# The Naked Scientists Forum

### Author Topic: Red shift  (Read 6048 times)

#### rosy

• Neilep Level Member
• Posts: 1018
• Chemistry
##### Red shift
« on: 10/04/2006 18:11:10 »
Right... I thought that because I don't have a problem with how Doppler shift works for sound (like what happens when an ambulance or police car or whatever goes past) that I understood how red shift works with light (like stars spreading out through the universe).
I realised about 1 o'clock last night that actually I have no idea at all how it works.

I understand this:
Whilst the siren thing works because if the source is moving away from you the distance successive peaks have to travel between the source and your ear is equal to the wavelength of the sound emitted plus the change in distance between start and end points during the period of the note.
So the new wavelength is now
wavelength(1) = wavelength(0) + T * velocity of sound source
(velocity measured in the frame of the observer)
So as the source moves away pitch goes down.

But my problem is this:
Red shift is supposed to be due to the expansion of the universe, right? And this universe expansion is happening everywhere all the time but we don't notice because, essentially, it's happening sort-of between atoms so a metre is in some way getting longer but we can't tell because everything's getting longer/bigger?
Or have I misunderstood that?
Anyway, if I am right about the first bit, how come it's being observed in the red shift? How come that isn't just lost in the general everything-getting-bigger and not actually observed?

• Neilep Level Member
• Posts: 2175
• Scallywag
##### Re: Red shift
« Reply #1 on: 10/04/2006 19:13:15 »
you might find this usefull

http://www.phys.ufl.edu/~rfield/PHY2061/images/chp39_2.pdf

What you do speaks so loudly that I cannot hear what you say.

#### rosy

• Neilep Level Member
• Posts: 1018
• Chemistry
##### Re: Red shift
« Reply #2 on: 10/04/2006 20:32:02 »
Cool. Thanks. Any chance someone could explain that in slightly more words and slightly less symbols? Bits of the notation look familiar from the first year physics course three years ago... but my grip on what it meant was tenuous at best then and is now non-existent...

• Neilep Level Member
• Posts: 2175
• Scallywag
##### Re: Red shift
« Reply #3 on: 10/04/2006 20:38:40 »

http://www.astro.ucla.edu/~wright/doppler.htm

What you do speaks so loudly that I cannot hear what you say.

• Neilep Level Member
• Posts: 2175
• Scallywag
##### Re: Red shift
« Reply #4 on: 10/04/2006 20:43:13 »
found thes notes thay may help too

Sound travels about 350 metres per second. A train travelling at 120 kilometres per hour is moving 33 1/3 metres per second, or roughly one tenth the speed of sound. From Doppler's formula, we can expect the frequency of sound approaching the train to be shifted to 10% higher pitch, and sounds receding shifted to 10% lower pitch, roughly the difference between two adjacent white keys on a piano keyboard. If a pipe organ in a church near the train tracks is playing this note, a passenger on a (very quiet Swiss) train approaching the church at 120 km/hour will hear the note shifted upward by about 10% in frequency. After the train passes the church, the note will be heard shifted down by the same percentage. Here's the three notes played together so you can hear the difference more clearly.

The reason we notice the Doppler shift of sound but not of light is simply the consequence of the tremendous difference in the speed of sound and the speed of light. There's an apocryphal story about the physicist who tried to beat a ticket for running a red light by telling the judge that at the speed he was approaching the signal, the red light was Doppler shifted so it appeared green. The judge pondered this for a few minutes and tore up the red light ticket. Then, seeing as the physicist would have to be driving about a quarter of the speed of light to see a red signal as green, the judge fined him 269 million dollars for speeding, one dollar for each kilometer per hour over the limit

What you do speaks so loudly that I cannot hear what you say.

#### harryneild

• Full Member
• Posts: 88
##### Re: Red shift
« Reply #5 on: 10/04/2006 20:47:23 »
I think that the expansion of the universe is only noticable when at the distances between galaxy clusters. I don't think that the distance between atoms expands measurably, if at all. I think i read about this when someone asked if the reason that people are getting more overweight was because the universe is expanding.

"Knowledge has to be improved, challenged, and increased constantly, or it vanishes." Peter F. Drucker
« Last Edit: 05/04/2007 22:13:33 by harryneild »

#### another_someone

• Guest
##### Re: Red shift
« Reply #6 on: 10/04/2006 20:48:18 »
quote:
Originally posted by rosy

I understand this:
Whilst the siren thing works because if the source is moving away from you the distance successive peaks have to travel between the source and your ear is equal to the wavelength of the sound emitted plus the change in distance between start and end points during the period of the note.
So the new wavelength is now
wavelength(1) = wavelength(0) + T * velocity of sound source
(velocity measured in the frame of the observer)
So as the source moves away pitch goes down.

But my problem is this:
Red shift is supposed to be due to the expansion of the universe, right? And this universe expansion is happening everywhere all the time but we don't notice because, essentially, it's happening sort-of between atoms so a metre is in some way getting longer but we can't tell because everything's getting longer/bigger?
Or have I misunderstood that?
Anyway, if I am right about the first bit, how come it's being observed in the red shift? How come that isn't just lost in the general everything-getting-bigger and not actually observed?

OK, I suspect my understanding is scarcely more than yours, but where I think you went wrong is in your assumption of universal expansion.

Expansion is only happening on a big scale, not on a small scale.  The distances between atoms, and between chunks of matter close by, is not expanding because the forces between matter (gravity, electrostatic, etc,) is holding it tightly together.  It is only the objects that are far enough apart already so as not be holding a tight grip on one another that are driving apart (if the rapid haste of expansion can really be described with such grace as a mere drifting apart).

George

#### rosy

• Neilep Level Member
• Posts: 1018
• Chemistry
##### Re: Red shift
« Reply #7 on: 10/04/2006 22:23:10 »
Oh, ok... that was what I'd been assuming up until last night. Then I remembered that I thought I'd seen something somewhere on the forum that seemed to imply that that wasn't what was going on. Sadly, I forget in which thread and who said it.
Anyone else have any thoughts?

#### sia

• Jr. Member
• Posts: 12
##### Re: Red shift
« Reply #8 on: 14/04/2006 01:18:58 »
quote:
Originally posted by rosy

(---)
But my problem is this:
Red shift is supposed to be due to the expansion of the universe, right?

Hi rosy,
To interpret red shift as a function of expansion is right -- but not as expansion of the universe, but as expansion of the spectral lines' spectrum.

Hubble and co. discovered this displacement phenomenon but didn't understand how to interpret it. And, as the only known interpretation was the Doppler theory, Hubble provisionally speculated in this idea by multiplying the spectral lines’ displacement with the speed of light.
That is the explanation behind the fluctuating Hubble-parameter.

But, there is a very simple universal entropy-law that gives the right explanation for this displacement phenomenon, and it is the same for wave-propagated energy impulses in electrodynamics and hydrodynamics and aerodynamics (light-waves, water-waves and sound-waves).

You can see further explanations on my web-page.
But I have temporarily made a pause in my writings because I have found some ideas in my discoveries that are possible to secure by patent.

Ingvar, Sweden
« Last Edit: 14/04/2006 11:29:52 by sia »

#### mtheory

• First timers
• Posts: 3
##### Re: Red shift
« Reply #9 on: 21/04/2006 00:08:12 »
The reason that atoms do not fly apart with the expansion of the universe is because matter isn't in motion relative to itself.  It's the actual fabric of space that's moving.  All matter is just stuck to it.  Imagine an ant on the surface of a balloon.  The ant's atoms won't disperse as the balloon is blown up.  But the ant would be moving away from another ant that was also on the surface of the balloon as the balloon's surface got larger, due to the balloon being blown up.

#### sia

• Jr. Member
• Posts: 12
##### Re: Red shift
« Reply #10 on: 21/04/2006 01:44:19 »
quote:
Originally posted by mtheory

It's the actual fabric of space that's moving (...) as the balloon is blown up.

You are right, the matter does not fly apart.
But!
The universe is not an expanding ballon.
It is the radiation that is "expanding" by wave-elongation.
This is called redshift, which no-one has found any empirical explation for, until now.
It is called entropy, the (lost) energy that is propelling the radiation forward.

Ingvar, Sweden

#### mtheory

• First timers
• Posts: 3
##### Re: Red shift
« Reply #11 on: 21/04/2006 01:58:58 »
quote:
Originally posted by sia

quote:
Originally posted by mtheory

It's the actual fabric of space that's moving (...) as the balloon is blown up.

You are right, the matter does not fly apart.
But!
The universe is not an expanding ballon.
It is the radiation that is "expanding" by wave-elongation.
This is called redshift, which no-one has found any empirical explation for, until now.
It is called entropy, the (lost) energy that is propelling the radiation forward.

Ingvar, Sweden

I agree.  I wasn't trying to explan the RedShift (sorry, the post was a bit off topic).  I was just explaining why matter doesn't fly apart.  There are some theories in theoretical physics that do treat the universe as a 4D finite and closed universe.  The balloon analogy doesn't do it justice, but it is a convenient and simple description.

Chris

#### ukmicky

• Moderator
• Neilep Level Member
• Posts: 3011
##### Re: Red shift
« Reply #12 on: 21/04/2006 02:04:15 »
The reason why matter within galaxies and solar systems dosent fly apart from each other as the universe expands is because the force which is expanding space is a weaker force than gravity and therefore within areas of high mass like within our galaxy/local cluster gravity is able to overcome the force's causing the universe to expand.

Michael
« Last Edit: 21/04/2006 02:07:32 by ukmicky »

#### mtheory

• First timers
• Posts: 3
##### Re: Red shift
« Reply #13 on: 21/04/2006 02:20:18 »
quote:
Originally posted by ukmicky

The reason why matter within galaxies and solar systems dosent fly apart from each other as the universe expands is because the force which is expanding space is a weaker force than gravity and therefore within areas of high mass like within our galaxy/local cluster gravity is able to overcome the force's causing the universe to expand and holds everything together.

Michael

Sorry, either I don't understand your point or I don't agree.

The expansion of the universe doesn't cause a repulsive force on matter or energy within the universe.  The expansion of the universe is the expansion of space and time.  It only 'drags' the matter and energy contained within it.

Can you be more specific, or give an example of, the effect you are referring to?

Chris

#### ukmicky

• Moderator
• Neilep Level Member
• Posts: 3011
##### Re: Red shift
« Reply #14 on: 21/04/2006 02:48:32 »
what don't you understand. Two distant galaxies two points of mass are dragged IF YOU LIKE apart yes from each other as the space in between them expands. however within a galaxy, two points of mass,two star systems are not dragged apart from each other like the two galaxies as the gravity within the galaxy which they reside is stronger than the expansion force "dark energy" and so is able to hold everything together.

http://www.fnal.gov/pub/inquiring/questions/expandinguniverse.html

PS
I don't really like the word drag because the galaxies are not really being dragged anywhere but rather the space in between the galaxies is being acted upon by what they call dark energy. Most of the galaxies are getting a free ride as the space in between them expands.
but that's probably just my interpretation of the word drag
Michael
« Last Edit: 23/04/2006 00:15:23 by ukmicky »

#### another_someone

• Guest
##### Re: Red shift
« Reply #15 on: 21/04/2006 03:49:43 »
quote:
Originally posted by mtheory
Sorry, either I don't understand your point or I don't agree.

The expansion of the universe doesn't cause a repulsive force on matter or energy within the universe.  The expansion of the universe is the expansion of space and time.  It only 'drags' the matter and energy contained within it.

Can you be more specific, or give an example of, the effect you are referring to?

Can I possibly draw an analogy.

If you have a bomb that explodes, and that bomb is surrounded by an iron casing, then the bomb will not cause every atom of the casing to pull apart from every other atom, but rather the casing will fragment into lots of small pieces of shrapnel, where each piece of shrapnel will be drawn apart from every other piece if shrapnel by the force of the explosion, but each piece of shrapnel will still hold together in itself.

One can think, in very crude terms, of the individual galaxies being like pieces of shrapnel released from the Big Bang, each piece drawing away from each other piece, and yet each piece still being held together by its own force of gravity, and so retaining its internal structure and size.

I am not saying that the analogy is exact, but hopefully reasonable enough to help you understand how the overall can expand, while the smaller parts of it still do not expand.

George

#### harryneild

• Full Member
• Posts: 88
##### Re: Red shift
« Reply #16 on: 23/04/2006 00:48:08 »
I have a question about red shift. The expanding universe is causing the light rays to 'stretch' into lower wavelength wave with less energy. Where does this energy go?

"Knowledge has to be improved, challenged, and increased constantly, or it vanishes." Peter F. Drucker

#### ukmicky

• Moderator
• Neilep Level Member
• Posts: 3011
##### Re: Red shift
« Reply #17 on: 23/04/2006 02:10:09 »
quote:
Originally posted by harryneild

I have a question about red shift. The expanding universe is causing the light rays to 'stretch' into lower wavelength wave with less energy. Where does this energy go?

"Knowledge has to be improved, challenged, and increased constantly, or it vanishes." Peter F. Drucker

Ok i probably wrong here and im sort of trying to remember something from a long time ago so i'm basically guessing  but isnt the photons energy conserved and as it redshifts it changes into potential energy. ok I dont know lets wait for someone who does.

Michael

#### another_someone

• Guest
##### Re: Red shift
« Reply #18 on: 23/04/2006 04:43:29 »
quote:
Originally posted by harryneild

I have a question about red shift. The expanding universe is causing the light rays to 'stretch' into lower wavelength wave with less energy. Where does this energy go?

This is how I see it – and I'm not a physicist.

It is not the expanding universe (at least not directly) that is causing the red shift.

The expanding universe is causing the distant galaxies to be running away from us, so that even as the light is emitted from its source, it already appears to us to be red shifted, because its source is receding.

I suppose that if one is to look for some loss of energy, it would somehow be bound up with the kinetic energy of the receding galaxy, but that would imply that the galaxy is somehow gaining energy (maybe that is some of this mysterious dark energy – though I doubt it).

George

#### The Naked Scientists Forum

##### Re: Red shift
« Reply #18 on: 23/04/2006 04:43:29 »