Quanta Magazine

How does objective reality come about? The deepest and most vexed issue posed by the theory is still being argued over a century later. There are possible explanations for how observations of the world yield classical results, drawing on different interpretations of quantum mechanics.

We may be able to get rid of at least one proposal. The idea that the collapse of quantum possibilities into a single classical reality is not just a mathematical convenience but a real physical process has been tested by recent experiments. There is no evidence of the effects predicted by the simplest models.

It is not yet certain if physical collapse does not occur. The models could still be modified to escape the constraints placed on them by the null results, according to some researchers. Sandro Donadi, a theoretical physicist at the National Institute for Nuclear Physics in Trieste, Italy, who led one of the experiments, doubts that the community will keep modifying the models. The biggest mystery of quantum theory appears to be getting closer to being solved.

What Causes Collapse?

There is a central dilemma of quantum theory. A quantum object is described by a mathematical entity called a wave function and all that can be said about it is encapsulated by it. A wave function does not describe a physical wave. Predicting the various possible outcomes of measurements made on the object and the chance of observing any one of them in an experiment is what it is called a "probability wave."

The wave function predicts the statistical distribution of outcomes if many measurements are made on the same object. There is no way to know what the outcome of a measurement will be. What makes a particular observation? According to John von Neumann, when a measurement is made, the wave function collapses into one of the possibilities. The process is random but has a bias. The collapse has to be manually added to the calculations due to the fact that quantum mechanics doesn't seem to predict it.

It works well enough. It seemed like a sleight of hand to some researchers. Einstein said it was like God playing dice to decide what becomes real. Physicists just had to accept a fundamental difference between the quantum and classical regimes, according to the interpretation by the Danes. Physicists now call wave function collapse an illusion and say that all outcomes are realized in a number of branching universes.

The fundamental cause of the wave function collapse is not known, according to Inwook Kim, a physicist at the Lawrence Livermore National Laboratory. How does it happen?

Physicists from Italy suggested an answer in 1986. They wanted to know if the wave equation was not the whole story. They theorize that a quantum system can jump into one of the system's observable states on a timescale that depends on the system's size. An atom in a quantum superposition will stay that way for a long time. Bigger objects collapse into a classical state almost instantly when they interact with a measurement device. The first physical-collapse model that involved gradual, continuous collapse was called the GRW model. Magdalena Zych is a physicist at the University ofQueensland in Australia.

There is a wave function collapse that causes this. Adding a mathematical term to the Schrdinger equation is suggested by the GRW and CSL models. Roger Penrose of the University of Oxford and Lajos Disi of Etvs Lornd University proposed a possible cause of the collapse in the 1980's and 90's. Their idea was that if a quantum object is in a superposition of locations, each state will feel it. The attraction causes the object to collapse. The fabric of space-time can be changed in two different ways at the same time if a superposition of localities is used. In a conflict between quantum mechanics and general relativity, quantum will crack first.

The Test of Truth

The ideas have always been very speculative. Physical-collapse models have the virtue of making observable predictions and thus being testable and falsifiable.

If there is a background perturbation that causes quantum collapse, all particles will interact with it, even if they are in a superposition. There should be consequences that are visible. Catalina Curceanu said that the interaction should create a zigzagging of particles in space.

The models suggest that this diffusive motion is small. The motion of the particle will cause a process called bremsstrahlung. A lump of matter should emit a very faint stream of electrons, which are predicted to be in the X-ray range. The Disi-Penrose model has been shown to emit radiation from any model of collapse.

The test is not easy in practice. An experiment must involve a lot of charged particles in order to get a signal from the predicted signal. The background noise comes from sources such as radiation and Cosmic rays. The most sensitive experiments, such as those designed to detect dark matter signals, are the only ones that can satisfy those conditions.

A student at Hamilton College in New York proposed using germanium-based experiments to detect a CSL signature of X-ray emission. He was struck by lightning while hiking in Utah and died. The idea was that germanium's protons and electrons should emit the radiation that ultrasensitive detectors pick up. There are instruments with the required sensitivity online.

A germanium detector was used to test the Disi-Penrose model by a group of people in Italy and Hungary. Gran Sasso, a mountain in the Apennine range of Italy, is where the IGEX detectors are located.

The physicists were able to see no emission at a sensitivity level that ruled out the simplest form of the Disi-Penrose model. The parameters within which various CSL models could still work were placed strong bounds. The original GRW model survived by being whisked away.

The 2020 result was confirmed and strengthened by an experiment called the Majorana demonstrator, which was established to search for hypothetical particles called Majorana neutrinos, which have the curious property of being their own antiparticles. The research facility is located in a former gold mine in South Dakota. It has more germanium detectors than IGEX and can detect low-energy X-rays. Kim, a member of the team, said that the limit was more strict than before.

A Messy End

Physical-collapse models are still alive and well. Kim said that the various models made different assumptions about the collapse. There is still hope despite the exclusion of most plausible possibilities in experimental tests.

Current tests assume that white noise is uniform at all frequencies and that the physical entity perturbing the wave function is a noise field. The easiest assumption is that. It is1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556 The emission spectrum will need to be measured at higher energies to test the more complicated models.

The Majorana demonstrator experiment is winding down but the team is forming a new collaboration with an experiment called Gerda to create another experiment. It will have a bigger germanium detector array. Kim said that legend may be able to push the limits on CSL models further. There is a proposal for testing these models in space-based experiments that don't suffer from noise from the environment.

Falsification is hard work and rarely ends well. A version of the Disi-Penrose model in which there is no radiation at all is being worked on by Roger Penrose.

Some people think that the writing is on the wall for this view of quantum mechanics. We need to rethink what models are trying to achieve and see if the motivating problems can be solved with a different approach. Since the first collapse models were proposed, we have learned a lot about quantum measurement. She said that she thinks we need to go back to the question of what these models were created for decades ago.

The most complicated thing you could possibly imagine is inside the protons.