Brian Collett

Wavefunction Collapse and Random Walk

Wavefunction collapse models modify Schrödingers equation so that it describes the rapid evolution of a superposition of macroscopically distinguishable states to one of them. This provides a phenomenological basis for a physical resolution to the so-called “measurement problem.” Such models have experimentally testable differences from standard quantum theory. The most well developed such model at present is the Continuous Spontaneous Localization (CSL) model in which a universal fluctuating classical field interacts with particles to cause collapse. One “side effect” of this interaction is that the field imparts energy to the particles: experimental evidence on this has led to restrictions on the parameters of the model, suggesting that the coupling of the classical field to the particles must be mass-proportional. Another “side effect” is that the field imparts momentum to particles, causing a small blob of matter to undergo random walk. Here we explore this in order to supply predictions which could be experimentally tested. We examine the translational diffusion of a sphere and a disc, and the rotational diffusion of a disc, according to CSL. For example, we find that the rms distance an isolated 10−5 cm radius sphere diffuses is ≈(its diameter, 5 cm) in (20 sec, a day), and that a disc of radius 2 ⋅ 10−5 cm and thickness 0.5 ⋅ 10−5 cm diffuses through 2πrad in about 70 sec (this assumes the “standard” CSL parameter values). The comparable rms diffusions of standard quantum theory are smaller than these by a factor 10−3±1. It is shown that the CSL diffusion in air at STP is much reduced and, indeed, is swamped by the ordinary Brownian motion. It is also shown that the spheres diffusion in a thermal radiation bath at room temperature is comparable to the CSL diffusion, but is utterly negligible at liquid He temperature. Thus, in order to observe CSL diffusion, the pressure and temperature must be low. At the low reported pressure of 5 ⋅ 10−17 Torr, achieved at 4.2°K, the mean time between air molecule collisions with the (sphere, disc) is ≈(80, 45)min. This is ample time for observation of the putative CSL diffusion with the standard parameters and, it is pointed out, with any parameters in the range over which the theory may be considered viable. This encourages consideration of how such an experiment may actually be performed, and the paper closes with some thoughts on this subject.

What Brown saw and you can too

A discussion of Robert Brown’s original observations of particles ejected by pollen of the plant Clarkia pulchella undergoing what is now called Brownian motion is given. We consider the nature of those particles and how he misinterpreted the Airy disk of the smallest particles to be universal organic building blocks. Relevant qualitative and quantitative investigations with a modern microscope and with a “homemade” single lens microscope similar to Brown’s are presented.

Constraint on collapse models by limit on spontaneous x-ray emission in Ge

The continuous spontaneous localization (CSL) model modifies Schrödingers equation so that the collapse of the state vector is described as a physical process (a special interaction of particles with a universal fluctuating field). A consequence of the model is that an electron in an atom should occasionally get “spontaneously” knocked out of the atom. The CSL ionization rate for the 1s electrons in the Ge atom is calculated and compared with an experimental upper limit for the rate of “spontaneously” generated x-ray pulses in Ge. This gives, for the first time, an experimental constraint on the parameters which characterize this model (the GRW parameters and the relative collapse rate of electrons and nucleons). It is concluded that the values assigned to the GRW parameters by GRW may be maintained only if the coupling of electrons to the fluctuating field is 0.35% or less than the coupling of nucleons, suggestive of a mass-proportional (and therefore gravitational) collapse mechanism. For other allowed values of the GRW parameters, it is still argued that nucleons should collapse more rapidly than electrons.

Two‐dimensional photon counter for x‐ray imaging

This paper characterizes an imaging x‐ray detector formed by coupling a gadolinium oxysulphide phosphor to the input of an optical imaging photon detector. The device is small, light, easy to use, and features a direct digital readout. It exhibits fairly high efficiency (40%–80%) and high resolution (160 μm‐width point spread function) near the center of its 40‐mm active area. There is a small amount of pincushion distortion which seems to be associated with a loss in resolution toward the edge of the active area. The device has very low noise and can be used at x‐ray fluxes down to about 0.1 x‐ray/mm2/s without loss of accuracy but it is count rate limited at 105 x rays/s over the active area and so is not usable in high‐flux situations such as are often found at synchrotrons. We have used the device to record good diffraction patterns from striated rabbit muscle in 30 min on a rotating anode x‐ray generator: less than one tenth the time needed under similar conditions when using film.

aCORN: An experiment to measure the electron-antineutrino correlation coefficient in free neutron decay

We describe an apparatus used to measure the electron-antineutrino angular correlation coefficient in free neutron decay. The apparatus employs a novel measurement technique in which the angular correlation is converted into a proton time-of-flight asymmetry that is counted directly, avoiding the need for proton spectroscopy. Details of the method, apparatus, detectors, data acquisition, and data reduction scheme are presented, along with a discussion of the important systematic effects.

The aCORN Backscatter-Suppressed Beta Spectrometer

Backscatter of electrons from a beta spectrometer, with incomplete energy deposition, can lead to undesirable effects in many types of experiments. We present and discuss the design and operation of a backscatter-suppressed beta spectrometer that was developed as part of a program to measure the electronantineutrino correlation coefficient in neutron beta decay (aCORN). An array of backscatter veto detectors surrounds a plastic scintillator beta energy detector. The spectrometer contains an axial magnetic field gradient, so electrons are efficiently admitted but have a low probability for escaping back through the entrance after backscattering. The design, construction, calibration, and performance of the spectrometer are discussed.

Proposed Measurement of the Beta-Neutrino Correlation in Neutron Decay

Currently, the beta-neutrino asymmetry has the largest uncertainty (4 %) of the neutron decay angular correlations. Without requiring polarimetry this decay parameter can be used to measure λ (ga/gv), test Cabibbo-Kobayashi-Maskawa (CKM) unitarity limit scalar and tensor currents, and search for Charged Vector Current (CVC) violation. We propose to measure the beta-neutrino asymmetry coeffcient, a, using time-of-flight for the recoil protons. We hope to achieve a systematic uncertainty of σa / a ≈ 1.0 %. After tests at Indiana University’s Low Energy Neutron Source (LENS), the apparatus will be moved to the National Institute of Standards and Technology (NIST) where the measurement can achieve a statistical uncertainty of 1 % to 2 % in about 200 beam days.