Scientists have yet another proof that Einstein was wrong
Measurement of one of photon properties affects the measurement result of another, proved scientists from Vienna (among them two Poles: Radek Łapkiewicz and Marcin Wieśniak). It is another proof of the accuracy of the theory, which Albert Einstein refused to accept.
Einstein refused to accept the proposals of quantum mechanics, which
describes the laws governing the smallest particles of matter. The main
problem brilliant physicist had with this theory would not surprise
anyone. Every layman, whom scholars try to explain quantum mechanics,
struggles with it: it is simply difficult to understand how it is
possible that the property of quantum system comes into existence only
when it is measured.
In quantum mechanics, until the time of
measurement the system may simultaneously exist in many states, in a
so-superposition. "This is a situation similar to that in which one man
would be able to go both right and left at the same time" - Radek
Łapkiewicz explained.
One could say that the measurement not
so much discloses the value of the particle property (e.g., its momentum
or position), but actually creates it. Observer decides which of many
possible properties he will "bring to life."
Einstein did not
want to accept that. He believed in the existence of objective reality,
independent of whether anyone observes it or not. Memories of frequent
discussion on the subject with a brilliant colleague have been written
by physicist, who worked on quantum mechanics, Abraham Pais. "I recall
that during one walk Einstein suddenly stopped, turned to me and asked
whether I really believed that the moon exists only when I look at it." -
Pais wrote.
However, the observations of physicists confirmed that measurements have
real impact on the behaviour of particles, and it is not only the
result of invasive measurement method changing the object properties.
Crown evidence are increasingly accurate observations of pairs of
entangled particles. Measurements performed on one of them immediately
affect the other, regardless of the distance between particles (this
phenomenon was observed even at distances exceeding 100 km).
Measurements made on each of the particles give completely random
results, but when we compare the results of both particles, it turns out
that they are identical.
Scientists have long attempted to
explain these phenomena without using quantum mechanics, but by creating
alternative theories, so-called hidden variables theories. In these
theories, systems have well defined properties that do not depend on
observation.
Łapkiewicz emphasised that these theories are not
widely popular among scholars, who generally recognize quantum mechanics
as a good description of the world. Until now, however, there was no
experiment (other than the one involving entangled particles), which
would exclude them. Now a group of young scientists from the University
of Vienna, supervised by Prof. Anton Zeilinger (who earlier conducted
groundbreaking experiments with entangled photons), designed and
conducted such experiment.
"According to the theory, which our
experiment refutes, measurement is a verification of pre-existing
properties (independent of our measurement). This can not be reconciled
with quantum mechanics, if we examine a system with at least three
distinguishable states (the so-called qutrit)" - Łapkiewicz explained.
This experiment could be called a test of strength between quantum
mechanics and alternative concepts. The result is not surprising, but it
is crucial for physics.
"Until now this result was theoretical.
We worked on the assumption that physics is an experimental science, so
we should consider an experiment that would tangibly exclude the
existence of hidden variables. We found the easiest method to conduct
this experiment" - said another of the experiment authors Dr. Marcin
Wieśniak, now an employee of the University of Gdansk.
Wieśniak,
who was responsible for theoretical part of the research, explained that
the experiment used inequality proposed by a group of Turkish
mathematicians. Alexander A. Klyachko, M. Ali Can, Sinem Binicioglu and
Alexander S. Shumovsky published a paper on this subject in early 2009.
"When
Radek Łapkiewicz found this article, it was quite fresh. I think it was
two or three months after publication" - Wieśniak recalled.
Inequality
used by experimenters is similar to that found by John Stewart Bell in
the 1960s. This inequality is satisfied, if the results of measurements
of one property are independent of what other properties of the particle
are being measured at the moment. Reversing this, some theories of
hidden variables can not be true (that is, inequality is not satisfied)
if the level of correlation between the results of measurements of two
different properties of the same particles exceeds the limit value.
Satisfied
inequality would mean that theories alternative to quantum mechanics
may be right - its violation, that quantum mechanics is correct.
Bell
formulated the first of these inequalities, relating to entangled
particles, and presented a theoretical proposal for an experiment that
could verify the theories of hidden variables. Later experiment ended in
favour of quantum mechanics, while showing the phenomenon of distant
objects entanglement.
Now, in turn, it has been demonstrated that
even without entanglement measurement results can not be described
using the hidden variables independent of measurements - the results
obtained by experimenters violated the inequality. It came as no
surprise to anyone.
Wieśniak reserved, however, that experiment
would not convince all supporters of theories alternative to quantum
mechanics. "It is very strong and very simple in its nature proof
against the so-called hidden variables. But I can not say that we have
definitively eliminated alternative descriptions. It is a battle with,
in some sense, conspiracy theories" - he said.
However, physicists do not feel that they have performed their experiment just for fun.
"Our
experiment is a bit like Bell’s experiment. If someone believes in
quantum mechanics, this result should not be a surprise. However, the
observation of Bell inequality violations proved to people that
surprising quantum effects are available to us and we may want to
consider using them. A concept was born of using strange quantum
mechanics properties in information processing and tranfer. Protocols
for cryptography, teleportation, and quantum computing algorithms have
been developed and demonstrated. It may be the same here. There was a
result on paper, and we showed that there was a way to see it in the
lab" - emphasised Łapkiewicz.
The experiment was prepared and
conducted by a team composed of: Radek Łapkiewicz, Peizhe Li, Christoph
Schaeff, Nathan K. Langford, Sven Ramelow, Marcin Wieśniak, under
supervision of Anton Zeilinger. Paper describing the course of the
experiment and its results has been recently published in the journal
Nature. (PAP)
last modification: 2011-07-15
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