GRAVITATIONAL WAVES: AN INTRODUCTION TO RIPPLES OF SPACE-TIME

 

 

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Aaditya Bhattacharya^[1]*

Abstract

The article explores and explains the evolving concept of “gravitational waves”. It presents the importance of and recent progress in the contemporary knowledge of “gravitational waves”. It provides a brief overview of the interferometer – an instrument used to measure these gravitational waves. The article elucidates how the detection of “gravitational waves” has helped astronomers, physicists, and scientists to amass conclusive scientific evidence to provide evidence for theories like the “Big Bang” and Einstein\’s “Theory of Relativity”. A reference to the works of LIGO and Virgo observatories and their findings have been discussed to explain the impact of “gravitational waves.”

Keywords

Astrophysics; Gravitational Waves; Gravity Waves; Accelerated Masses; LIGO and Virgo Observatories; Blackhole; Neutron star.

Introduction

The landmark invention of the modern telescope in 1609 by astronomer Galileo Galilei marked a milestone in human advancement for detecting light from distant bodies in space. It opened new doors to discover and understand the nature and behavior of celestial bodies and many complex and unknown phenomena in space, which was not possible earlier due to limitations of resources and technology. For a considerable amount of time, making observations primarily using one’s vision (with or without a telescope) was the only way astronomers could provide strong evidence about how the Universe works. Humans are bound by the limitation to only observe substances interacting with light. This barrier was proved to be evident when it was discovered by scientists that light travels enormous distances in the universe and reaches Earth as waves with longer wavelengths that were invisible and could not be observed by optical methods. When light covers huge distances in space, its wavelength gets stretched which in turn converts it into longer wavelength electromagnetic radiation such as infrared. These cannot be detected by optical telescopes. Therefore, there was a compelling need to develop technology to enable the detection and study of such other waves in the electromagnetic spectrum. The evolution of telescopes from detection in the visible spectrum to invisible spectrum has helped astronomers avoid cosmic barriers and accurately study many phenomena. For instance, the Spitzer telescope was a major invention that detected infrared radiation from celestial bodies at enormous distances. This was one of many telescopes that brought technology and astronomy together. The Fermi Gamma-ray Space Telescope was launched in 2008 to study energetic phenomena taking place in the universe such as gamma-ray bursts, pulsars, and diffuse gamma-ray emission. However, yet another limitation astronomers face is that there remain many barriers to detect radiation in the Electromagnetic Spectrum, which dilutes the image of the universe. Electromagnetic radiation (ER) interacts with matter and hence can be absorbed, reflected, refracted, or bent. This reduces the prospect of gathering concrete evidence for a theory to be accurately proven [1]. Besides that, it prevents astronomers from discovering and detecting celestial bodies and other relevant phenomena in the universe. The challenge was accepted by the astronomers and scientists whose relentless and diligent efforts in the race to attain absolute assertiveness in predictions, invented and devised new mechanisms and phenomena. Research of “Gravitational waves” (GW) turned out to be quite promising.

Gravitational waves, also recognized as gravity waves, are disturbances in the fabric of Space-time [2]. The existence of gravitational waves was first predicted by Albert Einstein in 1916 in his “General Theory of Relativity” [2]. Einstein theorized that when enormous masses are in acceleration (like a system of binary neutron stars [3]), this would disrupt space-time in such a way that waves of undulating space-time propagate in all directions away from the source [2]. This may be visualized as ripples or waves on the surface of water, though one cannot observe gravitational waves. Hence they are also referred to as “ripples” of the surface of Space-time. Gravitational waves interact very weakly with matter in space, which helps astronomers design a fairly clear image of the universe. The waves carry cosmic information that is free of distortions (unlike ER, owing to its tendency to interact with matter). The Laser Interferometer Gravitational-wave Observatory (LIGO) located in The U.S. and the Virgo interferometer located in Italy are currently the only two interferometers used to detect gravitational waves in space-time. These observatories have been working together, focusing on the detection of gravitational waves in space-time to observe and understand various cosmic phenomena. When GWs are produced by rotating pairs of neutron stars or black holes, they are detected by the LIGO and Virgo observatories.

Importance and future scopes of Gravitational-wave research:

The current focus of the LIGO and Virgo collaboration is on binary black holes and neutron stars. These are such powerful events that the gravitational waves that release thousands, millions, or billions of light-years from our planet, can still be detected from Earth. These GWs help us understand such energetic phenomena, and provide solid proof of Einstein’s Theory of Relativity. Gravitational-wave detection is now on the path of satisfactorily providing evidence of some of the earliest and unimaginable cosmic phenomena like the “Big Bang”. The technology for gravitational wave detection has started to evolve and currently, there are plans of building an observatory in space called “LISA” (Laser Interferometer Space Antenna), which would provide very accurate results, furthering our understanding of the universe.

“Gravity”: Do we know enough about it?

A common person is aware of this “attractive force” called gravity, but it goes much deeper than that. The basic definition of gravity according to physics, is that an attractive force is exerted by everything which has a certain mass that influences its surroundings. However, things get more peculiar when we start to learn about modern physics which includes the study of Einstein’s famous “Theory of Relativity”. Space-time is a fabric – as suggested by Albert Einstein – where an object with mass will exert the force of gravity in such a way that the space curves around the object. One can visualize this with a very simple experiment. Take a ball and place it on a stretched bed-sheet or cloth. The observation is, it creates a depression in the bed-sheet due to its weight. Similarly, due to an object’s mass, it exerts gravity of its own and hence creates a depression on the surface of space-time fabric. The idea has been pictorially depicted in Figures 1 & 2 below, for better comprehension.

But when one considers large masses like neutron stars the influence of gravity on Space-time increases and it increases even more if the star is rotating or is in an orbit with another star recognized scientifically as Binary Star Systems. They are extremely dense and highly accelerated and due to this energetic phenomena, distortion of space-time occurs forming “Ripples” on the surface of space-time fabric which are released in all directions outwards from the source. One can visualize this phenomenon by taking a tumbler of water and then putting a hand blender in it. The observation in this case will be pretty simple to conclude; when the blender is switched on, its blades spin and produce acceleration, due to which ripples or waves can be seen on the surface of the water. This is similar to the above astronomical phenomena of the formation of gravitational waves on the space-time fabric. The only complexity remaining is that they can only be detected, not seen. An attempt to pictorially explain the cosmic phenomenon is done below in Figures 3 & 4.

For a deeper understanding of the concept of gravitational waves, it is mandatory to understand the answers to the following inquiries:

1. What causes gravitational waves?
2. What can be the impact of gravitational waves on earth? Does every detected gravitational wave have a potential adverse impact on earth? What kind of accelerated masses are helping scientists detect Gravitational waves?
3. What are the instruments currently used to detect the gravitational waves? How do they work?
4. Is there any theory based on gravitational wave evidence to prove or at least claim our evolution?

What causes gravitational waves?

Gravitational waves can be caused by many cosmic phenomena, but to date, the detections revealing the sources of gravitational waves are limited to rotating and colliding neutron stars, black holes, and supernova explosions. There can be many other cosmic phenomena causing distortions in space-time. However, due to the current limitations, the discoveries of the existing sources are considered the best possible way to analyze and understand gravitational waves. The detected waves are not of massive magnitude to cause distortions to earth due to the loss of energy of these waves over enormous distances in space.

What can be the impact of gravitational waves on earth? Does every detected wave have a potential adverse impact on earth? What kind of accelerated masses are helping scientists detect gravitational waves?

One can say a “distortion effect” from gravitational waves causes the earth to expand at one part and compresses at another (provided, it reaches Earth, after traveling an enormous distance in Space). However, scientists have not recorded any significant impact so far on earth, caused by such a cosmic phenomenon. This is because they occur many light years away from earth so the Gravitational waves from that particular phenomena lose a significant amount of their initial energy. Therefore, we hardly observe or record any distortions. This would only be possible if the earth was sufficiently close to the site of phenomena. Instead what the earth receives is so small in magnitude, that scientists can hardly detect it without proper instruments like the laser interferometer.

Back in 2015, scientists in the LIGO and Virgo collaborations were successful in detecting these waves released 1.3 billion years ago due to the collision of two orbiting massive black holes. This provided supporting evidence for Einstein’s Theory of Relativity. Other than this LIGO has detected 50 distortions which include other massive stellar objects like rotating asymmetric neutron stars.

What are the instruments currently used to detect the gravitational waves? How do they work?

The Interferometer was developed by a scientist Albert Michelson in the 19th century. The instrument is used in many different research experiments to measure minute changes in experiments. As the name suggests, it works on the principle of producing interference patterns from two or more sources of light by reflecting them on mirrors placed at certain specified distances and angles [4]. With the invention of the “laser” (Light Amplification by Stimulated Emission of Radiation), the basic model of Michelson’s interferometer has evolved. The latest invention being the laser Interferometer, and also the largest being deployed in the US by “Laser Interferometer Gravitational-Wave Observatory” or LIGO for short. It works on the principle of generating interference patterns from a single source of laser light.

How does the Interferometer work?

The laser light from the single source travels down two arms of the vacuum chamber (which are placed perpendicular to each other) by the reflection of a mirror which splits the laser into two separate beams. The mirror splitting the beam into two is placed at a 45° angle from the source of the laser. The vacuum chamber helps to avoid the interaction of lasers with particles of matter or gas in order to prevent distortions to the beam.

The laser light hits the surface of two mirrors [5], (each placed at the very end of the two arms respectively) and gets reflected. The laser beams retrace their path back from the two consecutive arms down through which they were initially propagated.

The two laser beams reach the first mirror placed in front of the source. The laser beams recombine back at the base. The Rays lineup in such a way that they cancel each other out. The peaks and valleys of the two light waves align oppositely (i.e. the valley is aligned just above the peak) so that when they both are added together we get a zero or they have a nullifying effect.

When a gravitational wave hits the interferometer it distorts space-time; Consequently, the mirrors move from their original position (for about 1/10^th of the diameter of a proton). This interferes with the two laser beams, disturbing their original alignment, and thus, they no longer cancel each other out. As a result, in the newly disturbed condition (parallax error), the laser beam falls on a detector. This is proof of a gravitational wave being detected. A series of continued distortions, which each time needs to be measured with maximum precision provide evidence that the signals are gravitational waves. A diagrammatic representation of the types of interferometers can be seen in Figures 5 & 6.

So far, LIGO has made a number of confirmed detections of gravitational waves and is improving its technology with each passing day for more precise measurement of this cosmic phenomenon. To ensure the accuracy of data received, LIGO operates two observatories, each placed at a distance of 1900 miles. If a signal is picked by both the observatories with approximately the same intensity, then the chances of it being a gravitational wave increases.

Is there any theory based on the existence of gravitational waves that are proved or to be proven in the future?

The scientifically reliable detection of the GWs came from the detection of a black hole collision back in 2015. It was detected that two binary black holes collided and formed a massive black hole approximately 1.3 billion years ago. To be precise, the gravitational waves were detected on September 14, 2015, at 5:51 am EDT. The LIGO observatory in Livingstone detected the gravitational waves 7 milliseconds earlier than the LIGO observatory in Hanford. This provided definitive proof of Einstein’s “Theory of Relativity”. According to Einstein’s predictions, a pair of massive objects (like binary orbits of neutron stars or orbiting Black holes for instance), emit or lose their energy of rotation by emitting gravitational waves. Now, considering the case of orbiting Black holes specifically, the expulsion of energy in the form of gravitational waves is much higher, which also means that the rotational energy of the orbit decreases. As a result, a single and massive black hole is created, converting a portion of the combined black holes’ mass into energy. This notion conforms to Einstein’s formula E=mc². This energy is emitted as a final strong burst of GWs. It is these gravitational waves that LIGO observed [6].

The detection was also converted into sound and made available as a video in the public domain by Caltech and MIT.

Considering Physicists success in detecting gravitational waves it is still not near to satisfy theories with full accuracy. Hence, it requires physicists and engineers to continuously develop mechanisms and technologies to address the theory of GWs.

Conclusion

This article discussed the fundamental knowledge on the concept, functioning, and impact of gravitational waves and their research. Gravitational-wave research is now extremely popular, especially after the first detection in 2015. Rainer Weiss, Barry Barish, and Kip Thorne were honored with the Nobel Prize in Physics for the amazing use of interferometer and converting it into inferences on one of the simplest but advanced operating mega structures on earth. Scientists look forward to analyzing even more accurate data received viz. Detections of gravitational Waves. LIGO and Virgo collaboration has proven records to have the capability to detect the gravitational waves from the “Big Bang” 13.7 billion years ago. To date, gravitational waves remain one of the strongest evidential proof scientists have found to Einstein’s theory of Relativity. Gravitational-wave research also provides an insight into the most violent, complex, and mysterious phenomena including Supernova, Magnetar [7] collisions, and Rotating Asymmetric Neutron stars. The inter Black hole collision (most probably stellar black holes) back in 2015 in the U.S. gave the first definitive proof of the existence of space-time fabric. The collision happened approximately 1.3 billion years ago and its “ripples” were detected on September 14, 2015, which also provides experimental proof establishing the fact that GWs or the “ripples” in space-time travel at the speed of light. Periodic advancements and incorporation of sophisticated techniques and technologies will help scientists in accurately measuring stellar phenomena and bring us one step closer to the ultimate detection of gravitational waves released just after the big bang which will provide evidence for the theory. The “ripples” caused after the “Big Bang” 13.7 billion years ago can now be found as evidence to support theories like the “Big Bang” and “Einstein’s theory of Relativity”. The modification of Michelson\’s interferometer to the Laser Interferometer for measurement of the gravitational waves mark a great scientific advance of the modern era.

References

1. \”Why Detect Them?\”. 2020. LIGO Lab | Caltech. https://www.ligo.caltech.edu/page/why-detect-gw
2. \”What Are Gravitational Waves?\”. 2018. LIGO Lab | Caltech. https://www.ligo.caltech.edu/page/what-are-gw
3. \”Neutron Stars\”. 2018. National Geographic. https://www.nationalgeographic.com/science/space/solar-system/neutron-stars/
4. \”What Is An Interferometer?\”. 2020. LIGO Lab | Caltech. https://www.ligo.caltech.edu/page/what-is-interferometer
5. The mirrors are of the world’s finest quality and experimental specifications, to avoid any shortcomings to the experiment, Wolchover, Natalie. 2020. \”To Make The Perfect Mirror, Physicists Confront The Mystery Of Glass\”. Quanta Magazine. https://www.quantamagazine.org/to-make-the-perfect-mirror-physicists-confront-the-mystery-of-glass-20200402/
6. \”Gravitational Waves Detected 100 Years After Einstein\’s Prediction\”. 2020. LIGO Lab | Caltech. https://www.ligo.caltech.edu/news/ligo20160211
7. Kaspi, Victoria M., and Andrei M. Beloborodov. 2017. \”Magnetars\”. Annual Review Of Astronomy And Astrophysics 55 (1): 261-301. doi:10.1146/annurev-astro-081915-023329.

Figure References

1. Cover Page Figure: Artistic representation of Binary Black hole system (Credit \”Tidal Forces Carry The Mathematical Signature Of Gravitational Waves\”. 2019. MIT Technology Review. https://www.technologyreview.com/2019/12/14/131574/tidal-forces-carry-the-mathematical-signature-of-gravitational-waves/.)
2. Figure1:Earth’s representation of Space-Time (Credit: \”Could Quantum Mechanics Explain The Existence Of Spacetime?\”. 2019. Discover Magazine. https://www.discovermagazine.com/the-sciences/could-quantum-mechanics-explain-the-existence-of-spacetime.)

3. Figure 2: An experiment where a ball is placed on a stretched sheet, similar to the neighboring image of Earth. (Credit: \”Where Does Gravity Come From? – Universe Today\”. 2013. Universe Today. https://www.universetoday.com/75705/where-does-gravity-come-from/.)

4. Figure 3: An artist\’s impression of gravitational waves generated by binary neutron stars. (Credit: \”NSF’S LIGO Has Detected Gravitational Waves\”. 2016. NASA. https://www.nasa.gov/feature/goddard/2016/nsf-s-ligo-has-detected-gravitational-waves/.)

5. Figure 4: Ripples on the surface of water similar to that of Gravitational Waves. (Credit: \”Water Ripples Free Stock Photo – Shotstash\”. 2020. Shotstash. https://shotstash.com/photo/water-ripples/.).

6. Figure 5: A Diagrammatic representation of Michelson Interferometer. (Credit: \”The Michelson Interferometer – A Laser Lab Alignment Guide\”. 2018. Wiredsense. https://www.wiredsense.com/tutorials/the-michelson-interferometer-a-laser-lab-alignment-guide.)

7. Figure 6: A Diagrammatic representation of Laser Interferometer by LIGO. (Credit:\”What Is An Interferometer?\”. 2020. LIGO Lab | Caltech. https://www.ligo.caltech.edu/page/what-is-interferometer.)

 

1. ^* Delhi Public School, Neelbad, Bhopal, Madhya Pradesh, 462024, India. Email: [email protected] ↑