# The range of light

It occurs to me that the empirical evidence shows that there is a point out in space where light stops coming from.

Putting aside the expansion of the universe for a second, and focusing strictly on the evidence:

• what would the universe look like if light had a finite range?

• isn't that what the universe looks like?

Recall Hubble's Law,$v = H_0D$

The range of light is $H_0 D = c$

Also recall that Edwin Hubble stressed the point that $v$ is apparent recessional velocity. It is the apparent recessional velocity, not the actual recessional velocity. He proposed that rather than Doppler shifts, these redshifts are a "new principle of nature".

Ignoring the theories and strictly examining the empirical evidence, does it make sense that light could have a finite range?

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What evidence exactly are you asking to focus on, given that you are not considering the expansion of the universe? –  David Z Dec 20 '11 at 21:24
Light redshifts until it disappears. That's empirically true, and seems consistent with the hypothesis that light has a finite range. –  mobydikc Dec 20 '11 at 21:28
"Light redshifts until it disappears." Uhm...no. "That's empirically true" No it's not. In fact we can see all the way to the last scattering surface in every direction. The reason we can't see beyond that is not that light is worn out but that the universe we're looking at is opaque at that time. –  dmckee Dec 20 '11 at 22:06
Your equations are trivially the same, and both refer to velocities not ranges (distances?) could you edit? –  Nic Dec 21 '11 at 13:28
I think the range equation makes sense rearranged: D = c/H0 gives the distance at which the Doppler shift would reduce the frequency of incoming light to zero. Answers can then address whether at some point in the future when there is more history since opacity, this limit might apply. –  trichoplax Jan 28 at 23:50

## 1 Answer

What you're talking about is the tired light hypothesis. This was considered back in the 1930s but has now been conclusively disproven for reasons explained in the Wikipedia article. Basically, tired light is inconsistent with the observed level of blurring of cosmological observations, with time dilation observed in distant astrophysical processes, and with trends in observed brightness of distant objects, plus it completely fails to explain the measurements of the CMB energy distribution made by COBE and WMAP.

Of course, this isn't really my field, so perhaps someone else who is more experienced with this stuff can come in and give a more complete answer.

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Tired light never suggests that light has a finite range, even in a perfect vacuum. Tired light relies on interactions between the photon's source and the observer, not light having a naturally finite range. –  mobydikc Dec 22 '11 at 1:00
Actually "tired light" is a blanket term that applies to all sorts of effects, known or unknown, which could limit the range of light. –  David Z Dec 22 '11 at 1:27
I've looked and can't find any tired light hypothesis that claims in a perfect vacuum of sufficient length, light has a finite range and identifies it as Hubble's Limit. –  mobydikc Dec 22 '11 at 3:47