Electromagnetic radiation
From Wikipedia, the free
encyclopedia
Electromagnetic radiation (often abbreviated E-M radiation or EMR) is a
form of energy exhibiting wave like behavior as it travels through space. EMR
has both electric and magnetic field
components, which oscillate in phase perpendicular to each other and perpendicular to
the direction of energy propagation.
Electromagnetic radiation is
classified according to the frequency
of its wave. In order of increasing frequency and decreasing wavelength,
these are radio waves, microwaves, infrared radiation, visible light,
ultraviolet radiation,
X-rays and gamma rays
(see Electromagnetic
spectrum). The eyes of various organisms
sense a small and somewhat variable window of frequencies called the visible spectrum.
The photon is the
quantum of the electromagnetic interaction and the basic "unit" of
light and all other forms of electromagnetic radiation and is also the force carrier
for the electromagnetic force.
Physics
Theory
Shows three electromagnetic modes
(blue, green and red) with a distance scale in micrometres along the x-axis.
Main article: Maxwell's equations
James Clerk Maxwell first formally-postulated Electromagnetic waves.
These were subsequently confirmed by Heinrich Hertz.
Maxwell derived a wave form
of the electric and magnetic equations,
thus uncovering the wave-like nature of electric and magnetic fields, and their
symmetry. Because the speed of EM waves predicted by the wave equation
coincided with the measured speed of light,
Maxwell concluded that light itself is an EM wave.
According to Maxwell's equations, a time-varying electric field
generates a time-varying magnetic field
and vice versa. Therefore, as an oscillating electric field generates an
oscillating magnetic field, the magnetic field in turn generates an oscillating
electric field, and so on. These oscillating fields together form a propagating
electromagnetic wave.
A quantum theory
of the interaction between electromagnetic radiation and matter such as
electrons is described by the theory of quantum electrodynamics.
Properties
Electromagnetic waves can be
imagined as a self-propagating transverse oscillating wave of electric and
magnetic fields. This diagram shows a plane linearly polarized wave propagating
from right to left. The electric field is in a vertical plane and the magnetic
field in a horizontal plane.
The physics of
electromagnetic radiation is electrodynamics.
Electromagnetism is the physical phenomenon associated with the theory of
electrodynamics. Electric and magnetic fields obey the properties of superposition. Thus, a field due to any particular particle or
time-varying electric or magnetic field contributes to the fields present in
the same space due to other causes. Further, as they are vector fields, all magnetic and electric field vectors add
together according to vector addition. For example, in optics two or more
coherent lightwaves may interact and by constructive or destructive
interference yield a resultant irradiance deviating from the sum of the
component irradiances of the individual lightwaves.
Since light is an oscillation it is
not affected by travelling through static electric or magnetic fields in a
linear medium such as a vacuum. However in nonlinear media, such as some crystals,
interactions can occur between light and static electric and magnetic
fields — these interactions include the Faraday effect
and the Kerr effect.
In refraction, a wave crossing from
one medium to another of different density alters
its speed and direction upon entering the new medium. The ratio of the
refractive indices of the media determines the degree of refraction, and is
summarized by Snell's law. Light disperses into a visible spectrum as light is shone through a prism because of the wavelength
dependent refractive index of the prism material (Dispersion).
EM radiation exhibits both wave
properties and particle properties at the same time (see wave-particle duality). Both wave and particle characteristics have been
confirmed in a large number of experiments. Wave characteristics are more
apparent when EM radiation is measured over relatively large timescales and
over large distances while particle characteristics are more evident when
measuring small timescales and distances. For example, when electromagnetic
radiation is absorbed by matter, particle-like properties will be more obvious
when the average number of photons in the cube of the relevant wavelength is
much smaller than 1. Upon absorption of light, it is not too difficult to
experimentally observe non-uniform deposition of energy. Strictly speaking,
however, this alone is not evidence of "particulate" behavior of
light, rather it reflects the quantum nature of matter.
There are experiments in which the
wave and particle natures of electromagnetic waves appear in the same
experiment, such as the self-interference of a single photon. True single-photon experiments (in a quantum
optical sense) can be done today in undergraduate-level labs.[2]
When a single photon is sent through an interferometer,
it passes through both paths, interfering with itself, as waves do, yet is
detected by a photomultiplier or other sensitive detector only once.
Wave model
Electromagnetic radiation is a transverse wave
meaning that the oscillations of the waves are perpendicular to the direction
of energy transfer and travel. An important aspect of the nature of light is frequency.
The frequency of a wave is its rate of oscillation and is measured in hertz, the SI
unit of frequency, where one hertz is equal to one oscillation per second. Light usually has a spectrum of frequencies which sum
together to form the resultant wave. Different frequencies undergo different
angles of refraction.
A wave consists of successive
troughs and crests, and the distance between two adjacent crests or troughs is
called the wavelength. Waves of the electromagnetic spectrum vary in size, from
very long radio waves the size of buildings to very short gamma rays smaller
than atom nuclei. Frequency is inversely proportional to wavelength, according
to the equation:
where v is the speed of the
wave (c in a vacuum, or less in other media), f is the
frequency and λ is the wavelength. As waves cross boundaries between different
media, their speeds change but their frequencies remain constant.
Interference is the superposition of two or more waves resulting in a
new wave pattern. If the fields have components in the same direction, they
constructively interfere, while opposite directions cause destructive
interference.
Particle
model
Because energy of an EM wave is
quantized, in the particle model of EM radiation, a wave consists of discrete
packets of energy, or quanta, called photons.[3]
The frequency of the wave is proportional to the particle's energy. Because
photons are emitted and absorbed by charged particles, they act as transporters
of energy. The
energy per photon can be
calculated from the Planck–Einstein
equation:[4]
where E is the energy, h
is Planck's constant, and f is frequency. It is commonly expressed in the
unit of electronvolt
(eV). This photon-energy expression is a particular case of the energy levels
of the more general electromagnetic oscillator, whose average energy,
which is used to obtain Planck's radiation law, can be shown to differ sharply
from that predicted by the equipartition principle at low temperature, thereby establishes a failure of
equipartition due to quantum effects at low temperature.[5]
As a photon is absorbed by an atom, it excites the atom,
elevating an electron to a higher energy level.
If the energy is great enough, so that the electron jumps to a high enough
energy level, it may escape the positive pull of the nucleus and be liberated
from the atom in a process called photoionisation.
Conversely, an electron that descends to a lower energy level in an atom emits
a photon of light equal to the energy difference. Since the energy levels of
electrons in atoms are discrete, each element emits and absorbs its own
characteristic frequencies.
Together, these effects explain the
emission and absorption spectra of light. The dark bands in the absorption spectrum are due to the
atoms in the intervening medium absorbing different frequencies of the light.
The composition of the medium through which the light travels determines the
nature of the absorption spectrum. For instance, dark bands in the light
emitted by a distant star are due to the atoms in the star's atmosphere. These bands
correspond to the allowed energy levels in the atoms. A similar phenomenon
occurs for emission. As the electrons descend to lower energy levels, a
spectrum is emitted that represents the jumps between the energy levels of the
electrons. This is manifested in the emission spectrum of nebulae. Today, scientists use this phenomenon to observe what
elements a certain star is composed of. It is also used in the determination of
the distance of a star, using the red shift.
Main article: Speed of light
Any electric charge which
accelerates, or any changing magnetic field, produces electromagnetic
radiation. Electromagnetic information about the charge travels at the speed of
light. Accurate treatment thus incorporates a concept known as retarded time
(as opposed to advanced time, which is unphysical in light of causality),
which adds to the expressions for the electrodynamic electric field
and magnetic field. These extra terms are responsible for electromagnetic
radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same
frequency as the electric current. At the quantum level, electromagnetic
radiation is produced when the wavepacket of a charged particle oscillates or
otherwise accelerates. Charged particles in a stationary state
do not move, but a superposition of such states may result in oscillation,
which is responsible for the phenomenon of radiative transition between quantum
states of a charged particle.
Depending on the circumstances,
electromagnetic radiation may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light),
wavelength,
and frequency.
When considered as particles, they are known as photons, and each has an energy related to the frequency of the
wave given by Planck's relation E = hν, where E is the energy of the
photon, h = 6.626 × 10−34 J·s is Planck's constant, and ν is the frequency of the wave.
One rule is always obeyed regardless
of the circumstances: EM radiation in a vacuum always travels at the speed of light,
relative to the observer, regardless of the observer's velocity. (This
observation led to Albert Einstein's development of the theory of special relativity.)
In a medium (other than vacuum), velocity factor or refractive index
are considered, depending on frequency and application. Both of these are
ratios of the speed in a medium to speed in a vacuum.
Main article: Thermal radiation
The basic structure of matter involves charged particles bound together in many different
ways. When electromagnetic radiation is incident on matter, it causes the
charged particles to oscillate and gain energy. The ultimate fate of this
energy depends on the situation. It could be immediately re-radiated and appear
as scattered, reflected, or transmitted radiation. It may also get dissipated
into other microscopic motions within the matter, coming to thermal equilibrium and manifesting itself as thermal energy
in the material. With a few exceptions such as fluorescence,
harmonic generation, photochemical reactions and the photovoltaic effect, absorbed electromagnetic radiation simply deposits its energy
by heating the material. This happens both for infrared and non-infrared
radiation. Intense radio waves can thermally burn living tissue and can cook
food. In addition to infrared lasers, sufficiently intense visible and ultraviolet lasers can
also easily set paper afire. Ionizing electromagnetic radiation can create
high-speed electrons in a material and break chemical bonds, but after these
electrons collide many times with other atoms in the material eventually most
of the energy gets downgraded to thermal energy, this whole process happening
in a tiny fraction of a second. That infrared radiation is a form of heat and
other electromagnetic radiation is not, is a widespread misconception
in physics. Any electromagnetic radiation can heat a material when it is
absorbed.
The inverse or time-reversed process
of absorption is responsible for thermal radiation. Much of the thermal energy
in matter consists of random motion of charged particles, and this energy can
be radiated away from the matter. The resulting radiation may subsequently be
absorbed by another piece of matter, with the deposited energy heating the material.
Radiation
is an important mechanism of heat transfer.
The electromagnetic radiation in an
opaque cavity at thermal equilibrium is effectively a form of thermal energy,
having maximum radiation entropy. The thermodynamic
potentials of electromagnetic radiation can be
well-defined as for matter. Thermal radiation in a cavity has energy density
(see Planck's Law) of
Differentiating the above with
respect to temperature, we may say that the electromagnetic radiation field has
an effective volumetric heat capacity
given by
Electromagnetic
spectrum
Electromagnetic
spectrum with light highlighted
Legend:
γ = Gamma rays
HX = Hard X-rays
SX = Soft X-Rays
EUV = Extreme ultraviolet
NUV = Near ultraviolet
Visible light
NIR = Near infrared
MIR = Moderate infrared
FIR = Far infrared
Radio waves:
EHF = Extremely high frequency (Microwaves)
SHF = Super high frequency (Microwaves)
UHF = Ultrahigh frequency
VHF = Very high frequency
HF = High frequency
MF = Medium frequency
LF = Low frequency
VLF = Very low frequency
VF = Voice frequency
ULF = Ultra low frequency
SLF = Super low frequency
ELF = Extremely low frequency
γ = Gamma rays
HX = Hard X-rays
SX = Soft X-Rays
EUV = Extreme ultraviolet
NUV = Near ultraviolet
Visible light
NIR = Near infrared
MIR = Moderate infrared
FIR = Far infrared
Radio waves:
EHF = Extremely high frequency (Microwaves)
SHF = Super high frequency (Microwaves)
UHF = Ultrahigh frequency
VHF = Very high frequency
HF = High frequency
MF = Medium frequency
LF = Low frequency
VLF = Very low frequency
VF = Voice frequency
ULF = Ultra low frequency
SLF = Super low frequency
ELF = Extremely low frequency
Generally, EM radiation (the
designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region
we perceive as light, ultraviolet, X-rays and gamma rays. Arbitrary electromagnetic waves can always be expressed by
Fourier analysis in terms of sinusoidal monochromatic waves which can be
classified into these regions of the spectrum.
The behavior of EM radiation depends
on its wavelength. Higher frequencies have shorter wavelengths, and lower
frequencies have longer wavelengths. When EM radiation interacts with single
atoms and molecules, its behavior depends on the amount of energy per quantum
it carries. Spectroscopy can detect a much wider region of the EM spectrum than the
visible range of 400 nm to 700 nm. A common laboratory spectroscope
can detect wavelengths from 2 nm to 2500 nm. Detailed information
about the physical properties of objects, gases, or even stars can be obtained
from this type of device. It is widely used in astrophysics.
For example, hydrogen atoms emit radio waves of wavelength 21.12 cm.
Soundwaves are not electromagnetic
radiation. At the lower end of the
electromagnetic spectrum, about 20 Hz to about 20 kHz, are
frequencies that might be considered in the audio range, however,
electromagnetic waves cannot be directly perceived by human ears. Sound waves
are the oscillating compression of molecules. To be heard, electromagnetic
radiation must be converted to air pressure waves, or if the ear is submerged,
water pressure waves.
Light
Main article: Light
EM radiation with a wavelength
between approximately 400 nm and 700 nm is directly
detected by the human eye and perceived as visible light. Other wavelengths, especially nearby infrared (longer than
700 nm) and ultraviolet (shorter than 400 nm) are also sometimes
referred to as light, especially when visibility to humans is not relevant.
If radiation having a frequency in
the visible region of the EM spectrum reflects off of an object, say, a bowl of
fruit, and then strikes our eyes, this results in our visual perception
of the scene. Our brain's visual system processes the multitude of reflected
frequencies into different shades and hues, and through this not-entirely-understood
psychophysical phenomenon, most people perceive a bowl of fruit.
At most wavelengths, however, the
information carried by electromagnetic radiation is not directly detected by
human senses. Natural sources produce EM radiation across the spectrum, and our
technology
can also manipulate a broad range of wavelengths. Optical fiber
transmits light which, although not suitable for direct viewing, can carry data
that can be translated into sound or an image. To be meaningful both
transmitter and receiver must use some agreed-upon encoding system - especially
so if the transmission is digital as opposed to the analog nature of the waves.
Main article: Radio waves
Radio waves can be made to carry
information by varying a combination of the amplitude,
frequency and phase of the wave within a frequency band.
When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by
exciting the electrons of the conducting material. This effect (the skin effect)
is used in antennas. EM radiation may also cause certain molecules to absorb
energy and thus to heat up; this is exploited in microwave ovens.
Radio waves are not ionizing radiation, as the energy per photon is too small.
Derivation
Electromagnetic waves as a general
phenomenon were predicted by the classical laws of electricity
and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources
(charges or currents) then you will find that, along with the possibility of
nothing happening, the theory will also admit nontrivial solutions of changing
electric and magnetic fields. Beginning with Maxwell's equations in free
space:
where
One solution,
,
is trivial.
To see the more interesting one, we
utilize vector
identities, which work for any vector, as
follows:
To see how we can use this, take the
curl of equation (2):
Evaluating the left hand side:
where we simplified the above by using equation (1).
Evaluate the right hand side:
Equations (6) and (7) are equal, so
this results in a vector-valued differential equation for the electric field, namely
|
Applying a similar pattern results
in similar differential equation for the magnetic field:
.
|
where
c0 is the speed of the wave in free space and
f describes a displacement
Or more simply:
Notice that in the case of the
electric and magnetic fields, the speed is:
Which, as it turns out, is the speed of light
in vacuum. Maxwell's equations have unified the vacuum permittivity ε0, the vacuum permeability μ0, and the speed of light itself, c0.
Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism.
But these are only two equations and
we started with four, so there is still more information pertaining to these
waves hidden within Maxwell's equations. Let's consider a generic vector wave
for the electric field.
Here
is the constant amplitude, f is any second
differentiable function,
is a unit vector in the direction of propagation, and
is a position vector. We observe that
is a generic solution to the wave equation. In other words
,
for a generic wave traveling in the
direction.
This form will satisfy the wave
equation, but will it satisfy all of Maxwell's equations, and with what
corresponding magnetic field?
The first of Maxwell's equations
implies that electric field is orthogonal to the direction the wave propagates.
The second of Maxwell's equations
yields the magnetic field. The remaining equations will be satisfied by this
choice of
.
Not only are the electric and
magnetic field waves traveling at the speed of light, but they have a special
restricted orientation and proportional magnitudes, E0 = c0B0,
which can be seen immediately from the Poynting vector.
The electric field, magnetic field, and direction of wave propagation are all
orthogonal, and the wave propagates in the same direction as
.
From the viewpoint of an
electromagnetic wave traveling forward, the electric field might be oscillating
up and down, while the magnetic field oscillates right and left; but this
picture can be rotated with the electric field oscillating right and left and
the magnetic field oscillating down and up. This is a different solution that
is traveling in the same direction. This arbitrariness in the orientation with
respect to propagation direction is known as polarization. On a quantum level, it is described as photon polarization.
More general forms of the second
order wave equations given above are available, allowing for both non-vacuum
propagation media and sources. A great many competing derivations exist, all
with varying levels of approximation and intended applications. One very
general example is a form of the electric field equation,[6]
which was factorized into a pair of explicitly directional wave equations, and
then efficiently reduced into a single uni-directional wave equation by means
of a simple slow-evolution approximation.
What are electromagnetic waves?
Electricity can be static, like what holds
a balloon to the wall or makes your hair stand on end. Magnetism can also be static like a refrigerator magnet. But when they change or move together, they make waves - electromagnetic waves. |
|
Electromagnetic
waves are formed when an electric field (shown as blue arrows) couples with a
magnetic field (shown as red arrows). The magnetic and electric fields of an
electromagnetic wave are perpendicular to each other and to the direction of
the wave. James Clerk Maxwell and Heinrich
Hertz are two
scientists who studied how electromagnetic waves are formed and how fast they
travel.
|
Electromagnetic Waves have different wavelengths
When you listen to the radio,
watch TV, or cook dinner in a microwave oven, you are using electromagnetic
waves.
|
|
Radio waves, television waves, and
microwaves are all types of electromagnetic waves. They differ from each
other in wavelength. Wavelength is the distance between one wave crest to the
next.
|
|
Waves in the electromagnetic
spectrum vary in size from very long radio waves the size of buildings, to very
short gamma-rays smaller than the size of the nucleus of an atom.
Did you know that electromagnetic
waves can not only be described by their wavelength, but also by their energy
and frequency? All three of these things are related to each other mathematically. This means that it is correct to talk about the energy of
an X-ray or the wavelength of a microwave or the frequency of a radio wave. The
electromagnetic spectrum includes, from longest wavelength to shortest: radio
waves, microwaves, infrared, optical, ultraviolet, X-rays, and gamma-rays.
Microwaves
Microwaves have wavelengths that
can be measured in centimeters! The longer microwaves, those closer to a foot
in length, are the waves which heat our food in a microwave oven.
|
|
Microwaves are good for
transmitting information from one place to another because microwave energy
can penetrate haze, light rain and snow, clouds, and smoke.
Shorter microwaves are used in
remote sensing. These microwaves are used for radar like the doppler radar
used in weather forecasts. Microwaves, used for radar, are just a few inches
long.
|
|
This
microwave tower can transmit information like telephone calls and computer data
from one city to another.
How do we "see" using Microwaves?
Radar is an acronym for "radio
detection and ranging". Radar was developed to detect objects and
determine their range (or position) by transmitting short bursts of microwaves.
The strength and origin of "echoes" received from objects that were
hit by the microwaves is then recorded.
Because radar senses
electromagnetic waves that are a reflection of an active transmission, radar
is considered an active remote sensing system. Passive remote sensing refers
to the sensing of electromagnetic waves which did not originate from the
satellite or sensor itself. The sensor is just a passive observer.
|
|
What do Microwaves show us?
Because microwaves can penetrate
haze, light rain and snow, clouds and smoke, these waves are good for viewing
the Earth from space.
The ERS-1 satellite sends out
wavelengths about 5.7 cm long (C-band). This image shows sea ice breaking off
the shores of Alaska.
|
The JERS satellite uses
wavelengths about 20 cm in length (L-band). This is an image of the Amazon
River in Brazil.
|
This is a radar image acquired
from the Space Shuttle. It also used a wavelength in the L-band of the
microwave spectrum. Here we see a computer enhanced radar image of some
mountains on the edge of Salt Lake City, Utah.
|
In the 1960's a startling discovery
was made quite by accident. A pair of scientists at Bell Laboratories detected
background noise using a special low noise antenna. The strange thing about the
noise was that it was coming from every direction and did not seem to vary in
intensity much at all. If this static were from something on our world, like
radio transmissions from a nearby airport control tower, it would only come
from one direction, not everywhere. The scientists soon realized they had
discovered the cosmic microwave background radiation. This radiation, which
fills the entire Universe, is believed to be a clue to it's beginning,
something known as the Big Bang.
The image above is a Cosmic
Background Explorer (COBE) image of the cosmic microwave background, the pink
and blue colors showing the tiny fluctuations in it.
Did you know that if you had a
sensitive microwave telescope in your house that you would detect a faint
signal leaking out of your microwave oven, and from various other man-made
sources, but also a faint signal coming from all directions that you pointed
it? This is the Cosmic Microwave Background!
|
Radio Waves
Radio waves have the longest
wavelengths in the electromagnetic spectrum. These waves can be longer than a
football field or as short as a football. Radio waves do more than just bring
music to your radio. They also carry signals for your television and cellular
phones.
|
|
|
The antennae on your television
set receive the signal, in the form of electromagnetic waves, that is
broadcasted from the television station. It is displayed on your television
screen.
Cable companies have antennae or
dishes which receive waves broadcasted from your local TV stations. The
signal is then sent through a cable to your house.
Why are car antennae about the
same size as TV antennae?
|
Cellular phones also use radio
waves to transmit information. These waves are much smaller that TV and FM
radio waves.
Why are antennae on cell phones
smaller than antennae on your radio?
|
|
How do we "see" using Radio Waves?
Objects in space, such as planets
and comets, giant clouds of gas and dust, and stars and galaxies, emit light at
many different wavelengths. Some of the light they emit has very large
wavelengths - sometimes as long as a mile!. These long waves are in the radio
region of the electromagnetic spectrum.
Because radio waves are larger
than optical waves, radio telescopes work differently than telescopes that we
use for visible > light (optical telescopes). Radio telescopes are dishes
made out of conducting metal that reflect radio waves to a focus point.
Because the wavelengths of radio light are so large, a radio telescope must
be physically larger than an optical telescope to be able to make images of
comparable clarity. For example, the Parkes radio telescope, which has a dish
64 meters wide, cannot give us any clearer an image than a small backyard
telescope!
In order to make better and more
clear (or higher resolution) radio images, radio astronomers often combine
several smaller telescopes, or receiving dishes, into an array. Together, the
dishes can act as one large telescope whose size equals the total area
occupied by the array.
|
|
The Very Large Array (VLA) is one
of the world's premier astronomical radio observatories. The VLA consists of
27 antennas arranged in a huge "Y" pattern up to 36 km (22 miles)
across -- roughly one and a half times the size of Washington, DC.
|
|
The VLA, located in New Mexico, is
an interferometer; this means that it operates by multiplying the data from
each pair of telescopes together to form interference patterns. The structure
of those interference patterns, and how they change with time as the earth rotates,
reflect the structure of radio sources in the sky.
What do Radio Waves show us?
The above image shows the Carbon
Monoxide (CO) gases in our Milky Way galaxy.
Many astronomical objects emit radio
waves, but that fact wasn't discovered until 1932. Since then, astronomers have
developed sophisticated systems that allow them to make pictures from the radio
waves emitted by astronomical objects.
Radio telescopes look toward the
heavens at planets and comets, giant clouds of gas and dust, and stars and
galaxies. By studying the radio waves originating from these sources,
astronomers can learn about their composition, structure, and motion. Radio astronomy
has the advantage that sunlight, clouds, and rain do not affect observations.
|
Gamma-rays
Gamma-rays
have the smallest wavelengths and the most energy of any other wave in the
electromagnetic spectrum. These waves are generated by radioactive atoms and in
nuclear explosions. Gamma-rays can kill living cells, a fact which medicine
uses to its advantage, using gamma-rays to kill cancerous cells.
Gamma-rays
travel to us across vast distances of the universe, only to be absorbed by the
Earth's atmosphere. Different wavelengths of light penetrate the Earth's
atmosphere to different depths. Instruments aboard high-altitude balloons and
satellites like the Compton Observatory provide our only view of the gamma-ray
sky.
Gamma-rays
are the most energetic form of light and are produced by the hottest regions of
the universe. They are also produced by such violent events as supernova
explosions or the destruction of atoms, and by less dramatic events, such as
the decay of radioactive material in space. Things like supernova explosions
(the way massive stars die), neutron stars and pulsars, and black holes are all
sources of celestial gamma-rays.
How do we "see" using gamma-ray light?
Gamma-ray
astronomy did not develop until it was possible to get our detectors above all
or most of the atmosphere, using balloons or spacecraft. The first gamma-ray
telescope, carried into orbit on the Explorer XI satellite in 1961, picked up
fewer than 100 cosmic gamma-ray photons!
Unlike
optical light and X-rays, gamma rays cannot be captured and reflected in
mirrors. The high-energy photons would pass right through such a device.
Gamma-ray telescopes use a process called Compton scattering, where a gamma-ray
strikes an electron and loses energy, similar to a cue ball striking an eight
ball.
|
This image shows the CGRO
satellite being deployed from the Space Shuttle orbiter. This picture was
taken from an orbiter window. The two round protrusions are one of CGRO's
instruments, called "EGRET".
|
What do gamma-rays show us?
If you could see gamma-rays, the
night sky would look strange and unfamiliar.
The gamma-ray moon just looks like
a round blob - lunar features are not visible. In high-energy gamma rays, the
Moon is actually brighter than the quiet Sun. This image was taken by EGRET.
|
Credit:
D.J. Thompson, D.L. Bertsch (NASA/GSFC),
D.J. Morris (UNH), R. Mukherjee (NASA/GSFC/USRA) |
The familiar sights of constantly shining stars and galaxies would be replaced by something ever-changing. Your gamma-ray vision would peer into the hearts of solar flares, supernovae, neutron stars, black holes, and active galaxies. Gamma-ray astronomy presents unique opportunities to explore these exotic objects. By exploring the universe at these high energies, scientists can search for new physics, testing theories and performing experiments which are not possible in earth-bound laboratories.
If you could see gamma-rays, these
two spinning neutron stars or pulsars would be among the brightest objects in
the sky. This computer processed image shows the Crab Nebula pulsar (below
and right of center) and the Geminga pulsar (above and left of center) in the
"light" of gamma-rays.
|
The
Crab nebula, shown also in the visible light image, was created by a supernova
that brightened the night sky in 1054 A.D. In 1967, astronomers detected the
remnant core of that star; a rapidly rotating, magnetic pulsar flashing every
0.33 second in radio waves.
Perhaps
the most spectacular discovery in gamma-ray astronomy came in the late 1960s
and early 1970s. Detectors on board the Vela satellite series, originally
military satellites, began to record bursts of gamma-rays -- not from Earth,
but from deep space!
Today, these gamma-ray bursts,
which happen at least once a day, are seen to last for fractions of a second
to minutes, popping off like cosmic flashbulbs from unexpected directions,
flickering, and then fading after briefly dominating the gamma-ray sky.
|
Gamma-ray
bursts can release more energy in 10 seconds than the Sun will emit in its
entire 10 billion-year lifetime! So far, it appears that all of the bursts we
have observed have come from outside the Milky Way Galaxy. Scientists believe
that a gamma-ray burst will occur once every few million years here in the
Milky Way, and in fact may occur once every several hundred million years
within a few thousand light-years of Earth.
Studied
for over 25 years now with instruments on board a variety of satellites and
space probes, including Soviet Venera spacecraft and the Pioneer Venus Orbiter,
the sources of these enigmatic high-energy flashes remain a mystery.
By
solving the mystery of gamma-ray bursts, scientists hope to gain further knowledge
of the origins of the Universe, the rate at which the Universe is expanding,
and the size of the Universe.
X-rays
As the wavelengths of light
decrease, they increase in energy. X-rays have smaller wavelengths and
therefore higher energy than ultraviolet waves. We usually talk about X-rays in
terms of their energy rather than wavelength. This is partially because X-rays
have very small wavelengths. It is also because X-ray light tends to act more
like a particle than a wave. X-ray detectors collect actual photons of X-ray
light - which is very different from the radio telescopes that have large
dishes designed to focus radio waves!
X-rays were first observed and
documented in 1895 by Wilhelm Conrad Roentgen, a German scientist who found
them quite by accident when experimenting with vacuum tubes.
A week later, he took an X-ray
photograph of his wife's hand which clearly revealed her wedding ring and her
bones. The photograph electrified the general public and aroused great
scientific interest in the new form of radiation. Roentgen called it
"X" to indicate it was an unknown type of radiation. The name
stuck, although (over Roentgen's objections), many of his colleagues
suggested calling them Roentgen rays. They are still occasionally referred to
as Roentgen rays in German-speaking countries.
|
|
The Earth's atmosphere is thick
enough that virtually no X-rays are able to penetrate from outer space all the
way to the Earth's surface. This is good for us but also bad for astronomy - we
have to put X-ray telescopes and detectors on satellites! We cannot do X-ray
astronomy from the ground.
How do we "see" using X-ray light?
What would it be like to see X-rays?
Well, we wouldn't be able to see through people's clothes, no matter what the
ads for X-ray glasses tell us! If we could see X-rays, we could see things that
either emit X-rays or halt their transmission. Our eyes would be like the X-ray
film used in hospitals or dentist's offices. X-ray film "sees"
X-rays, like the ones that travel through your skin. It also sees shadows left
by things that the X-rays can't travel through (like bones or metal).
When you get an X-ray taken at a
hospital, X-ray sensitive film is put on one side of your body, and X-rays
are shot through you. At a dentist, the film is put inside your mouth, on one
side of your teeth, and X-rays are shot through your jaw, just like in this
picture. It doesn't hurt at all - you can't feel X-rays.
|
|
Because your bones and teeth are
dense and absorb more X-rays then your skin does, silhouettes of your bones
or teeth are left on the X-ray film while your skin appears transparent.
Metal absorbs even more X-rays - can you see the filling in the image of the
tooth?
|
|
When the Sun shines on us at a
certain angle, our shadow is projected onto the ground. Similarly, when X-ray
light shines on us, it goes through our skin, but allows shadows of our bones
to be projected onto and captured by film.
This is an X-ray photo of a one
year old girl. Can you see the shadow of what she swallowed?
|
We use satellites with X-ray
detectors on them to do X-ray astronomy. In astronomy, things that emit X-rays
(for example, black holes) are like the dentist's X-ray machine, and the
detector on the satellite is like the X-ray film. X-ray detectors collect
individual X-rays (photons of X-ray light) and things like the number of
photons collected, the energy of the photons collected, or how fast the photons
are detected, can tell us things about the object that is emitting them.
To the right is an image of a real
X-ray detector. This instrument is called the Proportional Counter Array and
it is on the Rossi X-ray Timing Explorer (RXTE) satellite. It looks very
different from anything you might see at a dentist's office!
|
What does X-ray light show us?
Many things in space emit X-rays,
among them are black holes, neutron stars, binary star systems, supernova
remnants, stars, the Sun, and even some comets!
The Earth glows in many kinds of
light, including the energetic X-ray band. Actually, the Earth itself does not
glow - only aurora produced high in the Earth's atmosphere. These aurora are
caused by charged particles from the Sun.
Credit:
Polar, PIXIE, NASA
|
To the left is the first picture
of the Earth in X-rays, taken in March, 1996 with the orbiting Polar
satellite. The area of brightest X-ray emission is red. The energetic charged
particles from the Sun that cause aurora also energize electrons in the Earth's
magnetosphere. These electrons move along the Earth's magnetic field and
eventually strike the Earth's ionosphere, causing the X-ray emission. These
X-rays are not dangerous because they are absorbed by lower parts of the
Earth's atmosphere. (The above caption and image are from the Astronomy
Picture of the Day for December 30, 1996.)
|
Recently, we learned that even
comets emit X-rays! This image of Comet Hyakutake was taken by an X-ray
satellite called ROSAT, short for the Roentgen Satellite. (It was named after
the discoverer of X-rays.)
|
The Sun also emits X-rays - here
is what the Sun looked like in X-rays on April 27th, 2000. This image was
taken by the Yokoh satellite.
|
Many things in deep space give off
X-rays. Many stars are in binary star systems - which means that two stars
orbit each other. When one of these stars is a black hole or a neutron star,
material is pulled off the normal star. This materials spirals into the black
hole or neutron star and heats up to very high temperatures. When something
is heated to over a million degrees, it will give off X-rays!
|
The above image is an artist's
conception of a binary star system - it shows the material being pulled off the
red star by its invisible black hole companion and into an orbiting disk.
Credit:
X-ray (NASA/CXC/SAO);
Optical (NASA/HST); Radio: (CSIRO/ATNF/ATCA) |
This image is special - it shows a
supernova remnant - the remnant of a star that exploded in a nearby galaxy
known as the Small Magellanic Cloud. The false-colors show what this
supernova remnant looks like in X-rays (in blue), visible light (green) and
radio (red).
|
No comments:
Post a Comment