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Mass extinctions, Safety glass stress, Magnets & laser
light
Q: How often do mass extinctions and ice ages occur? Is
there one on the horizon? Will humans eventually fall prey to mass extinction?
—Brad S.
A: There’s not a place in the universe that’s safe forever; the
universe is telling us, "Spread out, or wait around and die." —astronaut
Buzz Aldrin and John Barnes in their novel, "Encounter with Tiber."
 Mass
extinctions.
[Dave Seal and Paul Chodas of JPL]Jupiter image showing Comet
Shoemaker-Levy’s impact (artist rendition)
Life has overcome five mass extinctions during its four-billion year history.
The first occurred 450 million years ago, shortly after land plants evolved. It
coincided with an ice age and wreaked terrible species destruction. The last
happened 65 million years ago when a giant meteor collided with Earth and wiped
out the dinosaurs. Averaging the times, we get a mass extinction about every 90
million years but the events tend to cluster.
Ice ages.
The first ice age happened 2,200 million years ago when life consisted of
only one-celled creatures living in the ocean. Then, ice blanketed land near the
South Pole that became Africa and the Middle East. Glaciers covered even regions
close to the Equator. The next ice age, 650 million years ago, may have covered
all Earth in ice—a snowball. If so, that ice age killed all life except a few
beings huddled around volcanic vents on the sea floor.
The third ice age caused a mass extinction 450 million years ago. The last
ice ages began about two to three million years ago, occurred every 100,000
years, and covered 30 percent of Earth.
Mass extinctions of the future (and maybe the present).
More big meteors will crash into Earth, wiping out many species. "Earth is an
unsafe neighborhood," says William E. Burrows, aerospace historian. The
next glacial maximum will probably hit in another 80,000 years, killing more.
However, we may be in the middle of a mass extinction "It is happening now,
and we, the human race, are its cause," says Richard Leakey, African
anthropologist. In the past 400 years, 611 species have disappeared. For the
past 3.5 billion years, life has survived a steady extinction rate of one
species per year. Humans have been killing plants and animals at a much faster
rate by polluting their habitats, hunting them, or clearing the land for
farming.
Human survival.
We have colonized Antarctica and probably have the skills to survive an ice
age—as a species. But either an ice age or a meteor impact could devastate our
civilization. The fossil record shows that mammals (like us) rarely survive more
than four million years. It is prudent to spread out and colonize the Moon—for
starters.
Further Surfing:
U of California,
Berkely: A chart showing geological time periods and descriptions of each period
Ralph Taggart, Michigan
State U: Extinction of the dinosaurs and impact hypothesis
Richard Leaky,
The sixth extinction
Q: How is safety
windshield glass made? Why does it break into little rounded
pieces instead of big dangerous shards? —Lanney, Sandia Park, New Mexico
A: A manufacturer sandwiches a sheet of vinyl between two sheets of
shaped, annealed glass (a strong glass formed by heating, bending, and slow cooling) to form windshield glass. It’s called laminated safety glass. The
glass maker bonds the sandwich together by applying heat and pressure in a
special oven. The surface can withstand great impact, but, if it breaks, the
broken glass pieces stick to the vinyl and don’t fly around.
Why safety glass crumbles instead of shatters is a property of monolithic tempered glass
usually used in side and back windows. By monolithic I mean a single
homogeneous, solid glass sheet. Tempered glass isn't used in windshields because
it could obscure forward vision when it breaks into small pieces.
To make tempered glass, the manufacturer heats glass, raising its temperature to 1200 °F
(650°C). The maker pops the glass out
of the oven and cools it fast (to 500°F
or 260°C) by
blasting cool air over the surfaces. (That’s the trick: different cooling rates
inside and out. The rapid cooling
stresses the glass.) The center mass then cools more gradually,
compressing the surfaces and edges as it cools and establishing a tensile stress
in the glass midplane. The compression strengthens the
outside faces significantly. The differential cooling establishes a
residual stress pattern in the glass.
The compressive
stresses in the surface and edges balance the tensile stress in the midplane of
the glass, says Robert Fiedor of PPG Industries. So, the glass stays together
through this strong balance.
However, if an impact breaks the glass later, it will break along the
residual stress pattern and form granular glass pebbles. The greater the initial
stress, the more safety pieces of smaller size, says Fiedor.
"When properly tempered to DOT [Department of Transportation] specs, the
glass will break into 1/4" x 1/4" safety pieces," says Sheree Funkhouser of
Peninsula Glass Company.
By the way, auto designers have improved automobile safety in a number of
ways.
- In 1966, windshield manufacturers increased the thickness of plastic used
in windshield laminating to 0.030 inches. This quadrupled the windshield
penetration resistance.
- Seat belts and air bags---now standard equipment in cars---have lessened
the probability of hitting your head against the windshield.
- Flying glass particles from the windshield are a minimal concern since the
broken glass sticks to the vinyl part of the windshield sandwich.
Further Surfing:
Alumax Bath Enclosures:
Tempered glass
PPG Industries: Car
safety glass demo
![[NASA] Laser light reflected by mirrors but unbent by magnets](images/laser1.jpg) Q:
Can a magnetic field interrupt the path of a laser? Can a magnet bend light?
—"Bending light", Vancouver, British Columbia, Canada
[NASA] Laser light reflected by mirrors but unbent by magnets
A: A magnet can bend the path of any moving charged particle. In fact,
that’s how your TV screen displays a video picture. A magnet deflects a beam of
electrons to create a video pattern on the screen. Light, however, has no charge
and therefore its path is unaffected by a magnet.
But, you mention a magnetic field, which includes changing fields. If it’s a
changing field, things get more complicated. Any changing magnetic field
generates a changing electric field and that produces an electromagnetic wave.
Electromagnetic (EM) waves cannot interact directly with light photons since
photons have no charge. EM waves do not bend light, at least enough that we can
measure. If radio waves, for example, bent light appreciably then a transmitting
radio station would look blurry. But stations don’t go blurry.
Actually, electromagnetic waves can bend light through an indirect, quantum
effect—but to such a tiny degree that we cannot measure it. This quantum effect
(called Delbr ück
scattering) "is a process where, for a short time, the photon disintegrates into
an electron and positron pair," says Norbert Dragon, physicist at the Institute
for Theoretical Physics in Hanover, Germany. The charged pair interacts with an
EM wave and then recombines into the photon with a changed direction. Thus, the
EM wave bends the light.
"More probably the charged pair will annihilate into two or more photons—this
process has been observed under extreme conditions—but, then, the light ray is
not bent but rather split into several rays," says Dragon.
A positron (an anti-electron) has the same mass and charge magnitude as an
electron of ordinary matter but the anti-electron has a positive charge. It
quickly reacts with an electron. The two annihilate each other and produce two
or more photons in the observed cases.
Further Surfing:
Fraunhofer Institute
for Lasertechnology ILT: Laser principles
U of South Wales: Quantum electromagnetics
(Answered March 21, 2003)
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