Commentary

Einstein's Greatest Blunder

Einstein is a central character of early 20th century physics, having developed the theories of special and general relativity, of the corpuscular nature of light, and of the concept of detailed balance in the interaction of light with mater. From this work he has gradually become a scientific icon both in the general public and in the scientific community. This reverence is so strong that ideas developed by others are at times attributed to Einstein. The most striking example of this that I have seen occurred several years ago, after Einstein was selected by Time magazine as the man of the century, when a news broadcaster stated that among Einstein's achievements was the invention of television.

But when we look beyond Einstein the icon to Einstein the man, we see one characteristic that is very common in all men. Einstein tended to see what he wanted to see. Examples of this are his criticism of quantum mechanics by the phrase that God does not play with dice, and his lack of surprise at the confirmation of general relativity through the observed bending of light around the limb of the Sun. This one tendency got him in trouble when he invented the cosmological constant.

Einstein had the preconception that the universe was static; he believed that the distances between the galaxies was fixed for all time. But this view is not sustainable in classical gravitational theory, because all objects with mass are attracted to each other. If the universe were static at one instance in time, then its self-gravitation would cause it to collapse onto itself at later times. For cosmology conform to his preconception, Einstein had to create a mechanism to counter the universe's tendency to collapse, so he added a term to General Relativity, his theory of gravity, that counterbalanced the gravitational force of mater. This term, which effectively gives the vacuum a repelling force, is the cosmological constant. Einstein set its value to precisely counteract the gravitational force of mater.

When Einstein introduced his cosmological constant, however, he was making a deep mistake, because we was trying to make the universe conform to his expectations, rather than looking to the universe to guide him in developing his theory. But the universe seldom follows our preconceptions. Observations distant galaxies proved that the universe was not static, but expanding; with this discovery, there was no longer a need for a cosmological constant. This lead Einstein to refer to the cosmological constant as his greatest blunder.

Einstein's mistake was in violating the spirit of science. He came to believe that one can know the universe through pure reason. But science asserts the opposite: we only know the universe though observation and experimentation. The cosmological constant should not have been added to general relativity, because there was no observation that demanded its existence.

The trap of believing in intuitive pure reason afflicts many theorists. Often we see a theory justified through its mathematical properties, usually expressed as an aesthetic quality, such as beauty. In various scientific communities, theorists are motivated by an ideology; so in cosmology, many theorists, and observer for that matter, are motivated by the ideology that the universe must have the closure density,¹ while in particle physics physicists are motivated by the ideology of grand unification, the belief that all physics can be reduced to a single, relatively simple equation. In other cases, the justification for a theory is a prejudice, as it was with Einsteins belief in a static universe. Most often, the justification of a theory is a personal preoccupation, as when a theorist attempts to explain a phenomena with the latest physics that the researcher investigated; when you only have a hammer, you try to solve all problems with that hammer.

This disease is a matter of degree, because each of these reasons? an aesthetic, an ideology, a prejudice, or a preoccupation?is how the scientific community develops a large store of theories to test. The error is then not in being guided to some extent by these factors, but making them the dominant factor, especially when unnecessary theories are developed, or one theory becomes dominant to the point of excluding other reasonable theories.

The cosmological constant never truly died. It has been revived in the inflationary theories of the early universe, which are motivated by a desire to give a faux-explanation for the uniformity we see in the universe. But more recently, it has been revived as one of the explanations for a recent result in observational cosmology.

Recent studies of type Ia supernovae, which is believed to be the nuclear explosion of a degenerate-dwarf star, have found that supernovae at large redshift are brighter than expected for cosmological models that have only gravitational deceleration from matter.² This can be interpreted as the universe undergoing a period of acceleration in its expansion rate. This can be done with a cosmological constant set such that the vacuum has a repulsive effect. In this theory, the universe decelerates through its early life until the gravitational effects of the vacuum overwhelm the gravitational effects of matter. In later times, the vacuum becomes dominant, and the universe accelerates its expansion. In this theory we happen to live at the time when the two forces are about equal.

The cosmological constant is not the only explanation under consideration for the supernovae results. One is that the observations are a consequence of correcting for the effects of redshift in the observations. The fact that the conditions of the universe change at high redshift, particularly the chemical composition, suggests the second possibility that the effect may be a consequence of type Ia supernovae behaving differently at high redshift than at low redshift. Both have the advantage of naturally tying the observed effect to a redshift relative to us; in contrast, it is pure luck of the effects of a cosmological constant are appearing now, when life is arising to observe it. So there are a variety of possibilities that must be considered.

Is the introduction of the cosmological constant a mistake? Not if it is one of many ideas to consider. But within the cosmological community, there is a flocking to the cosmological constant theory that has all the earmarks of a theory being accepted for the wrong reasons. First, many of those advocating the cosmological constant also are drawn to the idea of the universe being at the closure density, and in fact argue that the presence of the cosmological constant makes the critical-density universe fit the observations better. Second, many embrace the cosmological constant as though its existence is now proven; this is particularly true within the literature produced by NASA. Advocates of the cosmological constant like to point ironically point to Einstein's statement about his greatest blunder, using this irony as a form of argument through authority. But Einstein did make a mistake, and I can't help but thinking that, in the way the cosmological constant is again being embraced, the same mistake is being made again.