Gravity measured for first time using a portable 'atomic clock'


Scientists have used a portable “atomic clock” to measure gravity for the first time.

The device makes use of small changes in the flow of time at different altitudes to measure the Earth’s gravity.

Generally speaking, atomic clocks are complex, delicate pieces of scientific equipment that are confined to labs and therefore cannot be used to make measurements in the field.

The use of a portable device enables scientists to make measurements that can help them understand planetary changes such as rising sea levels and melting ice sheets.

Dr Helen Margolis, a physicist at the National Physical Laboratory who contributed to the research, explained that atomic clocks are – in essence – similar to the clocks people use every day.

“Any kind of clock needs several parts of it; you need something that ticks, and something that keeps the ticks constant,” she said.

“So in an optical atomic clock, which is what we used in this experiment, the thing that ticks is a laser – and we basically keep its frequency stable by shining that laser onto an atom.”

The laser resonates with that atom at a very specific frequency, therefore “keeping the ticks constant”.

This device can be used as a measure of gravity at a specific site due to a phenomenon described by Einstein’s theory of relativity. Specifically, a clock in a strong gravity field will run slow compared with a clock in a less strong gravity field.

“That means if you have got a clock closer to the surface of the Earth it will run slow compared to one at the top of a mountain,” said Dr Margolis.

“Because our clocks are so precise we can translate a measurement of their frequency into a measurement of gravity potential.”

Other gravity clocks are available. The current standard clocks used in laboratories around the world employ microwaves to tell the time, but the scientists behind this work say optical versions that use lasers are set to be the “atomic clocks of the future”.

“Optical clocks are deemed to be the next generation atomic clocks – operating not only in laboratories but also as mobile precision instruments,” said Dr Christian Lisdat, a researcher at the Physikalisch-Technische Bundesanstalt (PTB) who also contributed to the research.

The work emerged from collaboration between European scientists, and was published in the journal Nature Physics.

Having developed a clock that could be transported around with relative ease, scientists from PTB drove it to the French Modane Underground Laboratory, an institution located in a road tunnel 1,700m below a mountain top.

They then compared measurements taken there with those from a second clock in Turin, around 1000m above the first.

The test determined the “gravity potential difference” between the two sites was consistent with measurements taken using other techniques, meaning the portable clock should be a viable method for measuring height changes of the Earth’s surface.

Currently, different labs around the world use different reference levels to measure the Earth’s surface.

In the past this has led to problems. The Hochrhein Bridge between Germany and Switzerland, for example, was constructed using different sea level measurements in the two countries, resulting in a 54cm discrepancy between the two sides.

The researchers hope the use of portable atomic clocks can help to achieve consistency between national height systems and therefore prevent such mistakes.

By measuring changes in gravity, and by association changes in elevation, climate scientists should also be able to employ portable atomic clocks to determine changes at the Earth’s surface associated with climate change.

“This will enable them to deduce all sorts of things like ice sheet masses changing with time, and monitoring changing sea levels,” said Dr Margolis.

As it stands, such measurements are made using satellite observations. While this is an effective technique, it has a very broad resolution and does not allow scientists to examine specific sites.

The new technique will allow scientists to zoom in on areas where environmental changes are taking place and make incredibly precise measurements, allowing them to effectively forecast the effects of climate change.

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