The super-clocks that define what time it is. How do we know the time... precisely? The question is more complex than it first appears, says Richard Fisher.

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I'm looking at a warning sign inside a laboratory in London. "Do not touch the maser," it reads. It's attached to a tall black box, on wheels, mounted in a steel protective case. Turns out it's a pretty important box, and the sign is there for a reason. It's not dangerous, but if I were to meddle with the device, it might just disrupt time itself.

It's one of a few such devices held at the National Physical Laboratory in south-west London, helping to ensure that the world has an accurate shared sense of seconds, minutes and hours. They're called hydrogen masers, and they are extremely important atomic clocks. Along with around 400 others, placed all around the globe, they help the world define what time it is, right now, down to the nanosecond. Without these clocks – and the people, technology and procedures around them – the modern world would slowly drift into chaos. For many industries and technologies we rely upon, from satellite navigation to mobile phones, time is the "hidden utility".

So, how did we arrive at this shared system of timekeeping in the first place, how does it stay accurate, and how might it evolve in the future? The answers involve looking beyond the clockface to explore what time actually is. Dig a little deeper, and you soon discover that time is more of a human construct than first appears.

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https://www.bbc.com/future/article/2...hat-time-it-is