Transcript 20Oct_2014

Reading
Unit 26, 27, 28, 29, 30
The Michelson-Morley Experiment
• Two scientists devised
an experiment to detect
the motion of the Earth
through the “aether”
– Light should move
slower in the direction
of the Earth’s motion
through space
– Detected no difference
in speed!
– No aether, and the
speed of light seemed
to be a constant!
Einstein’s Insights
• Albert Einstein started from the
assumption that the speed of light
was a constant, and worked out the
consequences
– Length does indeed contract in the
direction of motion, by a fraction
equal to the Lorentz factor
– Time stretches as well, also by the
Lorentz factor
• Moving clocks run slow
• Moving objects reduce their length
in the direction of motion
Special Relativity
• Time dilation and length
contraction depend on the
observer!
– To an observer on Earth,
the spacecraft’s clock
appears to run slow, and
the ship looks shorter
– To an observer on the ship,
the Earth appears to be
moving in slow-motion,
and its shape is distorted.
• The passage of time and
space are relative!
Possibilities for Space Travel
• Example: A spacecraft leaves
Earth, heading for a star 70 lightyears away, traveling at .99c
– To an observer on Earth, it takes
the spacecraft 140 years to get to
the star, and back again
– To passengers on the ship, it only
takes 20 years for the round-trip!
• This means that high speed travel
to the stars is possible, but comes
at the cost of friends and family…
General Relativity: Mass Warps Space
• Mass warps space in its
vicinity
• The larger the mass, the bigger
“dent” it makes in space
• Objects gravitationally
attracted to these objects can
be seen as rolling “downhill”
towards them
• If the mass is large enough,
space can be so warped that
objects entering it can never
leave – a black hole is formed.
Another underappreciated female in science?
Mileva Maric
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Einstein Nobel Prize was for photoeffect,
which was the theme of diploma work by
Mileva. To what extend she inspired Einstein
or collaborated with him is a subject of
debates.
Telescopes
• Telescopes have been
used for hundreds of years
to collect light from the
sky and focus it into an
eyepiece. An astronomer
would then look through
this eyepiece at planets,
nebulae, etc.
• The human eye is not very
sensitive to dim light, and
was replaced in astronomy
by the film camera.
• Film is sensitive to only
around 10% of the
impinging light, and is
usually replaced by a…
The Charge-Coupled Device (CCD)
•
•
The CCD, similar to those
found in commercial digital
cameras and phones, utilizes
the photoelectric effect to
collect around 75% of the
visible light that is focused
on it!
It has revolutionized
astronomy – images can be
recorded and downloaded to
a computer anywhere in the
world for analysis
•
The science of developing new methods for
sensing, focusing and imaging light in
astronomy is called instrumentation
Outside the visible spectrum
•
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Many objects of astronomical interest are
visible only in wavelengths other than the
visible!
Much can be learned from studying a star,
planet or nebula in multiple wavelengths.
Radio telescopes can be used from the ground
to image pulsars and other bodies
•
Observations in other wavelengths
require instrumentation to be lifted
above the Earth’s atmosphere.
X-ray, Gamma ray and infrared
wavelength telescopes are currently
in orbit!
Modern Telescopes
•
•
Modern telescopes are designed to
collect as much light as possible, and
must be built to exacting standards.
Collected light is of nanometer
wavelength, so the telescopes must
be extremely precise to keep the
waves coherent for maximum
efficiency
Radio Telescopes
• Radio telescopes,
like the one in
Arecibo, Puerto
Rico, collect radio
waves from
astronomical objects
and events
Radio Telescopes
• Radio telescope
arrays to achieve
large collecting
areas
National Radio Astronomy Observatory (U.S.A.)
Size Matters!
•
•
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Aperture size is very important
when collecting light!
A large collecting area allows
astronomers to image dim and
distant objects.
For a telescope with an aperture
a distance D in diameter,
Collecting Area =
p
4
´ D2
Refracting Telescopes
• Telescopes that use lenses to focus
light are called refracting telescopes,
or refractors.
• Large refractors are difficult to build!
– Glass is heavy, and glass lenses must be
supported only by their rims, a difficult
engineering problem
– Glass sags under its own weight,
defocusing the light!
– Refractors suffer from chromatic
aberration, a blurring effect due to
changes in the focal plane of the lens
for different wavelengths of light
Reflecting Telescopes
• Reflecting telescopes, or
reflectors, use a curved
mirror to focus light
• Mirrors can be supported
from behind, and so can be
much larger than refractors
• Larger sizes mean that more
light can be collected and
focused, allowing
astronomers to image
dimmer or more distant
objects
• Most modern telescopes are
reflectors.
Different styles of reflectors
X-Ray reflectors
•
•
X-rays only reflect at glancing angles,
otherwise they are absorbed or pass
through the mirror!
X-Ray mirrors are designed to gently
reflect the incoming photons, focusing
them at the end of a long tube-shaped
array of mirrors
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Chandra
Very Large Mirrors
• Reflectors can be made very large
if multiple mirrors are used as the
primary mirror.
• The Keck Telescope uses 36 large
mirrors to create a single huge
primary.
• The positions of the mirrors are
precisely measured by lasers, and
can be individually adjusted to
keep them perfectly aligned.
Diffraction and Resolution
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•
•
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Some stars that appear to be single
bodies to the unaided eye are, when
viewed through a telescope, found to
be two separate stars.
The telescope is able to separate the
two stars, while the human eye is
not.
The telescope, then, has better
resolution than the human eye.
The telescope’s resolution is better
because it has a larger aperture, and
light is diffracted less as it passes
through it.
•
•
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Diffraction is a rippling effect due to the finite
size of an aperture.
Light waves approach the aperture as flat plane
waves, similar to the straight water waves seen
above.
As the waves pass through the aperture, the
waves become curved.
Diffraction Effects
•
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Diffracted light waves can interfere with, or
cancel, each other.
This results in a diffraction pattern, a blurring
of the image as it passes through the
telescope.
Larger apertures have less diffraction, and
therefore higher resolution than smaller
apertures.
For observing light of wavelength nm, the
smallest separation angle arcsec a telescope
can resolve is related to the telescope aperture
Dcm by:
 arcsec
0.02  nm

Dcm
Interferometers
• To counter diffraction effects (and
build telescopes with higher
resolution), astronomers use
interferometers.
• Signals from these arrays of
widely-separated telescopes are
added together to create images
with very high resolution.
• In fact, the resolution is equivalent
to that of a single telescope with an
aperture as large as the separation
in the array!
Before and After
• Before:
– What looks like a single
star…
• After
– …is actually two stars!
Atmospheric Absorption
• The Earth’s atmosphere absorbs most
of the radiation incident on it from
space
• This is a good thing for life – high
energy photons would sterilize the
planet!
• This is not a good thing for astronomy,
however!
• Visible, radio and some infrared
wavelengths are not absorbed readily
by the atmosphere
– Optical and radio telescopes work well
from the ground
• Gamma Rays, X-rays, and UV
photons are absorbed
– Observatories for these wavelengths
must be kept above the Earth’s
atmosphere!
Ground- and Space-based Observatories
Light Pollution
• Ambient light from
cities are a real problem
for optical astronomy.
• This light pollution
washes out images in
telescopes.
• Research telescopes are
built far from cities to
reduce the effects of
light pollution
• It is getting harder to
find good locations for
telescopes!
Atmospheric Effects
• Air refracts light just like glass or
water, but to a lesser degree.
– Cool air refracts light more than
warm air
– Pockets of cool air in the
atmosphere create moving lenses
in the sky, shifting the light rays
randomly
– This causes a twinkling effect,
called scintillation.
• A stable atmosphere causes less
scintillation
– We say the seeing is good.
Adaptive Optics
• Some observatories
measure the amount
of atmospheric
turbulence with
lasers, and then
adjust the mirrors in
their telescopes with
tiny motors to
eliminate the effect
• This technique is
called adaptive
optics
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Spitzer Space Telescope
James Web
Space
telescope
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Edwin Hubble
Hubble Telescope
Observatories in Space