Reports from the Heart of the Machine
Editors: Xiao Zhou, Zhang Qian
After a rigorous 16-year test, Einstein's theory of general relativity still stands.
General relativity is an important theory of gravitation that Einstein completed in 1915 and formally published in 1916. This theory has very important applications in astrophysics: it directly deduces that some massive star will end up as a black hole.
Since its official publication in 1916, the physics community has never stopped experimentally validating this theory. Among them, researchers from the University of East Anglia (UEA) and the University of Manchester jointly conducted a 16-year experiment.
The international team of researchers observed a pair of pulsars through seven radio telescopes around the globe to conduct some of the most rigorous general relativity tests to date. It turns out that general relativity has withstood the test.
The study was published December 13 in the journal Physical Review X. "Over the past 16 years, our observations of double pulsars have proven to be surprisingly consistent with Einstein's general theory of relativity, with an accuracy of less than 99.99 percent." The author of the paper said.
Address: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.041050
Although the experiment has been very precise, dr. Robert Ferdman, one of the authors of the paper, from the UEA School of Physics, cautioned: "Although Einstein's theory of general relativity has proven to be very successful, we know that this is not the final conclusion of the theory of gravity."
Why challenge general relativity?
Although general relativity has been proposed for more than 100 years, scientists around the world are still struggling to find flaws in the theory. The research team believes that general relativity is incompatible with other fundamental forces described in quantum mechanics, so the most rigorous testing of general relativity should be carried out as much as possible to find flaws in the theory.
"A deviation in general relativity would be a major discovery that would open a new window in physics' current theoretical understanding of the universe," the researchers said. This may help us eventually discover a unified theory of the fundamental forces of nature."
How did the experiment work?
Led by Michael Kramer at the Max Planck Institute for Radio Astronomy, an international team of researchers from ten countries conducted the most rigorous test of Einstein's theory to date.
The study is based on experiments on a double pulsar discovered by team members in 2003, and it is currently the most accurate laboratory used to test Einstein's theory. Although general relativity was conceived at a time when such extreme stars and the techniques used to study them were unknown.
Dr. Ferdman said: "A pulsar is a highly magnetized, rotating dense star that emits a beam of electromagnetic radiation from its magnetic poles." Pulsars are more massive than the Sun, but they are only about 15 miles in diameter, so their density is very high, and the radio beams they produce sweep across the sky like lighthouses.
The double pulsar discovered by the research team consists of two pulsars that orbit each other at a speed of about 1 million kilometers per hour in just 147 minutes. One of the pulsars (the host star) rotates very fast, about 44 times per second, while the other pulsar (companion) rotates for about 2.8 seconds. Their motion around each other could almost be used as a perfect gravitational laboratory, testing gravitational theory in the presence of very strong gravitational fields.
The study tested the building blocks of Einstein's theory, the energy carried by gravitational waves. It is 25 times more accurate than the Hulse-Taylor pulsar discovered by Nobel laureates and 1,000 times more accurate than the gravitational wave detectors currently in use.
Not only are these observations consistent with theory, but the study also observes a number of phenomena that were previously impossible to study.
Beginning with the discovery of the double pulsar system, Jozoll · The Banks Observatory's Lovell Telescope monitors it every few weeks. This high-quality and frequent observation provides an excellent data set that can be combined with data from observatories around the world.
Professor Ingrid Stairs of the University of British Columbia, one of the authors, said: "We track the propagation of radio photons emitted from one pulsar and track their motion in the strong gravitational field of another pulsar (companion star)."
The study was the first to observe how light is delayed by the strong space-time curvature around the companion star, and detected that light was deflected at a tiny angle of 0.04 degrees. Never before had such an experiment been conducted at such a high curvature of space-time.
Professor Dick Manchester, from Australia's national science agency CSIRO, said, "The fast orbital motion of compact objects like this – which is 30% more massive than the Sun but only 24 km in diameter – allows us to test many different predictions of general relativity – 7 in all!"
"In addition to gravitational waves and light propagation, our precision allows us to measure the effects of 'time dilation,' which causes clocks to run more slowly in the gravitational field."
"When considering the effect of electromagnetic radiation emitted by fast-rotating pulsars on orbital motion, we even need to consider Einstein's famous equation E = mc^2. This radiation is equivalent to losing 8 million tons of mass per second! It may seem like a lot, but it's actually only a small fraction of the mass of a pulsar per second."
The researchers also measured with one millionth of an accuracy that the orbit changed its direction, a well-known relativistic effect that is also found in Mercury's orbit but is 140,000 times stronger here.
They realized that at this level of accuracy, they also needed to consider the effect of the pulsar's rotation on the surrounding space-time, which was "dragged" along with the rotating pulsar.
Another lead author of the study, Dr Norbert Wex of MPIfR, said: "Physicists call this the Cold Ze-Tyre effect or reference frame drag. In our experiments, this meant that we needed to consider the internal structure of pulsars as neutron stars. Thus, for the first time, our measurements allow us to use techniques that precisely track the rotation of neutron stars (pulsar timing techniques) to obtain constraints on neutron star expansion."
Pulsar timing technology combined with a sophisticated interferometry system to determine its distance with high-resolution imaging yields a value of 2400 light-years with an error of just 8%.
Professor Adam Deller, from the University of Swinburne in Australia, who is also part of the team, said that "the combination of different and complementary observation techniques adds to the extreme value of the experiment." In the past, similar research has often been hampered by our poor knowledge of the distance to these systems."
Now, the situation is completely different. In addition to pulsar timing and interferometry, information obtained from interstellar media effects is carefully considered.
Professor Bill Coles of the University of California, San Diego, agrees: "We gathered all possible information about the system and came up with a consistent picture. This picture involves many areas of physics research, such as nuclear physics, gravity, interstellar media, plasma physics, and so on. This is very unusual."
Paulo Freire, also from MPIfR, said, "Our results complement other experimental studies that test gravity under other conditions or observe different effects, such as using gravitational wave detectors or event horizon telescopes." They also complement other pulsar experiments, such as our timing experiment on pulsars in a stellar triple system, which provides an independent and superior test of the universality of free fall."
Professor Kramer added, "We have reached an unprecedented level of precision. Future experiments with larger telescopes will push humanity further afield. Our research has shown how such experiments need to be conducted and what subtle effects need to be taken into account. One day we will find a deviation from general relativity."
Reference links: https://scitechdaily.com/challenging-einsteins-greatest-theory-in-16-year-experiment-theory-of-general-relativity-tested-with-extreme-stars/
ETH Zurich DS3Lab: Building data-centric machine learning systems
The DS3Lab Lab at ETH Zurich is comprised of Assistant Professor Ce Zhang and 16 PhD and postdoctoral fellows, Ease.ML project: how to design, manage, and accelerate data-centric machine learning development, operation, and operation processes, and ZipML: Designing efficient and scalable machine learning systems for new hardware and software environments.
From December 15th to December 22nd, 11 guests from the DS3Lab Lab at ETH Zurich will share 6 sessions: Building Data-Centric Machine Learning Systems, as follows: