In 1993 the Nobel Prize in physics was awarded to two gentlemen by the names of Russell A. Hulse and Joseph H. Taylor Jr. They had discovered a new type of pulsar by accident while studying pulsars through a 300m radio telescope in Arecibo, Puerto Rico. They had no idea of what this discovery would come to mean in later years of physics, which is the second reason to why these men received the Nobel Prize for work they had completed nearly 20 years earlier.
Antony Hewish and Jocelyn Bell, both of Cambridge University were the first discoverers of pulsars back in 1967. They were awarded the Nobel Prize for their discovery of these cosmic oddities back in 1974. Binary Pulsars were thought to have existed, but there was no data available to support their existence due to the poor searching methods. Once doctors Hulse and Taylor discovered the Binary Pulsar system PSR 1913+16 they were awarded the Nobel Prize in 1993, after many years of finally coming to understand the importance of their discovery on the future experimentation of relativity.
Dr. Russell A. Hulse was born in New York City in 1950. He held a strong interest in science since a very young age, which led to him attending the Bronx High School of Science in 1963. He graduated with his Bachelors in Physics in 1970 from Cooper Union College. He received his Ph.D from the University of Massachusetts in Amherst in 1975, which is when he worked with Taylor to discover PSR 1913+16, the binary pulsar system. He has since worked with the Princeton University’s plasma physics laboratory developing computer codes to model the behaviour of impurity ions in the plasmas of the thermonuclear fusion devices.
Dr. Joseph H. Taylor Jr. was born in Philadelphia in the year 1941. He attended Moorestown Friends School in New Jersey; where he fell in love with math. He graduated from Haverford College with his Bachelors in Physics and then went on to receive his Ph.D in Astronomy from Harvard in 1968. He then became a professor at the University of Massachusetts in Amherst where he worked with Hulse.
Each of these gentlemen had their own motivation for Pulsar research starting with Hulse who wanted to gain information on properties of interstellar space and discover pulsars with very short periods, which was the most difficult property to detect at that time. Taylor’s goals were to increase accuracy of timing measurements of pulsar periods, search for more pulsars by planning an extensive pulsar survey, and find proof of a binary pulsar, but this was not the main purpose of his research.
They each shared motivation though, such as gaining more complete statistics on pulsar periods, know their distribution in the galaxy and understand what role pulsrars played in stellar evolution. They also wanted to improve pulsar detection methods which turned into the high sensitivity method. This method is described as using a more powerful telescope and the incorporation of digital computer techniques.
The discovery of the Binary Pulsar PSR 1913+16 was completely accidental. The data that helped the discovery was collected by the Arecibo radio telescope in Puerto Rico. This is the largest single-element radio telescope in the world, and is operated by Cornell University in agreement with the National Science Foundation. The combination of the computerized search system and Arecibo telescope achieved a higher sensitivity for pulsar detection. They composed a more comprehensive investigation of pulsar dispertion, period and pulse width by using more intensive computer analysis.
The completion of the extensive pulsar survey had uncovered a total of 40 new pulsars. They had made two separate 5 to 15 minute observations of the 40 pulsars’ periods. They had expected that the periods of the two observations should be the same or differ within a small, expected experimental error. What they had observed however was that all the periods but one differed within a small experimental error. The exception was PSR 1913+16. The period differed by 27 microseconds, which was much greater than their expected error.
Hulse and Taylor were convinced that there was some kind of and error in the measurement of the period. They proposed to obtain an even higher time resolution to record the period of PSR 1913+16 and developed a new computer program for the Arecibo CDC 3300 computer to better analyze the data. The result yielded two different periods once again. This led the men on a drastic measure.
They decided to record the period observations over a two-day period. This is when Hulse realized that the period variations were due to Doppler shifts in the pulsar period due to the pulsar’s orbital velocity around a companion star.
(Diagram 1: Pulsation Period vs. Time; Upper line is Day 1; Lower Line is Day 2)
PSR 1913+16 did in fact turn out to be a binary pulsar system. But what is the importance of this and how does a binary pulsar work? The orbit of the system works like any other binary system, at every instant in time the two stars are on opposite sides of an imaginary line that passes through the centre of mass. The Apastron is the position in which the stars are the furthest apart and the periastron is the position in which the stars are closest together. Due to the elliptical orbits, the stars will move slower when in their apastron positions than in their periastron positions. The force of gravity is greater in the periastron position, hence the stars will be traveling with greater velocities.
The importance of these binary pulsars is their usefulness in conducting experiments on the theory of relativity, mainly gravity waves; which I will come back to. Albert Einstein published the General Theory of Relativity in 1915. This theory describes the curvature of space-time and the distribution of matter throughout space using field equations. In layman’s terms, gravity results from massive objects bending the space-time geometry.
Gravitational Waves were predicted in Einstein’s theory of General Relativity. Gravitational waves are disturbances in the curvature of space-time caused by the motions of matter. G – waves propagate at or near the speed of light, not through space-time but as the space-time fabric itself oscillating, creating a wave-like pattern. According to Einstein accelerating masses moving relatively to each other lose energy and their period of revolution decreases due to the emission of gravitational waves. When these waves hit an object they cause minute movements and distortions of matter, such as length change and time dilation.
Relativity is a very abstract and uncertain field so it is very popular to be tested, and in 1919 that’s exactly what an expedition led by British astronomer Arthur Eddington set out to do. He set out to observe the deflection of light by the sun during a solar eclipse. Eddington’s results convinced him that Einstein was quite right in that the deflection of light did in fact match Einstein’s predictions.
But what exactly are pulsars? Where do they fit in with the cosmic world? It turns out that pulsars are neutron stars that emit beams of radiation that sweep through the Earth’s line of sight. A neutron star is about 20km in diameter and has a density of around 1.4 times greater than that of our Sun. This means that a teaspoonful would weigh a billion tons on Earth. Because these pulsars are so massive they bend the space-time fabric to a much higher degree, thus having stronger gravitational interactions. Neutron stars are formed when massive stars, four to eight times more massive than our own explode and leave behind the collapsed remnants of the central region. When this central region collapses it causes protons and electrons to combine to form neutrons, hence the name, neutron star.
A Pulsar works in a very specific way. It is a rotating neutron star. They were discovered in 1967 by graduate student Jocelyn Burnell as radio sources that blinked on and off at a constant frequency and period of time. Pulsars are unique in that they have jets of particles streaming from their magnetic poles. These ‘jets’ produce the powerful beams of light that are observed from Earth as blinking due to the pulsar’s rotational character. Due to the misalignment of the magnetic and rotational axes, the beams of light sweep around as the pulsar rotates. We see pulsars only when the beams ‘sweep’ over the Earth. Such pulses give neutron stars the characteristic name ‘Pulsars’. These pulses appear at the same rate as the rotation of the neutron star, thus appearing to periodically. However, these periods are so miniscule that they cannot be witnessed with the naked eye you must instead use radio telescopes to record the pulses.
Taylor and Hulse recorded these pulses using the 300m radio-telescope in Puerto Rico for the Binary system PSR 1913+16 in 1974. This pulsar differed from previously discovered pulsars in that it was comprised of two very small astronomical bodies, two separate pulsars, each with a radius of about 10km separated by a very short relative distance, rotating about their centre of mass. The binary system discovered had an orbit time of less than 8 hours. The new binary pulsar also had an extremely accurate pulse period, thus could be used as a clock. This ‘clock’ is more accurate than the atomic clocks we have here on Earth. The importance of this discovery was that this binary system could be used to study gravitational forces in addition to the many other fundamental laws of physics.
It was discovered after quite some time of studying PSR 1913+16 that the orbit period was decreasing; and the two neutron stars were rotating around each other at an increasing speed in a decreasing orbit length. The minute change that occurred corresponded to a reduction of the orbit period by about 75 millionths of a second every year. It was presumed that this change was being caused by the emission of energy in the form of gravity waves, just as Einstein had predicted back in 1916. The relationship between the theoretical and observed values of the orbital path of the binary system show promise towards the existence of gravity waves. The radiation from a binary pulsar is too weak to be observed from Earth. However, if hypothetically the binary pulsars were to approach the each other so as to cause violent disturbances of matter, they would produce gravitational radiation that could be observed from the Earth.
Due to Hulse and Taylor’s research and discovery Gravity Wave Astronomy has now become a possibility. Their discovery also provides an excellent ‘lab’ for the study of gravitational waves and other theories of Einstein’s Relativity.
So why are Binary Pulsars so special? Well, Binary Pulsars are now known to be the most efficient time keeping devices in the known universe, thus far. They are the best ‘laboratories’ for conducting experiments on Einstein’s theories of relativity and the distortion of space-time.
Over the past few years of monitoring PSR 1913+16 it has been noted that there has been a decline in the period of revolution and missing energy from the system that is causing this decline. The theory behind this is that the lost energy is given off in gravity waves, which are fluctuations in the space-time fabric. From the data collected on PSR 1913+16 we can calculate that the difference in period over time is equivalent to the energy lost in the system. This value of energy loss is equivalent to the theoretical loss in energy value that can be calculated using Einstein’s equations on relativity to predict gravity waves. Therefore we assume that the lost energy is given off in gravity waves.
What can gravity waves do, and why are they so important to research? Well, they are still just part of Einstein’s theory of general relativity. But if they are real then it is believed that they can distort matter (e.g. Length) and time. They work like ripples in a pond in that they weaken over the farther the distance they traverse. This explains why they are so weak once they reach Earth making them very hard to find prime evidence for their existence.
As of now we have two operational labs/techniques to detect and search for gravity waves. The first is the Parks Telescope located in Australia. This is a large radio telescope that picks up frequencies, which reach the Earth from space and are stored in a super-massive memory system to be analyzed for fluctuations in the frequencies. These fluctuations can tell us many things such as whether we are observing a single pulsar, binary pulsar and if the gravity waves are reaching us. The different patterns in frequencies recorded tell us the periods of the system and it is when you see a fluctuation in these patterns that you have promising evidence of gravity waves. This satellite used to be used strictly in finding pulsars for the pulsar survey.
The next system is LIGO, or the Laser Interferometer Gravitational-Wave Observer. This is a more extensive search for G-waves in that it is a much larger facility, in fact it is composed of two facilities, located in Washington and Alabama in order to omit fluctuations from noise in one of the facilities. The way LIGO works is that a unit shoots a laser that hits a beam splitter and divides the beam into two beams separated by 90 degrees. The beam then travels through a long pipeline, in order to help omit noise, this pipeline is 2.5 miles long from the beam splitter to the mirror that the beam is reflected off of. The way that the laser beams oriented is such that they experience destructive interference so that the photodetector receives no light. The theory behind this setup is that since gravity waves can distort length that when one strikes the pipe and mirrors they will cause just enough distortion so that the light detector can detect light because there is no longer destructive interference in the laser beam. Unfortunately this lab has not had very successful results and has experienced too much error from being on the Earth. So in 2014 they plan on launching LISA into outer space to collect data on gravity waves.
LISA or the Laser Interferometer Space Antenna will be located in space trailing behind the Earth’s path of revolution. LISA will be composed of three separate satellites that will all be launched into space and spread out to form an equilateral triangle with side lengths of 5 million km. They stay in formation by using similar laser technology to that of LIGO, except now it is used to keep the satellites in place, rather than be bent and detected. Each of the satellites have a gold cube inside of their internal structure which is suspended and will move when the satellite gets hit by a gravity wave. There have been many precautionary actions made to omit as much error as possible. One way to be efficient and lower the cost of this mission is that only one of the satellites will have a memory drive and an antenna to send the collected data back to NASA. The other two satellites will send their collected data to the first satellite to be stored and then sent every 8 hours. This project is scheduled for launch in 2014 and is designated to replace LIGO due to all of the error that setup experiences from being on Earth.
(Object 2: Schematic of internal LISA structure)
LISA will not just tell us information on gravity waves but will be able to tell us more about the history of the universe due to the information on gravity waves collected.
(Object 3: Diagram of the timeline LISA can detect)
So what will the detection of gravity waves do for us? First off they will tell us more about the fundamental laws of physics such as gravity and force. We will also gain more information on large-scale events in the universe such as the big bang, the formation and death of stars, also known as supernovas. Finally the detection of gravity waves will make Einstein’s theory of relativity and make it a law of physics, proving Einstein to be right the entire time.
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