How general is the General Theory of Relativity?

Whether it's a feather, an apple, or a brick: in a vacuum, when there is no more friction and only gravity acts, all bodies fall at the same speed. Einstein's General Theory of Relativity, specifically the equivalence principle, predicts this and it aligns with the current worldview of physics. Yet, there are doubts – at least regarding the extremes. Space experiments with quantum sensors are now supposed to bring clarity.

"On the large scale of galaxies, the laws of gravity do not explain why the universe has developed as we know it," says Andreas Wicht, head of the Laser Metrology Working Group at the Ferdinand-Braun-Institut, Leibniz Institute for High-Frequency Technology (FBH). "And on the microscopic level, below one hundred micrometers, there is no experimental verification of the validity of the gravitational law as we know it."

The aim of the joint project QUANTUS III, involving the universities of Hanover, Hamburg, Ulm, Mainz, Darmstadt, Bremen, and the HU Berlin, as well as the FBH, is to verify the validity of the equivalence principle at the atomic level. "Specifically, we ask ourselves: Do rubidium atoms fall exactly as quickly as potassium atoms?" Andreas Wicht is developing the laser technology platform for a so-called atom interferometer, a quantum sensor that is soon to be used in space.

Since the late 1980s, "atom fall experiments" have been conducted on a laboratory scale. However, the suspected differences in fall velocities are so minimal – at best in the tenth decimal place – that very long measurement times are needed for sufficiently sensitive measurements. These can only be achieved in space. But the roughly 2 x 2 meter measurement tables are far too unwieldy for this. Since the mid-1990s, the German Aerospace Center (DLR) has been promoting experiments and related technologies. Experiments have already been carried out on the Bremen Drop Tower, which allows four seconds of free fall in a vacuum tube about 100 meters high. "This demonstrated that the experiment works in principle. But the measurement time is still far too short."

How does the "fall experiment" work? "The atoms essentially act as sensors," explains Wicht. In microgravity, the atoms along with the measurement device are in free fall. First, the thermal motion of the atoms of both species is slowed down with light pulses of specific frequency until the atoms are almost stationary because they are cooled close to absolute zero. With further laser pulses, the atoms are manipulated into specific states that can only be described using quantum physics, which is why they are also called quantum optical sensors. The effect of the laser pulses and thus the measurement result of the quantum optical sensor depends very sensitively on the frequency and phase of the light pulses. If the atoms of these two species were accelerated differently, the frequency and phase of the laser pulses would need to be adjusted differently for both species to account for the Doppler effect resulting from this different acceleration. "This difference, if it exists, must be measured," Wicht explains.

At FBH, the "toolkit of light" has been developed for this purpose—a kind of synchronized light organ made of various spectrally narrow-band diode lasers, micro-mirrors, and other miniaturized optical components. "We can integrate two of these chips onto a ceramic base. Micro-optics are built around them," Wicht explains. "The complete laser system will consist of six or eight such modules and will be about 1000 times smaller in volume than a conventional product." It will be hermetically sealed, with only the optical fiber leading to the experimental chamber protruding.

The system has already proven its rocket compatibility at the Bremen Drop Tower. Experiments are scheduled for April 2015 on a high-altitude rocket that will ascend to about 100 kilometers. Until re-entry into Earth's atmosphere, there will be a six-minute window of microgravity for the experiment. Later, such experiments are to be conducted on a satellite or the ISS space station.

A pretty big effort just to disprove Einstein, or not? Wicht laughs. "One might think so. But when GPS was developed, no one would have thought it would be available on every smartphone today. And beyond atomic experiments, the technology of quantum sensors may not become an everyday tool, but at least find some specialized technical applications." Besides velocity and acceleration measurements, they can also be used for density measurements and very precise location determinations on Earth.

The British government recently invested 270 million pounds in the commercialization of quantum sensors. While today, precise location determination requires access to a GPS satellite, which could fail, quantum sensors navigate GPS-free, needing only a known location as a reference point. Unlike GPS, this also works in the depths of the oceans from submarines. "The second major application area will be exploration," Wicht adds. "Atoms in the sensor fall faster over ore deposits than over normal rock. Oil reservoirs and groundwater levels can also be mapped."

Great Britain is investing, the USA is planning experiments on the ISS – and what is Germany doing? The Program Committee for Optical Technologies identified quantum sensing as a promising future technology within the framework of the Photonics 2020 agenda for the BMBF. "But that's just on paper," Wicht says. "What we ultimately need is money for further research, a political decision." Currently, Germany is doing relatively well because the DLR funds these efforts. "But we must be careful not to lose our good position."

For fundamental researchers like Andreas Wicht, commerce is not the priority. They are interested in pure physics. Albert Einstein would probably have smiled at the fact that the Earth is not enough to grasp the limits of his brilliant ideas.