Time-symmetric formulation of quantum theory provides new understanding of causality and free choice

July 28, 2015, Université libre de Bruxelles
Credit: Vera Kratochvil/public domain

The laws of classical mechanics are independent of the direction of time, but whether the same is true in quantum mechanics has been a subject of debate. While it is agreed that the laws that govern isolated quantum systems are time-symmetric, measurement changes the state of a system according to rules that only appear to hold forward in time, and there is difference in opinion about the interpretation of this effect.

Now theoretical physicists at the Université libre de Bruxelles have developed a fully time-symmetric formulation of quantum theory which establishes an exact link between this asymmetry and the fact that we can remember the past but not the future – a phenomenon that physicist Stephen Hawking has named the "psychological" arrow of time.

The study offers new insights into the concepts of free choice and causality, and suggests that causality need not be considered a fundamental principle of physics. It also extends a cornerstone theorem in due to Eugene Paul Wigner, pointing to new directions for search of physics beyond the known models. The findings by Ognyan Oreshkov and Nicolas Cerf have been published this week in the journal Nature Physics.

The idea that our choices at present can influence events in the future but not in the past is reflected in the rules of standard quantum theory as a principle that quantum theorists call "causality".

In order to understand this principle, the authors of the new study analyze what the concept of choice in the context of quantum theory actually means. For example, we think that an experimenter can choose what measurement to perform on a given system, but not the outcome of the measurement. Correspondingly, according to the principle of causality, the choice of measurement can be correlated with outcomes of measurements in the future only, whereas the outcome of a measurement can be correlated with outcomes of both past and future measurements. The researchers argue that the defining property according to which we interpret the variable describing the measurement as up to the experimenter's choice, while the outcome not, is that it can be known before the actual measurement takes place.

From this perspective, the principle of causality can be understood as a constraint on the information available about different variables at different times. This constraint is not time-symmetric since both the choice of measurement and the outcome of a measurement can be known a posteriori. This, according to the study, is the essence of the asymmetry implicit in the standard formulation of quantum theory.

"Quantum theory has been formulated based on asymmetric concepts that reflect the fact that we can know the past and are interested in predicting the future. But the concept of probability is independent of time, and from a physics perspective it makes sense to try to formulate the theory in fundamentally symmetric terms", says Ognyan Oreshkov, the lead author of the study. To this end, the authors propose to adopt a new notion of measurement that is not defined only based on variables in the past, but can depend on variables in the future too. "In the approach we propose, measurements are not interpreted as up to the 'free choices' of agents, but simply describe information about the possible events in different regions of space-time", says Nicolas Cerf, a co-author of the study and director of the Centre for Quantum Information and Communication at ULB.

In the time-symmetric formulation of quantum theory that follows from this approach, the principle of causality and the psychological arrow of time are both shown to arise from what physicists call boundary conditions – parameters based on which the theory makes predictions, but whose values could be arbitrary in principle. Thus, for instance, according to the new formulation, it is conceivable that in some parts of the universe causality may be violated.

Another consequence of the time-symmetric formulation is an extension of a fundamental theorem by Wigner, which characterizes the mathematical representation of physical symmetries and is central to the understanding of many phenomena, such as what elementary particles can exist. The study shows that in the new formulation symmetries can be represented in ways not permitted by the standard formulation, which could have far-reaching physical implications. One speculative possibility is that such symmetries may be relevant in a theory of quantum gravity, since they have the form of transformations that have been conjectured to occur in the presence of black holes.

"Our work shows that if we believe that time symmetry must be a property of the fundamental laws of physics, we have to consider the possibility for phenomena beyond those conceivable in standard . Whether such phenomena exist and where we could search for them is a big open question", explains Oreshkov.

Explore further: Physicists publish solution to the quantum measurement problem

More information: Operational formulation of time reversal in quantum theory, Nature Physics, DOI: 10.1038/nphys3414

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21 comments

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francishayden
not rated yet Jul 28, 2015
Entanglement, 'spooky action at a distance', compromises causality in a very easily imagined way.

If we create two entangled particles and take one to Pluto then when we examine the one on Earth the state of the one at Pluto will instantaneously be decided and if it is examined a moment later we can predict what it will be.

But for an an observer moving rapidly through the solar system the order of these events could be reversed and it would therefore be the examination of the Pluto particle that causes the Earth particle to decide its state. And we know from Einstein that both views are equally valid.

Thus we cannot say which is the cause and which is the effect.
bluehigh
5 / 5 (3) Jul 28, 2015
Get a consensus then you will know for sure. That's how science works, right?
bluehigh
4 / 5 (4) Jul 28, 2015
It's of no value what you can 'say'. Cause always precedes effect by definition. It you observe otherwise then you've made a mistake.
docile
Jul 28, 2015
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docile
Jul 28, 2015
This comment has been removed by a moderator.
Returners
2 / 5 (4) Jul 28, 2015
Thus, for instance, according to the new formulation, it is conceivable that in some parts of the universe causality may be violated.


This would be a disaster for science. It would mean you cannot measure the distance to distant objects, and you can't trust observations of motion to tell you anything at all about the environment, and you can't say anything at all about what "Stars" are, etc, etc.

Exampels:

"What causes the CMB?"
"I don't know, maybe it violates causality."

"What causes objects in space to orbit on antoher?"
"I don't know, maybe it violates causality."

The stupidity that makes it into physorg articles is shocking.
docile
Jul 28, 2015
This comment has been removed by a moderator.
Returners
1 / 5 (1) Jul 28, 2015
But for an an observer moving rapidly through the solar system the order of these events could be reversed and it would therefore be the examination of the Pluto particle that causes the Earth particle to decide its state. And we know from Einstein that both views are equally valid.

Thus we cannot say which is the cause and which is the effect.


Not true.

In Relativity, events that are unrelated can change order in certain changes of reference frames.

Related events do not change their order of progression from one reference frame to another. The only difference is the amount of time that passes between the two events, as well as supposed things like length contraction and relativity of mass, etc.
ichisan
1 / 5 (3) Jul 28, 2015
This is all pseudoscience and crackpottery. In order to have a direction in time, there has to be motion in time. And motion in time implies a velocity in time which would have to be given as v = dt/dt. That's it.

The concept of a time dimension along which we are moving in any direction is a conceptual disaster. There is only the the ever changing present. Physicists need to wake the hell up.
Bloodyorphan
1 / 5 (2) Jul 29, 2015
Time can definitely be transcended, the question is "how do we do it consistently".

We have this concept that memory is a physical attribute of neurons, when in fact it may just be a very consistent window to the past, based on our physical biological existence and physical location.

I know for a fact that thought can transcend time. I'm living it :-)
antialias_physorg
3.4 / 5 (5) Jul 29, 2015
Entanglement, 'spooky action at a distance', compromises causality in a very easily imagined way.

Not really, as it doesn't transfer information. Causality is the notion of information transfer. Your "Pluto experiment" does not deal in classical information transfer.
They put the difference between classical and quantum information pretty succinctly in this part:
The researchers argue that the defining property according to which we interpret the variable describing the measurement as up to the experimenter's choice, while the outcome not, is that it can be known before the actual measurement takes place.

(Note: This is why entanglement cannot be used for transitting messages - which would be classical information. But it does allow for encryption - which is not a form of information)
Quantum information is not bound by causality.
Returners
1 / 5 (2) Jul 29, 2015
This is all pseudoscience and crackpottery. In order to have a direction in time, there has to be motion in time. And motion in time implies a velocity in time which would have to be given as v = dt/dt. That's it.

The concept of a time dimension along which we are moving in any direction is a conceptual disaster. There is only the the ever changing present. Physicists need to wake the hell up.


If there wasn't at least one time dimension, neither Newtonian Dynamics nor Relativity would make such good predictions about the bending of light around the mass of the Sun.

dt/dt = 1, which is a unitary operator, which doesn't make it invalidated.

Additionally, there may be higher dimensions which time is differentiated with respect to those dimensions, for example.

Please note that whenever I make a hypothesis I don't demand that everyone believe it as "settled science" the way the DM and DE crowd does.
Returners
3 / 5 (2) Jul 29, 2015
But it does allow for encryption - which is not a form of information)
Quantum information is not bound by causality.


Encryption is a form of information.

lol. What else do you think it would be?

Encryption is the process of disguising one form of information as another form of information.

In this case, "information" is actually stored in the apparatus, in that all possible outcomes are stored in the apparatus, not the particles/waves, and it gives you the false impression that "spooky action" has taken place.
docile
Jul 29, 2015
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docile
Jul 29, 2015
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antialias_physorg
3.7 / 5 (3) Jul 29, 2015
Zeph..you still have a bit of ways to go to understand QM. Pick up a book first. Your lacking some pretty fundamental pieces.
charlimopps
1 / 5 (1) Jul 29, 2015

The stupidity that makes it into physorg articles is shocking.


That's not what they're arguing. They're suggesting that time is purely an artifact of our psychology. That the entire premise of "Events" is wrong. The question "What caused the CMB?" Is like asking "How big is purple" the sentence has no real scientific meaning. If you think the arguments dumb, then you're kind of our of the loop with regard to where theoretical physics has gotten us. We're at the point now where it's becoming more and more obvious that the biggest hurtle we have to understanding reality is our own limited psychology. We were designed to breed and reproduce, not understand the fundamental underpinnings of the universe.
Returners
1 / 5 (1) Jul 29, 2015
Charlimopps:

If causality doesn't exist, then where do babies come from?
Jeweller
not rated yet Aug 01, 2015
Isn't that a bit like saying "Somewhere else in the Universe, Alice in Wonderland is a true story"
docile
Aug 02, 2015
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Returners
not rated yet Aug 02, 2015
Isn't that a bit like saying "Somewhere else in the Universe, Alice in Wonderland is a true story"


The mainstream physicists actually believe something like that anyway. They believe, for example, that if you travel far enough in any given direction, you will eventually encounter a region of space that is an exact copy of ours, with an exact copy of Earth and an exact copy of you and everyone else on it.

In String Theory, more or less any conceivable universe is alleged to exist and be just as "real" as our own universe, so in string Theory, it is actually very likely that Alice in Wonderland is to be viewed as a true story, because "somewhere" in the multi-verse there would exist a "real" universe with the same laws as the fictional universe of Alice, and the same characters, etc.

I don't believe that, but String Theorists apparently do.

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