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mysteriousquantumphysics · 4 years ago

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An unorthodox way of "realist" thinking

The conflict arising from the violation of Bell's inequalities is often reduced to a binary one: Either we have to give up "realism" or we are forced to give up "locality". Though I asked the same binary question in an older entry, this way of stating the problem ignores important subtleties of possible interpretations of quantum mechanics. In the following I will present you an approach of interpreting quantum theory that relies on the notion participatory realism (you can find a more general entry about realism here). We will study the approach called QBism, developed primarily by Chistopher Fuchs, Rüdiger Schack and David Mermin.

What is QBism?

The term Qbism is an abbreviation for Quantum Bayesianism - therefore denoting the bayesian understanding of probability as the fundament of this interpretation. To be more precise: The underlying interpretation of probability is the one of Bruno de Finetti. A view in which probabilities are merely personal degrees of belief and have nothing to do with the representation of physical objects. De Finetti claims radically: PROBABILITY DOES NOT EXIST. In the same manner, QBists state: QUANTUM STATES DO NOT EXIST. [1]

Quantum states are therefore no elements of the observer independent physical reality. Quantum mechanics itself is regarded to be rather an extension of probability theory than a theory about elements of reality. According to the binary categorization above, they give up "realism". Instead they regard quantum mechanics to be a strictly "local" theory, since "no agent can move faster than light". [4]

Solipsism? Instrumentalism? No - It's participatory realism!

Following the simplified binary categorization, QBists seem to give up "realism" (because they deny the quantum state to be an object of the observer inependent physical reality). As a result QBists are often accused to be instrumentalist or even solipsist - since their radically subjective view of quantum mechanics as a "single user theory" might suggest so. But the opposite is the case; QBists regard their interpretation neither as instrumentalist nor as solipsist. Why? Though denying the reality of the quantum state, they do not deny the existence of a world "out there": In Qbist thinking the Born rule is a statement about the world, it is empirical [3]. In addition the subjective character of the theory is not supposed to be a defect, it rather abolishs the strict division between reality and observer - consequently the observer becomes part of reality as well. As Fuchs put it: "That’s not less reality, that’s more." [2]

Randomness and participation in Qbist thinking

In a quantum measurement the "irreducible randomness" or "genuine autonomy"[2] of the world manifests itself. There are no hidden variables or whatsoever that predict with any certainty the measurement outcomes - measurements are rather "moments of creation" [1] in which the agent's participation plays a crucial role. This perspective definitely does not deny realism: "in such a quantum measurement we touch the reality of the world in the most essential of ways" [2].

Then, what are the laws of the world "out there"?

We already encountered the terms "irreducible randomness" and "genuine autonomy" [2] Their meaning can be subsumed in a very strong statement QBists do about the nature of physical reality - they go along with John Wheeler's declaration: "The only law of nature is that there is no law". [2]

The main point I want to convey with this entry is, one needs to be aware that we should not ask oversimplified questions, because doing so dangerously narrows our horizon of possible answers. Bell's theorem does not simply give us the possibility to decide between "realism" and "locality". There are much more possible facettes to explore.

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References

[1] Fuchs: QBism, the Perimeter of Quantum Bayesianism

[2] Fuchs: On participatory Realism

[3] Fuchs, Schack: QBism and the Greeks: why a quantum state does not represent an element of physical reality

[4] Fuchs, Mermin, Schack: An introduction to QBism with an Application to the Locality of Quantum Mechanics

#mysteriousquantumphysics #quantum physics #quantum #physics #math #studyblr #physicsblr #quantum mechanics #scienceblr #science #philosophy #QBism

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ontological-network · 3 years ago

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Several articles related to #qbism

#qbism #science

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superpte · 3 years ago

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QBism Dismissed; Wigner's Friend Elucidated?

QBism Dismissed; Wigner’s Friend Elucidated?

QBism is amusing, much ado about nothing. People making different Quantum experiments see different results. They can call these results “reality”. But that’s abusing language. In truth, what they found were different REALITIES. Nothing surprising there. **** According to the QBism point of view, many, but not all, aspects of the quantum formalism are subjective in nature. For example, QBism says…

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#QBism #Wigner&039;s Freind

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a-circle-is-round · 4 years ago

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tigersharks · 2 years ago

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quantum measurement probability lecture hurt my brain

#qbism is. interesting but confusing.#words from the deep

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qbism · 3 months ago

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#qbismparis #timesless #grandemesure #handcrafted #madeinfrance #newprojects we introduce a new project : QBISM TIMELESS, I dedicate this Overshirt to #Haroldmanning #uniquepiece #madewithlove❤️ #guncheck #cottonlining @Haroldmanning

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sterwood · 7 years ago

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Hey @alyesque, I know you’re super busy right now (you and me both, lmao), but I’m just tagging you in a link to some stuff on a theory of wave-particle duality and other quantum phenomena and stuff that I’ve found interesting before but that I haven’t explored in depth. I see the standford encyclopedia page aligns QBism (the name of the theory) with pragmatism, so there may be something there. But....here’s some stuff:

https://plato.stanford.edu/entries/quantum-bayesian/

https://www.quantamagazine.org/quantum-bayesianism-explained-by-its-founder-20150604

https://www.nature.com/news/physics-qbism-puts-the-scientist-back-into-science-1.14912

#only doing this publicly so others can see if they want to #reference #so I can find this later #I always forget the term 'qbism'

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imatree-worldblog · 4 years ago

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EP03 - Buddhism is not a kind of religion, it’s a core to understand the science as a being.

Study quantum mechanics allows me to link many things happened in our nature together, regardless the tangible or the intangible one. There is no absolute correct answer in this nature, and there is no absolute right or wrong in this statement too. Every single individual, as small as quantum, as big as cosmos, having a superposition of possibility to exist in particular way. All these can be structured as a wave function, with the guidance of Schrödinger equation to make predictions, but still, there is no absolute in there. We cannot strangle the other possibilities because of the outcome we measured, because what it gives is just a guidance to your focal point. This recalls a saying in ‘Being Peace’ by Thich Nhat Hanh, “Buddha’s teaching is only a raft to help you cross the river, a finger pointing to the moon. Don’t mistake the finger for the moon. The raft is not the shore. If we cling to the raft, if we cling to the finger, we miss everything”.

Frankly speaking, I will put Buddhism as a wave function, because it is an epistemology; the result needs not be, or cannot be showed in a physical system; and its existence and ideology are differed to every individual observer, as similar to quantum mechanics - they are correspondingly resonated to each other; we are unable to have a Heisenberg Cut to perceive Buddhism as either microscopic or macroscopic, instead it is a nonduality as advocated in the idea of Buddhism - nil compartmentalization, both microscopic and macroscopic are representing an unity. And this core of idea can spread out to be applied in different contexts. For instance, in complying the nature of quantum mechanics, the theory makes its contribution in several areas, namely quantum computing, purported applications such as quantum leadership, quantum psychology, quantum love, quantum happiness..... These seemingly relate to our real life without the effort of understanding it microscopically, we are them as a whole, and they are us as a whole. As same as quantum mechanics, the objectives of Buddhism can also be differed from countries due to their unique culture, history, and spirit. In America, there is American Buddhism; in Vietnam, there is Vietnamese Buddhism; in Japan, there is Japanese Buddhism..... Different languages, different cultures, but they all share the same ideology to understand the livings and world.

In Buddhism, we tend not to take side between two, we understand each and reconcile them as a whole, because they are in nature a system with entanglement. To reconcile them, the negotiation has to be taken out to make the scattering ideas collapse into a concentrated result - just like how the wave function behaves before and upon the measurement.

Buddha’s teaching does not consist assertion, its existence is not to be the highness among the other principles, doctrines, or ideologies; neither dictates nor rules the world. It is rather a compass for human’s internal awakening and understanding, and so bringing the peace to every beings to have a sustainable life in this world - Buddha, Dharma, and Sangha.

The science, testify the nature of Buddhism, gives many credible explanations for most of the happenings on earth. In the meantime it constructs the world, it is also destructing the world too. In the concept of entanglement, we are in the superposition of the results, we are the result, the result is us; our intention makes the result, and the result is our intention - the result appears in our observing position (Quantum Bayesianism, or QBism). Our world is a system of particles, we are the particles, why can’t we collapse our intention like a position in the wave function and bring our world to the positive world among the Many-Worlds? Buddhas, the awaken one could be the Buddha. Each of us is buddha, but depends on the degree of awakening. Buddha won’t be taking side on science right? Even when science leads to constructive scientific products and destructive scientific products, they reconcile it right?

Think about it. Bye la.

#quantum mechanics #quantum physics #sustainability #environment #buddhism #humanity #wavefunction #superposition #mindfulness #our world #inner peace #harmony #schrodinger #lifestyle #war and peace #greenfuture #dharma #sangha #buddha

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sciencespies · 4 years ago

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Physicists just found a new quantum paradox that casts doubt on a pillar of reality

https://sciencespies.com/physics/physicists-just-found-a-new-quantum-paradox-that-casts-doubt-on-a-pillar-of-reality/

Physicists just found a new quantum paradox that casts doubt on a pillar of reality

If a tree falls in a forest and no one is there to hear it, does it make a sound? Perhaps not, some say.

And if someone is there to hear it? If you think that means it obviously did make a sound, you might need to revise that opinion.

We have found a new paradox in quantum mechanics – one of our two most fundamental scientific theories, together with Einstein’s theory of relativity – that throws doubt on some common-sense ideas about physical reality.

Quantum mechanics vs common sense

Take a look at these three statements:

When someone observes an event happening, it really happened.

It is possible to make free choices, or at least, statistically random choices.

A choice made in one place can’t instantly affect a distant event. (Physicists call this “locality”.)

These are all intuitive ideas, and widely believed even by physicists. But our research, published in Nature Physics, shows they cannot all be true – or quantum mechanics itself must break down at some level.

This is the strongest result yet in a long series of discoveries in quantum mechanics that have upended our ideas about reality. To understand why it’s so important, let’s look at this history.

The battle for reality

Quantum mechanics works extremely well to describe the behaviour of tiny objects, such as atoms or particles of light (photons). But that behaviour is … very odd.

In many cases, quantum theory doesn’t give definite answers to questions such as “where is this particle right now?” Instead, it only provides probabilities for where the particle might be found when it is observed.

For Niels Bohr, one of the founders of the theory a century ago, that’s not because we lack information, but because physical properties like “position” don’t actually exist until they are measured.

And what’s more, because some properties of a particle can’t be perfectly observed simultaneously – such as position and velocity – they can’t be real simultaneously.

No less a figure than Albert Einstein found this idea untenable. In a 1935 article with fellow theorists Boris Podolsky and Nathan Rosen, he argued there must be more to reality than what quantum mechanics could describe.

The article considered a pair of distant particles in a special state now known as an “entangled” state. When the same property (say, position or velocity) is measured on both entangled particles, the result will be random – but there will be a correlation between the results from each particle.

For example, an observer measuring the position of the first particle could perfectly predict the result of measuring the position of the distant one, without even touching it. Or the observer could choose to predict the velocity instead. This had a natural explanation, they argued, if both properties existed before being measured, contrary to Bohr’s interpretation.

However, in 1964 Northern Irish physicist John Bell found Einstein’s argument broke down if you carried out a more complicated combination of different measurements on the two particles.

Bell showed that if the two observers randomly and independently choose between measuring one or another property of their particles, like position or velocity, the average results cannot be explained in any theory where both position and velocity were pre-existing local properties.

That sounds incredible, but experiments have now conclusively demonstrated Bell’s correlations do occur. For many physicists, this is evidence that Bohr was right: physical properties don’t exist until they are measured.

But that raises the crucial question: what is so special about a “measurement”?

The observer, observed

In 1961, the Hungarian-American theoretical physicist Eugene Wigner devised a thought experiment to show what’s so tricky about the idea of measurement.

He considered a situation in which his friend goes into a tightly sealed lab and performs a measurement on a quantum particle – its position, say.

However, Wigner noticed that if he applied the equations of quantum mechanics to describe this situation from the outside, the result was quite different. Instead of the friend’s measurement making the particle’s position real, from Wigner’s perspective the friend becomes entangled with the particle and infected with the uncertainty that surrounds it.

This is similar to Schrödinger’s famous cat, a thought experiment in which the fate of a cat in a box becomes entangled with a random quantum event.

For Wigner, this was an absurd conclusion. Instead, he believed that once the consciousness of an observer becomes involved, the entanglement would “collapse” to make the friend’s observation definite.

But what if Wigner was wrong?

Our experiment

In our research, we built on an extended version of the Wigner’s friend paradox, first proposed by Časlav Brukner of the University of Vienna. In this scenario, there are two physicists – call them Alice and Bob – each with their own friends (Charlie and Debbie) in two distant labs.

There’s another twist: Charlie and Debbie are now measuring a pair of entangled particles, like in the Bell experiments.

As in Wigner’s argument, the equations of quantum mechanics tell us Charlie and Debbie should become entangled with their observed particles. But because those particles were already entangled with each other, Charlie and Debbie themselves should become entangled – in theory.

But what does that imply experimentally?

Our experiment goes like this: the friends enter their labs and measure their particles. Some time later, Alice and Bob each flip a coin. If it’s heads, they open the door and ask their friend what they saw. If it’s tails, they perform a different measurement.

This different measurement always gives a positive outcome for Alice if Charlie is entangled with his observed particle in the way calculated by Wigner. Likewise for Bob and Debbie.

In any realisation of this measurement, however, any record of their friend’s observation inside the lab is blocked from reaching the external world. Charlie or Debbie will not remember having seen anything inside the lab, as if waking up from total anaesthesia.

But did it really happen, even if they don’t remember it?

If the three intuitive ideas at the beginning of this article are correct, each friend saw a real and unique outcome for their measurement inside the lab, independent of whether or not Alice or Bob later decided to open their door. Also, what Alice and Charlie see should not depend on how Bob’s distant coin lands, and vice versa.

We showed that if this were the case, there would be limits to the correlations Alice and Bob could expect to see between their results. We also showed that quantum mechanics predicts Alice and Bob will see correlations that go beyond those limits.

Next, we did an experiment to confirm the quantum mechanical predictions using pairs of entangled photons. The role of each friend’s measurement was played by one of two paths each photon may take in the setup, depending on a property of the photon called “polarisation”. That is, the path “measures” the polarisation.

Our experiment is only really a proof of principle, since the “friends” are very small and simple. But it opens the question whether the same results would hold with more complex observers.

We may never be able to do this experiment with real humans. But we argue that it may one day be possible to create a conclusive demonstration if the “friend” is a human-level artificial intelligence running in a massive quantum computer.

What does it all mean?

Although a conclusive test may be decades away, if the quantum mechanical predictions continue to hold, this has strong implications for our understanding of reality – even more so than the Bell correlations.

For one, the correlations we discovered cannot be explained just by saying that physical properties don’t exist until they are measured.

Now the absolute reality of measurement outcomes themselves is called into question.

Our results force physicists to deal with the measurement problem head on: either our experiment doesn’t scale up, and quantum mechanics gives way to a so-called “objective collapse theory“, or one of our three common-sense assumptions must be rejected.

There are theories, like de Broglie-Bohm, that postulate “action at a distance”, in which actions can have instantaneous effects elsewhere in the universe. However, this is in direct conflict with Einstein’s theory of relativity.

Some search for a theory that rejects freedom of choice, but they either require backwards causality, or a seemingly conspiratorial form of fatalism called “superdeterminism”.

Another way to resolve the conflict could be to make Einstein’s theory even more relative. For Einstein, different observers could disagree about when or where something happens – but what happens was an absolute fact.

However, in some interpretations, such as relational quantum mechanics, QBism, or the many-worlds interpretation, events themselves may occur only relative to one or more observers. A fallen tree observed by one may not be a fact for everyone else.

All of this does not imply that you can choose your own reality. Firstly, you can choose what questions you ask, but the answers are given by the world. And even in a relational world, when two observers communicate, their realities are entangled. In this way a shared reality can emerge.

Which means that if we both witness the same tree falling and you say you can’t hear it, you might just need a hearing aid.

Eric Cavalcanti, Associate Professor (ARC Future Fellow), Griffith University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

#Physics

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