宇宙と光のこと ~天文学を読み解くヒント集~

重力波プロジェクト推進室長 ラファエレ・フラミニオ博士 インタビュー [1/4]

Einstein’s Theory of General Relativity is 100 years old. Why it is still at the forefront of science?


That question has a very long answer, but General Relativity is gaining more and more importance actually. One should start from the beginning in the sense that the Theory of General Relativity when it was first published was considered something really strange. It was a mathematical construction which only very few people could understand, with little application to everyday life, or even to scientific pursuits.

たしかに、アインシュタインの理論は百年以上解決できなかった水星の近日点移動の問題をうまく説明しました。それに、恒星からの光線が、太陽の近くを通るときに太陽の重力場によってわずかに曲げられることも示されました。光線の湾曲は1919年の皆既日食時にエディントン卿 *1が観測したもので、この観測はアインシュタインが理論を発表してから数年後に行われています。しかし、ほかには実際にこの理論を検証できるような効果があるとは思われませんでしたし、またこの理論の持つさまざまな側面を確かめることができるようになるなどとも思われませんでした。

Einstein successfully explained the orbit of Mercury, which had been a question for centuries. Then there was this verification of the fact that the light coming from a star is deflected by the Sun as it passes near the Sun. This was the eclipse experiment done by Edington in the 1919, a few years after Einstein published his theory. But basically people thought there was no real possibility to have a visible effect, or check many aspects of the theory.


The development of General Relativity also was due to the fact that he was using mathematical tools which were very new, and only a few people knew about them. So the conditions were such that General Relativity was kind of on the side of science. There was a community of people, experts on General Relativity which were quite disconnected from other fields of physics. There were so few people who understood this theory that when someone said to Edington that it seems there are only 3 people who understand General Relativity, he answered, “Who is the third one?”


And this started to change with Hubble’s discovery of the expansion of the Universe. Einstein understood that his equation could explain that expansion. And from there, this field of science which now we call cosmology started relying more and more on General Relativity.


Then as people observed more and more of the Universe by looking at the sky with new instruments like radio telescopes, gamma ray observatories, and X-ray observatories, they discovered more and more objects which had not been visible before and in which the effects of General Relativity are visible. For example, the X-ray emissions from matter accreting around a black hole. So today, this field of General Relativity is linking back to astronomy, astrophysics, and physics, mainly because of what we are learning in the fields of astrophysics and cosmology.


We are now at the stage where General Relativity is entering everyday life. Because GPS, a global positioning system, could not work without corrections for General Relativity. Our position would be wrong by a hundred meters in less than an hour if General Relativistic corrections were not applied to the Global Positioning System.

How do gravitational waves fit into the history of General Relativity?

重力波は、かつては観測することなどとてもできないだろうと考えられていた一般相対性理論の帰結の一つです。1950年代にいたるまで、原理的に観測できるかどうかさえ定かではありませんでした。50年代に入ってようやく、ヘルマン・ボンディ *2が思考実験によって重力波がエネルギーを伝えることを示したので、原理的には検出できることがわかりました。重要なのは、重力波の効果の大きさがどれくらいか、そしてそれを実際に検出することができるのか、ということです。そして、60年代から重力波を捉える実験が始まったのです。

Gravitational waves were one of those consequences of General Relativity that people thought we will never be able to observe. Up to the 1950s, people were not even sure if it was even possible in principle to observe them. And it was only in the 50s, that Hermann Bondi came with his thought experiments showing that gravitational waves carry energy and so, in principle they should be detectable. The point was how big the effect is and if we can detect them in practice. And then, from the 60s people started trying to detect these waves.

重力波望遠鏡 Gravitational Wave Detectors

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*1. エディントン卿(Sir Arthur Eddington, 1882-1944):イギリスの天体物理学者。恒星の内部構造や進化の理論研究において大きな功績を挙げた20世紀前半の第一人者。一般相対性理論の重要性を早くから見出し、1919年5月29日の皆既日食の観測隊を率いて、太陽の至近を通る恒星の光が曲げられることを検証した。

*2. ヘルマン・ボンディ (Sir Hermann Bondi, 1919-2005):オーストリア生まれのイギリスの理論物理学者。フレッド・ホイルらとともに定常宇宙論を提唱したことでも知られる。一般相対性理論に基づき、重い天体の運動から重力の波が放射されエネルギーを運び出すことを導いた。