## Reynolds’ World

What’s it like for little things like bacteria to move around? How do they swim from place to place?

We know that swimming feels different from walking. Part of it is the feeling of being suspended, where instead of the firm solidity of the earth and the  insubstantial give of air, we have the water on all sides, supporting not just our feet but our legs, arms, and body. But also, it’s a lot harder to move through water! The same quality that makes us feel supported also impedes movement, so that even a very efficient swimmer will be easily outpaced by someone strolling along on dry land.

Scientists have a way to quantify that  difference, using a measure called the Reynolds number. The Reynolds number compares how strong inertial forces are in a fluid, which come from the particle size and the weight of the particles, with the viscosity of the fluid. If a fluid has low inertial forces compared to its viscosity, it has a low Reynolds number, and if it has high viscosity compared to its inertial forces, then its Reynolds number is low. So fluids with a high Reynolds number are easier to move through, and fluids with a low Reynolds number are harder to move through. The pitch of the Trinity pitch drop would have a very low Reynolds number! And fluid flow in high Reynolds number environments tends to have more chaos, vortices and eddies that can arise because of how easy it is to move light things that don’t stick together, like molecules of air.

So it turns out that what strategy you use to move in a low Reynolds number environment is different from what you’d use in a high Reynolds number environment. Of course, we already know that, because if we try to walk or run in water, it doesn’t work very well! Running is a great way to get around when you are moving through thin air with the solid ground beneath you, but humans have developed various modes of swimming for water, that take advantage of our anatomy and account for the different nature of water.

But remember, we are largely made up of water! So what about our moving cells and bacteria, which have to get around in a low Reynolds number environment all the time? And keep in mind that our cells are very small, subject to molecular forces and a lot closer to the size of water molecules than we are. Not surprisingly, there are different forms of swimming that take place in our cells. One of the most common is using a rotating propeller, a little like the blade on a helicopter, to move forward. These structures are called flagella and are common on the surface of various types of cells, to use rotary motion as a way of easily moving through the high Reynolds number environment.

So the next time you are walking around with ease, take a moment to imagine how different it is for everything moving from place to place in and around your cells. It is a whole different world, right inside our own!

## Quantum Worldview

I have always loved the kind of science fiction where you think about a world that is largely like our own, but in some fundamental way different. What if we all lived underwater, or if the force of gravity was lower, or if the sun were a weaker star? To me, that’s what the world of quantum physics is like: in a lot of ways it’s similar to our own world, in fact it’s the basis of our world! But it’s also crazy and strange. So what would it be like if we were quantum creatures, if we could actually see how everything around us is quantized?

Well first, there’s what it means to be quantum. A quantum of anything is a piece that can’t be subdivided any further, the smallest possible unit. But this implies a sort of graininess, where rather than a continuous stream of, say, light, we start to see at the small scale that light is actually composed of little chunks, quanta of light. Imagine being able to see how everything around you is made of discrete pieces, from light to sound to matter. When the sun came up, you’d see it getting lighter in jumps. When you turn up your music, you’d hear each step of higher volume. And as your hair grew, you’d see it lengthening in little blips.

And at the quantum scale, the wave nature of everything becomes indisputably clear. We normally think of waves as something that emerges from a lot of individual objects acting together, like the water molecules in the sea, or people in a crowd. But if you look at quanta, you actually find that those indivisible packets of light or sound or matter are also waves, waves in different fields of reality. That’s hard to get your head around, but think of it this way: as a quantum wave, if you passed right by a corner, you could actually bend around the back of it a little the way that ripples going around a rock in water do. Things like electrons and photons of light actually do this, so for example the pattern below is made by light going through a circular hole, and the wave diffraction is clearly visible.

Amazingly, as a wave, you could actually have a slight overlap with the person next to you. This gets at something that’s key to quantumness: the probabilistic nature of it. Say I thought you were nearby, and I wanted to measure your position somehow to see how close you were to me. I’d need to get a quantum of something else to interact with you, but because it would be a similar size to you, it would slightly change your position, or your speed. We don’t notice the recoil when sound waves bounce off us in the macroscale world, but if we were very small we would! So there is actually a built in uncertainty when dealing with quantum objects, but we can say there’s a probability that they are in one place or another. So as a quantum creature, you can think of yourself as a little wave of probability, that collapses to a point when measured but then expands out again after. When I’m not measuring you, where are you really? Well I can’t say physically, and this is why you can have a little wave overlap with your neighbour without violating the principle that you can’t both be in the same place at the same time.

And imagine that you’re next to a wall. As a wave you may have a little overlap of probability with that wall. And if the wall is thin enough, as a quantum object there is actually some chance that you’ll pass through the wall entirely! This is called quantum tunnelling, and actually it’s happening all the time in the electronics inside your phone. Modern microelectronics work in part because we can use effects from the quantum world in our own, larger world!

It’s difficult to imagine a world where everything happens in discrete chunks, where I can see myself as a wave, where I don’t know where I am until someone else interacts with me. But this is the world at the quantum scale, and it’s not science fiction!

## Top Ten Popular Science Books

I recently got a request to recommend some popular science books that don’t assume any scientific knowledge on the part of the reader. I was surprised at how hard it was to think of books, because to be honest, most pop science books do seem to assume that you have some fluency in science ideas or jargon, if at a lesser level than a scientist would. I’ve read some very popular books about biological topics that I found dry or hard to get through, because even though I’m a scientist I don’t know very much biology. But I came up with the following ten books, which explore different aspects of science in strongly accessible ways:

These books will give a nice overview of some of the great stuff that’s out there in popular science reading. (Note: the links above are affiliate links, just something we’re trying out!) Of course, I’m always interested in other people’s recommendations too, so have at it in comments if you like!

## A Chat About Women In Science

What do Erin and I talk about behind the scenes of this very blog? Well, plenty, but often we discuss science communication and women in science. We decided to post this recent chat we had, with references! This is how the sausage is made, people.

Jessamyn:  !!!
YES
Erin:  I question whether the overarcing discussion is still so humanities-unfriendly (given the proliferation of things like Bright Club over here), but he’s got a lot of good points
I also think it’s worth thinking about why the humanities’ contributions have been overlooked
mainly because I think they are viewed as ‘lesser’ by a lot of scientists
which is kind of ironic given the way the tide has turned
Jessamyn:  yeah
I think that putting science up on a pedestal has both isolated it and decreased its perceived value
and being all shirty about the social sciences and how they have ‘physics envy’
Erin:  which is ridiculous!
we should be able to appreciate the value of other areas of study and hopefully see how they can benefit our own
Jessamyn:  being interdisciplinary and collaborative have huge payoffs
but they aren’t ego payoffs
Erin:  absolutely
but I don’t think scientists feel like they are being egotistical
Jessamyn:  they may not label it as that
but they are imposing a moral value system that conveniently places their work at the top
and then building a hierarchy of status based on that
Erin:  oh! how funny it ended up like that
Jessamyn:  sorry guys it just HAPPENS that my stuff is the most important EVER
Erin:  as defined be ME AND ALL MY BUDDIES
Jessamyn:  not everyone is smart enough to do what I do, sadly
so I have to tell you what’s important and what’s not
ME
I AM IMPORTANT
Erin:  sadly the current environment of academia does not help dispel that. PIs and researchers are constantly being driven to justify their work
and it’s only very recently that outreach and collaboration have had any place in that justification
Jessamyn:  yeah
I love doing all this stuff but I’m very aware that it’s not going to do much to help me get a faculty position
research publications do that
I think some departments are more into outreach than others, and I’m hoping one of those will place a higher value on someone like me… but it’s not a universally valued thing the way research pubs are
Erin:  I was talking to a PhD student today who is great, she is part of the bioscience team and is also spearheading a college-wide blog initiative. and the comments from her colleagues have just been so dismissive. “Oh, you’re good at talking to people? you should quit your position and go into publishing or communications.”
Jessamyn:  yes!
after I won that physics communication award people asked me if I wanted to take the outreach coordinator’s job
and I’m like, no, I want to do research and some outreach, and I want her job to be coordinating lots of scientists like me
and I think most scientists should be doing this stuff
at varying levels, but I mean… COME ON
Erin:  yeah, oh god, don’t get me started
and it’s like “overall that might be a good amount of outreach but my god it’s unfair to the few people who are willing”
even if they enjoy it
Jessamyn:  my boss was also commenting to me and a gender equality committee yesterday that it’s generally female scientists who end up being great communicators
which, yes, because (a) I think female scientists are much more aware that they are unlikely to be able to be the single-minded scientist, if only because there is the ‘WHAT ABOUT BABEEEZ’ thing from so early on that men can just skate on past, assuming that a partner will do the bulk of the work
and (b) women are socially conditioned to value communication and language skills more than men are, and more ostracized if they fail to develop those skills, so
Erin:  yeah, I’ve definitely read studies and it goes: men without kids < men with kids < women without kids < women with kids in terms of scientists who engage with outreach efforts
Jessamyn:  makes sense
I mean I guess you could also say, well women in science know about the importance of role models
so they would be more invested in providing that, to get more scientists and more girls into science
but then that ends up being effectively another tax on being a woman in science
Erin:  yeah, especially since they’re also required to sit on all the committees, and be on all the brochures, and the panels, etc etc…
Jessamyn:  exactly
Erin:  :-/
AND WHO IS MORE LIKELY TO END UP IN THE HUMANITIES? (to tie this in to the beginning point)
Jessamyn:  yes!!!
and more of those people are women
I hated the title of this, using storytelling to bring women to science, but it’s very interesting
I like how it challenges the framing that women need better sci/math skills
when it’s also, we just lose those women who go off and pursue other things they are good at
Erin:  yeah!
science needs better women skills!
Jessamyn:  yes!
stop phoning it in, science!

For the most part I don’t write that much about science communication here, because my posts on this blog are one demonstration of what I feel science communication can be! But I spent the end of last year thinking a lot about outreach, and seeing how my outreach philosophy is different from other communicators who are doing great work, and I wanted to explain that a little more.

I’ve always found science fascinating as a lens for understanding the world and appreciating its beauty. But I think that in science and engineering, and especially my field of physics, there’s an inherent tension. On the one hand, you have the beauty and awe that science help illuminate, and the excitement of increasing your own realm of knowledge, or even pushing the boundaries of the knowledge of mankind. That is all exciting and lofty and many people who aren’t into science still see the appeal, because curiosity about the world around us is something every child starts out with. But on the other hand, there’s often an elitism in science, a sense of scientists as gatekeepers of truth high up in a hierarchy, which is encouraged by the media at times and even some scientists.

When I tell people I’m a scientist, or a physicist, a lot of times they tell me a story about the one bad physics teacher they had, who ruined all of science for them. This apparently happens a lot, and I do get that teachers can make or break a subject at times. (My first physics teacher was not stellar.) But it’s not like bad English teachers ruin reading and writing for anyone. “If it weren’t for that middle school teacher harping on verb tenses all the time, I would probably be a Proust scholar by now, but as it is I don’t even remember how to read.” But I think culturally, communication and language and the arts derived from those things are considered fundamental, in a way that science and math used to be but no longer are. It should be as much a mark of education to know some basic science as it is to have read some of the classic novels or to know the Beethoven symphonies! I’m never going to be one of those people who makes the argument that science literacy is more important than other forms of cultural literacy, but why isn’t it at least equivalent? I think that’s a direct result of our having tried to set science apart as a better, higher thing. When you put something up on a pedestal, it gains status but loses accessibility. Science is now considered less relevant for everyone to know, even though it’s just as foundational as it ever was.

But I don’t fundamentally believe that scientific ideas are out of reach for a layperson. There’s no insurmountable math barrier or smartness barrier, science is a topic like many other topics. And I mean that a layperson can understand basically any scientific idea, not just the vague and descriptive ones. Math is a great language for explaining science, if you know how to speak it. But actual language also does the trick! You just have to be willing to think about the best way to use it.

Only being willing to explain physics using math is a failure of imagination. And sure, maybe an explanation that doesn’t use math is going to be missing some things, but so is a math explanation that gives no qualitative interpretations. If you have no science background, and I’m telling you about electrons, you may not come to understand electrons in exactly the way that I do. But that’s as much because we are different people with different experiences and conceptual ways of thinking as it is because I have spent time studying physics.

There is a saying that you can’t teach someone physics, you can only help them to learn it for themselves. And while I agree that it’s the student who has to mentally grapple with and eventually accept the tricky topics in science (and life), that doesn’t mean there’s no point trying to teach! Each person comes to understand concepts, whether it’s particle-wave duality or mind-body duality, on their own terms. If someone is asking me to help them find those terms for a concept I know a little about, I can’t make the leaps for them, but I can try different approaches to facilitate that understanding. And I love doing that; it usually expands and reforms my own understanding as well.

## Thermoelectrics and the Movement of Electrons

Often the big picture, like a river winding down a canyon or flashes of lightning in the sky, can be understood by looking at the small-scale behavior of each component, like molecules of water rushing to lower elevation or electrons seeking to equalize potential. I’ve always liked that approach to understanding nanoscale physics phenomena, like electricity or heat. But if you think about it, since electricity is the movement of charge carriers in response to an electric field, and hotter particles also move around more, shouldn’t there be some interaction between the two? Can something develop an electric current as a result of being hot? Or can driving electricity result in the generation of heat?

If you’ve ever felt an electronic device heat up in your hand or on your lap, then you know the answer is yes! Running an electric current through some materials, like resistors, generates waste heat, which is a big practical problem for electronics manufacturers. But there are some materials that can do the opposite, converting heat to electricity. Extracting usable electricity from waste heat is especially impressive when you think about the reduction in entropy involved; turning the high-entropy disordered heat back into an ordered electrical potential is a strong local reduction of entropy. This is how 90% of the electricity in the world is generated, but at a low efficiency. Materials where a temperature difference creates an electrical potential are called thermoelectrics, and in addition to being really practically important, thermoelectrics are a great illustration of how important it is to understand what’s happening at the nanoscale.

The most common thermoelectric device is one where two different metals are pressed together, creating a junction. Each metal is a conductor, and will have its own electrons, which can freely move across the junction. But the electrons experience different forces in each conductor: they may find it easier or more difficult to move through the material, based on the physical properties of the metal itself. So applying a voltage across the junction will affect the electrons in each material differently, and can cause one metal’s electrons to move faster than the other’s. This difference in electron speeds, or a difference in how easily the electrons transfer their energy to the atomic nuclei in the metal, or just slow diffusion of electrons across the junction, can all lead to a temperature difference between the two metals. Thus heat can be produced at the junction, and it can even be removed given the right material properties. Heat generation or removal at an electrical junction is called the Peltier effect, and is the basis of some nanoscale refrigerators and heat pumps.

Conversely, if a temperature difference already exists across a junction of two conductors, you can imagine the faster moving electrons in the hotter material, interacting with the slower moving electrons in the colder material at the junction. For the right combination of material properties, an electrical potential will be induced by the differing temperatures, which is called the Seebeck effect. But it’s the same mechanism as the Peltier effect above, namely that both heat and electric fields induced the movement of charge carriers, and so of course the two effects have some interaction with each other.

It’s not just metals that exhibit thermoelectric behavior, though. Semiconductors can also be used as thermoelectrics, and actually have a broader range of thermoelectric behavior because their carrier concentration varies more widely than that of metals. Heat and electric field affect the charge carriers in every material, it’s just that some materials have properties that result in a more interesting and usable phenomenon.

Thermoelectric materials can be used as heat pumps and refrigerators, as I mentioned above. But the thermoelectric effect can also be used to measure temperature, by putting two metals that react differently to temperature together and then measuring the induced electric potential. This is how thermocouples work, which are incredibly common. And it all comes from the fact that both heat and electricity cause motion at the nanoscale.

## Small World

It’s a New Year, and I went ahead and tried something new! This is a video version of some of the cool nanoscale and quantum things I’ve written about before, created in collaboration with director/editor Kevin Handy. I hope you enjoy it; I had a blast making it.

## DART of Physics and Outreach

I’ve spent much of the last couple months working on this project called DART of Physics, which has put ads with provocative statements about physics up on the DART, the light rail line here in Dublin. Each of the statements has a page on the DART of Physics website explaining it in more detail, and they are all worth checking out:

In addition, there’s a DART of Physics blog that has some posts from me, guest posts, and in general just a ton of things about physics. The campaign has been running for 6 weeks and has another 2 to go, so it’s worth checking out all the posts we’ve got up so far. And we’re on twitter and facebook if you want to ask questions or suggest blog topics or anything. We have some amazing creative partners at Language and iQ Content, in addition to our core team from the Schools of Physics, Chemistry, and Education here at Trinity College Dublin:

And, while I’m talking about outreach and linking things, I wrote an article recently about the overlap and tension between science and improvised comedy, two things I love. I’m very proud of it! I really enjoy writing about science, both scientific concepts and the culture of science, and was very honored this week to win the Institute of Physics Early Career Physics Communicator Award. I feel strongly about the importance of science communication, that science is a fascinating lens through which we can view the world around us, and one that should be accessible to anyone whether they are a scientist or not. And the best thing about the IOP event was talking to other people who felt the same way!

## What’s in a flame?

We see fire all around us. Well, hopefully not all around us, but in fireplaces and candles and bonfires. So what’s in a flame?

A flame is the visible part of a sequence of chemical reactions called combustion, like the tip of an iceberg that indicates much more is present below the surface. To start combustion, we need oxygen, heat, and a material sensitive to both which acts as a fuel. When the oxygen and fuel are raised to a high enough temperature, the oxygen molecules interact with the molecules of fuel, exchanging electrons and swapping atoms while releasing energy. These atomic changes can make the material look very different, as you can see if you compare a piece of wood with a piece of charcoal.

The released energy, which comes from the changing configuration of atoms and chemical bonds in the burning material, can be in the form of heat or in the form of light. It’s only the hottest part of the flame that emits light, the part which is a plasma. What color the light appears as to us depends on the energy of the particles of light being released, which in turn depends on what elements were present in the fuel. If you have a propane flame and you add sodium or copper, you can see the color of the flame change. This happens because combustion of those materials releases different amounts of energy than combustion of propane.

As materials undergo combustion in the flame, the resultant gas expands. This means that the molecules move further apart, and the gas becomes less dense. Here on earth, the presence of gravity means that denser gases are pulled downward and lighter gases rise upward, so the hottest gas products will rise up and away from the fire. This is why flames often look like they are rippling upward, even though their base never moves. But experiments have been done in space to show that, without gravity, hot gases do not rise away from the flame. So flames in space are more or less spherical, and are usually all blue which would indicate the hottest part of a flame on earth.

What a flame looks like can tell us a lot about what a material is made of, how much oxygen is in the air, and even whether there is gravity or not! But fundamentally, flame is a visual sign that the burning material is being chemically changed into something new.

## The Colors of Noise

Talking about the ‘science’ of something often evokes ideas like precision, or a very controlled study where every parameter is meticulously accounted for. But the natural world is not necessarily precise or controlled; evolution gives the quickest functional solution, rather than the most elegant. And as with entropy, management of disorder and messiness is often simpler to achieve than full control. So even as we attempt to study and engineer our natural environment, we have to accept some level of randomness and chaos, or even perhaps take advantage of it as nature does.

It’s with that in mind that I’d like to talk about noise. Colloquially, noise refers to sounds, but specifically disordered or cacophonous ones. This comes from the idea of noise as a sort of randomness that’s part of a signal carrying information, like static on a phone line or a video. Those examples of noise are like random factors that affect the timbre or perception of the signal itself, but one can also think about noise more generally in nature as a perceivable, measurable result of some inherently random physical process. Such processes are called ‘stochastic’, as opposed to ‘deterministic’ processes where the probability is determined or fixed by the initial conditions.

But as you might imagine, there are lots of different physical phenomena that might lead to noise. So the noise resulting from stochastic processes has a sort of fingerprint that can be read out and may give useful information about the system the noise came from. For example, noise on a telephone line may be different from noise in a photodetector, even if both come from random things that electrons are doing in the material. But how to tell the two apart? Variations in noise are usually analysed by separating out the noise by frequency, which is to say looking at what parts of the signal occur once per second, versus twice per second, versus a hundred or a thousand times per second. This is described as the ‘power spectrum’ of the noise, but more lyrically, the most common power spectra for naturally occurring noise are called the colors of noise.

The power spectrum above shows noise that has the same power at all frequencies, which is called white noise, because of the idea that white light contains all the visible colors in equal proportions. White noise comes from thermal processes, which is to say atoms using the heat and kinetic energy around them to jump up to higher energy states and then back down. And since thermal processes don’t have any preference for one frequency of activity over another, the frequency spread of thermal noise is flat.

There’s also brown noise, which isn’t named for the color brown but rather for the scientist Robert Brown. He was a botanist who discovered that pollen grains in water moved in a random pattern when observed under a microscope, and initially could not explain the mechanism. Many decades later, Einstein explained the movement as a result of molecules in the water buffeting the pollen grains, causing a random movement of the grain itself to be visible even though the water molecules were too small to see. This movement is called Brownian motion, or a random walk, and so Brownian noise is characterized by enhanced low frequencies as you’d get in a random walk. Auditory brown noise sounds like a waterfall, softer than the other colors of noise. Here’s an animation of the pollen grains colliding with smaller water molecules, where you can see the randomness that we can call noise:

Pink noise, or flicker noise, decreases in power as the frequency of the noise increases. It’s also called 1/f noise, because the power of pink noise goes inversely as the frequency f. Pink noise comes from the trapping and detrapping of charge carriers like electrons, which can be counted as a stochastic process that is more likely at lower frequencies. Because the human ear is less sensitive to higher frequency noises, pink noise is often used as a reference signal in audio engineering.

But just as with light, our senses and processing of noise can distort it between its physical origin and our perception of it. So for example, we may perceive white noise, which has a flat power spectrum, as louder for frequencies that our ear can better detect. I mentioned before that the human ear is less sensitive to higher frequencies, but overall that sensitivity can be mapped to create a power spectrum that will appear flat given our sensory distortion. This sort of noise is called grey noise.

There are other colors of noise, such and blue and violet noise which increase in power at higher frequencies, green noise which is the selected center of the power spectrum for white noise, and black noise which is a fancy way of saying silence. You can listen to some colors of noise here. There is also a type of noise specific to small numbers of some countable event like trapping and detrapping, where the discreteness of the event becomes important. Fluctuations in a small number matter more than fluctuations in a large number. This is called shot noise, and is common in any signal that has a small number of countable events, like measuring individual electrons or photons. But noise is just a consequence of the inherent randomness in our physical world, an avatar revealing the exact physical mechanism behind its own creation.