Explain which direction is left, and which direction is right, to an English-speaking alien from Alpha...

physics/left.right.aliens New

New entry, edited from material collected for the archive and not yet in a released version.

Problem

Explain which direction is left, and which direction is right, to an
English-speaking alien from Alpha Centauri.

1.  This alien is completely fluent in English.  Thus, the alien has the
    words "left" and "right" in its vocabulary; however, you do NOT know
    if the alien is referencing these words as we do, or if it does the
    opposite (ie. switch the directions left and right).
2.  You can't see the alien, and it can't see you.  You are in
    communication via an instantaneous interstellar voice-only radio.
    The alien is 4 light years away.
3.  The alien understands all scientific principles which we hold to be
    "true."  In other words, it will understand when you say "entropy,"
    or "quantum mechanics," or "ozone degradation via chlorofluorocarbon
    catalysis," or any other scientific mumbo jumbo.
4.  The alien is highly intelligent and if you were to see a picture of
    it you would swear on this newsgroup that it was a chimpanzee.

You must explain which direction is left and which direction is right
without using left and right as references in your solution.  If you
think there is no solution, prove it.

-- Dave Oei <itdave@chip.ucdavis.edu>

Show solution

This puzzle is from Martin Gardner's book "The Ambidextrous Universe",
which is about symmetry in physical theory.  Assuming that you and the
alien have no way of agreeing on common objects such as visible stars,
galaxies and so on, then if physical theory were perfectly left-right
symmetric then there would be no method of conveying the meanings of
"left" and "right" purely through exchange of information.

But physical theory is not perfectly left-right symmetric, and there are
symmetry-breaking events.  Gardner discusses one experiment, in which an
excited Cobalt-60 atom can be observed to emit more particles from its
south magnetic pole than its north.  This allows the identification of
electromagnetic north, and it's simple to get from there to "left" and
"right".

-- Gareth Rees <gdr11@cl.cam.ac.uk>

[ Editor: a second posting, "Left or right? (some wrong solutions)": ]

A lot of solutions have required the aliens to already know the names
for two directions.  Just as with "left" and "right", the names for
these directions are arbitrary, and the aliens are just as much in the
dark with respect to them as they are with respect to "left" and
"right".  These names for directions include:

  which angles are positive and which are negative
  which motion is clockwise and which anticlockwise
  which way is east and which is west
  the difference between |_ and _| (or 'b' and 'd') [1]
  which magnetic pole is north and which south [2]

[1] Sending the aliens pictures won't help, because [any sensible
statement of] the puzzle assumes that the aliens are so far away that we
can't send them physical objects or point to objects in the sky that we
both can see, but can only communicate by sending pure information [4].
Pictures must be encoded as information streams, but then how do we know
that the aliens haven't reconstructed the pictures left/right reversed?

[2] It is not the case that magnets attract objects with electric charge
(if it were, then we could define magnetic north with respect to the
charge on an electron [3]); in order for the electromagnetic effect to
happen, either the magnet or the electric charge must be moving, and the
direction of movement determines the effect.  You still have to specify
a direction.

[3] To do even this we would have to assume that the aliens don't live
in an antimatter galaxy (and thus understand "positron" when we say
"electron").  Given what physicists think about proportions of matter
and antimatter in the universe, this seems a reasonable assumption,
though.

[4] This necessary condition makes the puzzle rather unrealistic.  The
aliens are so far away that we can't point to objects in the sky that we
can both see, but they are close enough that we can talk to them!  Think
of the puzzle as illustrating by analogy the concept of symmetry in the
laws of nature.

-- Gareth Rees <gdr11@cl.cam.ac.uk>