Experiment in space intends to test the equivalence principle with so far unrivalled accuracy testen

09.05.2016

Were Galileo, Newton and Einstein mistaken? At least just a little? Thanks to them, today we know that heavy mass and inertial mass are different approaches to describe the same physical effect: mass. Thus in vacuum, all objects fall to the ground with equal acceleration, no matter how heavy they are. This 400-year-old discovery has thus far withstood every test. Scientists confirm the so-called equivalence principle with an accuracy of 10-13, thus accurate to one ten billionth. However, modern physical theories, such as the string theory, assume that far behind the decimal point the proof could be waiting that heavy mass and inertial mass differ from one another. French scientists now want to put the principle to the test again – with the active support of the "Wissenschaftlicher Gerätebau" (Scientific Instrumentation Department) of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig. It was there that two pairs of hollow cylinders were developed and produced to an accuracy within micrometers. These cylinders have been orbiting the Earth in a satellite since 25 April 2016 within the scope of the MICROSCOPE project. Thus the law of gravitation is to be tested with so far unrivalled accuracy.

Microscope-Satellit (Das Foto wird von der französischen Raumfahrtorganisation CNES zur Verfügung gestellt: http://cnes.photonpro.net/cnes/categories/654)

Mithilfe zweier Zylinder soll an Bord eines Satelliten im Rahmen des Microscope-Projektes überprüft werden, ob sich schwere und träge Masse unterscheiden.

When you hold an object in your hand, you notice how it presses downwards. Earth's gravitational force pulls it to the ground. If you open your hand, it falls – that is caused by the so-called heavy mass. If we were to fall together with the object, we would no longer feel the gravitational force. But it would still be working. However, it is cancelled out through an opposing force: inertial force. This force resists acceleration. Why heavy mass and inertial mass cancel out exactly is still an enigmatic mystery. Therefore some scientists believe that the law of gravity cannot be correct. The string theory predicts an uncertainty in the range from 10^-14 to 10^-17 for the equivalence principle. Via the MICROSCOPE project, physics could advance into these ranges. Therefore, a measurement uncertainty of up to 10^-15is to be made possible. This could not be measured on Earth because it wouldn't be possible to simulate free fall in a vacuum for a long enough period of time. The situation is different in space: On board a satellite, the cylinders (test masses) will "fall" during the next two years on a sun-synchronized orbit around the Earth at a height of approx. 700 kilometers. This will show whether one of the test masses will experience a different acceleration.

In order for this to succeed, the test objects have to have the same shape. "We have selected two cylinders mounted inside each other, since these have the same center of gravity", explains Dr. Daniel Hagedorn, Head of the Working Group for Surface Technology at PTB. The masses of the first pair of cylinders are composed of the same material: platinum-rhodium (PtRh10). In the case of the second pair of cylinders, the outer cylinder is composed of a titanium alloy (TiAl6V4), a material well known in aircraft construction. The inner cylinder is, just as the first pair of cylinders, composed of platinum-rhodium. "If we see a difference in the acceleration, it will be with the heterogeneous pair of cylinders", says Hagedorn. What is measured would then be the shift of the center of gravity of the cylinders. The uniform pair serves as a control. If both show a reading, then it is due to the instruments. To ensure that the experiment succeeds, the test masses must always have the same alignment in space. The satellite makes sure that the path and angle are correct. Electrostatic force fields support the cylinders.

To fabricate the cylinders exactly uniform was the greatest challenge for the Braunschweig instrument builders. They measured and worked at it for more than five years. "That was an incredibly complex process", says Hagedorn. To bring the surfaces to perfection, it was necessary to first of all field test the rotational speed, the amount of cooling lubricant, the type and shape of the polycrystalline diamonds used for cutting, the tool wear, and much more. Because up to now, no one has ever attempted to achieve the required accuracy of less than three micrometers. "We were bold enough to try it", says Hagedorn. With success: As to shape, it was possible to achieve an accuracy of approx. one micrometer. The mean surface roughness lies within the range of a few ten nanometers.

Ninety percent of this venture is financed by the French space agency CNES. The institutes ONERA (Office national d’études et de recherches aérospatiales - The French Aerospace Lab) and OCA (Observatoire de la Côte d’Azur - Côte d’Azur Observatory) are responsible for the research. In addition to PTB, the Zentrum für Angewandte Raumfahrttechnologie und Mikrogravitation (ZARM) (Center of Applied Space Technology and Microgravity) in Bremen also supports the project. Tests were conducted there as to whether the cylinders were suitable for the experiment at zero gravity. The Deutsches Zentrum für Luft- und Raumfahrt (German Aerospace Center) helped finance this. The test masses will be in orbit for two years – and in this time they will circle the Earth more than 1000 times. As to whether the resulting data will upset our understanding of time and space remains to be seen. ms/ptb

Contact at PTB

Dr. Daniel Hagedorn, Head of PTB Working Group 5.54 Surface Technology, telephone: +49 (0) 531 592-5540, e-mail: daniel.hagedorn@ptb.de

For further information:

· MICROSCOPE project https://microscope.cnes.fr/en/MICROSCOPE/index.htm

· Press photos for downloading http://cnes.photonpro.net/cnes/categories/654

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Experiment in space intends to test the equivalence principle with so far unrivalled accuracy testen

French scientists are putting the law of gravitation to the test. To this end, PTB supplies highly precise test masses