omenti

a century of einstein's gravity

In a few weeks we'll be at the centennial of the publication of Einstein's paper on general relativity.  Special relativity came out about a decade earlier nailing down what is relative and what is absolute about time, space and motion.  It is beautifully elegant and even simple. The only math is high school level geometry and with a bit of dedication and logic a high school student can understand the basics.  

 

General relativity is a different beast.  Mathematically complex - Einstein needed serious help with the math - it is traditionally first introduced as a junior or senior level course for physics majors with a much deeper dive in grad school.  But it is both beautiful and important, so the challenge is to describe the basics accurate enough.   Randall Munroe of xkcd fame has a nice little introduction in  November 18 issue of The New Yorker - The Space Doctor's Big Idea.   He restricts himself to a vocabulary of the 1,000 most commonly used words in American English - a feat in it's own right.  read it!

 

Einstein said he only had a few original thoughts - 'the happiest thought of my life' came to him in 1907 when he was working at the patent office in Bern.

If a person falls freely he will not feel his own weight.

Most people wouldn't go much further, but Einstein saw that an accelerating reference frame was equivalent to a gravitational field.  From there things quickly into some difficult math, but Randall paints some of the results.  My approach usually begins with: 

down is the direction in which time runs more slowly...

In any event there will be several explanation in the popular press in the next month.  If you haven't been exposed give them a read - start with Randall's.  You can get a rough overview - good enough to follow some fascinating astrophysics a bit better.  And here you have a leg up on Einstein.  In his day it was felt the effects of general relativity would be very difficult to detect.  It was all about getting closer to Nature.  Now we have that, but also a powerful tool to probe a Universe much more exotic than the human imagination could conjure up by itself.   It sometimes holds that the equations of Nature seem to know much more than the people who discover them. 

 

If you've had an introductory physics class in college you might enjoy taking a look at Relativity : the Special and General Theory by Albert Einstein from 1916.  He was a good writer and the translation is good. Many of the explanations by others follow it's form.  

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Recipe Corner

 

nope, nothing from me.  'too close to Thanksgiving.

 

 

particle physics you can use

A few days ago I learned a parent of a close friend will be having a PET scan in the near future.    There are several non-invasive methods of looking into a living body each with its own niche.  Most were developed by physicists using instrumentation and techniques developed for studying physics - a nifty side effect that has impacted millions of people.

 

Positron Emission Tomography is right out of particle physics with a bit of biochemistry mixed in for good measure.  You need a cyclotron, gamma ray detectors and computational techniques that were mostly well developed by the 60s and further refined since then.  Tomography is the reconstruction of images from image sections that were created by some form of penetrating wave.  CT scans, MRI, fMRI, and PET are commonly used in medicine and biology, but the technique is broadly applicable elsewhere.  Underground water, pockets of oil and natural gas, even the interiors of pyramids.  

 

Positron emission is the sexy part...

 

There are times when you want to map out regions of the body that happen to have very high metabolic rates - active cancer sites for example.  You want the cancer stand out from surrounding tissue.  Bonus points for being able to place it in three dimensions and measure metabolic rates in as much detail as possible.  To do this you fool part of the metabolic process.

 

A tracer called fluorodeoxyglucose does the trick (I never remember what it stands for and had to look it up - I just remember it as [¹⁸F]FDG).   The name gives a hint.  It is a glucose-like molecule that your body will happily substitute for garden variety glucose.  An oxygen atom has been replaced by a radioactive isotope of fluorine with nine neutrons and nine protons in its nucleus - fluorine 18 or ¹⁸F.  About 98% of the time it undergoes a positive beta decay to oxygen 18 with eight protons and ten neutrons.  Effectively a proton has been converted into a neutron lowering the atom's atomic number by one and turning it into oxygen, with a positron and an electron neutrino leaving the nucleus to make the physics work out.  Directly detecting the neutrino is very difficult, but fortunately the positron provides the useful signal.

 

A positron is a bit of antimatter - an anti-electron.  It has a positive charge in contrast to the electron's negative charge but both have the same rest mass - 511 keV (thousand electron volts).¹  Since opposite charges attract and it is moving out into a world of matter its life is numbers.  After a very short trip - usually less than a millimeter, it runs into an electron and the two of them annihilate becoming two packets of pure energy.  These packets, or gamma ray photons, each have an energy of 511 keV and leave in opposite directions at the speed of light.  Some will be stopped by flesh and bone, but mostly they just keep cruising.

 

To catch and detect the gammas a tube shape detector surrounds the person.  It is composed of segments of a scintillator material  - usually an inorganic crystal -  that glows when a gamma ray strikes it.  The brightness of the glow is proportional to the gamma ray's energy and a photodetector makes note of the time and the energy deposited.

 

Remember that I said the gamma rays left in opposite directions?  That was in the rest frame of the positron.  The positron was moving relative to the patient so the gamma rays will not be seen as 180° apart, but nearly so.  The scintillator array has segments lighting up randomly in position and energy.  When two segments light up at the 511 keV annihilation signature and a straight line running through the person's body connects them,  the detector's logic says - 'aha!  I know how to draw a line roughly through the point in the body where the annihilation took place.  Correlated events are saved to memory - all uncorrelated scintillator flashes are ignored. 

 

The process continues and thousands of events are noted allowing a 3d image can be constructed. Higher intensity corresponds to a higher metabolism.  For a good image you need a lot of ¹⁸F - a dose of roughly 14 millisievert or about twice a chest CT scan or 100 chest X-Rays.²   The ¹⁸F has a half life of about 1.8 hours.  24 hours after it is injected it has gone through a bit over 13 half lives - only (1/2)¹³ of it remains about about 1/8,192 of the original amount. Its radiation level is very low at this point and it is safe for normal handling.³  But that presents a problem - at such low dosages it is useless as a radioactive tracer.  Even with overnight shipping you would have to make dangerously large and expensive amounts of it to be useful.  You need an on-site cyclotron.

 

A cyclotron is a simple particle accelerator.  Old cathode-ray tubes are simple linear accelerators (much more fun to use for that purpose than for watching TV).  The got up to about 25 keV ... not enough to be very useful in nuclear medicine. More powerful accelerators tend to be long.  A neat trick found in the thirties was to make them circular - the cyclotron accelerates a charged particle, usually a proton, up to a few million electron volts in its simple form.  The CERN accelerator uses a similar principal refined over the decades.  Most medical cyclotrons are used to make ¹⁸F for on-site FDG.  A few million dollars of kit allows you accelerate a non-trivial number of protons (tens of milliamps)to about 99.5% the speed of light and direct them to a target of ¹⁸O enriched water (a different sort of heavy water).  The clock is now ticking and you have to do some rather fast chemistry to make the FDG ..  this isn't exactly easy, but is done every day at a few hundred sites around the world. 

 

So if you or a loved one needs the technique you have instrumentation physics people at Brookhaven National Lab and several universities around the world and the budget for fundamental science to thank...

 

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¹  The positron and electron have some other differences, but they're not factors in nuclear medicine. Physicists measure mass-energy in a variety of units - an electron volt is very small.  A gram is about 5.6 * 10³² eV.  Technically it is energy change when you move an electron across a potential difference of one volt. 

² Not to worry - while that sounds a lot, the body can handle a few a year.  The danger from radiation is much lower than exploratory surgery!

³ Some of it remains in the body, much of it is excreted.

 

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Recipe corner

 

Thanksgiving is coming -- what more can I say?  If you're not an American it still makes sense to have a holiday devoted to family and friends and a bonding meal.  A warning though...  a few years ago I counted the pre-meal goodies before guests and again just before the meal.  The average person had about 1,200 calories of nuts, chocolate, and other treats before dinner!

scaling the s curve

Around the time cellphones began to become available a story began to circulate.  

Early on a Saturday morning in Lancaster County, Pennsylvania an Amish horse-drawn buggy and a Mercedes were in an accident.  Both vehicles were too damaged to move and a passenger was injured.  Waiting for help to drive by in this rural area might take too long. but fortunately one of the Amish had a cellphone.

The story is apocryphal, but the Amish are often called techno-selectives.  The Amish and a few other groups like the Hutterites are conservative Christians known for their resistance to much of modern culture. Old Order Amish reject reject electricity from a  public power grid, radio, tv, cars, and contemporary fashion.   They operate as  collectives and, given the realities of land and supply prices, have become good at business as a survival skill.  Some collectives are more progressive than others and have moved into businesses not focused on community support and agriculture adapting modern tools along the way.  

 

Generally the view is technology is usually neutral, but you shouldn't have it if it doesn't promote fellowship.  Harold Rheingold noted: For them, it is not just how you use the technology, it is what kind of person you become when you use it.¹  Church leaders evaluate technologies rejecting them outright, allowing them with specific restrictions or allow them unconditionally.   Rollerblades, boats and barbecues are fine as they bring families and community together while cars, radios and tvs are seen as keeping people apart or as counterproductive distractions.  Cellphones may be used in a business, but not within a home. A computer might be part of a dairy business, but it might be in a barn and run by batteries that are recharged with portable generators. The rule making is fascinating as the concept of contemplating how a technology will impact your life is foreign to most of us.  

 

Cellphones not withstanding, the decision to adopt a technology conditionally or unconditionally usually comes when it very mature and most of its new users are technology laggards - the late adopters.  A technologies market share over time often has a characteristic S shape.²  The point a technology begins to take off, where it reaches something like a halfway point or a bit beyond, how quickly it grows and its saturation point are fascinating questions for business. Horace Dediu actively asks these questions on his excellent Asymco blog.

 

Technologies tend to saturate well below the 100% adoption level and although it is a very useful model,  the curve isn't quantitatively predictive.  I find it very useful to look at the outliers.  Why do a very few technologies to very well as well and why many never seem to live up to original expectations"  The questions the Amish elders ask form a useful framework.  As I get older I'm asking these questions of the things I buy and getting rid of items that fail to serve me well, but this questioning is only in the last decade and is specific to my tastes. To understand it more generally a variety of models can be useful.  

 

Moving away from thinking about people as simple consumers and users (or non consumers or users) of services and devices, you can think of them as networks that are embedded in larger networks.  Relationships between people, services, institutions, culture, the law, etc. that are mediated by technologies and how these relate to non-technical relationships.   The approach is  too broad to consider every possible connectino. Rather the trick is to work out the dominate factors for the problem you're studying.³    It is usually qualitative framework, but you can use the results build semi-quantitative models.  

 

Apple is an interesting example for consideration.  Getting deep into detail is beyond the scope of the blog, but this framework shifts it from being a maker of products that may be vulnerable to commodification to a complex web that involved personal style, how others in the social and institutional groups you interact view you, ecosystems of services involving Apple and others, and so on.  The iPhone has become central to many users and is a useful lens to flesh out connections.  By weighting the network connections you simply the web and begin to look for connected relationships.  Links between the nodes can be examined for stickiness and you can imagine how they may evolve over time. Some actions of the company, like focusing on personal privacy, are of particular interest and may give insight into future direction.

 

Failure can be equally interesting.  An  example I leave to you is to think about VW and the human centered networks of American vs European diesel buyers...

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¹ Harold's fascinating January 1999 piece in Wired:  Look who’s talking

² The curve is a sigmoid function of the form f(x) = 1/(1 + e^-x)^. . Note the denominator is huge for large -x and approaches 1 for large x.  A more general form is f(x) = C/(1 + e^{-k(x -x₀)}), where C is the maximum value of the curve at saturation - eg the total population, k is related to how steep the curve is, and x₀ is the 50% point of the curve.

This general form is very common in many disciplines - if you've taken a course in differential equations it is just the solution for f' = f(1-f).

³ This is the fun of the framework.  It is very similar to the approach used in physics.

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Recipe Corner

 

I'm a huge sweet potato fan and love to roast them.  It is great for warming the house as the weather chills. The measurements are best guesses here.  I started out with equally sized seasonings and adjusted later.

 

Baked Sweet Potatoes

 

Ingredients

 

° 4 medium sweet potatoes

° 1 standard can (15oz) chickpeas drained

° 1/2 tbl olive oil

° 1/2 tsp cinnamon, coriander, cumin and paprika

° 1/2 tsp coriander

° 1/2 tsp cumin

° 1/2 tsp paprika (smoked would be good!)

° sea salt

(sauce)

° 1/4 cup hummus (tahini would also be good)

° juice of 1/2 lemon

° 1 tsp dried dill

° 3 garlic cloves minced

° water to thin

(toppings)

° diced cherry tomatoes

° chopped parsley

° whatever seems interesting... lemon juice, chili sauce...

 

Technique

 

° preheat oven to 400°F

° cut sweet potatoes in half lengthwise 

° toss the chickpeas with olive oil and place on a baking sheet (I line the sheet with aluminum foil)

° rub the sweet potatoes with olive oil and place face down the the baking sheet

° roast for about 25 to 30 minutes until the chickpeas are golden brown

° while roasting is going on make the sauce by whisking all of the ingredients in a bowl with a bit of water to make it thin enough to pour.  Adjust the seasonings.  

° toss the toppings if you're using them

° when ready serve the sweet potatoes flat side up, smoosh down the insides a big and top with chickpeas, sauce and add the garish.

  

hammers and nails

Several pieces have been written about Intel’s fashion partnership initiative. This  from the 2-November New York Times.  Generally they seem to imply that these partnerships will find the correct mix of art and technology. 

 

I find the results to date wanting .. sort of here’s some tech, how to I build a container or wire it in somehow? I’m much more impressed by new materials and fabrication techniques. Work on smart fabrics that self regulate temperature for example. Groundwork is being laid for the future. A few serious designers like Iris van Herpen are exploring the space, but it will take time to find the right voice. 

When I was a student a strange little publication called the Journal of Irreproducible Results published a few joke science and technology papers. It evolved and began to add unusual real papers and paved the way for the very real Ig Nobel Prize awards. The classic JIR paper was originally published in the Cal Engineer by Charles Seim in the mid fifties - A Stress(ed) Analysis of a Strapless Evening Gown.¹ Over the years it had become legendary and known simply as SAOASEG. 

The trigger for remembering the SAOASEG was a racked piece recommended by Jheri - Why Are Sports Bras So Terrible?

 

...

While there are suddenly a ton of options out there for women, not all of them are good. "There is no piece of clothing that is more difficult to design well than a sports bra," says LaJean Lawson, a breast researcher and consultant for Champion Athletics. "There are so many different parameters. It's the most hooked into cultural stereotypes. You have to think about sweat, support, chafing, straps, slippage, and then looking cute. That's a really long list of conflicting design requirements."

 

Lawson has been studying breasts, their movement, and the bras that contain them for Champion since 1984, when she tested the original Jogbra. In 1987, she evaluated the seven sports bras Champion had on offer using a 16mm film camera. Today, she employs the same methods that White's lab does, using complex camera tracking to test hundreds of bras both from Champion and other companies. "I just ordered $1,000 worth of Victoria's Secret sports bras to test in the lab," Lawson says, "and they called me and asked, ‘Who are you, and why are you buying all these bras?'"

...

A very real and serious design problem no one has yet sorted out. Design and materials are currently not up to the task, but a good deal of experimentation is underway to observe and better understand the problem. The tech future of fashion will come from seamlessly solving real problems rather than being involved in a nail hunt. We're beginning to see this and collaborations are centrally important, but they have to be deeper than Intel’s.  

 

Over the centuries a lot of technology has been driven by textiles - arguably programming had its roots in looms.   New classes of textiles are being developed that have the ability to self-regulate skin temperature along with other human centered properties.²  New manufacturing techniques are being developed that will better enable fit and mass customization and infrastructures for automated provenance reports are being designed.  A good deal of change is in the wind - change much deeper than the fast fashion that is the topic of the day.

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¹ Recently republished on Neatorama.  The book of the title is great with other classics  like The Postal System Input Buffer Device  - a very nerdy look at post box design...

² this is a very hot and cool area.  APRA-E has fueled the fire with grants to develop clothing that can reduce heating and cooling loads.  Clothing has always been something of a microclimate and improvements are possible.   UCSD's ATTACH project.

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just a quick post - nothing dramatic on the recipe from other than enjoying roasting vegetables.  The trick is to get enough heat. I like the variety that comes with roasting somewhat different sizes - so be sloppy with your chopping.