Why are there gravitational waves

The secret of gravitational waves

A fateful night shift in Hanover

It is the night of September 14, 2015. The data analyst Marco Drago is sitting in front of his computer in Hanover and evaluating measurement data. Drago belongs to a global group of physicists who have set themselves the task of proving a 100-year-old theory by Albert Einstein: the theory of gravitational waves.

For this purpose, mile-long measuring instruments were built in the USA. It is the "Laser Interferometer Gravitational Wave Observatory", LIGO for short.

The giant interferometers are in the US cities of Livingston and Hanford. Four kilometers long, double tubes set up at right angles, in which there is an absolute vacuum and precise mirrors reflect laser beams.

When the LIGO's computer system sounded the alarm that evening, Marco Drago had no idea that he was the first person in the LIGO's 40-year history to see the signal of a real gravitational wave. The 100 year long search is over. A new gateway to the universe is open.

What are gravitational waves?

Albert Einstein had also examined gravitation in his general theory of relativity and described it as an effect of space and time. In his mind, space and time are interwoven like a rubber blanket. Every energy, every mass in our universe - be it a sun, a planet or a hamster - creates a dent in this cloth. The deeper the dent created, the greater the gravity, i.e. the attraction of the object.

If the objects move on this cloth, which Einstein calls space-time, then they generate waves in space-time. Kind of like a boat moving across the calm surface of a lake and causing waves.

In spacetime, these waves mean that both space and time are briefly compressed and stretched. Einstein calls these waves in space-time gravitational waves. Since gravitational waves have no mass, they travel through space-time at the speed of light. It took 100 years for Einstein's theory to be proven by the LIGO.

Where do the gravitational waves come from?

The gravitational wave that caused our earth to oscillate in space-time on September 14, 2015, originated 1.4 billion years ago far behind the Magellanic Cloud in the constellation Swordfish. This constellation can only be seen from the southern hemisphere of the earth.

Two black holes came closer and collapsed into one another. They circled each other on increasingly narrow spiral paths. This movement created the gravitational wave that was supposed to be measured on earth.

The physicists at LIGO can draw conclusions about the two black holes based on the shape and frequency of the gravitational wave. One of the black holes was almost 30 times as heavy as our sun, the other had 35 solar masses.

After the two black holes had moved 350 kilometers together, they merged into a new black hole in just 0.2 seconds. This has a mass of 62 solar masses.

The other three solar masses were radiated in the form of a gravitational wave. The wave had an incredible performance at the beginning. 36,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 watts. There is no word for this extremely large number, which is why physicists abbreviate it to 3.6 times 10 to the power of 49 watts. The wave spreads equally in all directions at the speed of light, thus distributing its energy.

Measure gravitational waves with a 150 year old invention!

We humans cannot feel gravitational waves. In fact, measuring instruments are needed that can measure a change in length by a thousandth of the diameter of a proton: an unimaginable tiny amount.

But it is possible with a trick: you need an interferometer. The interferometer goes back to Albert A. Michelson. The Prussian physicist constructed a device in 1883 to generate interference using two light beams, i.e. a change between the wave crest heights and wave trough depths.

Since light has a wave character, it is possible to generate interference with light. To do this, the light rays must have the same wavelength. If the two light rays are in the same wave cycle, then the wave crest with wave crest, wave valley with wave trough add up. But if they are out of sync, the wave crests and troughs are completely or partially extinguished. A typical interference pattern arises. There are areas of reinforcement and areas of total extinction. This creates light and dark patterns of light on a screen.

How can a laser interferometer measure gravitational waves?

A laser interferometer takes advantage of the fact that laser light always oscillates with the same wavelength. The laser beam is sent through a semi-transparent mirror so that the beam splits at 90 degrees. Both laser beams now make the journey of the same length to the mirror at the end of the beam tube and from there back again.

Only the rays from the first semi-transparent mirror are directed to a detector. Since both beams traveled the same length, their cycle is also the same. The detector does not see any interference.

However, a gravitational wave stretches both time and space. The result: when a gravitational wave hits the interferometer, the two laser beams in the two beam tubes lose their rhythm. This creates an interference pattern, the signal of a gravitational wave. The LIGO detector software then calculates the typical wave pattern, which can ultimately also be reproduced acoustically.

Real wave or false wave - five months of uncertainty

Searching for gravitational waves is not easy. Every bump, every passing truck, even the surf of the distant ocean is measured by the LIGO detectors.

There are fine wave patterns that need to be distinguished. The physicists quickly filtered out the earthly waves, but what does a gravitational wave look like for the measuring instruments?

For this purpose, mathematical models were developed, like templates that simulate a rotating neutron star, for example, or two black holes colliding with each other. These templates are then placed over the measured data and the decision can be made: is it a gravitational wave or not.

In addition, deceptively real test waves were and are played into the system again and again. This is how you want to practice emergencies - from measurement to verification. For days the LIGO employees carried out calculations and analyzes until it was only announced at the end that it was just a test.

Therefore, the signal from September 15, 2015 will be kept secret for the time being. Because nobody knows at first whether it is a real signal or a simulated one. Then it turns out: It's not a test! It is the first time in human history that a gravitational wave has been measured.

The physicists of the LIGO network, however, come to the conclusion that this news can only be announced after careful scientific investigation. So it happened that five months later, in February 2016, the public was informed of the discovery. Only then can all the results and investigations really be published. This includes a chirping noise that is created when the measured gravitational wave is transformed into the audible frequency range.

The Nobel Prize in Physics

Until February 2016, astronomy relied on optical, infrared and radio telescopes to uncover the mysteries of the universe. The problem was always that you could only discover objects that emit light, no matter how weak it was. Dark objects such as black holes could only be examined indirectly.

With the discovery of gravitational waves, the universe can now not only be seen but also heard. With the investigation of gravitational waves, black holes can be detected directly for the first time. But other massive objects, such as exploding stars, supernovae, or neutron stars, can now be examined more closely. 100 years after Einstein's prediction, scientists now have the opportunity to understand and research their universe in a completely new way.

The dedication and decades of search for the gravitational waves were rewarded with the Nobel Prize in Physics in 2016. The award winners Rainer Weiss, Barry Barish and Kip Thorne are the founders and directors of the LIGO. You will receive the award on behalf of the many thousands of scientists and engineers who have proven Albert Einstein's theory: with proof of gravitational waves.