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New quantum paradox throws the foundations of observed reality into question

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 behavior of tiny objects, such as atoms or particles of light (photons). But that behavior 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.

Read more: Einstein vs quantum mechanics ... and why he'd be a convert today

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.

Read more: Schrödinger's cat gets a reality check

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?

Read more: Quantum physics: our study suggests objective reality doesn't exist

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.

Read more: The universe really is weird: a landmark quantum experiment has finally proved it so

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.

This article was originally published at The Conversation. The publication contributed the article to Live Science's Expert Voices: Op-Ed & Insights.

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  • rod
    Very interesting report here on QM and *observed reality*. I enjoy *observing* using my telescopes. On 05-September, I was out from 2000 - 2300 EDT. I viewed Jupiter's Great Red Spot rotate into view and by 2218 EDT, the Great Red Spot crossed Jupiter's central meridian. The predicted time for Jupiter's Great Red Spot crossing the central meridian at Jupiter was published by Sky & Telescope web report and September magazine, page 50. The waning gibbous Moon and Mars ascended in the east sky in Pisces, about 1-degree angular separation and was very enjoyable to view.

    Q: Based upon this report by space.com, are my telescope observations showing reality in nature or something else?
    Reply
  • Geoman
    What controls an ant colony? Ants colonies are very sophisticated structures. Ants perform multiple duties, and will change their "jobs" as necessary. But ants have only 250,000 neurons. Far too little to actually "think" or "decide". The answer is the virtual super intelligence - ant behavior is governed by very simple rules, and the interaction of those rules creates an ant colony intelligence that controls the colony.

    If ants interacting and controlled by very simple rules, can do it, why not something even smaller? Like individual particles?

    What if the individual particles are part of an emergent super intelligence that is created by the interaction of the particles, and is designed to determine the maximum entropy and information generated in the system. The mistake is to assume that the only observers are the people. Each particle is also an observer, and will decide to change it's properties in accordance with the rules of the universe as determined by the emergent super intelligence.

    Once you grant each particle with the power to "decide" it all becomes quite simple. Each particle displays a very narrow, but almost unfathomably huge, super intelligence. Each particle is calculating the correct state it needs to be in at any time using fundamental laws.
    Reply
  • rod
    The space.com report here ask "If a tree falls in a forest and no one is there to hear it, does it make a sound? Perhaps not, some say."

    My observation. A philosophy of science is reported in the space.com report where *objective reality* can never be known. Applying the QM view of reality in this report to my telescope observations I documented in post #2, looks like QM provides a view of nature where what I observed was not *objective reality* but perhaps, something else. For example, Jupiter's Great Red Spot just appears to follow a predictable path, similar to Mars rising along with the Moon in Pisces. There could be another reality here that I was viewing following the QM view of science in the report it seems thus a geocentric universe could also be reality, as well as the Earth is flat.
    Reply
  • Helio
    rod said:
    The space.com report here ask "If a tree falls in a forest and no one is there to hear it, does it make a sound? Perhaps not, some say."
    But notice the final statements by those scientists...

    "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....

    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."

    Science is objective based. Theories are falsifiable. Many theories that keep passing the tests because they produce the same reliable results over and over. At some point laws are formed, which is what engineers want to have.

    What we call "truths" and things "real" are side issues from science. They are philosophical terms, and not to be taken as serious science terms. Science is a conversation with nature. If everyone with normal hearing hears a tree fall then maybe they physics the describes that fall is worthy of application to our world. I don't think any are saying that some trees are silent when they fall.

    My observation. A philosophy of science is reported in the space.com report where *objective reality* can never be known.
    That's reasonable when we are trying to stretch science to become real, as if an absolute. We take many things to be real, of course, but science isn't allowed, IMO, to go beyond their own magesteria.

    For example, Jupiter's Great Red Spot just appears to follow a predictable path, similar to Mars rising along with the Moon in Pisces.
    Right, but if every telescope from all locations can confirm a match, you now have objective evidence to help decide what it is everyone is seeing.

    There could be another reality here that I was viewing following the QM view of science in the report it seems thus a geocentric universe could also be reality, as well as the Earth is flat.
    A flat earth model is a scientific one, thus it can be falsified, and has been to the point it has been sent to Sillyville. Thus, they too will be regarded as actually being silly (at best).

    A geocentric model, if taken only on a mathematical basis, does work since choosing a different reference frame here or there isn't a violation, at least on a math level. On a physics level, however, the model must present objective evidence that present causation for the effects observed. Since the gravity model does such an incredibly nice job for our favored model then a modified geocentric model is only useful for working on projects on or near the Earth itself.
    Reply
  • rod
    It seems some are hand waving here attempting to avoid the many references to measurement and reality in the space.com report and paper. The space.com does say near the end "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."

    *the absolute reality of measurement outcomes themselves is called into question.* I see no difference made here between the quantum universe and macro universe, perhaps they are all entangled and no objective reality exist - seems the intent and direction here :)
    Reply
  • John R MacDaddy
    The solution to this so-called 'quantum paradox' of entangled photons seems rather simple to me.
    The answer lies in DARK ENERGY. Science hasn't caught up to how 'entangled' our baryonic universe is with Dark Energy. Science has only (relatively) recently realized photons travel in waves, not singly. The medium photons use to travel through is Dark Energy. That is the missing 'connection' between the 2 observed photons. When one photon is 'measured', this destroys the entire wave and therefore all associated photon particles. Imagine a bolos (Spanish throwing weapon). Now imagine the leather attaching the balls is invisible Dark Energy. When one ball hits something, the other is obviously affected. Same with photons.
    I'm fairly convinced that stars and planets absorb dark energy and heat and light is the byproduct. This is why a distant, small moon or planet can have a molten core. It's not gravity generating the heat by itself. Of course, science is hobbled by the limitations of their Scientific Method when trying to measure or observe something that is invisible and untouchable.

    Mod Edit for Content
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  • Helio
    rod said:
    It seems some are hand waving here attempting to avoid the many references to measurement and reality in the space.com report and paper.
    It's not hand waving, it's finger pointing, but not at you. :) I'm trying to show that science doesn't go in the direction of arguing for absolutes or what is "real".

    The space.com does say near the end "What does it all mean?
    Right, and "meaning" is a philosophical question. It may lead to a new scientific model at some point, to be fair, but they aren't stating one now that I can tell, but I could be wrong. Perhaps it's something to look for in the future superior to Bell.

    They aren't saying, apparently, that any specific macro model (e.g. GR) is in question, and they even suggest a hearing problem to properly explain why one may not hear the tree fall.

    *the absolute reality of measurement outcomes themselves is called into question.* I see no difference made here between the quantum universe and macro universe, perhaps they are all entangled and no objective reality exist - seems the intent and direction here :)
    That claim may be fair in philosophy, but, as I'm trying to emphasize, science doesn't deal with absolute realities. Things either work or they don't. Science is fine with formulating new models or tweaking others to make them work for us, if they are falsified.
    Reply
  • ARTGLICK
    The "tree in the forest" analogy is a poor choice when dealing with QM. A sound is scientifically defined as the vibration of air molecules (or some other medium) and that vibration occurs whether or not a human listener is present. How dare we puny humans think that our presence or contribution is required for something to exist?

    On the other hand, there's no way to divorce the nature of the observer from the nature of their perception, even in the macro world. Do you hear "yanny" or "laurel"? What color is the dress, really?
    Reply
  • rod
    FYI. The report here cites the paper and we see in the report, "Quantum mechanics vs. common sense...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."

    Applying to the macro universe we have the trend now in science that indicates a theory can never be falsified unless there is perhaps a preponderance of evidence. 'The Idea That a Scientific Theory Can Be ‘Falsified’ Is a Myth It’s time we abandoned the notion', https://www.scientificamerican.com/article/the-idea-that-a-scientific-theory-can-be-falsified-is-a-myth/?fbclid=IwAR2lSosMZeHq_uqEZ0POqqg2cx6zp2PDb9fnqFqXKHXUC5KZUifShYOvw6Y, 07-Sep-2020

    The space.com report cited 'A strong no-go theorem on the Wigner’s friend paradox'. "Does quantum theory apply at all scales, including that of observers?..We discuss how this new theorem places strictly stronger constraints on physical reality than Bell’s theorem."

    Perhaps like the QM cat is dead and alive at the same time, the earth can be flat and round, the Sun can move around the Earth and the Earth can move around the Sun. I like observing the Galilean moons with my telescopes. However, it seems what I think I see, may not be what *really happened* now :)
    Reply
  • Helio
    rod said:
    FYI. The report here cites the paper and we see in the report, "Quantum mechanics vs. common sense...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."
    Yes, the QM world is bizarre, but the macro world less so. But the mechanics of QM offers very reliable results, else it would not be respected as it is.

    Applying to the macro universe we have the trend now in science that indicates a theory can never be falsified unless there is perhaps a preponderance of evidence. 'The Idea That a Scientific Theory Can Be ‘Falsified’ Is a Myth It’s time we abandoned the notion', https://www.scientificamerican.com/article/the-idea-that-a-scientific-theory-can-be-falsified-is-a-myth/?fbclid=IwAR2lSosMZeHq_uqEZ0POqqg2cx6zp2PDb9fnqFqXKHXUC5KZUifShYOvw6Y, 07-Sep-2020
    Agreed, the veracity of the falsification must be considered. The Jesuits were quick to end a 2000 year model that claimed Venus orbited between the Earth and the Sun as soon as the "preponderance of the evidence" justified the falsification, though it was troubling to do so. There were some who argued that their (built by Galileo) telescopes were faulty, and some of the early ones were problematic. But the quality of a telescope can be tested terrestrially as well. Once all the phases of Venus were clearly observed, they had enough evidence and they agreed the model was debunked.

    The key is to reach a reasonably clear picture of just how strong the falsification claim truly is. It is more difficult to falsify things indirectly, such as how, when and where fossils should form.

    Perhaps like the QM cat is dead and alive at the same time, the earth can be flat and round, ...
    Well, it doesn't hurt to consider that statement. GR may agree with you (slightly) since the Earth can be considered to be traveling in a straightline that just happens to bring us back to where we were last year. :) The important point is to understand how the models work, and how they don't.
    Reply