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Savannah Mitchem

The Ugliest but Most Talented Bats

Bats are not blind. In fact, they see just as well as humans do, and like humans, their night vision is mediocre. But at night, insects abound and competing predators are fast asleep, creating the perfect opportunity for bats to feast. During seasons when they are not hibernating, clouds of carnivorous bats fly out from caves and trees to hunt for insects using a process called echolocation that allows them to navigate and locate prey with sound. While hunting, they can eat up to 1200 insects an hour, making them indispensable to farmers.

A little over half of bat species echolocate. Most of these echolocating bats fall into a group called “microbats.” Although they are some of the smallest bats out there (it is not uncommon for a microbat to be the size of a human thumb), microbats are also some of the ugliest. Unlike their cuter counterparts, megabats, who basically look like foxes with wings and often only eat fruit, microbats have beady little eyes, wrinkled pig noses, and oversized ears. Consider them the pugs of the bat world. It is partly these oversized ears, however, that allow them to distinguish between objects half a millimeter apart.

GHOST BAT Diclidurus albus Yavari River Peru
Peruvian Ghost Bat (the most attractive of the microbats)

The process of bat echolocation starts in the throat. Bats emit a clicking sound, almost like a chirp, through their mouths into their surroundings. These sounds, referred to as “calls”, are mostly ultrasonic, meaning they are at frequencies that the human ear cannot detect. And this is fortunate for us, because many bats (like the Myotis Lucifugus) produce calls as loud as 120 decibels. For reference, this is louder than a fire alarm ten centimeters from your ear. Bats are also mostly deaf to their own calls. The bones that connect the ear drum to the inner ear detach from each other when the bat emits its call so that no vibration is sent to the inner ear.

The calls permeate the surrounding area, reflecting off of trees, vines, other bats, and prey as the bat hunts. The sound waves then return to the bat’s ears after being changed by the surroundings. It processes the discrepancies between ingoing and outgoing sound waves to construct a mental map of the area. Bats are only able to process one of these call-and-echo pairs at a time, rather than a continuous stream of sound.

Bat calls range between 0.2 to 100 milliseconds in duration. As the bat moves closer to its prey, the calls get shorter so the bat can more quickly process information about the prey’s location. The amount of time between successive calls also decreases as the bat approaches the prey. This is because the sound does not have to travel as far out and back, so the bat is able to update its mental map of the insect’s location more frequently.

There are two main strategies of calls that bats use when echolocating. The first is frequency modulated sweeps. This is when the bat’s outgoing signal covers a broad range of frequencies, usually starting from high frequencies and going to low all in one call. This is good for when the bat is in an area with a lot of noise, such as trees or other bats. There is more information going out, including different harmonics, and therefore more information can be gained from the resulting echoes.

The second call strategy is using constant frequency tones rather than changing the frequency within the call. This is used when a bat is flying in an open area without much interference. It requires less energy from the bat than frequency modulation does. Another advantage of constant frequency calls is the detection of doppler shift, or the shifting of the frequency of sound waves as the source of the sound moves relative to the observer. When the source is moving toward the observer, each wave is emitted at a distance closer to the observer than the previous wave. This causes the sound waves to reach the observer at a quicker and quicker rate (or frequency!). Think about when an ambulance drives past you on the sidewalk. The pitch of its siren is raised until it passes you, and then it gets lower.

Similarly, a bat call goes out at one frequency, and the echo from a moving insect comes back at a lower or higher frequency. From this change, the bat can not only tell that the insect is moving, but it also knows its speed. Not to mention that tiny vibrations from insect wings even allow bats to know if their prey is in flight.

bat calls
Bat calls of two different species. Notice how they get more intense the closer they are to the prey. Also notice the more frequency modulated calls in the top graph and the more constant frequency calls (though really a combo of the two) in the bottom graph. Source 

Even cooler is that bats compensate for doppler shift by raising or lowering the frequency of their calls so that the echo comes back at the optimal range of frequencies it can hear. This phenomenon was discovered by strapping a bat to a swing, facing it toward a wall, and recording the way the bat lowered and raised its call frequency depending on if it was going toward or away from the wall. Interestingly, different bat species that live in the same areas have evolved to hear best at frequencies distinct from one another so as not to interfere with the others’ hunting. How sweet of natural selection.

Before writing this, my experience of bats consisted only of the scene in The Office where Meredith contracts rabies after Dwight traps a bat in a bag around her head. And although they do carry disease and can look unsettling, microbats are incredible and important animals that can teach humans a great deal about using sound to navigate. For example, check out this article about a blind human man that uses echolocation!

References and cool stuff:

  •  http://chargedmagazine.org/2013/12/badass-biology- bat-echolocation/
  •  https://www.scientificamerican.com/article/how-do- bats-echolocate-an/
  •  http://www.bats.org.uk/pages/echolocation.html
  •  http://news.nationalgeographic.com/2015/07/150804-microbats-animals-bats-science/

Symmetry

We regularly see symmetry throughout nature and in what we create. The wings of a butterfly, the face of a loved one, and even the triangular Whataburger building across the street from me right now all display this common motif. Symmetry often provides an important underlying structure of the laws of nature, and it characterizes our universe and its ability to be studied and understood.

The type of symmetry we are most familiar with is geometrical symmetry, or symmetry in shapes. A square, for example, can be cut multiple ways down its middle to produce two identical halves. Stated another way, the square’s shape is identical to the shape of its mirror image when oriented certain ways. You may remember finger painting on a piece of paper in elementary school and folding it in half before it dries in order to create an intricate symmetrical pattern. This same type of symmetry often shows up in biological systems. The right side of your body is roughly a mirror image of the left.

In a broader sense, symmetry is defined as the ability of some property of an object or law to stay the same when something is done to it. In the case of the square, its appearance does not change when we change our point of view by flipping it over its middle or rotating it through multiples of 90 degrees. Similarly, a perfect sphere remains the same any way you spin it about its center.

Just like these shapes and objects have symmetry, physical phenomena can also have symmetry. For example, the path of a ball thrown into the air creates a parabola, and this parabola is a mirror image of the one the ball would make if time were reversed. Therefore, the path is symmetric in regards to the direction of time. The laws of nature themselves also have certain symmetries. For example, the laws describing the physical world do not change depending on where you are in the universe. They are the same whether you are over here or over there. In addition, these laws do not change with time. The same laws that dictated the behavior of the early universe dictate physical behavior now. Finally, a rotated system in space obeys the same physical laws as a non-rotated system. Note that I said a rotated system, as opposed to one that is rotating (this will be relevant later on).

Often we take these facts for granted. Of course the laws of physics have the same affect on a system whether you have rotated it or not. In fact, you could think of rotating a system as if it were merely you having rotated your point of view around a stationary system. Likewise, it is intuitive to us that if you drop a ball under certain conditions, and then took the ball to a different place with identical conditions, the ball would drop in the exact same way.

It didn’t have to be this way, though. There is nothing logically impermissible about a universe with laws that change with time, position, and angular orientation. Thankfully, we live in a universe where its behavior can be studied through experimentation. If these symmetries of the physical laws did not exist, there would be no point in doing an experiment multiple times or in multiple places because the laws might change between trials. Conclusions could not be drawn. The scientific method would be pointless. (Not to mention there is the chance we wouldn’t exist in any form in a universe without symmetry.)

These symmetries also have crucial implications in regards to conservation laws. Emmy Noether, an early 20th century German mathematician (described by Einstein as the most important woman in the history of mathematics), proved the theorem that with every symmetry, there is a corresponding conserved quantity. For example, the fact that the physical laws are symmetrical with respect to position means that momentum is conserved. Likewise, we know energy is conserved because of the time-symmetry of the laws. (The proof of Noether’s Theorem requires knowledge of higher level calculus, so you will just have to trust me on this one.)

Symmetry also shows up in quantum mechanics and relativity. For example, the speed of light is the same with respect to you no matter how fast you are moving! This is called Lorentz invariance. The laws acting on a moving system are the same as those acting on a stationary system, but fast motion causes observed lengths and times to expand and contract in order to keep the speed of light constant. In particle physics, for every particle there exists a type of anti-particle with identical mass but opposite charge. The curious thing is that there is, for reasons still unknown, a much higher amount of matter in the universe than antimatter. We know this because when a particle meets its anti-particle, they annihilate each other, and look how much matter there is around you that isn’t annihilated! This is what is called a “broken symmetry.”

Let’s talk more about what is not symmetric. The physics of accelerating systems is not the same for systems moving at a constant velocity. This includes rotating systems, as rotation is a type of acceleration. For example, the fact that the Earth is spinning causes what are called the Coriolis and Cetrifugal forces. These forces, though small and often neglected in calculations, are responsible for physical phenomena that would not exist on Earth if it were not rotating. For example, the Coriolis force is largely responsible for typhoons, and the Centrifugal force is what makes one weigh slightly (less than one percent) less at the equator than at the poles of the Earth.

Another non-symmetry is that of scale. Richard Feynman, in a lecture of his on symmetry, used the example of a small model of a cathedral made of matchsticks. If the model cathedral were scaled up to the size of a real cathedral, it would collapse because of the weakness of the wood to stand the increased amount of gravitational force from the Earth. He points out that some might say that if the Earth is having an effect on the system, it should be included in the system and scaled up as well. “But then it is even worse,” he says, “because the gravitation is increased still more!”

So, what are we to make of the fact that our universe is often almost symmetric? Scientists are baffled when they discover unexpected broken symmetries, such as the matter and antimatter discrepancy and other broken symmetries in particle physics. We think of symmetry as perfection, and anything less than complete symmetry as unnerving and disappointing.

In the same lecture as referenced above, Feynman describes an intricate gate in Neiko, Japan that has carved into it elaborate details that are completely symmetrical except for one small design on one side that was purposefully carved upside down. The story is that it was made slightly off from perfect because the men who built it did not want to make God jealous of their perfection. Feynman lightheartedly goes on to say, “We might like to turn the idea around and think that the true explanation of the near symmetry of nature is this: that God made the laws only nearly symmetrical so that we should not be jealous of His perfection!”

The snowflake photograph was done by Alexey Kljatov.

Here are some links if you want to read more about symmetry!

Feynman’s Lecture

Symmetry Magazine

A good article

Hairs Themselves are Bald, and Other Reasons Violins Can Make Sound


One of my violin instructors once told me that there were tiny hairs all over the violin bow that each grabbed the string and plucked it, and that this series of plucking is what we actually hear when someone bows a string. He played his violin gorgeously under this assumption despite the fact that it was irredeemably false.

             The violin is an intricate machine that goes through a complex process to produce its sound. This process begins with the interaction between the bow and the strings. Violin strings were originally made of animal gut, but most modern strings are made of metals such as aluminum, steel, and sterling silver. The violin has four strings tuned to the notes E, A, D, and G (from highest to lowest frequency). For reference, the G here is the one just below middle C on the piano.

Most bows are made of horse hair and rubbed with rosin (essentially solidified tree sap). The rosin keeps the coefficient of static friction between the bow and string high, while keeping the coefficient of kinetic friction relatively low. In simpler terms, it is hard for the string to become unstuck from the bow, but not hard for the string to slip and slide on the bow once it is unstuck.

This relationship produces a phenomenon called the “slip-stick” effect which is illustrated here (animation by Heidi Hereth). As the bow is drawn across a string, the string sticks to it and is pulled along with it. At some critical point, the force of the bow overcomes the static friction force that kept the string stuck to the bow, and the string is released. It slides all the way down to a point where it is picked up again, and the process repeats. This cycle occurs hundreds of times per second when playing a note on the violin. It is this pattern of tension and release caused by the bow that is “felt” by the bridge, the wooden piece that holds up the strings and connects them to the belly of the violin. The bridge vibrates, transferring this information to the rest of the body and eventually to your ears.

The pitch of the note you hear depends directly on the fundamental frequency at which the string vibrates. Frequency, when in the context of waves, is the number of full cycles that occur in a certain amount of time. This frequency is affected by the string’s tension, which is controlled by tuning the strings with the pegs that stretch or release the string. As the tension increases, the frequency increases. Even if you don’t have a violin handy, you can still hear this effect when plucking a rubber band stretched to different tensions.

The density of the string (the mass per unit length) is also a factor in the frequency of vibration. The lower three strings, A, D, and G, generally have a metal wire core and are wound in additional wire. This additional wiring increases the density of the string without affecting the tension. Higher mass per length causes the string to move more slowly, and therefore to vibrate at a lower frequency. Finally, the length of the vibrating string affects the frequency. When a violinist presses the string down to the fingerboard, she decreases the length of the part of the string that is free to vibrate, increasing the frequency as a result.

The fundamental frequency is the lowest frequency at which a certain string can vibrate. The violin string can also vibrate at harmonic frequencies, integer multiples of the fundamental. Generally speaking, the string will vibrate as a composite wave of the fundamental and many of its harmonics. The speed, pressure, and placement of the bow all have effects on the harmonic overtones of the produced sound. The combination of the fundamental and harmonic frequencies allows us to distinguish the sound of a violin from other instruments that make sound at the same fundamental frequencies. The harmonic makeup of the violin’s sounds characterizes its smooth and wilting timbre in contrast to, say, the brighter and raspier trumpet.

 

me-and-lucia
Me as a whipper snapper violinist in Italy. Mrs. Lucia was not messing around.

One interesting feature of the violin is that certain harmonic frequencies can be isolated from the fundamental and the lower harmonic frequencies and played by themselves. There are certain points on a bounded vibrating string called “nodes” where the string does not move, but either side of it is oscillating back and forth. Certain harmonics have nodes at specific places on the string, namely rational fractions of the total length. By placing a finger lightly (so as not to dampen the lower part of the string from vibrating at all) in one of these places, the string is forced to vibrate at a frequency that has a node in that place and where the string around it oscillates. Due to the geometry of these nodes and their corresponding harmonic frequencies, any harmonic lower than the one with a node in that specific place, including the fundamental, is unable to show up in the total makeup of the sound. The result is a light, high end whistle.

Even more complex and interesting physics happens between the time the vibrations reach the bridge and the time the sound waves reach your ears, but that is for another post. We have enough information now to prove my well-intentioned instructor wrong.

Violin strings vibrating by themselves make little to no sound. They must be attached to the body through the bridge for the vibrations to be amplified into audible sound waves. If the rapid plucking of the string by the “micro-hairs” (not to be confused with micro-taters which are equally as fun and infinitely more real) is what transfers the vibrations to the bridge, then that would mean that the faster you moved your bow across the string, the higher frequency the note would be.

It would also mean that the harmonic overtones so crucial to the unique sound of the violin would not exist, because any varying wave pattern you transfer to the bridge would be due to sporadic changes in the velocity of the bow rather than the relationship between the fundamental frequency, its integer multiples, and their geometries. Not to mention rosin would suddenly seem like a counterproductive measure. And finally, it would mean that horse hairs have horse hairs, which seems at least a bit off.

The Old and Slippery Neutrino

“She was radioactive for seven days

How I wanted to be holding her anyways

But the doctors, they told me to stay away

Due to flying neutrinos and gamma rays”

-Andrew Bird “Puma”

Indifferent toward you and your goings on, ten trillion incredibly small particles called neutrinos pass through your body each second. You never knew this because neutrinos are equally disinterested in how your body’s particles are charged to attract and repel, and they are so tiny (a neutrino’s size is to a speck of dust as a speck of dust is to our solar system) that gravity is largely unable to divert them from their mission: to do nothing but travel in a straight line toward the edge of the universe.

(I’m kidding, they aren’t sentient.)

Neutrinos are subatomic particles produced during radioactive breakdown of atoms. The majority of the neutrinos in the universe have been here since the beginning. They are also created by stars, our own particle accelerators, and phenomena in the atmosphere.

These elusive particles are, as the name suggests, neutral in charge. Actually, they were originally named the neutron before what we now call the neutron was discovered. Enrico Fermi then decided to be cute and call them neutrinos, which means “little neutron” in Italian. Because they are neutral, they are immune to one of the strongest forces in our universe, the electromagnetic force. This means that other particles do not attract or repel them with positive or negative charge. The only force that has any significant effect on neutrinos is the weak subatomic force, and for this force to act on a neutrino would require the neutrino to be extremely close to other particles, a rare occurrence.

In fact, a neutrino could pass through light-years of lead and still never come in contact with another particle. This is because matter is mostly nothing. Though it appears that you, or your table, or your soup, or your grandma are made of something relatively substantial and dense, the atoms that make up these things are pretty much entirely vacuum. And so the neutrino presses on, traveling at very near the speed of light and rarely interacting with any matter at all.

So how do we know anything about them?

During a reaction called beta decay, a neutron becomes a proton and an electron. In 1930, Wolfgang Pauli studied these reactions and found that the energy and momentum the proton and electron gain is less than the original energy and momentum of the neutron. This is inconsistent with the fact that energy and momentum are always conserved, so Pauli hypothesized about a “Ghost Particle” that is also spit out during a beta decay and holds the missing energy.

Decades later in 1959, the electron neutrino was discovered by Clyde L. Cowan and Frederick Reines with a huge vat of heavy water. I would like to point out that part of their experiments were done at the Savannah River Plant in Augusta, GA. (I used to live in Augusta, and I am moving there again soon. Plus my name is Savannah, so I feel like I was part of this experiment.) They received the Nobel Prize for this work in 1995, one year before I was born. (Okay I’m done.)

The particle Cowan and Reines discovered, the electron neutrino, is just one of three different “flavors” of neutrinos. There are also muon and tau neutrinos, which correspond to the muon and tau particles. The flavor of a neutrino becomes apparent only when it goes through a decay process to create other charged particles. And here, just as it does with the entire “small realm” of the universe, quantum makes it weird. We can only either know a neutrino’s flavor or information about its relative mass, but not both.

Neutrino mass has been a topic of intrigue, especially as of late. The Nobel Prize in Physics 2015 was given to Takaaki Kajita and Arthur B. McDonald “for the discovery of neutrino oscillations, which shows that neutrinos have mass.” This discovery was ground-breaking in the world of particle physics, because the Standard Model of particle physics, which has been widely accepted by particle physicists for the past century, predicted that neutrinos had zero mass!

“Neutrino oscillation” is the changing of a neutrino from one flavor to another. In 2001, it was observed at the SNO detector in Canada that two-thirds of the electron neutrinos coming to the detector from the sun had oscillated into the other two flavors before arriving. These oscillations involve quantum mechanical phenomena, and they require a non-zero neutrino mass, though the value of the mass is still unknown.

The fact that neutrinos are massive (tiny, but massive) gives rise to questions about its nature in relation to other particles and the rules they obey. Is the neutrino exempt from the general symmetry rules of the universe? Unlike other particles, does it gain its mass from a source other than the Higgs field? Does the neutrino experience time the same way other particles do? Do neutrinos have anything to do with dark matter (matter that mathematically should be in the universe but has not yet been detected)? Could neutrinos’ neutral charge and non-zero mass be the reason matter has been able to overpower antimatter, leaving us with the universe as we know it?

I’ll expand on that last question. Particles have anti-particles associated with them, and just as particles make up matter, anti-particles make up anti-matter. These anti-particles have the same mass but opposite charges (except in the case of the neutron’s anti-neutron which has a neutral charge but is made of anti-quarks as opposed to quarks) as their matter partners. Neutrinos are unusual particles in that they are not made of any smaller particle that we know of, and they are also neutrally charged. So what could be different about the neutrino and the antineutrino? It could be that a neutrino its own antiparticle. This would raise deep questions about symmetry, but it may answer questions, too.

Any time anti-matter collides with matter, both are annihilated, leaving nothing. It is unknown why the anti-matter and matter in the universe haven’t cancelled each other out yet, or why there would be an unequal amount of each. The answer may involve dark matter, and neutrinos and antineutrinos could provide some insight.

Despite our many unanswered questions about neutrinos, we know an impressive amount about them considering how difficult they are to detect. Ice Cube is a neutrino observatory in the South Pole whose website describes their detectors as “Butterfly Nets for Ghosts.”

Note: Just today, June 22, 2016, two critically ill researchers were rescued from the Amundsen-Scott South Pole Station in which Ice Cube is located. The names of the researchers were not released, but there is a good chance they worked on Ice Cube. This is the third time ever that a successful rescue has been made during the South Pole’s winter season lasting from February to October. It is generally much too dark and cold to risk flying a plane there.

To detect these slippery particles, the observatories are built underground, shielded from the noise of cosmic rays. Some detectors, such as Super-Kamiokande in Japan, use enormous vats of water with photo-multiplier tubes lacing the sides to capture the effects neutrinos leave when they interact with the atoms in the tank. Super-K is filled with 50,000 tons of water and detects about 18 neutrinos a day. It was the primary detector used by the 2015 Nobel Laureates to prove the neutrino’s non-zero mass. The original Kamiokande detector, before Super-K, detected 19 out of the supposed billion trillion trillion trillion trillion neutrinos resulting from the explosion of a star in the Large Megallanic Cloud.

Neil neutrino
Neil Degrasse Tyson floating around in Super K (Credit: facebook.com/COSMOSonTV)

A similar detector called ANTARES lies 2.5 kilometers under the Mediterranean Sea. It consists of twelve long, floating strings of photomultiplier tubes sensing neutrinos that go through the water around it.

Those photomultiplier tubes detect what’s called Cherenkov Radiation. When a neutrino contacts other particles, their energy and mass will turn into different particles themselves. These new particles (electrons, muons, or taus depending on the neutrino’s flavor) travel at a speed faster than light inside that medium. These new charged leptons affect the electrons in the atoms around it as it travels, often leaving a blue ring around its path. This distortion is what the photomultiplier tubes detect. But all in all, only about a few thousand neutrinos are detected a year, and many of these detections have no information about the neutrino’s flavor.

As we discover more about the neutrino, I hope to write updates about them and post them here. These little buggers have opened my eyes to how mind-boggling particle physics can be, and I hope they’ve intrigued you all, too. Regardless though, they’ll keep passing through you by the trillions, slipping through the cracks of your matter and wandering the universe without an atom for a home.

“Do you see particles in the air?

Unguided particles in the air

Do you see particles in the air?

Nobody notices, nobody cares”

-Andrew Bird, “Puma”

I care, Andrew.

 

References:

Super K Website:

http://www-sk.icrr.u-tokyo.ac.jp/sk/index-e.html

Ice Cube Website:

https://icecube.wisc.edu/info/neutrinos

Nobel Prize Info:

https://www.nobelprize.org/nobel_prizes/physics/laureates/2015/press.html

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