A Satellite Equivalence-Principle Experiment Using DC Cancellation

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A Free-Fall Satellite Test of the Equivalence Principle (EP)

The paper referenced in this link describes the first report at the 8th Marcel Grossmann Meeting in Jerusalem in 1997 of a satellite free-fall EP experiment using DC Cancellation (PDF). This paper is a very short description of the experiment, and a much more detailed version (PDF) has been submitted for publication. The text below gives a brief introduction to the concept of the Equivalence Principle and why testing it is important.

The Equivalence Principle

Gravity has a very unusual property. At a given point in space, the gravitational attraction of all objects divided by their mass is identical. The weight as shown on a scale is different, but (neglecting the air) all objects fall identically. This is known as the Equivalence Principle. (See the Eöt-Wash group's webpage for more exact definitions).

Drop a dime and pocket knife together from a standing position onto a carpet, and they will fall exactly together and strike the carpet at the same time. This is one of the deepest and most significant experiments in physics.

It is not always obvious that all objects fall together because air resistance can make leaves and feathers, for example, seem to fall slower. In a high-school General-Science class, the teacher will sometimes do a demonstration in which a long glass vertical tube about 3 inches in diameter is evacuated, and a feather and a lead weight are released from the top together by an electromagnet. When the air is removed, they fall exactly together.

The History of Equivalence-Principle Tests

Aristotle and Galileo

The confusion caused by air resistance is very old. Aristotle taught that heavier objects fall faster than light objects, and this was dogma for 2000 years. It was Galileo ‡ who first did experiments to show that all dense objects fall together. Stillman Drake the famous Galileo expert at first though that the story of Galileo dropping the 10 pound and one pound cannon balls from the Leaning Tower of Pisa was apocryphal. Later he came to the conclusion that the famous experiment actually happened, but that is still controversial today ‡.

Newton: Mechanics, Gravity, the EP, and Tests of the EP

In the early 1800's Bessel also did pendulum experiments, and tested the EP to an accuracy of about 10^-5.

Eötvös Does Even Better with a Torsion Balance

Around 1905, Eötvös used a torsion balance to measure the EP to an accuracy of about 10^-8. Einstein, like Newton, made the EP the foundation of his theory of Gravity, General Relativity.

Modern High-Accuracy EP Measurements

Around 1960 Dicke decided that he could improve in Eötvös' result using the Sun as a source. In 1964 he reported a measurement of the EP with an accuracy of about 10^-11. In 1972 Braginsky and Panov using similar techniques, reported 10^-12. In 1981 Keiser and Faller measured 10^-10 using a floated apparatus, and in 1987 Niebauer et al. measured 10^-10 using a Drop Tower.

Using laser ranging, the orbit of the moon has also measured the EP to about 10^-12.

In 1990 and 1999 Adelberger reported 10^-12 and 10^-13 respectively using a torsion balance.

The Need for Even More Accuracy

It has now been 85 years since General Relativity was published in 1916, and no experiment has ever been at variance with it. One would think that if an effect is 10^-12, we should just call it zero; but there are a number of reasons not to do this.

The most significant problem with General Relativity is that it does not fit into Quantum Mechanics. It is intellectually very unsatisfying to have two mutually incompatible theories, one which very accurately describes the Universe in the small and the other which very accurately describes the Universe in the large.

Furthermore there are problems in the Standard Model, the description of the Universe in the small, which are best solved with Supersymmetry. General Relativity does not include a Supersymmetric theory of gravity.

String Theory is becoming more and more accepted and is a generalization of Yang-Mills Theory (the base of the Standard Model) AND General Relativity. In String Theory, there are scalar fields which violate the EP. An important question is why the effects of these fields is not large. The standard answer is that they are massive, and therefore, have a very short range. Another possibility, however, has been proposed by Damour, Nordtvedt, and Polyakov, (T. Damour and A. M. Polyakov, Nucl. Phys. B423, 532, (1994)). They show that under certain conditions, the expansion of the Universe dilutes these fields. Thus they could be small but long range and hence measurable by a very accurate satellite EP experiment.

All of this is best summarized in a paper by T. Damour, Testing the Equivalence Principle: Why and How?, Clas. and Quant. Grav. 13, (1996), A33-A41 and in a second paper Equivalence Principle and Clocks also by Damour.

Doing a High-Accuracy test of the Equivalence Principle in a satellite is one of the most important experiments in Gravitational Physics yet to be done.