What are Gravitational Waves? Part 1– Astronomy

Gravitational waves: the hubbub around LIGO’s February announcement still hasn’t died down. The question isn’t even whether it will win the Nobel Prize, but which of its scientists it will be awarded to. But, what exactly are gravitational waves? What causes them and why are they such a big deal?




Let’s turn back the clock one hundred years. The year is 1914. A few months after Charlie Chaplin makes his film debut, Einstein comes up with the theory of General Relativity, which states that massive objects like stars or planets curve the very fabric of space and time. Smaller masses fall into this curved spacetime: this is what we call gravity.

Some regions of spacetime known as black holes are so dense that they exert gravity strong enough nothing, not even light, can escape.

If two black holes get too close to each other, they start exerting gravity on each other.The closer they get the faster they orbit until finally they collide. The energy emitted by this collision is massive- yet the signal it gives off are so weak by the time they reach the earth we’ve never been able to prove that these collisions happen.

That is, until a project known as LIGO, the Laser Interferometer Gravitational Wave Observatory, detected two black holes colliding. These black holes were roughly THIRTY TIMES the mass of the sun– each! At one point it even released more power than that released by all the stars in the universe — combined.

This collision was so huge and catastrophic that we could detect it from one billion light years away. What we heard were the “aftershocks” of the event: gravitational waves, ripples from massive objects accelerating that cause space to expand and contract.

Gravity is a weak force, and gravitational waves are nearly impossible to detect, since they are drowned out by other signals in the universe. However this collision was so strong, LIGO was able to “listen” to it happen.

Written by: Jessica Karch
Animated and illustrated by: Maxine Nelson
Narrated by: Ciara McGovern
Special thanks to Brian Dawes
Music Credit: “New Dawn”, Bensound.com



The STEM Chicksmas Day Ten: Graphene

feature_The Stem Chicksmas Physics 2010

For the 12 Days of “The STEM Chicksmas” we’re highlighting 12 scientists who have contributed something innovative and exciting to their field. It is the season of giving, and these brilliant minds have given incredible gifts to the scientific community! This year we’re looking at 12 Nobel Prize winners from the past 15 years in the fields of Physics, Chemistry, and Physiology or Medicine.

Day Ten: The 2010 Nobel Prize in Physics.

Source: Nobel Prize Summary, Nobel Prize Speed Read, and Nobel Prize Popular Information

What if you could win a Nobel Prize with a pencil and some tape? The winners of the 2010 Nobel Prize Andre Geim and Konstantin Novoselov did just that. They isolated and characterized a structure called graphene from graphite, the stuff pencils are made from, discovering a new class of stable “two dimensional” materials.

Graphite is a layered material, unlike other crystal lattices which are made of unit cells that extend in three dimensions. Each layer of graphite is made up of carbons arranged in repeating hexagons, like a honeycomb. The layers are held together by van der Waals forces, which are relatively weak interatomic interactions. Scientists have been trying for decades to isolate a single layer of graphite, called graphene, without success. Geim and Novoselov succeeded through a simple method called “exfoliation.” You lay a piece of scotch tape down on a piece of graphite. You then stick and unstick the scotch tape with a second piece of scotch tape over and over, each time yielding fewer layers of carbon, until you have a single layer. The mechanical strength of sticking and unsticking the tape is enough to overcome the van der Waals interactions without disrupting the stronger covalent bonds within the layer. This layer is “two dimensional”—it has macroscopic width and length, but it is only one atom thick. Consequently, its properties only extend in two dimensions, making it for all intents and purposes two dimensional.

Geim and Novoselov’s method was also remarkable because of the surface area it was able to produce. They were able to produce reliably large areas of intact graphene with few or no defects. Because of this feat, they were able to characterize their material and perform various tests on it. They discovered that graphene is conductive (read more about conductivity in our post on conductive polymers). Graphene is also very mechanically strong. A common example people use is that a single sheet of graphene is strong enough to hold up a cat! At least, according to calculations, because we’ve never produced a sheet large enough to try. It is very flexible, partially because the intralayer bonding is so strong. Graphene is also almost transparent.

Graphene’s unique properties make it the source of a lot of research into future applications. One example is organic solar cells. Graphene is basically transparent, so it could be coated onto more conventional solar panels without preventing light from going through, and contribute its conductive properties to enhances the solar cell’s energy conversion efficiency. Another potential application is in electronics—we could use graphene to make flexible conductors. The future of graphene is very exciting and a common subject of research in materials and condensed matter physics. However, because it is currently impossible to produce industrial sized layers of graphene (the “large” layers mentioned earlier are a few microns wide), many of these applications remain theoretical. The discovery of graphene also showed that it is possible to make stable two dimensional materials, which was previously thought of as impossible. Two dimensional materials is a rising field that many scientists are studying intently (including this author’s former research group!).

The discovery of graphene has opened up exciting new research and pioneered new paths. However, best of all, it shows that science doesn’t always need fancy and expensive equipment—sometimes all you need is some household objects and ingenuity.

Read more at the Nobel Prize website or read the award winning work here.

The STEM Chicksmas Day Seven: Neutrino Oscillations

The Stem Chicksmas Physics 2015

For the 12 Days of “The STEM Chicksmas” we’re highlighting 12 scientists who have contributed something innovative and exciting to their field. It is the season of giving, and these brilliant minds have given incredible gifts to the scientific community! This year we’re looking at 12 Nobel Prize winners from the past 15 years in the fields of Physics, Chemistry, and Physiology or Medicine.

Day Seven: The 2015 Nobel Prize in Physics.

The STEM Chicksmas Nobel Prize 2015 Physics
Source: Nobel Prize Summary, Nobel Prize Popular Information, and Invisibles of European ITN

What’s your favorite flavor of neutrino? Neutrinos belong to a category of fundamental particles called leptons. (To read more about particles and their role in the Standard Model of physics, check out our post on the Higgs Boson.)  There are three types, or in physicist parlance “flavors” of leptons, the electron, muon, and tau particles. These flavors can be found in both neutral and charged particles—the charged particles are simply named after the flavor (e.g. electron) and the neutral particles are neutrinos (e.g. electron neutrinos).

Leptons only interact with three of the four fundamental forces: the weak force, gravity, and the electromagnetic force. Neutrinos, being neutral, do not interact with the electromagnetic force. Furthermore, the weak force is, as the name implies, weak and a short-range interaction, and gravity has a very weak effect at the subatomic level. Neutrinos thus pass through matter as if it weren’t there. Detecting them, even now, is not an easy task. So when scientists in the 60s noticed that they only detected one third of the total number of neutrinos they expected to come from the sun, they figured that there was a problem with their model or their detection techniques. However, this one third remained consistent. No research group could account for the missing two thirds empirically.

Two sources of neutrinos are solar fusion and cosmic radiation. Muon neutrinos, for example, come from cosmic radiation, whereas solar fusion only produces electron neutrinos. Some physicists speculated that neutrinos traveling from the sun “changed flavor” on their way to Earth, which is why we could not detect them. (The detection methods were usually specific to electron neutrinos.) However, this would only be possible if the neutrinos had mass, and according to the Standard Model they were massless.

Why? Something important to keep in mind is that in quantum physics, particles are also waves—this is called wave-particle duality. Light is the best known example—it is simultaneously a wave and a photon. Waves are also capable of interfering with each other. When two wavelengths travel at different frequencies, they go out of phase with each other. When waves are out of phase, the amplitudes and shape of the superimposed wave is like an average of the composite waves. This is relevant for neutrinos because neutrinos also oscillate. Because the three flavors of neutrons have slightly different masses, they also move through space at different rates and frequencies. The case for neutrinos is more complicated, because both mass and flavor (specifically the interaction with their charged counterpoints) affect their quantum mechanical properties. The result is, however, that as neutrinos move through space, they oscillate and change flavor. If you look at our graphic, you see that the peaks of the different neutrino flavors shift in relation to each other as they move across the x-axis. This happens only if neutrinos have mass.

2015 Nobel Laureates in Physics Takaaki Kajita and Arthur B. McDonald were able to prove that neutrinos have mass.  Kajita and McDonald worked at two different detectors. At the Super-Kamiokande, neutrinos passed through a tank of very pure water that was deep within the ground. When muon and electron neutrinos passed through the water, they would sometimes collide with an atom nucleus or an electron in the water. When they did this, they would make charged particles (muons and electrons). The charged particles would move faster than the speed of light in water (75% of the speed of light in vacuum), and generate a blue light called the Cherenkov light. This light could be measured by detectors in the tank and the neutrino source determined. One curious aspect was that there were way more muon-neutrinos coming from the atmosphere straight above the detector than from those that traveled through the earth, ie those that traveled a further distance. This was explained at the Sudbury National Observatory. They ran a similar experiment on electron neutrinos from solar radiation, and using heavy water (water with deuterium instead of hydrogen), they detected the full number of neutrinos suggested by the Standard Model in all three flavors, not just electron-neutrinos.

The discovery at the Super-Kamiokande suggested that the distance traveled was related to the flavor of neutrino expressed. This makes sense when considering the probability of neutrinos’ waves going out of phase. The discovery at the Sudbury National Observatory accounted for the infamous missing two thirds of neutrinos. The fact that neutrinos have mass is important because it violates certain ideas set forth by the Standard Model, such as lepton conservation. Furthermore, it is important because we still don’t fully understand the role neutrinos play in the universe. There are billions and billions of neutrinos, and we don’t really know what they do. Some theorize that neutrinos play a role with hot dark matter. However, discovering neutrinos have mass is one step forward towards fully understanding particles and the universe.

Read more at the Nobel Prize website or check out the award winning work here (Kajita) or here (McDonald).

The STEM Chicksmas Day Six: The God Particle

The Stem Chicksmas Physics 2013

For the 12 Days of “The STEM Chicksmas” we’re highlighting 12 scientists who have contributed something innovative and exciting to their field. It is the season of giving, and these brilliant minds have given incredible gifts to the scientific community! This year we’re looking at 12 Nobel Prize winners from the past 15 years in the fields of Physics, Chemistry, and Physiology or Medicine.

Day Six: The 2013 Nobel Prize in Physics.

The STEM Chicksmas Nobel Prize 2013 Physics
Source: Nobel Prize Summary and Nobel Prize Popular Information

Have you ever thought about where mass comes from? The answer isn’t as obvious as it first seems. A theory called the Standard Model says the universe is made of 17 elementary particles called quarks, leptons, and bosons; and four fundamental forces called the gravitational, weak, electromagnetic, and strong forces. Electrons and neutrinos, for example, are both types of leptons and the protons and neutrons that make up atoms are themselves made from quarks and get their mass from quarks’ binding energy. A force is defined by an exchange of a type of boson called gauge bosons. For example, light is the exchange of photons, which are the bosons associated with the electromagnetic force. However, a problem with the Standard Model physicists struggled to explain is that none of these gauge bosons should have mass, but of the four gauge bosons, only photons are massless. So, where did their mass come from?

In 1964, the 2013 Nobel Laureates in Physics Peter Higgs and François Englert proposed the idea of a field that interacts with the bosons to give them mass. As the bosons move through this field, called the Higgs Field, they interact with it. The more strongly they interact with it, the more potential energy the field transfers to them. Because energy and mass are related, the more energy the particles get, the more mass they have. If the particle does not interact with the field, they are massless, like the photon. Here’s one way to picture this. Imagine a room full of scientists throwing a party. A grad student walks in, and no one pays her any attention because she’s just a grad student. Now imagine Einstein walks into the room. Everyone immediately mobs him and tries to engage him in conversation. Like a photon, the grad student passed through the room unnoticed, whereas Einstein, like the other gauge bosons, gained the “mass” of a bunch of physicists. This is one way of imagining the coupling of particles and the Higgs field.

However, proving the existence of the Higgs Field was not an easy task. To do so, physicists concentrated their efforts on finding a particle called the Higgs Boson. The Higgs Boson is a quantum excitation of the Higgs Field. If the Higgs Boson exists, the Higgs Field must exist as well. Much like how if we see a light it must come from a light source, if we see a Higgs Boson it must come from the Higgs Field. The Standard Model also predicts that the Higgs Boson should have certain properties, such as a mass of 125-127 Gev/c2, zero spin, and positive parity (which means that if you flip the orientation of space like in a mirror, the Higgs boson still acts the same). This is easier said than done. The Higgs Boson is now often referred to as the “God particle,” which appears to be named so because the Higgs Field gives mass to other particles. However, this name is actually an abbreviation—originally it was referred to as the goddamn particle, because it was so goddamn hard to find.

Experiments done at the Large Hadron Collider (LHC) near Geneva strongly suggested the existence of the Higgs field. The LHC is a particle collider, which means it operates by smashing high energy beams of protons together at very high velocities. Lower energy particle accelerators are used to make certain radioactive isotopes. However, the scientists working at the LHC had bigger fish to fry—they were looking for the Higgs Boson. A high energy particle collider is needed to find the Higgs boson since E=mc^2 means that the energy of the proton beams determines the mass of the particles it can create. The Higgs boson was predicted to have a relatively high mass so only the LHC had enough energy to produce it. In 2012, LHC scientists successfully discovered a particle that agreed with the Standard Model predictions– they discovered the Higgs Boson!

Discovery of the Higgs Boson is so exciting because it confirms a lot of fundamental questions about the universe. It validates the Standard Model and explains how certain particles get their mass. It also poses a lot of mathematical questions about symmetry. Scientists believe that the Standard Model isn’t a complete picture of the universe—it doesn’t explain gravity, dark matter, or dark energy.. However, by getting a better understanding of it and the Higgs Field, we are one step closer to developing a “Theory of Everything.”

Read more at the Nobel Prize website or the winning work here (Higgs’ 1964 paper) and here (LHC publication).

The STEM Chicksmas Day Two: Expanding Universe

The STEM Chicksmas Nobel Prize 2011 Physics

For the 12 Days of “The STEM Chicksmas” we’re highlighting 12 scientists who have contributed something innovative and exciting to their field. It is the season of giving, and these brilliant minds have given incredible gifts to the scientific community! This year we’re looking at 12 Nobel Prize winners from the past 15 years in the fields of Physics, Chemistry, and Physiology or Medicine.

Day Two: The 2011 Nobel Prize in Physics.

Source: Nobel Prize Summary and Nobel Prize Popular Information

Almost 14 billion years ago, the universe was infinitely dense and hot, and then it started expanding exponentially in what we call the Big Bang. Fast forward to today, and the universe is still expanding.  Up until 1998, however, most scientists thought this expansion was slowing down. It’s a logical conclusion—something can’t expand forever, right? However, research done by Saul Perlmutter, Brian Schmidt, and Adam Riess proved otherwise.

When Einstein was devising his theory of general relativity, he was so troubled by the idea that gravity should eventually cause the universe to contract that he added a term called the “cosmological constant,” which would allow for a static universe. When Hubble made observations that led to the conclusion that the universe was expanding, Einstein considered adding the cosmological constant to be the “biggest blunder” of his life. For most of the 20th century, astronomers thought the cosmological constant was zero.

The experiments performed by the prize winners’ respective research groups were simple enough in theory—identify a distant supernova that is sufficiently and consistently bright and take two photographs of it three weeks apart. (This is of course much less simple in practice.) They were hoping to measure the rate at which the universe’s expansion was slowing down, so they graphed the supernovae’s brightness (dependent on distance) versus its redshift (dependent on the object’s velocity: a light source continually emits light as waves, and as the object moves away from the observer the waves expand and thus become “redder”). However, after three weeks the supernovae were actually less bright than expected, which meant they had moved further away than they should have.  The best explanation for this phenomenon was if the expansion of the universe was accelerating rather than decelerating!

When we say the universe’s expansion is accelerating, we mean that the space itself between galaxies is actually increasing. Scientists aren’t sure what the cause of this is, so they refer to this unknown force as “dark energy.” One leading hypothesis is that space intrinsically has energy—in this theory, dark energy is represented by Einstein’s confusing cosmological constant. Quantum mechanical fluctuations could be the cause of this energy associated with empty space. However the prediction from quantum mechanics disagrees wildly with the observed value in the universe. This remains an unresolved problem in modern physics. Another confusing aspect of this dark energy is that it has only been the dominant force for the last 6 billion years—except for the time directly after the Big Bang, the universe’s expansion had been slowing down! Why that changed is unknown. One more astounding fact is that dark energy accounts for 68% of all of the universe’s mass-energy! Matter, like you, me and the Earth, makes up less than 5%.

The discovery that the universe’s expansion is accelerating rocked cosmology to its core. It found a place for the cosmological constant, something even Einstein couldn’t resolve, and it fundamentally changed our view of the universe. It also gives us a picture of what the fate of the universe might be—galaxies infinitely far apart from each other, and a cold and lonely place.

To read more, check out the Nobel Prize website here or the award winning papers here (Riess, Schmidt, et. al) and here (Perlmutter)