Presentation by Geoff Grayer at the Newbury Astro meeting on 4 March 2016.
Journey of a Gravitational Wave Credit: Courtesy Caltech/MIT/LIGO Laboratory.
Most of us heard all the hype on radio and TV on 11 February after the announcement of the detection of gravitational waves. The BBC rushed together a radio programme (which I listened to twice) and, unfortunately, got some of their facts completely wrong. I will try to clarify these and add a bit of history.
It was said several times in the programme that gravitational waves were predicted by Einstein. It is true that gravitational waves emerge naturally from Einstein’s equations describing the distortion of space by gravity in his Theory of General Relativity. In a similar way, electro-magnetic waves emerge from Maxwell’s equations describing the relation between electricity and magnetism. But the wave theory of light, explaining refraction, reflection, diffraction and interference, was described by Huygens in 1678, 200 years before Maxwell. In a similar way, you don’t need General Relativity to infer that there must be gravitational waves. As soon as one accepts that nothing can propagate faster than light then, any time something moves, its gravitational field changes; this change then has to spread out just like the disturbance of a stone dropped in a pond.
It is only relatively near, massive objects like Earth, the Moon and the Sun, which we recognise affect us gravitationally. This is because, of all the known forces, gravity is by far the weakest. It has, however, three redeeming features which make it so significant in the cosmic scale.
First, gravity is a long-range force. Like electro-magnetic radiation it reduces by distance according to the inverse square law. Incidentally, this implies that the mediator of the gravitational force, known as the graviton, is massless like the photon. Otherwise it would have a finite range, as do the nuclear forces. A massless graviton also means that gravitational waves propagate at the speed of light. The graviton is rarely mentioned because we don’t have a satisfactory quantum theory of gravity and, for all but the most extreme situations, gravitational quantum effects are unmeasurable.
Secondly, in contrast to the electric force where unlike charges attract and like charges repel, gravity is only attractive. There is no anti-gravity, despite the fertile imagination of some science fiction writers!
Third, unlike electro-magnetic radiation which can get scattered or absorbed by dust and gas - or be obscured or deflected by large bodies - there is nothing we know of which can absorb or obscure gravitational attraction.
All this means that, if there is enough matter, eventually gravity will overcome all other forces. The result is a Black Hole.
Black holes might seem to be a modern invention. In fact, in 1784, shortly after Newton published his theory of gravity, the Rev. John Mitchell speculated that there might be objects so massive that light could not escape from them – as did Laplace a decade later. Einstein did not like the concept of a black hole; he hoped that some force would stop them forming. After all, what mathematician wants a singularity, even if it is ‘hidden’?
During his journey to this country in 1930, the young Indian astrophysicist Chandresakhar worked out that any star greater than 1.4 solar masses will eventually collapse – and no force could stop it. This idea was anathema to Sir Arthur Eddington, who agreed with Einstein, declaring to the Royal Society that, “there ought to be a Law of Nature preventing stars acting in this way”!
But let’s get back to the detection of gravitational waves. I remember, about 50 years ago when I was a graduate student, going to an Institute of Physics meeting in Edinburgh. I don’t remember anything about the papers given there, but I do remember being taken to a basement room of the University and being shown one of the first generation of gravitational wave detectors. It consisted of a large metal cylinder, covered with strain gauges, and suspended on damped springs to avoid the effects of vibrations from seismic events – or the trams which then plied the city streets. Of course, it couldn’t have possibly worked, but that is said with the benefit of hindsight.
One reason for this is that the wavelength of gravitational waves is necessarily very long – many kilometres. Light waves have wavelengths measured in nanometres because they are radiated by sub-nuclear particles, which have very low mass, and hence can be accelerated very fast.
Detectable gravitational waves can only originate from the motion of very large masses which, even if they are as dense as black holes or neutron stars, have diameters of several kilometres. Hence the lengths of the arms of the interferometers used for these detections need to be of comparable length, as this determines the wavelengths to which they are sensitive. Otherwise, modern gravitational wave detectors are, in effect, scaled up versions of the interferometer used by Michelson and Morley in their epoch making experiment of 1887 which showed the constant velocity of light. The main difference is the use of a laser light source, essential for those long arms, which of course wasn’t invented in their day.
This long wavelength has a further consequence. The energy carried by a wave is proportional to its frequency, which is why gamma and X-rays are so dangerous whilst radio quanta are (relatively) harmless. Long wavelength means low frequency, and low energy, which makes gravitational waves even more difficult to detect.
I should mention one further property of gravitational waves. I have mentioned electro-magnetic waves and ripples on a pond. Both these are transverse waves; their motion being perpendicular to their direction. Gravitational waves are more like the pressure waves of sound, or compression type seismological waves, so they have no polarisation.
I have left the most difficult bit until last. I have been talking blandly about ‘gravitational waves’ as if it was easy to understand what they are. We are told that mass ‘distorts’ space-time, so you might think that a gravitational wave distorts everything in its path in the same way. So how can you detect a gravitational wave if your ‘ruler’ is distorted, along with the baseline of the detector? The BBC programme obviously thought in this way; they explained that any form of accurate ruler would be affected, along with the distance, which is why a laser was used to detect the change in length. They did not attempt to explain why the laser beam was not affected. In fact, they got it completely wrong!
Let me give you two quotes. Eddington, who wrote a book on General Relativity which was praised by Einstein, warned his students that if they thought they could visualise ‘the curvature of space’ they might be quite certain that they were wrong.
Sir Edmund Whittaker, another famous astrophysicist of the time and one-time Astronomer Royal of Ireland (yes, there was such a thing in those days) said, ‘It is not space that is curved, but the geometry of space’. Well, no-one ever claimed General Relativity was easy!
The sound of two black holes colliding. Credit: Courtesy Caltech/MIT/LIGO Laboratory
At the BAA’s Autumn Weekend meeting at Rutherford Appleton Laboratory last September, Prof Andrew Taylor spoke about Euclid, ESA’s Dark Energy space mission. Just before his talk I asked him about this problem.
He explained that the subject had been widely discussed in the 1970s, especially concerning the question of how gravitational waves could carry energy. The problem springs from our picture of a set of fixed co-ordinates in space, which harks back to the old idea of an ‘aether’.
In fact, solid objects are not affected by a gravitational wave because the inter-atomic forces holding them together are so much stronger than gravity. It is the photons in the laser beam which ride the gravitational wave, as they are not constrained, thus changing the interference pattern.
Finally, hopefully these gravitational detectors will enable more information to be obtained about black holes, binary systems involving black holes and neutron stars, and Active Galactic Nuclei. However, they are unlikely to reveal more about those elusive black holes thought to exist at the centres of most galaxies.
Geoff is a retired experimental particle physicist who worked at CERN for 11 years before returning to work at the Rutherford Appleton laboratory. He has been a member of the BAA since 1958 and has written a number of articles and papers on astronomy related subjects.