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December 6, 2021

Thermal Timekeeping
Constructing a clock that measures temperature could unite the conflicting conceptions of time in the quantum and cosmic realms.
by Sophie Hebden
FQXi Awardees: Gerard Milburn
February 28, 2012
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Gerard Milburn and postdoctoral researcher Uzma Akram,
University of Queensland
Time hasn’t always been the exacting march of seconds, minutes, and hours that we take for granted. In the fourteenth century, people lived with two sorts of time: solar time, which began at sunrise and divided daylight into twelve hours—and was all that mattered if you worked the land—and the new mechanical clock time. In summer the solar hour was longer than the clock hour, and in winter it was shorter.

So perhaps we shouldn’t be too perturbed when physicist Gerard Milburn says that he wants to redefine time once more, even if—mind-bogglingly—he wants to construct a clock that marks its ticks based on…temperature. As strange as it sounds, this new view of thermal time could help the quest for a theory of quantum gravity, uniting the physics of the very large, governed by Einstein’s theory of general relativity, and the quantum realm of the very small.

Thanks to Einstein’s theories of relativity, physicists are already familiar with time’s ability to stretch and contract, in response to the strength of gravity nearby or to the speed of an object you are observing. But while Einstein’s theories work brilliantly on a large scale, there’s no easy way to include gravity and the relative nature of time at the atomic scale. In the quantum regime, the uncertainty principle prevents you from defining space and time too closely. "This tells you that time and space are kind of lumpy—they are not smooth and flowing but in some sense jittery and noisy," says Milburn, who is based at the University of Queensland, in Brisbane, Australia.

The $64,000 question—or in this case the $64,250 question to be addressed with a grant from FQXi—is: What is time in General Relativity? The usual thing is to identify some physical thing in the theory that acts as a proxy for time, Milburn explains. So, with his former grad student Nick Menicucci, now at the University of Sydney, in New South Wales, Australia, he has been searching for a property that can mark time equally well in both the cosmic and quantum regimes. "Thermal time" fits the bill, they argue, because it is based on the physics of thermodynamics, which uses statistics to describe how flows of heat and work done on a system change its temperature, volume, and pressure. "Thermodynamics is an incredibly fundamental theory, it applies across all physical theories," says Milburn, "including any future quantum theory of gravity."

You know exactly how
a system will tick once
you push it away from
thermal equilibrium.
- Gerard Milburn on thermal time
Thermal time is regulated by something reaching a uniform temperature. For example, if you dip one end of a steel bar into a bucket of ice and take it out, it takes a while for both ends to have the same temperature again. This balanced state is called thermodynamic equilibrium, explains Milburn. So far, so good, but how does that help us identify a new kind of clock? "It sounds counter-intuitive because, by definition, a system in equilibrium doesn’t change at all, so in that sense it doesn’t have any time," admits Milburn.

But the key point is that when you have a gravitational system in thermodynamic equilibrium, you can look at the probability of finding it with a particular energy. Physicists describe the way a quantum system’s energy evolves with a mathematical term known as the Hamiltonian, which, in turn, completely determines the flow of time for that system, says Milburn. "Once you know this, you know exactly how a system will tick once you push it away from thermodynamic equilibrium."

Forget Time

The idea of relating time to thermodynamics was actually first introduced by Carlo Rovelli and Matteo Smerlak, at the Centre for Theoretical Physics in Marseille, France. Rovelli outlined his "Thermal Time Hypothesis" in his award-winning FQXi essay, "Forget Time." As an experimental physicist, Milburn’s first thought on reading about this was "How can we measure that?" recalls Menicucci. "It’s an extremely useful approach in physics," Menicucci adds. "Some of the effects that Rovelli talked about actually disappear or become more interesting when you consider how to build a clock to measure them."

Other independent researchers, such as physicist Erik Verlinde, at the University of Amsterdam, have also found signs, in recent years, of an underlying link between thermodynamics, gravity, and Einstein’s theory of general relativity (see "The Myth of Gravity"). "That’s why I think thermal time is an idea worth pursuing," says Milburn.

The race to find a theory of quantum gravity could be hotting up,
thanks to thermal time.

Credit: Konstantin Yuganov
Milburn is hoping to extend their understanding of thermal time to space as well, and end up with a theory that works like general relativity but is valid everywhere—in short, deriving a thermal spacetime. Thermodynamics could, they hope, provide a natural way to describe the jitteriness of spacetime on quantum scales because its mathematical framework is already well-equipped for describing how thermal fluctuations cause atoms and molecules to jiggle.

Although they haven’t yet worked out the details, they are encouraged by their progress so far, as are some other physicists. Tim Ralph, who leads a research centre on quantum computing at the University of Queensland, says it is an interesting approach to putting quantum mechanics and general relativity together.

But attempting to redefine time is an ambitious task and others, while impressed by the project’s audacity, urge caution. "The idea of replacing spacetime by something as yet unspecified in which something like thermodynamics exists, yet which can recover in some approximation physics in spacetime, remains intriguing but elusively vague to me," says Ted Jacobson, a gravitational theorist of the University of Maryland, College Park.

Smerlak, commenting on a draft paper, agrees the team must be careful not to "jump ahead of themselves" by claiming to have constructed a protocol for thermal clocks, but adds that they are following "a very interesting track."

Ever the experimentalist, Milburn understands that to win over skeptics, his ideas must eventually be testable, and it is something he is working on: "We might even be able to propose an experiment to probe the thermodynamic approach to quantum gravity by checking to see if gravity has a noisy effect on quantum states of light," he says. "But this is all conjecture at the moment."

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Recent Comments

You are mistaken, as unification in physics (that is, gravity, inertia, and electromagnetism together in an equivalent and balanced fashion) necessarily involves HALF gravity and half inertia, as this is the MIDDLE distance in/of space. This gives us invisible and visible space in equilibrium and balance.

Hi Amrit,

How are you? Fine I hope. I asked me but where were you ? happy to see you again on this platform.

About you post,

Indeed but with a complexification of mass by evolutive optimization. The numerical change on the entropical arrow of time is universal and deterministic. A little if we said that the spheres of mass polarises the spheres of light. That is why a main different sense of rotation is essential consideringt the linear light.If we take my equations with the...

on the micro and macro level time is a numerical order of change.......

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