# Tired light

1. Feb 4, 2011

### zonde

"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."

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.

2. Feb 4, 2011

### Tanelorn

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.

Last edited: Feb 4, 2011
3. Feb 4, 2011

### bcrowell

Staff Emeritus
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.

4. Feb 5, 2011

### zonde

No, expansion and doppler is just enough.

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. Feb 5, 2011

### zonde

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.

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. Feb 5, 2011

### Chalnoth

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. Feb 5, 2011

### zonde

Well yes, photons have to be coherent but they don't have to be emitted that way. They can acquire coherence along the way.

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. Feb 5, 2011

### bcrowell

Staff Emeritus
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. Feb 5, 2011

### Chalnoth

Given the very limited interactions that photons undergo when traveling through space, this is highly unlikely 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. 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. Feb 7, 2011

### zonde

But it sounds just fine to me. So if you do not provide any arguments I suppose we can discuss it no further.

11. Feb 7, 2011

### zonde

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.

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.

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. Feb 7, 2011

### Chalnoth

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.

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.

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. Feb 7, 2011

### zonde

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?

Well, then it's not exactly "tired light" model you are speaking about. It's something else.

From paper Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant

"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, $$\Omega _{M}$$, 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 ($$\Omega _{\Lambda}$$), 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).

14. Feb 7, 2011

### Chalnoth

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 $\tau$ (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 $e^{-\tau}$ times the light that was emitted.

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.

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

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.

15. Feb 7, 2011

### bcrowell

Staff Emeritus
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. Feb 8, 2011

### Tanelorn

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?

Last edited: Feb 8, 2011
17. Feb 8, 2011

### Chalnoth

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. Feb 9, 2011

### zonde

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.

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. Feb 9, 2011

### Chalnoth

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. Feb 9, 2011