If a tree falls in a forest and no one is there to hear it, does it make a noise? Maybe not, some say.
And as one is to hear there? If you think that means it of course the make a noise, you may have to repeat that opinion.
We have found a new paradox in quantum mechanics – one of our two most fundamental scientific theories, along with Einstein’s theory of relativity – that casts doubt on some common sense ideas about physical reality.
Quantum mechanics vs common sense
Take a look at these three statements:
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When someone observes an event, it is really took place.
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It is possible to make free choices, or at least, statistically random choices.
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A choice made in one place may not immediately affect a distant event. (Physicists call this “location”.)
These are all intuitive ideas, and also many are believed even by physicists. But our research, published in Nature Physics, shows that not all of them can be true – if quantum mechanics has to break even at some level.
This is the strongest result to date in a long series of discoveries in quantum mechanics that have fueled our ideas about reality. To understand why it is so important, let’s look at this history.
The struggle for reality
Quantum mechanics works extremely well to describe the behavior of small objects, such as atoms or light particles (photons). But that behavior is … very strange.
In many cases, quantum theory does not provide definitive answers to questions such as “where is this part right now?” Instead, it only offers a chance at where the particle can be found if it is observed.
For Niels Bohr, one of the founders of the theory a century ago, this is not because we do not lack information, but because physical properties such as “position” do not actually exist until they are measured.
And what’s more, because some properties of a particle cannot be observed perfectly at the same time – such as position and velocity – they cannot really at the same time.
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 that there must be more to reality than quantum mechanics could describe.
Read more: Einstein vs quantum mechanics … and why he would be a convert today
The article considered a few particles remotely in a particular state now known as a “trapped” state. If the same property (say, position as velocity) is measured on both scattered particles, the result will be random – but there will be a correlation between the results of each particle.
For example, an observer measuring the position of the first part could perfectly predict the result of measuring the position of the distance, without even touching it. Or the observer could choose instead to predict the speed. This had a natural explanation, they argued, if both properties existed before they were measured, contrary to Bohr’s interpretation.
In 1964, however, Northern Irish physicist John Bell found that Einstein’s argument was refuted by performing a more complex combination. different measurements on both particles.
Bell showed that if the two observers randomly and independently choose between measuring one or another property of their particles, such as position and velocity, then the average results cannot be explained in a theory where both position and velocity pre- existing local properties were.
That sounds unlikely, but experiments have now definitively shown that Bell’s correlations occur. For many physicists, this is proof that Bohr was right: physical properties do not exist until they are measured.
But that begs the crucial question: what is so special about a ‘measurement’?
The observer, observed
In 1961, the Hungarian-American theoretical physicist Eugene Wigner conceived a thought experiment to show what is so difficult about the idea of measurement.
He considered a situation in which his friend walks into a tightly sealed lab and takes a measurement on a quantum particle – the position of it, say.
However, Wigner noted that when he applied the equation of quantum mechanics to describe this situation from the outside, the result was very different. Instead of measuring the friend who makes the position of the particle real, the friend from Wigner’s perspective is polluted with the particle and tainted with the uncertainty surrounding it.
This is similar to Schrödinger’s famous cat, a thought experiment in which the fate of a cat in a box is contaminated with a random quantity of events.
Read more about: Schrödinger’s cat gets a reality check
For Wigner, this was an absurd conclusion. Instead, he believed that once an observer’s consciousness was involved, the filming would “collapse” to make the friend’s observation definitive.
But what if Wigner was wrong?
Our experiment
In our research, we built on an extended version of the friend paradox of the Wigner, first proposed by Časlav Brukner of the University of Vienna. In this scenario, there are two physicists – they call Alice and Bob – each with their own friends (Charlie and Debbie) in two remote labs.
There’s another twist: Charlie and Debbie are now measuring some twisted particles, as in the Bell experiments.
As in Wigner’s argument, the equation of quantum mechanics tells us that Charlie and Debbie must be contaminated with their observed particles. But because those particles were already familiar with each other, Charlie and Debbie themselves would have to be entangled – in theory.
But what does that mean experimentally?
Read more: Quantum physics: our study suggests that objective reality does not exist
Our experiment goes like this: the friends go into their labs and measure their particles. A while later, Alice and Bob each spin a coin. When it’s hollow, they open the door and ask their friend what they saw. If it is tail, they perform another measurement.
This different measurement always gives a positive outcome for Alice as Charlie is familiar with his observed part in the way calculated by Wigner. Neither for Bob and Debbie.
However, upon each realization of this measurement, every record of her friend’s observation in the lab is blocked in order to reach the external world. Charlie or Debbie will not remember that they saw anything in the lab when they woke up from total anesthesia.
But did it really happen, even if they did not 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 in the lab, regardless of whether Alice or Bob later decided to open their door or not. Also, what Alice and Charlie see does not have to 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 that Alice and Bob could expect to see between their results. We have also shown that quantum mechanics Alice and Bob predict that will show correlations that go beyond those limits.
Photo by Kok-Wei Bong
Next, we conducted an experiment to confirm the quantum mechanical predictions with pairs of scattered photons. The role of each measurement of each friend was played by one of two paths that each photon can take in the setup, depending on a property of the photon named “polarization”. That is, the path ‘measures’ the polarization.
Our experiment is only a proof of principle, because the “friends” are very small and simple. But it opens up the question of whether the same results would remain with more complex observers.
We can never do this experiment with real people. But we argue that it will one day be possible to make a conclusive demonstration if the “friend” is an artificial intelligence on a human level that runs in a massive quantum computer.
What does it all mean?
Although a final test may be decades away, if the quantum mechanical predictions continue to hold true, this has strong implications for our understanding of reality – even more so than the Bell correlations. For one, the correlations we discover cannot be explained by simply saying that physical properties do not exist until they are measured.
Now the absolute reality of measurement outcomes is even called into question.
Our results force physicists to deal with the head of the measurement problem: either our experiment does not scale, and quantum mechanics gives way to a so-called ‘objective collage theory’, or if one of our three assumptions of common sense has to be rejected.
Read more: The universe is really strange: a landmark quantum experiment has finally proven it that way
There are theories, such as the Broglie-Bohm, that postulate “action at a distance,” in which actions may have direct effects elsewhere in the universe. However, this is in direct conflict with Einstein’s theory of relativity.
Some seek a theory that rejects freedom of choice, but they instead demand causality, as 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, several observers could not agree when of where something happens – but what bart was an absolute fact.
In some interpretations, such as relational quantum mechanics, QBism, or the interpretation of many worlds, events themselves can only take place relative to one or more observers. A fallen tree observed by one may not be a fact for everyone else.
All this does not mean that you can choose your own reality. First you can choose which questions you ask, but the answers are given by the world. And even in a relational world, when two observers communicate, their realities are familiar. In this way, a shared reality can emerge.
Which means that if we both witness the same tree falling and you say you can not hear it, you may just need a hearing aid.
