Re: Lightspeed (was Re: phlogistontravel)

continuing my mission to educate the masses on the wonders of special
rel, an explanation of redshift with all but the most basic maths
stripped out. :)

Buddy Murphy wrote:
>
> Then how come we can tell how fast an object is moving away by it "red
> shift".

light is a wave. the color, as has been mentioned, depends on the
wavelength, or, more relevantly in this case, the frequency.
(wavelength=speed of light/freqency)
one way the freqency can be measured is by timing the arrival of ther
peaks of the wave.

so; imagine a ship travelling toward us emitting light. every nanosecond
(say) it produces a peak.
however, in the time it's taken between peaks, it's moved toward us.
this means that the second peak takes a shorter time to get to us
(because it does not have as far to travel), and has effectively 'caught
up' slightly with the pulse that had been emitted earlier, but had
longer to go.
thus, when we measure the peaks, the interval between them appears to
have decreased; giving a higher frequency, and bluer light.

similarly for a ship moving away from us appears redder.

that's the basics. unfortunately, it's not quite as simple as that.
(when is it ever!)
this is true for reflected light (which is usually most relevant),
however as mentioned in the other email, objects that themselves emit
light are subject to time dilation as well. thus the time between peaks
in the ship's rest frame is shorter than the time between peaks, so
light emitted will be dimmer and redder than predicted. this, however,
is a smaller effect than the doppler effect mentioned above, but i
thought i should include it for completeness. ;-)


--
rad.

stand still while i mock you!
                -- dogbert