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Red shift amidst all this motion?

Posted: 26.06.2006, 16:02
by Startyger
Okay, here's a question, i've always wanted to have answered... maybe i'm an idiot.. ut, i'd still like to know the answer.

Amidst all the motion of worlds, stars, stellar systems, galaxies, galactic clusters, helixes of galactic clusters etc.... how do w e know what specific motion is being recorded via red n blue shifts here on little Earth, espcially, since we've not established a true map of our own local group?

Posted: 26.06.2006, 16:21
by selden
Each element (hydrogen, helium, gold, etc) emits specific colors when it's heated. This pattern of colors (called spectral lines) is unique to each element. You can determine what elements are present in a remote object by looking at the object's spectrum. You can determine what colors an element emits by heating it in a lab here on Earth and using a spectrograph to look at the light.

If its spectral lines are bluer than what we see when we heat that element in a lab on Earth, then it is moving toward us. When the pattern of an element's spectral lines are redder than they should be, then the object is moving away from us. How much those lines are shifted tells us how fast it is moving.

This also is known as the Doppler effect. Something similar happens with sound: a horn blowing on a car moving away from us seems to have a lower frequency than when it is stationary.

Does this answer your question?

No..not really..

Posted: 26.06.2006, 16:31
by Startyger
This, i knew... i was referring to how we determine what particular motion is being observed.. when nearly all we're observing is moving.. and not, in the same direction? For example, that everything's expanding, when we're moving in one direction, the solar system is moving in its direction, the milky way is moving in its direction, the local group is moving in its direction.. and other things.. are moving in thier directions...

how are motions attributed to orbits, revolutions, and 'random activities' tossed out to determine a 'greater' motion of the universe?

Posted: 26.06.2006, 17:24
by selden
I think the answer is "very carefully". :)

Each motion has to be carefully measured and taken into account. "Vector arithmetic" is involved when combining velocities: velocity includes the direction as well as the speed of each motion.

The doppler effect yields the radial velocity relative to the observer. When the observer is on the surface of the Earth, measurements at different times of the day and of the year will reveal the rotational speed of the Earth on its axis (which repeats with a 24 hour period) and its revolutional speed around the sun (which repeats with a 1 year period). The velocity of the Sun in its orbit around the galaxy is somewhat harder to determine (you have to measure the doppler shift relative to many different stars and other remote objects and take them all into account) and the Milky Way's trajectory through intergalactic space is even more difficult.

Of course, observations from satellites and from telescopes carried by balloons or high altitude aircraft have even more motions to take into account.

When extremely precise measurements are involved, one should also take into account gravitational effects.

Sorry: I don't know enough of the details to describe things much better than this. I'll have to refer you to appropriate text books. :)


Posted: 28.06.2006, 12:40
by Startyger
Yeah.. i was wondnering if computer models were being used as the time differential due to the speed of light, the distance it has to travel, gravity, dark matter, etc etc.. come into play. Thanks for your answer... back to the research.