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Einstein's equivalence principle tested near a supermassive black hole

  Einstein's equivalence principle tested near a supermassive black hole
Galactic center photograph. Picture credits: European Southern Observatory (ESO).

The GRAVITY Collaboration, a research team of several renowned institutes, including the Max Planck Institute, the LESIA Paris Observatory and the European Southern Observatory, has recently tested part of the Einstein Equivalence Principle, namely the Local Positon Invariance (LPI) in the Near the galactic center supermassive black hole. Their study, published in Physics Review Letters (PRL), examined the dependence of various atomic transitions on gravitational potential to define an upper limit for LPI injury.

"The general theory of relativity and, in general, all the gravitational metric theories rely on the equivalence of inertial mass and gravitational mass, formalized in the Einstein equivalence principle," said Maryam Habibi, one of the researchers who carried out the study. org. "General relativity is the best theory of gravity we have, but there are still many unanswered puzzles that are closely linked to our incomplete understanding of gravity."

The principle of equivalence, a crucial component of Einstein's general theory of relativity, states that the gravitational force that occurs in a small space-time domain is equivalent to the pseudo-force experienced by an observer in an accelerated frame of reference. The examination of this principle is central because it may lead to interesting observations and broaden our current understanding of gravity.

"Einstein's equivalence principle consists of three main principles," explained Habibi. "One of them, referred to as Local Position Invariance (LPI), states that measurements outside of gravity should be independent of the location in space-time (indicated by gravitational potential) at which they are performed testing the LPI principle. "

Earlier observations indicate that most, if not all, massive galaxies contain a supermassive black hole that is typically at the center of a galaxy. The mass of the supermassive black hole of the Milky Way is 4 million times larger than that of the Sun. It thus generates the strongest gravitational field in the galaxy and is therefore the ideal place to search for unexplored phenomena and to test general principles of relativity.

Star S2, one of the brightest stars in the innermost region of the Milky Way, has its closest encounter with the galactic center supermassive black hole at a distance of 1

6.3 light hours. In other words, the star takes 16 years to make a full orbit around the black hole, which is extremely short in astronomical timescales. S2 moves into and out of the gravitational field of the black hole. Therefore, the GRAVITY team decided to test a part of the Einstein equivalence principle.

"As predicted and shown in an earlier study published in June 2018, as we approach the S2 star to the black hole, we observe the" gravitational redshift "in the star's light," Habibi explained. "Gravitational redshift occurs because the strong gravity on the stellar surface slows the oscillation of the light waves, stretches them, and makes the star appear redder than normal from Earth."

To test the LPI principle of Einstein, the researchers used two different types of atoms: Stern atmosphere of S2: hydrogen and helium atoms. The LPI principle states that the rotational displacement of the gravitation of a star flying in and out of a strong gravitational field depends only on the gravitational potential and does not depend on other parameters such as the internal structure of the atom.

  Testing Einstein's Equivalence Principle Near a Supermassive Black Hole
The picture shows one of ESO's Very Large Telescope (VLT) unit telescopes directing a laser beam at the Milky Way to create an artificial star. Picture credits: European Southern Observatory (ESO).

"We have measured the frequency change of the light of these atoms moving through a varying potential," said Habibi. The vibration of the light waves was measured by adjusting the line-of-sight speed of the S2 spectrum separately using the hydrogen and helium spectral lines. By measuring the difference in frequency change for both atoms, we could specify an upper limit for the wavelength of LPI injury during pericentre passage. If there was an obvious violation of LPI, we would have had to measure very different vibrations of light waves from the helium and hydrogen lines.

The principle of equivalence and the general theory of relativity are merely theories, so they need to be tested to verify their validity: So far, most researchers have performed tests on the Earth and in the solar system.

However, these theories should also be used in extreme scenarios can be tested because it can determine whether they persist and lead to more conclusive evidence such tests could preclude some of the principles that shape our current understanding of gravity or indicate violations of the general theory of relativity.

"Testing the Equivalence Principle in all different regimes is important because different alternative theories of gravitation in extreme conditions predict an injury, "said Felix Widmann, another researcher involved in the study, to Phys.org." For me, the most meaningful result of our study is that we the equivalence prince In this most extreme case, you could test p near a supermassive black hole that is over 20 thousand light-years away. The limits we set to injury are not very restrictive, but they are in a gravitational regime that was previously untested.

Habibi, Widmann, and their colleagues were among the first to test part of the equivalence principle near the Milky Way's central supermassive black hole, and their work provides valuable insights into the validity of General Relativity, particularly the LPI principle.

"The past year has been extremely successful in GRAVITY collaboration," Widmann said, "for the first time we observed relativistic effects in the orbit of a star around a supermassive black hole and used that star to test the principle of equivalence. We also observed material that circles very close to the black hole, another observation that would have been impossible without GRAVITY. But this is more of a beginning than an end to us.

With the optimal season for observing galactic centers just around the corner, researchers at the GRAVITY collaboration will continue to focus their telescopes on S2 and the supermassive black hole of the galactic center, which, according to Widmann, could soon become more subtle relativistic Seeing effects in the orbit of S2, which makes it possible to re-test the general theory of relativity, in their future observations, researchers also hope they will flare up more around the black hole, as this would allow further study to gain their understanding of the galactic black hole and the black holes of the Milky Way in general.

"With future telescopes such as the Extremely Large Telescope, which has a mirror with a diameter of 39 m, we can perform similar experiments and after a million smaller effects possible violations of the LPI look for it today, "added Widmann. "This will allow us to test the other part of the Einstein equivalence principle, called the weak equivalence principle, which states that an object in the free fall of gravity is physically equivalent to an object that accelerates with the same force in the absence of gravity The Galactic Center is a unique observatory and with GRAVITY and future telescopes we want to learn as much as we can about it. "

First successful test of Einstein's general theory of relativity near a supermassive black hole (Update)

Further information:
A. Amorim et al. Testing the Einstein Equivalence Principle Near the Galactic Center of the Supermassive Black Hole, Physical Review Letters (2019). DOI: 10.1103 / PhysRevLett.122.101102

Detection of the gravitational redshift in the orbit of the S2 star near the massive black hole in the galactic center, Astronomy & Astrophysics (2018). DOI: 10.1051 / 0004-6361 / 201833718

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