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.

3 thoughts on “The Doppler Effect

  1. Among all the absolutely asounding things about the universe is that these unbelievably immense things called galaxies are moving. Where are they moving to? What is there for them to move into?

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