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Das Internationale Einheitensystem (SI)

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At present, the kelvin is literally based on no more than water – on the triple point of water, to be more precise. The base unit of temperature is therefore dependent on a material whose properties may vary. But this is about to change: In the fall of 2018, the kelvin, as well as all the other units of the International System of Units (SI), will be redefined to rest on a solid and invariable...

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The universe of physics is experiencing great changes: by 2018, scientists want to place all physical base units on solid, unchangeable foundation in the form of fundamental constants. The units "meter" and "second" were well ahead of schedule; now the kelvin, the kilogram, the mole and the ampere are next in line. The research activities of the Physikalisch-Technische Bundesanstalt (PTB) are...

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In the past few years, optical atomic clocks have made spectacular progress, becoming 100 times more precise than the best caesium clocks. So far, their precision has been available only locally, since frequency transfer via satellite cannot provide sufficient resolution. This has recently changed thanks to a new direct optical connection between France and Germany, established by joint work of...

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Nach fast 130 Jahren wird das internationale Ur-Kilogramm im Herbst 2018 voraussichtlich in den wohlverdienten Ruhestand geschickt. Dann kommt die Welt zu einem metrologischen Gipfeltreffen zusammen, um über die Neudefinition des Kilogramms zu entscheiden. Wie der Meter und die Sekunde soll künftig auch das Kilogramm über unveränderliche Naturkonstanten definiert werden. Dazu zählen...

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Atomic clock experts from the Physikalisch-Technische Bundesanstalt (PTB) are the first research group in the world to have built an optical single-ion clock which attains an accuracy which had only been predicted theoretically so far. As early as 1981, Hans Dehmelt, who was to be awarded a Nobel prize later, had already developed the basic notions of how to use an ion kept in a high-frequency...

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Researchers from the Physikalisch-Technische Bundesanstalt (PTB) have thoroughly analyzed the noise processes in their optical lattice clock with neutral strontium atoms. This analysis proves that their optical atomic clock has reached the best stability worldwide thanks to a newly developed laser system whose frequency is extremely stable. This allows high-precision measurements in a short time...

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The interaction of thermal energy from the environment with motional degrees of freedom is well known and often referred to as Brownian motion (also thermal motion). But in the case of polar molecules, the internal degrees of freedom – in particular the rotational quantum state – are also influenced by the thermal radiation. So far, the detection of the rotational state was only possible by...

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The second and the metre have already surged ahead. The kilogram, the ampere and the other physical base units are now trying to catch up. This is one way of describing what is currently happening in the world of metrology, in the world of measuring. The International System of Units is facing a redefinition of its foundations: fundamental constants are to serve as the defining reference values...

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[ptb/es] Are the fundamental constants really constant? Recent investigations carried out at the Physikalisch-Technische Bundesanstalt (PTB) have shown that one essential fundamental constant - namely the mass ratio of protons to electrons - can have changed only by a maximum of one part in a million over the age of our solar system (i.e. extrapolated over approx. 5 billion years). Previously,...

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