specific curved space-time.
When large masses move unexpectedly, some of this space-time curvature ripples out spreading in much the way as ripples on the surface of an agitated pond. When two dense objects in space orbit each other, space-time is stirred by their motion and gravitational energy ripples throughout the universe.
In 1974 Joseph Taylor and Russell Hulse found such a pair of neutron stars in our galaxy. One of the stars beams regular pulses of radio waves toward Earth. Taylor and his colleagues were able to use these radio pulses, like the ticks of a very precise clock, to study the orbit of neutron stars. Over two decades, these scientists watched for and found the tell-tale shift in timing of these pulses, which indicated a loss of energy from the orbiting stars – energy that had been carried away as gravitational waves.
The result was just as Einstein’s theory predicted. The discovery of Hulse and Taylor represents strong indirect evidence of gravitational waves and on February 11, 2016, the LIGO (Large Interferometer Gravitational Wave Observatory) and Virgo Scientific Collaboration announced they had made the first observation of gravitational waves. The observation itself was made on 14 September 2015, using the Advanced LIGO detectors. The gravitational waves originated from a pair of merging black holes. After the initial announcement the LIGO instruments detected two more confirmed, and one potential, gravitational wave events.
In August 2017, the two LIGO instruments, and the Virgo instrument, observed a fourth gravitational wave from merging black holes, and a fifth gravitational wave from a binary neutron star merger. Several other gravitational-wave detectors are planned or under construction now.
In general terms, gravitational waves are radiated by objects whose motion involves acceleration and its change, provided that the motion is not perfectly spherically symmetric (like an expanding or contracting sphere) or rotationally symmetric (like a spinning disk or sphere). A simple example of this principle is a spinning dumbbell. If the dumbbell spins around its axis of symmetry, it will not radiate gravitational waves; if it tumbles end over end, as in the case of two planets orbiting each other, it will radiate gravitational waves. The heavier the dumbbell, and the faster it tumbles, the greater is the gravitational radiation it will give off. In an extreme case, such as when the two weights of the dumbbell are massive stars like neutron stars or black holes, orbiting each other quickly, then significant amounts of gravitational radiation would be given off.
- Two objects orbiting each other, as a planet would orbit the Sun, will generate.
- A spinning non-axisymmetric planetoid with a large bump or dimple on the equator will generate.
- A supernova will generate except in the unlikely event that the explosion is perfectly symmetric.
- An isolated non-spinning solid object moving at a constant velocity will not radiate. This can be regarded as a consequence of the principle of conservation of linear momentum.
- A spinning disk will not generate. This can be regarded as a consequence of the principle of conservation of angular momentum. However, it will show gravitomagnetic effects.
More technically, the third time derivative of the quadrupole moment (or the l-th time derivative of the l-th multipole moment) of an isolated system’s stress–energy tensor must be non-zero in order for it to emit gravitational radiation. This is analogous to the changing dipole moment of charge or current that is necessary for the emission of electromagnetic radiation.
During the last 100 years astronomy has been revolutionized, using new methods for observing the entire universe. Astronomical observations were originally made using visible light. Galileo Galilei pioneered the use of telescopes to enhance these observations. However, visible light is only a small portion of the electromagnetic spectrum, and not all objects in the distant universe shine strongly in this particular band.
More useful information may be found, for example, in radio wavelengths. Using radio telescopes, astronomers have found pulsars, quasars, and made other unprecedented discoveries of objects not formerly known to scientists. Observations in the microwave band led to the detection of faint imprints of the Big Bang, a discovery Stephen Hawking called the “greatest discovery of the century, if not all time”. Similar advances in observations using gamma rays, x-rays, ultraviolet light, and infrared light have also brought new insights to astronomy. As each of these regions of the spectrum has opened, new discoveries have been made that could not have been made otherwise.
Astronomers hope that the same holds true of gravitational waves and unveil more of the deep secrets of our universe.