The strength of an interaction is related to the rate, or probability, of the emission or absorption of its quantum. For example, a nucleus takes about 10^ -12 second (a millionth of a billionth of a second) to emit a photon. In comparison the beta decay of a neutron takes 12 minutes—about 10^ 14 times longer. It can be calculated that the time necessary for the emission of a graviton by a nucleus is 10^60 seconds, or 10^53 years! This is slower than the weak interaction by a factor of 10^58.
Now, neutrinos are themselves particles with an extremely low probability of absorption, that is, interaction, with other types of matter [see "The Neutrino," by Philip Morrison; SCIENTIFIC AMERICAN, January, 1956]. They have no charge and no mass. As long ago as 1933 Niels Bohr inquired: "What is the difference between [neutrinos] and the quanta of gravitational waves?" In the so-called weak interactions neutrinos are emitted together with other particles. What about processes involving only neutrinos-say, the emission of a neutrino- antineutrino pair by an excited nucleus? No one has detected such events, but they may occur, perhaps on the same time scale as the gravitational interaction. A pair of neutrinos would furnish a spin of two, the value calculated for the graviton by Dirac. All this is, of course, the sheerest speculation, but a connection between neutrinos and gravity is an exciting theoretical possibility.
Gravity and Electromagnetism
In the laboratory diary of Michael Faraday appears the following entry in 1849: "Gravity. Surely this force must be capable of an experimental relation to electricity, magnetism and other forces, so as to build it up with them in reciprocal action and equivalent effect. Consider for a moment how to set about touching this matter by facts and trial." The numerous experiments he undertook to discover such a relation were fruitless, and he concludes that part of his diary with the words: "Here end my trials for the present. The results are negative. They do not shake my strong feeling of the existence of a relation between gravity and electricity, though they give no proof that such a relation exists." Subsequent experimental efforts have not been any more successful.
A theoretical attack aimed at bringing the electromagnetic field into line with the gravitational field was undertaken by Einstein. Having reduced gravity to the geometrical properties of a space-time continuum, he became convinced that the electromagnetic field must also have some purely geometrical interpretation. However, the "unified field" theory, which grew out of this conviction, had hard going, and Einstein died without producing anything so simple, elegant and convincing as his earlier work. Today fewer and fewer physicists are working at unified-field theory; most are persuaded that the effort to geometrize the electromagnetic field is futile. It seems, at least to the author, that the true relation between gravitational and electromagnetic forces is to be found only through an understanding of the nature of elementary particles-an understanding of why there exist particles with just certain inertial masses and not others-and of the relation between the masses and the electric and magnetic properties of the particles.
As a sample of one of the basic questions in this field, consider again the relative strength of gravitational and electromagnetic interactions. Instead of comparing the times required for emission of quanta, let us compare the actual strength of the electrostatic and gravitational forces between a pair of middleweight particles, say pi mesons. Computation shows that the ratio of electrostatic to gravitational force equals the square of the charge on an electron divided by the square of the mass of the particles times the gravitational constant : e2 / M2 C. For two pi mesons the value is 10^40. Any theory that claims to describe the relation between electromagnetism and gravity must explain this ratio. It should be pointed out that the ratio is a pure number, one that remains unchanged no matter what system of units is used for measuring the various physical quantities. Such dimensionless constants, which can be derived in a purely mathematical way, often turn up in theoretical formulas, but they are usually small numbers such as 2π, 5/3 and the like.