Before beginning our discussion about the search for planets around other stars,
we need to go through a brief primer on the Doppler Effect.
Christian Doppler was an Austrian physicist working in Prague,
who in the year 1848 proposed that the observed frequency of a wave depends upon
the relative speed of the source of the waves and the person observing the waves.
This could apply to light waves, sound waves, or
indeed, any kind of oscillatory phenomenon.
It was confirmed experimentally for
sound waves in 1845 by the Dutch chemist, Buys Ballot.
It was also then confirmed for
light waves in 1848 by the French physicist Hippolyte Fizeau.
The Doppler Effect is very important to us in astronomy because it gives us
one of the most important ways,
in fact in many ways the only way we have to measure the speeds of objects
when we can't see their actual motion, we can only measure the bulk light from them.
To see how the Doppler effect works, let's begin with the most direct experience that
most of us have with the Doppler effect everyday, the Doppler Effect in sound.
The Doppler Effect is the shift in the frequency of a wave
when the source is moving either towards or away from you.
Starting with an example here of a stationary source.
For example we have two cats sitting with a mouse squeaky toy between them.
The oscillator in the squeaky toy is emitting a high pitch squeak.
Both of the cats are stationary relative to each other and
stationary relative to the mouse.
Each of the cats hears exactly the same pitch.
I've represented the peaks of the sound waves
by the green circles appearing to radiate from the mouse toy.
If now, the mouse toy was to be wound up and set into motion,
there would be a very different sound heard by each of the different cats.
The cat on the left is in the direction of motion of the mouse,
the mouse is moving from right to left on the screen.
And as a consequence, as it moves along,
at each position of the green dot shown it emits a squeak.
The green waves in the direction of motion tend to bunch up.
Because the waves are closer together, we would refer to this as coming by us
very frequently, and therefore the sound wave perceived by the cat on the left, in
the direction of the mouse's motion, would perceive a high pitch for the squeak.
The cat on the right sitting in the direction that the mouse is moving away
from, the waves appear to be stretched out.
A long distance between the wave peaks gives the impression of a low pitch,
a smaller frequency.
By being able to sense both the change in pitch, higher or lower, and
the magnitude of that change, one could perceive whether the sound source
is moving toward you, higher pitch, or away from you, lower pitch.
And by measuring the shift in frequency, one can measure the speed
in meters per second, kilometers per hour, or whatever the appropriate unit may be.
Let's see an actual demonstration of this using an oscillator set into motion.
We're going to demonstrate it with sound because the speed of sound is slower and
the effect is much more pronounced, and
the ear is remarkably good at sensing small changes in pitch.
I have in my hand a small electronic oscillator that plays a pure tone
of 620 hertz frequency.
[SOUND] [SOUND] [SOUND] When the tone generator is stationary
you hear a pure tone with no modulation at all.
But if I was to set this in motion by using this string and
swinging over my head, you would hear a very distinct modulation in pitch.
Higher as it comes towards you, lower as it goes away.
[NOISE] And
then steady
as we stand still.
The Doppler Effect in light is just like the Doppler Effect in sound waves.
If a light source is moving toward you, the waves will appear to bunch up.
Bunched up light waves are bluer in color.
Whereas if you are on the position where this light source is moving away from you,
the light waves will appear to be stretched out.
Longer wavelength, or lower frequency light, looks redder to the eye.
And we don't always measure this with the eye, we also measure this with devices
called spectrometers that make precision measurements of the wavelength of light.
The size of the shift of wavelength, and we're going to use wavelength,
because I'm an optical spectroscopist, and that's what we do.
The size of the shift is going to proportional to the line of sight
velocity of the source.
Lambda is the symbol that physicists use to measure the wavelength of light.
If I take the observed wavelength of light minus the emitted wavelength and
divide it by the emitted wavelength.
So, I'm now not looking at the whole shift, but the fractional shift.
That's going to be equal to the speed of the source relative to c,
the speed of light.
In order to get a very large shift in light, either a blue shift or a red shift,
the source must be moving at a reasonable fraction of the speed of light.
However, our ability to measure wavelengths
in high precision spectrometers can be very, very good.
We can actually measure the speeds of objects to a very,
very tiny fraction of the speed of light.
Current state of the art in astronomical spectographs can be getting down to
measuring speeds of centimeters per second, a very slow walk.
This gives us a very high precision way to measure line of sight, or
we call it radial velocities.
The term radial is because we see ourselves at the center and we think of
the object either as moving radially away from me or radially towards me.
The Doppler Effect however is not just confined to astronomy or cat toys.
A very practical everyday application of the Doppler Effect in light waves
is Doppler weather radar.
Here, instead of using light, or at least visible light, we're going to use a low
frequency form of light, microwaves emitted by a microwave radar emit antenna.
The microwave radar antenna bounces pulsed microwaves off of distant clouds or
precipitation falling from those clouds.
The frequency shift in the received signal that's
measured by the Doppler radar system tells us the speed and
direction of the clouds and winds in the distant location.
The signals strength can also be use to give us an idea of
how many clouds there are, whether there are heavy clouds or light clouds, and
if there's any precipitation, rain or snow falling.
The strength of the signal returned will be stronger if there are a lot of water
droplets or snowflakes for the signal to bounce off of.
A bit of weather radar animation showed over here on the right,
this is a reconstruction using computers of the Doppler radar signals.
Doppler Radar is an everyday part of how many of
us keep track of the weather in our day.
We can see whether a rainstorm or snowstorm is coming towards us, or whether
the rain is about to pass over us and it's safe to go home without using an umbrella.
Another everyday example of Doppler Effect in electromagnetic radiation
is a traffic radar gun used by law enforcement.
Hopefully this is not something you encounter everyday, but
it is used as a way for a law enforcement officer
to measure the speeds of cars to see if they're complying with speed limits.
The hand-held gun, which can be either mounted on a dashboard or
perhaps even held by a police officer,
emits a very narrow beam of microwaves of known frequency.
By pointing the radar gun towards a moving vehicle and
pressing the trigger, it emits this narrow beam and then measures
the frequency of the microwave signal reflected off of the moving vehicle.
A tiny computer inside of the gun measures the frequency difference and
converts that to a line of sight speed.
This will tell the police officer whether you are moving towards or
away from the officer and at what speed.
If you're exceeding the speed limit then you're probably going to get a ticket.
In astronomy we use the observed Doppler shifts of absorption and emission lines
and the spectra of objects to measure their radial speeds relative to the Earth.
For example, let's say we have a star over here at the right which is at
rest with respect to us, it's neither moving toward us nor moving away from us.
We observe it with our telescope and pass the light through a spectrograph
breaking the light into its rainbow of component colors from blue to red.
The spectra of stars are crossed by dark lines called absorption lines.
The wavelengths of these lines are very well known because we
associate each of these absorptions with atoms or molecules known on the Earth, and
we know what their wavelength should be when they're at rest.
If now we observe a second star with the same spectral spectrum, but
now that star is moving away from us with some speed,
all of the lines in the spectrum will be shifted to redder wavelengths than we will
observe in the same type of star that is at rest.
We say that this object has a spectrum that is redshifting.
By measuring the amount of the redshift, we can measure the speed away from us.
Similarly, if another star of the same spectral type,
is moving towards us, all of the lines will be shifted towards bluer colors or
they will be blueshifted relative to the spectrum of the star at rest.
By measuring the amount of the blueshift,
I can measure the speed of the star coming towards me.
Put together, both the size of the shift and the sense of the shift, whether blue
or red, tells me the speed and direction of the star's motion relative to Earth.
This Doppler Effect in stellar spectra is going to be very important to us
when we attempt to measure small
changes in the speed of a star, due to the fact that the star and
an unseen mass of planet are orbiting about their common center of mass.
We're going to call this Doppler or radial velocity method.