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The Doppler Effect
Learn about the Doppler Effect
by
Astronomers
study the light from distant objects in order to understand them.
Light
moves through space at 299,000 kilometers per second, and its path can be
deflected by gravity as well as absorbed and scattered by clouds of material in
the universe.
Astronomers
use many properties of light to study everything from planets and their moons
to the most distant objects in the cosmos.
Delving
into the Doppler Effect
One
tool they use is the Doppler effect. This is a shift in the frequency or
wavelength of radiation emitted from an object as it moves through space.
It's
named after Austrian physicist Christian Doppler who first proposed it in
1842.
How
does the Doppler Effect work?
If
the source of radiation, say a star, is moving toward an astronomer on Earth (for example),
then the wavelength of its radiation will appear shorter (higher frequency, and
therefore higher energy).
On
the other hand, if the object is moving away from the observer then the
wavelength will appear longer (lower frequency, and lower energy).
You
have probably experienced a version of the effect when you heard a train
whistle or a police siren as it moved past you, changing pitch as it passes by
you and moves away.
The
Doppler effect is behind such technologies as police radar, where the
"radar gun" emits light of a known wavelength.
Then,
that radar "light" bounces off a moving car and travels back to the
instrument. The resulting shift in wavelength is used to calculate the speed of
the vehicle.
(Note: it is actually
a double shift as the moving car first acts as the observer and
experiences a shift, then as a moving source sending the light back to the
office, thereby shifting the wavelength a second time.)
Redshift
When
an object is receding (i.e. moving away) from an observer, the peaks of the
radiation that are emitted will be spaced farther apart than they would be if
the source object were stationary.
The
result is that the resulting wavelength of light appears longer. Astronomers
say that it is "shifted to the red" end of the spectrum.
The
same effect applies to all bands of the electromagnetic spectrum, such as radio, x-ray or gamma-rays.
However,
optical measurements are the most common and are the source of the term
"redshift".
The
more quickly the source moves away from the observer, the greater the redshift.
From
an energy standpoint, longer wavelengths correspond to lower energy radiation.
Blueshift
Conversely,
when a source of radiation is approaching an observer the wavelengths of light
appear closer together, effectively shortening the wavelength of light. (Again,
shorter wavelength means higher frequency and therefore higher energy.)
Spectroscopically,
the emission lines would appear shifted toward the blue side of the optical
spectrum, hence the name blueshift.
As
with redshift, the effect is applicable to other bands of the electromagnetic
spectrum, but the effect is most often times discussed when dealing with
optical light, though in some fields of astronomy this is certainly not the
case.
Expansion
of the Universe and the Doppler Shift
Use
of the Doppler Shift has resulted in some important discoveries in astronomy.
In
the early 1900s, it was believed that the universe was static.
In
fact, this led Albert Einstein to
add the cosmological constant to
his famous field equation in order to "cancel out" the expansion (or
contraction) that was predicted by his calculation.
Specifically,
it was once believed that the "edge" of the Milky Way represented the
boundary of the static universe.
Then, Edwin
Hubble found that the so-called
"spiral nebulae" that had plagued astronomy for decades were not nebulae at
all.
They
were actually other galaxies. It was an amazing discovery and told astronomers
that the universe is much
larger than they knew.
Hubble
then proceeded to measure the Doppler shift, specifically finding the redshift
of these galaxies.
He
found that that the farther away a galaxy is, the more quickly it recedes. This
led to the now-famous Hubble's Law,which says
that an object's distance is proportional to its speed of recession.
This
revelation led Einstein to write that his addition
of the cosmological constant to the field equation was the greatest blunder of
his career.
Interestingly,
however, some researchers are now placing the constant back into general relativity.
As it
turns out Hubble's Law is only true up to a point since research over the last
couple of decades has found that distant galaxies are
receding more quickly than predicted.
This
implies that the expansion of the universe is accelerating. The reason for that
is a mystery, and scientists have dubbed the driving force of this
acceleration dark energy.
They
account for it in the Einstein field equation as a cosmological constant (though
it is of a different form than Einstein's formulation).
Other
Uses in Astronomy
Besides
measuring the expansion of the universe, the Doppler effect can be used to
model the motion of things much closer to home; namely the dynamics of
the Milky Way Galaxy.
By
measuring the distance to stars and their redshift or blueshift, astronomers
are able to map the motion of our galaxy and get a picture of what our galaxy
may look like to an observer from across the universe.
The
Doppler Effect also allows scientists to measure the pulsations of
variable stars, as well as motions of particles traveling at incredible
velocities inside relativistic jet streams emanating from supermassive black holes.
Edited and updated by Carolyn
Collins Petersen.
John P. Millis, Ph.D
Chairman, Department of Physical
Sciences and Engineering at Anderson University
Ph.D. in Physics and Astronomy at Purdue
University
Conducts astronomical research at the
VERITAS observatory
Experience
John Millis is a former writer for
ThoughtCo. He has taught physics and astronomy at the collegiate level since
2001 and is currently the chair of the Department of Physical Sciences and
Engineering at Anderson University, in Anderson Indiana. He teaches a wide
variety of courses while maintaining an active research program in high energy
astrophysics.
John's research focus is on pulsars, pulsar
wind nebulae, and supernova remnants. Using the VERITAS gamma-ray observatory
in southern Arizona, he studies the very high energy radiation from these
dynamic sources to extract information about their formation and emission
mechanisms. In 2010, he co-founded a small consulting business, Aurum
Consulting, LLC, assisting with biological testing, chemical formulations, and
product development. John Millis wrote on Space and Astronomy topics for the
Dotdash/About.com networks for three years.
Education
Dr. Millis received his bachelor of science
in physics, with a mathematics minor from Purdue University in West Lafayette,
Indiana. He remained at Purdue for the completion of his Doctor of Philosophy
degree, where he focused on High Energy Astrophysics.
John P. Millis, Ph.D
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