Watching the Observers

Who is in charge, the observers, or the observed?

That, in a nutshell, is the question that quantum physicist Ivette Fuentes, of the University of Vienna, Austria, is seeking to answer.

Fuentes, like many other physicists, is intrigued about the nature of quantum gravity—the elusive theory that would unite the quantum physical rules of the microworld with general relativity, the laws of gravity on cosmic scales. Unlike many others striving toward the same goal, however, Fuentes and her colleagues are on this seemingly esoteric quest because they believe it could have tangible consequences in the lab today for our ability to measure time and space precisely and to make accurate astronomical observations. Fuentes notes that observers—the people making these experimental measurements—have profoundly different roles, and limitations, in these cornerstone theories of modern physics, and this could be the key to understanding how the theories interact.

In the usual way of thinking about general relativity, observers play a passive role. "When observers make measurements of the system, they don’t really affect it," Fuentes explains, "but their observations depend on their state of motion." In quantum mechanics, by contrast, an observer takes an active part in what happens. The mere act of choosing when and how to make a measurement can irrevocably change the properties of the underlying system being measured. "So you can’t separate the observer from the physical system that’s being observed," Fuentes says.

These two versions of an observer are not necessarily incompatible. But they are also not clearly the same thing. So, with the help of physicists David Bruschi of the University of York, UK, and Stefano Mancini of the University of Camerino, Italy, and an FQXi grant of almost $85,000, Fuentes is asking, is there a way to make these two ways of observing agree? And how does this affect the precision of measurements we take for granted?

"Quantum theory can tell us at the end what we can say and what we cannot about the universe," says Mancini.

The team’s approach may sound modest at first. Rather than trying to build a complete quantum gravity theory from scratch, they are looking at current theoretical and experimental methods and calculating their limitations—due to quantum, gravitational or other relativistic effects.

Radiation Bath

Thanks to quantum uncertainty, there’s a limit to how precisely physicists can measure the properties of tiny quantum objects. You famously cannot know both the position of a particle and its momentum simultaneously. This uncertainty is usually thought to be unimportant in large-scale astronomical observations, however. For instance, astronomers have learned huge amounts about the origins of the universe by precisely measuring the temperature of light particles in the cosmic microwave background—a radiation bath created roughly 400,000 years after the big bang that still pervades today’s universe.

But the team argues that astronomers must ultimately face the fact that the particles of light themselves are subject to quantum uncertainty, which will limit the precision of their measurements of the radiation. Bruschi blames this "blurriness" on the quantum nature of the spacetime fabric itself. The team wants to find out what this means for our attempts to pin down the basic properties of the universe.

They also want to know what precision limits mean for "quantum clocks"—devices developed almost a decade ago that use single ion vibrations to track time more accurately than international standard atomic clocks. Some physicists hope to one day use such devices to precisely measure general relativistic effects and the properties of spacetime. But there may be a problem. Typical discussions of general relativity depict observers as simple, point-like, unphysical non-entities. But real devices are complex, have a size, and follow laws of physics. How might this affect what we can observe? "The moment you put in real things to measure space and time, quantum mechanics doesn’t allow you to go beyond a limit," Fuentes notes.

This could have profound implications. General relativity supposes that spacetime is smooth, but this quantum measurement limitation could define a grainy structure for spacetime’s fabric that might become a core feature of a new theory of quantum gravity, Fuentes explains: "Is that somehow the starting point of a quantized spacetime?” (See also "Wrinkles in Spacetime.")

Physicist Julian Barbour, an expert on time at Oxford University, UK, agrees that to truly understand time, we must consider the quantum laws that describe the motion of real clocks.

The project could also have an important impact in the field of quantum metrology, which aims to exploit quantum effects to improve measurement techniques. "Hopefully we’ll get interesting things that can be tested," says Tim Ralph, an expert on quantum optics at Queensland University in Australia. He also hopes that the project will yield "some useful techniques for improving metrology in useful situations."

The team hopes their work will connect quantum and gravity, theory and experiment, practical and fundamental—and finally start to bring questions that were once only intellectual musings, into the lab: "We are really at a tipping point," says Bruschi, "where these questions are becoming not just questions for a theoretician."