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FQXI ARTICLE

September 29, 2021

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.

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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...

DURGA DAS DATTA. wrote on June 30, 2016

Do we exactly know that flow of time is in continuous progression or in bits of Planck time ? The possibility of loosing causal for an effect can not be ruled out. As such quantum effects become probabilistic in nature.We should be doubtful about a super computer on such loosing information in quantum bits.

Do we exactly know that flow of time is in continuous progression or in bits of Planck time ? The possibility of loosing causal for an effect can not be ruled out. As such quantum effects become probabilistic in nature.We should be doubtful about a super computer on such loosing information in quantum bits.

STEVE DUFOURNY wrote on March 23, 2016

:) you are cool in fact ,a realjedi of the sphere ,I like :) we are all linked indeed since the begining of the big polarization after all.Like jedis travellers froms stars and galaxies.We imrpove and we create, we evolve and we increase in mass entropy .We are all linked in a dance of rotations and spherisation.let's sing together the alleluia :)

:) you are cool in fact ,a realjedi of the sphere ,I like :) we are all linked indeed since the begining of the big polarization after all.Like jedis travellers froms stars and galaxies.We imrpove and we create, we evolve and we increase in mass entropy .We are all linked in a dance of rotations and spherisation.let's sing together the alleluia :)

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