RECENT ARTICLES

A radical theory replaces the cosmic crunchers with fuzzy quantum spheres, potentially solving the black-hole information paradox and explaining away the Big Bang and the origin of time.

Why physics and astronomy communities must take diversity issues seriously in order to do good science.

Einstein’s relativity pushes physicists towards a picture of the universe as a block, in which the past, present, and future all exist on the same footing; but maybe that shift in thinking has gone too far.

Resolving the black hole firewall paradox—by calculating what a real astronaut would compute at the black hole's edge.

Defining a ‘quantum clock’ and a 'quantum ruler' could help those attempting to unify physics—and solve the mystery of vanishing time.

FQXI ARTICLE

June 22, 2018

Blurring Causal Lines

Quantum experiments mix past and future on the microscopic scale—opening the door to faster computers and revising our notion of causality.

FQXi Awardees: Caslav Brukner

February 29, 2016

Caslav Brukner

University of Vienna

Thus psychoanalyst-detective Max Liebermann is introduced to his first body in Frank Tallin’s crime novel

"All events are causally ordered such that for every pair of events you can say that one event is cause or effect of the other," says quantum physicist Caslav Brukner. Causality is so ingrained in the texture of our lives, and our brains, that it is hard to imagine letting go of it. Yet this is exactly what Brukner addressed with the help of an FQXi grant of over $63,000. The implications of this idea could be enormous: we might find that space, time and causality are not the basic building blocks of nature. It would have practical consequences too—potentially helping us to build quantum computers to outperform today’s devices.

In present day Vienna at the Institute for Quantum Optics and Quantum Information at the University, Brukner heads a small group of researchers who try to understand the foundations of quantum theory, which governs the behaviour of atoms on the smallest scales. For instance, a quantum particle can be in multiple places at the same time; before it is observed, it exists in a

"In all our physical theories we assume a well-defined causality," says Brukner. That’s even true for Einstein’s special and general theories of relativity, which mess with many of our everyday conceptions by stating that time is another dimension, like the three dimensions of space. Einstein taught us that, depending on how two people move relative to one another, they may disagree on the order in which they perceive two independent events to have occurred. Causality is still sanctified in relativity, however, because there’s no way any observer could perceive an effect to have taken place before its cause.

But Brukner argues that to truly understand nature and the world we live in—and perhaps to eventually combine quantum theory and general relativity into one framework—we may have to get away from this safe sense of order. "If we believe in the validity of quantum mechanical laws, and we believe in the validity of general relativity, we need to think of a situation in which the causal order is not well-defined," says Brukner.

Removing Causal Order

In research published in

The work challenges

established views on

space and time.

established views on

space and time.

- Stefan Wolf

But here was the trick. To allow the physicists to create a superposition between the order of events in Alice and Bob’s laboratories, Brukner’s team conjectured that the two laboratories were

"In absence of a global causal order the fact that the particle enters each laboratory only once would not allow us to conclude in which order the particle visits the labs," says Brukner. "We can talk about situations in which we can not see in advance whether A is before B, or B is before A and we can have something which we roughly identify with superposition of causal orders."

Superfast Computing

Removing causality is more than just an interesting conceptual shift; it may also have benefits for those attempting to build super fast computers that exploit quantum laws to outperform today’s machines. While our personal computers store information as bits that can either be 1 or 0, quantum computers—which were first proposed in the 1980s—use quantum bits, or qubits, that can represent a 1, 0, or any superposition of these states. The ability of qubits to hold multiple states at once would allow quantum computers, in theory, to solve problems more quickly than classical ones.

Before or After?

Experiments test whether quantum particles obey causality.

Credit: Philip Walther’s group

It took a couple of years for the ideas of Brukner and his team to be tested, however. "Nowadays, theory is very much advanced as compared to experiment," says Stefan Wolf, an expert on quantum information at the University of Lugano in Switzerland. "The reason is that this is often a very big step, where one faces both technological difficulties and conceptual challenges."

But now, experiment is catching up. In a study published in August 2015 in

While this work might lead to more advanced quantum computers, what makes Brukner even more excited is the idea that we can learn something fundamental about nature. He argues that the difficulties in developing a unified framework for quantum theory and Einstein’s theory of gravity might partially lie in our inability to incorporate the lack of global causality in the framework. "I hope this will help people conceptually to develop new ideas towards this goal."

"The work is very fundamental and touches on the deep questions on the global causal understanding of quantum theory and challenges the established views on space and time," adds Wolf.

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!

function ValidatePostText_main () {
form = document.addPostForm_main;
var recaptcha = $("#g-recaptcha-response").val();
if (recaptcha === "") {
event.preventDefault();
alert("The reCaptcha Box below must be checked before you submit the form");
}
else if (form.postText_main.value == '') {
alert ("The post contains no text");
return false;
}
else {
return true;
}
}

**Your name:**
(optional)

Recent Comments

read all article comments

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 (10

^{100}) and subscript (A_{2}) 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

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

Attachments
[hide]

You may optionally attach up to two documents to your post. To add an attachment, use the following feature to browse your computer and select the file to attach. The maximum file size for attachments is 1MB.

Once you're done adding file attachments, click the "Submit New Post" button to add your post.

You may optionally attach up to two documents to your post. To add an attachment, use the following feature to browse your computer and select the file to attach. The maximum file size for attachments is 1MB.

Once you're done adding file attachments, click the "Submit New Post" button to add your post.

THUY LIEN wrote on December 15, 2017

Good article, thanks for sharing.

hell let loose

metal gear survive

far cry 5 release date

Good article, thanks for sharing.

hell let loose

metal gear survive

far cry 5 release date

ANONYMOUS wrote on June 2, 2017

The political SOP by Pres. Trump to the nouveau riche, of reneging on the Paris climate accords mixes past and future macroscopic events.

The political SOP by Pres. Trump to the nouveau riche, of reneging on the Paris climate accords mixes past and future macroscopic events.

STEVE DUFOURNY wrote on June 30, 2016

Hello ,perhaps it is just an effect of rotations ,clocks of quantum and cosmological 3D sphères Inside this 3D universal sphere.The senseor rotation os gravitation seems correlated in a simplistic vue and it is irreversible entropically speaking.Now of course it is a tool with our relativity permitting to class our evolution in seeing simply our past more we go far in the analyse at two scales.We see our future with the cosmological scale and we study our past in analysing the steps towards the...

Hello ,perhaps it is just an effect of rotations ,clocks of quantum and cosmological 3D sphères Inside this 3D universal sphere.The senseor rotation os gravitation seems correlated in a simplistic vue and it is irreversible entropically speaking.Now of course it is a tool with our relativity permitting to class our evolution in seeing simply our past more we go far in the analyse at two scales.We see our future with the cosmological scale and we study our past in analysing the steps towards the...

read all article comments