1. can someone please explain what redshift is in layman's terms, if thats possible, i dont hold any qualifications in the sciences, well any really, i read books and watch science documentaries, but the first time ive encountered red/blueshift was here. ill be greatful if anyone can spare a few minutes to try and explain. thanks in anticipation of your patience.

2.

3. Redshift and its opposite blueshift are instances of what is known as Doppler effect.

Doppler effect is very easy to understand if we talk first of sound waves.

One of the properties of waves is what is called frquency. Frequency is "how many times does the wave happen in a second". if the frequency are 100 Hz (Herz), this means the wave "happens" 100 times per seoncd -there are 100 waves per second. That is if a source is before you, you will be hit by 100 waves in a second... but, what if the source is moving?

If it moves away, it sitll will be emitting 100 waves per second. But waves have a limited speed -they take a time to reach you. And as the source is farther, then they take longer to reach you. They reach you "delayed",a and this means that you no longer get 100 waves in a second. You may get the first 99... but as they're being "delayed" as distance increases, the 100th wave won't reach you that second, but the next one... From your point of view, the frequency of the waves decreases as they move away from you -but only as long as they move.

The opposite happens when they get close to you. In the same given second, you will the first 100 waves... and an extra one that has "catched up" with them as has been emitted closer and has reached you sooner. You will notice as if frequency increased, as you're getting more waves per second as the source closes to you and each new wave must travel a shorter distance.

Summarizing: when waves are being emitted each one farther than the earleir, their perceived frequency shrinks as each new wave must travel a larger distance and is "delayed" in the process. Conversely, if they get closer, frequency increases as each wave travels a shorter distance than the earlier and so waves "catch up" wiht the earlier ones, so you perceive more waves per second, aka frequency increases.

Well then, this is true for ANY wave. When light waves are being emitted from an object that moves away from you, their frequency is decreased.

This is the phenomenon we call "redshift", as red light haves the lower frequency of all visible right (and blue the higher frequency).

Actually, redshift is caleld thsi way because fo antohr phenomenon...

Light is not emitted in every possible frequency by every possible matherial; due to certain properties of matter, there are certain frequencies which can't be emitted by certain elements. This causes that the light spectre is not a continuous stirip of rainbow colors, but it's interrupted by black lines of "absent light", frequencies whcih are being blocked by an element which is emitting that light. Every element has a series of very precisely determined "absent frquencies", whcih are called absorbtion stripes. This provides a mean to identify elements by just looking at how they affect the light spectre, what absorbtion stripes they cast. And then...

What happens if you look to a star... or a galaxy far away... and you KNOW where the absorbtion stripes should be... and they are... just they are palced closer to red than they should? THIS is what is precisely what is called redshift, and it means that the light source is moving away from you... or vice versa... or both you are getting away from each other, thus moving the absorbtion stripes corresponding to the elements from which the stars and galaxies are built from... :wink:

4. Lucifer's explanation seems to be the one to go with. Here's another small words attempt...

Imagine a light source (a star or even a single point) moving away from you at half the speed of light (ignore, for now, the relativistic effects).

Now imagine (this is not what actually happens, but is just to help understand the phenomenon) that a single light wave, from crest to crest, or trough to trough, is generated by this source.

From the start of the generation, the first crest, to the end, the second crest, some time will have passed. Therefore, the light source will, in that time, have moved away from you (at half the speed of light).

Therefore, while the light 'wave' may have had a frequency (to which the wavelength 'w' is inversely proportional) of 'h' in terms of the energy levels of the eletrons that emit it, this will change as the wavelength, from crest to crest, is 1.5 times what the original 'should' have been.

The frequency, because of this change of wavelength, will appear to be only 66.666% of what it 'should' have been.

Now since (in the visible spectrum) the frequency goes UP from red to blue, and therefore wavelength goes UP from blue to red, with the wavelength 1.5 times what it should have been, the light from the sources will appear to have 'shifted' from the blue end to the red end of the spectrum. This, in essence, is the Red Shift.

One or two caveats:

1. How do we know what colour the light 'should' have been? (Or, why do we not assume that the colour we see is the colour of light emitted without any shifting?) Simply because of spectral analysis. The spectral lines, for instance, of Hydrogen are uniquely distinctive not just because of where they fall on the spectrum, but also because of their pattern (like tally marks fading towards one end, and then echoes of those marks endless repeating more and more faintly further along). We identify the patterns, and then noticed that they're in a redder portion of the spectrum than we expected - ergo, red-shift.

2. The Red Shift works whether you are moving away from the light source, or it is moving away from you, or both - that's relativity for you.

Hope this helps.

5. thanks guys, i think im going to mull over both of your posts over a bottle of red when the kids are in bed.

6. Light is a wave. It never speeds up or slows down, at least in terms of how fast it's moving.

What it does instead is bob up and down more quickly (the frequency gets higher or lower). I mean that the wave reaches its high point, and its low point, more times per second.

If light is emitted by, or bounces off of something that is moving toward or away from us, then instead of actually speeding up or slowing down, it changes frequency.

An object like a baseball would speed up or slow down, in addition to changing direction. The harder the batter hits the ball, the more it speeds up. If he were hitting a beam of light, the light would go up in frequency instead of speed, which would make it look more blue.

A redshift kind of implies he's swinging away from the pitcher instead of toward him.

7. Just a simple question: Light is a longitudinal wave? Meaning it is made up of areas of higher and lower densities of photons? Then for red shift to occur the distance between, say, the denser areas have get longer.

8. sorry kalster i dont understand your question. im only just getting to grips with red/blue shift, when its simplified like this it makes a lot of sense. now i understand the basics i can look more into it, dont think ill be answering any questions just yet though :-D

9. Originally Posted by KALSTER
Just a simple question: Light is a longitudinal wave? Meaning it is made up of areas of higher and lower densities of photons? Then for red shift to occur the distance between, say, the denser areas have get longer.
My understanding is that light behaves like a transverse, not a longitudinal wave. In any case, even if it were a longitudinal wave, you cannot think of that wave in terms of photon density - you cannot simply identify the wave and particulate nature of light with each other.

10. Ok, the old duality thing. So how to imagine light? Sound can be discribed like I tried with light I think.

11. Originally Posted by KALSTER
Ok, the old duality thing. So how to imagine light? Sound can be discribed like I tried with light I think.
Sound can, indeed. Unfortunately for us, thanks to what we know of quantum physics, we cannot 'imagine' light in any way that makes intuitive sense to our stone age brains. I just tend to call them all 'wavicles' and let them interact how they will.

It is possible, howver, if you wish, to think of them as transverse waves (only) when investigating phenomena like the red shift. In such a case it is best to stop thinking about photons!

Similarly if you wish to use photons (say you are examining the photoelectric effect), then it is best to not think about waves, but simply individual photon energy.

12. i was doing so well too :?

13. Originally Posted by beats666
i was doing so well too :?
Don't give up. It took me years to come to my accommodation with it - and it's not scientifically respectable, it simply helps my brain work its way through it: I think of elementary particles (and hence matter-energy) as probability waves (which is nothing new - that's what quantum theory is supposed to be about) which means that for any given 'particle', at any given time, there is a finite probability of its engaging in an interaction somewhere in the universe.

Practically speaking, for particles like electrons, the probability peaks are usually within the regions that we call the orbitals of the atom in question. For photons, these tend to be the scintillation screen (or whatever other device you use) that measures the results of your double-slit versus single-slit set-ups.

The way I think of it is that, when a double-slit changes to a single-slit, the universe has changed, and the probability waves of the photon in question will therefore change - giving rise to the differences we see. But it took years to come to this semi-intuitive manner of thinking, and it may not work for everyone. All the best with that.

14. Originally Posted by KALSTER
Ok, the old duality thing. So how to imagine light? Sound can be discribed like I tried with light I think.
I'm not sure. I always get the impression that during the "compression" part, the force (an electric or magnetic field) is only exerted in one direction perpendicular to the direction the wave is traveling, instead of in all directions perpendicular to it. (which would make it different from sound, I think)

I could be wrong about that. It would be nice to know, and I think light would make a lot more sense if I'm wrong.

15. Originally Posted by kojax
Originally Posted by KALSTER
Ok, the old duality thing. So how to imagine light? Sound can be discribed like I tried with light I think.
I'm not sure. I always get the impression that during the "compression" part, the force (an electric or magnetic field) is only exerted in one direction perpendicular to the direction the wave is traveling, instead of in all directions perpendicular to it. (which would make it different from sound, I think)

I could be wrong about that. It would be nice to know, and I think light would make a lot more sense if I'm wrong.
You are right - which is why light can be polarised.

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