GRAVITATIONAL WAVES

DEVI VIJAYAN 18222382903 PHYSICAL SCIENCE SREE NARAYA GURU KRIPA BED COLLEGE, POTHENCODE

“We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the Universe. That makes us something very special.” – Stephen Hawking

CONTENTS • INTRODUCTION • HOW DOES GRAVITY WORKS? • GENERAL THEORY OF RELATIVITY • PREDICTION BY A GENIUS • WHAT ARE GRAVITATIONAL WAVES?

• SOURCES OF GRAVITATIONAL WAVES • TYPES OF GRAVITATIONAL WAVES 1. Continuous Gravitational Waves 2. Compact Binary In spiral Gravitational Waves 3. Stochastic Gravitational Waves 4. Burst Gravitational Waves

• FIRST MEASUREMENT • DETECTION OF GRAVITATIONAL WAVE – LIGO • WHAT IS LIGO? • LIGO INTERFEROMETER – STRUCTURE

• LIGO INTERFEROMETER – WORKING • DETECTION AFTER 100 YEARS! • RINGING OF SPACETIME • A NEW WINDOW ON THE UNIVERSE!

INTRODUCTION Gravity really does exist,” Newton stated in 1687. Before Newton, no one had heard of gravity, let alone the concept of a universal law. In his book Philosophiae Naturalis Principia Mathematica, Newton described gravity as an ever-present force, a tug that all objects exert on nearby objects. The more mass an object has, the stronger its tug. Increasing the distance between two objects weakens the attraction. With his equations, Newton was able to explain for the first time why the Moon stays in orbit around Earth. To this day, we use Newton’s math to predict the trajectory of a softball toss or of astronauts landing on the Moon. In fact, all everyday observations of gravity on Earth and in the heavens can be explained quite precisely with Newton’s theory.

HOW DOES GRAVITY WORKS? “Gravity must be caused by an agent acting constantly according to certain laws. But whether this agent be material or immaterial, I have left to the consideration of my readers.”ISAAC NEWTON The truth is, Newton could describe gravity, but he didn’t know how it worked. For 300 years, nobody truly considered what that agent might be. Maybe any possible contenders were intimidated by Newton’s genius

GENERAL THEORY OF RELATIVITY Albert Einstein wrote in his memoirs. “Newton, forgive me, you found the only way which, in your age, was just about possible for a man of highest thought and creative power.” In 1915, after eight years of sorting his thoughts, Einstein had dreamed up an agent that caused gravity. And it wasn’t simply a force. According to his theory of General Relativity, gravity is much weirder: a natural consequence of a mass’s influence on space.

PREDICTION BY A GENIUS Gravitational waves were first predicted in Albert Einstein’s 1915 theory of gravity, known as general relativity. One of the main conclusions of this theory is that gravity emerges as a result of the fact that objects of mass warp the very fabric of spacetime. This is analogous to taking a stretched rubber sheet and placing balls of different masses upon it creating "warps" in its fabric. The bigger the mass of the ball the more extreme the warp it causes. The greater the mass of an object the more extreme the warping of space it causes, so a star warps spacetime more than a planet, and a black hole warps it more than a star.

WHAT ARE GRAVITATIONAL WAVES? Gravitational waves are ‘ripples’ in space-time caused by some of the most violent and energetic processes in the Universe. They travel at the speed of light

SOURCES OF GRAVITATIONAL WAVES Any object with mass that accelerates (which in science means changes position at a variable rate, and includes spinning and orbiting objects) produces gravitational waves, including humans and cars and airplanes etc. The Universe is filled with incredibly massive objects that undergo rapid accelerations (things like black holes, neutron stars, and stars at the ends of their lives)., the gravitational waves produced by these massive objects can be detected.

TYPES OF GRAVITATIONAL WAVES In order to understand the types of gravitational waves, LIGO scientists have defined four categories of gravitational waves, they are: 1. Continuous Gravitational Waves 2. Compact Binary In spiral Gravitational Waves 3. Stochastic Gravitational Waves 4. Burst Gravitational Waves

1. CONTINUOUS GRAVITATIONAL WAVES Continuous gravitational waves are thought to be produced by a single spinning massive object like a neutron star. Any bumps on or imperfections in the spherical shape of this star will generate gravitational waves as it spins. If the spin-rate of the star stays constant, so too are the gravitational waves it emits. That is, the gravitational wave is continuously the same frequency and amplitude (like a singer holding a single note). That’s why these are called “Continuous Gravitational Waves”.

Artist's depiction of a super dense and compact neutron star. [Credit: Casey Reed/Penn State University]

2. COMPACT BINARY INSPIRAL GRAVITATIONAL WAVES Compact binary inspiral gravitational waves are produced by orbiting pairs of massive and dense (“compact”) objects like white dwarf stars, black holes, and neutron stars. There are three subclasses of “compact binary” systems in this category of gravitational-wave generator: • Binary Neutron Star (BNS) • Binary Black Hole (BBH) • Neutron Star-Black Hole Binary (NSBH)

Binary Neutron Star inspiral. [Credit: Albert Einstein Institute (AEI)]

3. STOCHASTIC GRAVITATIONAL WAVES

4. BURST GRAVITATIONAL WAVES

Astronomers predict that there are so few significant sources of continuous or binary inspiral gravitational waves in the Universe that LIGO doesn’t worry about the possibility of more than one passing by Earth at the same time (producing confusing signals in the detectors). However, we do presume that many small gravitational waves are passing by from all over the Universe all the time, and that they are mixed together at random. These small waves from every direction make up what is called a “Stochastic Signal”, so called because the word, ‘stochastic’ means having a random pattern that may be analysed statistically but not predicted precisely. These will be the smallest and most difficult gravitational waves to detect, but it is possible that at least part of this stochastic signal may originate from the Big Bang. Detecting relic gravitational waves from the Big Bang will allow us to see farther back into the history of the Universe than ever before.

The search for 'burst gravitational waves' is truly a search for the unexpected—both because LIGO has yet to detect them, and because there are still so many unknowns that we really don’t know what to expect! For example, sometimes we don’t know enough about the physics of a system to predict how gravitational waves from that source will appear.

FIRST MEASUREMENT Twenty years after the death of Einstein in 1954, physicists Russell Hulse and Joseph Taylor used the Arecibo Radio Observatory in Puerto Rico to discover a tantalizing hint at gravitational waves in the form of a pair of rapidly spinning neutron stars or pulsars.

Knowing a pulsar should emit gravitational waves robbing the binary pulsars of energy and forcing their orbits together, the duo tested Einstein’s theory of general relativity by watching the binary pulsars for eight years. This revealed that the orbits of the pulsars tightened by precisely the amount predicted by general relativity if they were emitting gravitational waves. The discovery would earn Hulse and Taylor the 1993 Nobel Prize in Physics.

Arecibo Observatory's cable-suspended science platform, as seen before damage accrued in 2020. (Image credit: UCF)

DETECTION OF GRAVITATIONAL WAVE -LIGO In 2015, scientists detected gravitational waves for the very first time. They used a very sensitive instrument called LIGO (Laser Interferometer GravitationalWave Observatory). These first gravitational waves happened when two black holes crashed into one another. The collision happened 1.3 billion years ago. But, the ripples didn’t make it to Earth until 2015!

WHAT IS LIGO? LIGO stands for "Laser Interferometer Gravitationalwave Observatory". It is the world's largest gravitational wave observatory and a marvel of precision engineering. Comprising two enormous laser interferometers located 3000 kilometres apart, LIGO exploits the physical properties of light and of space itself to detect and understand the origins of gravitational waves (GW).

LIGO LIVINGSTON

LIGO INTERFEROMETER STRUCTURE The LIGO observatories shoot laser beams down 4kilometer-long (2.4-mile-long) tubes that have been equipped with mirrors. Theoretically, gravitational waves should produce a tiny shift in the paths of the light beams. That should result in a characteristic interference pattern when the two beams meet. Two facilities were built 1,900 miles (3,000 kilometres) apart, in Louisiana and Washington, to provide a double-check on any scientific findings.

LIGO FACILITY IN WASHINGTON STATE).

LIGO-INTERFEROMETER WORKING Gravitational waves cause space itself to stretch in one direction and simultaneously compress in a perpendicular direction. In LIGO, this causes one arm of the interferometer to get longer while the other gets shorter, then vice versa, back and forth as long as the wave is passing. The technical term for this motion is “Differential Arm” motion, or differential displacement, since the arms are simultaneously changing lengths in opposing ways, or differentially.

As described above, as the lengths of the arms change, so too does the distance travelled by each laser beam. A beam in a shorter arm will return to the beam splitter before the beam in a longer arm, then the situation switches as the arms oscillate between being longer and shorter. Arriving at different times, the waves of light no longer meet up nicely when recombined at the beam splitter. Instead, they shift in and out of alignment or “phase” as they merge while the wave is causing the arm lengths to oscillate. In simple terms, this results in a flicker of light emerging from the interferometer

DETECTION AFTER 100 YEARS! The first direct observation of gravitational waves came on September 14, 2015, 100 years after the publication of Einstein’s theory of general relativity. The twin LIGO interferometers, located in Livingston, Louisiana, and Hanford, Washington observed gravitational waves from the merger of two black holes, each about 30 times the mass of our sun. The signal from the event designated GW150914, with the “GW” prefix standing for “Gravitational Wave,” was detected at 5:51 a.m. Eastern Daylight Time (09:51 UTC), according to a statement from LIGO announcing the breakthrough(opens in new tab) issued on Feb. 11, 2016. As of Now, the LIGO-Virgo-KAGRA collaboration has detected at least 90 gravitational wave signals from a variety of different events. This included the first detection by LIGO and Virgo of a neutron star-neutron star merger(opens in new tab) on August 17, 2017, designated GW170817.

Black hole collision reveals clues to early

RINGING OF SPACETIME The way gravitational waves propagate also seems analogous to the sound waves, but in the near vacuum of space where there isn’t a medium like air there can’t be sound so gravitational waves remain silent. However, when gravitational waves are detected here on Earth their frequencies can be converted into sound thus enabling us to hear the “ringing of spacetime” that is created from the collision and merger of black holes or neutron stars, or a mix of the two.

MIT scientists have captured the "ringing" of a newly-formed black hole, in the form of gravitational waves, depicted in this artist's illustration.

LIGO converted the frequency of the gravitational waves detected on September 14, 2015, from the merger of two black holes, each about 30 times the mass of our sun to sound. This allowed us to “hear” the ringing of spacetime created by two colliding cosmic titans. As the black holes spiral closer together the frequency of the gravitational waves increases creating increasing the frequency of the gravitational waves which when converted to sound resemble a bird’s “chirp.”

Two black holes, the product of the destruction of a massive star, spin, move and collide with one another. (Illustration: SXS University of Maryland)

A NEW WINDOW ON THE UNIVERSE! The detection of gravitational waves has provided astronomers with a way of looking at the universe via gravitational wave astronomy. Astronomers have traditionally used the electromagnetic spectrum and its range of frequencies from lowenergy ultraviolet light through visible light to highenergy gamma rays as the basis of astronomy, but objects like black holes and dark matter that do not emit light remain invisible to this form of astronomy. Additionally, while electromagnetic radiation interacts strongly with matter, being absorbed, reflected, refracted, or bent as it travels the cosmos, gravitational waves only interact with matter weakly. LIGO says that as a result of weak interaction these ripples in spacetime travel great distances across the universe unimpeded and free of distortions. These gravitational waves, therefore, carry uncorrupted information about the incredibly distant events that created them allowing astronomers to “hear” phenomena in the universe that conventional astronomy can’t “see.”

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