The scientific research team of the National Institute of Standards and Technology (NIST) recently announced the results of a 10-year experiment, which gave a new value to the gravitational constant "G", one of the most basic and difficult to accurately measure constants in physics, and revealed a potential reason for the long-term "inaccuracy" of gravity measurements.
Gravity is the weakest of the four fundamental interactions in nature, which makes it one of the most difficult physical quantities to accurately measure. NIST physicist Stephan Schlamminger said that the scientific community has been tracking the gravitational constant for more than 200 years, but the dispersion of the existing 16 main measurement results is still very large, with a typical uncertainty of about 10 parts per million, which is far inferior to the accuracy level of other basic constants.
The gravitational constant, also known as the "Big G" by the physics community, describes the strength of the gravitational force between two masses. For the public's daily life, small changes in G will not have a perceptible impact, but for physicists, locking in its precise value as much as possible will help further understand the nature of gravity and promote the exploration of a unified physical theory.
In this work, Schramminger’s team chose to replicate the experimental path rather than completely overturn and design a new solution. They transported the same set of equipment used in a famous gravitational constant experiment conducted at the International Bureau of Weights and Measures (BIPM) in France in 2014 from France to the NIST laboratory in Gaithersburg, Maryland, USA, in an attempt to reproduce the experiment in different environments and examine whether there were systematic biases hidden in the results of that year.
The BIPM experiment in 2014 gave one of the most "deviant" G values at the time, so the replication experiment is expected to reveal the details behind such abnormal results. The NIST team officially started the measurement work in 2016. The entire project lasted for 10 years. It was not only a scientific measurement, but also a long-term polishing of ultra-precision weak force measurement technology.
The latest published data shows that the value of the gravitational constant given by the team is6.67387±0.00038×10−11m3kg−1s−2, the relative standard uncertainty is5.7×10−5. Compared with the BIPM experimental results in 2014, this value is about 0.0235% lower. In the field of high-precision measurement, this difference cannot be ignored. At the same time, the result is also slightly lower than the G value recommended by CODATA 2018, but it is still difficult to clearly explain the source of the deviation.
What is even more breakthrough is that when the researchers repeatedly deduced the experimental conditions, they discovered a factor that was often ignored before—the influence of residual air in the vacuum chamber. According to the design, in order to eliminate interference as much as possible, the experiment needs to be conducted in a near-perfect vacuum environment, but the team found that no matter how the gas is pumped, a small amount of gas will always remain in the container, forming the so-called "vacuum pressure."
This residual gas will exert an extremely small force on the experimental device, thereby affecting the final measured G value. However, this effect has not been systematically included in the analysis of many previous experiments. Schramminger pointed out that this discovery is expected to help explain why the G values given by different experiments have been inconsistent for a long time, but it is still too early to draw conclusions. It is also necessary to review each experimental plan one by one to verify how they handle details such as residual gas.
When talking about the difference between the new results and the existing recognized values, Schramminger said that the team currently tends to believe that the deviation may come from the superposition of multiple cumulative effects rather than a single factor. However, what effects and respective weights are not yet accurately dismantled. Relevant research papers have been published in Metrologia, a journal in the field of metrology, and have been independently fact-checked.
This work did not bring an end to the debate over the gravitational constant, but it clearly demonstrated the complexity of the problem: even if it took ten years, reused the same device, and worked carefully in different laboratories, the final G value was still significantly different from previous results. From the perspective of the scientific community, this is both a setback and a motivation - it reminds researchers that if they want to fully understand this "most familiar and unfamiliar" natural constant, more and more detailed experiments, longer-term persistence and more acute error identification capabilities are needed.