Galactic collisions explained
Perhaps you can explain why Andromeda and the Milky Way are going to
collide (“Milky Way will be hit head-on,”
SN: 7/14/12, p. 10). Galaxies, as is always
written, are rushing away from each
other at ever-increasing speeds. How
do things collide when there is never
anything to collide with? Either galaxies
are rushing outward in all directions
with the same impetus from the point
of the Big Bang, or they are not.
Bruce Smith, via e-mail
Galaxies do indeed collide with one
another even though the universe is
expanding at an ever-accelerating
rate. In the case of the Milky Way and
Andromeda, the two galaxies are massive
enough and close enough that they are
gravitationally attracted to one another.
So they are headed for a collision even
as both galaxies continue to be carried
along on the flow of the expanding universe. You can think of it like two boats
running into one another in the middle
of a river; they collide even though both
are also being carried downstream by the
current. — Alexandra Witze
Clarifications
The high energy (100 billion billion
electron volts) of a particle mentioned
in “Chasing a cosmic engine” (SN:
7/14/12, p. 16) refers to the particle’s
energy when it hit the atmosphere,
not when it reached detectors on the
ground. The article should have said
that in 1962, the particle slammed into
the atmosphere, creating a shower of
particles that rained down on an array
of detectors in Volcano Ranch, N.M.
The image of human papillomavirus
shown in “Catching a cancer” (SN:
7/28/12, p. 22) is a photo illustration
based on a micrograph and shows
a single virus particle at multiple
magnifications.
Send communications to: Editor, Science News,
1719 N Street, NW, Washington, D.C. 20036 or
editors@sciencenews.org. Letters subject to editing.
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