[Marxism] Quantum Experiment

Louis Proyect lnp3 at panix.com
Sat Nov 16 08:53:03 MST 2019


On 11/16/19 10:19 AM, Marla Vijaya kumar via Marxism wrote:
> 
> Louis,            The subject is extremely interesting and I am working on the philosophical aspect of it.
> But unfortunately, it requires subscription.
> Can you post the text?
> Vijaya Kumar M

Vijaya, this was from Alternet, which does not have paywalls. In any case:

Quantum physics: our study suggests objective reality doesn’t exist
November 14, 2019 7.40am EST

Authors
Alessandro Fedrizzi
Professor of Quantum Physics, Heriot-Watt University

Massimiliano Proietti
PhD Candidate of Quantum Physics, Heriot-Watt University

Alternative facts are spreading like a virus across society. Now it 
seems they have even infected science – at least the quantum realm. This 
may seem counter intuitive. The scientific method is after all founded 
on the reliable notions of observation, measurement and repeatability. A 
fact, as established by a measurement, should be objective, such that 
all observers can agree with it.

But in a paper recently published in Science Advances, we show that, in 
the micro-world of atoms and particles that is governed by the strange 
rules of quantum mechanics, two different observers are entitled to 
their own facts. In other words, according to our best theory of the 
building blocks of nature itself, facts can actually be subjective.

Observers are powerful players in the quantum world. According to the 
theory, particles can be in several places or states at once – this is 
called a superposition. But oddly, this is only the case when they 
aren’t observed. The second you observe a quantum system, it picks a 
specific location or state – breaking the superposition. The fact that 
nature behaves this way has been proven multiple times in the lab – for 
example, in the famous double slit experiment (see video below).


In 1961, physicist Eugene Wigner proposed a provocative thought 
experiment. He questioned what would happen when applying quantum 
mechanics to an observer that is themselves being observed. Imagine that 
a friend of Wigner tosses a quantum coin – which is in a superposition 
of both heads and tails – inside a closed laboratory. Every time the 
friend tosses the coin, they observe a definite outcome. We can say that 
Wigner’s friend establishes a fact: the result of the coin toss is 
definitely head or tail.

Wigner doesn’t have access to this fact from the outside, and according 
to quantum mechanics, must describe the friend and the coin to be in a 
superposition of all possible outcomes of the experiment. That’s because 
they are “entangled” – spookily connected so that if you manipulate one 
you also manipulate the other. Wigner can now in principle verify this 
superposition using a so-called “interference experiment” – a type of 
quantum measurement that allows you to unravel the superposition of an 
entire system, confirming that two objects are entangled.

When Wigner and the friend compare notes later on, the friend will 
insist they saw definite outcomes for each coin toss. Wigner, however, 
will disagree whenever he observed friend and coin in a superposition.

This presents a conundrum. The reality perceived by the friend cannot be 
reconciled with the reality on the outside. Wigner originally didn’t 
consider this much of a paradox, he argued it would be absurd to 
describe a conscious observer as a quantum object. However, he later 
departed from this view, and according to formal textbooks on quantum 
mechanics, the description is perfectly valid.

The experiment
The scenario has long remained an interesting thought experiment. But 
does it reflect reality? Scientifically, there has been little progress 
on this until very recently, when Časlav Brukner at the University of 
Vienna showed that, under certain assumptions, Wigner’s idea can be used 
to formally prove that measurements in quantum mechanics are subjective 
to observers.

Brukner proposed a way of testing this notion by translating the 
Wigner’s friend scenario into a framework first established by the 
physicist John Bell in 1964. Brukner considered two pairs of Wigners and 
friends, in two separate boxes, conducting measurements on a shared 
state – inside and outside their respective box. The results can be 
summed up to ultimately be used to evaluate a so called “Bell 
inequality”. If this inequality is violated, observers could have 
alternative facts.

We have now for the first time performed this test experimentally at 
Heriot-Watt University in Edinburgh on a small-scale quantum computer 
made up of three pairs of entangled photons. The first photon pair 
represents the coins, and the other two are used to perform the coin 
toss – measuring the polarisation of the photons – inside their 
respective box. Outside the two boxes, two photons remain on each side 
that can also be measured.


Researchers with experiment. Author provided
Despite using state-of-the-art quantum technology, it took weeks to 
collect sufficient data from just six photons to generate enough 
statistics. But eventually, we succeeded in showing that quantum 
mechanics might indeed be incompatible with the assumption of objective 
facts – we violated the inequality.

The theory, however, is based on a few assumptions. These include that 
the measurement outcomes are not influenced by signals travelling above 
light speed and that observers are free to choose what measurements to 
make. That may or may not be the case.

Another important question is whether single photons can be considered 
to be observers. In Brukner’s theory proposal, observers do not need to 
be conscious, they must merely be able to establish facts in the form of 
a measurement outcome. An inanimate detector would therefore be a valid 
observer. And textbook quantum mechanics gives us no reason to believe 
that a detector, which can be made as small as a few atoms, should not 
be described as a quantum object just like a photon. It may also be 
possible that standard quantum mechanics does not apply at large length 
scales, but testing that is a separate problem.


There may be many worlds out there. Nikk/Flickr, CC BY-SA
This experiment therefore shows that, at least for local models of 
quantum mechanics, we need to rethink our notion of objectivity. The 
facts we experience in our macroscopic world appear to remain safe, but 
a major question arises over how existing interpretations of quantum 
mechanics can accommodate subjective facts.

Some physicists see these new developments as bolstering interpretations 
that allow more than one outcome to occur for an observation, for 
example the existence of parallel universes in which each outcome 
happens. Others see it as compelling evidence for intrinsically 
observer-dependent theories such as Quantum Bayesianism, in which an 
agent’s actions and experiences are central concerns of the theory. But 
yet others take this as a strong pointer that perhaps quantum mechanics 
will break down above certain complexity scales.

Clearly these are all deeply philosophical questions about the 
fundamental nature of reality. Whatever the answer, an interesting 
future awaits.



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