From hart@soliton.physics.arizona.edu Mon Oct 20 15:14:45 1997 Date: Wed, 24 Sep 1997 13:53:09 -0700 (MST) From: Samuel Hart Subject: Re: Black Holes and the Doppler shift On Sat, 20 Sep 1997 wrote: > I've been trying to study cosmology, and in some of my readings, I > encountered what is called the "The Doppler Shift". I can't quite > understand the concepts behind it, if you could explain to me in simple > terms and whether there's any relation to Black Holes. The Doppler Effect is simply the frequency increase or decrease with respect to the relative motion of one object (the transmitter) to another object (the receiver.) It's something everyone encounters in everyday life. Let's say that you're standing on the side of the road, and a car is approaching. As it's approaching, you hear the sound of its engine. When it passes you, the sound of the engine now sounds lower than it was before. Also, the sound from the car seems to fade into the distance more slowly than it did as it approached. This is the Doppler Effect. What is happening here is that the sound producing object is in motion while it's producing the sound. Thus, the waves propagating outward from the direction of travel wont have as much time to move as those coming out the back do. The image at http://www.physics.arizona.edu/~hart/tifun/doppler.gif illustrates this. In this image, Vs is the vector showing the direction of the sound source (s), and the circles represent the wave-crests emanating from s. As you can see, in the direction of Vs, the wave-crests are clumped together, whereas in the opposite direction, they are spread apart. Thus, the frequency increases in the direction of travel, decreases the other way. What does this have to with cosmology? Well, a similar effect occurs with light waves. (Though, it is important to note that it is not the _same_ effect, simply because sound waves [like all mechanical waves] need a medium for their propagation, and light waves do not.) As an object that is emitting light moves (relatively) away from us, the frequency of its light that we receive is shifted lower. Conversely, if the source is moving (relatively) towards us, then we view its frequency shifted higher. In Doppler observations in astronomy, the wavelength is usually more easily measured than the frequency, but the effect still holds. This effect is used to determine the relative speed of extra-solar objects to us. The way that it works is scientists first catalogue the light spectra of various elements. Then they look for similar spectra in extra-solar objects (incidentally, also a major way that star compositions are determined.) When they find one, they compare its spectra with the known, observed one, on Earth. If the spectra from the star is similar in placement of the spectral lines, except they are all shifted in one direction or another, then you can be certain of the direction (relative to Earth) of travel. For example, lets say you measure the spectrum of element A here on Earth and find it looks like the image at http://www.physics.arizona.edu/~hart/tifun/s1.gif then you measure the spectrum of star X and get something like http://www.physics.arizona.edu/~hart/tifun/s2.gif From this you could tell that element A is part of the chemical composition of star X. Furthermore, from the shift in the spectral lines towards more energetic frequencies, you could tell that star X is moving towards us. So the Doppler Effect in cosmology aids in determining the speed and direction (relative to Earth) of various extra-solar objects. I hope this helped, and I didn't ramble too much! ;) -- Sam Hart http://www.physics.arizona.edu/~hart/ Web Page Highlights: Video Game History, Black Hole Simulation, & more.