### The Appearance of Dilated Time

I've written a bit lately on the appearance of objects moving at relativistic speeds[6].  There are some very intersting non-intuitive results.  For example, a Lorentz contracted sphere will still look, (visually), like a sphere, not an ellipsoid, (see Penrose, Teller, and Boas)[1][2][3].  It turns out there's an analogous phenomenon in realtion to time dilation.

Yesterday over at my ChipDesignMag column[4] I mentioned Brian Greene's explanation of time dilation from his book "Fabric of the Cosmos"[5].  In that description he mentions that if Lisa, who didn't change her velocity  looked at Bart's watch as he sped away, she would see his watch moving more slowly because of time dilation.  Well, sort of.  It is true that Bart would be moving more slowly through time, but if Lisa could look at Bart's watch, that's not necessarily what she would see.  In this month's Physics Teacher, Frank Wang[7] of LaGuardia Community College points out that if Bart's watch actually flashed a light each time it clicked a second off, then Lisa would see Bart's seconds as being shorter than hers as he approached and longer than hers as he passed her and moved away because of a special relativistic Doppler effect.  It's all summed up rather brilliantly in a graph from the article:

As Bart approaches Lisa from the lower left corner, the light pulses, (moving on the dashed lines), reach Lisa, (who is moving along the solid vertical line), at shorter intervals.  As Bart passes Lisa and moves away, they reach her at longer intervals.

References:
1.  Penrose on Lorentz contracted spheres
http://dx.doi.org/10.1017%2FS0305004100033776
Penrose R. (1959). The apparent shape of a relativistically moving sphere, Mathematical Proceedings of the Cambridge Philosophical Society, 55 (01) 137. DOI:

2.  Boas on Lorentz contracted spheres
http://dx.doi.org/10.1119%2F1.1937751
Boas M.L. (1961). Apparent Shape of Large Objects at Relativistic Speeds, American Journal of Physics, 29 (5) 283. DOI:

3.  Terrell on Lorentz contracted spheres
http://dx.doi.org/10.1103%2FPhysRev.116.1041
Terrell J. (1959). Invisibility of the Lorentz Contraction, Physical Review, 116 (4) 1041-1045. DOI:

4. ChipDesignMag column
http://chipdesignmag.com/carter/2013/03/21/fringe-science-and-the-science-of-meanness/

5.  Fabric of the Cosmos

6.  http://copaseticflow.blogspot.com/2013/03/lorentz-contraction-accidental.html

7.  Frank Wang's "Physics Teacher" article
http://dx.doi.org/10.1119%2F1.4792010
Wang F. (2013). Moving Clocks Do Not Always Appear to Slow Down: Don't Neglect the Doppler Effect, The Physics Teacher, 51 (3) 154. DOI:

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### Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems, there seem to be fewer explanations of the procedure for deriving the conversion, so here goes!

What do we actually want?

To convert the Cartesian nabla

to the nabla for another coordinate system, say… cylindrical coordinates.

What we’ll need:

1. The Cartesian Nabla:

2. A set of equations relating the Cartesian coordinates to cylindrical coordinates:

3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system:

How to do it:

Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables.

The chain rule can be used to convert a differential operator in terms of one variable into a series of differential operators in terms of othe…

### The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1.  While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero.  This is typically explained in the following way.  While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare.  That doesn't stop us from looking for them though!

Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2].  Cabrera was utilizing a loop type of magnetic monopole detector.  Its operation is in concept very simpl…

### Unschooling Math Jams: Squaring Numbers in their own Base

Some of the most fun I have working on math with seven year-old No. 1 is discovering new things about math myself.  Last week, we discovered that square of any number in its own base is 100!  Pretty cool!  As usual we figured it out by talking rather than by writing things down, and as usual it was sheer happenstance that we figured it out at all.  Here’s how it went.

I've really been looking forward to working through multiplication ala binary numbers with seven year-old No. 1.  She kind of beat me to the punch though: in the last few weeks she's been learning her multiplication tables in base 10 on her own.  This became apparent when five year-old No. 2 decided he wanted to do some 'schoolwork' a few days back.

"I can sing that song... about the letters? all by myself now!"  2 meant the alphabet song.  His attitude towards academics is the ultimate in not retaining unnecessary facts, not even the name of the song :)

After 2 had worked his way through the so…