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## Lagrange, Laplace, and Legendre: which one is which?

Anyone who studies physics and/or mathematics has often encountered the following conundrum:

How do you distinguish 18th-century French mathematicians with surnames beginning with an “L”? (I call these E.C.F.M.W.S.B.W.A.L.’s)

For example, you might recall that an E.C.F.M.W.S.B.W.A.L. invented the calculus of variations, some time around 1760.  Was it Legendre?  Lagrange? Laplace?  Or maybe you remember that an E.C.F.M.W.S.B.W.A.L. was the father of probability theory, and worked on the Buffon needle problem.  Was that Laplace?  Legendre? Lagrange?

So as a public service, I’ve sorted this out for you.  I henceforth talk about these three great mathematicians, and hope to distinguish them in your mind.

Lagrange: perhaps the best mathematician of the 1700’s.

Lagrange is the oldest of the E.C.F.M.W.S.B.W.A.L.’s, born in 1736.  Some call him the greatest mathematician of the century, although I might give that title to Euler.  In any case, he’s responsible for a host of discoveries: he pretty much invented an entire branch of mathematics, the calculus of variations; he used this tool to reformulate classical mechanics (think L = T – V) making it suitable for non-Cartesian coordinates, such as polar; he invented Lagrange multipliers, an elegant way to deal with constraints in differential equations; and he introduced the f(x),f'(x),f”(x)…notation for derivatives.

His greatest work was Mécanique analytique; all of the above achievements are found in this book.  Hamilton described the work as a “a scientific poem,” for its elegance is astounding.

Lagrange

Lagrange was rigorous and abstract: he bragged that the Mécanique analytique did not have a single diagram.  To Lagrange, math was an art; the aesthetics of a theory took precedence over utility.

Laplace: the “applied” mathematician

Laplace was seven years younger than Lagrange, born in 1749.  He also is associated with classical mechanics, but unlike Lagrange, he did not reformulate the field per se.  Rather, he took Newtonian mechanics to its “apex” with his work Mécanique céleste.  This work is brilliant, but it’s also clunky and difficult.  It analyzes the orbits of all known bodies in the solar system, and concludes that there is no need of God to keep the whole mess going.  In fact, Napoleon supposedly asked why Laplace didn’t mention God in the Mécanique céleste.  He reportedly said “I have no need for that hypothesis.”

Laplace

Laplace didn’t place as much emphasis on “beauty” in mathematics.  To him, math was just a tool.  Not surprisingly, he contributed to the “applied” field of probability theory; in fact, he’s arguably the founder of probability theory as we know it today.

Legendre: the elliptic integral guy

Although highly regarded in his day, Legendre (b. 1752) is really a tier below the first two guys.  Basically, he worked out how to do some elliptic integrals, and he introduced the Legendre transformation, which is used in many branches of physics.  For example, you can go back and forth between the Hamilton and Lagrange approaches of classical mechanics by means of Legendre transformation.  Also, such transformations are ubiquitous in thermodynamics (think U → H → A → G).

Legendre is also know for the portrait debacle.  Only a single known image of Legendre exists, and that image is not flattering:

Legendre

Every other supposed portrait of Legendre is actually the picture of some obscure politician, because of a mistake which has propagated forward for 200 years.

In summary:

Lagrange: the beauty of math; reformulated mechanics in the Mécanique analytique

Laplace: math as a tool; Newtonian mechanics reaches its zenith in Mécanique céleste; probability theory

Legendre: the creepy looking elliptic integral guy

Note: I have not mentioned Lavoisier (b. 1743) because he was a chemist.  But if you really need him:

Lavoisier: a chemist who was guillotined in the French Revolution.

[Note added Dec. 4, 2014]  I could have included L’Hopital (French, died 1704) but all he did was write a textbook.  Laguerre was French, but he was born in 1834;  Lebesgue was French, but he was born in 1875.

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## Virgin Galactic’s ridiculous, utterly unscientific SS2 graphic

Recently Virgin Galactic suffered a horrible setback: their SS2 “spaceplane” crashed, killing one and injuring another. My deepest sympathies go out to their families; this blog post is not meant to disrespect these brave men in any way.

My beef is with the graphic-design bozos at Virgin Galactic, who give us this laughable graphic:

It looks nifty, sure. But the science (as represented by this travesty) is weak to say the least. In fact, I’ll say more: the science in this graphic is laughable.

First of all, notice how there’s a dotted line that says “edge of space”. It’s like the Mason-Dixon line: on one side, you can buy sweet tea, on the other side, you can’t. It’s nice how they colored space “black” and colored “not space” blue. Thanks. That clarifies things.

In point of fact, of course, there is no “Edge of space”. The atmosphere decreases gradually as you move away from the Earth. Where do you draw the line? Should it be the upper limit of human survivability, around 10,000 meters, or maybe the upper limit of commercial airline flights, at around 18,000 meters? The Fédération Aéronautique Internationale (FAI) puts “space” at 100,000 meters, but that is arbitrary. Nothing special “happens” at that height.

Secondly, notice how the graphic says that there’s “zero gravity” at that height. Sigh. Don’t they go over this in 6th grade?

There’s plenty of gravity in space; at least, where satellites orbit. (I discuss this at greater length in an earlier post.) At 100,000 meters, the acceleration due to gravity g has the value of 9.5 m/s2, compared to 9.8 m/s2 at sea level. That’s not “zero gravity.”

I’m sure what they meant was that the plane is traveling in some parabolic arc, and at that the top of that arc the plane is in free fall, so (momentarily) people on the plane experience the absence of any normal force, otherwise known as a state of “apparent weightlessness”. Oh, who am I kidding. They didn’t mean that…they meant what they said, and what they said was nonsense.

I’m not pointing any fingers for the SS2 disaster, and anyway, the NTSB will figure it out eventually. Until then, don’t rely on the Virgin Galactic design team to know anything beyond 6th grade physics.

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