The Mad Scientist Diary

The ideas, experiences, and projects of a mad scientist-in-training.

When powdered hot chocolate is stirred into a cup of milk, the pitch produced by tapping on the mug changes drastically as the powder is stirred in. This phenomenon is often called the hot chocolate effect, but it can also be observed with other drinks such as instant coffee or cold beer. 

In every case, the change in pitch is caused by the presence of air bubbles in the liquid. These bubbles are stirred into the mixture along with the powder ( giving the hot chocolate a thicker, foamy texture). However, since the speed of sound is much higher in water (or milk) than in air, these air bubbles lower the effective speed of sound in the hot chocolate.  This in turn lowers the pitch at which the mug resonates when tapped. 

Over time the gas bubbles dissipate, accounting for the steady rise in the pitch of the tapping after the stirring stops. Stirring the hot chocolate again reintroduces the bubbles, starting the effect all over again. You can even hear the pitch lowering as you stir by listening to the sounds made as the spoon accidentally hits the side of the mug.

Quantum theory predicts that single photons can interfere with themselves, just like waves, when passing through a pair of slits. This may seem weird enough, but photons don’t always act like waves! In fact, we can change whether a photon acts like a particle or a wave merely by looking at it.
The image on the left was captured while sending a beam of single photons through a pair of slits. As discussed in my last post the photons are acting like waves, producing an interference pattern. In this experiment we know nothing about the individual photons passing through the apparatus. Most importantly, we don’t know which of the two slits an individual photon has passed through.
In the image on the right, I modified the experiment so that I could tell which slit each individual photon had passed through. Suddenly, the interference pattern disappears! By making the measurement I forced the photons to act like particles, which do not interfere. 
Amazingly, the only change made between the experiments above was adding a filter between the slits and camera that detected the interference pattern. This filter changed how the photons interacted with the slits, even though they passed through the slits first! I think this is one of the coolest examples of the weirdness of quantum mechanics.

Quantum theory predicts that single photons can interfere with themselves, just like waves, when passing through a pair of slits. This may seem weird enough, but photons don’t always act like waves! In fact, we can change whether a photon acts like a particle or a wave merely by looking at it.

The image on the left was captured while sending a beam of single photons through a pair of slits. As discussed in my last post the photons are acting like waves, producing an interference pattern. In this experiment we know nothing about the individual photons passing through the apparatus. Most importantly, we don’t know which of the two slits an individual photon has passed through.

In the image on the right, I modified the experiment so that I could tell which slit each individual photon had passed through. Suddenly, the interference pattern disappears! By making the measurement I forced the photons to act like particles, which do not interfere. 

Amazingly, the only change made between the experiments above was adding a filter between the slits and camera that detected the interference pattern. This filter changed how the photons interacted with the slits, even though they passed through the slits first! I think this is one of the coolest examples of the weirdness of quantum mechanics.

In my last post, I explained how Thomas Young’s double slit experiment convinced the world in the 19th century that beams of light behaved like waves. A little over a century later, interference patterns like the one above shook up the world once again. This image was produced while allowing only a single photon pass through the slits at a time, confirming the quantum mechanical prediction that single particles of light behave like waves, and are able to interfere with themselves!
I took the picture above with a special camera that is sensitive to single photons. In order to create a beam of single photons, we start with a normal laser beam. We then pass the beam through a series of filters, each of which weakens the beam by a factor of one hundred. After two such filters, the beam is so weak that only a single photon is passing through the slits at a time. 
Quantum theory says that, until we observe it, a photon does not have a definite location but is instead spread out as a cloud of possible locations called the wave function. Since this probability distribution behaves like a wave, it interferes with itself as it passes through the slits, creating a probability distribution that looks like an interference pattern. When each photon is measured by the camera, its wave function collapses, causing it to “chose” a single pixel to trigger on the camera. However, over time the number of photons seen builds up according to this probability distribution, making visible the interference pattern pictured above. 
Single particles interfering with themselves is pretty weird. However, interference experiments can get even weirder! It turns out that if we try to watch which slit the photon passes through, the interference pattern disappears altogether. I’ll explain more about this behavior in my next post. 

In my last post, I explained how Thomas Young’s double slit experiment convinced the world in the 19th century that beams of light behaved like waves. A little over a century later, interference patterns like the one above shook up the world once again. This image was produced while allowing only a single photon pass through the slits at a time, confirming the quantum mechanical prediction that single particles of light behave like waves, and are able to interfere with themselves!

I took the picture above with a special camera that is sensitive to single photons. In order to create a beam of single photons, we start with a normal laser beam. We then pass the beam through a series of filters, each of which weakens the beam by a factor of one hundred. After two such filters, the beam is so weak that only a single photon is passing through the slits at a time. 

Quantum theory says that, until we observe it, a photon does not have a definite location but is instead spread out as a cloud of possible locations called the wave function. Since this probability distribution behaves like a wave, it interferes with itself as it passes through the slits, creating a probability distribution that looks like an interference pattern. When each photon is measured by the camera, its wave function collapses, causing it to “chose” a single pixel to trigger on the camera. However, over time the number of photons seen builds up according to this probability distribution, making visible the interference pattern pictured above. 

Single particles interfering with themselves is pretty weird. However, interference experiments can get even weirder! It turns out that if we try to watch which slit the photon passes through, the interference pattern disappears altogether. I’ll explain more about this behavior in my next post. 

The image above shows what happens when you shine a laser through a pair of very small and very close together slits. This distribution of light is called an interference pattern, and is the quintessential example that light consists of waves. 
Like waves on the ocean, light waves have both crests and valleys. When a crest meets a crest, they reinforce one another to form a still larger crest. However, when a crest meets a valley, they completely cancel out. This process is called interference. In the double slit experiment described above, each slit acts as a source of light waves. As the two waves cross they interfere, creating dark spots where crests meet valleys. These dark spots then become the dark bands of the interference pattern. 
For around a hundred years after this experiment was first performed by Thomas Young in 1803, it was seen as proof that light must be a wave rather than a stream of tiny particles. However, quantum mechanics now tells a different story. In my next post, I’ll describe the bizarre behavior we observe when we allow only a single photon, a single particle of light, to pass through the slits at a time.

The image above shows what happens when you shine a laser through a pair of very small and very close together slits. This distribution of light is called an interference pattern, and is the quintessential example that light consists of waves. 

Like waves on the ocean, light waves have both crests and valleys. When a crest meets a crest, they reinforce one another to form a still larger crest. However, when a crest meets a valley, they completely cancel out. This process is called interference. In the double slit experiment described above, each slit acts as a source of light waves. As the two waves cross they interfere, creating dark spots where crests meet valleys. These dark spots then become the dark bands of the interference pattern. 

For around a hundred years after this experiment was first performed by Thomas Young in 1803, it was seen as proof that light must be a wave rather than a stream of tiny particles. However, quantum mechanics now tells a different story. In my next post, I’ll describe the bizarre behavior we observe when we allow only a single photon, a single particle of light, to pass through the slits at a time.

As a fun side project this summer, I fixed this old water bottle rocket launcher that was used in an old physics class. The physics department’s PREP program (a summer camp for high school girls) then built rockets for the launcher, which we fired off on the university’s main quad. 

Unlike the simple water rocket launchers I built as a kid, this launcher is capable of shooting rockets up to several hundred feet into the air. The rockets (made out of soda bottles) are placed on the pipe at the top of the launcher. They are pressed down against an o-ring to create an airtight seal, then wedged in place with a metal clip. The soda bottle can then be pressurized (through the pipe) to up to 100 psi. When the clip holding the rocket down is finally pulled away the pressure is released, propelling the rocket upwards on a column of water. This rapid decompression of the bottle vaporizes some of the water inside, forming a miniature cloud inside the bottle! 

We had mixed results with our first round of rockets, but the whole process was so much fun that I plan on building some of my own this semester. 

Thanks to Rebecca Galasso, another undergraduate at the University of Rochester, for the fantastic blast off image above! 

The picture above is the culmination of my last two summers of work: a trapped cloud of extremely cold rubidium atoms!

These atoms were cooled in a magneto-optical trap or MOT (described in a previous post here) that uses lasers to create an “optical molasses” that slows the atoms down. The resulting cloud is less than a millimeter in diameter (the bright spot in the center of the hazy purple cloud in the second image) but contains many thousands of rubidium atoms.

While I have not yet measured the temperature of these atoms, typical temperatures in a MOT are measured in hundreds of micro Kelvin above absolute zero. Put in perspective, that makes these atoms several thousand times colder than outer space. 

At such incredibly low temperatures atoms start exhibiting strange quantum behavior. Studying the properties of these clouds of cold matter allows scientists to probe deep into the mysteries of quantum mechanics and will hopefully help answer fundamental questions about its laws. 

The video above shows the traces of individual subatomic particles in a cloud chamber here at the University of Rochester.

A cloud chamber is a sealed container containing a fine mist of very cold alcohol vapor. The vapor forms a thin cloud that is on the verge of crystallizing to form snow. Just as clouds in the atmosphere are triggered to precipitate by small particles of dust, the alcohol vapor in the cloud chamber will form snow when a charged subatomic particle passes through. The snow formed by an individual particles forms a white streak in the chamber as seen in the video above.  

The majority of the particle tracks seen in the video are emerging from the rod in the center of the screen which contains the radioactive element thorium. Each time a thorium atom decays, a fragment consisting of two protons and two neutrons (an alpha particle) is emitted. These particles form the traces seem emanating from the rod: each trace indicates that another atom has just split. 

If you look carefully in the video, you may also see some traces that are clearly not coming from the rod. The majority of these particles are likely solar muons created high above the earth where the radioactive solar wind bombards the atmosphere. 

The beautiful crystal shown above is a polarizing beam splitter cube (PBSC) that will be part of the magneto-optical trap that I am building. The purpose of the cube is to split a beam of light into two beams with different polarizations. 
The polarization of light can be visualized by thinking about waves travelling along a string. If the end of the string is moved up and down, a vertical wave will travel along the string. This wave is said to be vertically polarized. Similarly, waving the string from left to right would produce horizontally polarized waves. Light waves behave much these waves on a string and can also be vertically or horizontally polarized. 
A polarizing beam splitter cube consists of two triangular prisms glued together to form a cube. When a light beam shone through one face reaches the boundary between the two prisms, one polarization passes straight through while the other is reflected at a ninety degree angle. The two output beams then exit through separate faces. The colors are caused by a chemical anti-reflection coating that helps minimize reflections of the light off the surfaces of the cube. 

The beautiful crystal shown above is a polarizing beam splitter cube (PBSC) that will be part of the magneto-optical trap that I am building. The purpose of the cube is to split a beam of light into two beams with different polarizations. 

The polarization of light can be visualized by thinking about waves travelling along a string. If the end of the string is moved up and down, a vertical wave will travel along the string. This wave is said to be vertically polarized. Similarly, waving the string from left to right would produce horizontally polarized waves. Light waves behave much these waves on a string and can also be vertically or horizontally polarized. 

A polarizing beam splitter cube consists of two triangular prisms glued together to form a cube. When a light beam shone through one face reaches the boundary between the two prisms, one polarization passes straight through while the other is reflected at a ninety degree angle. The two output beams then exit through separate faces. The colors are caused by a chemical anti-reflection coating that helps minimize reflections of the light off the surfaces of the cube. 

Here’s another successful test of our hybrid rocket motor! This test shows off one of the coolest properties of a hybrid rocket: variable thrust.

The thrust is controlled by the flow of oxygen and can thus be set by simply adjusting the pressure of oxygen coming from the regulator into the back of the motor. The motor can even be turned off and then back on again over short periods of time, as shown in the video. 

Conventional solid fuel rockets (like the solid rocket boosters used to launch the space shuttle) cannot be stopped once they are lit, making them pretty dangerous. Hybrid rocket motors may someday offer a much safer alternative. 

The first test of our hybrid rocket motor was a success! 

In order to light the motor we allowed a small amount of oxygen to flow into the chamber and then inserted a lit match into the nozzle. We then turned up the flow of oxygen until the acrylic ignited. Once lit, the thrust of the motor is determined by the amount of oxygen we are letting into the chamber.