No area of physics causes more confusion, not just among the general public but also
among physicists, than quantum mechanics.
On the one hand, it's the source of New Age mythology, and has enabled hucksters to peddle
new self-help cures; on the other, for the philosophically inclined, it has provided
some illusory hope of free will in an otherwise deterministic universe.
Of the aspects of quantum mechanics that confuse and dismay observers, perhaps nothing approaches
the property called entanglement.
Einstein, who never really accepted entanglement's existence, called it, derisively, spooky action
at a distance.
Unfortunately for Einstein, entanglement, "spooky" or not, is apparently real, as researchers
in the Netherlands demonstrated last week, just in time for Halloween.
In doing so, the researchers affirmed once again that quantum mechanics, as strange as
it may seem, works in every way we can test it.
To understand just how spooky entanglement really is, it helps to step back and think
about what happens to sensible, "classical" objects when you separate them.
(Classical objects are large enough, or interact strongly enough with their environments, for
quantum-mechanical effects to wash out.)
Imagine that I have a detonator and a bomb.
If I separate them across the street from each other and activate the detonator, it
can trigger the bomb only by sending a signal at the speed of light or slower.
Only after the bomb receives the signal will it detonate.
Quantum theory, however, suggests that objects which have been carefully prepared together
and placed into a combined quantum state can, even when separated across the galaxy, remain"entangled,"
as long as neither has any significant interactions with other objects to break the entanglement.
If I perform a measurement on one of two entangled objects, the state of the other object will
be instantaneously affected, no matter how far apart the two objects are.
In itself, this may not seem that spooky.
After all, if I separate two identical twins across the galaxy and then observe that one
twin has red hair, I have instantaneously determined that the other twin has red hair,
too.
The real spookiness comes in only when you consider what a measurement in quantum mechanics
really involves.
Imagine a pair of electrons.
Electrons behave as though they are spinning; moreover, because they carry an electric charge,
they act like small magnets, which means that, using electromagnetic radiation, its possible
to manipulate their spins.
Our pair of electrons, for example, can be set up so that their spins point in opposite
directions along the same axis.
We can say that such electrons are anti-aligned.
Now, suppose that, later, I try to measure the axis about which one of the electrons
is spinning.
If it's spinning in one direction around that axis, it acts like a magnet with its north
pole pointing in a direction we can call up; if it's spinning in the other direction, then
its north pole would point down.
Because the electrons are anti-aligned, if I discover that one is pointing up, then I
know that its partner must be pointing down.
There's a catch, however.
Quantum mechanics says that the actual spin direction of either electron is not determined
in advance of the measurement; the only thing that's for sure is that the spins are anti-aligned.
Even stranger, until they have been measured, both electrons are actually spinning up and
down at the same time.
Their measured state is probabilistic: all that can be said is that there's a fifty-fifty
probability that, once one of the electrons is measured, it will be "fixed" in a state
of spinning up or down.
Because the two electrons are in a single quantum state because they are entangled the
moment I measure the spin of one electron, I fix the direction of spin of the other electron.
It's as though, by flipping one coin, and coming up "heads," I force another coin to
come up "tails."
As long as the two electrons remain entangled, then this link endures even if they are separated
across the galaxy.
If I measure one electron in my lab, the second electron is affected by the measurement of
the first electron with no time delay instantaneouslyeven though a signal travelling at the speed of
light would take millenia to cross the distance between them.
That instantaneous link is the spooky action at a distance of which Einstein was so skeptical.
In his day, of course, no one had actually observed entanglement; it was just a prediction
of quantum theory.
In 1935 Einstein, along with two collaborators, Boris Podolsky and Nathan Rosen, wrote a famous
paper arguing that entanglement was so crazy that, if quantum mechanics predicted it, the
whole system had to be flawed.
No system that made such predictions could possibly describe the universe accurately.
Einstein and his collaborators were thinking classically, imagining that the electrons
were separate objects when separated.
But, quantum-mechanically, the two electrons are part of a single quantum state, no matter
how far apart they are.
They are not independent, well-separated objects.
Indeed, quantum mechanics tells us that, until we measure the position of either electron,
we cannot say for certain where it is located.
It can be, in some sense, everywhere at once.
Over the years, some physicists have channeled Einstein's skepticism.
They have proposed that there is some hidden classical mechanism we don't know about, which
is, somehow, fixing the results so as to create the appearance of entanglement.
Perhaps, for example, the laboratory setup is somehow predetermining the spin-directions
of the electrons before they are separated.
This conjecture has presented physicists with an impasse: How could we ever know that some
hidden mechanism wasn't at work?
In 1964 a physicist named John Bell found a way around it.
He wrote a beautiful paper demonstrating that, if an entanglement experiment worked reliably,
a specific set of measurements could be made on the particles which produced a quantitative
result that couldn't be explained by any classical mechanism which might predetermine the spins
before they were measured.
Over the past half-century, many groups have used Bell's Theorem in trying to affirm that
quantum-mechanical entanglement is real.
All the while, however, skeptics have pointed out that, in each experiment, there may have
been subtle loopholes.
It's been possible, for example, that systems which were supposed to be separated may have
actually been coupled in hidden ways.
Last week, a team of researchers led by a physicist named Ben Hensen, at Delft University
of Technology, in the Netherlands, reported the results of an experiment designed to lay
these concerns to rest.
They performed simultaneous measurements on a pair of entangled electrons separated by
1.3 kilometers, far enough that no signal, even one travelling at the speed of light,
could get from one detector to the other in time to interfere with their measurements.
They also devised a way of independently checking that the electrons being measured were, in
fact, entangled.
Both these aspects of their experiment are significant and novel: they remove key loopholes
which skeptics had argued might bias previous experiments.
Needless to say, the new results agreed precisely with both the predictions of quantum theory
and of Bell's Theorem.
Entanglement now appears to be an empirically closed case at least, until someone can convincingly
argue that there is some loophole not ruled out by this experiment (which some physicists
have already begun to do).
If that happens, other researchers will inevitably attempt to produce an even better experiment
in which Bell's Theorem will be satisfied while the new loophole is ruled out.
The cycle will repeat until there are no loopholes left or until those that do remain seem so
implausible that it isn't worth the effort to explore them.
I say this with confidence because, even though these direct measurements of quantum spookiness
are important to perform, we can already be sure that quantum mechanics is the correct
description of the world at the smallest, most fundamental scales.
In fact, we test that assumption every day.
Most of our modern technology depends on it.
The semiconductor transistors that govern the behavior of our cell phones, our computers,
our cars, and many other electronic devices are based on quantum-mechanical principles
associated with the electronics that drive their computing capability.
Those principles hinge, indirectly, on the spookiness being measured explicitly in the
Delft experiment.
So, spooky or not, our lives in the modern world rely on quantum mechanics in many ways.
Entanglement is so spooky that it's tempting, when thinking about it, to draw nonsensical
conclusions.
Deepak Chopra, for example, keeps implying that quantum mechanics means that objective
reality doesn't exist apart from conscious experience.
The truth, however, is that consciousness is irrelevant to the act of measurement, which
can be done by machines, or even by single photons.
If consciousness matters, then the inner thoughts of the experimenter who operates the machines
would also have to be reported when we write up the results of our experiments.
Wed need to know whether they were daydreaming about sex, for example.
We don't.
The machines can record data and print it out whether or not a person is in the room,
and those printouts, which behave classically, don't change when the humans come back.
Similarly, last week, the Pulitzer prize-winning writer Marilynne Robinson published an essay
in which she challenges the nature and relevance of modern science.
The essay argued that entanglement raises fundamental questions about time and space,
and therefore about causality.
She went on to say that this called into question the ability of science to explain reality
as a whole.
It's easy to understand how Robinson arrived at this incorrect idea: when a measurement
of one electron here can instantaneously affect the measurement of another electron on the
opposite side of the universe, faster than the speed of light, it does seem as though
causality has been thrown out the window.
But the lessons of quantum mechanics aren't that simple.
The truth is that nature contains a cosmic Catch-22.
Consider our two experimenters one located here on Earth, and the other stationed at
the edge of the Milky Way.
Separately, they measure the spins of their entangled electrons.
Nothing about what they measure will point to a measurement going on thousands of light-years
away.
Each of them will measure their electrons spinning up fifty per cent of the time and
down fifty per cent of the time.
Nothing will suggest that the spins of their electrons are correlated with electrons anywhere
else.
The only way they can realize this is if they communicate with one another.
But that communication can happen, at its very fastest, at the speed of light.
Entanglement may be instantaneous, but it produces no signals that can be detected instantaneously.
Its detection still follows the ordinary laws of cause and effect.
Quantum mechanics reveals that nature is indeed spooky at its smallest scales.
If we are careful, we can detect that spookiness.
But, happily, its presence doesn't mean that all bets are off.
The fundamental laws that govern the universe don't disappear in a quantum haze.
The strangest thing about entanglement may be that it fits so neatly into our broader
understanding of the universe.