Einstein's theory of general relativity created a revolution in the field of Newton's dynamics. The theory derives from the idea that gravity is actually a distortion in space-time. The idea of gravitational wave (GW) has existed for nearly a century, but only now is evidence coming to light. Research institutes around the world anxiously await the detection of GWs, which may prove to be a key in understanding how things such as black holes or even the universe itself were created. Dr. Koji Arai, Assistant Professor of the TAMA project at the National Astronomical Observatory of Japan, took the time to explain GWs and the impact they've had on his research.
GWs are a phenomenon that result in the distortion of space-time. Think about ripples on the surface of water or sound vibrations in the air, spreading out as waves. GWs, however, don't spread out through space via something. Instead, space-time themselves are distorted as the wave passes. You may find it somewhat difficult to imagine.
According to the theory of general relativity, gravity originates from this distortion of space-time. When a mass exists, space-time is distorted. This is gravity. And if a mass moves at high velocity, the distortion of space-time will change over time to become a wave. Einstein proved this mathematically in 1916, hypothesizing the existence of GWs.
The presence of a mass itself distorts space-time, and when any mass moves, GWs are generated and propagated at the speed of light. They are also emitted from us, but are very weak.
Think about the galaxy. Four fundamental forces exist in the universe. One is gravity, which exists between all bodies of matter even between the two of us. The second is electromagnetic force, which binds an atom's core to its electrons. Thirdly, there is "weak force," which enforces neutron's beta decay. Finally, there is "strong force," which stabilizes the core of the atom. Gravity is actually the weakest of all four forces, even weaker than the "weak force."
In areas of the galaxy where many stars are clustered close together, the electromagnetic, strong and weak forces (the three forces other than gravity) do not have much influence because they either cancel each other out or are only effective at extremely close distances. Among these four forces, only gravity has no positive or negative aspect, so it rules everything in the astronomical world. This also means that GWs can only be detected at an astrodynamic level of mass and movement.
Hulse and Taylor discovered a pair of pulsars in 1974, and after 15 years of observation, indirectly confirmed the existence of GWs. Let me explain. A star bigger than a certain mass collapses under its own weight at the end of its lifetime (a phenomenon known as gravitational collapse), thereby triggering a supernova explosion. Pulsars, or neutron stars, are formed from the leftover neutrons of a supernova. The radius of a pulsar is only 10km, but its weight is 1.4 times that of our Sun, and Hulse and Taylor discovered a pair of pulsars orbiting each other at an enormous velocity. This provided an ideal opportunity to observe GWs. They continued observing the period of the two pulsars' orbits. If GWs were present, energy would be gradually radiated by GWs and orbiting would grow increasingly fast. The two would then gradually grow closer and finally, collide. Hulse and Taylor did not detect actual GWs, but observed changes in the pulsars' orbit that matched the estimated calculation of change they believed a GW would make to within 0.1%.
Such collisions of astronomical bodies have not been detected, but supernovae have been observed many times. Ancient Chinese documents describe how a "star suddenly became incredibly bright." A supernova that occurred in 1987 was observed both by optical telescope and the detection of neutrinos. Supernovae were previously thought to occur in a galaxy only once every few decades. If you are observing several dozen galaxies, however, you may be able to catch a supernova as frequently as once a year. The further away the galaxies you observe, the better your chances. In fact, more than 100 supernovae a year have been observed with optical telescopes.
Yes. According to a calculation derived from Einstein's theory of general relativity, the propagation speed of GWs is the same as the speed of light. However, when a supernova occurs, a star's composition turns into gas and fills all of the space surrounding it, so it takes some time for the light to escape. GWs, on the other hand, are able to be propagated through all matter, so GWs are thought to reach the earth before light. Someday we hope to observe the details of the exact moment of explosion.
If neutrinos reach earth before a GW, we have to consider the possibility that the speed of GWs is different than the speed of light. In that case, we may have to make adjustments to the theory of general relativity. If the speed of a GW can be proven to be the same as the speed of light, it would support the theory of general relativity.
TAMA300 is an interferometric detector that uses laser beams. Within each arm of its L-shaped tubes, a pair of mirrors is suspended at a distance of 300m from each other. In those two opposing mirrors, laser light is accumulated. Fluctuation of the mirror distance is detected from changes to the interference condition.
The longer the tubes are (the greater the distance between the two mirrors), the higher the accuracy of the interference becomes. The largest facilities in the world even have four-kilometer-long tubes.
Interferometric GW detectors throughout the world
| 1. GEO600, Germany & Great Britain L=600m |
2. VIRGO, Italy & France L=3km |
| 3. TAMA300, Japan L=300m |
4. LIGO, USA L=2km (Hanford1) L=4km (Hanford2) |
| 5. LIGO, USA L=4km (Livingston) |
|
Yes. The method you just mentioned is called "delay line," when lasers are reflected in a zig-zag pattern to attain the same effect as stretching the distance between mirrors.
TAMA300 employs the "Fabry-Perot" method. The reflectance of the front mirror is 98.8%. That means the transmissivity is 1.2% and almost the entire laser is reflected. However, utilizing the optical resonance, lasers can be increasingly accumulated within a Fabry-Perot resonator. In addition, 1.2% of the light that enters the front mirror leaks out, but the rest is accumulated within the resonator.
Summary of Fabry-Perot resonator
Just as with the delay line method, a detector's sensitivity improves when more laser beams accumulate. TAMA300 is able to achieve a practical efficiency 300 times the actual length of the facility.
Nikon Engineering (in Japanese language only) supplied us with the suspension systems for optical parts such as mirrors. Through extensive basic research, we determined the required specifications, such as the length of wire that suspends the mirrors. However, to design an actual assembly, we had to consider various facts not only performance but also compatibility, installation and maintenance. That process required engineering. We chose Nikon Engineering because they are used to receiving such detailed, customized orders, and we felt comfortable discussing specifications with them. We have 11 suspension systems made by Nikon Engineering in this facility.
To accumulate light within the resonator, incoming and outgoing light has to be resonated, or match the peaks of the phases perfectly. To achieve that, the position of the two mirrors becomes crucial. To cancel internal vibration, caused by the adjustment of the mirrors, and external vibrations, such as seismic ground movement, we also need stabilizing optical parts such as mirrors for long-term observation.
Image of the observable range by GW detector
Observable range: LCGT (planned at Kamioka mine, Gifu): 200Mpc,
LIGO/VIGO: 20Mpc,
GEO600: 1Mpc
TAMA300: our galaxy
Mpc: Distance unit used in astronomy
1pc (per sec): 3.26 light years (30,857 billion km)
M (Mega) indicates one million times
We began installation in 1997 and started operation in 1999. In 2001, we maintained a continuous observation for 50 days a world first. Now, we have succeeded in increasing sensitivity up to 2 x 10-21
√Hz at around 1kHz and are able to continue observing at a band of 100Hz to 3kHz.
At present sensitivity levels, however, we can only monitor our own galaxy, where the possibility of a GW-scale event is very low. If we increase sensitivity further, more galaxies can be observed and chances for detecting GWs increase. We are trying to raise the sensitivity of TAMA300 to as close as possible to the theoretical limit of 2 x 10-22√Hz.
It is relatively easy to increase sensitivity at the high-frequency range, but there is a limit to increasing sensitivity at the low-frequency range due to the fact that TAMA300 is located in an urban area and is susceptible to seismic noise. To overcome this, we are proposing the LCGT (Large-scale Cryogenic GW Telescope) project, creating a large-scale detector in a mine in Gifu, Japan. The length of tubes in the LCGT will be 3km, 10 times that of TAMA300. By increasing the sensitivity of TAMA300 to its theoretical limit and by applying this know-how to a large-scale detector, we are hoping to get closer to detecting GWs.
TAMA300 observation data
Shows the relation between the frequency and sensitivity
The sensitivity is improved from February 1999 (pink line) to black line.
The gray line at the bottom is the value of the immediate goal
First of all, we would need to examine the theory of general relativity. For example, testing of general relativity by observing light bending around the edge of the Sun or the gradual shift of the orbit of the planet Mercury around the Sun have both confirmed that general relativity is valid under weak gravitational fields. However, under a strong gravitational field, the validity of general relativity has not yet been confirmed. When very dense and heavy stars such as neutron stars collide, the collision takes place and GWs generate under a strong gravitational field. By observing GWs in such conditions, we may be able to determine the reliability of the theory of general relativity.
In addition, the structure or condition of neutron stars has not been fully understood. There is a possibility of solving this by observing the waveforms of GWs at the moment of collision. We are more likely to understand the hardness or composition of a star when we understand the impact it makes when it hits another star - is it soft and pliable or hard and unbending? Furthermore, if we can detect a GW made long ago, when the universe was young, we may be able to better understand the mechanics of the Big Bang.
I have been interested in science since I was young, and majored in physics at university. As I continued studying, I began to think that I wanted to experiment and see something through a device I created myself. I eventually came to the conclusion that astronomy was the place for me.
When I had to choose a project in university, I was also interested in X-ray astronomy or gamma ray astronomy, but I wondered whether there would be anything new left to discover by the time I got involved. For that reason, GWs seemed attractive since they were not established as a field of astronomy. It seemed fun to have the possibility of playing an active role in this field, where everything I tried would be new.
I appreciate that people are expecting so much of this field. We have not proven anything yet and there is so much ahead of us to overcome before we reach our goal. I'm very happy just to be able to head toward our goal, overcoming each individual challenge.
It was 1916 when the existence of GWs was first predicted, and by the late 1960's, observations had begun. For researchers who started observations then, it has already been more than 30 years. Some people must be thinking: "It's been too long. GWs, show yourself now, please!" However, when large-scale detectors around the world start to operate and GWs are actually detected, they will blossom into a giant field of study. I'm confident that day is near.
Mirror suspension system on display at the National Science Museum
Opening its doors in 1877, the National Science Museum, located in Ueno Park, Tokyo, was the first national institution in Japan dedicated to supporting scientific research and learning opportunities for the public. The grand opening for its new annex was held in November 2004. An example of the Nikon mirror suspension system is now on display on the B3 level of the annex under the theme: "The Natural World-The Universe, Matter, and the Laws of Physics." We hope you will visit our exhibition to see scientific principles in action.
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