Negative energy solves atomic clock puzzle
A serious contradiction between theory and experiment has so far prevented atomic clocks from becoming more accurate. Now it turns out that what was missing was negative energy.
The world's most accurate clocks are now even more accurate. Two research groups have independently succeeded in precisely calculating a tiny interference factor in the interaction of a strontium atom with light, which had previously led to a contradiction between theory and experiment in time measurements. Two physical properties of the atom, the polarisabilities of the electric quadrupole moment (E2) and the magnetic dipole moment (M1), lead to a small shift in the energy states when interacting with the laser light in the atomic clock. So far, however, measurements have shown that this shift deviates from the theoretically expected value.
The teams led by Sergey G. Porsev from the University of Delaware and Fang-Fei Wu from the Wuhan Institute of Physics and Mathematics have now come to the conclusion that electronic quantum states with negative energy completely resolve this contradiction. This means that strontium-based atomic clocks can now achieve higher accuracies than the 1 : 10¹⁸ previously achievable. These clocks could therefore now be accurate enough for previously unattainable applications, such as gravitational wave measurements with satellite constellations.
High-precision strontium atomic clocks are based on a specific energy transition at a wavelength of 698 nanometres in this atom. In order to utilise its frequency for exact time measurements, the atoms are held in a standing wave of laser light, the wavelength of which is set so that it shifts the frequency of the transition as little as possible. However, a certain interaction with the laser light cannot be avoided, and this depends on the polarisabilities of the electric quadrupole moment and the magnetic dipole moment of the strontium atom. Attempts to calculate these two quantities have so far led to contradictions with the measurement data: According to calculations, E2 is larger, according to measurements M1.
The teams led by Porsev and Wu have now carried out these calculations again. They also took into account quantum states with negative energy, which occur as solutions to the quantum mechanical equations for the electrons of the atom. These are generally omitted because real particles have positive energies. In the calculations of E2 and M1, however, a mathematical method is used that adds up all possible states of the particles involved in order to obtain the polarisability of the overall system. And, as it turned out, all actually means all. Even those with negative energy.
The fact that quantum states of negative energy cannot be neglected in such theoretical procedures had already been shown in the calculation of other quantum properties. As the experts discovered, it also makes a considerable difference in atomic clocks whether these states are included in the total sum - but not for both variables considered. While the states with negative energy hardly play a role in E2, they make up the most important articles in M1. This correction means that M1 is now larger than E2 and therefore the theoretical calculations no longer fundamentally contradict the measured properties of the strontium atom. This means that the interaction between light and the atom can now be calculated much more precisely - and therefore also the exact frequency that is used to measure time.
Spectrum of Science
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© liulolo / Getty Images / iStock (detail) In the processes in atomic clocks, particles with negative energy must also be taken into account.
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