In one of the letters written to the Infeld group in Warsaw Einstein wrote:
"A new manner of thinking is essential if humankind is to survive."

  

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Paradox of Langevin's Traveler 
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PROBLEM

Interpreting the time contraction of the 
Special Relativity Langevin imagined his famous
Traveler who after having moved during 2 years
along a straight line, with the constant speed 
v=(799/800)/c would find Earthians older by
40 years.

That takes us a bit in deep waters. In SR 
movement is strictly relative. If observer O1
of inertial referential R1 observes some speed
determined phenomenon in another inertial R2,
then O2 of R2 will observe an equivalent 
phenomenon in R1.

To somebody flying fast close to us earth will 
look like a flat disk. But, then, to us, this 
flying fellow will look just as flat.

So, in spite of the recognized authority of 
Langevin, we see that something must be wrong: 
if Earthian gets 40 years older during Traveler's 
2 years, then Traveler gets symmetrically 40 
years older during Earthian 2 years. Each gets 
at the same time younger and older than the other 
one. A pretty kettle of fish!

And, to make it easier, the Traveler could never
have been among us, nor can he land on the Earth 
to enjoy his lovely youth: both start and landing
mean acceleration and in the Special Relativity
there ain't no such animal; inertial referentials
move with respect to one another with constant 
speed along straight lines and that's all.

With General Relativity we could fairly easily 
pin the Traveler down (we shall restate him there
as the "Twins Paradox"), but all textbooks quote 
him as part and illustration of the Special, 
so with Special we must deal.
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COMMENTS

So, we see again how most textbooks dump 
crap on poor students (see Infinitesimals & 
Differentials). The Traveler is no example,
nor illustration of Special Relativity, but a
warning against silly textbooks.

But what about Langevin himself? Was he as 
silly, as textbooks quoting him? 

Certainly not. Langevin was a great scientist 
and when he talked about his Traveler, he talked 
to the club: Einstein, Lorentz, Bohr, etc. 
These people knew that he would not talk flat 
crap and they could read between the lines. 
He was not addressing poor science fiction writers 
who jumped on the opportunity to depict supermen 
coming back from the space young and fit, just 
in time to prevent old idiots to blow up the planet.
I was not there and cannot say for sure what he 
really meant, but my guess would be that he
predicted such Travelers as the meson. 
Now, meson's relative lifetime T increases with 
its relative energy E (Eo being its rest energy):

T = To(E/Eo) = To * sqrt(1-v^2/c^2)

That would mean that even assertions of best 
scientists should be carefully interpreted 
before being dumped on innocent students.
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