Can someone explain redshift and blueshift?

[Manok101]

I've been thinking about it lately and I don't understand how we know galaxies are moving at all let alone where, and that's my question, how do we know that's what these two things mean?

[Parus major]

[Multi-Coated]

Good link , thanks.

[Si W]

Doppler effect - Wikipedia, the free encyclopedia

The Doppler effect for electromagnetic waves such as light is of great use in astronomy and results in either a so-called redshift or blue shift. It has been used to measure the speed at which stars and galaxies are approaching or receding from us, that is, the radial velocity. This is used to detect if an apparently single star is, in reality, a close binary and even to measure the rotational speed of stars and galaxies.

The use of the Doppler effect for light in astronomy depends on our knowledge that the spectra of stars are not continuous. They exhibit absorption lines at well defined frequencies that are correlated with the energies required to excite electrons in various elements from one level to another. The Doppler effect is recognizable in the fact that the absorption lines are not always at the frequencies that are obtained from the spectrum of a stationary light source. Since blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blue shift and those of a receding astronomical light source exhibit a redshift.

Among the nearby stars, the largest radial velocities with respect to the Sun are +308 km/s (BD-15°4041, also known as LHS 52, 81.7 light-years away) and -260 km/s (Woolley 9722, also known as Wolf 1106 and LHS 64, 78.2 light-years away). Positive radial velocity means the star is receding from the Sun, negative that it is approaching.

[scogyrd]

Is there any experiments they can do in a lab to prove redshift is what it is said to be or is it the result of a "thought experiment"?

[Odd Dob]

"Thought experiment"? It was an observable phenomena that needed explaining. Experimental science. It's not an idea that came from nowhere.

[Si W]

Is there any experiments they can do in a lab to prove redshift is what it is said to be or is it the result of a "thought experiment"?

Yes, that's how they found what frequency each chemical emits or absorption at in a lab, and a pattern emerged, so when they took the spectrum of a stars or galaxies the shift towards the blue or red tells them if its moving towards us or away, and the further the absorption line moves, the higher the radial velocity or speed it's moving.

Edit: I'll let you do the math scogyrd http://en.wikipedia.org/wiki/Redshift

[scogyrd]

"Thought experiment"? It was an observable phenomena that needed explaining. Experimental science. It's not an idea that came from nowhere.

I didn't say it saying that it came from nowhere, or that it doesn't exist. I just wondered how we really know that it means the galaxies are moving away from us.

[scogyrd]

Yes, that's how they found what frequency each chemical emits or absorption at in a lab, and a pattern emerged, so when they took the spectrum of a stars or galaxies the shift towards the blue or red tells them if its moving towards us or away, and the further the absorption line moves, the higher the radial velocity or speed it's moving.

Thank you.

[Odd Dob]

Heh, sorry. What I meant was that it isn't just a stand alone theory on it's own that somebody made up without reference to empirical data (what I understood by you using the words "though experiment"). It's a model based on corroborated evidence from many sources/theories. I did try and write a longer explanation but it got complicated so I cut my post short and it sounded a bit stroppy by accident.

[scogyrd]

Heh, sorry. What I meant was that it isn't just a stand alone theory on it's own that somebody made up without reference to empirical data (what I understood by you using the words "though experiment"). It's a model based on corroborated evidence from many sources/theories. I did try and write a longer explanation but it got complicated so I cut my post short and it sounded a bit stroppy by accident.

There's no need to apologise, no offence was taken. It was probably my fault for not wording the question very well in the first place anyway.

[Manok101]

Hmm I'll check out the links when I don't feel like passing out. Thanks for giving me something to read/watch.

[Superewza]

Is there any experiments they can do in a lab to prove redshift is what it is said to be or is it the result of a "thought experiment"?

Not in a lab but...

• Stand in the middle of a reasonably long road
  
• Get a friend to drive along it at a constant speed
  
• Listen to the change in pitch of the sound
  
• ???
  
• Profit
  

Exactly the same principle. That and the space between galaxies is expanding too, but that's just complicated. The car will also get slightly more blue and red as it moves towards/away from you, but good luck trying to measure that

[scogyrd]

Not in a lab but...

• Stand in the middle of a reasonably long road
  
• Get a friend to drive along it at a constant speed
  
• Listen to the change in pitch of the sound
  
• ???
  
• Profit
  

Exactly the same principle. That and the space between galaxies is expanding too, but that's just complicated. The car will also get slightly more blue and red as it moves towards/away from you, but good luck trying to measure that

How does this prove it though? Sound and light are two completely different things aren't they?

Sorry if I'm being thick.

[inksmithy]

How does this prove it though? Sound and light are two completely different things aren't they?

Sorry if I'm being thick.

No, essentially sound and light are exactly the same thing, but they are occurring on different wavelengths. The only real difference between them is the equipment we use to detect them.

Slow the frequency of the reforms of visible light enough and you will eventually start hearing them.

Alan

[scogyrd]

No, essentially sound and light are exactly the same thing, but they are occurring on different wavelengths. The only real difference between them is the equipment we use to detect them.

Slow the frequency of the reforms of visible light enough and you will eventually start hearing them.

Alan

I was taught at school that sound is basically vibrations travelling through the air (or any other medium), although it's been a few years now so it's a bit sketchy, is this wrong?

And, if we can slow down light to create sound can we speed up sound to produce light?

Sorry for all the questions. I'm just trying to get my head round this.

[Si W]

I was taught at school that sound is basically vibrations travelling through the air (or any other medium), although it's been a few years now so it's a bit sketchy, is this wrong?

And, if we can slow down light to create sound can we speed up sound to produce light?

Sorry for all the questions. I'm just trying to get my head round this.

The principal of Doppler effect of sound and light is the same, but that's it, you can't hear sound in space as sound needs air as a medium to travel though, in the vacuum of space sound has no medium to travel though, but light obviously does.

Sound is not part of the electromagnetic spectrum, it's just air vibrating, no air, no vibration, so no sound, light is completely different.

[cervesca]

The shift is based on a similiar phenomen with light and sound, as both are essentially waves. The comparison between them is more an analogy, as sounds waves are vibrations in matter (eg air) travelling at a much smaller velicity (300 m/s in air) than light. As the speed of sound is much lower these effects are more obvious in everyday life.

Its observed expmerimentally (in electromagnetic radiation eg light) as when the velocity towards and away from us is small, the shifts are small - This is used in radar to work out speeds of planes etc. Its also used to measure the speeds to and away from us of stars in our neighbourhood and is used for measuring binary stars as they take turns coming towards and away from us.

Another way to think of is is to pretend your a boat travelling through the sea - the faster travel towards the waves, the more peaks and troughs you see - counting the number you pass per second gives you the frequency.

[ollypenrice]

Sound and light are not, I wouldn't say, the same thing but they are both waves and being waves get their wavelengths compressed or extended if the source is approaching or receeding.

A caution here. In cosmology there is a formal distiction to be made between the redshift due to the expansion of the universe (the cosmological redshift) and the redshift due to the proper motion of the galaxy in question. They both arise because the light from the galaxy is being stretched but if the stretching is caused by the proper motion of the galaxy it is called a Doppler shift. If it is caused by the expansion of the universe it is not so called.

A galaxy may well, in its proper motion, be trying to head our way (in which case there is a blueshifting Doppler effect) but this may be entirely overwhelmed and turned into a net redshift by the expansion of space between us and it.

There is a glorious tale (maybe false) of Carl Doppler getting a French Horn player to play middle C on a train flatbed while going past a musician with perfect pitch who was to estimate the change in pitch. Trouble was, the train drowned the French horn. So then an entire penguin suited orchestra was loaded onto the flatbed...

Olly

PS Sorry Cervesca, we crossed.

[FraserClarke]

Sound and light (radiation) are different types of waves -- but the same basic principles of wave mechanics apply. Your understanding of sound is right scogyrd. You can't "slow down" a light wave and hear it, and neither could you speed up a sound wave and see it -- but the principles of how the waves work are the same.

Anyway, if you want to test this in a lab with light, take a light source with a nice well defined emission line (e.g. a laser) and whizz it round your head. You can then measure the shift in the position of that line as it comes towards/goes away from the detector. Probably the easiest way to observe this though it to take spectra of two stars in orbit around each other -- you'll see a beautiful periodic change in the position of the absorption lines as the go round their orbits. Equally, you see the doppler shift of things like radios on planes, satellites in orbit, planets in orbit around the sun, etc.

By the way, the doppler effect is one of the main ways we detect extra solar planets, by watching how the planet pulls the star around in its orbit. The signals from that are tiny -- just a few meters per second in some cases. That's about as quickly as you can walk!!

Edit: gah! must type quicker four posts in 5 minutes -- we must be on to something

[inksmithy]

Well, I'm prepared to accept I am wrong, I always thought sound and light were on the same spectrum for some reason.

Reading the posts above and thinking about it, I feel pretty silly.

I love education.

Alan

[scogyrd]

I love education.

I agree, and thanks for the replies everyone. They've certainly helped clear things up a bit.

It's not that I didn't believe in redshift, it's just that I didnt really understand how we knew it worked. The analogy about speeding cars just didnt make any sense to me as I didnt see how it related to light, and Carl Sagan talking about magic cameras and thought experiments didn't help much either. This is why I wasn't sure if it was based on anything tangible.

I'm off to build a new base for my dob now so I'll have plenty of time to think about it and will come back if I have any more questions.

[Ad Astra]

The easiest Doppler shift experiment I know of involves only a battery operated buzzer and a meter or so of string.

Tie the buzzer to the string, turn it on, and have a mate a few meters away whirl the buzzer around their head on the string. You, the distant observer, will note a distinct change in frequency as the buzzer rotates first toward you, then half a revolution later, away from you. If you have a microphone that hooks into your computer, you should be able to see the frequency change in the sound very nicely.

Now swap positions with your mate and whirl the buzzer yourself. You will hear no change in sound. At the center of rotation, the buzzer is never moving either toward or away from you.

If you record the sound and put it through a frequency analyzer program, you will see the +f, no shift, -f, no shift patter repeat endlessly. This is the same pattern that astronomers look for to identify spectroscopic binary stars (stars that are too close to be resolved optically). The spectrum splits into two (binary A is moving toward you, binary B is moving away), then re-merges into a single pattern (both A and B are moving perpendicular to you). The time of the merge-split-merge event gives the orbital period of the system.

I've taken the liberty of uploading a couple of worksheets from my astronomy class on binary stars, I'm not sure which one, but one (or more) have diagrams on them that will help to explain this.

Cheers,

Dan

Astro_Wksht_12-1_Discovering_Binary_Stars_-_print.doc

Astro_Wksht_12-2_Eclipsing_Binary_Stars.doc

Astro_Wksht_12-3_Evolving_Binary_Stars.doc

[ollypenrice]

Well, I'm prepared to accept I am wrong, I always thought sound and light were on the same spectrum for some reason.

Reading the posts above and thinking about it, I feel pretty silly.

I love education.

Alan

Maybe you were thinking of radio waves and light waves as being the same stuff. They are - but we think of radio as sound for obvious, though false, reasons!

Olly

[inksmithy]

That could be it yeah.

It's all good, is rather look silly and learn than look foolish and remain ignorant.

Alan