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How do you check the accuracy of an atomic clock that 鈥渨ill lose less than 1 second every 40 billion years鈥�, as per a recent New Scientist article?

Pat French
Longdon-upon-Tern, Shropshire, UK

Atomic clocks are based upon quartz crystals. These crystals vibrate at a very uniform rate when an electric current is passed through them. By measuring minute variations in those vibrations over months or years, scientists can predict a clock鈥檚 accuracy.

The value of those variations is so small that it needs to be projected over tens of billions of years in order to be stated in seconds for the benefit of us readers. One second in 40 billion years is a little more meaningful to most of us than 25 picoseconds per year.

Shimon Kolkowitz
Herst Chair Associate Professor of Physics, University of California, Berkeley, US

No clock made by humans can tick for 40 billion years. After all, the universe is a 鈥渕ere鈥� 14 billion years old so far.

The statement that an atomic clock is so accurate that it can tick for 40 billion years without losing a second is a mathematical metaphor intended to convey just how accurate these clocks really are, using a timescale we are familiar with: the second.

The clock in question has been characterised to have an accuracy of 8 parts in 10¹⁹. This is roughly the same as the ratio of 1 second to the number of seconds in 40 billion years (~1.26 脳 10¹⁸ seconds). There are also ~1.26 脳 10¹⁸ nanoseconds in 40 years, so it would be equally meaningful to say that the clock could tick for 40 years without losing a nanosecond, or that it could tick for an hour without losing more than roughly 3 femtoseconds (there are 10¹⁵ femtoseconds in 1 second). These statements are all equivalent.

The ideal way to check if a clock really is this accurate is to compare it to an even more accurate clock, but as no such clock exists, you instead need two independent clocks. After building them, you would compare them to each other over the course of hours or days. If after 1 hour of ticking independently the two clocks still agree with each other on the elapsed time to better than 3 femtoseconds, then they have the claimed accuracy of 8 parts in 10¹⁹, and you could argue that if they did tick for 40 billion years straight they would agree with each other to within 1 second.

To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.

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