Physicists Debate Existence of Elusive 5th Fundamental Force

A fifth fundamental force governing matter, first postulated in January, may not exist after all, according to some leading physicists, including a University of California, Riverside, professor.

The hypothetical fifth force would have opposed the effects of gravity over short distances, causing some objects to fall to earth faster than others. For instance, the theory holds that, in the absence of wind resistance, a feather would fall faster than a lead weight.

But that theory is based on an erroneous interpretation of a classic experiment conducted 77 years ago by Hungarian physicist Roland von Eotvos, according to physicists Shu-yuan Chu of UC Riverside and Robert H. Dicke of Princeton University in a paper published in the current Physical Review Letters.

Eotvos determined the value of the so-called gravitational constant--a measure of the acceleration of objects due to gravity--by suspending weights of different composition from the ends of a balance. A combination of the Earth’s gravity and its centrifugal force would cause the balance to twist, and the amount of twist revealed the gravitational constant.

He obtained a very accurate value for the gravitational constant but observed certain discrepancies in his data that he attributed to limitations in the experimental apparatus.

But when physicist Ephraim Fischbach and his colleagues at Purdue University reexamined Eotvos’ data, they found that the discrepancies could be explained by invoking a new force, which they called “hypercharge.”

Chu and Dicke argue in their paper that most of the discrepancies could have arisen from slight differences in temperature within Eotvos’ laboratory.

In a paper in the same journal, however, Fischbach and his colleagues contend that they have already considered the effects of such temperature differences and have shown that they are insignificant.

Slight Discrepancies

The four known forces--gravity, electromagnetism and the strong and weak forces that govern the structure of the atom--are thought to explain all things in nature. In recent years, however, scientists have found slight discrepancies in certain physics experiments that suggest that an unknown and previously unsuspected force may also be influencing the data.

Fischbach reexamined Eotvos’ data because there are no other appropriate experiments that examine the acceleration due to gravity over such short distances.

His results and the postulation of the existence of hypercharge were highly controversial, but they were sufficiently plausible that many other physicists have begun experiments searching for its existence. None of the results from those experiments is available yet.

Chu and Dicke, in contrast, propose that slight differences in temperature within Eotvos’ laboratory could have created what Chu, in a telephone interview, called “a gentle wind” blowing past the apparatus.

Fifth Force Unnecessary

Because the objects on the ends of the balance are different sizes, in order to make their weights balance, the wind itself could twist the balance. Chu and Dicke calculated that such a temperature difference could account for most of Eotvos’ discrepancies without the need for a hypothetical fifth force.

In rebuttal, Fischbach and his colleagues noted that Eotvos’ experiments were conducted over a period of three years, therefore making it unlikely that a temperature difference would have had the same effect on all experiments.

The Fischbach group also noted that Chu and Dicke’s calculations do not account for all the data. For most of the materials studied by Eotvos, the twist resulting from the temperature difference should be similar to that caused by hypercharge. When platinum was used as one of the objects, however, the twists should be different.

Discrepancies Remain

Significantly, the group said, Chu and Dicke’s calculations do not account for discrepancies in the platinum experiments.

Another physicist who is conducting experiments measuring gravity over short distances, Riley Newman of the University of California, Irvine, said by telephone that, “both Chu and Dicke’s paper and the response are valid.” He also noted that the data for platinum is “particularly crucial.”

“But rather than raking over the coals of (such an old) experiment again and again,” he concluded, “it is better to wait until some of the data from more modern experiments are available.”