Explained: the Doppler effect (w/ Video)

Aug 03, 2010 by Morgan Bettex
In every star’s outer layer, certain atoms are absorbed by light at different wavelengths, and this absorption appears as dark lines in the different colors of the star’s spectrum. Researchers use these lines as convenient markers to measure the size of the Doppler shift over time. For a so-called “red shift,” the lines move toward the red end of the spectrum (as displayed in the middle row in this graphic), and for a blue shift, the lines move toward the blue end (as displayed in the bottom row in this graphic).

Many students learn about the Doppler effect in physics class, typically as part of a discussion of why the pitch of a siren is higher as an ambulance approaches and then lower as the ambulance passes by. The effect is useful in a variety of different scientific disciplines, including planetary science: Astronomers rely on the Doppler effect to detect planets outside of our solar system, or exoplanets. To date, 442 of the 473 known exoplanets have been detected using the Doppler effect, which also helps planetary scientists glean details about the newly found planets.

The , or Doppler shift, describes the changes in frequency of any kind of sound or produced by a moving source with respect to an observer. Waves emitted by an object traveling toward an observer get compressed — prompting a higher frequency — as the source approaches the observer. In contrast, waves emitted by a source traveling away from an observer get stretched out.

In astronomy, that source can be a star that emits ; from our vantage point, Doppler shifts occur as the star orbits around its own center of mass and moves toward or away from Earth. These wavelength shifts can be seen in the form of subtle changes in its spectrum, the rainbow of colors emitted in light. When a star moves toward us, its wavelengths get compressed, and its spectrum becomes slightly bluer. When the star moves away from us, its spectrum looks slightly redder.

To observe the so-called red shifts and blue shifts over time, planetary scientists use a high-resolution prism-like instrument known as a spectrograph that separates incoming light waves into different colors. In the outer layer of every star, certain atoms are absorbed by light at different wavelengths, and this absorption appears as dark lines in the different colors of the star’s spectrum. Researchers use the shifts in these lines as convenient markers by which to measure the size of the Doppler shift.

This video is not supported by your browser at this time.
Video: Doppler shift. From NASA/JPL

If the star exists by itself — that is, if there is no exoplanet or companion star in its stellar system — then there will be no change in the pattern of its Doppler shifts over time. But if there is a planet or companion star in the system, the gravitational pull of this unseen body or star will perturb the host star’s movement at certain parts of its orbit, producing a noticeable change in the overall pattern and size of Doppler shifts over time. In other words, the pattern of a star’s Doppler shifts can change over time as a result of gravity affecting the star’s motion. “If this shift is large, then it must be caused by another star pulling it, but if this shift is small, then it is likely caused by a low-mass body like an exoplanet,” explains Joshua Winn, an assistant professor in MIT’s Department of Physics. As part of his work at MIT’s Kavli Institute for Astrophysics and Space Research, Winn studies the relationship between an exoplanet’s orbit and its parent star’s rotation for clues about how the planet may have formed.

How a planet’s changes over time can also shed light on the planet’s orbital period (the length of its “year”), the shape of its orbit and its minimum possible mass. Recently, Kavli postdoc Simon Albrecht used the Doppler effect to detect color shifts in the light absorbed by an exoplanet, which indicated strong winds in the planet’s atmosphere.

As depicted in this image, astronomers can detect an exoplanet thanks to the changes in Doppler shifts that the planet’s gravitational pull exerts on its host star. Those shifts are seen as red or blue color changes in the spectrum of light emitted by the star. Image: NASA

Doppler shifts are used in many fields besides astronomy. By sending radar beams into the atmosphere and studying the changes in the wavelengths of the beams that come back, meteorologists use the Doppler effect to detect water in the atmosphere. The Doppler phenomenon is also used in healthcare with echocardiograms that send ultrasound beams through a body to measure changes in blood flow to make sure that a heart valve is working properly or to diagnose vascular diseases. Police also rely on the Doppler effect when they use a radar gun to bounce radio beams off of your car; the change in frequency between the directed and reflected beams provides a measure of your car’s speed.

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User comments : 21

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RayVecchio
5 / 5 (2) Aug 03, 2010
Hold on... Light absorbing the atoms??? it is I am getting old but or else I switched universes again?
T2Nav
5 / 5 (1) Aug 03, 2010
In the movie, shouldn't the absorbtion gaps be moving up and down the color spectrum as the star wiggles back and forth? The movie just shows them staying in place relative to the colors.
Kedas
5 / 5 (2) Aug 03, 2010
Don't worry still the same universe it is
"certain atoms are absorbing light at different wavelengths"

And the movie is also wrong they should keep the background fixed, otherwise nothing is changing.

This is all assuming that I didn't switched universe together with you.
MustaI
1.8 / 5 (5) Aug 03, 2010
You're right. The dispersion of ripples at the water surface illustrates, the Doppler shift is virtually indistinguishable from dispersion at intrinsic perspective.

http://www.pitt.e...ples.jpg

Eezyville
1 / 5 (1) Aug 03, 2010
No. You guys switched universes. Welcome to an alternate reality. Its almost the same as the one you came from except there's an alien envasion happeneing tomorrow morning. They're using the CME to knock out our defenses and striking when we're weak.
RayVecchio
5 / 5 (1) Aug 03, 2010
Arrgghh!!! It is that new atom absorbing light!!! don't look at the light whatever you do!
Mr_Man
5 / 5 (1) Aug 03, 2010
Arrgghh!!! It is that new atom absorbing light!!! don't look at the light whatever you do!


Whatever, it's not like you can out run the light. Just let it happen, man.
ShadowRam
not rated yet Aug 03, 2010
How do we know our own gravity of the solar system isn't 'misrepresenting' the amount of shift?

Aka, things are closer than they appear affect.
kaypee
not rated yet Aug 03, 2010
The video shows the speed of light varying (follow the wave pattern near the earth) which is certainly does not do.
Sanescience
not rated yet Aug 03, 2010
Physics fail. The full length of the light beam shouldn't change as the star rotates either.
Jigga
1.7 / 5 (6) Aug 03, 2010
How do we know our own gravity of the solar system isn't 'misrepresenting' the amount of shift?.
Because it would affect it in the same way for all stars observed.
The full length of the light beam shouldn't change as the star rotates either.
How/why do you think so?
franco_bonafe
not rated yet Aug 04, 2010
Plenty of mistakes, but the worst thing is that most of them appear to be misconceptions of the theory. "Atoms absorbed by light" is awfully wrong. I'm quite surprised.
SincerelyTwo
5 / 5 (1) Aug 04, 2010

The quality of Physorg articles has gone off a cliff, this is just embarrassing.
frajo
3 / 5 (2) Aug 04, 2010
The quality of Physorg articles has gone off a cliff, this is just embarrassing.
The articles are not generated by PhysOrg. It's up to you to evaluate their respective qualities.
Or would you prefer to have the PhysOrg staff decide for you which articles are worth reading?
Ethelred
5 / 5 (1) Aug 08, 2010
genastropsychicalst :

Out of curiosity and a vain hope for a sane response.

Just what the heck is your point?

Do you have one?

Is this a bizarre experiment?

If so I think I know what the result will be. I am willing to discuss it in PMs.

And to respond to you in kind instead with a vain hope.

Also including plus even and or.

Ethelred
TabulaMentis
not rated yet Aug 08, 2010
Ethelred:

In a Physorg article titled: Revised theory of gravity doesn't predict a Big Bang, you mentioned the following: One way to get rid of singularities is to assume that there is limit how small anything can be in the Universe. The Planck length will do nicely.

My question is: What about sub-Planck lengths that Brian Greene has talked about in one or two of his books?
frajo
not rated yet Aug 09, 2010
In a Physorg article titled: Revised theory of gravity doesn't predict a Big Bang, you mentioned the following: One way to get rid of singularities is to assume that there is limit how small anything can be in the Universe. The Planck length will do nicely.

My question is: What about sub-Planck lengths that Brian Greene has talked about in one or two of his books?
You asked the same question in the abovementioned article. It was comment #198, I think. Ethelred and others answered that question in the following comments.
Why don't you read their answers there and continue the discussion there?
TabulaMentis
not rated yet Aug 09, 2010
Frago:
When I checked yesterday there were not comments.
My mistake. I figured people had moved on to newer articles.
Ethelred
5 / 5 (2) Aug 12, 2010
genastropsychicalst

seems to have been banned. All the posts are gone. Now we may never no what the heck was going on.

What a tragedy.

Ethelred
rlingenf
not rated yet Aug 18, 2010
"In every star’s outer layer, certain atoms are absorbed by light at different wavelengths..."

Does Einstein know about this?
Ethelred
5 / 5 (1) Aug 18, 2010
Does Einstein know about this?
No. He died a long time ago. He doesn't know ANYTHING anymore.

However he did know it in a slightly different and vastly more CORRECT form, as opposed to that mess. At guess someones fingers got behind their head. I do that all the time. Usually it results in dropped words rather than words getting switched around.

Ethelred