Sunday, December 22, 2019

A quantum experiment suggests there’s no such thing as objective reality (English)

UPDATE 27-12-2019:

In 1961, the Nobel Prize-winning physicist Eugene Wigner conceptualized a thought experiment revealing a little-known paradox of quantum mechanics. Wigner’s thought experiment demonstrated a strange quirk of the universe. It allows for two observers to experience two different realities from the same event.
This paradox potentially creates a rift in the foundation of science itself, calling into question the nature of measurement. Can objective facts even exist? Scientists carry out experiments to establish objective facts, but if they experience different realities how can they agree on what these facts might be?
Wigner’s thought experiment was put to the test by a team of physicists at Heriot-Watt University in Edinburgh with the results being published in February 2019. The experiment tested for the validity of observer-independence at the quantum level, similarly using photon polarization...The results of that experiment lends considerable strength to Wigner’s thought experiment and interpretations of quantum theory that are observer-dependent.

Physicists have long suspected that quantum mechanics allows two observers to experience different, conflicting realities. Now they’ve performed the first experiment that proves it.

...In a new study, called ‘Experimental test of local observer independence’, researchers from Heriot-Watt University in Edinburgh demonstrate via a unique experiment that two different observers are entitled to their own facts...

In 2018, Časlav Brukner at the University of Vienna demonstrated that, under certain assumptions, Wigner’s idea can be used to formally prove that measurements in quantum mechanics are subjective to observers. Scientist performed another thought experiment, but this time there wasn’t just Bob and Sally, there was one more pair of friends watching the same quantum experiment. To get it right, we suggest to watch the video that explains the paradox...

Still, these experiments remained purely hypothetical, until now...

Back in 1961, the Nobel Prize–winning physicist Eugene Wigner outlined a thought experiment that demonstrated one of the lesser-known paradoxes of quantum mechanics. The experiment shows how the strange nature of the universe allows two observers—say, Wigner and Wigner’s friend—to experience different realities.

Ниже есть продолжение.

Since then, physicists have used the “Wigner’s Friend” thought experiment to explore the nature of measurement and to argue over whether objective facts can exist. That’s important because scientists carry out experiments to establish objective facts. But if they experience different realities, the argument goes, how can they agree on what these facts might be?

That’s provided some entertaining fodder for after-dinner conversation, but Wigner’s thought experiment has never been more than that—just a thought experiment.

Last year, however, physicists noticed that recent advances in quantum technologies have made it possible to reproduce the Wigner’s Friend test in a real experiment. In other words, it ought to be possible to create different realities and compare them in the lab to find out whether they can be reconciled.

And today, Massimiliano Proietti at Heriot-Watt University in Edinburgh and a few colleagues say they have performed this experiment for the first time: they have created different realities and compared them. Their conclusion is that Wigner was correct—these realities can be made irreconcilable so that it is impossible to agree on objective facts about an experiment.

Wigner’s original thought experiment is straightforward in principle. It begins with a single polarized photon that, when measured, can have either a horizontal polarization or a vertical polarization. But before the measurement, according to the laws of quantum mechanics, the photon exists in both polarization states at the same time—a so-called superposition.

Wigner imagined a friend in a different lab measuring the state of this photon and storing the result, while Wigner observed from afar. Wigner has no information about his friend’s measurement and so is forced to assume that the photon and the measurement of it are in a superposition of all possible outcomes of the experiment.

Wigner can even perform an experiment to determine whether this superposition exists or not. This is a kind of interference experiment showing that the photon and the measurement are indeed in a superposition.

From Wigner’s point of view, this is a “fact”—the superposition exists. And this fact suggests that a measurement cannot have taken place.

But this is in stark contrast to the point of view of the friend, who has indeed measured the photon’s polarization and recorded it. The friend can even call Wigner and say the measurement has been done (provided the outcome is not revealed).

So the two realities are at odds with each other. “This calls into question the objective status of the facts established by the two observers,” say Proietti and co.

That’s the theory, but last year Caslav Brukner, at the University of Vienna in Austria, came up with a way to re-create the Wigner’s Friend experiment in the lab by means of techniques involving the entanglement of many particles at the same time.

The breakthrough that Proietti and co have made is to carry this out. “In a state-of-the-art 6-photon experiment, we realize this extended Wigner’s friend scenario,” they say.

They use these six entangled photons to create two alternate realities—one representing Wigner and one representing Wigner’s friend. Wigner’s friend measures the polarization of a photon and stores the result. Wigner then performs an interference measurement to determine if the measurement and the photon are in a superposition.

The experiment produces an unambiguous result. It turns out that both realities can coexist even though they produce irreconcilable outcomes, just as Wigner predicted.

That raises some fascinating questions that are forcing physicists to reconsider the nature of reality.

The idea that observers can ultimately reconcile their measurements of some kind of fundamental reality is based on several assumptions. The first is that universal facts actually exist and that observers can agree on them.

But there are other assumptions too. One is that observers have the freedom to make whatever observations they want. And another is that the choices one observer makes do not influence the choices other observers make—an assumption that physicists call locality.

If there is an objective reality that everyone can agree on, then these assumptions all hold.

But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.

Of course, there is another way out for those hanging on to the conventional view of reality. This is that there is some other loophole that the experimenters have overlooked. Indeed, physicists have tried to close loopholes in similar experiments for years, although they concede that it may never be possible to close them all.

Nevertheless, the work has important implications for the work of scientists. “The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them,” say Proietti and co. And yet in the same paper, they undermine this idea, perhaps fatally.

The next step is to go further: to construct experiments creating increasingly bizarre alternate realities that cannot be reconciled. Where this will take us is anybody’s guess. But Wigner, and his friend, would surely not be surprised.

Ref: : Experimental Rejection of Observer-Independence in the Quantum World


What is objective reality? This is quite a philosophical question, but you don’t doubt yourself when it comes to distinguishing day and night, apples and oranges, or left and right. So how surprised would you be, if you and your friend looked outside the window, and they saw day, while you definitely saw night? (To be clear, this imaginary situation isn’t possible beyond the polar circle). Doesn’t make any sense, right? It’s dark, you see the moon and the stars, you’re definitely observing night, so based on your measurements (observations), you say that it’s night outside, and your friend is stupid, and you want to make some new friends.

Well, quantum mechanics won’t help you find new friends, but it can help you make peace with the fact that there is no objective reality. Buckle up, it’s quantum physics time!

There’s a fundamental principle in this mysterious science, it’s called quantum superposition. It states that, much like waves, any two quantum states can be added together and the result will be another valid quantum state. Remember Schrödinger's cat? The scenario presents a hypothetical cat that is simultaneously both alive and dead, so a cat is in a state of quantum superposition. However, as soon as any observer checks on the poor animal, the magical superposition state vanishes, and an observer either sees a dead or an alive cat.

The thing with a cat was always a thought experiment (There’s no evidence of Erwin Schrodinger murdering his pets in the name of science), however, there were numerous actual experiments that demonstrated the principle of superposition.

The most famous one is a double-slit experiment. It was first performed on light by Thomas Young in early 19th century. To prove its wavelike nature, scientist fired pure-wavelength light through a sheet with two slits in it, as the light passed through the slits it split in two distinct waves, after passing thought the slits these waves interfered and created an interference pattern, or a series of light and dark fringes on the screen behind the sheet. Without diffraction and interference, the light would simply make two lines on the screen.

About a century later, scientists did quite a similar experiment, but this time the experiment featured electron beam gun that fired electrons through the double-slit apparatus. If particles were sent one at a time, it resulted as a single particle appearing on the screen, as expected. Remarkably, however, an interference pattern emerged when these particles were allowed to build up one by one. The experiment, which was later conducted on whole atoms and even molecules, showed that a particle can behave as a wave, the phenomena was called ‘wave-particle duality’.

There’s one problem, though. We can’t see such behavior, we can only see the result of many particles behaving this way. As soon as we peek at the particles, they stop behaving as waves and pretend to be good old particles, so elegantly described by classical physicists...

But what if an observer of a quantum experiment is being observed? This thought experiment was proposed by the physicist Eugene Wigner in 1961. The scenario involves an indirect observation of a quantum measurement. An observer, let’s call him Bob, observes another observer, let’s call her Sally, who is performing a quantum measurement on a physical system, to make it easier let’s do a cat experiment again. The cat is in superposition, it’s either dead or alive. As soon as Sally peeks at it, the state of superposition comes to an end and the cat is being either dead or alive. Bob, however, stays out of the room, Sally doesn’t let him in to check on the cat. So for Bob the whole room with Sally and the experiment is in the state of superposition, moreover, Sally and the cat are entangled, connected.

Bob can verify this superposition using a so-called ‘interference experiment’ — a type of quantum measurement that allows him to unravel the superposition of an entire system, confirming that two objects are entangled.

When Bob and Sally compare notes later on, Sally will insist she saw a definite outcome. And here when it’s all go ‘pretty quantum’. In most of the interpretations of quantum theory, the resulting statements of the two observers contradict each other.

In 2018, Časlav Brukner at the University of Vienna demonstrated that, under certain assumptions, Wigner’s idea can be used to formally prove that measurements in quantum mechanics are subjective to observers. Scientist performed another thought experiment, but this time there wasn’t just Bob and Sally, there was one more pair of friends watching the same quantum experiment. To get it right, we suggest to watch the video that explains the paradox.

Still, these experiments remained purely hypothetical, until now. Researchers at Heriot-Watt University in Edinburgh have actually performed the test experimentally on a small-scale quantum computer made up of three pairs of entangled photons. The first photon pair represented the quantum experiment, the other two played the outcomes of the experiment — measuring the polarization of the photons — inside their respective box. Outside the two boxes, two more photons remained on each side.

What, do you think, was a result of all this state-of-the-art extremely complicated nonsense? Well, to quote the researchers who managed to bring the thought experiment to life: ‘this result implies that quantum theory should be interpreted in an observer-dependent way’. The experiment therefore demonstrates that, at least for local models of quantum mechanics, we need to rethink our notion of objectivity.

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