Collapsing Physics: Q&A with Catalina Oana Curceanu

In 2015, physicist Catalina Oana Curceanu, of the National Institute of Nuclear Physics, in Frascati, Italy received an FQXi grant of $85,000 to investigate how the uncertain fuzziness of the microscopic quantum realm transitions to the definite macroscopic world we see around us. In particular, she and her colleagues are carrying out experiments to test how measurements force quantum systems to take definite properties—as she explains to Carinne Piekema.

You plan to test a proposed solution to the so-called "quantum measurement problem." What is the measurement problem?

The theory of quantum mechanics is very successful in describing the world and phenomena on a microscopic scale (electrons, atoms, and even molecules), but it starts to be questionable whether the same theory can describe macroscopic bodies, or aggregates of many, many atoms.

The superposition principle tells us that microscopic bodies can be in various possible states at the same time (Schrödinger’s famous cat is both dead and alive). This is described mathematically using a "wavefunction," which comes from solving an equation derived by Erwin Schrödinger. However, when one performs a measurement only one definite answer arrives (the cat is either dead or alive): the wavefunction has collapsed. This is the famous "measurement problem."

The question is then: how does the wavefunction collapse to generate the event we see? Our FQXi project deals with exactly this question.

Different models have been suggested to explain this collapse from the micro to macro worlds. Which collapse model are you using and why?

We aim to measure signals from a theory that modifies the Schrödinger equation, adding terms that induce the collapse of the wavefunction in a very natural way. The specific model we use is the "continuous spontaneous localization" model. This has the nice feature that it allows microscopic systems to remain in superposition for a long time, while it immediately collapses the wavefunction of big bodies. This collapse is thought to be induced by the interaction of the particles with a special collapsing "field."

How can you test whether this hypothesized field exists, in the lab?

What we measure is the spontaneous radiation resulting from the interaction of particles with the collapsing field. Imagine for example a free electron moving in space: if there is no field to induce a collapse, the electron will go in a straight line forever. If, however, there is the interaction with this collapsing field, it will cause the electron to zigzag. Whenever the trajectory changes, the system emits radiation.

This emission is not present in standard quantum mechanics, but is a unique feature of the collapse model and we are trying to measure it.

Where does the collapsing field that causes the radiation come from?

This radiation is not due to some field that is known, but would appear to be a weird phenomenon in which energy does not seem to be conserved. Of course, in reality, energy is conserved, but we would need to know the theory "beyond standard quantum mechanics" to conserve it properly. A hypothesis is that this field could be related to the gravitational field.

Which quantum systems are you testing in the lab?

We use an ultrapure germanium detector and measure the radiation emitted by the detector itself, searching for the spontaneous radiation emitted by the electrons as well as the protons of germanium atoms.

These must be very small signals in an environment that has a lot of radiation from other sources. How are you able to create circumstances clean enough to measure spontaneous radiation?

We try to reduce the influence of cosmic radiation as much as possible, so we do our experiments in the belly of the Gran Sasso d’Italia mountain. The LNGS laboratory—three huge cathedral-like spaces in the mountain connected by galleries—is located half way in the 10km long tunnel that connects the cities of L’Aquila and Teramo. In the mountain, cosmic rays are reduced by a factor of a million with respect to ground experiments.

To reduce the background even further, we use ultra pure germanium instruments. However, there is still some radiation from the materials we use to perform our experiments and from the environment (as radon). So we use Monte Carlo simulations for data analyses to see which part of the X-ray spectrum we measure does not come from the collapse, but from radionuclides present in the setup materials.

Do you have any results yet?

We have done preliminary measurements and are now analysing the data. We are finding out what part of our signal can be ascribed to the residual background. Some very interesting results are coming out which we hope to publish soon.

What happens if you don’t see any radiation? Would that allow you to rule out such collapse models?

Collapse models are characterised by the so-called "lambda-parameter," which describes the number of interactions of particles with the collapsing field per second. There are two limits proposed in theory. One is conservative (proposed by physicist GianCarlo Ghirardi at the University of Trieste, in Italy and others) in which lambda is 10^-17 interactions per second. The other, put forward by physicist Steve Adler at the Institute of Advanced Studies, in Princeton, New Jersey, is 10^-9. We can already exclude the latter and we hope to approach the other limit too. If we could somehow exclude Ghirardi’s limit then either the collapse models as they are have to be discarded, or more likely, they need to be modified. We might see shadows of a new physics.

Do you think we are close to fully understanding the relationship between the quantum and classical worlds?

I believe there is a continued search for deeper understanding, but I don’t believe there is a theory of everything yet. It might also be that we are with quantum mechanics where Newton was with respect to Einstein. With the numerous experiments that are going on today, I hope we might get a hint of a new theory and I hope to contribute by performing nice experiments. It is also a lot of fun!