Search FQXi


RECENT FORUM POSTS

Steve Dufourny: "Hi Peter,Adel, Colin, You know when I speak about spheres I insist, about..." in Manipulating the Quantum...

Peter Morgan: "Colin, I forgot to discuss the Planck energy scale that you mention,..." in Manipulating the Quantum...

Jeffrey Schmitz: "Please read my essay. Yes, my essay is in the bottom half of the rating,..." in FQXi Essay Contest 2016:...

Steven Andresen: "Darwinian Universal The nature of the interaction between space and..." in Alternative Models of...

Steven Andresen: "Darwinian Universal The nature of the interaction between space and..." in Complexity levels and...

Steven Andresen: "Darwinian Universal The nature of the interaction between space and..." in Alternative Models of...

Steven Andresen: "Darwinian Universal The nature of the interaction between space and..." in Is the Past Infinite?

Steven Andresen: "Darwinian Universal The nature of the interaction between space and..." in Theories of Everything,...


RECENT ARTICLES
click titles to read articles

Watching the Observers
Accounting for quantum fuzziness could help us measure space and time—and the cosmos—more accurately.

Bohemian Reality: Searching for a Quantum Connection to Consciousness
Is there are sweet spot where artificial intelligence systems could have the maximum amount of consciousness while retaining powerful quantum properties?

Quantum Replicants: Should future androids dream of quantum sheep?
To build the ultimate artificial mimics of real life systems, we may need to use quantum memory.

Painting a QBist Picture of Reality
A radical interpretation of physics makes quantum theory more personal.

The Spacetime Revolutionary
Carlo Rovelli describes how black holes may transition to "white holes," according to loop quantum gravity, a radical rewrite of fundamental physics.


FQXI ARTICLE
June 28, 2017

Watching the Observers
Accounting for quantum fuzziness could help us measure space and time—and the cosmos—more accurately.
by Brendan Foster
FQXi Awardees: Ivette Fuentes
June 13, 2017
Bookmark and Share


Ivette Fuentes
University of Vienna
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.


Blurred Lines
Could quantum effects make us question data from the sky?
Credit: Planck/ESA
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."

Comment on this Article

Please read the important Introduction that governs your participation in this community. Inappropriate language will not be tolerated and posts containing such language will be deleted. Otherwise, this is a free speech Forum and all are welcome!
  • Please enter the text of your post, then click the "Submit New Post" button below. You may also optionally add file attachments below before submitting your edits.

  • HTML tags are not permitted in posts, and will automatically be stripped out. Links to other web sites are permitted. For instructions on how to add links, please read the link help page.

  • You may use superscript (10100) and subscript (A2) using [sup]...[/sup] and [sub]...[/sub] tags.

  • You may use bold (important) and italics (emphasize) using [b]...[/b] and [i]...[/i] tags.

  • You may also include LateX equations into your post.

Insert LaTeX Equation [hide]

LaTeX equations may be displayed in FQXi Forum posts by including them within [equation]...[/equation] tags. You may type your equation directly into your post, or use the LaTeX Equation Preview feature below to see how your equation will render (this is recommended).

For more help on LaTeX, please see the LaTeX Project Home Page.

LaTeX Equation Preview



preview equation
clear equation
insert equation into post at cursor


Your name: (optional)



Important: In order to combat spam, please select the letter in this menu between 'N' and 'P':




Recent Comments


Your infinities are pretty much the same as mainstream science infinities and do explain anything that you want them to explain...that is what infinities do and that is what makes them so alluring.

By explaining everything, infinities actually explain nothing and so there is not useful future for infinity...


Excellent article. Keep writing such kind of information on your site. Im really impressed by your blog. Hello there, You’ve performed a fantastic job. I’ll certainly digg it and in my view suggest to my friends. I am sure they will be benefited from this web site. FM Radio Live Online Streaming


What’s up, everything is going perfectly here and of course every one is sharing information, that’s in fact fine, keep up writing.TV Live Online Stream

read all article comments

Please enter your e-mail address:
Note: Joining the FQXi mailing list does not give you a login account or constitute membership in the organization.