
Quantum effects are normally important only on
atomic scales or smaller. Gravity is the dominant
force for very massive bodies. Under what
conditions will quantum effects become important
for gravity?

Question Date: 20040415   Answer 1:
The more massive an object, the more gravity it
exerts. Mass has nothing to do with volume, except
in that other forces (electromagnetism, nuclear
forces)resist an object being made too small. If
the mass,and gravity, are enough to overpower
these other forces, everything collapses to a
point (called a black hole), and at that scale,
quantum effects become important. Just how that
works is, well, the first person who figures that
out is guaranteed to get the Nobel Prize!
  Answer 2:
It turns out that recent experiments discovered
that gravity can have a quantum effect. Gravity is
a very week force, particularly at small distance
scales, making it extremely difficult to see its
quantum effects.The larger things are the more
subtle the quantum effects are. Even small
molecules are almost too large to see quantum
effects clearly. So how did physicists measure
the quantum effects for gravity? Ultracold
neutrons are very slowmoving, uncharged
particles.Physicists isolated these neutrons from
the effects of the other three fundamental forces
(electromagnetism, and strong and weak nuclear
forces) and followed the progress of hundreds of
these ultracold neutrons falling from the top of a
detector to the bottom. The team of
physicists discovered that the particles only
existed at certain heights! They didn't move
continuously, but rather jumped from one height to
another, just as quantum theory predicts! This
research was done just over 2 years ago, and the
implications are still being explored.
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