Does CMBR include free photons?

[Q-reeus]

Even a high quality laser (small divergence) will spread to an enormous diameter at cosmological distances.

Unless diffuse scattering is significant (hugely unlikely imo), what matters is the relative size of that diameter - i.e. beam angle remains small. [Whoops: on second thoughts, all that matters is the probability of absorption - and photons in a spherical pulse has essentially the same chance of survival as for those in a narrow laser beam]

Still, averaging over this area which might contain spots of no absorption which might show minute power discrepancies versus the complete absorption of the sun.

Why would you conclude non-absorption would be minute in deep space directions, rather than the other way around? Astonomers have no trouble seeing in sharp detail (limited only by instrument resolution) distant galaxies back to 13 odd billion years ago. And that from times when dust and gas was far more concentrated than now or especially into the distant future.

Absent such experimental evidence...

Making the reasonable assumption that absorption this side of horizon crossing is statistically unlikely, in #17 was provided a modified version of OP's experimental test in #1 that is rediculously easy to implement. One that should give an easily observable positive if that 2nd conjecture: "Emission probabilities in the directions of absorbers increase to compensate for the dark spots." is correct. Even moderately powered laser pointers are now illegal to own where I live, but having kept an old, low power one, I can confirm there is zero visual evidence for the above conjecture, based on the 'test' of #17. But let's have feedback from various 'labs' - like independent corroboration to confirm things.

 

[Q-reeus]

If photons depend on some kind of retrocausal mechanism - the Wheeler-Feynman story - then this suggests either all the emitted CMB must be absorbed by particles within the lightcone, or perhaps the event horizon itself is some kind of ultimate reflector - perhaps emitting Hawking radiation in the manner of black holes to do some late-in-the-story mopping up of stray CMB photons.

There is some detailed theory behind that last idea, or does it fall into the retrocausality speculative bracket?

 

[skippy1729]

Emission probabilities in the directions of absorbers increase to compensate for the dark spots. Skippy

This sounds like the answer.

Why would action-at-a-distance seem more plausible than a completely absorbing universe?

 

[apeiron]

Why would action-at-a-distance seem more plausible than a completely absorbing universe?

I thought a future cosmological event horizon looks a problem for either. If I stress the transactional interpretation, it is is true that is what I personally am more interested in (and what I thought was also DrC's motivation).

But either way, event horizons seem to change the game. I note for instance that Hoyle-Narlika siezed on the absorber issue to revive a steady-state cosmology .

http://rmp.aps.org/abstract/RMP/v67/i1/p113_1

One way or another, it looks to be an important constraint on viewpoints. If your theory demands a completely absorbing universe, the question arises of are there sufficient absorbers in the far future?

 

[skippy1729]

I agree, either way we have a consistency constraint relating quantum theory and cosmology.

 

[DrChinese]

I agree, either way we have a consistency constraint relating quantum theory and cosmology.

That's sort of the issue I am highlighting. The idea of an accelerating expansion was more or less farfetched until the last 2 decades led to the many new data points that support that. So looking at the subject again, it seems to me that either we will be shocked to learn we can peer into the future, or that something with existing theory is off. So just to recap, it appears to me (lacking an explanation otherwise) that one of these is wrong:

a) The universe is in an accelerating expansion;
b) Every photon emission requires a future photon absorption;
c) There is no way to probe the future location of mass in our vicinity.

 

[TrickyDicky]

it appears to me (lacking an explanation otherwise) that one of these is wrong:

a) The universe is in an accelerating expansion;
b) Every photon emission requires a future photon absorption;
c) There is no way to probe the future location of mass in our vicinity.

Ok, c) seems just a filler, so we have GR versus QED...hmm, would you mind taking your pick first? I'm curious what you'd get rid of to begin with.

 

[apeiron]

one of these is wrong:

a) The universe is in an accelerating expansion;
b) Every photon emission requires a future photon absorption;
c) There is no way to probe the future location of mass in our vicinity.

But why can't b) be right, and then the cosmological event horizon now be given some ultimate role as the absorber of last resort?

Lineweaver - in http://www.mso.anu.edu.au/~charley/papers/EganLineweaverApJOnline.pdf
- calculates the CEH to be easily large enough to do the job.

The CMB amounts to 10^88 degrees of freedom, while the CEH will have 10^122.

It would seem like the CEH would be as smooth and even as possible, so not have the kind of material dark spots that are motivating your thought experiment.

All we need now is a plausible mechanism for how the event horizon does the job. And others here would have a better idea than me if there is one.

 

[DrChinese]

But why can't b) be right, and then the cosmological event horizon now be given some ultimate role as the absorber of last resort?

Lineweaver - in http://www.mso.anu.edu.au/~charley/papers/EganLineweaverApJOnline.pdf
- calculates the CEH to be easily large enough to do the job.

The CMB amounts to 10^88 degrees of freedom, while the CEH will have 10^122.

It would seem like the CEH would be as smooth and even as possible, so not have the kind of material dark spots that are motivating your thought experiment.

All we need now is a plausible mechanism for how the event horizon does the job...

I don't see how the CEH serves as an absorber. There is nothing there. Sure, there may be paths by which absorption occurs, but they would be few. And that leads to physically testable predictions too (I think).

According to the referenced article, the current CEH is 15.7 billion LY out and the radius of the entire universe is about 46 billion LY out. However, they don't expect the CEH radius to change much from the current value.

 

[apeiron]

I don't see how the CEH serves as an absorber. There is nothing there.

Yes, I don't see exactly how either. But as you say, if emitters require absorbers - and this seems essential to the transactional interpretation, even if we dismiss QED as "just a model" - then something must do the job. And CEH arguments now say it can't be future particles of matter it appears.

There is a lot of speculation about event horizon as "entropic screens" - not just Sarfatti (who is pretty cranky), but also Verlinde with his entropic gravity. So "nothing there" can be possibly a concrete constraint in fact.

How it works is the open question. But again, that is why I drew attention to Lineweaver in particular. He asserts that the heat death of the universe has a maximum entropy now defined by a stable CEH.

Once even black holes have decayed to a redshifting fizzle of photons, all that will be left is "blackbody radiation with a wavelength the span of the visible universe". Which means that something is actually left.

But in this scenario of Lineweaver's, where are the emitters and absorbers? You only seem to have the "two sides" of the CEH sustaining a blackbody radiation of almost absolute 0 degrees K.

Lineweaver, along with Davies, is an authority on this issue. So he seems to think this is possible. On the other hand, I haven't found any proper publication of these ideas, just the paranthetical notes in other more general papers. So either it is still work in progress, or an idea that does not pan out.

But the CEH imposes a fundamental constraint now on transactional/retrocausal style explanations of particle interactions. Posters here accept that it seems. The question is whether it is possibly also the solution? Maybe the better informed have ideas here.

 

[fleem]

I've been away a while and have only skimmed the many new posts, so please forgive if these comments have been mentioned.

It seems strange to me that some scientists embrace the belief that all the energy in a black hole still completely "exists" in the eyes of the rest of the universe (the black hole still conserves energy, angular momentum, etc. in our eyes) yet they find it easy to believe there is no interaction between us and things that have gone outside the CEH. IMO it seems more plausible that it is best to think of the energy in a black hole as residing on its event horizon rather than a singularity. And in the eyes of beings that have fallen into a black hole, the rest of the universe appears as if its plastered on the surface of their CEH. So I wonder if we would never see things blink out as they move past the CEH, and the universe is closed. Or if the universe is open, then there is still interaction with those things that have moved past the CEH because those interactions are faster than light (just as there are interactions that can reach up through a black hole's event horizon). Of course, this is still a large-scale (classical mechanics) picture of things, and we know classical mechanics is technically wrong. In any case I do think photons are really simply local interactions in reference frames moving at c, so there has to be an emitter and an absorber.

To wax more philosophical, I think its important to choose a rigorous definition of "exist". If our definition includes things that don't interact with us, then one can claim some pretty kooky things "exist" but never interact with us. Please note I am not saying such a definition is wrong. For all I know reality came about because chaos allows ALL rules to exist, but we see only self-consistent rules because only those rules evolved. It might be that any consistent set of rules possible, actually exists, and science is simply the attempt at noting all those rules.

 

[sheaf]

I was just wondering about one of the considerations that kicked off this thread - namely that it's questionable whether we can talk of a photon's existence if it doesn't interact with matter. Presumably, when we have a satisfactory quantum theory of gravity, any photon will act as a source for gravity regardless of whether it is ultimately absorbed by matter. In that sense, the photon's existence will be (at least in principle) detectable. When we use our single photon calcium ion source to fire one photon per hour off into the void, presumably they will generate some sort of gravitational trace ?