In March this year, a paper published in the top international academic journal Nature caused a sensation in the academic community: an international team led by Professor Du Lingjie of Nanjing University reported that the "projection" of gravitons in condensed matter was observed for the first time. Everyone has been exposed to atoms and electrons in middle school physics class, but what are gravitons? Why is it so important to "paint" gravitons?
Gravitons that matter
What exactly is a graviton?
The consensus in physics today is that there are four types of interactions in the universe: electromagnetic, strong, weak, and gravitational. Scientists speculate that these interactions are transmitted by messenger medium particles, such as photons, the electromagnetic force we are most familiar with. Gravitons, on the other hand, are the medium particles in which gravity interacts.
Of course, to understand gravitons, we also need to go back a little bit in the history of physics.
In 1913, Albert Einstein proposed the theory of gravitational field, which believed that there is a gravitational field around any object with mass. The gravitational field propagates through gravitational waves, and the medium through which gravitational waves propagate is the graviton. In 2016, human beings directly detected gravitational waves for the first time during the collision and merger of two black holes, confirming Einstein's prediction. It's just that what the graviton looks like, scientists still don't know.
Previously, the international physics community pinned its hopes on a series of large scientific devices to explore the universe, such as the American Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European "Virgo" gravitational-wave detector, to directly discover the traces of gravitons in the universe. However, in response to the old saying, "Flowers are planted intentionally, but flowers do not bloom, and willows and willows are planted unintentionally", what surprised the physics community was that the first place to see the trace of gravitons was actually a small condensed matter physics laboratory on the earth.
A story of unintentional willows
Du Lingjie received specialized physics training, starting with quantum physics and later moving to the field of semiconductor physics. His fate with gravitons began with his postdoctoral research period.
In 2016, Du Lingjie, who graduated with a Ph.D., came to Columbia University to devote himself to the research of low-temperature optics. It was here that he began to come into contact with one of the frontier topics of condensed matter physics, the fractional quantum Hall effect. This effect can only be observed under the conditions of low temperature and strong magnetic fields, and Du Lingjie's laboratory has the conditions.
Professor Du Lingjie (second from left) discusses the experimental work with students
In the first half of 2019, he and his collaborators observed a new kind of collective excitation in the fractional quantum Hall effect—that is, the energy transition of the collectivity of a large number of electrons. For example, it is like a tranquil lake that suddenly stirs up countless and diverse ripples. Unexpectedly, the most excited about this result were their peers in the field of theoretical physics, who thought it might be evidence of the existence of fractional quantum Hall effect gravitons.
The reason why the fractional quantum Hall effect has attracted countless condensed matter physicists is that it is related to many extremely subtle phenomena in quantum mechanics. Among them, there is the "fractional quantum Hall effect graviton" with a law similar to the graviton. Some scientists joke that it is the "projection" of gravitons in the condensed matter world. However, just as it is difficult for us to catch our own shadows, scientists have not had much to do with this mysterious "projection" for decades.
Du Lingjie, who is about to return to China, will be the one who grasps the opportunity of this "portrait"?
The moment when the "window" is opened
In 2019, Du Lingjie returned to his alma mater, Nanjing University. He started from scratch, and the big problem in his heart was the fractional quantum Hall effect graviton.
It was not easy to obtain the liquid nitrogen necessary for the experiment, which forced him to change the technical route; The humid environment in southern China often detracts from the laser's effectiveness, and he and his colleagues have to figure out how to debug it...... Overcoming one difficulty after another, Du Lingjie set up a two-story experimental device at NTU, and at this time, it was August 2022.
It's time to sprint for your ideals. "On December 17, 2022, I locked myself in my room and reviewed all the experimental data for 4 months, and that afternoon, I found the faint signal in the fractional quantum Hall effect that was most likely graviton excitation." Du Lingjie still clearly remembers the time when the opportunity finally knocked on the door. Based on this signal, Du Lingjie and his team measured graviton excitation as they wished.
Artistic imagination of the experimental process
However, the process of convincing his fellow physicists to submit his findings to Nature was a tortuous one—during the year-long review period, Du Lingjie had to repeatedly respond to the reviewer's questions about "insufficient evidence".
In July 2023, at an international conference held in San Sebastian, Spain, an authoritative physicist asked him: The spin of graviton excitation is 2, so is all excitation of spin 2 graviton?
For example, if we see a light on the top of a mountain in the dark, do we think that someone there has polished a match? Or maybe it's just a gathering of fireflies.
"That's a question that stopped me." Du Lingjie said frankly, "What are the characteristics of graviton excitation in the fractional quantum Hall effect? This question is also related to its characteristic energy, and the problem of characteristic energy is like a 'hard bone', which has been avoided in condensed matter physics for more than 30 years, and everyone has avoided it before, and we have also avoided it in the first draft, which is the reason why the reviewer wants to reject the manuscript. ”
To confirm the characteristic energies of graviton excitation, the excitation of smaller momentum needs to be measured, which must be premised on redesigning the experiment. Fortunately, the "dry route", which had to be adopted due to the lack of liquid nitrogen, was just conducive to detecting lower excitation momentum, and everything was like providence.
In January 2024, at an international conference on low-temperature physics held in Singapore, in front of the world's top experts in the study of fractional quantum Hall effect, Du Lingjie showed new evidence observed in gallium arsenide quantum wells, fully responding to previous doubts from the perspectives of spin, momentum and energy. Hearing the warm applause, Du Lingjie understood that he had pushed open a new "window" to explore graviton physics.
What else can you see through the window? Du Lingjie and his companions are full of curiosity and strength.
Source: Half Moon Talk