More accurate clocks may add more disorder to the universe
All clocks create entropy, but accuracy might come at a higher cost
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Entropy – or disorder – is created every time a clock ticks. Now scientists working with a tiny clock have proven a simple relationship: the more accurate a clock runs, the more entropy it generates. “If you want your clock to be more accurate, you’ve got to pay for it,” said Natalia Ares, a physicist at the University of Oxford. “Every time we measure time, we are increasing the universe’s entropy.”
As we go forwards in time, the second law of thermodynamics states that the entropy of a system must increase. Known as the ‘arrow of time’, entropy is one of the few quantities in physics that sets time to go in a particular direction – from the past, where entropy was low, to the future, where it will be high.
This tendency for disorder to grow in the universe explains many things, such as why it’s easier to mix ingredients together than separate them out, or why headphone wires get so intricately tangled together in our trouser pockets. It’s also through this growing disorder that entropy is wedded so intimately to our sense of time.
A famous scene in Kurt Vonnegut’s novel Slaughterhouse-Five demonstrates how differently entropy makes one direction of time look to the other by playing World War II in reverse: bullets are sucked from wounded men, and fires are shrunk, gathered into bombs, stacked in neat rows and separated into composite minerals as the reversed arrow of time undoes the disorder and devastation of war.
This intimate connection between time and entropy has fascinated scientists for decades. Machines such as clocks also produce entropy in the form of heat dissipated to their surroundings. Physicists have been able to prove that a tiny quantum clock – a type of atomic clock that uses laser-cooled atoms that jump at highly regular intervals – creates more disorder the more accurately it measures time. But until now it has been very difficult to prove that larger, more mechanically complex clocks create more entropy the more accurate they get, even if the idea sounds good in theory.
“Clocks are like little steam engines – you need to put work into them to measure time,” Ares said, where the “work is the energy transfer needed to make mechanical devices like clocks run. In order to get that regular tick you have to get the machine going. That means you need to invest in entropy production.”
To test this idea, researchers have built a simplified clock made up of a 50-nanometrethick, 1.5-millimetre-long membrane stretched between two tiny posts that they vibrated with pulses of electricity. By counting every flex up and down as a tick, the team showed that more powerful electrical signals made the clock tick more regularly and accurately, but at the cost of adding more heat – and therefore more entropy. Perhaps if clocks didn’t produce any entropy, they’d be just as likely to run backwards as they do forwards, and the more entropy they generate the more they’re protected from stutters and backwards fluctuations.
“We don’t know for certain yet, but what we’ve found is that there’s a proportional relationship between accuracy and entropy,” Ares said. “It might not always be a linear relationship for other clocks, but it does look like the accuracy is bounded by the laws of thermodynamics.” Aside from being useful for designing clocks and other devices in the future, these findings are laying the groundwork for further exploration of how the large-scale laws of thermodynamics apply to tiny nanosized devices.
“If you want your clock to be more accurate, you’ve got to pay for it. Every time we measure time, we are increasing the universe’s entropy” Natalia Ares