I'd like to start a short series of posts on what I'm doing this summer. Like most all of the first year physics graduate students, I've found a research group at Cornell for the summer, and if everything goes well, I'll continue working with them after the summer is done. This is good for three reasons. First, I'm getting paid, so I can do things like pay rent. I'm not all about the money, but having a place to live is a big deal to me. Second, I'm getting a chance to explore a new area of research, with limited expectation of commitment on my part. Third, if everything works out, I've found the group I'll be working with the for the next 4-6 years. I'd like to spend a few posts here on The Virtuosi discussing first the physics I'm considering, and then what I actually do. Today I'm going to talk about the most exciting sounding piece of my work, cryopreservation. What do we mean when we talk about cryopreservation? For many of us our first thought may be Han Solo frozen in carbonite. This is the basic idea. Cryopreservation is the freezing biological samples, and thawing them, while they retain biological viability. At least, that's the idea. At the moment, cryopreservation is the freezing and thawing biological samples, and hoping that some of them remain biologically viable. Even though the methods have been in development since the 1950s, successes are limited even for such simple objects as spermatozoa or oocytes (sperm & eggs), let alone larger and much more complicated objects like Harrison Ford. Why would we want to cryopreserve something? (and did I just invent a verb?) The idea is fairly simple. If we cool down biological samples, biological functions slow down or completely stop. The hope is that if we get samples sufficiently cold, we can suspend biological (and all other) functions indefinitely. This would allow indefinitely preservation of many things. Eggs and sperm for reproductive purposes (both human and other animals), plant seeds, blood (for transfusions), vaccines, drugs (no more pesky shelf life for pharmaceuticals until you unfreeze them). The perfect frozen strawberries. The list of possibilities is huge. And according to my dictionary, cryopreserve is a well established verb. Why is it so hard? It's easy to stick something in the freezer. That seems to work on my food. However, it doesn't work well on biological samples. If we just stick them in the freezer many of the cells are damaged irreparably. We'd like to know why. To answer this we're going to need to take a step down to the cellular level. I'm not a biologist, so I'm not going to attempt to explain cell structure to you. For our current purposes it is sufficient to know that cells have an inside and an outside (then, doesn't most everything?), separated by a semi-permeable membrane, and that both inside and outside of the cells there is water. We need to take a brief detour now into the realm of water. Water is a fantastically amazing substance, and I'm not just saying that because we (and most/all life on earth) would all die without it. Water is fascinating from a pure physics perspective. Now, when I refer to water, I'm going to use it to mean H2O, in any phase. Colloquially we usually mean liquid water when we say water, we say ice we mean crystalline frozen water, and steam when we mean vaporous water. I'm going to call all of these things 'water' and I will specify what phase I mean by saying 'frozen, liquid, vapor'. I will refer to frozen crystalline water as ice. So what's so fascinating about water? Well, first off, it is one of the only substances that expands when it crystallizes (forms ice). Most objects become more dense when they transform from a liquid to a solid. Water doesn't. This is why ice floats. There are many other fascinating aspects of water, for example, that liquid water is most dense at 4 C. This is mostly responsible for why fish survive in the winter. Ice forms on top of lakes/rivers, and the 4C water stays at the bottom of the lake because it is the densest. But for our purposes, what we really need to know is that water expands on forming ice. When we freeze our cells, this expansion of water can cause massive problems. Everything in the cell besides the water contracts on cooling. The water crystallizes and expands (by about 9%). If you've ever frozen something liquid in a very full container and seeing it split the container open, the idea is very similar. The intracellular water can crystallize and split open the cell membrane. Once you've split the cell membrane, there's no way (that I've seen) to successfully revive that cell. It turns out that ice formation outside the cell isn't such a big deal, it usually just pushes the cells around. The water in the cells is what is most likely to cause damage to our biological tissues when we freeze them. Early studies showed survival rates between 10^-6% and \~50%. Those are not good odds. How do we prevent this damage? There are two methods currently in practice to prevent damage on cooling. The first of these is slow cooling. It was discovered that if you cool your sample at a rate of \~1 C/min very little to no intracellular ice forms, though intercellular ice still forms. The water in and between cells is not pure water, but contains all kinds of things (proteins, salts, etc). This means that it starts to crystallize not at 0 C, but somewhere lower. This is the reason we salt sidewalks in the winter (at least, in places where we get snow). The salt lowers the freezing point of water, causing the ice/snow to melt even if the temperature is below 0 C. Of course, if the temperature gets low enough it will still freeze. So our intra and intercellular water crystallizes below 0 C. It turns out that the intercellular water starts to crystallize before the intracellular water. This means we have ice formation outside the cell while the water inside the cell is still liquid. Earlier I told you that we needed to know that the membrane of a cell was semi-permiable. This is why. Some water diffuses through the membrane. At room temperature, the liquid water outside the cells diffuses into the cell at the same rate that the liquid water inside the cell diffuses out of the cell. Now, if we start crystallizing the intercellular water, there will be less liquid water moving into the cell. But since the intracellular water is still liquid it will still be diffusing out at the same rate. This results in a partial dehydration of the cell, more liquid water moving out than in. This is a basic example of osmotic pressure. With less liquid water in the cell, when it does crystallize, the ice crystals that form are smaller, and less likely to damage the cell membrane. This method is not perfect, because extreme dehydration of cells will also damage them, due to cell shrinkage and extremely high solute concentration (this phenomena is called osmotic shock). The second method that is considered is ultrafast cooling. Here we must take another diversion into physics land. You may have noticed my emphasis on ice as crystalline water. It turns out that when you freeze water, forming a crystal is not the only option available to it (and we're not even going to discuss the variety of crystals it can form). It can also form a glass. Now, if you're not a physicist, you're probably not familiar with glass as a type of substance, but rather as the thing you put in your windows and drink out of. A glass, to a physicist, is, essentially, a solid without long range order. The best way to think about of it would be to imagine you took liquid water and stopped time for the water (not freezing it in the conventional sense). That's a glass. It looks like liquid water, has the same density as liquid water, but is a solid. It's a very strange concept, and I'm going to save a more thorough discussion for a later post, because that takes me into a whole different research area I'm working on this summer. For our purposes, it is sufficient to know that we can reach a solid state of water at cold temperatures that has the same density as liquid water. Based on our discussion above, the advantages should be immediately obvious. Without the expansion of ice to worry about, the solidification of water is nowhere near as damaging to our cells. By vitrifying (turning to a glass) our liquid water, we should see much less damage to the cells. To achieve vitrification, you have to cool down the cells much faster (depending on the substance >1000 C/min). How do we do this? The conventional method is to dunk the cells directly into liquid nitrogen, or put them into a very cold stream of gas (around liquid nitrogen temperatures). I should note that I'm skipping over a very large field of putting cryopreservants into cells and freezing them. However, the effect of cryopreservants is to prevent ice crystallization, and so this is just a method of vitrification. It just makes it easier to achieve by lowering the rate you have to cool the sample at. So that's it? It may seem like that's the solution. Cooling fast enough to vitrify the liquid water, and we'll get good cryopreservation of our cells. The rest is just technology development. Unfortunately, it's not that easy. It turns out that warming is a harder problem to solve than cooling. When we warm up glassy water, it turns back into supercooled liquid water (liquid water way below the freezing point of water). I'll discuss this in more detail in my next post on my research. So now we've got really cold water, and it will start forming ice, unless we warm up fast enough to prevent that, very similar to how we had to cool down very quickly to prevent ice formation. The problem is, experiments suggest that 'fast enough' is at least 10-100 times faster than cooling down. Not only that, but on cooling down, we're really trying to get very very cold. On warming up, we don't want to get much over room temperature, otherwise we'll fry the cells. So we have to warm really really quickly, in an extremely well controlled manner. So far, this has proved very challenging to do. A recent study with mouse oocytes achieved a \~90% success rate with their freezing/thawing cycles, and that is the best that I've seen. 90% is pretty good, but we'd like to do better. So what's your research this summer? It turns out that my research isn't so much into cryopreservation. As I will talk about next post, it is more into the physics of crystallization and glass formation of aqueous substances, where there is a lot of fundamental physics still to be understood. However, one of the most exciting eventual applications is cryopreservation. We've already developed a fast cooling system, and we're working on a fast warming system that will allow us to study crystallization and glass formation, and hopefully extend the techniques we develop into the field of cryopreservation. That's way cool! Yes. Yes it is. Even if that is a horrible pun. Note: The question and answer format was inspired by Chad over at Uncertain Principles and his research blogging.

I was outside talking with Alemi last week and we were both startled to realize that the frozen white tundras of Ithaca had somehow transformed into fields of green. Apparently the snow was a temporary fixture that covered real live grass. Neato, gang! The joy at seeing green grass led quickly to surprise, confusion and then anger. Why the heck is grass green? Well, things look a color when they reflect back that color. So grass is green because its pigments (chlorophyll) absorb only a certain range of the visible spectrum, reflecting back the greenish bits. But if I know anything about approximating the sun as a blackbody, I know that it has a peak output of around 550 nm (i.e. green) light. So what's going on? Why are plants blatantly rejecting the most abundant kind of light? Since my initial confusion rested on the assumption of the sun as a blackbody, I decided to take a closer look at the actual spectrum of the sun. Below is a graph showing the frequency dependence of solar radiation incident on the top of the atmosphere and at sea level. Since plants typically don't live in space, we are most concerned with the sea level plot. From this plot, it looks like the incident radiation from the sun is fairly level beyond about 450nm or so. Just going on this graph alone, it looks like plants could just absorb reddish light and do alright for themselves. But do they? Let's take a look at the absorption spectrum of chlorophyll. As it turns out there are a bunch of different "flavors" of chlorophyll (chlorophyll a, b, c, and d). As far as I could gather, only a and b are important (a is found in just about everything that plays the photosynthesis game and b is found in plants and green algae). So we need to find the absorption spectrum of chlorophyll a and b. After looking for a while at very qualitative drawings, I found this neato-toledo site, which actually gives real live data. Plotting the results, we get the figure below. Comparing with our handy dandy wavelength-to-color converter below we see that there is a big peak in absorption of both chlorophyll a and b in the dark blue and lesser peaks in the red. So how do these absorption lines correspond to the incident light at sea level given above. Well, its kind of tough to check by eye, but it looks like chlorophyll has its biggest peaks right below the plateau in the solar spectrum. Marking the wavelengths of the absorption peaks makes this clear (a = blue, b = green). It seems like plants are using a sub-optimal band of the spectrum! So how do we reconcile this? Well, let's first start at The Beginning. The first photosynthetic organisms developed and spent the first billion or so years of their existence living and evolving in the oceans. The solar spectrum we have been using so far has been accurate only in air at sea level. Presumably it would be favorable for the organism to be able to survive at some finite depth in the ocean and not merely at the surface. Thus we must consider the effects of water on our incident solar radiation. A plot of the absorption spectrum of water is shown below (note the log-log scale). Lo and behold, the minimum absorption of visible light in water occurs towards the far end in the blue. And this is exactly where our biggest absorption peak in of chlorophyll is! Comparing our two graphs we see that the ratio of incident blue light to incident green light at see level is at worst about a third. But we see that for each meter traveled in water, green light is absorbed almost ten times more than blue light. Thus, an organism that lives a few meters underwater and wants to harness solar energy would probably do best to focus on that blue light. [WARNING: The next bit is speculative and I haven't taken a bio class since high school] As much as I gather about chlorophyll from Wikipedia and other semi-reputable sites, chlorophyll a is found in just about anything that photosynthesizes, BUT chlorophyll b is only found in plants and green algae. And apparently, land plants are largely descended from green algae (which are aquatic, but typically around the surface and around the shoreline). Now take a look at the chlorophyll absorption spectra again. The chlorophyll b spectrum is sort of squished in more towards the middle (towards 550 nm maybe?) than chlorophyll a. In fact, on the graph where I have drawn lines on the solar spectra graph where the chlorophyll peaks are, we see that chlorophyll b just barely gets up to that plateau region. This suggests to me that chlorophyll a was working just fine for the early aquatic plants, but once they reached land and got out of all that water it became an advantage to utilize light closer to the peak solar output. Thus, plants that had chlorophyll b in addition to a had a slight advantage over their b-less brethren. Or so I shall continue to shamelessly speculate (and, apparently, alliterate). Anyway, I thought that was kind of cool, but if I have made some horrible error or mangled some biology, please let me know!

Those of us originating on the right side of the atlantic ocean are familiar with a little quirk of international flights: the flights home are shorter. Specifically, going from Tel Aviv to New York takes about one hour longer than going the other way around. This is an oddity, and the very first explanation that comes to mind the rotation of the Earth. After all, our naive image of a plane going up in the air might be something a little like a rock being thrown up from a moving cart, and we would imagine the plane to pick up some relative speed by not rotating as fast as the Earth. Is this a factor in the plane's movement? This gives us a perfect chance to use the Earth Units we introduced a few weeks ago. Specifically I'll use the Earth meter (equal to the radius of the Earth, which I'll dub e-m) and Earth second (one day, e-s). First we want to figure out velocity of the airplane compared to the ground. When it is grounded, the plane and the Earth's surface both have an angular velocity of 2π 1/e-s; they do one revolution per day. This means the plane's linear velocity is 2π e-m/e-s, and its angular velocity once it's airborne is $$2 \pi \cdot \frac{(1\; \rm{m_\oplus})}{(1\; \rm{m_\oplus}+A)} \;\rm{s_\oplus^{-1}},$$ where A is the altitude. That's the one number I'm going to pull out of thin air here; that being the thin air of the cabin where they always announce that we have attained a cruising altitude of 30,000 feet. In real people units, that's about 9,000 meters, or 9 kilometers - round it up to 10. Going back to the Earth day post 1 e-m is about 6380 km, so that the angular velocity of the airplane is about 0.9984 (2π) 1/e-s, and relative to the ground it is $$0.0016 \cdot 2 \pi \;\rm{s_\oplus^{-1}}$$ So, over a journey of length of about 0.5 e-s, the overall distance traveled due to this effect would come to about $$0.0008 \cdot 2\pi \;\rm{m_\oplus}.$$ Tel Aviv and New York are both in the mid-northern hemisphere and seven time zones apart, so a first-order estimate of the distance between them would be about $$\frac{7}{24}\cdot 2\pi \;\rm{m_\oplus} \approx 0.29\cdot 2\pi \;\rm{m_\oplus}.$$ Overall, it looks like this effect is negligible. Indeed, anyone who gives the matter a second thought would notice that the planes should go faster when traveling westwards, as the Earth spins eastwards toward the rising sun. Anyone who looks even further into the matter finds that eastbound and westbound planes simply take different routes across the Atlantic, leaving us with a rather more mundane and less exciting explanation. Still, I won't complain if it makes my flight any shorter. Now, if you'll excuse me, I have some beaches to catch up with.

What we usually do here at the The Virtuosi is take an interesting problem, and work out the physical principles behind what we're seeing. Or pose a question and try to answer it. Now, I'm a big fan of this kind of thing, which is why I've done so much of it. But I worry that it might give a slightly skewed view of physics. Sure, physics explains things. That's why we do it. But not everything in physics is laser guns and solar sails. There are a lot of interesting physics phenomena that the general public will never hear about, because they're just too, well, esoteric. What I'm going to do is occasionally talk about such effects, and, for some of them, give you applications for these strange effects you might see on a day-to-day basis. Today I'm going to examine the Hall effect. The Hall effect is simple, as these things go, once we understand the pieces. The first piece is that magnetic fields deflect moving electrically charged particles. I don't think I can give you a good simple reason for this, you're just going to have to trust me (for those interested, I'd argue that the relativistic transformation of a magnetic field is an electric field, and that will certainly deflect an electrically charged particle). This is a piece of the Lorentz force. The next piece that we need to know is that opposite electrically charged particles attract. So a positively charged particle attracts a negatively charged particle. Knowing those two things we can detail the Hall effect. Take the slab pictured above. We run an electric current through it. Conventionally we take current as moving positively charged particles. There is a magnetic field into the screen. This deflects the positive charges up the screen, as shown, with some upward force. Over some time, we will accumulate positive charges at the top. Because there is no net charge in our slab, this must leave a region of negative charge at the bottom. These regions of charge will attract, and cancel out the force from the magnetic field. This charge separation results in a voltage differential between the two sides of the slab, which is what we actually measure. The Hall effect has some nifty consequences physically. I mentioned that conventionally we take current to be positive particles moving. A microscopic picture of our conductors will tell us that, in general, electrons are what we consider to be flowing in an electric current. Now, it turns out that our magnetic field will deflect electrons moving opposite our current direction (negative current moving backwards is the same as positive current moving forwards) to the same side as our hypothetical positive particles got deflected to. This generates a charge differential with negative and positive charges on the opposite sides of the slab (shown below), which means the voltage is negative what we would have measured above! This means we expect to get a certain sign of the measured Hall voltage, which we can predict. This sign would correspond to negative particles (electrons) being the moving charge carriers in substances. It turns out that there are some substances (some semiconductors) where the sign of the Hall voltage is opposite what we expect from electrons. This means that in those substances the current is being carried by positive particles! I won't explain what that means here (I may address that in a later post), but I hope you can see why that would be fascinating. We expected to have electrons moving, and it turns out that something else is really doing the moving. The Hall effect is an experimental result that helped suggest a whole new way of thinking about conduction in materials. Beyond being very interesting physics, there are some applications to this effect. It is an easy way to create a magnetic field sensor. Take a slab of material, run a current through it, and measure the voltage on the sides. Where do we use magnetic field sensors? Well, they sometimes show up as a way to tell if something is open or closed. Put a magnet in your lid, and a Hall effect sensor in the lip the lid rests on. When it is closed, you'll measure a voltage, and when it is open you won't. Now you can tell if it is open or closed. A little imagination, and you can see how this would be useful for all kinds of switches. Hit the switch, move your magnet, and change your voltage. According to Wikipedia, Hall effect switches are used in things as diverse as paintball guns and go-cart speed controls. They could also be used as a speed or acceleration measurement in a rotating system. Attach a magnet to the rotating object, put a sensor at a fixed location, and measure how the voltage in your sensor changes as the object sweeps past it. There are many more applications, but this is just to give you a taste of how this seemingly esoteric physics concept may show up in your everyday life. It's not just the interesting problems we often work on this blog, physics is everywhere. In many different guises

One thing that I've wanted to quantify since reading Intelligent Life in the Universe, an outstanding book by Carl Sagan and I.S. Shklovskii, is the idea of exogenesis. Exogenesis is the hypothesis that life formed elsewhere in the universe and was somehow transferred to earth in the form of some small seed or spore. Now since E.T. E. coli presumably do not have little tiny jetpacks or other means of active transport, they would need to traverse the cosmos in some passive way. One such way would be solar sailing. Way back in Solar Sails I, we derived equations describing the maximum speeds and time-of-travel for various distances for a given solar sail. Each of these equations was a function of the surface mass density of the sail, which is just the amount of mass per unit cross-sectional area. All we need to know is the cross-sectional area and mass of a given object and we can apply these equations to just about anything! Assume we have some spherical blob with the density of water (1g/cm^3). The effective sigma of this blob would just be the mass divided by the cross-sectional area. In other words, $$ \sigma = \frac{m}{Area} = \frac{\frac{4}{3}\pi r^3 \rho}{\pi r^2} = \frac{4}{3}\rho r .$$ Rearranging to get r in terms of the other variables, we have $$ r = \frac{3\sigma}{4\rho} . $$ Plugging in our density of 1g/cm^3 and a suitable sigma (10^-4 g/cm^2), we get $$ r \le 0.75 \times 10^{-4} cm = 0.75 \mu m .$$ Check out this fun site to see what kind of critters can fit in this blob. From the previous post, we saw that for a sigma of 10^-4 g/cm^2, our sail would get to the nearest stars on a timescale of order 10,000 years. Thus if our blob has a radius of less than about a micron, it could spread to hundreds of stars in around 10,000-100,000 years. Even if it would take millions of years, that would be almost no time at all on the cosmic scale. Just based on this calculation it all seems fairly feasible. In making these calculations I have neglected several important aspects of the problem. First, in no way have I actually calculated any sort of probability of this happening. Additionally, I would have to see how likely it is for some blob to reach planetary escape velocity (presumably just by riding that tail of the Boltzmann distribution). Finally, and perhaps most important of all, I have not given any sort of motivation or mechanism by which some living body could survive hundreds of thousands of years in the vacuum of space with constant radiation exposure. But I have heard that some forms of life are totally extreme (especially if they drink this). Even though such a process seems possible, it certainly doesn't seem like the easiest way to get life on earth. I prefer the much more satisfying "amino acids + lightning + magic = life" model. But it does offer some interesting possibilities. Suppose we as people think that people are super awesome and therefore people should be everywhere. We do some bio magic and put whatever DNA we want into viruses, which we then pack into as many micron spheres as we can make. We then point them at the nearest stars and have them disperse. What would the probabilities be that they land somewhere habitable? Are there any ethical considerations in doing this? Is it a galactic faux pas?