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explain it in 60 seconds
are extremely difficult to measure.
While we know precisely how
much an electron weighs, we have little information on the mass of
its neutral partner, the electron neutrino. The same is true of the
muon neutrino and tau neutrino.
For a long time scientists thought neutrinos might be massless.
Then experiments revealed that the three types of neutrinos can
transform into each other, a process known as neutrino oscillation.
According to quantum theory, this is possible only if neutrinos have
mass. Cosmological observations and laboratory-based experiments
indicate that the masses of the three neutrino types must be extremely
small: The electron, the lightest charged elementary particle, is at
least a million times heavier than any neutrino.
Physicists think the origins of neutrino masses are closely tied to
subatomic processes that took place right after the big bang.
Determining which neutrino types are heaviest and lightest—the neutrino
mass ordering—is a first step toward revealing these processes.
So far, neutrino oscillation experiments have provided some information
on the differences in mass between the different neutrino types.
Future experiments, requiring accelerator-produced, high-intensity
neutrino beams traveling at least 500 miles through the Earth, will
tell us what the neutrino mass ordering is.
Gary Feldman, Harvard University
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This issue features the work of
physicist and photographer David
Kirkby, whose photographs of
ordinary objects aim to give people
insights into physics. Here, a
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the invisible force that is accelerating
the expansion of the universe.
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