Relativity Just Passed a Major Test Involving the Most Accurate Clocks Ever

Yep, Einstein is still right.

27 MAR 2017

Science loves nothing better than to find new ways to kick well-tested theories even harder and see if they can't be made to bleed a little.

Take time dilation for instance - the idea that time depends on your relative speed and gravity's pull. While it's been tested using highly accurate caesium atomic clocks, physicists have now put it to the test using even more accurate strontium atomic clocks, and they've found that Einstein still stands victorious.


Researchers from the Paris Observatory in France set up a bunch of the latest-generation strontium atomic clocks around Europe to test if their different speeds as the Earth spins affects their relative times - just as Einstein's special relativity predicts.

Given the number of times this theory has been tested, it's unlikely anybody would expect a radical refutation of special relativity in a single blow; however, even a slight contrast on a tiny level could provide physicists with clues as to how how the powerful but incompatible theories of general relativity and quantum mechanics can be married together.

For anybody who's a little confused, physicists use general relativity to predict how massive things like galaxies and stars behave, and quantum mechanics to predict how very tiny particles will interact.

Both are incredibly accurate, but oddly, the two ideas don't play well together.

Usually that's not a big deal, as they typically focus on different scales, but when it comes to massive objects and tiny scales, such as what happens on the inside of black holes, physics gets weird, and even well-tested mathematics falls apart.

In really basic terms, general relativity deals mostly with gravity and space, while its cousin - special relativity - deals with time and space.


The "relativity" parts of those theories describes how the rules of the Universe appear to people located in different times and spaces.

According to a relativity rule called Lorentz invariance, all physical laws will be the same, regardless of whether you're moving, standing still, in the middle of a city, or floating in space.

Unfortunately, there's a slight hitch - the same bunch of rules says light has a 'one speed only' policy.

If light can only go one speed in a vacuum, two people moving at different velocities or affected by the gravitational warping of space-time will have to agree on that speed.

That means they'll both have to agree that light appears to be travelling at about 300 million metres per second (or just under 1 billion feet per second), but their perspective on each other's time is slowed down instead.

This odd little quirk of physics called time dilation has been measured over and over again - even forcing us to take it into account in the mathematics of our GPS satellites.

To measure the subtle differences in how time appears to tick slower depending on how fast you seem to be moving, you need a clock that ticks with extreme precision.

This experiment used four optical lattice clocks based on the 'ticking' of a few thousand strontium atoms, which are bathed in the light of an extremely stable laser in such a way that they're forced to switch energy levels about 430 trillion times a second.

The clocks are so stable they don't lose or gain a second over 15 billion years, making them three times more accurate than the previous generation's caesium atom clocks.

Two of the strontium clocks were housed at the Paris Observatory, one at Braunschweig in Germany, and a fourth at Teddington in the UK. All were then connected using optic fibres, which carried the laser to kick the atoms into ticking.

Thanks to the fact they sit at different latitude on the globe, the three cities move at different speeds as the globe spins, which technically should mean time seems to flow different differently from one another's perspective.

By coordinating the clocks over a distance with the same laser, the team could accurately detect any variations in their frequency that could tell them which clocks were ticking slightly faster or slower than the others.

Using these measurements, the researchers calculated a parameter called 'alpha' to be less than 0.0000001 - close enough to the zero needed for them to conclude that the Lorentz variation was still intact.

The research is currently awaiting review on the pre-publication website, but while the results don't break any radical new grounds, they should still be taken with a grain of salt until they've cleared peer-review.

But what if that number was big enough to say Lorentz variation had been violated?

"The immediate consequence would be that nobody would believe it," theoretical physicist Sabine Hossenfelder, who wasn't involved in the research, told Anil Ananthaswamy at New Scientist.

"Quantising gravity, [the nature of] dark matter, and dark energy - these are three big questions for which Lorentz invariance violations would be an extremely important hint as to the nature of the underlying theory," said Hossenfelder.

It seems like every other day, Einstein's work is getting tested on either the small scale with atomic clocks or the massive scale of distant galaxies.

It's not that people don't trust the guy; more that with such tantalising, strange discoveries as dark matter, or that two of our best ideas in physics are irreconcilable, it's hoped that there's some wiggle room on some level.

For now, there isn't a sign of even a small amount of wiggle room for relativity to be wrong enough to make room for quantum mechanics.

Maybe we just need to kick that theory a little harder next time.

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