how is the doppler effect a valid evident to the idea that our universe is expanding?

[jerryyu]

The more distant a galaxy, the greater the redshift. But is that really a vaild support for the expanding universe? The farther a galaxy is, its light will have to travel farther distances. The farther it has to travel, it will have a greater chances of interacting with atoms/molecules in space-as a result of the interaction, it will have less energy(thus resulting a red wavelength). So is the red shift really doppler effect or simply just the interaction of light?

[John Cuthber]

"as a result of the interaction, it will have less energy"

Why?

 

There are a limited number of ways photons interact with mater.

They may be absorbed, they might be reflected and they might be scattered with, or without a shift in wavelength.

Any of those processes would mean that the photon wouldn't reach us.

 

The photons that get here didn't interact with anything on the way. That's how they got this far.

Edited December 12, 2010 by John Cuthber

[michel123456]

(...)The photons that get here didn't interact with anything on the way. That's how they got this far.

Not even with other photons?

[John Cuthber]

I can't see how such an interaction would fail to change the direction of the original photon. Anyway the OP was talking about interactions with matter.

[michel123456]

I can't see how such an interaction would fail to change the direction of the original photon. (...)

 

If you take the photon as particle, you are right. But if you take the photon as a wave, would he change direction?

[Sisyphus]

The more distant a galaxy, the greater the redshift. But is that really a vaild support for the expanding universe? The farther a galaxy is, its light will have to travel farther distances. The farther it has to travel, it will have a greater chances of interacting with atoms/molecules in space-as a result of the interaction, it will have less energy(thus resulting a red wavelength). So is the red shift really doppler effect or simply just the interaction of light?

 

Just as a note, that redshift isn't the doppler effect. There are three kinds of redshift: doppler effect due to relative motion of the emitter and receiver, gravitational redshift from "climbing" out of a gravity well, and cosmological redshift, which is due to the expansion of space through which the light is traveling. It's the third which we talk about when we talk about the expanding universe.

 

What you're suggesting is basically a "tired light" cosmology, which for various reasons have been discredited. I don't know a lot about it, but check out the "criticisms" section of the linked article, which lists some basic problems. Basically expansion successfully predicts a lot of observations that tired light does not (time dilation, ages of stars, etc.), and there are no known viable mechanisms for "tiring" light that would result in what we actually see.

[michel123456]

Just as a note, that redshift isn't the doppler effect. There are three kinds of redshift: doppler effect due to relative motion of the emitter and receiver, gravitational redshift from "climbing" out of a gravity well, and cosmological redshift, which is due to the expansion of space through which the light is traveling. It's the third which we talk about when we talk about the expanding universe.

 

What you're suggesting is basically a "tired light" cosmology, which for various reasons have been discredited. I don't know a lot about it, but check out the "criticisms" section of the linked article, which lists some basic problems. Basically expansion successfully predicts a lot of observations that tired light does not (time dilation, ages of stars, etc.), and there are no known viable mechanisms for "tiring" light that would result in what we actually see.

 

What is this paper, referenced in the wiki article, from D.L.Mamas talking about?

"A new theoretical model is presented which accounts for the cosmological redshift in a static universe. In this model the photon is viewed as an electromagnetic wave whose electric field component causes oscillations in deep space free electrons which then reradiate energy from the photon, causing a redshift. The predicted redshift coincides with the data of the Hubble diagram. The predicted redshift expression allows for the first time distance measurements to the furthest observable objects, without having to rely on their apparent magnitudes which may be subject to cosmic dust. This new theoretical model is not the same as, and is fundamentally different from, Compton scattering, and therefore avoids any problems associated with Compton scattering such as the blurring of images."

Edited December 12, 2010 by michel123456

[John Cuthber]

If you take the photon as particle, you are right. But if you take the photon as a wave, would he change direction?

The photon will do exactly as it pleases no matter whether I take it as a particle, a wave or an ice-cream cone.

 

 

The question stands.

Does that interaction change the direction of the photon?

If it does then the photon won't reach us.

[michel123456]

The photon will do exactly as it pleases no matter whether I take it as a particle, a wave or an ice-cream cone.

Exactly. And that helps. Because taking the photon as a particle must give the same result as taking the photon as a particle.

 

 

The question stands.

Does that interaction change the direction of the photon?

I don't know. What I know is that when light interacts when light, it behaves like a wave.

 

If it does then the photon won't reach us.

I don't think so. It means the image will be seen elsewhere (like refraction) and probably that the image will be blurred.

[swansont]

 

I don't think so. It means the image will be seen elsewhere (like refraction) and probably that the image will be blurred.

 

If it is seen elsewhere, it doesn't reach us.

[michel123456]

If it is seen elsewhere, it doesn't reach us.

 

Lost.

Begin with : it reaches us. Then I mean the visible position of source is different from the actual position

(image from here)

[Sisyphus]

You're not talking about refraction, you're talking about random scattering.

[swansont]

Refraction requires non-normal incidence of a medium with a different index, and you need to explain how all — not some — of these images are refracted, and how this shifts the wavelength of the light we see.

[Janus]

You're not talking about refraction, you're talking about random scattering.

 

On top of that, refraction and random scattering are frequency dependent. So you wouldn't get those nice clean spectrums where all the spectral lines are still properly spaced though shifted.

[michel123456]

Truly, I engaged on a slippery road.

 

What I was thinking is that, if you take for example the well known 2slit experiment, where there is interference of light, and if you put your eye in place of the screen*, not on the direct path, but on one of the side stripes, would you observe the source? And if yes, would it be blurred or clear and sharp?

 

*don't do that.

[swansont]

Truly, I engaged on a slippery road.

 

What I was thinking is that, if you take for example the well known 2slit experiment, where there is interference of light, and if you put your eye in place of the screen*, not on the direct path, but on one of the side stripes, would you observe the source? And if yes, would it be blurred or clear and sharp?

 

*don't do that.

 

Diffraction depends on the wavelength: you could see a signal at one wavelength but not another. It would be an exceedingly curious observation.