Observations of light coming from a star zipping in orbit around the humongous black hole at the center of our galaxy have provided fresh evidence backing Albert Einstein’s 1915 theory of general relativity, astronomers said on July 25.
Researchers studied a star called S0-2, boasting a mass roughly 10 times larger than the sun, as it travels in an elliptical orbit lasting 16 years around the supermassive black hole called Sagittarius A* residing at the center of the Milky Way 26,000 light years from Earth.
They found that the behavior of the star’s light as it escaped the extreme gravitational pull exerted by the black hole, with 4 million times the sun’s mass, conformed to Einstein’s theory’s predictions. The famed theoretical physicist proposed the theory, considered one of the pillars of science, to explain the laws of gravity and their relation to other natural forces.
Photo: Reuters
照片：路透
While Einstein’s theory held up in the observations of this star, astronomer Andrea Ghez of the University of California, Los Angeles said it may not be able to fully account for what happens in the most exotic possible gravitational environments like those of black holes. These extraordinarily dense celestial entities exert gravitational fields so strong that no matter or light can escape.
The study detected a co-mingling of space and time near the black hole as predicted by Einstein’s theory. Isaac Newton’s 17th century law of universal gravitation could not account for these observations, Ghez said. “Newton had the best description of gravity for a long time but it started to fray around the edges. And Einstein provided a more complete theory. Today we are seeing Einstein’s theories starting to fray around the edges,” said Ghez, who led the study published in the journal Science. At some point a more comprehensive theory of gravity may be required, she said.
The study, relying heavily on data from the Keck Observatory in Hawaii, focused on an effect called gravitational redshift. Einstein’s theory foresees the wavelength of electromagnetic radiation including light lengthening as it escapes the pull of gravity exerted by a massive celestial body like a black hole.
Photons — particles of light — expend energy to escape but always travel at the speed of light, meaning the energy loss occurs through a change of electromagnetic frequency rather than a slowing of velocity. This causes a shift to the red end of the electromagnetic spectrum, a gravitational redshift.
(Reuters)
天文學家於七月二十五日指出，有一顆恆星繞著我們銀河系中心的巨大黑洞迅速運行；經過觀察，它發出的光線提供新證據，支持艾伯特．愛因斯坦於一九一五年提出的「廣義相對論」。
科學家研究的這顆恆星名為「S0-2」，其質量約為太陽的十倍大，它在週期長達十六年的橢圓形軌道上，繞著坐落在銀河中心的超大質量黑洞「人馬座A*」運行，距離地球兩萬六千光年。
科學家發現，隨著這顆恆星的光線逃離「人馬座A*」黑洞──質量達到太陽的四百萬倍──所施加的極端引力，其呈現出的狀態符合愛因斯坦的理論預測。這位著名的理論物理學家提出廣義相對論，以解釋重力法則和其它自然力的關聯，今日被視為科學理論的一項支柱。
儘管愛因斯坦的理論得到這顆恆星觀察結果支持，加州大學洛杉磯分校的天文學家安德烈婭‧葛孜指出，這項理論或許無法完全解釋像黑洞這樣、可能是最奇特的重力環境中發生的事件。這類密度極大的天體產生的重力場極為強大，以至於沒有物質或光線能夠逃逸。
這份研究偵測到空間和時間在黑洞附近發生「混合」，正如愛因斯坦的理論所預測。葛孜表示，艾薩克‧牛頓在十七世紀提出的「萬有引力定律」並沒有辦法解釋這些觀察結果。由葛孜主持的這份研究近日發表於期刊《科學》，她進一步解釋：「在過去很長的一段時間，牛頓對重力的描述可說是最好的，但是他的理論後來逐漸受到磨損。之後，愛因斯坦提供了更完整的理論，但是今天我們看到愛因斯坦的理論也開始出現磨損。」葛孜表示，在未來的某個時間點，我們會需要更全面的重力理論。
主要仰賴夏威夷凱克天文台觀測資料的這份研究，聚焦在「重力紅移」效應。愛因斯坦的理論預見電磁輻射的波長，包括光線在逃離黑洞這類巨大天體施予的引力時發生的拉長現象。
光子──也就是光線的粒子──在逃離黑洞時會耗費能量，但運行速度仍然維持在光速，這意謂著能量的損耗是來自電磁頻率改變，而不是減緩速度。這個效應造成電磁光譜會往紅色端方向偏移，也就是所謂的「重力紅移」。
（台北時報章厚明譯）
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