The discovery represents an exciting advance from theory to observed phenomena.
Johns Hopkins University astrophysicists have developed a method for observing a quantum phenomenon theorized 90 years ago using data from the Sloan Digital Sky Survey and the Gaia Space Observatory. The team has investigated the radio-mass relationship of white dwarf stars, looking at evidence from quantum mechanics and Einstein’s theory of general relativity in their data.
Neutron stars and a quantum phenomenon
At the heart of every white dwarf star, the dense stellar object that remains after a star has burned its fuel reserve as the end of its life cycle approaches, lies a quantum conundrum: as White dwarfs add mass, shrink in size until they become so small and compact that they cannot sustain themselves, collapsing into a neutron star. This puzzling relationship between the mass and size of a white dwarf, called the mass-radius ratio, was first theorized by Nobel Prize-winning astrophysicist Subrahmanyan Chandrasekhar in the 1930s.
A quantum phenomenon theory
The combined data sets in the new research provided observations of more than 3,000 white dwarfs for the team to study. “The mass-radius relation is a spectacular combination of quantum mechanics and gravity, but it’s counterintuitive for us—we think that as an object gains mass, it should get bigger,” explained Nadia Zakamska, an associate professor in the Department of Physics and Astronomy who supervised the student researchers. “The theory has existed for a long time, but what’s notable is that the dataset we used is of unprecedented size and unprecedented accuracy. These measurement methods, which in some cases were developed years ago, all of a sudden work so much better, and these old theories can finally be probed.”
Einstein’s theory of general relativity
The team obtained its results using a combination of measurements, primarily including the gravitational redshift effect, which is the change in wavelengths of light from blue to red when light moves away from an object. It is a direct result of Einstein’s theory of general relativity. Scientists also had to explain how a star’s movement through space could affect the perception of its gravitational redshift.
Just as a fire truck siren changes pitch according to its movement relative to the listener, light frequencies also change based on the movement of the light-emitting object relative to the observer. This is referred to as the Doppler effect. It is a “noise” that confuses and complicates the estimation of the gravitational redshift effect, says study collaborator Sihao Cheng, a fourth-year graduate student.
The Doppler effect
To account for variations caused by the Doppler effect, the team classified the white dwarfs in their set of samples by radio. They then equated the redshifts of stars of a similar size, effectively discovering that no matter where a star is found or where it moves relative to Earth, it can be assumed to have an intrinsic gravitational redshift of a specific value.
This is equivalent to an average measurement of all tones of all fire trucks moving in a given area at any given time; Any fire truck, no matter which direction it is moving, can be expected to have an intrinsic tone to that average value. As revealed by the researchers, the intrinsic gravitational redshift values can be used to study stars that will be observed in future data sets.
The researchers reveal that upcoming data sets that are larger and more accurate will allow for a better fit of their measurements and that these data may contribute to future analysis of the chemical composition of white dwarfs. The study represents an exciting advance from theory to observed phenomena.
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