omenti

natural conditions

a minipost

Natural conditions impact outdoor sports.  Tokyo in late July and early August is "interesting" - it's very hot with humidity rated as oppressive about 60% of the time.  Pleasant weather doesn't return until the Fall, but TV demands drive the Olympic schedule.  Some endurance events will probably be run at night and athletes will be adding high temperature conditioning to their training routines in the next month or two.   Excessive heat has important physics and physiology consequences and the understanding of physiology has changed a lot in the past decade (you can drink too much in endurance events and electrolyte drinks are ineffective).  Perhaps something for future posts, but for now let's stick with the wind.  

 

During the Olympics Tokyo will probably have consistent winds around 7 mph mostly from the South or East.  To establish a record events like the 100 and 200 meter sprint, 110 meter hurdles and the long and triple jumps record the wind velocity and require a trailing windspeed to be less than 2 meters per second - about 4.5 mph - measured at the track.  The wind doesn't matter much within a race, but qualifications can be biased if the wind changes much between qualifying events. 

 

You'd think a tailwind would always be advantageous for hurdlers, but they have to mentally adjust their strides and launch points for a variety of winds and only the best can handle a variety of conditions.  Long jumpers have to adjust their approach to launch as close to the takeoff point as possible and not foul. Assuming they succeed, a trailing wind can be a big advantage.  Except for record setting pole vaulting is similar.  The height they clear is related to the square of takeoff speed so a tailwind can be a big help.  Oddly there's no wind assistance limit for records. Perhaps an approaching hurricane would be useful in world record attempts.

 

Closed road course races are problematic.  One might think tailwind and headwind portions average out, but if the wind is at a constant velocity, the headwind portions are always slower (here's a simple explanation).  Of course there's the larger problem of shoe performance so maybe we're at a point where records aren't terribly meaningful in running.

 

For some sports the wind adds an interesting dimension.  The density of a beach volleyball is low compared to other most other sports balls, so the wind makes a big difference.  Additionally volleyballs have interesting aerodynamic properties in airspeeds between 14 and 17 meters per second - speeds commonly seen in the women's game and exploited by the better players.  Teams must to adjust for windspeed and direction, but events often go on in strong  and gusty winds - sometimes with heavy rain, which is not unlikely in Tokyo during the Olympics.  

 

Many other sports are impacted by the wind, but last Spring the final project in a science of sports class I taught got my attention.  An amateur bicycle racer/biology major proposed human powered flight.   You would charge a battery by frantically pedaling  generator.  After some amount of time you'd take flight adding whatever power you could muster with your legs to what the motor could produce. Finding the right limits for weight,  charging times, and allowable technologies would make or break the sport.  It probably wouldn't be tmuch of a spectator sport, but it would make a great project for engineering students. 

 

the third wave

Electric cars are beginning to take off.  If you need to replace your old car they may be a good choice as maintanence is potentially much less expensive.  They're usually a bit more environmentally friendly than a conventional car of the same size, but not by much. There are much more cost effective ways to lower your carbon footprint. One is replacing local commutes and errands with a bicycle. 

 

We may be at the beginning of a third wave of bicycling in the US.  This time it's a bit different with pedestrian, human powered, and human-electric hybrid bicycles in a category that's called active transportation.  I'm not so sure it will take hold in the US this time - after all, our second wave fizzled in the 70s.

 

The first wave came with the invention of the safety bicycle in the 1880s.  Suddenly cheap transport that could extend your walking range by a factor of three or four appeared and bicycle fever swept the US and much of Europe.  I've written about it a few times and won't say much other than it was an important component of women's suffrage in England. It also lead to widespread mass production and engineering leading directly to the automobile and airplane.  

 

The second wave began in the late 1960s - first in the Netherlands, followed by the US, and slightly later by Denmark.  None shared a common trigger.  The Netherlands was a reaction against the beginning of affluence that came in the mid 60s that flooded the streets with cars.  The narrow streets, children playing and cars were a nasty combination and people reacted strongly closing streets and going back to the bicycles they had used since the war.  The US boom was fueled by the green movement - particularly in university towns.  Denmark's was a reaction to the oil crisis.  The Netherlands and Denmark stayed the course building physical and social infrastructure that favored cycling in cities and small towns. Progress was slow, but by the early 90s the bicycle had become an important piece of transportation.

 

The US started much more aggressively, but fizzled for a number of reasons shortly after the mid 70s. Carlton Reid wrote a piece on the US boom that failed - a short recommended read!

 

Now something has changed - the ebike.

 

A first ride is astonishing.  They're not motorbikes - you pedal and an electric motor adds power to your muscle power.  You can set the level of added power to levels better than any Olympic bike racer. Hills flatten and the distance you're willing to ride doubles on average.  The efficiency of a person on a regular bike is amazingly high - an ebike typically improves it by a factor of two. You're still get exercise and it's much more practical than a Peloton if exercise is the goal.  Currently there are more customers than bicycles..  

 

So they make sense for you?  Probably the most important consideration is safety. Women and children are usually seen as the two indicator species - when at least half of the adult riders are women and most of the children walk or bike to school..   Women and children already dominate bike riding in the Netherlands and Denmark.  Bicycle racks were removed from schools in my New Jersey town as bike riding was considered too dangerous by the school board.  They were afraid of lawsuits if they encouraged bicycle use.  Some of you live in areas with very low traffic densities and safe conditions. For others the question is how to build a safe transportation infrastructure. 

 

You don't want to share the road with drivers who aren't aware of you, don't know what it's like to be on a bike, and may have more legal protections than you.  A variety of approaches can create safe cycling - certainly European counties have taken differing approaches.  A commonality is traffic calming.. speed limits should be 20 mph or less if bikes and cars share a lane.  Bikes on more heavily traveled streets should have physically separated lanes.  Paris has shown this isn't difficult or expensive.  The most cost effective way to come up with real estate for separate bike lanes is to eliminate car parking and the experiment has been very popular to the point where the changes are becoming permanent.  The city is on its way to removing about 70 percent of parking.  

 

Assuming the risk of cycling is low enough for you, it's important to find a good ride as there are a lot of poorly built bikes out there.  A good ebike that should last a decade will set you back at least $2.5k.  These usually have a similar design to a conventional bike  They're great for commuting and just riding around, but consider a cargo bike if you're serious about replacing a car on local trips.  The past few years has seen a revolution in properly engineered  and practical electric cargo bikes.  Good bikes in this class tend to be spendy starting around $4k.  I have a favorite, but these are individual choice and spening time at a good bike shop is important.  

 

a promotion video of a very good cargo bike illustrating some use cases - you can easily carry two hundred pounds of cargo without worrying about balance, power or overloading.

 

Back to that third wave - will it continue or just fade?  Many regions in Europe already see a lot of cycling and the trend will only increase as automobile use is minimized in the cities.   North America is more difficult as the infrastructure and expectation supporting cars and city layouts are somewhat rigid.   Emerging cities, mostly outside of North America and Europe,  have a choice of city layout and transportation infrastructure.  It will be fascinating to see how and where active transportation fits in.  And there are other practical but non-conventional transportation modes - and I'm not talking about self driving vehicles...  

 

 

ultra high energy cosmic travelers and beach volleyball

In 1912 Victor Hess was testing a hypothesis that this new fangled radiation came from rocks in the earth.  He built a simple detector and made a series of ascents in a hydrogen balloon.  The higher he went, the higher the radiation levels.  The radiation was coming from above rather than below - he had discovered cosmic rays.  

 

Cosmics proved frustrating to study. Very high energy particles - usually protons or light nuclei -  hit the upper atmosphere where they'd strike a nitrogen or oxygen atom. That produced a shower of particles which would strike other atoms spreading the shower.  It was difficult to say where the initial particle came from or even how much energy it had other than very high energy cosmic rays produced showers that spread out more than lower energy events.

 

By the 1960s physicists started building arrays of small water tanks and spreading them across large open areas.  The tanks could detect small flashes of light made by fast moving particles moving faster than the speed of light in water.  More tank signals at the same time meant a larger shower.  By seeing how much area had been impacted by the shower it was possible to infer the original cosmic's energy.  

 

By the 80s more sophisticated techniques began to lift the curtain a bit.  Cosmics cover an enormous range of energy ranging from that of particle accelerators of the 1970s and up.. way up.  Nature had built some amazingly power accelerators and we were building detectors for them.  Learning where these accelerators are and how they work may teach us about the fundamental nature of the Universe.  The focus shifted to extreme events over 10¹⁸ electron volts - that's over 100,000 times more energy than the highest energies produced in the Large Hadron Collider at CERN.  These are among the fastest objects with mass in the Universe.

 

The really powerful ones are fairly rare.  A square kilometer of the Earth will see about one 8 EeV (an EeV is 10¹⁸ electron volts - an exaelectron volt) per year.  One array is about 3,000 square kilometers so it would see nearly ten of the 8 EeV events per day.  It's effectively an enormous digital camera with car sized pixels spread over a huge area.   Even higher energy cosmic rays occur less frequently - you expect a 100 EeV event per square kilometer per century.

 

It turns out the amount of kinetic energy an apple falling from a table is about one joule - that's about 6 EeV.¹   These tiny incident cosmic rays have energies in ranges we're familiar with.  Sarah can serve a beach volleyball at around 80 kilometers per hour giving it a kinetic energy of about 70 joules or 420 EeV.   In 1991 an event known as the Oh My God particle was detected in Utah with an energy of about 320 EeV.  That's only a bit less than Sarah's serve and about the same as a 60 mph baseball.  It's almost certain higher energy cosmic rays exist, but we just haven't found any yet.

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¹ An electron volt is a tiny amount of energy that's used mainly to study energies at the molecular and atomic level.  A light photon useful for photosynthesis is on the order to 2 electron volts.  A joule something more on the human scale..  70 for Sarah's beach volleyball serve and 140 for a 100 mph.  fastball.

 

right out of buckaroo banzai

mini-post

You probably know Pluto isn't considered a planet anymore.  In the 90s objects of similar size were discovered in a region outside the orbit of Neptune known as the Kuiper Belt.  Several of their features set them apart from the first eight planets so it made sense to call them by a different name - dwarf planets.  One of the major figures in these discoveries is an astronomer who specializes in the solar system and how it came to be.  Mike Brown killed Pluto. Neil deGrasse Tyson only popularized Pluto's demotion.  Mike took a lot of flak including hate mail from children.  His young daughter told him he'd have to go and discover the ninth planet to make up for it.  

 

Konstantin Batygin joined Mike in the discovery of an unusual clustering of the orbits of some extreme trans-Neptunian objects.  These objects are well beyond the Kuiper Belt - generally over one hundred times farther from the Sun than we are.¹  They hypothesized a very large ETNO five to ten times as massive as the Earth in a very elongated orbit 200 to 400 times as far from the Sun as we are was causing the clustering. 

 

The hunt for Planet Nine has been a huge challenge as it would be extremely dim.  There are ideas where it may be, but so far nothing.  The search continues and other discoveries are being made.

 

The real reason for the post is to show scientists have lives. Konstantin happens to play guitar in a small rock group.  Eduardo Marturet and composed a piece of music for symphony orchestra and electric guitar in honor of Einstein's birthday.  So why not have a physicist as guest soloist?   This past Sunday Konstantin joined the Miami Symphony Orchestra by video link for the performance.  Right out of Buckaroo Banzai.

If the piece isn't to your taste you can scroll around - say to about the five minute mark - and watch Konstantin play with his slate blackboard in the background (physicists and mathematicians  - even the young ones - still use chalk)

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¹ A quick way to think of the size of the solar system is to consider the distance of planets from the Sun in terms of astronomical units.  One AU is the average distance of the Earth from the Sun.  So moving out from Earth and rounding:

Earth                   1.0

Mars                    1.5

Jupiter                5

Saturn                 10

Uranus                20

Neptune              30

Pluto                    30 - 50 

hacks and homebrew

About twenty years ago a Summer student I had a few years earlier called to talk about digital cameras.  She was waiting for the cost of "good enough" sensors to drop, but was excited by the prospect of eliminating the film costs.  Soon we were talking about the total cost of taking a photograph.  There was the cost of a trip if you wanted one from the North rim of the Grand Canyon and a photo of the Earth from the Moon would cost billions - less if a robot took it.  There was an interesting optimization question:  how far up could you go to take a photograph for a fixed amount of money?  One of us suggested five hundred dollars.  An airline ticket could get you to nearly 40,000 feet with some planning.  But a weather balloon?  Now we had something to think about!  An hour of dreaming and scheming quickly went by.  It was a fun little gedanken project to play around with.  I moved on to other things the next day and mostly forgot about it.  She didn't.

 

A month later, the phone rang - something about scoring a dozen "Barbie's First Cameras" for ten dollars.  "That's four hundred and ninety dollars to go...you in?"  The cameras were cardboard disposables with with a plastic lens and shutter assembly and about a dozen exposures worth of color Kodak film.  They were pink.  Of course I was in. 

 

I found a damaged Trimble GPS with a broken screen for fifteen dollars.  The electronics worked so it was only a matter of getting the signal out and sending it to the ground.  She was responsible for programming an ancient Z80 microprocessor and working out a mechanism to push the shutters.  I repurposed an old amateur radio.  We modified a small styrofoam box with a battery and resistors to keep everything warm.  Then it was finding a weather balloon and helium.  

 

This leaves out dozens of details. Many were ugly, but good enough rules.  This would be easy these days with electronics available to hobbyists. You could do a much better job for under a hundred bucks without the balloon, or just send up an old smartphone.

 

Three of the cameras worked.  This is the best photo from 26,900 meters above Ohio.  

 

The goal of engineering is to find a good, hopefully great, optimization to a problem with several competing requirements.  Usually it's impossible to optimize everything. A car might have cost, performance, fuel economy, safety, time to market, and appearance as desirables. There's no way to maximize everything.  Usually the problem is constrained - a supercar throws cost and fuel economy out the window and turns out to be easy to execute.  Mass market cars are much more difficult.  Great engineers excel at this type of optimization. I don't have the necessary talent and experience.  Fortunately I live in a different problem space.  

 

Experimental science is generally underfunded.   Not only is it difficult to justify buying some gizmo, but sometimes the gizmo you really need doesn't exist.  You need to modify something you already have or can borrow or just build something.  The result is often crude, but it works.   Many physics departments make sure you can qualify for a key to the metal and electronics shops. These days knowledge of 3d printing is almost assumed.  More specialized skills and rules of thumb come with what amounts to the apprenticeship otherwise known as the PhD program. 

 

I don't have an exhaustive knowledge of homebrew equipment, but you see it almost everywhere.   Here are a few I've used or have built

 

° We had a particle detector with an internal atmosphere of argon, krypton and xenon. A cheap way to measure their relative concentrations is to measure the speed of sound in the gas.  In the 70s Polaroid had a camera with an ultrasonic rangefinder.  It sent out a short ultrasonic pulse and timed the return of the reflection to adjust the focus.  Damaged cameras were cheap and we had existing electronics that did a much better timing job.  There are probably a dozen other ways to do this, but it worked.  I later used one of these transducers to listen to bat calls. It led to a series of bat detectors I built as a hobby.  Again - not elegant, but workable and I learned enough to ask informed questions about bats.

° If you need a high voltage, low current power supply, the flyback transformer in an old CRT color TV gives you about 25,000 volts and is free if you can find one in a junk pile.  

° Many experiments need to minimize the effect of the Earth's magnetic field.  Usually a set of three Helmholtz coils - three pairs of circular wire loops current runs through.  Each pair generates a uniform magnetic field.  Three of these pairs, each at right angles to each other, allow you to create a field that counteracts the Earth's field.    I once used Hula Hoops as forms to wind the wire.

° Immersion cookers (sous vide) in a styrofoam lined box are a good way to keep a liquid bath at a stable temperature above room temperature.  Much cheaper than commercial heated water baths.

° Some biology labs I've visited use household refrigerators with plastic baffles and better thermostats saving thousands per over lab grade refrigerators.

° The performance of a small anti-vibration table can be improved by "floating" it on powerful speaker magnets with coils connected to an audio amplifier and simple feedback circuit.

° Home photo scanners (Canon, Epson, whatever) can be hacked to build the guts of a table that can position something lightweight in two dimensions to better than a tenth of a millimeter.  An "x-y" table.   For the price accuracy and repeatability of these scanners are remarkable.

° At Bell Labs in Murry Hill, NJ a prototype x-ray lithography tool created a x-ray beam by shining a beam of electrons on an iron target.  The target? - a VHS tape driven by the tape mechanism taken out of a VCR.   The iron particles embedded in the tape were perfect for the job.  After about ninety minutes a fresh blank tape would go in. 

° Back to my student days at Brookhaven National Laboratory.  One experimental area produced a clean beam of neutrinos. A beam of protons with a target producing a lot of particles including neutrinos. Neutrinos rarely interact with matter - a beam of them would lose about half its intensity after passing through through a wall of lead a light year thick. The problem is some of the other particles will travel several kilometers through the air.  The trick is to put a lot of mass between where they're produced and your experiment.  Somehow BNL managed to convince the government they should store about 50,000 tons of armor plates from WWII ships in a line at the lab ... you know, just incase there's a strategic need for iron in the future.  It's covered with dirt and has nice wild flowers growing on it.  

° Commercial camera lenses can be amazing.  The University of Toronto's Dragonfly array uses two clusters each comprised of 24 400mm F2.8 telephoto lenses to make the equivalent of a 1 meter f 0.4 refractor.  The later would probably cost more than ten times as much - if it was possible to even make it.   It does things other instruments can't and has made important contributions to astrophysics.  

 

I'll stop here. Experimentalists often have to be cheap.  Sometimes it's invention, sometimes it's just getting something working.  It's definitely not engineering - a good engineer would throw up their hands.

 

looking at the algorithmic economy

minipost

If you think about how society and the Internet, you probably know about Joan Donovan of Harvard's Shorenstein Center.  Among other things she's known for rejecting the notion of the "attention economy" as being somewhat gas lighting.  Instead she uses the phrase "algorithmic economy."

Andy Revkin of Columbia interviewed her this past Friday as part of his Sustain What series.  She talks about the possibility of a path to a public interest Internet while safeguarding speech.  Also how disinformation memes develop and need to be watched and how the current control of interests on the Internet fail.   You may or may not agree with her, but she's one of the deeper thinkers in this area and this is a particularly clear introduction to what she's about.  Andy does a great interview job, pushing back here and there to go deeper.  An excellent way to spend an hour.

leaps of faith

minipost

Yesterday I had my first jab of a Covid-19 vaccine - specifically the Pfizer mRNA vaccine.  Despite political, production, and distribution issues several vaccines appeared and more are coming.  Going from decoding the SARS-CoV-2 genome to first injections took only about a half year and a year later a few hundred million doses have been produced. While some corners were safely cut, the real story - at least for the new mRNA vaccines - began nearly thirty years earlier.  

 

Katalin Karikó's work will certainly win her a Nobel Prize next year.   Here's a good telling of the story.  A major point is the work was judged unworthy and finding support was nearly impossible.  After about ten years another player, Drew Weissman, entered and the two of them managed to crack the final bits of the problem.  He'll certainly be sprinkled with the Swedish holy water too. The mRNA approach is important well beyond the current pandemic - it may mark a fundamental change in medicine.  It wouldn't have happened - at least not as soon as it did - without a certain type of reserch.  

 

Curiosity-driven fundamental research requires a certain type of scientist, time and resources.  While much of it fails to produce an immediately useful result,  it has an established track record of changing the world.  Industries don't do this kind of work anymore.  Most universities shy from it - there's a tendency to want results-focused work.  When it's funded you usually find it in physics and astronomy  rather than the biological sciences and that's a problem.   These leaps are driven by hope and curiosity.  They open doors that often require more than a decade of work - usually by other people in different organizations.   Expanding this work is how you create radical change.  I'd never argue results-driven research should be cut - after all, that's where the next five to twenty years come from.  But I'd love to see the Katalin Karikós of the world given more of a space.  And then there are the spin-offs - a good deal of the instrumentation used in medicine came out of  fundamental physics - but that's another set of stories.

 

my that's a big watch you have

 

That watch you're wearing - what does it do?

Sarah always wears the same watch.  A large round face that must be 60mm across and a simple band.  I was curious.

It tells time and it's easy to read at a glance.  She replied

That's about what I expected - just a simple analog watch.  About the farthest athing from a fitness tracker or wearable computer.  As a world champion Olympian on the hunt for gold, Sarah doesn't leave much to chance. But before going into why she doesn't use wearables, it's probably best to backtrack a bit.

 

For the majority of people fitness trackers can be useful.  If you give them to people, about three out of five increase their activity levels for at least a few months.  If they buy on their own it goes up to about four out of five.  It's generally agreed that physical exercise of any kind is healthy and many MDs and medical researchers consider exercise a powerful drug with few negative side effects (you can always injure yourself if you aren't careful).  It is possible that these devices will have a positive long term impact on the population.

 

Less known is what kind of exercise is best for a given person.  Here we get into personal tastes and capabilities.  My undergrad school had a physical education requirement.  These were once more common with roots in the muscular Christianity movement of the late 19^th century.  I'm far from an athlete, but was surprised to find the first meetings were a series of one on one tests and a discussion of what seemed right.  The teacher was more of a physical trainer than anything.  Half joking he told us the college wanted people to be healthy so they could live long productive lives and leave a lot of money to the school.  His job was to find a few activities that we might enjoy enough to continue for decades.   He was trying to help us figure out our own internal version of a fitness tracker.   For me it came down to cycling and rowing.

 

More than a few people buy expensive exercise kit only to quit after a few months.  Everything from simple treadmills to Pelotons.  I'd go with what I learned in college - hire a personal trainer for a couple of sessions to (a) find out what kind of exercise you like, (b) learn some proper form, and (c) learn how to set and keep goals.  Some people respond well to online sessions, but they're often only one-way without analysis of your form.  Some personal trainers consult via video chat and may be the best choice in a pandemic.  Of course that's if you're serious.  If you're the type who responds to closing circles, you can get good results with a fitness tracker or Apple Watch.  Or you can use your bicycle. 

 

What about more serious athletes?  There are a number of tools available to amateurs and pros.  I've had chats with some serious sports science people - the type who work with elite athletes.  They'll tell you it boils down to measurement:

What do you measure?
How do you measure it?
What does it mean?

Basically the same things I worry about in physics and probably many of you do in yours.

 

Much of this goes beyond what a smartwatch can do.  Power ergometers on bicycles and rowing machines, gps tracking at the team level, heart rate analysis, enzyme measurements, muscle fatigue tests, and much more. Sometimes movements are analyzed with thousand frame per second photography.  Many of these reading are combined in an attempt to measure something.  In many sports it's been counter productive (again at the elite level) there aren't direct measurements of what you'd really like to measure - whatever it takes to create the best performance.  You start out not knowing exactly what you're doing and from there it only gets worse - often the advice is about as good as a coin-flip. 

 

Elite athletes also have access to excellent coaches who know what to measure and how to use it in context. This can vary wildly from sport to sport and even from athlete to athlete in the same sport.  These people can answer those three questions for the technologies they use. 

 

Beyond tech the mental game is often decisive.  Humans have dozens of internal senses (we have way more than five senses).  Our interoceptive system has this fantastically powerful neural network in our skull for analyzing complex information from around our body in real time. Some athletes train themselves to be aware of and learn how to read this information.  It's part of the mental game that separates the best from everyone else.  There are a variety of ways to get there including mindfulness and it's an active area of exploration.

 

Watching Sarah is fun.  In a match she's performing some experiments on the other side early on while having an intense conversation with her teammate Melissa. at some point a switch seems to come on and she's playing at another level.   She says she really doesn't notice it - much of it is in a state of flow.  She manages to do this even under intense pressure and knowing how to adjust to whatever state her body is in.  Of course the other top players are doing something similar, which is what makes the sport interesting. 

 

It took her years and years to get to this point.  Fortunately none of us have to do anything like that as we're usually more interested in health and feeling good rather than crushing the competition.  Go with whatever motivates you.  Even brisk walking is great.  Get a fitness tracker if that's a positive motivation.   But maybe the best spent money is a few sessions with a physical trainer to explore the space.

 

Or you can get a dog that you have to walk and enjoy extra mental health benefits along with the exercise.

 

The impact of tech in sport is another long conversation I'm only learning a bit about. 

 

power different

Recently a friend sent a piece on an energy harvesting device that coverts some of your body's waste heat into electricity.  The description was sparse, but it mentioned the average person produces about one hundred watts of waste heat and went on to say that is enough to power a laptop.  

 

umm..  Both true, but there are some problems ...

 

The average adult metabolizes about 100 watts on average to stay alive.  That's about the same as eating 2000 calories of food a day.   The average adult has about 1.7 square meters of skin area.¹  This means we need to get rid of about 0.006 watts per square centimeter of skin area.  Six milliwatts of power per square centimeter.  The photo of the device showed a ring like affair with some fins - maybe two square centimeters of radiator area.  Let's round that to about ten milliwatts for the device - hardly enough to deal with a laptop, which are on the order of 20 watts or even a smartphone which are a few watts.  That assumes all of the waste heat can be harvested with one hundred percent efficiency.  Here things begin to go down hill quickly.

 

A thermoelectric converter on human skin has a theoretical efficiency of about 5% at room temperature.²  This reduces the six milliwatts per square centimeter down to 0.3 mw/cm².  Even if you covered your entire body, you'd only harvest five watts and you'd be shivering.   And that's unrealistically optimistic as these devices rarely have more than single digit efficiencies on top of their thermal efficiency in this temperature range.  Even optimistically we're down to about 0.03 mw/cm² or 30 microwatts/cm².  Something like an armband might have 100 cm² giving us 3 milliwatts.

 

This isn't reason to abandon hope.  There are hardware devices with sub-milliwatt power requirements. Small energy harvesting devices that harvest mechanical or solar energy also exist and make it practical to put tiny sensors with tiny transmitters nearly anywhere as long as your receiver is nearby. The micro part of the Internet of Things (which has its own issues).  It turns out this is a rich area for college engineering projects.

 

But what about larger power demands where it's inconvenient to change batteries?  Spacecraft that travel past the orbit of Mars generally rely on radioisotope thermoelectric generators (RTGs) because the Sun is too dim to make solar power practical.  An RTG puts a thermoelectric converter on something hot - a radioactive material that decays rapidly.  Ideally it's an alpha emitter so you don't need heavy shielding.  There are a number of candidates, but Plutonium-238 (Pu-238) is the most common American choice with a half life of about 88 years. Not only do these generators produce electricity, the extra heat is used to warm the spacecraft's warm.   The current generation of Martian rover is RTG powered. 

 

There are a number of RTG stories.³  I don't want to go on forever, but the nuclear powered pacemaker and artificial heart are among the more interesting.  

 

A problem with early pacemakers was longevity. Batteries had to be replaced every eighteen months or so necessitating expensive and risky surgery.  In the late 60s and early 70s two types of tiny radioisotope generators were manufactured for use by several pacemaker manufacturers. They had enough shielding to be very safe and would last longer than even very young patients.  They were also very cost effective and a few thousand patients received them. Then, in the mid 70s, the practice suddenly halted.   The worry was devices might not be removed before cremation from patients who had died.  This would destroy the shielding and turn an area in and around the crematorium into a radioactive hot zone requiring evacuation and a very expensive cleanup.  

 

The radioactive heart implant never made it into a patient.  To make enough power to run the heart, the RTG generated about 50 watts of waste heat adding to a normal person's 100 watts.  It wouldn't be bad if it could be spread over a skin sized area, but that would be impractical.  It would be like holding an incandescent light bulb.   One of those interesting ideas that you wonder about afterwards...

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¹ The approximation medical researchers usually use is from Mosteller:  (height[cm] * weight[kg]/3600)^0.5  a

² The theoretical maximum efficiency of a heat engine is 1 - Tc/Th  where Te is the temperature of the environment - the air around the device in this case - and Th is the temperature of the radiator - the skin temperature.  We have to use an absolute temperate scale so Th = 310°K and Tc ~ 295°K.  So about 4.8% efficient.. let's call it 5, although it is probably half that with poor coupling to the skin and air. 

³ One could go on for a long time .. the CIA's RTG in the Himalayas is particularly interesting.  And then there's the Apollo 13 RTG that required last minute maneuvers as the limping spacecraft approached the Earth.   The very hot RTG is somewhere on the floor of the Pacific.

 

frozen bubbles

a minipost

 

First a short video

You can make this yourself.  You'll need a chilly day and a cold surface.  I had success as a teenager in Montana with outdoor temperatures ranging from -25°C up to about -10°C.  Colder worked but the effect wasn't as dramatic and warmer just didn't work. Some of you are in areas that are still seeing chilly weather. You could probably do it in a very cold home freezer if you opened it slowly enough that the warm room air didn't mix in very much.  Use a drinking straw to blow bubbles from a soap mixture onto a cold surface.  Then watch in amazement.  Some of the best views I had were at night using a light to illuminate just the area around the bubble.   Children's soap bubble soap works, but I've found better and you might experiment   My favorite recipe created a bit stronger bubble:  

200g water
35g corn syrup
35g dish soap
25g white sugar

Stir and set aside for awhile so the turbulence from stirring settles down.  The corn syrup makes a more robust bubble. The sugar  was based on a hunch that it would create nucleation sites.

 

Here's a piece on some people who looked into the underlying physics.  My basic toy model back then that was sort of on the right track but missed a central insight and lacked the experimental analysis.  Physics gives tools to understand a variety of phenomena.  The underlying physics is very well understood, but much of what happens around us isn't deeply understood as it hasn't been studied or the math is just too difficult.  Sand, for example, is extremely difficult at the fundamental level. 

 

Bubbles have had an important role in physics.  Newton was fascinated by their formation and optical properties.  Faraday (one of the three great scientific minds of the 19^th century and the last physicist who didn't use sophisticated mathematics) used bubbles in a creative measurement that a led him to develop the idea of field theory - something that James Clerk Maxwell (one of the other great 19^th century scientists) used to explain one of the four fundamental forces.  Fields are central to fundamental physics.  They were also part of particle physics experiments for about thirty years in the form of some of the most sophisticated and exotic photographic cameras ever made - including an early test that used beer. 

 

Probably things I should write about down the road, but for now try to freeze some bubbles if you have the opportunity!