Why does red-shift mean the Universe is expanding?

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Offline Thebox

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I am well aware of the Dopppler, but I have a slight misunderstanding in how red-shift could possibly mean expansion.   

When light slows down, it is quite evidently that the wave compresses to form its wavelength,

The faster light travels  the more light permeates and is a ''straighter wave''

Blue is slower than red, but white is faster than blue and red,

So red must be pulling rather than expanding because red is slower than white.

if it were expanding it would still be white.


p.s im off fishing , look forward to returning to your replies.



« Last Edit: 16/07/2016 13:20:03 by chris »

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Offline PhysBang

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Re: Why does red-shift mean the Universe is expanding?
« Reply #1 on: 16/07/2016 14:09:34 »
OK, so it is clear that thebox doesn't actually care about the real answer, since they have made it clear that they are only interested in their own personal view on the world. However, in the interests of those who might care, let's have a simple explanation.

Redshift could mean that everything is merely moving away from us and that we are seeing a doppler effect on light. Light doesn't slow down, but as an object emits light gets farther away, the peaks between the waves of emitted light get farther apart than they would normally be so the light gets shifted to a longer wavelength.

But there are other explanations. In General Relativity, we can describe the way to measure distances of space in the universe as a standard Euclidean geometry with one difference: the average distance between objects increases over time. (This works at the hugest scales, but not within galaxies because gravity holds galaxies together.) This leads to no motion between galaxies, since they do not change there position, but their distance changes and so this introduces a Doppler-like effect on the light.

One big advantage of the latter description over the former is that the latter does not require us to imagine that the Earth is the centre of the universe. In the latter description, every point of space is the same.

There are other advantages, too, but this gets into the entire science of cosmology and thus can get technical.

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #2 on: 16/07/2016 16:17:50 »
As PhysBang said, cosmological redshift means that galaxies are moving away from us. Observations of these galaxies led to the discovery that they aren't simply moving away from us but that, in general, each galaxy is moving away from every other galaxy. It's as if the galaxies were dots on a rubber sheet and the rubber sheet was being uniformly stretched in every direction. The mere presence of cosmological redshift itself does not imply that space itself is expanding. It just means that the universe, defined by the collection of galaxies, is spreading out. Why we think that space itself is expanding is for other reasons such as the fact that the general theory of relativity (GR) coupled with observations of cosmological redshift tells us that distant galaxies are moving away from us at speeds faster than the speed of light. That can only happen if space itself is expanding. GR is used to construct models of the universe and the model which describes the universe is one that leads to the Friedman metric and equations. For more on this please see: https://en.wikipedia.org/wiki/Metric_expansion_of_space

The expansion of space means that the distance between different points in space increases with time. The analogy that is commonly used to explain this phenomena is the surface of a sphere. Please keep in mind that the analogy is between our three-dimensional space and the surface of the balloon. It would be going outside the analogy to think that of the surface of the sphere as being embedded in a higher dimensional space. So when the surface of the sphere increases as the sphere itself gets larger the distance between any two points on the sphere increases. This means that as time goes on there is more and more surface area on the surface of the sphere. 2D beings living on the surface would be able to make measurements of the amount of space they had and would determine that there is more and more space as time increases. If our universe is a closed universe then there would be an ever increasing amount of space in the universe.
« Last Edit: 16/07/2016 16:24:33 by PmbPhy »

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Offline PhysBang

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Re: Why does red-shift mean the Universe is expanding?
« Reply #3 on: 16/07/2016 16:40:39 »
As PhysBang said, cosmological redshift means that galaxies are moving away from us.
I very carefully did not say this.

The idea of "the curvature of spacetime" in General Relativity is really important, despite how some people like to diminish it. The galaxies (or rather galaxy superclusters) of the standard cosmological model are not moving, they are staying still. It is the distance between these objects that is getting larger. This is an effect of the shape of spacetime as it changes over cosmological time, not the result of a force.

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #4 on: 16/07/2016 21:45:47 »
Quote from: PhysBang
I very carefully did not say this..
I was referring to this comment Redshift could mean that everything is merely moving away from us - So sue me.

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Offline evan_au

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Re: Why does red-shift mean the Universe is expanding?
« Reply #5 on: 17/07/2016 00:00:29 »
Quote from: TheBox
Blue is slower than red, but white is faster than blue and red
It is not quite clear what is being discussed here, since the sentence contains no nouns, only adjectives.

However, just to be clear: In the vacuum of space, blue light, red light and white light all travel at the same speed, which we call c.

If we regard the light emitted by a star as "white light", consisting of many wavelengths:
- If this star is observed by an astronomer who is moving away from the star, he will see the wavelengths of this light as being longer than when they were emitted, so the light of the star appears redder due to doppler shift.
- If this star is observed by an astronomer who is moving towards the star, she will see the wavelengths of this light as being shorter than when they were emitted, so the light of the star appears bluer due to doppler shift.

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I have a slight misunderstanding in how red-shift could possibly mean expansion
If you look at distant galaxies (as Edwin Hubble did), and notice that the more distant galaxies tended to have greater redshift, then you conclude that more distant galaxies are expanding away from us.

If you see that the expansion is roughly proportional to distance, then you conclude that these distant galaxies are also expanding away from each other. ie the whole visible universe is expanding.

This does not mean that blue shift is impossible; the "nearby" Andromeda galaxy is part of our local cluster of galaxies and has a blue shift; it is moving towards our Milky Way, and is expected to collide in about 5 billion years. This is interpreted as a local motion within our local galaxy cluster, and does not disprove the general expansion of the universe. 

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #6 on: 17/07/2016 12:26:34 »
I am now more confused than I started out .  OK, the permitivity of a medium slows light down , the waves become more compressed the slower the speed?

The light that passes through glass is more ''red'' than light that passes through brick?


added- ''in a race, the ''red'' tip is faster than a ''blue'' tip when relative to passing through something?
« Last Edit: 17/07/2016 12:52:41 by Thebox »

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #7 on: 17/07/2016 12:41:18 »
Quote from: evan_au
If we regard the light emitted by a star as "white light", consisting of many wavelengths:
- If this star is observed by an astronomer who is moving away from the star, he will see the wavelengths of this light as being longer than when they were emitted, so the light of the star appears redder due to doppler shift.
- If this star is observed by an astronomer who is moving towards the star, she will see the wavelengths of this light as being shorter than when they were emitted, so the light of the star appears bluer due to doppler shift.
To be more precise, astronomers must first break the starlight up into a spectrum so that they can identify the emission and absorption lines. The frequency of that light is compared to the spectra of what the starlight should be if the galaxy wasn't moving relative to us. Using that data the astronomers then determine what recession velocity it takes to do that and that's the velocity of the galaxy!

Quote from: evan_au
Quote
I have a slight misunderstanding in how red-shift could possibly mean expansion
If you look at distant galaxies (as Edwin Hubble did), and notice that the more distant galaxies tended to have greater redshift, then you conclude that more distant galaxies are expanding away from us.
The redshift means that the object emitting that light is moving away from us. When a source of light moves away from the observer that observer no sees the light as its stretched out due to the source moving away from us just like the tone of the whistle of a train decreases when it changes from a higher pitch to a lower pitch as the train approaches you and then passes you and is then moving away.

Quote from: evan_au
If you see that the expansion is roughly proportional to distance, then you conclude that these distant galaxies are also expanding away from each other. ie the whole visible universe is expanding.
I recommend exercising caution on interpreting the meaning of the relative motion of galaxies. I think people get confused on this point because its difficult to understand that its actually the space itself that is stretching out rather than the object getting further away in space. Recall my analogy with the sphere.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #8 on: 17/07/2016 12:55:34 »

The redshift means that the object emitting that light is moving away from us. When a source of light moves away from the observer that observer no sees the light as its stretched out due to the source moving away from us just like the tone of the whistle of a train decreases when it changes from a higher pitch to a lower pitch as the train approaches you and then passes you and is then moving away.


The Doppler effect I understand of sound, why do we compare this effect of light to sound ?

Couldn't the Doppler shift of sound and the redshift of light be two different things?


Doesn't white light passing through a Prism red-shift? blue-shift? etc

How is the Phase velocity shift  of the temporal distortion of the spectral colour red different to the Hubble observed red-shift of light?

« Last Edit: 17/07/2016 12:58:19 by Thebox »

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #9 on: 17/07/2016 13:55:48 »
Quote from: Thebox
The Doppler effect I understand of sound, why do we compare this effect of light to sound ?
Because each has the essential characteristics of what the Doppler effect is. Both light and sound waves move at a given speed. We use the letters v for sound and c for light. Both have a wavelength L, frequency f and time period R, related to each other in the same exact relationship; c(or v) = L/T = Lf

Quote from: Thebox
Couldn't the Doppler shift of sound and the redshift of light be two different things?
That question is meaningless. To make it meaningful you need to state what you mean by "different things" because the Doppler effect is not a "thing".

Quote from: Thebox
Doesn't white light passing through a Prism red-shift? blue-shift? etc
[/quite]
No. The amount of deflection of a beam of light passing through a prism depends on the frequency of the light.

Quote from: Thebox
How is the Phase velocity shift  of the temporal distortion of the spectral colour red different to the Hubble observed red-shift of light?
I'm sorry but I don't know what that sentence means

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Offline evan_au

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Re: Why does red-shift mean the Universe is expanding?
« Reply #10 on: 18/07/2016 22:47:10 »
Quote from: TheBox
I am now more confused than I started out .  OK, the permitivity of a medium slows light down , the waves become more compressed the slower the speed?
Yes, light travels slower when it is traveling through glass. But that is only while the light is traveling through the glass; all the light once again travels at c when it returns to a vacuum (like space).

And when we talk about the wavelength of light, we usually mean the wavelength in a vacuum. When we are talking about a different medium, we normally say so, eg microelectronics is exposed to UV light in a liquid because the wavelength of light is shorter in a liquid than in air, so you can draw finer lines, and fit more transistors on a chip.

Quote
Doesn't white light passing through a Prism red-shift? blue-shift? etc
No. The wavelength of the light before it enters the prism is exactly the same as the wavelength after it exits the prism.

What might be confusing you is that glass is a "dispersive medium", ie some wavelengths travel faster than others.
See: https://en.wikipedia.org/wiki/Dispersion_(optics)

Quote
in a race, the ''red'' tip is faster than a ''blue'' tip when relative to passing through something?
If I understand this correctly, in glass, blue light travels slower than red light, so blue light is bent more when it enters a glass prism, producing a rainbow. (Although there are other materials that work the opposite way, and are used in telecommunications to reassemble laser pulses which smear out in time after they have passed through an optical fiber.)

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The light that passes through glass is more ''red'' than light that passes through brick?
Light doesn't propagate through a brick.

But in very general terms, for transparent materials, a more-dense material like air, water or glass affects the wavelength more than a less-dense material like a vacuum.

But there are many exceptions (eg diamond). You can look at a table of refractive indices.
https://en.wikipedia.org/wiki/List_of_refractive_indices

But be aware that in many materials, the refractive index is dependent on wavelength.

Quote
Phase velocity [vs] Hubble observed red-shift of light?
As I understand it, the Phase Velocity becomes important when you have a dispersive medium.

However, cosmological redshift is viewed through the vacuum of space, which is not a dispersive medium. (Otherwise we would see stars with colored streaks, like a rainbow or a spectrograph.)

Phase velocity does not affect the Hubble redshift.
See: https://en.wikipedia.org/wiki/Phase_velocity

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #11 on: 21/07/2016 15:13:59 »
I've done some more thinking on this issue and came to a different understanding that I had before. Before I was wrong. I erroneously thought that by "expanding universe" that cosmologists were speaking of expanding space and that cosmological redshift was evidence of that. The correct understanding is that by an "expanding universe" galaxies are all moving away from each other. That is to say that if you were in an observer in one galaxy and observed another galaxy then you'd determine that light from that galaxy was redshifted and as such was moving away from you. You'd then determine through observations that the speed of the galaxy was a linear function of the distance to the galaxy.

It should be noted that the expansion of space is consistent with this determination but is not proved by it. Especially since nothing can actually be proved in science. So in the past when I claimed that cosmological redshift implied that space was expanding, I was wrong. It means that the universe is expanding. That space is expanding is another issue. It would take a force other than gravity to restrain a galaxy from moving away from ours even if it was moving at constant speed. Also, if space wasn't expanding then no galaxy would move away from us at speeds faster than the speed of light.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #12 on: 23/07/2016 08:13:13 »

Yes, light travels slower when it is traveling through glass. But that is only while the light is traveling through the glass; all the light once again travels at c when it returns to a vacuum (like space).

Yes, the glass has more permitivity than the vacuum of space and space allows light to permeate freely.



Quote
Doesn't white light passing through a Prism red-shift? blue-shift? etc
Quote from: Evan
No. The wavelength of the light before it enters the prism is exactly the same as the wavelength after it exits the prism.

Huh?  the white light going in comes out different colours, i.e different wavelengths, I can see red and blue among the colours, how is the red output not the same as redshift?






Quote
The light that passes through glass is more ''red'' than light that passes through brick?
Quote from: Evan
Light doesn't propagate through a brick.

Visible light doesnt propagate through a brick, what about x-ray light and such?




Quote
However, cosmological redshift is viewed through the vacuum of space, which is not a dispersive medium. (Otherwise we would see stars with colored streaks, like a rainbow or a spectrograph.)

At least you understand the ''gin-clear'' space, but according to science we don't actually view through anything do we because that is not how sight works...!

« Last Edit: 23/07/2016 08:15:29 by Thebox »

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #13 on: 23/07/2016 08:18:00 »
I've done some more thinking on this issue and came to a different understanding that I had before. Before I was wrong. I erroneously thought that by "expanding universe" that cosmologists were speaking of expanding space and that cosmological redshift was evidence of that. The correct understanding is that by an "expanding universe" galaxies are all moving away from each other. That is to say that if you were in an observer in one galaxy and observed another galaxy then you'd determine that light from that galaxy was redshifted and as such was moving away from you. You'd then determine through observations that the speed of the galaxy was a linear function of the distance to the galaxy.



Yes Pete that is correct like I have said many times before , there is no evidence of space expanding, the evidence is of the distance between Galaxies is increasing.

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #14 on: 23/07/2016 09:29:18 »
Quote from: Thebox
Yes Pete that is correct like I have said many times before , there is no evidence of space expanding, the evidence is of the distance between Galaxies is increasing.
I know what you said but you keep ignoring or forgetting what I said. Its  probably because you may not know what the term evidence means. Evidence of a theory is any information that is consistent with that theory. If space was expanding then Galaxies would be moving apart and we'd see each galaxy redshifted, the amount of redshift being determined by the distance that galaxy is from us. The further away the galaxy the greater the redshift. And that's exactly what we see. That means that cosmological redshift is evidence of expanding space.

Your problem has always been your lack of understanding of the scientific method and the philosophy of science. One of the most important points that a scientist learns when they study the philosophy of science is that proof is not part of the scientific method. That's because evidence of a hypothesis or theory can in theoretically be explained in more than one way. That's why cosmological redshift is said to be evidence that space is expanding and its not considered proof that space is expanding.

So before you make anymore of your false claims that there's no evidence that space is expanding, first learn what evidence is.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #15 on: 23/07/2016 13:04:49 »
Quote from: Thebox
Yes Pete that is correct like I have said many times before , there is no evidence of space expanding, the evidence is of the distance between Galaxies is increasing.
I know what you said but you keep ignoring or forgetting what I said. Its  probably because you may not know what the term evidence means. Evidence of a theory is any information that is consistent with that theory. If space was expanding then Galaxies would be moving apart and we'd see each galaxy redshifted, the amount of redshift being determined by the distance that galaxy is from us. The further away the galaxy the greater the redshift. And that's exactly what we see. That means that cosmological redshift is evidence of expanding space.

Your problem has always been your lack of understanding of the scientific method and the philosophy of science. One of the most important points that a scientist learns when they study the philosophy of science is that proof is not part of the scientific method. That's because evidence of a hypothesis or theory can in theoretically be explained in more than one way. That's why cosmological redshift is said to be evidence that space is expanding and its not considered proof that space is expanding.

So before you make anymore of your false claims that there's no evidence that space is expanding, first learn what evidence is.

What? you have  said in previous post you have now realised space is not expanding and the evidence is of objects. 
« Last Edit: 23/07/2016 13:07:44 by Thebox »

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #16 on: 23/07/2016 13:26:26 »
Quote from: Thebox
What? you have  said in previous post you have now realised space is not expanding and the evidence is of objects.
Wrong!!! I did not say that. I said that cosmological redshift implies that space is expanding. That was an incorrect assertion. It's evidence of spatial expansion, it doesn't imply it. Please read more carefully next time.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #17 on: 23/07/2016 13:34:58 »
Quote from: Thebox
What? you have  said in previous post you have now realised space is not expanding and the evidence is of objects.
Wrong!!! I did not say that. I said that cosmological redshift implies that space is expanding. That was an incorrect assertion. It's evidence of spatial expansion, it doesn't imply it. Please read more carefully next time.

No Pete, you said -

''I've done some more thinking on this issue and came to a different understanding that I had before. Before I was wrong. I erroneously thought that by "expanding universe" that cosmologists were speaking of expanding space and that cosmological redshift was evidence of that.''


There is no evidence of space itself expanding , the evidence of red shift is of Galaxies.

Pete please talk to me about something, I have a problem with red shift showing ''expansion'', we know that when light slows down it can change frequency, so when we observe red shift that must be light slowing down, now if something is travelling away from something, it is impossible for the ''leader'' to slow down the ''trailer'', the light should not red shift if something is moving away it should remain ''gin-clear'' a maxed out speed?

What permitivity causes the red shift?

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Offline jerrygg38

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Re: Why does red-shift mean the Universe is expanding?
« Reply #18 on: 23/07/2016 13:44:18 »


The Doppler effect I understand of sound, why do we compare this effect of light to sound ?

Couldn't the Doppler shift of sound and the redshift of light be two different things?

  Light is similar to radar waves which exhibit Doppler effects. A radar source moving toward us will have a higher frequency and a radar source moving away from us will have a lower frequency.
  The red shift of the far stars can be viewed from two perpectives. As the universe expands everything expands. the clocks and rulers expand. thus there is a common mode redshift. Yet white light will still appear white. The expansion also has a Doppler shift due to the fact that the distance to the far stars has changed with respect to the expanding universe. Thus it looks like a simple Doppler problem.
   To make matters worse space time is non-linear so the best we can do is work out linear approximations to space and time. Einstein's work tends to be a best fit approximation to space and time from a mathematical perspective. 

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #19 on: 23/07/2016 13:51:17 »


The Doppler effect I understand of sound, why do we compare this effect of light to sound ?

Couldn't the Doppler shift of sound and the redshift of light be two different things?

  Light is similar to radar waves which exhibit Doppler effects. A radar source moving toward us will have a higher frequency and a radar source moving away from us will have a lower frequency.
  The red shift of the far stars can be viewed from two perpectives. As the universe expands everything expands. the clocks and rulers expand. thus there is a common mode redshift. Yet white light will still appear white. The expansion also has a Doppler shift due to the fact that the distance to the far stars has changed with respect to the expanding universe. Thus it looks like a simple Doppler problem.
   To make matters worse space time is non-linear so the best we can do is work out linear approximations to space and time. Einstein's work tends to be a best fit approximation to space and time from a mathematical perspective.

Light can not work the same as sound and the doppler, where is the opposing force coming from to slow down the light to ''red''?




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Offline Colin2B

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Re: Why does red-shift mean the Universe is expanding?
« Reply #20 on: 23/07/2016 19:01:43 »
The Doppler effect I understand of sound

Apparently not:

Light can not work the same as sound and the doppler, where is the opposing force coming from to slow down the light to ''red''?
What opposing force, there isn't one in sound Doppler.
Light doesn't slow down for a Doppler shift and it doesn't need to.
You are so eager to push your own theories that you are failing to read and understand what is being said.
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #21 on: 23/07/2016 22:01:03 »
The Doppler effect I understand of sound

Apparently not:

Light can not work the same as sound and the doppler, where is the opposing force coming from to slow down the light to ''red''?
What opposing force, there isn't one in sound Doppler.
Light doesn't slow down for a Doppler shift and it doesn't need to.
You are so eager to push your own theories that you are failing to read and understand what is being said.

There is no theory just a question.  If light does not slow down then how do you observe ''red''? 

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #22 on: 24/07/2016 00:31:39 »
Quote from: Thebox
No Pete, you said -

''I've done some more thinking on this issue and came to a different understanding that I had before. Before I was wrong. I erroneously thought that by "expanding universe" that cosmologists were speaking of expanding space and that cosmological redshift was evidence of that.''
Then I was correct in the first place. Cosmological redshift is indeed evidence of space expanding. It just doesn't imply it.

the box still has to learn what evidence is and I'm not about to keep correcting you over and over while you ignore what I'm explaining to you and merely repeat erroneous claims.

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Offline jerrygg38

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Re: Why does red-shift mean the Universe is expanding?
« Reply #23 on: 24/07/2016 00:45:32 »




Light can not work the same as sound and the doppler, where is the opposing force coming from to slow down the light to ''red''?
   You are assuming that red light is slower than blue light. they both move at the same speed of C.  Draw a chain of sine waves on a piece of paper. If you are stationary and the wave flows toward you the light will be white. Now move toward the chain of waves. this will cause the peaks and valleys to happen faster. thus the light will be bluer. Now move away from the chain of waves. The peaks and valleys will happen slower. thus the light turned redder.
   The redshift shows that the waves coming toward us come from a source of light that is moving away from us.

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Offline evan_au

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Re: Why does red-shift mean the Universe is expanding?
« Reply #24 on: 24/07/2016 05:22:45 »
Quote from: TheBox
Quote from: Evan
The wavelength of the light before it enters the prism is exactly the same as the wavelength after it exits the prism.
Huh?  the white light going in comes out different colours, i.e different wavelengths, I can see red and blue among the colours, how is the red output not the same as redshift?
If you put light of a single color into a prism (eg from a laser), it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming light.

The prism will bend the light by a different angle depending on its color (ie its wavelength and frequency).

So:
  • if you shine a red laser into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming red light.
  • if you shine a blue laser into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming blue light.
  • But the red light will come out at a slightly different angle than the blue light
Now white light (eg from the Sun) is made up of many wavelengths all mixed together (including red and blue).
So:
  • If we shine the red light from the Sun into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming red light.
  • If we shine the blue light from the Sun into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming blue light.
  • But the red light will come out at a slightly different angle than the blue light (or green light). This is how a prism splits up white light from the Sun into its constituent colors.

The rainbow effect has nothing to do with redshift; in redshift, all colors have their wavelength (and frequency) changed compared to the wavelength when it was emitted.

Quote from: TheBox
we know that when light slows down it can change frequency, so when we observe red shift that must be light slowing down
I think you may be confusing frequency and wavelength?

In a vacuum, frequency and wavelength of light are directly related to each other, and the speed is constant = c. The speed of light does not slow down in space.

However, when you consider light traveling through different media (eg glass, as discussed in another recent thread), the speed of light is variable, and is less than c. This can cause the speed of light to slow down temporarily (until it returns to a vacuum, when it returns to its original speed=c).

While light is traveling through glass at <c, its wavelength is less than it is in a vacuum. But its frequency is the same as the frequency in a vacuum.

So, overall, seeing a red shift from a distant galaxy does not imply that light is slowing down.
Quote from: jerrygg38
there is a common mode redshift. Yet white light will still appear white.
A bit of nitpicking here (to reduce confusion for TheBox)...
White light from a star like the Sun has an overall "black body" spectrum, where some frequencies output more power than other frequencies.

Red-Shifted light from the Sun when observed by distant galaxies would appear slightly more red, because the peak of the spectral output is moved down to redder frequencies.

In practice, this form of redshift assumes that an astronomer knows what the peak spectral output was originally; a bit impractical for a very distant star.

Apart from the overall broadband "black-body" spectrum, there are narrow Fraunhofer lines that are absorbed by atoms in the Sun's outer atmosphere. A distant astronomer can work out which atoms these lines come from, and he knows (from laboratory measurements) at what frequencies these absorption lines occurred in the source star.

This allows astronomers to work out the Doppler shift and relative velocity of a distant galaxy; by measuring the velocity of many different galaxies, Hubble was able to deduce cosmological expansion.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #25 on: 24/07/2016 08:02:22 »
Quote from: TheBox
Quote from: Evan
The wavelength of the light before it enters the prism is exactly the same as the wavelength after it exits the prism.
Huh?  the white light going in comes out different colours, i.e different wavelengths, I can see red and blue among the colours, how is the red output not the same as redshift?
If you put light of a single color into a prism (eg from a laser), it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming light.

The prism will bend the light by a different angle depending on its color (ie its wavelength and frequency).

So:
  • if you shine a red laser into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming red light.
  • if you shine a blue laser into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming blue light.
  • But the red light will come out at a slightly different angle than the blue light
Now white light (eg from the Sun) is made up of many wavelengths all mixed together (including red and blue).
So:
  • If we shine the red light from the Sun into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming red light.
  • If we shine the blue light from the Sun into a prism, it will come out of the prism with exactly the same wavelength (and frequency & speed=c) as the incoming blue light.
  • But the red light will come out at a slightly different angle than the blue light (or green light). This is how a prism splits up white light from the Sun into its constituent colors.

The rainbow effect has nothing to do with redshift; in redshift, all colors have their wavelength (and frequency) changed compared to the wavelength when it was emitted.

Quote from: TheBox
we know that when light slows down it can change frequency, so when we observe red shift that must be light slowing down
I think you may be confusing frequency and wavelength?

In a vacuum, frequency and wavelength of light are directly related to each other, and the speed is constant = c. The speed of light does not slow down in space.

However, when you consider light traveling through different media (eg glass, as discussed in another recent thread), the speed of light is variable, and is less than c. This can cause the speed of light to slow down temporarily (until it returns to a vacuum, when it returns to its original speed=c).

While light is traveling through glass at <c, its wavelength is less than it is in a vacuum. But its frequency is the same as the frequency in a vacuum.

So, overall, seeing a red shift from a distant galaxy does not imply that light is slowing down.
Quote from: jerrygg38
there is a common mode redshift. Yet white light will still appear white.
A bit of nitpicking here (to reduce confusion for TheBox)...
White light from a star like the Sun has an overall "black body" spectrum, where some frequencies output more power than other frequencies.

Red-Shifted light from the Sun when observed by distant galaxies would appear slightly more red, because the peak of the spectral output is moved down to redder frequencies.

In practice, this form of redshift assumes that an astronomer knows what the peak spectral output was originally; a bit impractical for a very distant star.

Apart from the overall broadband "black-body" spectrum, there are narrow Fraunhofer lines that are absorbed by atoms in the Sun's outer atmosphere. A distant astronomer can work out which atoms these lines come from, and he knows (from laboratory measurements) at what frequencies these absorption lines occurred in the source star.

This allows astronomers to work out the Doppler shift and relative velocity of a distant galaxy; by measuring the velocity of many different galaxies, Hubble was able to deduce cosmological expansion.

Ok , I understand your reasoning for now.

You say white light is a mixture of frequencies, however I am sure we can look at white light in two ways

1)white light is a mixture of frequencies

2)A mixture of frequencies can be created from white light and white light is the ''base'' frequency.


How do we know number 2 does not apply? because to me the ''gin-clear'' appears to be one constant frequency, an equilibrium to sight and 'invisible'' rather than like the NSF white background these words are on, and why do we not call it invisible light because it is definitely not white like the background of NSF?

p.s I have said before about the angular distortion of light entering a prism and I was told this was wrong.

C.O.R.P   

Center of radiation pressure delta = delta P  = delta fr where P is pressure and fr is frequency?

is this how a prism works?






« Last Edit: 24/07/2016 09:00:11 by Thebox »

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Offline evan_au

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Re: Why does red-shift mean the Universe is expanding?
« Reply #26 on: 24/07/2016 23:16:43 »
Quote from: TheBox
2. A mixture of frequencies can be created from white light and white light is the ''base'' frequency.
How do we know number 2 does not apply?
If you split white light from the Sun into its constituent frequencies with a prism, you will see the individual frequencies to which we give names like red, yellow, green, blue, etc - the colors of the rainbow. Each one of these colors is visibly different from the original white light.

If you use a thermometer, you will also detect additional invisible frequencies like infra-red and ultraviolet past the visible edges of the spectrum.

No matter how you manipulate* one individual frequency of the rainbow by shining it through lenses, reflecting off mirrors, etc, you cannot reconstruct white light. This is because a single frequency does not possess the large variety of frequencies necessary to produce white light.

However, if you take all of the frequencies of the rainbow, and combine them together, you will have something that looks like the original white light. This experiment was first conducted by Isaac Newton.

So white light is not a single "base" frequency, but is the sum of many different frequencies.

*Today we have non-linear crystals that can double or halve the frequency of light; this does change the color of the light, but it will still be visibly different from the original white light.

Quote
NSF white background
What is "NSF"? National Science Foundation?

Quote
C.O.R.P is this how a prism works?
I am afraid that, as usual, I cannot understand the pseudo-geometrical construction in the diagram.

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Offline PmbPhy

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Re: Why does red-shift mean the Universe is expanding?
« Reply #27 on: 25/07/2016 13:55:21 »
More on the expansion of the universe. As some of you may know, Alan Guth is a very good friend of mine. I asked him about this and he responded. He said that I could post his response:

Quote from: Pete
I'm studying cosmology right now and am unable to determine whether or not by the expansion of the universe cosmologists mean that all galaxies are receding from all other galaxies or whether they mean that space is expanding. I can easily understand why you say that cosmological redshift implies those galaxies which are redshifted are moving away from us and that, in general, all galaxies are moving away from us and that observers in each galaxy make the same determination and refer to this as the expansion of the universe. But I don't see how the presence of cosmological redshift implies that space is expanding. Can you help clarify this issue for me, please? Thank you.

Quote from: Alan Guth
I think that if you ask 12 cosmologists this question, you'll get 15 different answers.  Whether or not the question is meaningful depends somewhat on what other assumptions one makes.

1) If the total volume of the universe is finite, then there is o way that all the galaxies could get further apart without the volume increasing.  This would apply to a closed universe, but we probably don't live in a closed universe.  There are also ways of defining periodic identifications in an open universe, making the volume finite, and then the volume of such a universe would have to increase with time to be consistent with the expansion that we observe.  But if the volume is infinite, then there is no unambiguous meaning to saying that it is getting larger.

2) Even if the universe is infinite, however, it is still easier to think in a language in which space is expanding.  General relativity allows you to assign coordinates any way you want, but for cosmology it is overwhelmingly simplest to use a coordinate system that expands with the average flow of matter.  In this coordinate system the universe looks (to high accuracy on large scales) homogeneous.  In this coordinate system the average velocity of matter is zero.  The volume contained within any region of coordinate space is getting larger with time.  But if you wanted to be stubborn, you could use a coordinate system that did not expand, but instead matter would be moving through it. In this coordinate system, however, the universe would not look at all homogeneous, and everything would be a mess.  Nobody ever uses such a coordinate system.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #28 on: 25/07/2016 21:40:46 »
Quote from: TheBox
2. A mixture of frequencies can be created from white light and white light is the ''base'' frequency.
How do we know number 2 does not apply?
If you split white light from the Sun into its constituent frequencies with a prism, you will see the individual frequencies to which we give names like red, yellow, green, blue, etc - the colors of the rainbow. Each one of these colors is visibly different from the original white light.

If you use a thermometer, you will also detect additional invisible frequencies like infra-red and ultraviolet past the visible edges of the spectrum.

No matter how you manipulate* one individual frequency of the rainbow by shining it through lenses, reflecting off mirrors, etc, you cannot reconstruct white light. This is because a single frequency does not possess the large variety of frequencies necessary to produce white light.

However, if you take all of the frequencies of the rainbow, and combine them together, you will have something that looks like the original white light. This experiment was first conducted by Isaac Newton.

So white light is not a single "base" frequency, but is the sum of many different frequencies.

*Today we have non-linear crystals that can double or halve the frequency of light; this does change the color of the light, but it will still be visibly different from the original white light.

Quote
NSF white background
What is "NSF"? National Science Foundation?

Quote
C.O.R.P is this how a prism works?
I am afraid that, as usual, I cannot understand the pseudo-geometrical construction in the diagram.

On your Naked scientist forum and you need to ask what NSF meant?

Ok I understand the colours and why we say/think white light is  a mixture of frequencies but I think you missed the point so will say it in an alternative mannerism.

White light is the whole, the whole is a singularity, an individual with no other characteristics than whole, not mixed like a rainbow road but a ''solid'' of one thing, however this one thing can be distorted into things that are different than the whole, special white 'paint'' than when slowed down by obstruction changes from the whole into a lesser white paint , it changes to blue or red but is still actually white, it is just slower white.


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Offline evan_au

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Re: Why does red-shift mean the Universe is expanding?
« Reply #29 on: 25/07/2016 22:19:13 »
Quote from: TheBox
On your Naked scientist forum and you need to ask what NSF meant?

Sorry, I missed it...
"NSF white background" is the white background of the Naked Scientists Forum webpage...

Quote
White light is the whole, ... an individual with no other characteristics than whole, not mixed like a rainbow road
That is the point of this discussion - white light does have different characteristics which can be separated out to form a rainbow.

Quote
light is ... a singularity
I suggest that you stop using "singularity", as it always comes out confused.

In this context, you possibly mean something like "an indivisible whole".

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Offline evan_au

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Re: Why does red-shift mean the Universe is expanding?
« Reply #30 on: 25/07/2016 22:26:21 »
Quote from: Alan Guth
if you wanted to be stubborn, you could use a coordinate system that did not expand, but instead matter would be moving through it.
So perhaps we could say "In the vicinity of our local galactic cluster, space is getting less dense over time. However, we don't see matter being destroyed - it is just moving outside our vicinity, and the same could be said of most other galaxy clusters in our visible universe."

So for the stubborn, a universal reduction in the local density of matter is equivalent to expansion of the universe.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #31 on: 26/07/2016 08:59:22 »
Quote from: TheBox
On your Naked scientist forum and you need to ask what NSF meant?

Sorry, I missed it...
"NSF white background" is the white background of the Naked Scientists Forum webpage...

Quote
White light is the whole, ... an individual with no other characteristics than whole, not mixed like a rainbow road
That is the point of this discussion - white light does have different characteristics which can be separated out to form a rainbow.

Quote
light is ... a singularity
I suggest that you stop using "singularity", as it always comes out confused.

In this context, you possibly mean something like "an indivisible whole".

1.
singularity - the state, fact, quality, or condition of being singular.

I do not think I misuse the word in any sense, if anything you are not being ambiguous with the word singularity and you only look at the word in this content -

''PHYSICS MATHEMATICS
a point at which a function takes an infinite value, especially in space–time when matter is infinitely dense, such as at the centre of a black hole.''


But yes, an ''indivisable'' whole.

''white light does have different characteristics which can be separated out to form a rainbow.''

Define separate?  coloured light is hardly a separation, it is still light and if not being compressed in wavelength and was allowed to permeate properly is still actually white light.

I do not think we actually separate it, I think we ''manipulate'' it by changing the wavelength/frequency from the wavelength/frequency whole , which to me is a perfect linearity whole  that extends in all directions from source and has an infinite ''wave-length''  and 0 frequency, but only zero because our devices are set up,calibrated at 0 and at an already equilibrium to the environment to begin with.








''


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Offline puppypower

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Re: Why does red-shift mean the Universe is expanding?
« Reply #32 on: 26/07/2016 12:14:28 »
If energy or EMR (electromagnetic radiation) is red shifting due to the expansion of the universal space-time, does that mean the EM force is also shifting, since these are two sides of the same coin? 

In other words, if we assume dark energy is causing the expansion of universal space-time, where space-time is expanding in all directions at the same time, have the size of atoms increased with increasing space-time? Or are the size of atoms constant and therefore exempt from universal space-time expansion? If exempt, how can the EM ignore dark energy and changes universal space-time? Also, if they are exempt, are the material based space-time contractions we see, sort of an illusion, since the EM based matter/atoms, which defines the phases of matter, is not doing anything by stays constant?

For example, say we had an isolated hydrogen atom, early in the universe, when space-time was far more contracted. Dark energy is acting, causing space-time to expand in all directions. Does the hydrogen atom expand along with space-time, such that the modern hydrogen atoms have additional energy levels due to the larger size? If not and the EM force and atoms are exempt, does the EM force, by being exempt, play an ever increasing role, helping to compensate for gravity and different references, since gravity is not exempt from space-time expansion as reflected by galaxies and can gravity can generate different references? 

The red shifted energy we measure is EM energy, so the answer of why red shift is dependent on what happens to the EM force and the size of atoms that make use of that force, as they absorb and emit EMR, as dark energy changes space-time and gravity.
« Last Edit: 26/07/2016 12:17:28 by puppypower »

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Offline evan_au

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Re: Why does red-shift mean the Universe is expanding?
« Reply #33 on: 26/07/2016 12:42:07 »
Quote from: TheBox
Quote from: evan_au
''white light does have different characteristics which can be separated out to form a rainbow.''
Define separate?
I mean we can separate white light into its constituent colors (the rainbow) using raindrops in the sky, a glass prism or a diffraction grating.

Quote
I do not think we actually separate it, I think we ''manipulate'' it by changing the wavelength/frequency from the wavelength/frequency whole

The three methods above are all "linear" techniques that do not generate any frequencies that weren't already there in the original white light. They just separate out the frequencies that were already there.

This is distinct from red-shift: if we move relative to the light source, the observed frequencies differ from the original frequencies by a percentage (the Doppler shift) without generating any new frequencies that weren't already there. This red-shifted white light can then be separated out into these new frequencies using a prism.

This is very distinct from "non-linear" optical materials, which manipulate the frequency of the incoming photons to produce photons at frequencies that were not present in the incoming light, typically the sum or difference frequencies. But these materials only work at very high light intensities, such as you can get from a focused laser.

Your cellphone and WiFi devices use non-linear circuits to manipulate microwave frequencies. These non-linear circuits produce sum and difference frequencies that were not present on the antenna. The difference frequency is lower than the frequencies at the antenna, and we can manipulate these lower frequencies more easily to decode the data.

If we were listening to music, we call non-linear effects like speaker overload "distortion", because it (unintentionally) manipulates the audible signal, generating sum and difference frequencies that were not present in the original music recording.
« Last Edit: 26/07/2016 12:57:28 by evan_au »

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Offline evan_au

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Re: Why does red-shift mean the Universe is expanding?
« Reply #34 on: 28/07/2016 22:26:06 »
Quote from: TheBox
I do not think we actually separate it, I think we ''manipulate'' it by changing the wavelength/frequency from the wavelength/frequency whole
It may be possible to illustrate the mixture of colors in white light by an example from chemistry: the mixture of chemicals in black crude oil.

You could consider black crude oil as a single thing, an indivisible whole.

However, when you put it into a petroleum refinery, they separate different chemicals from the crude oil by their boiling points. This does not change the nature of the components; if you mixed them together again, you would have the original black crude oil.  https://en.wikipedia.org/wiki/Oil_refinery#The_crude_oil_distillation_unit

This is similar to what happens when you split light into its constituent colors with a prism; it does not change the nature of the components; if you mixed them together again, you would have the original white light.

However, not all of the chemical components of crude oil are equally valuable - there is a great demand for lighter oils to power cars and trucks, for example. So long-chain molecules are reacted with hydrogen to turn them into shorter molecules; this is called "cracking". If you mixed all the components together after cracking, you would find something different from the original black crude oil.
See: https://en.wikipedia.org/wiki/Cracking_(chemistry)

This is similar to what happens with white light when we manipulate it with non-linear techniques. You can end up with color components that are half the wavelength of the original light. If you then mixed all the components together again, you would find something different from the original white light.

In this sense, "White light is a mixture of colors" is similar to the lesson in early chemistry lessons around "Black crude oil is a mixture of chemicals". And the lessons on the difference between a mixture and a chemical reaction.

It's as clear as Black and White!

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #35 on: 30/07/2016 09:53:41 »
Quote from: TheBox
Quote from: evan_au
''white light does have different characteristics which can be separated out to form a rainbow.''
Define separate?
I mean we can separate white light into its constituent colors (the rainbow) using raindrops in the sky, a glass prism or a diffraction grating.


Looking from a different ''angle'' I can perceive this differently, it means the same thing almost but there is a subtle difference I ''see'' from an alternative viewpoint.

Explaining is obviously my weakness, I will try though like I always do.

Let us look at a ''solid'', we can take the ''solid'' and split it down into different particles,  we can say the ''solid'' is a mixture of particles. 

Let us look at a tube of smarties, we can say the tube consists of a mixture of colours.

Let us look at a spinning spectral wheel that makes the colours seem visually white when in rotation, we can say that the white is a mixture of colours in motion but the colours do not actually mix.

(Sounds gibberish, I am trying to aid myself looking for the correct wording).

Let us look at the spectral wheel spinning compared to the surrounding space, I can see the now formed mixture of position of the colours as white , I see it is white relative to the reference frame of the surrounding clear space.  But I do not see the surrounding space to be white I see it to be '''gin-clear'' or clear space.

My brain at this stage acknowledges that the changing  position of the colour on the wheel rotating at a certain speed give the illusion of a white wheel. My brain is saying to me RGB ''flicker'',


(Pause in thought to explain that I use to be a silk screen printer, I used 4 colour process, coloured dots overlay to build up a picture, my mind keeps recalling this information about layers).


So far I have flickers and layers in my head and to you all this may seem complete gibberish and irrelevant but I try not to overlook ''things''.


(pause for a cigarette).

How can I explain the un-explainable that is in my head, I have no idea of how to explain it .


Lets try take a 1000w, split it into several different wattages,

We could say the different watt's is a mixture of 1000w,

added - what i am trying to say is that ''white'' light is not  a mixture of frequencies from my viewpoint, visible light (colour) is the same, it is all light and not  mixture of anything, colour is a process of the slowing down of the light by permitivity , refraction and other.

Temporal distortion is colour, we are not splitting the slight, we manipulate the light to make colour, the clear space light is exactly the same as blue light or red light or any other light, but the light in space is not been magnetically distorted by em fields permitivity , absorbsion etc.

aded- if we can imagine light passing through space is a whole and a mono type field, then things affect this mono type field,


Light cannot travel centripetally from a source?

added - sorry for the extended added content. I have just had a thought it may seem off topic but it is not,

A particle emits light, but a particle also absorbs light, so why does a particle not absorb its own light?  does a particle have a light capacitance it can absorb?

hfmax=?

...


added - I still have a problem with red shift, if light is red shifting , i.e  the wavelength is shorter,   then there is an opposing force needed to do this, the galaxies would have to be pushing the light rather than travelling away from the light, something is not making sense or there is something I am not understanding, but either way it seems contradictory.

added- considering Newtons third law where is this diagram in error?










 





 


 








« Last Edit: 30/07/2016 10:39:40 by Thebox »

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Offline PhysBang

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Re: Why does red-shift mean the Universe is expanding?
« Reply #36 on: 31/07/2016 14:23:26 »
Again, the problem here seems to be that thebox does not bother to read any physics and merely makes it up himself. That thebox does not then understand science is his fault, not the fault of scientists or science educators.

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Offline Thebox

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Re: Why does red-shift mean the Universe is expanding?
« Reply #37 on: 31/07/2016 16:11:16 »
Again, the problem here seems to be that thebox does not bother to read any physics and merely makes it up himself. That thebox does not then understand science is his fault, not the fault of scientists or science educators.

I do not ''see'' how that answers my question?

I will redefine for you, If an object is moving away from you stretching a piece of elastic, then how can the elastic possibly become ''slack''?

Added - I did you a diagram of the question if you are unsure.

« Last Edit: 31/07/2016 16:14:31 by Thebox »