# Red Shift: How do they recognize it?

I have never fully understood red shift of a star’s spectrum. Wouldn’t you need to see the star’s spectrum when it’s still AND compare it to the same star’s spectrum when it is moving away from the observer.

But! We have nothing to compare it to, so how can we observe a shift without a baseline against which to compare it??? Can some SDoper explain this more fully? There’s some step I’m missing in the logic here… - Jinx

Yes, you are. Each element has a unique emission spectrum composed of a series of lines. The patterns for each are well-defined and all we need to do is to see how far towards the red or blue ends of the spectrum a given pattern of lines has been shifted in order to measure the blue or red shift of a given body.

The short answer: Redshift is measured by looking at the full spectrum of light. By looking at the full spectrum we can see where the emission or absorption lines of different elements (hydrogen, helium, etc) are. Now if the object we are looking at is going away from us these lines and shifted from the “stationary” to the red end of the spectrum if the object is coming closer the lines and shifted towards then blue end of the spectrum. The same effect is noticeable when standing by a road and listening to cars driving past and noticing that the pitch of the noise the cars create rises as the car approaches and drops as the car goes away from you.

Each element either absorbs or emits light depending on it’s state a cool cloud might absorb light and we will see dark lines across the spectrum if the source of the light is stationary relative to our position then the spectrum will be ‘normal’. Or, if the element is fusing and therefore the source of the light. The spectrum will be darker and the light emitted will show up as bright lines across the spectrum and, once again if the source of the light is stationary relative to our position the lines will be in the same position. If the source of the light is going away the lines are shifted to the red end and likewise if the source is coming closer the lines are shifted to the blue end.

Of course the amout of the shift coresponds to the velocity of the light source

OK, what I don’t understand is:

I’ve always heard that you will always see light as approaching you at the standard speed of light, no matter what. If you still see this light as coming at you at “normal” speed, how can the perceived frequency be changed?

By changing the wavelength of the light.

As a source moves away from you, each successive wavecrest is further away from its predecessor. The change in the seperation of wavecrests is dependant on the velocity of the source. Since the wavespeed is dependant on both frequency and wavelength, effectively “stretching” the radiation’s wavelength will change the perceived frequency, so that the speed always remains constant.

The speed of light remains constant. What changes is the wavelength. If the source of the light is moving away from you, the distance between the crests of the wave increase. Since frequency is the wavelength / the speed of light, the frequency gets shifted toward red when the light is moving away, and towards blue when it’s moving closer.

A slight hijack: in the first Star Wars movie there is a scene where a red shift can be seen to occur through the window as Hans Solo accelerates his ship. The only thing is, IIRC, the point of view is through the front window of the ship as Millennium Falcon moves forward. Shouldn’t there have been a blue shift instead?

The front window? Yes, afaik it should.

It reminds me of my physics class:

Teacher: And the light from all stars is massively blue-shifted.
Shade and friend: [look at each other] DUUCCKKK!

Our teacher hated us.

BTW, anyone asking this question may be interested in the next obvious one: are we certain that the red-shift is due to the ‘doppler effect.’ For instance, a quick google shows the tired light theory being rebutted.

The same thing happens with doppler shifted sound; the speed at which it travels to your ears is the same (as dictated by the speed of transmission that is based on the way that air particles bang together), but the wavecrests are squished together (or stretched apart).

The same thing happens with doppler shifted sound; the speed at which it travels to your ears is the same (as dictated by the speed of transmission that is based on the way that air particles bang together), but the wavecrests are squished together (or stretched apart).

If they were moving through normal space, then yes, certainly. But we know nothing about travel through some hypothetical “hyperspace”, so there’s room for artistic liscense. Heck, Star Wars exercises license even in cases which we do understand, and when was the last time (outside of this board) that you heard someone complaining about hearing explosions in space?

And Doppler shift is only one of at least three known mechanisms for redshift. You can also have gravitational redshift (where light loses energy climbing out of a gravitational field) and cosmological redshift, where the space in between source and observer stretches while light is en route. This latter is often described in terms of a Doppler shift, but it’s a distinct effect.