Possibly the biggest science story of the year—or the most embarrassing slip up—was the claim to have found evidence of a gravitational wave from the birth of the universe. Meanwhile, the search for gravitational waves from more local events, such as neutron stars in tight orbits, continues.
But what are gravitational waves? The first thing is to distinguish them from gravity waves, which are altogether different. Gravity waves include such familiar concepts as water waves on the ocean. They occur when something disturbs a fluid and gravity draws it back, creating an oscillation around the mean level.
Gravitational waves, on the other hand, are altogether more exotic. Their existence relies on the concept of spacetime, as described by Einstein in his general theory of relativity. To us it looks like the world is three-dimensional, while time seems to be something utterly different. However, Einstein envisaged these as a single topological space.
Under what might be considered everyday conditions, spacetime is unnecessary and overly complicated to wrestle with. However, under conditions of extreme gravity or acceleration, time is altered in ways that mean it cannot be kept discrete from space. Sufficient acceleration changes the way time is observed. We can measure this in a minor way through tiny deviations in atomic clocks located on the ground (in Earth’s gravity) compared with those on satellites.
One of the predictions of General Relativity is that accelerating objects will produce gravitational waves unless their motion has certain forms of symmetry. In theory, any object—no matter how small—that accelerates in the appropriate way can produce a gravitational wave. These waves are commonly compared to ripples in the spacetime continuum. They spread outwards from the object that emits them at the speed of light, carrying away energy in the process.
In practice, however, the waves most objects emit are calculated to be so incredibly tiny that even the most sensitive equipment we could currently devise would not come close to detecting them.
However, the heavier an object is, and the faster it is traveling, the larger the gravitational wave. Some astronomical events should produce waves powerful enough to be detected. One possibility is a supernova explosion that is not perfectly symmetric. However, the shortage of supernovae in our galaxy over recent centuries makes these a challenging target. Consequently, the search for gravitational waves has focused on two possibilities.
On the one hand, there is the wave from the acceleration that is thought to have followed the Big Bang by trillionths of a second. On the other, there are pulsars and neutron stars in exceptionally close orbits—either with each other or with another very heavy object. While the scientific world waits on further evidence as to whether the supposed gravitational wave from the early universe was an error or not, we have strong indirect evidence from pulsars.
The energy carried away in gravitational waves has to come from somewhere, and the effect is to cause orbits to decay, with the objects getting steadily closer together. To see just how small the effect can be, it is believed the Earth’s distance from the sun is shrinking by 3.5x10-13m per year as a result of energy lost in gravitational waves. This means that over the lifespan of the planet we have moved 1.6mm closer to the sun, a distance utterly swamped by other influences.
The Hulse-Taylor binary, on the other hand, is a pulsar and ordinary star orbiting with a radius not much larger than that of the sun. Thirty years of observations have seen the orbits decay exactly as general relativity predicts if gravitational waves are carrying energy away—a finding that won a Nobel Prize.
In an effort to detect gravitational waves directly rather than infer their existence from orbital decay, scientists are establishing a set of interferometers with lasers that detect when the length of one arm varies even minutely relative to the other. A gravitational wave would stretch one arm and shrink the other, potentially bringing the lasers out of alignment. However, even the 2-4 kilometre-long arms of the world's most sensitive detectors have yet to produce an unambiguous result.
A schmatic of laser interfermoters, with an incoming beam split to travel down two arms at right angles before recombining, detecting relative changes in the length of each arm half a wavelength long.
A new generation of detectors will be even more sensitive, and it is hoped that these will pick up gravitational waves, not only to confirm General Relativity, but to provide us with a new set of tools to explore the events that produced the waves. At first we might not be able to detect much more than the existence of a wave, but it is hoped that with time we would be able to measure the frequency and changing amplitude of the waves to study them as we now do with light.
Earthquakes are a problem for these gravitational wave detectors, since they can produce similar distortions in the lengths of the arms. Some signals have been detected that could have been gravitational waves or something more local, which is why an international network of detectors is required. However, it is also possible that the passage of gravitational waves through the Earth could trigger earthquakes, turning the entire planet into a detection device.
Earlier this year, a study was published attempting to find evidence of a pattern in quakes that would indicate the influence of gravitational waves. Nothing was found, but the research team hope that lunar seismometers placed by the Apollo astronauts might prove more revealing. The low level of intrinsic activity on the Moon should make externally produced tremors stand out, allowing us to find unambiguous evidence where so many other techniques have failed.
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