Nobel Prize for Physics: The most sensitive “sensor” in the world

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Supermassive black holes with masses millions of times that of the sun emit gravitational waves when they collide that can be measured on Earth more than a billion years later. If that doesn’t qualify for a Nobel Prize. electronica congratulates American researchers Rainer Weiss, Barry Barish and Kip Thorne.

Approximately one hundred years ago, Albert Einstein surprised the world when he predicted gravitational waves as part of his general theory of relativity. They compress and stretch space similar to the way that a stone thrown into a quiet lake creates ripples on the surface. While gravity was still infinitely fast to Newton, Einstein’s gravitational waves spread at the speed of light—when masses accelerate. And the following rule applies: The greater the mass, the stronger the effect. However, the ability to measure it borders on a miracle, even when supermassive giants such as neutron stars or black holes are involved.

Apparently, gravitational waves have also impressed the Swedish Nobel Prize Committee. It already awarded the Nobel Prize in Physics for indirectly proving their existence in 1993. In 1974, American astronomers Joseph Taylor and Russell Hulse observed two neutron stars orbiting one another that kept getting closer together. They were able to show that the loss in kinetic energy corresponded exactly to the amount of energy “swallowed” by emitting the gravitational waves.

The first direct proof did not come until 41 years later in the autumn of 2015. In addition to recently honored Nobel Laureates Rainer Weiss, Barry Barish and Kip Thorne from the United States, more than a thousand researchers around the world were involved. In fact, technical components for the LIGO were developed in Germany—in the GEO600 gravitational wave detector operated by the Max Planck Institute for Gravitational Physics as well as various universities.

Nobel Prize for Physics with LIGO

Nobel Prize for Physics with LIGOSo-called interferometric detectors such as the LIGO (Laser Interferometer Gravitational Wave Observatory) in the United States are used to measure gravitational waves. It features two four-kilometer long tubes that are positioned at a right angle to one another, and when laser beams are emitted, they are simultaneously reflected by mirrors at the ends of the tubes. Because the tubes are exactly the same length, the light from both beams arrives back at the point of origin at the same time. Changing the positions of the mirrors makes it possible to adjust the system so that the wave valley of one beam precisely meets the wave peak of another, and both cancel each other out at the point of origin due to interference. In other words, the detector no longer registers any light.

So, if a gravitational wave from space travels through the earth and, therefore, through the LIGO, it rhythmically compresses and stretches space, making the detector slightly shorter and then longer, in alternation. And to a varying extent, because the tubes are positioned at right angles to one another. The result: The laser beams no longer completely cancel each other out at the point of origin. The detector detects light, which indicates the possible existence of gravitational waves.

Supernatural precision for cosmic events

It sounds simple, but it is more than just complicated. Because the amplitude of a gravitational wave is inversely proportional to the distance from the source, hardly anything is left by the time it reaches earth, even when it comes to events of cosmic proportions such as two black holes colliding. As a result, measurement precision is absolutely unique. Length differences of one thousandth of a proton can be detected in the LIGO. However, given these magnitudes, outside influences tend to be the rule rather than the exception. That is why the gravitational waves reach a second facility 3,000 kilometers away from the LIGO a fraction of a second later. Only if the results of both LIGOs “match” can researchers assume with reasonable certainty that they have actually observed gravitational waves.

It was just recently—on August 14, 2017—that both LIGO detectors in the United States and, for the first time ever, the French-Italian VIRGO detector near Pisa, which has a similar design, all observed gravitational waves at the same time. Once again, the event was triggered by the merging of two black holes.

Direct proof of gravitational waves opens up entirely new windows to the universe. They allow us to peer even further into the universe—possibly even as far as the Big Bang. It is a fascinating notion.

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Nobel Prize for Physics (Image: Johan Jarnestad/ The Royal Swedish Academy of Sciences).

2017 Nobel Prize for Physics awarded to LIGO black hole researchers. (Image: Johan Jarnestad/ The Royal Swedish Academy of Sciences).