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Shadow Play: Radiation pressure - not gravity - ma
Shadow Play
What drew matter together in the early universe to form galaxies and
clusters of galaxies? Prevailing theory, which holds gravity
responsible, has been challenged by two investigators at the
University of Arizona. Craig J. Hogan and Simon D. M. White write in
Nature that "mock gravity," an effect of radiation pressure, may have
precipitated the coalescence instead.
Radiation pressure is an analogue of the more familiar, readily
experienced kinds of pressure. Just as expanding gas in a cylinder
head exerts force on the vessel's walls and will cause a piston to
move if the pressure is sufficiently high, so light exerts force on
particles and will cause them to move if the radiation is sufficiently
intense. On the earth radiation pressure is negligible compared with
the strength of gravity. In deep space radiation pressure assumes
increasing importance.
If two particles lie in a beam of light so that the first particle
shadows the second, the first particle "feels" the radiation pressure
of the light, whereas the second particle does not. As a result the
first particle will move toward the second. In the space between
stars, where radiation comes more or less evenly from all directions,
two nearby particles can lie in each other's shadow. Mock gravity is
the attractive force the particles feel because of such mutual
shadowing. Hogan and White estimate that in space the force can be
200 times as strong as the gravitational attraction between the
particles.
If the mock-gravity theory is correct, interstellar dust particles
should produce a distinct pattern of infrared radiation. The Cosmic
Background Explorer satellite, which is scheduled for launch at the
end of the decade, should detect such a pattern if it exists.
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