The Doppler Effect

Everyone is familiar with the Doppler Effect; they just might not know the name.  We hear it every time we hear a police or fire truck siren.  You will note that the siren raises in pitch as the car is coming toward you and lowers as it travels away.  Sound waves are like light and water waves.  They have a wavelength, the distance from one crest to the next.  The closer the crests are to each other, or the shorter the wavelength, the higher the sound the wave makes.  Conversely, the longer the wavelength the lower the sound.

Doppler effect

Doppler effect (Photo credit: Wikipedia)

When the source of the sound is moving, such as when a siren is attached to a fire truck, as the truck comes toward us each wave that comes from the siren is slightly closer than the previous wave.  This has the effect of shortening the wavelength, raising the pitch of the siren.  Of course, the siren is emitting a steady pitch; we only perceive it to be higher or lower based on the movement of the fire truck relative to us.  As the truck moves away, each wave that is emitted is slightly farther away than the previous wave, so the wavelength is longer and the pitch drops.

The Doppler Effect is highly accurate and is the science behind radar guns.  A radar gun doesn’t actually emit radio waves (“radar” stands for RAdio Detection And Ranging) because the radio waves are too long to be reflected accurately by cars.  Radar guns actually use microwaves.  By measuring the change in frequency of the reflected microwave off a moving car, the gun calculates the speed of the car.

By the beginning of the 20th Century, the trifecta of spectroscopy, the Doppler Effect and more powerful telescopes allowed astronomers to make unparalleled strides in analyzing stars.  Their velocities could be measured accurately and by 1912 it was determined that  some stars were moving along at a few kilometers per second and some were zipping through the cosmos at over 50 km/sec.  To put this in perspective if a jet plane could travel 50 km/sec it would cross the Atlantic ocean in a couple of minutes.

In 1912 an amateur astronomer, Vesto Slipher, became the first person to measure the velocity of a nebula (recall that at this time the Great Debate over nebula vs. galaxy had not been resolved).  He discovered that the Andromeda Nebula was blue-shifted (meaning it is moving toward the Milky Way) to such an extent as to have a velocity of 300 km/sec.  Doubting his measurements he trained his telescope on the Sombrero Nebula (now galaxy) and discovered a red shift (meaning the Sombrero Galaxy is moving away from the Milky Way) that yielded a speed of 1,ooo km/sec., nearly 1% of the speed of light.  A plane traveling this fast would go from New York to London in about six seconds.

As more astronomers looked at the Doppler Effect on galaxies they discovered a very weird and unexpected result.  The vast majority of galaxies are moving away from the Milky Way.  Scientists expected some galaxies to be moving toward us while others moved away, but almost all were racing away as if the Milky Way had measles.

Various theories about why this is so were advanced but no consensus emerged.  Eventually Edwin Hubble, already famous for settling the Great Debate, would make another monumental discovery that would further support the Big Bang Theory.

Light: The DNA of the Universe

If you follow the various crime dramas on TV, you know that once the villain’s DNA is found the crime scene analysts can tell everything about him:  sex, age, height, weight, race, what he had for dinner last night and probably where he lives.  Whether that is real science or not, light tells just about everything there is to know about a star.

In 1842 French philosopher Auguste Comte tried to categorize certain things that would forever remain unknown.  Among those was “the chemical and mineralogical” makeup of stars.  He would be proven wrong within two years of his death.

Light is a form of energy.  Physicists think of light as being a wave, much like waves in water.  The wavelength of light is measured by the distance from the crest of one wave to the crest of the next.  The shorter the wavelength of light, the more energy it has and vice versa.  Humans tend to think of light only in terms of those wavelengths that the eye can detect, which range from the longest wavelengths of red to the shortest of violet.  Wavelengths longer than red are called infrared, while wavelengths shorter than violet are called ultraviolet.  Scientists, however, often lump all electromagnetic radiation under the heading of “light.”  Other forms of electromagnetic radiation are radio waves, which have wavelengths in the hundreds of meters (a meter is roughly a yard), to microwaves, exactly what are used in microwave ovens and are what the police really use in radar guns, and are still much longer than infrared light, to gamma and x-rays, which have very short wavelengths and thus high energy, which allows x-rays to penetrate solids.  The higher energy of the shorter wavelength light, such as x-rays and gamma rays, can cause damage to humans.   Gamma rays are one of the deadly emissions from a nuclear explosion.

Complete spectrum of electromagnetic radiation...

Complete spectrum of electromagnetic radiation with the visible portion highlighted (Photo credit: Wikipedia)

Since light is a form of energy scientists can use light to determine the temperature of a star.  An object heated to about 500 degrees Celsius (900 degrees Fahrenheit) begins to glow red, literally red-hot.  At 3,000 C (5,400 F), it begins to emit white light. So by analyzing the spectrum of light given off by a star, science can determine its temperature.

In 1752 Scottish physicist Thomas Melvill noticed that different substances emitted different colors when burned.  By carefully categorizing the colors emitted by the burning of various elements and comparing those to the light given off by a star the star’s composition can be deduced.  Melvill wasn’t expecting this nor was he looking for it.  His discovery illustrates the adage that most scientific discoveries are accompanied not by a cry of “Eureka!” but by a murmured “that’s funny.”

Each element has a distinctive color, which acts as its DNA, allowing it to be identified just by looking at it.  This combination of light, color and atoms is called spectroscopy.  The science of studying light emitted by objects is called spectroscopic emission.  The opposite phenomenon, absorption of certain light wavelengths by a substance, also exists and that is called spectroscopic absorption.  Spectroscopic absorption allowed scientists to determine the composition of the sun.  By noting which wavelengths were absorbed and therefore not visible in the light emitted from the sun, scientists could determine what elements were present in the sun that absorbs those wavelengths.

In addition to determining the composition and temperature of stars from starlight, the star’s velocity can also be found.  This stunning discovery was made by Thomas Huggins and his wife, Margaret, herself an accomplished astronomer and 24 years his junior.  When Thomas, at 84, was too feeble to clamber around a telescope, his young wife, only 60, was able to take over.

Science had long known that stars appear to change position in the sky relative to Earth and to other stars.  This movement across the sky is called proper motion.  However, proper motion is incremental and even with advanced technology is difficult to detect.  In addition, proper motion only tracks movement of a star laterally in the sky, as if all the stars were in the same plane.  Proper motion can tell nothing about the movement of a star either toward or away from the earth.

Mr. and Mrs. Huggins were able to combine spectroscopy with a piece of physics known as the Doppler Effect, after Christian Doppler, the Austrian physicist who discovered it, to determine the velocity of stars.  The Doppler Effect would become key in proving the Big Bang Theory.