MIT scientists find weird quantum effects, even over hundreds of miles

Neutrinos traveling 450 miles have no individual identities, according to MIT analysis.

In the world of quantum, infinitesimally small particles, weird and often logic-defying behaviors abound. Perhaps the strangest of these is the idea of superposition, in which objects can exist simultaneously in two or more seemingly counterintuitive states. For example, according to the laws of quantum mechanics, electrons may spin both clockwise and counter-clockwise, or be both at rest and excited, at the same time.

The physicist Erwin Schrödinger highlighted some strange consequences of the idea of superposition more than 80 years ago, with a thought experiment that posed that a cat trapped in a box with a radioactive source could be in a superposition state, considered both alive and dead, according to the laws of quantum mechanics. Since then, scientists have proven that particles can indeed be in superposition, at quantum, subatomic scales. But whether such weird phenomena can be observed in our larger, everyday world is an open, actively pursued question.

Now, MIT physicists have found that subatomic particles called neutrinos can be in superposition, without individual identities, when traveling hundreds of miles. Their results, to be published later this month in Physical Review Letters, represent the longest distance over which quantum mechanics has been tested to date. 

A subatomic journey across state lines

The team analyzed data on the oscillations of neutrinos — subatomic particles that interact extremely weakly with matter, passing through our bodies by the billions per second without any effect. Neutrinos can oscillate, or change between several distinct “flavors,” as they travel through the universe at close to the speed of light.

The researchers obtained data from Fermilab’s Main Injector Neutrino Oscillation Search, or MINOS, an experiment in which neutrinos are produced from the scattering of other accelerated, high-energy particles in a facility near Chicago and beamed to a detector in Soudan, Minnesota, 735 kilometers (456 miles) away. Although the neutrinos leave Illinois as one flavor, they may oscillate along their journey, arriving in Minnesota as a completely different flavor.

The MIT team studied the distribution of neutrino flavors generated in Illinois, versus those detected in Minnesota, and found that these distributions can be explained most readily by quantum phenomena: As neutrinos sped between the reactor and detector, they were statistically most likely to be in a state of superposition, with no definite flavor or identity.

What’s more, the researchers  found that the data was “in high tension” with more classical descriptions of how matter should behave. In particular, it was statistically unlikely that the data could be explained by any model of the sort that Einstein sought, in which objects would always embody definite properties rather than exist in superpositions.

“What’s fascinating is, many of us tend to think of quantum mechanics applying on small scales,” says David Kaiser, the Germeshausen Professor of the History of Science and professor of physics at MIT. “But it turns out that we can’t escape quantum mechanics, even when we describe processes that happen over large distances. We can’t stop our quantum mechanical description even when these things leave one state and enter another, traveling hundreds of miles. I think that’s breathtaking.”

Kaiser is a co-author on the paper, which includes MIT physics professor Joseph Formaggio, junior Talia Weiss, and former graduate student Mykola Murskyj.

A flipped inequality

The team analyzed the MINOS data by applying a slightly altered version of the Leggett-Garg inequality, a mathematical expression named after physicists Anthony Leggett and Anupam Garg, who derived the expression to test whether a system with two or more distinct states acts in a quantum or classical fashion.

Leggett and Garg realized that the measurements of such a system, and the statistical correlations between those measurements, should be different if the system behaves according to classical versus quantum mechanical laws.

“They realized you get different predictions for correlations of measurements of a single system over time, if you assume superposition versus realism,” Kaiser explains, where “realism” refers to models of the Einstein type, in which particles should always exist in some definite state.

Formaggio had the idea to flip the expression slightly, to apply not to repeated measurements over time but to measurements at a range of neutrino energies. In the MINOS experiment, huge numbers of neutrinos are created at various energies, where Kaiser says they then “careen through the Earth, through solid rock, and a tiny drizzle of them will be detected” 735 kilometers away.

According to Formaggio’s reworking of the Leggett-Garg inequality, the distribution of neutrino flavors — the type of neutrino that finally arrives at the detector — should depend on the energies at which the neutrinos were created. Furthermore, those flavor distributions should look very different if the neutrinos assumed a definite identity throughout their journey, versus if they were in superposition, with no distinct flavor.

“The big world we live in”

Applying their modified version of the Leggett-Garg expression to neutrino oscillations, the group predicted the distribution of neutrino flavors arriving at the detector, both if the neutrinos were behaving classically, according to an Einstein-like theory, and if they were acting in a quantum state, in superposition. When they compared both predicted distributions, they found there was virtually no overlap.

More importantly, when they compared these predictions with the actual distribution of neutrino flavors observed from the MINOS experiment, they found that the data fit squarely within the predicted distribution for a quantum system, meaning that the neutrinos very likely did not have individual identities while traveling over hundreds of miles between detectors.

But what if these particles truly embodied distinct flavors at each moment in time, rather than being some ghostly, neither-here-nor-there phantoms of quantum physics? What if these neutrinos behaved according to Einstein’s realism-based view of the world? After all, there could be statistical flukes due to defects in instrumentation, that might still generate a distribution of neutrinos that the researchers observed. Kaiser says if that were the case and “the world truly obeyed Einstein’s intuitions,” the chances of such a model accounting for the observed data would be “something like one in a billion.”  

So how do neutrinos do it? How do they maintain a quantum, identityless state for seemingly long distances? André de Gouvêa, professor of physics and astronomy at Northwestern University, says because neutrinos move so fast and interact with so little in the world, “relativistic effects — as in Einstein’s special theory of relativity —are huge, and conspire to make the very long distances appear [to the neutrinos] short.”

“The final result is that, like all other tests performed to date under very different circumstances, quantum mechanics appears to be the correct description of the world at all distance scales, weirdness not withstanding,” says Gouvêa, who was not involved in the research.

“What gives people pause is, quantum mechanics is quantitatively precise and yet it comes with all this conceptual baggage,” Kaiser says. “That’s why I like tests like this: Let’s let these things travel further than most people will drive on a family road trip, and watch them zoom through the big world we live in, not just the strange world of quantum mechanics, for hundreds of miles. And even then, we can’t stop using quantum mechanics. We really see quantum effects persist across macroscopic distances.”

MIT

Weighing the lightest particle

Neutrinos are everywhere. Every second, 100 trillion of them pass through your body unnoticed, hardly ever interacting. Though exceedingly abundant, they are the lightest particles of matter, and physicists around the world are attempting the difficult challenge of measuring their mass.   

For a long time, physicists thought neutrinos were massless. This belief was overturned by the discovery that neutrinos oscillate between three flavors: electron, muon and tau. This happens because each flavor contains a mixture of three mass types, neutrino-1, neutrino-2 and neutrino-3, which travel at slightly different speeds.

According to the measurements taken so far, neutrinos must weigh less than 2 electronvolts (a minute fraction of the mass of the tiny electron, which weighs 511,000 electronvolts). A new generation of experiments is attempting to lower this limit—and possibly even identify the actual mass of this elusive particle.

Where did the energy go?

Neutrinos were first proposed by the Austrian-born theoretical physicist Wolfgang Pauli to resolve a problem with beta decay. In the process of beta decay, a neutron in an unstable nucleus transforms into a proton while emitting an electron. Something about this process was especially puzzling to scientists. During the decay, some energy seemed to go missing, breaking the well-established law of energy conservation.

Pauli suggested that the disappearing energy was slipping away in the form of another particle. This particle was later dubbed the neutrino, or “little neutral one,” by the Italian physicist Enrico Fermi.

Scientists are now applying the principle of energy conservation to direct neutrino mass experiments. By very precisely measuring the energy of electrons released during the decay of unstable atoms, physicists can deduce the mass of neutrinos.

“The heavier the neutrino is, the less energy is left over to be carried by the electron,” says Boris Kayser, a theoretical physicist at Fermilab. “So there is a maximum energy that an electron can have when a neutrino is emitted.”

These experiments are considered direct because they rely on fewer assumptions than other neutrino mass investigations. For example, physicists measure mass indirectly by observing neutrinos’ imprints on other visible things such as galaxy clustering.

Detecting the kinks

Of the direct neutrino mass experiments, KATRIN, which is based at the Karlsrule Institute for Technology in Germany, is the closest to beginning its search.

“If everything works as planned, I think we'll have very beautiful results in 2017,” says Guido Drexlin, a physicist at KIT and co-spokesperson for KATRIN.

KATRIN plans to measure the energy of the electrons released from the decay of the radioactive isotope tritium. It will do so by using a giant tank tuned to a precise voltage that allows only electrons above a specific energy to pass through to the detector at the other side. Physicists can use this information to plot the rate of decays at any given energy.

The mass of a neutrino will cause a disturbance in the shape of this graph. Each neutrino mass type should create its own kink. KATRIN, with a peak sensitivity of 0.2 electronvolts (a factor 100 better than previous experiments) will look for a “broad kink” that physicists can use to calculate average neutrino mass.

Another tritium experiment, Project 8, is attempting a completely different method to measure neutrino mass. The experimenters plan to detect the energy of each individual electron ejected from a beta decay by measuring the frequency of its spiraling motion in a magnetic field. Though still in the early stages, it has the potential to go beyond KATRIN’s sensitivity, giving physicists high hopes for its future.

“KATRIN is the furthest along—it will come out with guns blazing,” says Joseph Formaggio, a physicist at MIT and Project 8 co-spokesperson. “But if they see a signal, the first thing people are going to want to know is whether the kink they see is real. And we can come in and do another experiment with a completely different method.”

Cold capture

Others are looking for these telltale kinks using a completely different element, holmium, which decays through a process called electron capture. In these events, an electron in an unstable atom combines with a proton, turning it into a neutron while releasing a neutrino.

Physicists are measuring the very small amount of energy released in this decay by enclosing the holmium source in microscopic detectors that are operated at very low temperatures (typically below minus 459.2 degrees Fahrenheit). Each holmium decay leads to a tiny increase of the detector’s temperature (about 1/1000 degrees Fahrenheit).

“To lower the limit on the electron neutrino mass, you need a good thermometer that can measure these very small changes of temperature with high precision,” says Loredana Gastaldo, a Heidelberg University physicist and spokesperson for the ECHo experiment.  

There are currently three holmium experiments, ECHo and HOLMES in Europe and NuMECs in the US, which are in various stages of testing their detectors and producing isotopes of holmium.

The holmium and tritium experiments will help lower the limit on how heavy neutrinos can be, but it may be that none will be able to definitively determine their mass. It will likely require a combination of both direct and indirect neutrino mass experiments to provide scientists with the answers they seek—or, physicists might even find completely unexpected results.

“Don't bet on neutrinos,” Formaggio says. “They’re kind of unpredictable.”

Symmetry Magazine

Is the neutrino its own antiparticle?

Almost every particle has an antimatter counterpart: a particle with the same mass but opposite charge, among other qualities.

This seems to be true of neutrinos, tiny particles that are constantly streaming through us. Judging by the particles released when a neutrino interacts with other matter, scientists can tell when they’ve caught a neutrino versus an antineutrino.

But certain characteristics of neutrinos and antineutrinos make scientists wonder: Are they one and the same? Are neutrinos their own antiparticles?

This isn’t unheard of. Gluons and even Higgs bosons are thought to be their own antiparticles. But if scientists discover neutrinos are their own antiparticles, it could be a clue as to where they get their tiny masses—and whether they played a part in the existence of our matter-dominated universe.
Dirac versus Majorana

The idea of the antiparticle came about in 1928 when British physicist Paul Dirac developed what became known as the Dirac equation. His work sought to explain what happened when electrons moved at close to the speed of light. But his calculations resulted in a strange requirement: that electrons sometimes have negative energy.

“When Dirac wrote down his equation, that’s when he learned antiparticles exist,” says André de Gouvêa, a theoretical physicist and professor at Northwestern University. “Antiparticles are a consequence of his equation.”

Physicist Carl Anderson discovered the antimatter partner of the electron that Dirac foresaw in 1932. He called it the positron—a particle like an electron but with a positive charge.

Dirac predicted that, in addition to having opposite charges, antimatter partners should have opposite handedness as well.

A particle is considered right-handed if its spin is in the same direction as its motion. It is considered left-handed if its spin is in the opposite direction.

Dirac’s equation allowed for neutrinos and anti-neutrinos to be different particles, and, as a result, four types of neutrino were possible: left- and right-handed neutrinos and left- and right-handed antineutrinos. But if the neutrinos had no mass, as scientists thought at the time, only left-handed neutrinos and right-handed antineutrinos needed to exist.

In 1937, Italian physicist Ettore Majorana debuted another theory: Neutrinos and antineutrinos are actually the same thing. The Majorana equation described neutrinos that, if they happened to have mass after all, could turn into antineutrinos and then back into neutrinos again. 

The matter-antimatter imbalance

 

Whether neutrino masses were zero remained a mystery until 1998, when the Super-Kamiokande and SNO experiments found they do indeed have very small masses—an achievement recognized with the 2015 Nobel Prize for Physics. Since then, experiments have cropped up across Asia, Europe and North America searching for hints that the neutrino is its own antiparticle.

The key to finding this evidence is something called lepton number conservation. Scientists consider it a fundamental law of nature that lepton number is conserved, meaning that the number of leptons and anti-leptons involved in an interaction should remain the same before and after the interaction occurs.

Scientists think that, just after the big bang, the universe should have contained equal amounts of matter and antimatter. The two types of particles should have interacted, gradually canceling one another until nothing but energy was left behind. Somehow, that’s not what happened.

Finding out that lepton number is not conserved would open up a loophole that would allow for the current imbalance between matter and antimatter. And neutrino interactions could be the place to find that loophole.

Neutrinoless double-beta decay

 

Scientists are looking for lepton number violation in a process called double beta decay, says SLAC theorist Alexander Friedland, who specializes in the study of neutrinos.

In its common form, double beta decay is a process in which a nucleus decays into a different nucleus and emits two electrons and two antineutrinos. This balances leptonic matter and antimatter both before and after the decay process, so it conserves lepton number.

If neutrinos are their own antiparticles, it’s possible that the antineutrinos emitted during double beta decay could annihilate one another and disappear, violating lepton number conservation. This is called neutrinoless double beta decay.

Such a process would favor matter over antimatter, creating an imbalance.

“Theoretically it would cause a profound revolution in our understanding of where particles get their mass,” Friedland says. “It would also tell us there has to be some new physics at very, very high energy scales—that there is something new in addition to the Standard Model we know and love.”

It’s possible that neutrinos and antineutrinos are different, and that there are two neutrino and anti-neutrino states, as called for in Dirac’s equation. The two missing states could be so elusive that physicists have yet to spot them.

But spotting evidence of neutrinoless double beta decay would be a sign that Majorana had the right idea instead—neutrinos and antineutrinos are the same.

“These are very difficult experiments,” de Gouvêa says. “They’re similar to dark matter experiments in the sense they have to be done in very quiet environments with very clean detectors and no radioactivity from anything except the nucleus you're trying to study."

Physicists are still evaluating their understanding of the elusive particles.

“There have been so many surprises coming out of neutrino physics,” says Reina Maruyama, a professor at Yale University associated with the CUORE neutrinoless double beta decay experiment. “I think it’s really exciting to think about what we don’t know.”

Fermilab/SLAC

A light in the dark

The MiniCLEAN dark matter experiment prepares for its debut.

Getting to an experimental cavern 6800 feet below the surface in Sudbury, Ontario, requires an unusual commute. The Cage, an elevator that takes people into the SNOLAB facility, descends twice every morning at 6 a.m. and 8 a.m. Before entering the lab, individuals shower and change so they don’t contaminate the experimental areas.

A thick layer of natural rock shields the clean laboratory where air quality, humidity and temperature are highly regulated. These conditions allow scientists to carry out extremely sensitive searches for elusive particles such as dark matter and neutrinos.

The Cage returns to the surface at 3:45 p.m. each day. During the winter months, researchers go underground before the sun rises and emerge as it sets. Steve Linden, a postdoctoral researcher from Boston University, makes the trek every morning to work on MiniCLEAN, which scientists will use to test a novel technique for directly detecting dark matter.

“It’s a long day,” Linden says.

Scientists and engineers have spent the past eight years designing and building the MiniCLEAN detector. Today that task is complete; they have begun commissioning and cooling the detector to fill it with liquid argon to start its search for dark matter.

Though dark matter is much more abundant than the visible matter that makes up planets, stars and everything we can see, no one has ever identified it. Dark matter particles are chargeless, don’t absorb or emit light, and interact very weakly with matter, making them incredibly difficult to detect.

Spotting the WIMPs

 

MiniCLEAN (CLEAN stands for Cryogenic Low-Energy Astrophysics with Nobles) aims to detect weakly interacting massive particles, or WIMPs, the current favorite dark matter candidate. Scientists will search for these rare particles by observing their interactions with atoms in the detector.

To make this possible, the detector will be filled with over 500 kilograms of very cold, dense, ultra-pure materials—argon at first, and later neon. If a WIMP passes through and collides with an atom’s nucleus, it will produce a pulse of light with a unique signature. Scientists can collect and analyze this light to determine whether what they saw was a dark matter particle or some other background event.

The use of both argon and neon will allow MiniCLEAN to double-check any possible signals. Argon is more sensitive than neon, so a true dark matter signal would disappear when liquid argon is replaced with liquid neon. Only an intrinsic background signal from the detector would persist. Scientists would like to eventually scale this experiment up to a larger version called CLEAN.

Overcoming obstacles

 

MiniCLEAN is a small experiment, with about 15 members in the collaboration and the project lead at Pacific Northwest National Laboratory. While working on this experiment underground with few hands to spare, the team has run into some unexpected roadblocks. 

One such obstacle appeared while transporting the inner vessel, a detector component that will contain the liquid argon or neon.

“Last November, as we finished assembling the inner vessel and were getting ready to move it to where it needed to end up, we realized it wouldn’t fit between the doors into the hallway we had to wheel it down,” Linden explains.

When this happened, the team was faced with two options: somehow reduce the size of the vessel, or cut away a part of the door—not a simple thing to do in a clean lab. Fortunately, temporarily replacing some of the vessel’s parts reduced the size enough to make it fit. They got it through the doorway with about an eighth of an inch clearance on each side.

“What gives me the energy to persist on this project is that the CLEAN approach is unique, and there isn’t another approach to dark matter that is like it,” says Pacific Northwest National Laboratory scientist Andrew Hime, MiniCLEAN spokesperson and principal investigator. “It’s been eight years since we starting pushing hard on this program, and finally getting real data from the detector will be a breath of fresh air.”

Fermilab

Nanotechnology World Association

What is a Neutrino?

A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with half-integer spin. The neutrino (meaning "little neutral one" in Italian) is denoted by the Greek letter ν (nu). All evidence suggests that neutrinos have mass but the upper bounds established for their mass are tiny even by the standards of subatomic particles.

Neutrinos do not carry electric charge, which means that they are not affected by the electromagnetic forces that act on charged particles such as electrons and protons. Neutrinos are affected only by the weak sub-atomic force, of much shorter range than electromagnetism, and gravity, which is relatively weak on the subatomic scale. Therefore a typical neutrino passes through normal matter unimpeded.

Neutrinos are created as a result of certain types of radioactive decay, or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms. There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. Each type is associated with an antiparticle, called an "antineutrino", which also has neutral electric charge and half-integer spin. Whether or not the neutrino and its corresponding antineutrino are identical particles has not yet been resolved, even though the antineutrino has an opposite chirality to the neutrino.

Most neutrinos passing through the Earth emanate from the Sun. About 6.5×1010 solar neutrinos per second pass through every square centimeter perpendicular to the direction of the Sun in the region of the Earth.

Physicists at the CERN laboratory have put the final nail in the coffin for the idea that neutrinos can travel faster than the speed of light. They also confirmed that the groundbreaking results from 2011 can be blamed on faulty equipment.

[partner id="wireduk"] Back in September 2011, a team of particle physicists detected neutrinos moving faster than the speed of light as they traveled from CERN to the Gran Sasso lab. They smashed the universal speed limit by 60 nanoseconds — a result that was constant, even after 15,000 repetitions of the process.

The results seem to run counter to a century’s worth of physics and would overturn Einsten’s special theory of relativity if true. As such, CERN called for more experiments to double-check the findings.

“When an experiment finds an apparently unbelievable result, it’s normal procedure to invite broader scrutiny,” CERN research director Sergio Bertolucci said at the time. “We need to be sure that there are no other, more mundane, explanations.”

At the International Conference on Neutrino Physics and Astrophysics in Kyoto on June 8, CERN research director Sergio Bertolucci presented results on the travel time of neutrinos from CERN to the INFN Gran Sasso Laboratory, on behalf of four experiments — Borexino, Icarus, LVD and Opera.

All four experiments measured a neutrino time of flight that was below the speed of light, confirming that neutrinos respect Einstein’s cosmic speed limit. The previous anomaly was “attributed to a faulty element of the experiment’s fibreoptic timing system.”

“Although this result isn’t as exciting as some would have liked,” said Bertolucci, “it is what we all expected deep down.”

“The story captured the public imagination, and has given people the opportunity to see the scientific method in action — an unexpected result was put up for scrutiny, thoroughly investigated and resolved in part thanks to collaboration between normally competing experiments.”

In March 2012, Antonio Ereditato — spokesperson of the Opera experiment — resigned from his post. That was following revelations that the results could have been affected by an unaccounted-for error, and a follow-up test from Icarus that ran counter to Opera’s findings.