Tired light

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zonde
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I read this in article "Misconceptions about the big bang" that was mentioned in another thread:
"For example, when a star explodes as a supernova, it brightens and then dims — a process that takes about two weeks for the type of supernova that astronomers have been using to map out space. During these two weeks, the supernova emits a train of photons. The tired-light hypothesis predicts that these photons lose energy as they propagate but that the observer always sees a train that lasts two weeks."
This article can be found http://astronomy.case.edu/heather/us211.07/misconceptions.pdf" [Broken] as well.

I have some doubts about such a simple prediction for tired light.
From quantum mechanics we know that photons are not like billiard balls. They have phase and as a result interference effects can take place.
Now if we increase wavelength and therefore decrease frequency phase differences inside photon train will change. As a result there should appear some interference effects that will change photon train on the whole not only individual photons.

Any comments?
 
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  • #2
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We can change the velocity of light with refraction, it slows down and we can separate frequencies because different frequencies bend at different rates. Also we can slow the light down for example in a coax cable ~0.7c. However, I cant think of many other ways of changing the frequency other than doppler.

Fluorescence is one: http://en.wikipedia.org/wiki/Fluorescence
Here's a list of ...esenses http://en.wikipedia.org/wiki/Luminescence


Are you trying to find alternative explantions for red shift? and thus the rate of expansion of the observable universe?
I doubt any of the above could produce black body radiation of the correct frequency.
 
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bcrowell
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I have some doubts about such a simple prediction for tired light.
From quantum mechanics we know that photons are not like billiard balls. They have phase and as a result interference effects can take place.
Now if we increase wavelength and therefore decrease frequency phase differences inside photon train will change. As a result there should appear some interference effects that will change photon train on the whole not only individual photons.
The coherence length of these photons is probably on the order of micrometers or less. The mean distance between photons, on the other hand, grows in proportion to the distance to the source, and will be extremely large when you're light-years away from the source. Therefore I don't think there is any way that individual photons could be interfering with one another. I don't think there is anything like a "photon train" composed of multiple photons here.
 
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zonde
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Are you trying to find alternative explantions for red shift? and thus the rate of expansion of the observable universe?
No, expansion and doppler is just enough.

I doubt any of the above could produce black body radiation of the correct frequency.
If photons lose energy it has to end up somewhere most likely as a heat energy. This is not very detailed answer but on the other hand the question is not about black body radiation.
 
  • #5
zonde
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The coherence length of these photons is probably on the order of micrometers or less. The mean distance between photons, on the other hand, grows in proportion to the distance to the source, and will be extremely large when you're light-years away from the source. Therefore I don't think there is any way that individual photons could be interfering with one another.
Yes, photons just by themselves can't interfere along the way. Something like that can happen if they are repeatedly undergoing quantum decoherence. Say they are repeatedly crossing surfaces between mediums with different refractive indexes. It's like in Quantum Zeno effect.

I don't think there is anything like a "photon train" composed of multiple photons here.
Of course there are different interpretations in Quantum mechanics.
But I think that ensemble interpretation is more down-to-earth than any other interpretation. And from perspective of this interpretation "photon trains" or photon ensembles are necessary to explain quantum effects and resolve quantum paradoxes.
 
  • #6
Chalnoth
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I have some doubts about such a simple prediction for tired light.
From quantum mechanics we know that photons are not like billiard balls. They have phase and as a result interference effects can take place.
Now if we increase wavelength and therefore decrease frequency phase differences inside photon train will change. As a result there should appear some interference effects that will change photon train on the whole not only individual photons.

Any comments?
The emitted photons would have to be coherent for any interference effects to be observable. But the photons emitted during a supernova are primarily thermal.

Anyway, we know that this sort of thing doesn't happen anyway, because very distance supernovae are too bright to be explained by tired light.
 
  • #7
zonde
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The emitted photons would have to be coherent for any interference effects to be observable. But the photons emitted during a supernova are primarily thermal.
Well yes, photons have to be coherent but they don't have to be emitted that way. They can acquire coherence along the way.

Anyway, we know that this sort of thing doesn't happen anyway, because very distance supernovae are too bright to be explained by tired light.
You mean that distant supernovae are too dim to be explained by tired light, right?

Anyways I am not sure that expanding universe can really help here. You see, if gravity and expansion are two opposite things then gravity is contraction of spacetime and the net effect of the two would be static universe.
That thing about gravity is quite obvious if you consider how effects of black hole are described i.e. space like time inside event horizon.
 
  • #8
bcrowell
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Anyways I am not sure that expanding universe can really help here. You see, if gravity and expansion are two opposite things then gravity is contraction of spacetime and the net effect of the two would be static universe.
That thing about gravity is quite obvious if you consider how effects of black hole are described i.e. space like time inside event horizon.
This doesn't sound to me like a correct characterization of how cosmology is described in GR. If you'd like to learn more about GR I'd be happy to try to help by pointing you to books at the appropriate level. Just post about your current level of study in math and physics.
 
  • #9
Chalnoth
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Well yes, photons have to be coherent but they don't have to be emitted that way. They can acquire coherence along the way.
Given the very limited interactions that photons undergo when traveling through space, this is highly unlikely to occur.

You mean that distant supernovae are too dim to be explained by tired light, right?
No, too bright. If the appearance of acceleration were explained by tired light, then the acceleration would continue into the past no matter how far back we look. But dark energy-based models of cosmology predict that the expansion in the past was decelerating, and when we look far enough away (and thus, back in the past), this deceleration is precisely what we see.
 
  • #10
zonde
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This doesn't sound to me like a correct characterization of how cosmology is described in GR.
But it sounds just fine to me. So if you do not provide any arguments I suppose we can discuss it no further.
 
  • #11
zonde
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Given the very limited interactions that photons undergo when traveling through space, this is highly unlikely to occur.
Assuming that photons undergo very limited interactions when traveling through space it is indeed highly unlikely to occur.
Assuming the opposite it is quite likely to occur.

No, too bright. If the appearance of acceleration were explained by tired light, then the acceleration would continue into the past no matter how far back we look.
Accelerated expansion can not be explained by tired light no matter what observation you make.
The question can be only about expansion (accelerated, decelerated or what ever) versus static universe of tired light model. And in that case distant light sources are too dim to be explained by naive version of tired light.

But dark energy-based models of cosmology predict that the expansion in the past was decelerating, and when we look far enough away (and thus, back in the past), this deceleration is precisely what we see.
There where no such predictions prior to observations. There were tunable parameters that could account for any observation of that type that would be made.
 
  • #12
Chalnoth
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Assuming that photons undergo very limited interactions when traveling through space it is indeed highly unlikely to occur.
Assuming the opposite it is quite likely to occur.
The statement that photons don't undergo many interactions in interstellar space isn't an assumption, but a direct result from the WMAP satellite: the photons emitted when the universe cooled from a plasma to a gas some 13.7 billion years ago have only lost about 8% of their brightness due to interactions in the interim. These are the photons that have been out there the longest, that have traveled the furthest.

Now, it is very possible for a significant amount of interaction to occur within the host galaxy of whatever emitted the light (some are so dusty that hardly any visible light gets through). But once the photons reach interstellar space, there is almost no further interaction.

Accelerated expansion can not be explained by tired light no matter what observation you make.
The question can be only about expansion (accelerated, decelerated or what ever) versus static universe of tired light model. And in that case distant light sources are too dim to be explained by naive version of tired light.
Er, no. The comparison is between a model that has continual deceleration + tired light vs. accelerated expansion. A tired light model predicts that the acceleration is only inferred due to a failure to account for this extra dimming of supernovae, and it predicts that the amount of extra dimming will therefore be proportional with distance.

But when we look far enough into the past, we actually see an apparent brightening of supernovae, which completely solidifies this as being impossible.

I'd also like to point out that supernovae are only the first experimental evidence of the acceleration. Today we have many more pieces of evidence, and they all fit a Lambda-CDM model of the universe.

There where no such predictions prior to observations. There were tunable parameters that could account for any observation of that type that would be made.
No. There is only one single "extra" parameter in the simplest dark energy cosmology, Lambda-CDM. And that model unambiguously predicts that until recently, the universe was decelerating, and acceleration is only a recent phenomenon.
 
  • #13
zonde
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The statement that photons don't undergo many interactions in interstellar space isn't an assumption, but a direct result from the WMAP satellite: the photons emitted when the universe cooled from a plasma to a gas some 13.7 billion years ago have only lost about 8% of their brightness due to interactions in the interim.
Are you saying that there is some disagreement between expected intensity of light at different wavelengths as compared with black body spectrum? And this disagreement amounts to 8%? Or something else?
Maybe you can give the source of your statement?

Er, no. The comparison is between a model that has continual deceleration + tired light vs. accelerated expansion. A tired light model predicts that the acceleration is only inferred due to a failure to account for this extra dimming of supernovae, and it predicts that the amount of extra dimming will therefore be proportional with distance.

But when we look far enough into the past, we actually see an apparent brightening of supernovae, which completely solidifies this as being impossible.
Well, then it's not exactly "tired light" model you are speaking about. It's something else.

No. There is only one single "extra" parameter in the simplest dark energy cosmology, Lambda-CDM. And that model unambiguously predicts that until recently, the universe was decelerating, and acceleration is only a recent phenomenon.
From paper http://arxiv.org/abs/astro-ph/9805201" [Broken]

"The time evolution of the cosmic scale factor depends on the composition of mass-energy in the Universe. While the Universe is known to contain a significant amount of ordinary matter, [tex]\Omega _{M}[/tex], which decelerates the expansion, its dynamics may also be significantly affected by more exotic forms of energy. Pre-eminent among these is a possible energy of the vacuum ([tex]\Omega _{\Lambda}[/tex]), Einstein’s “cosmological constant,” whose negative pressure would do work to accelerate the expansion (Carroll, Press, & Turner 1992; Schmidt et al. 1998). Measurements of the redshift and apparent brightness of SN Ia of known intrinsic brightness can constrain these cosmological parameters."

Does not sound like confirmation of some prediction.
It sounds exactly like results are used to do some fitting of parameter(-s).
 
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  • #14
Chalnoth
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Are you saying that there is some disagreement between expected intensity of light at different wavelengths as compared with black body spectrum? And this disagreement amounts to 8%? Or something else?
Maybe you can give the source of your statement?
This primarily comes from the correlation of the polarized photons and unpolarized photons in the CMB, for which the current best measurement is the WMAP seven-year release:
http://lambda.gsfc.nasa.gov/product...year/cosmology/wmap_7yr_cosmology_reprint.pdf

The parameter of interest is [itex]\tau[/itex] (the Greek letter tau), which is the optical distance to the surface of last scattering, which is constrained to be around 0.085, plus or minus about 0.015. This parameter is such that the amount of light we see is [itex]e^{-\tau}[/itex] times the light that was emitted.

Well, then it's not exactly "tired light" model you are speaking about. It's something else.
Pretty sure I'm talking about the only tired light models that were ever considered remotely likely as an explanation for the apparent accelerated expansion.

From paper http://arxiv.org/abs/astro-ph/9805201" [Broken]

"The time evolution of the cosmic scale factor depends on the composition of mass-energy in the Universe. While the Universe is known to contain a significant amount of ordinary matter, [tex]\Omega _{M}[/tex], which decelerates the expansion, its dynamics may also be significantly affected by more exotic forms of energy. Pre-eminent among these is a possible energy of the vacuum ([tex]\Omega _{\Lambda}[/tex]), Einstein’s “cosmological constant,” whose negative pressure would do work to accelerate the expansion (Carroll, Press, & Turner 1992; Schmidt et al. 1998). Measurements of the redshift and apparent brightness of SN Ia of known intrinsic brightness can constrain these cosmological parameters."

Does not sound like confirmation of some prediction.
It sounds exactly like results are used to do some fitting of parameter(-s).
What you're not getting is that these parameters have been measured independently with a wide variety of experiments today.

For example, see here:
http://hera.ph1.uni-koeln.de/~heintzma/Sp_Art2/S406.htm [Broken]
[PLAIN]http://hera.ph1.uni-koeln.de/~heintzma/k5/b3/Cosmic_Acc.gif [Broken]

Note that the different experiments all converge on the same values for these parameters. Granted, the supernova estimate is biased a bit towards the upper right, but this is likely due to a failure to take into account the effect of gravitational lensing on the supernova signals.
 
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  • #15
bcrowell
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But it sounds just fine to me. So if you do not provide any arguments I suppose we can discuss it no further.
I've asked a couple of times now that you post something about your background in math and physics. I'm not sure why you aren't willing to do that. There are some things that you clearly don't understand, and I would be happy to help, but I can't do that without knowing what your background is.
 
  • #16
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Since we were also earlier talking about dark energy cosmology and Lambda-CDM, I wanted to clarify the following:

Do we know, or is it perhaps assumed, that the effect of dark energy on space at any given moment of time is equal and homogenious throughout the whole universe?
Or do we see variation in the magnitude of the effect of dark energy both at different distances and directions, as well as at different points in time?
 
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  • #17
Chalnoth
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Since we were also earlier talking about dark energy cosmology and Lambda-CDM, I wanted to clarify the following:

Do we know, or is it perhaps assumed, that the effect of dark energy on space at any given moment of time is equal and homogenious throughout the whole universe?
Or do we see variation in the magnitude of the effect of dark energy both at different distances and directions, as well as at different points in time?
It's difficult to say for sure just yet. We just don't have the experimental accuracy to distinguish differences in the dark energy density in different places at present.
 
  • #18
zonde
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I've asked a couple of times now that you post something about your background in math and physics. I'm not sure why you aren't willing to do that. There are some things that you clearly don't understand, and I would be happy to help, but I can't do that without knowing what your background is.
Hmm, somehow I doubt that you are interested in any discussion.
If you would be then you would find out my level in physics and math along the way.

Instead you are offering to find me some reading. Well, I am not asking that.
Why should I bother answering you?

So maybe you can try to tell me what I "clearly don't understand" and I will tell you if I understand what I "don't understand".
 
  • #19
Chalnoth
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Hmm, somehow I doubt that you are interested in any discussion.
If you would be then you would find out my level in physics and math along the way.

Instead you are offering to find me some reading. Well, I am not asking that.
Why should I bother answering you?

So maybe you can try to tell me what I "clearly don't understand" and I will tell you if I understand what I "don't understand".
Well, you clearly don't understand the "tired light" ideas that have circulated in the cosmology community since the discovery of the accelerated expansion, either what they were proposing or what they were proposed to explain, you don't understand why far-away supernovae completely falsify these ideas, and you don't understand the very nature of cosmological parameter estimation.

You can "tell us" all you like what you do and do not understand, but I think I speak for most everybody here when I say we don't care what you say you understand, only what you can demonstrate you understand.
 
  • #20
zonde
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Well, you clearly don't understand the "tired light" ideas that have circulated in the cosmology community since the discovery of the accelerated expansion, either what they were proposing or what they were proposed to explain, you don't understand why far-away supernovae completely falsify these ideas, and you don't understand the very nature of cosmological parameter estimation.

You can "tell us" all you like what you do and do not understand, but I think I speak for most everybody here when I say we don't care what you say you understand, only what you can demonstrate you understand.
But Chalnoth, please calm down.
I didn't realized that we where discussing "tired light" ideas that have circulated in the cosmology community after the discovery of the accelerated expansion.

Could you tell me who were proponents of those ideas? Maybe if I will have decent source of information I will be able to somehow reach the understanding.
 
  • #21
Chalnoth
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But Chalnoth, please calm down.
I didn't realized that we where discussing "tired light" ideas that have circulated in the cosmology community after the discovery of the accelerated expansion.
Well, since you brought tired light up in the context of supernova, which are widely used as evidence for the acceleration of the expansion, it only makes sense that this is what you would be talking about.

Could you tell me who were proponents of those ideas? Maybe if I will have decent source of information I will be able to somehow reach the understanding.
Well, the only somewhat reasonable hypothesis that I saw along these lines was from photons oscillating into axions over large distances. Look up "supernova dimming through axions", and you'll find a fair amount on the subject.
 
  • #22
Let's start this discussion from another standpoint.

Doesn't the Lambda CDM Model, and in effect the theorized expansion of the universe based on the fact that the further something is, the more it's redshift, makes a huge assumption, the assumption that light travelling for billions of years is unchanged and all it's initial properties are conserved?

I would say that this a a cataclysmic assumption to make, specially considering that even in short timescales with little interference, light does alter it's properties, such as wavelenght for example.

Therefore, it is much more likely to leave the door open for the possibility that there exists some kind of correlation between redshift and the amount of time/distance light has traveled.

This would in a sense question the very nature of the big bang theory which is now ubiquitous in the scientific community, and I just think there is an overwhelmingly small amount of data to support it, specially when they completely ignore the issue that something might be causing the redshift on distant light.
 
  • #23
Chalnoth
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Let's start this discussion from another standpoint.

Doesn't the Lambda CDM Model, and in effect the theorized expansion of the universe based on the fact that the further something is, the more it's redshift, makes a huge assumption, the assumption that light travelling for billions of years is unchanged and all it's initial properties are conserved?

I would say that this a a cataclysmic assumption to make, specially considering that even in short timescales with little interference, light does alter it's properties, such as wavelenght for example.

Therefore, it is much more likely to leave the door open for the possibility that there exists some kind of correlation between redshift and the amount of time/distance light has traveled.

This would in a sense question the very nature of the big bang theory which is now ubiquitous in the scientific community, and I just think there is an overwhelmingly small amount of data to support it, specially when they completely ignore the issue that something might be causing the redshift on distant light.
This is worthless without a specific proposal for how the light is changed en route. All specific proposals have so far failed to match experiment.
 
  • #24
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The observation is that the frequency of certain known light sources has changed during the years since the big bang.
As far as we know every possible explanation for this has been considered.

After eliminating all other possibilities, the one remaining-no matter how unlikely-must be the truth and that is the expansion of the universe is accelerating.
 
  • #25
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Perhaps, without this further expansion of space the largest gravitationally bound structures and super clusters would eventually have collapsed into very dense supergalaxies larger and denser than the largest ellipticals which would I suspect have been very harmful to life. If so then perhaps it is for the best that the ultimate size of galaxies is limited by this expansion of space.

Thinking of the multitude of happenstances to make it happen, perhaps this Universe is well designed with the best of intentions after all :)
 
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