"The winners of the Nobel Prize in Physics were themselves surprised by the strength of the signal of the first gravitational wave event"

This is what Prof. Ehud Neckar, who was a postdoctoral researcher in the theoretical astrophysics department at Caltech, of which Thorne is one of the leaders, says in an interview with the scientist website. There is nothing more significant than the ability to examine basic laws of nature that were not accessible to us until now"

"The winners of the Nobel Prize in Physics were themselves surprised by the strength of the signal of the first gravitational wave event, but they would not despair if they did not discover anything in the current round of observations. The worries would begin if, even when the detectors are at their peak sensitivity, nothing was detected." This is what Prof. Ehud Naker of the Department of Physics and Astronomy of Tel Aviv University says in an interview with the Hidan website.

Prof. Naker studies the signals we may receive in visible light from neutron star mergers, or from one in which one of the parties involved is a black hole and the other a neutron star. About a decade ago, Prof. Necker spent four years (2005-2008) at the California Institute of Technology (Caltech) during his postdoctoral studies, and worked as a theoretician in the theoretical astrophysics department, one of whose heads was Prof. Thorne.

The concept of gravitational waves has been around for 100 years. Physicists realized that this was an inevitable consequence of Einstein's theory of general relativity. But the gravitational waves are so weak that even Einstein himself thought that their effect would be impossible to detect. This is not what Rainer Weiss of MIT, the winner of half of the prize, thought, who wrote a theoretical paper in the late XNUMXs in which he described how a detector should be built to detect gravitational waves. Since then, for about forty years, the three winners together with Ron Drever (DREVER) who passed away this year, included and improved the LIGO detectors and occasionally ran them in test runs in one of which the discovery was made.

Electromagnetic radiation is very easy to detect. It is very strong and every device around us produces and emits it all the time. In contrast, gravitational waves are very weak. Although we feel gravity as the Earth orbits the Sun, to measure the gravitational waves we need to take the Sun and accelerate it to a speed close to the speed of light. The universe is not full of suns that are accelerated to the speed of light, but there are such bodies that are compact enough like black holes or neutron stars - stars like the mass of the sun that are concentrated in a radius of 10 km and in terms of density are of a slightly lesser degree than black holes. In nature many stars come in pairs and this may also happen in the case of compact stars. Such stars are constantly emitting gravitational waves that cause their orbits to shrink and get closer to each other, until they finally merge."

"In the moment before the merger they move close to the speed of light. So they send out strong gravitational waves compared to what they sent out before, but still very weak, but like the Nobel laureates did, if you build the devices in the right way, they can be received."

"The three winners and many other people who are not recognized were visionaries and they built the detector in a systematic way for 40 years, during which time they added more and built more and made observations every so often. They knew that the detectors had to reach an insane level of sensitivity and it was impossible to do it all at once, they built and improved the sensitivity and it was always clear that the chance of them seeing anything was small, because we can estimate how many events are happening in the universe and at what distance from us and what is the chance of observing them for a year."

Weiss, an experimentalist from MIT, made a crucial contribution to the design, financing and establishment of the experiment and for this he received half of the award. Kip Thorne is a theoretical astrophysicist from Caltech whose theoretical calculations and those of the group he led predicted what a merger of black holes would look like. Barry Barish was the second director of LIGO, in the nineties at a time when it was not clear that LIGO would continue to exist, because all the plans were then only on paper. He managed to bring the project from a state of uncertain future, to a state where in 2002 there was a primary detector that performed measurements. In addition to the three winners, Ron Derber, who passed away this year, made an essential contribution to the success of the experiment. Derber, who was an experimental physicist at Caltech, introduced an improvement to the detectors without which the discovery could not have occurred.

"When I was at Clatech in 2005-2008, it was the first time the group activated the first full version of the detector. They made observations for a year and as expected did not see anything, but showed that they could reach the required sensitivity. At the same time as the impressive experimental efforts, they raised a group of over a thousand people who worked on the subject, including researchers participating in the great theoretical effort: to calculate how the merger of black holes should look in the observational data. The reason is that the signal is so weak that you have to dig through the noise to find it, but it's much easier to discover things when you know what you're looking for. In this field, Kip Thorne had a considerable contribution. He is a theoretical physicist and established strong research groups mainly at Caltech and Cornell to understand what gravitational waves would look like in the observational data. In the end the signal was much stronger and they didn't have to dig through the data to find it. It was a collision between two holes with masses of about 30 solar masses each and the event was so powerful that it was absorbed by the Earth when the gravitational waves moved to a distance of 1.4 billion light years."

According to Prof. Necker, the run during which the gravitational waves were discovered in February 2016 was another one of those test runs, and that the detector should reach its peak sensitivity during 2018. "The detector was first and foremost built to locate mergers involving a pair of neutron stars and not necessarily black holes. This is because, until the discovery, we only knew about the existence of pairs of neutron stars and could only accurately estimate their approximate fusion rate. On the existence of pairs of black holes we could only rest. A month ago, a run ended and the facilities enter a state of improvement for about a year. Even if nothing was discovered in the runs of 2016 and 2017 there was still something to look forward to. On the other hand, if even with the sensitivity that the detector would have in 2018, nothing would be discovered, we would have to suspect that something in our knowledge of physics or astrophysics is wrong, which of course did not happen."

What did we learn from the discovery?

Prof. Neker: "This is the first time we have measured gravitational waves. We have actually opened up a new sense of the universe. If until now we have sensed the universe through electromagnetic radiation (ie electric fields) and neutrinos which are particles that are measured only by the weak force. Measuring gravitational waves is a new way to know the universe. This opens the door to much greater discoveries.

In terms of physics, this is another confirmation of the correctness of Einstein's theory of general relativity, and also (as surprising as it sounds AB) one of the best, if not the best, evidence for the existence of black holes. Since every physical theory can be tested in an experimental way accessible to us and we cannot know if the same theory will also be valid in areas we have not tested, we have not been able to test general relativity in the field of strong gravity, but only the effect of weak gravity, such as that of the Earth or the Sun . In the vicinity of black holes where gravity is very strong, we don't even know if general relativity works or not. The discovery of gravitational waves allows us to examine the theory of general relativity in a new field that was not accessible to us before.

"There is nothing more significant than the ability to examine basic laws of nature that were not accessible to us now." Emphasizes Prof. Neker.

"Gravitational waves distort time and space. When a gravitational wave passes through the Earth the bar will shorten and lengthen as the wave passes through it. If you place rulers tilted at ninety degrees, when one of the rulers lengthens the other will contract and vice versa, the detectors work on this principle. The problem is that the differences are on the order of an atom over distances of many millions of kilometers, and any truck that passes by the detector may cause a false alarm."

"That's why it took so many years to build the detectors. The amount of thought put into successfully building such detectors is amazing. The three winners are the ones who won the job. They were the contractors, the founding thinkers, the scientific planners, and the conductors over the choir." concludes Prof. Neker."

See more on the subject on the science website:

• For the initial news about the awarding of the 2017 Nobel Prize in Physics to the discoverers of gravitational waves
• A collaboration between three observatories resulted in another discovery of gravitational waves (September 2017)
• A new detector will determine the origin of gravitational waves
• Proof of Einstein's theory of general relativity: "The discovery of gravitational waves opens a new window to the universe"
• First evidence for the discovery of gravitational waves
• Einstein's mistakes
• Gravitational waves: the most important discovery of 2016 according to the editors of Science magazine
• A third gravitational wave event has been discovered; Confirms the existence of a new type of black holes