Oooooobleck!

Maximizing the amount of sleep I get lately has been one of my top goals. Last night I got plenty of sleep and felt energetic and ready to head to class. I grabbed a quick breakfast to eat on my way/in class and then walked over to our class with Alex because Julia had already left. Alex and I were a little early today, so I had time to say hello to my friend Mikael before class began. Today was unique because Bill tied together many of the ideas we have been studying in the last few weeks into one large goal for the next couple of days – measuring the speed of light-.

To be a bit more specific, over the last several weeks, we have studied mechanics, optics, learned to use oscilloscopes and many other neat things. These topics are being united in a way because measuring the speed of light we will need to use our knowledge that we gained here to put together a device that is essentially a laser that sends out a beam of light that we split, reflect, and carefully time how long the pulse takes to return to our oscilloscope which we learned how to use last week.

To be honest, actually walking into the lab and seeing the dozens of wires and pieces of equipment that we had to hook together I felt like we wouldn’t be able to actually create the device that we needed to. With some excellent teamwork we actually managed to put together several of the circuits before we asked for help. Our teachers all have a nice approach to helping us where they try to steer us in the right direction and only interfering if we’re totally stuck. Today, Mary helped us several times when the wiring diagrams got a little overwhelming or we otherwise got stuck. I was very grateful for her help as well as Bill and Ryan’s and sooner than we expected we had constructed our light-speed measuring apparatus.

Next up, we were lucky to get to hear from another one of Penn’s faculty, Dr. Phil Nelson. Something that I’ve found great about the UPenn faculty we’ve gotten to listen to is that while they are very well educated and deeply into very specific research, they have all been excellent speakers as well. Phil spoke today about light and color. The first thing that he did was demonstrate how our eyes combine colors to make other colors. He showed us a pure yellow slide (which he proved was only yellow by shining it through a prism) and then showed us how he could overlap a red slide and a green slide to create an identical looking yellow. However, the mixed yellow could be separated back out into red and green. Then Phil began to throw some questions at us, why do our brains combine these colors instead of sorting them out? Can electronics make superior vision? And what are the applications of this knowledge?

Phil began to answer his first question by speaking about a man named Thomas Young. Young made many very accurate predictions about color and our eyes almost 200 years before his hypotheses could be confirmed. Perhaps the most consequential hypothesis that Young made is that the response of the photoreceptors in our eyes to light is equal to the Intensity times the Sensitivity. Thus our photoreceptors must sacrifice some clarity in order to be sensitive to a wider range of colors. For evolutionary reasons, humans have evolved to have three different types of photoreceptor cells that each specialize in decoding what we call red, green, and blue light. Because these photoreceptors are tuned to these colors, our brains must guess when something is in between these colors and that is where the red and green look yellow. Our brains see that the red and green photorecetors are reporting the same amount of light and ‘average it’ to yellow. Phil then talked about the many applications of this knowledge. Perhaps the most obvious is that LCD and Plasma screens each use different combinations of red, green, and blue light to make every color they display. The other applications are countless, and once again I was impressed by how articulate and relevant Phil was.

In the afternoon, it was time for my group to present what we have been working on for the last several days. As a brief synopsis, we have been experimenting (playing) with a mixture of cornstarch and water called oobleck. Oobleck is classified as a non-Newtonian fluid because it behaves as a fluid when you move something slowly through it, but it behaves as a solid when you quickly try to move through it. This has some interesting results, because you can slowly sink your hand into it and it’ll get wet and covered in oobleck, but if you punch it as hard as you can you’ll probably end up breaking your knuckles. Today, my group displayed these properties by making four huge tubs of oobleck and then having the entire class run across the surface. Watching them do this was a bit of a cruel natural selection process because the faster students made it out unscathed while those who hesitated sank in and then had to struggle to get out.

Overall, today was a lot of fun because I worked hard with my group in the morning and overcame obstacles, I enjoyed hearing another incredible lecture, and I had fun making a mess in the afternoon.

Oh, and my favorite thing that Bill said today was about how earning a PhD is beneficial, “By learning everything about something, you learn something about everything.” I don’t know if my educational interests will take me in the direction of a PhD, but I thought that quote really summed the idea of a PhD up nicely.

Man-made Quicksand in the Rain

Even though it had only been three days, while I was walking to class it felt like it had been a very long time since I had last been in the DRL (David Rittenhouse Labs). Today’s class was challenging, to say the least. We began class with Bill explaining how exactly we are going to measure the speed of light. In order to measure the speed of light, we will split a pulsed laser beam right at the tip of a laser pointer. Half of it will signal one channel on our oscilloscope. The other half of the beam will travel the length of the room and will be returned by a mirror. A lens will focus the returning beam on a photodiode that is connected to the second channel on our oscilloscope. The separation between Channel 1 pulse and the Channel 2 pulse indicates the time taken by light for the round trip. That doesn’t sound too complicated, right? In practice, however, it is pretty confusing. In order to get all of the apparatuses set up, a series of circuits have to be made, certain wires have to be connected in just the right places, and all of the pieces have to come together and work in unison so an accurate measurement can be obtained.

As Bill went through the procedure in his lecture, it seemed easy enough to follow, but this was not the case when we had to assemble everything. Progress was slow. We managed to get the laser working, but after that we were having trouble making our circuits. Thankfully Ryan, one of the TAs, was able to point us in the right direction and we were able to get everything connected so that it should have worked. However, it did not work for some reason. We spent the rest of our time in the lab making adjustments to the oscilloscope and our circuits, but to no avail. Before we could find the correct arrangement, it was time for our guest lecture. We will have to figure out our problem(s) tomorrow in class. Thankfully we get two days to do this lab.

Our guest lecturer was Dr. Phil Nelson, who is a part of Penn’s Nano-Bio Interface Center, and the Institute for Medicine and Engineering. His lecture was about light and its erratic behavior. The lecture was actually half bio-physics, half optics. He began by combining different colors of light and splitting white light into a rainbow by using a prism, but as the lecture continued he began talking about how our eyes and brain perceive colors. He explained that color is of vital importance our survival because it is a major factor in how we identify things in our environment. He also said that our eyes omit a lot of information and that they only pick up certain spectral colors. What I found the most interesting was his discussion of photons. He told us that photons arrive at random, no matter how hard we try to make a steady light. Their average rate corresponds to what we think of as brightness. In class, we have mostly been referring to light as a wave. We have acknowledged that it was also a particle, but this was the first time that we had every really analyzed light in that respect.

Phil Nelson discussing the interaction of different spectras of light

After lunch, we returned to the labs for the most fun part of the day. The ooblek group was finally going to get to show off what they have been learning for the past week by preparing small pools of the mixture. They prepared four tubs of the muck and, when it was all ready, they had people run across it. Most of the class ran across, save about eight or so people, and it was very fun to watch. I didn’t run because I was taking pictures and videos for almost the entire time, but it was fun nonetheless. Some decided to just run across, while others just walked across to see what it would be like to try to get out of quicksand, which is basically what ooblek is. We spent the entire second half of class outside the DRL playing in ooblek and it was one of our best sessions in class thus far.

Run!

Brian helps Bill mix the ooblek

After class, Onur, Brian, Julia, and I returned to the dorm and began our PowerPoint presentation on Newton’s Law of Cooling. It was actually easier than I had anticipated. We already knew everything we needed to know about the law, so we just had to organize our thoughts, data, and pictures. We finished preparing the presentation in about an hour. Now we just have to decide who will be covering which part(s) of the presentation and rehearse our information. I am confident that we will be able to do this with little difficulty.

A quick dinner was next on our agenda. Onur, Fred, Brian, and I headed over to the commons, while Julia went to go play squash with Abheek. When Onur and I exited the commons (Brian and Fred left dinner early to go to the gym), we discovered that it was raining. This was different rain than what we have been getting though. This rain actually fell for a good hour, maybe more! I am unsure of the exact time, but I do know that it was very refreshing and it got rid of the humidity in the air. It was actually such a welcome occurrence that Onur, Abheek, Julia, and I hung out in the middle of quad for the duration of the rain. It reminded Abheek of his home in London, just as much as it reminded me of rain back home in the Bay Area. It was a nice change, especially after that massive heat wave last week.

The rest of our evening was spent lounging in our dorm. Fred and Onur hung out for a while and we watched funny YouTube videos while we listened to music over Onur’s speakers. We are all well aware that our time together is getting shorter. Fred even began packing up clothes that he didn’t think he was going to wear in the next four days. It is sad to think about the fact that our family will be disbanding so soon, but our strong friendships and memories of the fun times that we have had over the past three weeks will make it easier to deal with. I look forward to the next few days, both because of class and the last few nights with friends. Tomorrow is another day, and I’m ready to face it, even if it means dealing with that confounded contraption we are using to measure the speed of light. On that note, good evening.

Physicists Are Not Afraid to Get Their Hands Dirty

I am continually forced to rethink my definition of "most exciting class ever" with Bill's physics course. You'd think roller coasters would prove hard to beat, but literally every class brings something new to get excited about. Today, it was more than something; it was everything. Even just the set up for the lab we are going to perform tomorrow to measure the speed of light got my blood pumping.

Admittedly, the first hour of class was incredibly hard to follow. Despite Bill's exceptional ability to eloquently explain complicated topics, describing how to build a circuit with a light-sensing diode or a laser controlled by a transistor and connected to function generator is no easy task.

Consequently, it wasn't until we broke up into our new lab groups and got to fiddle with the wires and the circuit boards that the diagrams we were given (on the left) started making any kind of sense.

At first it was confusing and slightly frustrating, especially because Bill specifically told us that he had it all set up last night so everything was working perfectly, but disassembled it just so we would have to put it back together. Once we transformed the diagrams and mess of wires and banana plugs into the mechanisms we would be using to perform our experiment, I understood why he did it. Even before the experiment began, I already felt a sense of pride and ownership regarding the data we are going to collect with the gadgets that we built ourselves. Not only that, but we have a much deeper understanding of how our tools work and therefore what the numbers that come up actually mean.
Each week it seems we delve even deeper into the realm of what experimental physics actually feels like. In this further step, not only are we collecting our own data, but we are building our own tools with which to do it and it feels amazing. If this ends up being what I do for a living, I will have no objections.

After the lab setup, guest speaker Phil Nelson spoke about the physics of human and superhuman color vision. He began with a puzzling paradox about yellow light. First he showed us pure yellow light, visible in a spectrum created by shining white light through a prism. Next he showed us a square of yellow light made by a projector. When he diffracted this yellow light with the prism, he did not get yellow light, instead he got red and green. The question that he posed was why our eyes were unable to distinguish between the two (true yellow light versus the red/green mixture).

To explain this phenomenon, he described to us the way our eyes perceive color. Specifically, that the photo receptors in our eyes are capable of distinguishing between blue, green and red wavelengths. These can be expressed as curves showing how many photons of each particular color are sensed. The reason we are unable to distinguish true yellow from the mixture now becomes clear: our eyes can only measure the relative presence of green and red light. Therefore, because yellow neighbors both green and red on the spectrum, true yellow light contains traces of wavelengths from both red and green and so is indistinguishable to our tri-color sensitive eyes.

He went on to suggest that if we create a machine with the ability to detect more than three discrete wavelengths, it proves useful in a number of fields. Spectral karyotyping, for instance, uses such machines to detect subtle differences in stain colors of chromosomes, allowing scientists to conduct studies regarding genetic mutation, cancerous cells, and the genetic history of evolution. Superhuman color detection also allows scientists to maps the connectomes of mouse brains. When Nelson mentioned this, it really got my attention because I had seen the images he had displayed before on a podcast - the very podcast that got me interested in neuroscience in the first place. I was not aware when I watched it however, how interdisciplinary the sciences that were used in developing the method were. In fact, it is remarkable how often the guest lectures tie into my prior knowledge, and also to each other. I am slowly realizing as the weeks progress that it is very difficult to find scientific research that is independent from a network of other fields, which to me is what makes science truly spectacular.

We took a long break for lunch today so that the Non-Newtonian Fluids group could set up their "demonstration". Really, it was more like two hours of the entire class playing with the vats of "oobleck" that they brewed up. The stuff is surprisingly entertaining. We ran our fingers through it, slowly at first sow that it flowed like a liquid, then quickly so it ripped like a solid. If you moved fast enough, you could form it into a ball, but once you stopped applying pressure and just let it sit in your hand, it would melt away and slide through your fingers. Needless to say, it wasn't long before we were all covered with the corn-starch/water mixture. To demonstrate the unique principle of more applied force creating a larger resists to that force, we punched it and slapped it and threw stuff at it, and then we started to get really creative. Bill dropped a bowling ball into one of the vats and the effect was counter intuitive (as non-Newtonian fluids often are). Instead of the six pound ball creating a splash in the tub of liquid, it almost bounced off the surface before slowly sinking in.

Of course, the next step was to line the three containers up and have people run across it. The different techniques that people used to stay on the surface varied greatly. Some students hopped from tub to tub like hopscotch. This worked for some people, but when I tried it I found that the time between jumps was so long that I started to sink in as I prepared for the next jump. The easier thing to do was to continuously stomp on it, as if you were smashing grapes while running across. A surprising amount of students chose to walk through it to see what it felt like. The trick to walking is to move as slowly as possible, because as hard as it is to get something to move quickly into the mixture, it proved even more difficult to yank a foot out.
Bill wrapped the day up nicely with a demonstration of how a pro does it:

When the excitement was over, I returned to the quad and rested for a while before going out to famous cheese steak restaurant Pat's King of Steak, for Emily, the Texan on our floor's birthday. I have to say I was disappointed with the quality of food they served there. For such a famous place, the sandwiches were just not tasty. I much preferred the random little shop we found in our first couple days in the city. But we ate cake, sang happy birthday, and got to see more of the city it was an altogether pleasant experience.

As we walked off the bus, we were surprised to see it had started raining in the time it took us to make the trip home. Instead of retreating into the dorms, I took of my sneakers and played some Frisbee in the wet grass. Even though I got soaked, the air was warm enough that I didn't freeze to death like I would have back at home.

Once I got myself dried off and cozy, I relaxed in my room for the rest of the night.