Although I will not be talking about the graviton much in this series, it is worthwhile to make a slight detour from the main story line to discuss the role of the gravity force. (For previous posts in this series, click on the Higgs folder just below the blog post title.)
I should point out that unlike with the other force fields, we have not as yet been able to find a way to ‘quantize’ the gravitational field, i.e., find a way to make its particle properties manifest, the way we have been able to do with the theories of the strong, electromagnetic, and weak forces. But despite this, we have been able to make great progress in understanding it. As this article points out, because we have the option of visualizing the behavior of fields using either a particle model or a wave model, we can choose which is the most convenient in any given context, and that has enabled us to overcome the lack of a quantized theory of gravity.
Theorists believe that both of these descriptions are valid, in much the same way that we can think of the force of electromagnetism as being either the product of a continuous field, or the exchange of numerous force-carrying particles called photons. For certain ‘classical’ calculations, the description of electromagnetism as a field is more workable than its ‘quantum’ description, and vice versa. The problem is that, although physicists have a workable theory of gravity that involves the gravitational field, and gravitational forces as a curvature of space-time, there is no currently believable quantum theory of gravity involving ‘gravitons’. We do not even know, a priori, whether there are such things as gravitons even though we have isolated the particles responsible for the other three forces in nature. Just as photons are ‘packets’ of the electromagnetic field, gravitons would be considered ‘packets’ of the gravitational field or space-time curvature.
In the comments to an earlier post, some had difficulty understanding how the exchange of particles gave rise to forces. As I said in response, different ways of looking at the same phenomenon enable one to overcome some of those conceptual difficulties. When one is dealing with dynamic situations where elementary particles are moving past or colliding with each other, then the particle exchange model of forces gives us a good way of visualizing the force between them. But when dealing with steady-state situations, like planets orbiting a star or the electrons in an atom, the field picture is easier to visualize.
The search for a theory of quantum gravity has been a major focus of physics theory for a long time dating back to Einstein but as yet no one has been successful in creating one. The reason it is so hard is that while fields represent vibrations of space, ever since Einstein we have understood gravity as a theory of the curvature of space itself caused by the presence of matter and energy.
While we have been able to construct quantized theories of the strong, electromagnetic and weak forces, this has been done by treating those fields as if they were vibrations in ‘flat’ space, i.e., space that does not have any curvature. A fully unified theory of quantized fields that includes gravity would take into account the curvature of space and this would require us to also develop theories of the vibrations of fields in curved space. As an analogy, think of the fields as ballet dancers and gravity as the stage on which they dance. As long as the stage is flat and inert, there is no problem in doing intricate maneuvers. But if the stage were undulating due to the presence and movement of the dancers, then ballet would become much harder and radically transformed from what it is now. The difficulty of choreographing dances in such a situation is analogous to the difficulty that theorists face in developing a unified quantum theory of gravity.
Despite the lack of a quantum theory of gravity, we postulate the existence of gravitons more or less in analogy with the particles associated with other forces. The gravity force has strong similarities with the electromagnetic force. They are both long (infinite) range, and the particles associated with them have zero mass and charge. As with the electromagnetic waves, we would expect to see gravity waves, but the gravity force is so much weaker than all the other forces that detecting its effects is really hard.
There is currently a massive experiment underway to try and detect gravitons via gravitational waves. You can read about the LIGO (Laser Interferometer Gravitational-wave Observatory) effort here. The experimental set up has strong similarities to the classic Michelson-Morley experiment to detect the ether using electromagnetic waves that was done at Case Western Reserve University in the late 19th century. It involves splitting a laser beam into two and sending each along perpendicular directions. The two beams of light are reflected from distant mirrors and when they reach the source again, the researchers look for changes in interference patterns that would be caused by tiny changes in the lengths traveled by each beam due to passing gravity waves.
The catch is that the predicted changes are so small that in order to detect them the mirrors in each arm have to be placed very far apart, of the order of two and a half miles, while the Michelson-Morley experiment could be done on a tabletop. There are three such interferometers, two in Washington state and the third in Louisiana. Since the gravity waves are arriving from extragalactic sources, all three detectors should see the same effect at the same time and this duplication will serve as a check against spurious results. (See this article for what kinds of sources produce gravitational waves.)
The LIGO experiment is as technologically challenging as the search for the Higgs but it has not received anywhere near the publicity as that search. This may be because gravity is not something new and exciting in the eyes of the media and no one has given the graviton a catchy name, like ‘the god particle’. Maybe we should call it ‘the devil particle’ or ‘the black hole particle’ and scare the hell out of people about what might happen if we found it.
Since gravity plays such a negligible role in particle physics due to the extreme weakness of the gravity force compared to the other three forces, the absence of a theory of quantum gravity has not as yet proven to be a significant hindrance except in one area, and that is with the early universe. In the period right at and just after the Big Bang, we had a situation where the entire mass of the universe was in the form of elementary particles, all compressed into a tiny volume, so we had elementary particles acted upon by massive gravitational forces causing huge curvatures of space-time. In order to properly understand what was going on then, we need a theory that of relativistic quantum fields in curved space-time and we don’t have one yet. It is this failure that has made physicists cautious about speculating about what happened in the time of the Big Bang or before.
This hesitancy has been seized upon by religious apologists as the ultimate gap that requires a god to explain. But as I said, despite the lack of a theory of quantum gravity, we know from general principles what the properties of the graviton are and can generate models of how the universe came to be and even what might have existed before, though such theories lack the confidence of statements about later stages of the universe.
Next: What makes the Higgs field so special