Category: physics

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astronomy Dark Energy physics

Shining Some Light on Dark Energy

You may have heard of the term dark energy but what is it, what does it do and why is it called dark? We know a black hole is called black because it is not visible to us and even light can not escape its gravitational pull. So is dark energy called dark because we can’t physically see it? Well, yes actually it is. We are unable to detect it directly and are only able to infer its existence by the effects it has on other objects. The effects are very real and until recently were not well understood. Let’s take a closer look at dark energy.

The Expansion of the Universe

The universe has been expanding since the Big Bang some 14 billion years ago. So what does this expansion look like? Are the galaxies flying away from each other never to be seen again? Well, sort of. Edwin Hubble noted in the 1920s that the farther away we look the faster galaxies seem to be moving away from us. All galaxies are receding from one another so no matter what direction we look in we will see galaxies receding from our own. Hubble’s Law tells us that the speed at which galaxies recede from each other are proportional to their distance so the farther away a galaxy is the faster it is moving away from us.

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Hubble’s Law shows that the rate at which galaxies are receding are proportional to their distance. Courtesy of space.fm

This idea that galaxies are moving away from each other doesn’t tell the entire picture. It turns out the galaxies themselves aren’t moving but rather the space between them is. The galaxies are not receding away from one another through space they are moving in space meaning that the space in between each galaxy is also moving. One common analogy is to think of the universe as a loaf of bread and the galaxies are represented by raisins. In this analogy a baker making a loaf of raisin bread will sprinkle raisins throughout the batter. Each raisin is some distance apart from every other raisin. When the dough is put into the oven what happens? The dough rises and expands. As the bread begins to cook the dough expands so the raisins become farther apart from each other. The raisins aren’t actually moving it is that the dough in between them is expanding.

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The raisin bread analogy of the expansion of the universe. Courtesy of openstax.org

A second useful analogy is to think of the early universe as deflated balloon. If you were to mark the balloon with several galaxies you would see them recede from one another as the universe, the balloon in this case, expanded due to inflation. Once again it is not the galaxies themselves that are moving but it is the space in between them that is expanding.

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The balloon analogy of the expansion of the universe. Courtesy of Forbes.com

What Does Dark Energy Actually Do?

As noted earlier the universe has been expanding ever since the Big Bang occurred. For many years scientists believed that gravitational forces would either slow the expansion down or even cause the universe to contract at some point in the future. If you stretch a rubber band and then release it what happens? The rubber band will contract back to its original size. Some scientists thought a similar fate awaited the universe. The Law of Universal Gravitation states that all matter in the universe is attracted to all other matter.

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Universal Law of Gravitation. Fg represents the gravitational attraction between two objects. G represents the universal gravitational constant (6.67E-11 Newtons kg-2 m^2. m1 and m2 represent the mass of two objects respectively and r^2 represents the distance between the two objects. Courtesy of Kahnacademy.org

Since the universe is filled with matter it seemed reasonable to think that the attractive forces between matter would cause the universe to slow down or possibly contract. The Hubble Telescope provided evidence, however, that the universe was actually expanding more slowly in the past then it is today. We now know that the rate of expansion of the universe today is actually accelerating!

In the 1990s scientists were surprised to learn that the expansion of the universe was expanding rather than slowing down. Many scientists now believe that this expansion is being driven by a force that acts opposite of the attractive force of gravity. They believe there is a repulsive force driving this accelerated expansion. This expansion appears to be occurring faster as the universe continues to expands. The term they gave this force is dark energy.

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This diagram reveals changes in the rate of expansion since the universe’s birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart at a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart. Courtesy of NASA/STSci/Ann Feild

How do we know that there is something driving this expansion? We know this because we can measure how dark energy impacts the expansion of the universe. What we don’t know is what dark energy is. Dark energy is distributed evenly throughout the universe. As a result of this distribution, dark energy does not appear to have any local gravitational effect rather dark energy effects the expansion of the entire universe as a whole. Scientists have been able to accurately measure the expansion rate of the universe and this in part has helped confirm the presence of dark energy. Most estimates have the universe comprised of 68% dark energy.

What is Dark Energy?

This is the million dollar question. Nobody knows for sure what dark energy is. They can measure how it impacts the expansion of the universe but nobody has been able to directly detect it or determine its composition. Albert Einstein determined that is was possible for more space to spontaneously be created. He developed a cosmological constant in an early draft of his gravitational theory. He would later call this his “greatest blunder.” This cosmological constant was needed by Einstein to show that the universe was static and not expanding. Some scientists today are reevaluating Einstein’s cosmological constant in hopes that it can be used to explain the very expansion it was created to refute. Others have hypothesized that dark energy is a fifth fundamental force they call “quintessence”. The original four fundamental forces are: gravity, electromagnetism, weak nuclear force and strong nuclear force. This potential fifth fundamental force is described as a fluid-like substance which is a repulsive force that may be driving the expansion of the universe. If you are looking to pick up your own Nobel Prize maybe detecting and describing dark energy will get you an invite to the award ceremony in Stockholm, Sweden to collect your prize.

Categories
physics quantum mechanics

The Many Worlds Theory of Quantum Mechanics

If you read my last post on the Thomas Young’s double slit experiment then you may recall the discussion of the collapse of the wave function. If you didn’t read that post, you should, but I will briefly describe the idea of the wave function collapse. The wave function equation is a formula which describes all the information of a particle. The square of the wave function gives you the probability of the location of a particle such as an electron or photon. Quantum mechanics differs from Newtonian mechanics in that it is probabilistic rather than deterministic. This means we can calculate the probability of the location or velocity of an object for example but we don’t know for sure until we make a measurement. In the double slit experiment we don’t know which slit a specific electron or photon travels through. We can use a detector to determine which slit it physically passed through but this measurement has consequences. When we measure or observe the particle we say the wave function collapses and the particle must travel through one slit or the other. Prior to making the observation the particle is in a state of superposition which means it is in all possible locations simultaneously. This idea is the most widely accepted theory regarding quantum behavior and is known as The Copenhagen Interpretation.

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Schrodinger’s wave equation

But what if making the measurement doesn’t cause the wave function to collapse. What if, rather, it causes one outcome to occur in our universe and causes all other possible outcomes to occur in a separate universes? This may sound like science fiction but it is considered to be a legitimate theory in quantum mechanics and is called The Many Worlds Theory.

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A depiction of the may worlds theory at the classical level. Courtesy of britannica.com

Many Worlds Vocabulary

Before we delve farther into this fascinating and controversial interpretation of quantum mechanics let’s review some terminology that will appear in this post. The meaning of superposition with respect to quantum mechanics is that a particle or group of particles such as electrons, photons, even molecule can exist across all possible states simultaneously. Decoherence in quantum mechanics is a way around the wave collapse idea. When quantum features interact with the classical realm then the wave function is no longer a smooth single quantum system. The wave function is said to be no longer coherent, that is smooth and continuous. Another way to think of this is that the particle in question is no longer in a state of superposition. When the decoherence occurs the particle must have a specific state. It is at this point, according to the many worlds theory that the universe branches off and all possible states of the particle occur but in different universes that can never interact with one another.

The World’s Most Famous Cat

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Schrodinger’s famous though experiment. Courtesy of science.howstuffworks.com

Schrodinger’s cat is a thought experiment developed by Erwin Schrodinger to illustrate what he perceived to be the ridiculousness of The Copenhagen Interpretation of quantum mechanics. In this experiment, a cat is placed in a box with a vile of poison. There is a particle that is undergoing radioactive decay and when the decay is complete it triggers a hammer to smash the vile of poison causing it to be released thus killing the cat. The particle itself is part of the quantum world and so it may or may not decay. A timer is set for one hour, at which time the decay of the particle would be complete. The idea is that once the hour is up, the cat is both alive and dead until we open the box to observe it. It is only by making an observation, opening the box, that we can determine its state. Schrodinger created this thought experiment in an effort to show that the idea of superposition of states is a fundamentally fatal flaw in The Copenhagen Interpretation. We know today that in the quantum world the superposition, does in fact occur, but does it occur at the macroscopic level? Here is a video of from minutephysics that describes Schrodinger’s cat experiment and briefly discusses the implications: https://www.youtube.com/watch?v=IOYyCHGWJq4

What Does The Many Worlds Theory Mean?

If we put science fiction aside for the moment we can look into what this interpretation actually says. As previously stated, The Many Worlds interpretation was developed in response to The Copenhagen Interpretation. What should be made clear is that both interpretations make the same predictions about the eventual outcome of a quantum system. If, for example, you were making quantum predictions about the double slit experiment both interpretations would arrive at the same conclusions. Where they differ is in how you arrive at your conclusion.

The Many Worlds Theory says that any time there is a quantum interaction between particles and the environment, which can be an observer or anything in the macroscopic world. The quantum system experiences decoherence and a single outcome occurs. At this point the theory predicts that “branches” or other universes are formed for every possible alternative of the particle’s superposition of states. It is important to note that once these branches are formed they are completely independent of each other and do not interact with each other. The Many Worlds theory concludes that all branches or universes are equally valid and real but that because quantum is inherently probabilistic some possibilities are more probable. So if we think back to the double slit experiment and measure an electron going through the slit on the left of the grate then at this point a new universe or branch is formed where the electron traveled through the slit on the right hand side.

Where this theory differs from The Copenhagen Interpretation is that in this interpretation there is no wave collapse. In The Many Worlds version, all possible outcomes occur, they just occur in other parallel universes. The elegance of this theory is that it does not require the collapse of the wave function. This is important because the wave function collapse is not derived from Schrodinger’s wave equation and this collapse must be added in order to satisfy The Copenhagen Interpretation. All theories agree that quantum systems follow Schrodinger’s equation when they are not being observed. The Many Worlds Interpretation says that these systems also follow the same equation when they are being observed.

Are There Copies of Us in the Other Universes?

Ok, enough about boring electrons creating and existing in other universes. Are there other versions of me in other branches or universes? According to physicist Sean Carroll at Caltech the answer is yes. “Its all the same. Many Worlds says, look, if an electon can be in a superposition, you can too.” He is quick to point out that a new universe is not created because you made a decision it only occurs when a quantum system interacts with the macroscopic environment.

Who Developed This Theory and Why?

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Hugh Everett III Courtesy of newscientist.com

In 1957 a graduate student by the name of Hugh Everett III published his now famous paper, which was an edited and abbreviated version of an earlier draft, titled “Relative State’s Formulation of Quantum Mechanics”. Everett was dissatisfied with The Copenhagen Interpretation of quantum mechanics specifically with the requirement of the wave collapse. The argument against the wave function is based on the idea that the wave function collapse appears completely arbitrary or random meaning it does not utilize any of the information contained in the wave function to determine which outcome is favored. Secondly, the wave function collapse does not originate from Schrodinger’s equation but rather must be added and some say it violates the Schrodinger equation.

Everett’s ideas were not readily accepted by the scientific community and shortly after publication Hugh Everett went to work for the defense department and never returned to his academic life. Albert Einstein and Erwin Schrodinger also left the field of quantum mechanics to pursue other topics in physics. Einstein had difficulty accepting the probabilistic nature of quantum mechanics. In a letter to colleague Max Born Einstein states “Quantum mechanics … delivers much, but does not really bring us any closer to the secret of the Old One. I, at any rate, am convinced that He does not play dice.” This quote is often abbreviated as “God does not play dice with the universe.” Maybe in another universe Einstein, Schrodinger, and Everett all stayed in the field of quantum mechanics and unraveled more quantum weirdness.

Categories
astronomy gravitational waves physics special relativity

Gravitational Waves

A brief history of gravitational waves

Albert Einstein predicted the existence of Gravitational waves in his famous 1916 paper describing general relativity. A century after Einstein’s prediction of these mysterious gravitational waves was made, proof of their existence was detected in September 2015. Einstein, as it turns out was not the first scientist to describe or predict gravitational waves but he was the first to accurately describe the phenomena. Einstein wrestled with the idea of gravitational waves for many years after publishing his paper on general relativity which indicated that these waves could, in fact, be a consequence of his theory of general relativity.

A British physicist Oliver Heaviside first proposed gravitational waves in 1893. In 1905 Henri Poincare predicted the existence of gravitational waves in his paper On Electron Dynamics where he states that a consequence of space-time geometry gravitation must produce waves that travel at the speed of light in a fashion close to electromagnetism. While there is some argument as to who first described the concept of gravitational waves it seems clear that Einstein was the first to correctly describe gravitational waves through his theory of general relativity.

What is a gravitational wave and what causes it?

A gravitational, wave according to NASA’s space place website, is “an invisible (yet incredibly fast) ripple in space. These waves travel at the speed of light through space-time which is “incredibly fast” indeed. These ripples in space physically alter the fabric of space-time as they travel. These waves stretch space in one direction and squeezes space in a direction perpendicular to the direction of stretch. These waves travel at the speed of light, in all directions, through space-time away from the source of the gravitational wave.

Gravitational waves are caused by massive objects which are accelerating around each other and may cause this ripple in the fabric of space-time when they eventually collide or merge with each other. Neutron stars or black holes are examples of objects that are massive enough to cause gravitational waves. Events which may be described as cataclysmic, such as the merger of two neutron stars or black holes or a neutron star going supernova likely produce the strongest of these waves.

How are gravitational waves detected?

The first gravitational waves that were verified were detected by LIGO (Laser Inferferometer Gravitational-Wave Observatory) located in Livingston, Louisiana and its twin inferferomter in Hanford, Washington on September 14th, 2015. The event that caused these waves is believed to be the merger of two black holes that occurred 1.3 billion years ago. The black holes reportedly collide at nearly .5c or 1/2 the speed of light to form a single massive black hole. The result is the release of an enormous amount of energy, in this case the amount of energy that was converted was equal to 3 times the mass of the sun. This process occurs in accordance with Einstein’s equation E=mc^2 which states that mass can be converted to energy. The mass that is converted to energy is discharged in the form of gravitational waves. It is these gravitational waves that were detected by the twin LIGO detectors in September of 2015.

Courtesy of Physics.Org: diagram of LIGO Interferometer and gravitational waves

The LIGO equipment consists of two 4 kilometer detector arms in an “L” configuration which can detect the distortion of space by as little as 1/10,000th the diameter of a proton. These distortions are the result of extremely violent events such as the merger of black holes, neutron stars, or a neutron stat going supernova. According to the LIGO Caltech website the 4 kilometer arms were “long enough that the curvature of the Earth was a factor in their construction.”

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An aerial view of LIGO Hanford and LIGO Livingston. Courtesy of LIGO Caltech

The Virgo interferometer is located in Italy which has arms that are 3 kilometers in length and there are plans for two more detectors, one will be located in India and will be a joint operation between LIGO and three research facilities in India. Another detector will be an underground detector called KAGRA located in Japan. Here is a quick link describing how LIGO detects gravitational waves https://www.sciencemag.org/news/2016/02/gravitational-waves-einstein-s-ripples-spacetime-spotted-first-time

Earth based interferometer can detect waves with a frequency of 30-400 hertz (Hz). These ground based detectors have the ability to detect waves that are longer than the 3-4 kilometer arms of the detectors. Space based interferometers which are slated for deployment in the 2030s are projected to be able to detect waves with a frequency of .1-100 milliHz. LISA, or Laser Interferometer Space Antenna consists of three probes that have the ability to detect waves to much lower frequencies than their ground based counterparts. Scientists are attempting to develop methods of detecting subtle variations from pulsars located within the Milky Way using “pulsar arrays” which are located in Europe, Australia, North America, and one being developed in China. These variations may be caused by the propagation of gravitational waves through our home galaxy. The pulsar arrays can detect frequencies from 1-320 nanoHz

Courtesy of NASA

What can we learn from gravitational waves?

So we know Albert Einstein predicted gravitational waves in his paper on general relativity and we know that the technology used to detect these waves is amazing but why should we study these waves? Why should we care about waves that may have been generated millions or billions of years ago?

Gravitational waves are unrelated to electromagnetic radiation and this allows us a fuller picture of events in the universe. Black holes for example are invisible to electromagnetic radiation but can be studied by the gravitational waves they create as they merge or collide with one another. Scientists are hoping to answer some fundamental questions regarding black holes and how they end up pairing and circling one another prior to colliding.

The study of gravitational waves led scientists to the origin of heavy elements in the universe. In 2017 scientists were able to witness two neutron stars merging by detecting the gravitational waves associated with the merger. Scientists were able to detect the heavy element strontium in the aftermath of the neutron star merger and the resulting explosion and burst of gamma rays known as a kilonova. LIGO Caltech defines a kilonova as “a phenomenon by which the material that is left over from the neutron star collision, which glows with light, is blown out of the immediate region and far out into space.” It is from this event that the scientists were able to prove that the heavy element strontium was created in the explosion of a neutron star. The study of gravitational waves may reveal information about the rate of expansion of the universe, the origin of black holes, and the composition of neutron stars.

Categories
physics special relativity

What’s so special about Special Relativity?

In 1905 Albert Einstein wrote a paper titled “On the Electrodynamics of Moving Bodies” which would drastically transform our understanding of the relationship between space and time. This paper introduced the world to the concept of special relativity. What makes this topic “special” is that the behaviors described in the paper apply to objects traveling in an inertial frame of reference. An inertial frame of reference is a frame in which the object being observed is moving at a constant or non accelerated speed. As an example, If I am standing still then I am in an inertial frame of reference. If a passerby is traveling by me in a vehicle at 55 miles per hour they are in a separate inertial frame of reference. Relativity is all about how objects move relative to one another. In order to describe the behavior of bodies moving in an accelerated frame of reference general relativity is needed. One of the major revelations of special relativity is the unification of space and time. Until Einstein published his now famous paper space and time were thought of as two independent coordinates. In 1908 a mathematician named Herman Minkowski developed a mathematical model based on Einstein’s paper and was the first to use the term space-time.

Postulates of Special Relativity

As noted above the effects of special relativity occur only in inertial frames of reference. If an object accelerates or changes direction then special relativity no longer applies and an object is subject to general relativity. There are two postulates of special relativity which provide the foundation for the entire theory. The first postulate is that there are no preferred inertial frames of reference and that all inertial frames of reference are equally valid and useful. This postulate is the basis for the idea that events that are simultaneous for an observer in one inertial frame of reference need not be simultaneous to an observer in another inertial frame of reference. The second postulate states that the speed of light in a vacuum is a constant and invariant quantity. This idea may sound simplistic but is very different than the way other things work in our daily life. If I were riding in a car traveling at 50 miles per hour relative to the ground and were to throw a ball at 10 miles per hour relative to the car than a bystander at rest on the sidewalk would see the ball traveling at 60 miles per hour relative to the ground. Light, on the other hand, travels only at the speed of light. If you were to measure the speed of light from the headlights of my vehicle at rest, you would measure the speed of light to be 670,616,629 miles per second. If you were to measure the speed of light on an object traveling at the speed of a man made satellite, 16,800 miles per hour, you would still measure the speed of light as 670,616,629 miles per hour. The speed would not be added to the speed of the satellites the way the speed of the ball was added to the speed of my vehicle. This fact will become significant in understanding the idea that time is not absolute and can change depending upon one’s frame of reference.

Courtesy of ilectureonline.com

Consequences of special relativity

There are three effects or consequences that occur as a result of special relativity. These effects do not become discernible until an object is traveling at some significant portion of the speed of light. The first effect is that the faster an object moves through space the slower it moves through time. This is called time dilation and has been repeatedly verified experimentally. Tests have been done using atomic clocks, one on the ground and one flown around the world in a plane. When they compare the clocks after the flight there is a slight disagreement between the clocks. This is not merely an issue of the clock not functioning properly, time is moving differently for each of these clocks. There is a simple mathematical equation that can be used to predict the time dilation between an object traveling at some significant portion the speed of light as compared to the time passage of an object in a rest frame of reference.

The above equation describes the degree to which special relativity varies from classical relativity. This equation can be used to determine time dilation, length contraction, and change in mass of an object.

This equation allows you to calculate the difference between special relativity and classical relativity of an object in a rest frame as compared to an object traveling at a high rate of speed. If you were traveling aboard a space ship traveling at say 60% percent the speed of light for one year, you would experience the passage of one year while people back on earth would experience 1.25 years. This means that you would have traveled 3 months into the future! As of now we have no ability to accelerate a space ship anywhere close to that rate of speed.

Effect number two is length contraction. As an object travels at relativistic speeds the object contracts in the direction of its motion. A passenger on the a space ship would not notice nor experience any difference in time or length in his frame of reference. An observer watching the space ship travel would, in fact, notice the length being contracted in the direction of its motion.

Effect number three is an increase in mass of an object as it approaches the speed of light. The faster an object moves the more mass it gains. Consequently, it is impossible for an object to reach the speed of light because it would become infinitely massive and require an infinite amount of energy to travel at that speed.

The World’s most famous equation!

Everyone is familiar with the famous equation E=mc^2 but what does it actually mean and why is it important? The “E” represents energy in this equation while “m” represents the mass of an object and c represents the speed of light squared. The equations states that the amount of energy you can obtain from an object is equal to the mass of the object multiplied by the speed of light squared. Even if the mass is extremely small the amount of energy available will be extremely large due to the large value of the speed of light squared. https://www.youtube.com/watch?v=hW7DW9NIO9M

It sounds simple-we can get large amounts of energy from everyday objects so we should never have to worry about energy ever again. The difficult part is how to obtain this energy from everyday objects. As it turns out we can harvest energy from very small objects in a process called nuclear fission. In nuclear fission a large amount of energy that can be obtained from a small amount of uranium contained in fuel rods. The splitting of the nucleus releases a large amount of energy energy. The release of energy in which is used to heat water and is eventually converted to electricity. Currently 29 nuclear reactors across the United States generate 20 percent of the country’s electricity by taking advantage of Einstein’s famous equation.

Categories
astronomy physics

Olber’s Paradox

Heinrich Wilhelm Olbers

Wilhelm Olbers was a German astronomer and physician who was born in 1758 and died in 1840. Olbers’ had several noteworthy contributions to the field of astronomy including developing a method to calculate the parabolic orbit of comets. Olbers’ primary focus was in searching for comets. According to Encylopedia Britannica a comet is ” a small body orbiting the sun with a substantial fraction of its composition made up of volatile ices.”

Olbers’ was the first to propose that a comet’s tail is always pointed away from the sun as a result of radiation pressure from the sun itself. Olbers’ is also credited with discovering a total of 5 comets as well as two asteroids, Pallas and Vesta. On March 6th, 1815 Olbers’ discovered a comet with a period of approximately 72 years. The period of the comet was calculated by his friend and colleague Carl Friedrich Gauss and verified by other astronomers of the time. Olbers comet, 13P/Olbers, next perihelion, which is its closet proximity to the sun, will be on June 6th, 2024.

A Scientific Paradox

A paradox, according to Merriam-Webster, is “a statement that is seemingly contradictory or opposed to common sense and yet is perhaps true.” Science is certainly not immune to experiencing a paradox. One of the most famous paradoxes is the grandfather paradox which appears to be a way for nature to forbid time travel to the past. It goes like this, suppose you go back in time and murder your grandfather before he and your grandmother conceive your father. By doing this your father and consequently, you will not be born. If you are not born then you could not have gone back in time to murder your grandfather so then you will be born. A resolution to this paradox is that if you kill your grandfather, you are actually killing him in different universe. Another possible resolution is based on quantum mechanics and the superposition of states. You can watch a short video from minute physics to see the description of the grandfather paradox and possible resolutions to the paradox. https://www.youtube.com/watch?v=XayNKY944lY This is just one example of a paradox in science there are many others you can find by executing a quick google search.

Olber’s Paradox

Courtesy of abyss.uoregon.edu

In 1826 Heinrich Wilhelm Olbers asked a deceptively simple question which became known as Olber’s Paradox. That question was “why is the night sky dark?” If the universe is both endless and populated with bright stars then the night sky should be blindingly bright at least as bright as the sun because every single line of sight must end at a star. If the universe is infinite then we should be able to see a star in every direction. Even if some stars that were further away were dimmer there would be more of them so the result should be a consistent luminosity across the sky.

So why isn’t the night sky uniformly bright?

Perhaps the most reasonable factor in the resolution to Olber’s paradox is that the universe is finite and that light from stars that are more than 13.7 billion years old are to far away for their light to have reached us yet. A second factor is that because the universe is not static but rather is expanding, light from distant galaxies is red-shifted into the non visible portion of the electromagnetic spectrum. The farther away a galaxy is from the earth the higher the red-shift of the visible light. So light from galaxies that are past a certain distance from us is red-shifted to the infrared portion of the spectrum which we can’t detect with our eyes. In summary, because the universe had a beginning, there aren’t stars in every direction, light from stars that are older than 13.7 billion years old hasn’t reached us yet, and because of the expansion of the universe resulting in red-shifting of visible light the night sky looks dark.

Categories
astronomy physics

Neutron Stars: What are they and why should we care?

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Courtesy of sciencealert.com

Neutron stars: One of the universe’s most exotic and bizarre objects.

Most people are familiar with black holes and understand that they are created when a massive star, one that is greater than 30 times the mass of the sun, dies. Stars that are similar in mass to our sun ranging from about .3 times the mass to 8 times the mass of the sun become red giants when they die. So what exactly is a neutron star? Neutron stars are the result of a collapsed star that is approximately 25-30 times the mass of our own sun. The star goes supernova at the end of the its life. When a star runs out of nuclear fuel and nuclear fusion within the core slows, the result is a decrease in pressure. This drop in pressure causes the star’s core to compress under the strain of it’s own gravitational forces. Without the offsetting pressure to maintain the stars structure the core collapses in a few thousandths of a second.

Neutron star via NASA
Courtesy of NASA

The core temperature of a star that has gone supernova may exceed billions of degrees Celsius or 100 000 000 000 K. The star undergoes a fantastic explosion that is called a supernova. The luminosity of a supernova may be up to 10 billion times greater that of our own sun. The supernova may even outshine its entire galaxy for a few days. The rate at which core collapse supernova occur is approximately 1 supernova per century per galaxy.

Once a star within the 25-30 times the mass of our sun has gone supernova, what happens to its left over core? One possibility is that the core stabilizes and becomes a neutron star. A neutron star is thought to be composed of a super-fluid an exotic friction free state of matter of neutrons. The electrons and protons inside the star have been compressed to create the neutrons found in the super-fluid state. A neutron star may be only 12 miles in diameter and have a mass of 1.3-2.5 the mass of the sun. So how did a star that was 25-30 the mass of our sun end up being a core that is only 1.3-2.5 times the mass of our sun? When the core of the star is collapsing the outer layers of the star get removed due to the large amount of energy that is released by the star. A large majority of the energy released by the star is in the form of neutrinos (99% of the energy) while the remaining energy released is in the form of light. The matter within a neutron star is so densely packed into the 12 mile diameter that a sample of the neutron star the size of a cube of sugar would weigh 1 billion tons.

Why should we care about neutron stars?

Scientists have long wondered where elements heavier than iron were created. Dying stars produce elements up to iron but little was known how the heavier elements were produced. As it turns out during a supernova, heavier elements including gold and platinum may be produced. In order for these heavier elements to be produced a neutron rich environment is needed. A process called ‘r-process’ or rapid neutron capture process is used to create these heavier elements. Neutron stars, as you might guess from the name, provide just such a neutron rich environment. One theory suggests that the merger of neutron stars is a mechanism for r-process and the creation of these heavy elements.

Interesting facts about neutron stars

  • The gravity of a neutron star is approximately 200 billion times greater than gravity on Earth.
  • The magnetic field of a neutron star is approximately 1 trillion times greater than on Earth.
  • The electric fields of a neutron star are 30 million times more powerful than a bolt of lightning.
  • Neutron stars rotate several hundred times per second with the fastest known neutron star, PSR J17482446ad, rotating over 700 times per second.
  • There are about 100 million neutron stars in the Milky Way galaxy.
  • There are 2 known neutron stars to host planets.