If a tree falls in forest & nobody is there to hear to it, does it make a sound? Perhaps not, some say. And if someone is there to listen to it? If you think that meaning it obviously did make a sound, you would possibly got to revise that opinion. They have found new paradox in quantum-physics – one among our two most fundamental scientific theories, along side Einstein’s theory of relativity – that throws doubt on some common-sense ideas about physical reality.
Quantum Mechanics VS Common Sense
Take a glance at these 3 statements:
1. When someone observes event happening, it really happened.
2. it’s possible to form free choices, or a minimum of, statistically random choices.
3. A choice made in one place can’t instantly affect a foreign event.
These are all intuitive ideas, & widely believed even by physicists. But their research, published in Nature Physics, shows they can’t all be true – or quantum physics itself must break down at some level.
This is the strongest result yet during a long series of discoveries in quantum physics that have upended their ideas about reality. To know why it is so important, let’s check out this history.
The battle for reality
In many cases, scientific theory doesn’t give definite answers to questions like “where is that this particle right now?” Instead, it only provides probabilities for where the particle could be found when it’s observed.
For Bohr , one among the founders of the theory a century ago, that’s not because lack information, but because physical properties like “position“ don’t actually exist until they’re measured. And what’s more, because some properties of a particle cannot be perfectly observed simultaneously – like position & velocity – they can not be real simultaneously.
No less a figure than Einstein found this concept untenable. In 1935 article with fellow theorists Boris Podolsky & Nathan Rosen, he argued there must be more to reality than what quantum physics could describe.
The article considered a pair of distant particles during a special state now referred to as an “entangled“ state. When an same property (say, position or velocity) is measured on both entangled particles, the result are going to be r&om – but there’ll be a correlation between the results from each particle.
For example, an observer measuring the position of the 1st particle could perfectly predict the results of measuring the position of the distant one, without even touching it. Or the observer could prefer to predict the speed instead. This had a natural explanation, they argued, if both properties existed before being measured, contrary to Bohr’s interpretation.
Bell showed that if the 2 observers randomly & independently choose from measuring one or another property of their particles, like position or velocity, the typical results can’t be explained in any theory where both position & velocity were pre-existing local properties.
That sounds incredible, but experiments have now conclusively demonstrated Bell’s correlations do occur. for several physicists, this is often evidence that Bohr was right: physical properties don’t exist until they’re measured.
But that raises the crucial question: what’s so special a few “measurement”?
The observer, observed
He considered a situation, which his friend goes into a tightly sealed lab & performs a measurement on a quantum particle – its position, say.
However, Wigner noticed that if he applied the equations of quantum physics to explain this example from the surface , the result was quite different. rather than the friend’s measurement making the particle’s position real, from Wigner’s perspective the friend becomes entangled with the particle & infected with the uncertainty that surrounds it.
This is almost like Schrödinger’s famous cat, an idea experiment during which the fate of a cat in box becomes entangled with a random quantum event.
For Wigner, this was an absurd conclusion. Instead, he believed that when the consciousness of an observer becomes involved, the entanglement would “collapse“ to made friend’s observation definite.
But what if Wigner was wrong?
In their research, they built on an extended version of the Wigner’s friend paradox, first proposed by Časlav Brukner of the University of Vienna. during this scenario, there are 2 physicists – call them Alice & Bob – each with their own friends (Charlie & Debbie) in 2 distant labs.
There’s another twist: Charlie & Debbie are now measuring a pair of entangled particles, like within the Bell experiments.
As in Wigner’s argument, the equations of quantum physics tell us Charlie & Debbie should become entangled with their observed particles. But because those particles were already entangled with one another , Charlie & Debbie themselves should become entangled – in theory.
But what does that imply experimentally?
Experiment goes like this: the buddies enter their labs & measure their particles. a while later, Alice & Bob each flip a coin. If it’s heads, they open the door & ask their friend what they saw. If it’s tails, they perform a special measurement.
This different measurement always gives a positive outcome for Alice if Charlie is entangled together with his observed particle within the way calculated by Wigner. Likewise for Bob & 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 won’t remember having seen anything inside the lab, as if awakening from total anaesthesia.
But did it really happen, albeit they do not remember it?
If the three intuitive ideas at the start of this text are correct, each friend saw a true & unique outcome for his or her measurement inside the lab, independent of whether or not Alice or Bob later decided to open their door. Also, what Alice & Charlie see shouldn’t depend upon how Bob’s distant coin lands, & the other way around .
They showed that if this were the case, there would be limits to the correlations Alice & Bob could expect to ascertain between their results. Also showed that quantum physics predicts Alice & Bob will see correlations that transcend those limits.
Next, they did an experiment to verify the quantum mechanical predictions using pairs of entangled photons. The role of every friend’s measurement was played by one among two paths each photon may take in setup, counting on a property of the photon called “polarisation“. That is, path “measures“ the polarisation.
They may never be ready to do that experiment with real humans. But argue that it’s going to at some point be possible to make a conclusive demonstration if the “friend” may be a human-level AI running during a massive quantum computer.
What does it all mean?
Although a conclusive test could also be decades away, if the quantum mechanical predictions still hold, this has strong implications for understanding of reality – even more so than the Bell correlations.
For one, the correlations they discovered can’t be explained just by saying that physical properties don’t exist until they’re measured.
Now absolutely the reality of measurement outcomes themselves is named into question.
Results force physicists to affect the measurement problem head on: either experiment doesn’t proportion , & quantum physics gives thanks to a so-called “objective collapse theory“, or one among 3 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 within the universe. However, this is often in direct conflict with Einstein’s theory of relativity.
Some look for a theory that rejects freedom of choice, but they either require backwards causality, or a seemingly conspiratorial sort of fatalism called “superdeterminism“.
Another way to resolve the conflict might be to make Einstein theory even more relative. For Einstein, different observers could disagree about when or where something happens – but what happens was absolute fact.
However, in some interpretations, like relational quantum physics , QBism, or the many-worlds interpretation, events themselves may occur only relative to at least one or more observers. A fallen tree observed by one might not be a fact for everybody else.
All of this doesn’t imply that you simply can choose your own reality. Firstly, you’ll choose what questions you ask, but the answers are given by the planet. And even during a relational world, when 2 observers communicate, their realities are entangled. during this way a shared reality can emerge.
Which means that if both witness same tree falling & you say you cannot hear it, you would possibly just need a hearing aid.