Does it create a sound if a tree falls in a forest and no one is there to hear it? Maybe not, according to some.
If someone is there to hear it, though? You may need to change your mind if you believe that proves it made a sound.
One of our two most basic scientific theories, along with Einstein's theory of relativity, has a new contradiction that calls into question several conventional notions about the nature of physical reality.
Common sense against quantum mechanics
Look at these three assertions:
When someone witnesses an event taking place, it really did.
It is conceivable to make decisions at will, or at least decisions that are statistically random.
A decision taken in one location cannot immediately impact a distant event. This is known as "locality" in physics.
These are all common, intuitive beliefs shared even among physicists. Our study, which was published in Nature Physics, demonstrates that they cannot all be accurate, or quantum physics must somehow fail.
This is the most significant outcome to yet in a long line of quantum mechanical findings that have challenged our perceptions of reality. Let's examine this history to see why it's so crucial.
The struggle with reality
When describing the behaviour of microscopic things like atoms or light particles, quantum mechanics does a fantastic job (photons). But such behaviour is very strange.
Quantum theory often fails to provide conclusive answers to inquiries like "where is this particle right now?" Instead, it just offers potential locations for the particle to be when it is seen.
That's not because humans lack knowledge, according to Niels Bohr, one of the theory's pioneers who lived a century ago, but rather because physical qualities like "position" don't genuinely exist until they are measured.
Furthermore, certain of a particle's characteristics, such as location and velocity, cannot be accurately observed and real at the same time.
Even Albert Einstein deemed this concept to be unworkable. He suggested that reality must be more complex than what quantum mechanics could capture in a 1935 piece co-authored with fellow theorists Boris Podolsky and Nathan Rosen.
The paper took into account a pair of far-off particles in a unique condition that is now referred to as a "entangled" state. When two entangled particles with the same attribute (such location or velocity) are measured, the outcome will be random, but there will be a correlation between the data from each particle.
Without ever touching the distant particle, an observer may, for instance, accurately anticipate the outcome of measuring the location of the first particle. The observer might also decide to guess the velocity. They contended that, in contrast to Bohr's view, this had a natural explanation if both attributes existed prior to being measured.
John Bell, a physicist from Northern Ireland, discovered in 1964 that Einstein's theory fell apart if you combined more detailed observations of the two particles.
Bell demonstrated that no theory where both position and velocity were pre-existing local attributes could account for the average findings if the two observers arbitrarily and independently choose between measuring one or the other property of their particles, such as position or velocity.
Although it seems impossible, experiments have now unequivocally shown Bell's connections actually exist. This is proof, in the eyes of many physicists, that Bohr was correct when he said that physical attributes don't exist until they are measured.
But it begs the fundamental query: what makes a "measurement" so unique?
The watcher watched
Theoretical physicist Eugene Wigner, who is Hungarian-American, developed a thought experiment in 1961 to demonstrate why the concept of measurement is so difficult to understand.
He imagined a scenario in which his buddy entered a lab that was hermetically enclosed and measured a quantum particle, maybe its location.
Wigner saw that the outcome was quite different if he used the quantum mechanics equations to explain this event from the outside. From Wigner's viewpoint, the friend gets entangled with the particle and infected with the uncertainty that surrounds it rather than the friend's measurement making the particle's location genuine.
This is comparable to Schrödinger's cat, a famous thought experiment in which a cat in a box's destiny is intertwined with a random quantum event.
This was a ridiculous conclusion to Wigner. Instead, he thought that if an observer's awareness becomes engaged, the entanglement will "collapse," solidifying the friend's perspective.
What if Wigner was mistaken, though?
Our investigation
The Wigner's buddy dilemma was initially put out by aslav Brukner of the University of Vienna, and we expanded on it in our study. In this situation, two physicists—call let's them Alice and Bob—are located in two different laboratories, each with their own friends—Charlie and Debbie.
Another wrinkle is that, as in the Bell experiments, Charlie and Debbie are now seeing a pair of entangled particles.
The quantum mechanical equations predict that Charlie and Debbie should entangle with their observable particles, similar to Wigner's claim. Charlie and Debbie should, in principle, become entangled since those particles were already connected to one another.
What does it, however, mean experimentally?
The pals enter their laboratories for our experiment and measure their particles. Later, Alice and Bob toss a coin for themselves. If it's a head, they open the door and inquire about what they observed with their companion. They take a different measurement if it's a tails.
If Charlie is, in fact, entangled with his seen particle in the manner predicted by Wigner, then this alternative measurement always results in a favourable result for Alice. For Bob and Debbie, it's the same.
Any realisation of this measurement, however, prevents any evidence of their friend's lab observation from ever leaving the lab. As though awakening from complete anaesthesia, Charlie or Debbie won't recall having seen anything inside the lab.
But even if they can't recall it, did it truly happen?
If the three intuitive hypotheses presented at the beginning of this essay are accurate, each buddy saw a genuine and distinct result for their measurement inside the lab, regardless of whether Alice or Bob ultimately chose to open their door or not. Additionally, neither what Alice and Charlie observe nor how Bob's far-off coin falls should have any bearing on the other.
We demonstrated that there would be restrictions on the correlations Alice and Bob may anticipate between their findings if this were the case. We also demonstrated how quantum physics foretells that Alice and Bob would see correlations outside of those restrictions.
Using pairs of entangled photons, we then conducted an experiment to verify the quantum mechanical predictions. One of two possible courses that each photon in the setup may take in the setup, based on a characteristic of the photon termed "polarisation," performed the role of each friend's measurement. That is, the polarisation is "measured" by the route.
Since the "friends" are so little and basic, our experiment is actually just a confirmation of the theory. However, it raises the possibility that more sophisticated observers may get the same outcomes.
We may never be able to conduct this test on actual people. However, we contend that if the "friend" is a human-level artificial intelligence operating in a huge quantum computer, it could someday be conceivable to provide a convincing demonstration.
What's the overall meaning?
Even more so than the Bell correlations, if the quantum mechanical predictions continue to hold true, this has significant consequences for our understanding of reality even if a definitive test may not be available for some decades.
One is that it is not sufficient to simply claim that physical attributes do not exist until they are measured to account for the relationships we found.
Currently, the absolute veracity of measuring findings is under doubt.
Our findings require physicists to confront the measurement issue head-on: either quantum mechanics is superseded by a "objective collapse theory" as a consequence of our experiment's inability to scale up, or one of our three common sense hypotheses must be disproved.
According to certain theories, such as de Broglie-Bohm, an action may have an immediate impact on another part of the universe. This, however, directly contradicts Einstein's theory of relativity.
Some people try to find a theory that forbids choice, but the theories they find either need backward causality or a "superdeterministic" kind of fatalism that seems to be a conspiracy theory.
Making Einstein's theory even more relative could be a further means of resolving the disagreement. Different witnesses may dispute as to the precise moment or location of an event, but Einstein believed that the event itself is an unchangeable reality.
Events themselves could, however, only be relative to one or more observers under certain interpretations, such as relational quantum mechanics, QBism, or the many-worlds interpretation. One person's observation of a fallen tree may not be shared by everyone other.
This does not mean that you can create the world you choose. The questions you ask are up to you, but the world will provide the answers. Additionally, when two observers interact in a relational reality, their realities are linked. A shared world might develop in this manner.
In light of this, you could only need a hearing aid if we both see the same tree falling and you claim not to be able to hear it.
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