Monthly Archives: February 2013

Solid or liquid

You may have heard of oobleck, but if not, you are missing out greatly. Ooobleck consists of 1 part water mixed with about 2 parts cornstarch. It is a non-Newtonian fluid. Don’t be intimidated by the name, it’s lots of fun.

Non-Newtonian fluids differ from Newtonian fluids in one simple way, they do not hold a constant viscosity. With most fluids, the ability to flow (viscosity) is a variable that is constant for any given temperature and pressure. The viscosity of non-Newtonian fluids, on the other hand, is also affected by stress.

What this means is that if you take a bowl of oobleck and flip it sideways, the fluid will slowly ooze out of the bowl like any other highly viscous material (syrup or honey for example). The difference comes when you decide to squeeze it in your hand. Syrup and honey will obviously ooze out of your hands and make a mess, but the non-Newtonian fluid does not follow suit. The stress applied by your grip increases the viscosity of the material so greatly that it is essentially in a solid state as you grip it in your hand. Once the stress is released though, it will return back to its oozing self.

Time for some fun. All you need is cornstarch, water, tape, cellophane, and a sub-woofer. The force caused by the low base of a sub-woofer turned up nice and loud causes the oobleck to jump around as it is fluctuating between the solid and liquid state due to the changing force on the material. IMG_0397 (The cellophane and tape are just so that your sub-woofer doesn’t end up a mess). How well this works also depends on the quality of the oobleck, so don’t be afraid to mess with the ratio to find the best combination (1 part water and a range of 1.5-2 parts cornstarch). If you’ve ever watched the Big Bang Theory, you may have seen the episode with oobleck in it. Mythbusters also had some fun running on a pool of this stuff.

Jerry

A long time ago, in a galaxy far, far away – more specifically, 950 light-years from Earth, about 5.58×1015 miles away – a star was born. Two actually. Twins.

Last week, on February 7, 2013, NASA released a time-lapse movie of a pulsing light in the sky taken by the Hubble Space Telescope. The star-system’s name is LRLL 54361. We’ll call it Jerry.

pulse star

Jerry is actually a binary system, with two stars at its center, gravitational bound in rotation with one another. He is still just an infant, though, both of his stars being protostars (the protostellar phase is early in the process of star formation). The flashing, pulsing light seen by scientists is called pulsed accretion. This system is one of only three “strobe-light” systems ever found; Jerry is actually “the most powerful stellar strobe found to date,” letting out a pulse every 25.34 days (Clara Moskowitz). “The strength and regularity of this accretion signal is surprising; it may be related to the very young age of the system, which is a factor of ten younger than the other pulsed accretors previously studied.” Jerry’s youth, though, provides both opportunities and difficulties to scientists.

Since this system is in its early stages of life, its stars are surrounded by a dense disc of gas and dust. This causes makes it hard for scientists to study Jerry’s twin stars, but it also is in large part the reason the system was found. Scientists believe that the bright pulsing is a result of the dust and gas. The two stars drag some of the gas and dust along around them, and when they come near to one another in their orbits, the materials are  collected by the other star, causing a blast of radiation – the pulsing, strobe nature of Jerry.

Furthermore, the surrounding disc of gas and dust create a “light echo.” The flashes of light (blasts of radiation just mentioned) propagate through the dust and gas and reflect toward earth, causing the light released to be enhanced, creating the increased brightness seen by scientists.

hubble                             spitzer

With the discovery of LRLL 54361 scientists hope to gain knowledge into the formation of stellar systems. Further studying of the system using the Hubble and Spitzer telescopes will lead to insights of the nature of pulsing stars. With luck and these huge telescopes, scientists can discover why the pulsing occurs and what determines the period of alternating luminosity. They will learn more about the binary systems, especially those in the early stages of life. These insights will help further our understanding of solar systems and even the origins of our entire universe.

Colder than Cold?

A team of physicists at the University of Munich, headed by Ulrich Schneider, have generated negative temperatures. We’re not talking Fahrenheit here folks, and I don’t mean Celsius. I am talking about Kelvin. The scale that starts at a true zero temperature, absolute zero that is. The point at which atoms are said to have zero kinetic energy, zero entropy; they are at a standstill. So how do you get below this?

It has to do with the way temperature and our universe work. At any temperature, the kinetic energy is just an average value. The majority of atoms exist at very low energy levels while a couple more energetic atoms are at higher levels. The distribution of these is represented by the Boltzmann distribution:

boltzmann

 

It shows that there is a range of velocities, with certain velocities having greater probabilities than others. The lower the temperature, the greater probability of finding atoms of a particular velocity (the red graph is not as wide, and has a lower average velocity, with a higher peak at that temperature). This distribution of atoms can be thought of using potential wells and hills.

temp well

At a low temperature, the majority of atoms, with low kinetic energies, will settle into the potential well as shown on the left figure. As temperature is increased, the Boltzmann distribution widens greatly, as seen in the first figure. This means that there is a very large range of kinetic energies for the atoms, represented by the middle of the second figure; atoms exist in a chaotic pattern, with many velocities, therefore particles exist at all energy levels, in the well, on the plain, and on the hill.

Looking at these two figures, we can picture what zero kelvin as well as infinite kelvin would look like. At zero kelvin, all of the particles would exist in the potential well. The probability graph would not be a curve. Every particle would have a velocity of exactly zero. At an infinite temperature, the probability graph would be spread out to the point that every velocity would have the same, extremely low, probability. It would look like a flat line close to zero on the y-axis extended forever in the x-direction. The particles therefore exist at all possible locations on the “well and hill” picture, similar to the middle of the second figure. So if that’s what infinite temperature looks like, how is the right most configuration of that picture formed?

It is negative kelvin.

The scientists at the University of Munich discovered a process that makes this possible.

“Schneider and his colleagues began by cooling atoms to a fraction above absolute zero and placing them in a vacuum. They then used lasers to place the atoms along the curve of an energy valley with the majority of the atoms in lower energy states. The atoms were also made to repel each other to ensure they remained fixed in place.

Schneider’s team then turned this positive temperature system negative by doing two things. They made the atoms attract and adjusted the lasers to change the atoms’ energy levels, making the majority of them high-energy, and so flipping the valley into an energy hill. The result was an inverse energy distribution, which is characteristic of negative temperatures.”

– Jacob Aron

temp flip

As I said earlier, the majority of atoms in any system exist with low kinetic energies, with a few particles at higher energy levels. The scientists at the University of Munich switched this, creating the existence of a system where the majority of the particles have high energies and only a few are at lower energy levels. This creates the configuration at the right of figure two.

“The resulting thermometer is mind-bending, with a scale that starts at zero, ramps up to plus infinity, then jumps to minus infinity before increasing through the negative numbers until it reaches negative absolute zero, which corresponds to all particles sitting at the top of the energy hill.”

Clay Dillow

This negative zero value is important because it is actually the highest possible temperature. All of the particles exist on the top of the hill, but they don’t like most hill representations do. Rather than being an unstable equilibrium, it is actually stable. Energy must be added in order to get the particles away from the stable equilibrium. Because the system was created in a vacuum, there is no energy for the particles to gain energy, therefore they stay in the equilibrium. Another way to look at it is in terms of kinetic energy. If the particles were to “roll” down the hill, they would have to gain kinetic energy. The problem is still the same though, there is no energy for them to gain in the vacuum.

And if you know anything about science, you probably know the next question that is asked (you are probably thinking it yourself). Why does it matter?

Studying this negative temperature scale is believed to give us insights into dark matter, which scientists believe have negative temperatures as well as negative pressures. Some hypothesize that this fact is why our universe is expanding at an accelerating rate. On the more practical side, scientists believe that the discovery of negative temperatures can lead to super efficient engines that can absorb energy from both hot and cold substances.