[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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