Arcadian Functor

occasional meanderings in physics' brave new world

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Marni D. Sheppeard

Thursday, August 27, 2009

The Fermi Debate

The Fermi debate appears to have convinced some people that any kind of variation in $c$ has been ruled out, or will soon be ruled out as more GRBs are observed.

But consider the Riofrio cosmology. If one must insist (sigh) on a classical picture of photons whizzing through an objective vacuum from GRB 090510 to us, then the Riofrio cosmology would say that all photons shift their speed in unison, as the cosmic epoch changes. They would all be measured locally at a speed $c$. This quantum cosmology is consistent with all known data.

The Loopie picture, on the other hand, defends its new position on the fence by pointing out that DSR breaks Lorentz symmetry through deformation, a supposedly subtle kind of symmetry (actually, it's pretty simple and there is no physical motivation for it). The linked paper begins with the line:
What is the fate of Lorentz symmetry at Planck scale?
There is a problem with this statement. Traditional string theorists make similar statements. The Riofrio cosmology does not impose a fixed scale on the non local theory of quantum gravity, which contains all possible values of $c$, $\hbar$ and $M_{\textrm{pl}}$. It is capable of recovering local Lorentz invariance precisely because it considers a fixed Planck scale an approximation.


Anonymous Anonymous said...

Dear Kea,
When you refer to ''cosmic time', in which frame of reference is this measured? If you are assuming this is the same for everyone, then you in effect stating that time is absolute, which we know to be false. Suppose that in the reference frame that 'cosmic time' is measured, the speed of light changed as you claim. However, in a different frame of reference, the speed of light would not change by the same amount since he measures a different amount of time. Thus, the two observers would disagree on the speed of light and Lorentz invariance is broken.

August 28, 2009 4:19 AM  
Blogger Kea said...

Anonymous, I do not assume that cosmic time is the same for all observers, although it is essentially the same for all human observers since we haven't really left Earth yet - the Pioneer anomaly being a very interesting case in point.

Your second implicit assumption - that this is the only kind of time - is also not assumed. Human observers on Earth occupy a fixed cosmic epoch, which does not conflict with the fact they must always observe photons locally to be travelling at c (as in the c for this epoch).

August 28, 2009 6:00 AM  
Anonymous Anonymous said...

It doesn't matter if cosmic time is 'essentially' the same for all observers. For Lorentz invariance to hold in this scenario, it has to be exactly the same for all observers. Thus, this idea either breaks Lorentz invariance by 1) assuming absolute time for all observers or 2) resulting in the speed of light being different for observers in different frames.

August 28, 2009 9:50 AM  
Blogger Matti Pitkanen said...

A comment about absolute time and Lorentz invariance. In General Relativity Lorentz invariance and the idea about absolute time are in conflict.

In TGD framework space-times are 4-surfaces in M^4xS (S=CP_2) and Lorentz group acts as isometries of the imbedding space. In zero energy ontology intersections of future and past light-cones -causal diamonds (CDs)- are basic objects: space-times are 4-surfaces in CDxCP_2. The light-cone proper time is a natural Lorentz invariant time variable and in a well-defined sense absolute. This time serves as cosmic time in TGD inspired cosmology but requires that the consideration is restricted inside single sub-cosmology.

This approach also allows to define what the measurement of c means as I have explained a couple of times. Many-sheeted space-time allows variation of light-velocity in this sense without breaking of Lorentz invariance. In cosmic scales the light-velocity measured in this manner increases slowly (the time taken to travel along curved space-time surface decreases since it becomes less curved as it flattens so that c approaches its maximal value).

This prediction is opposite to the hypothesis of Louise Riofrio. As also Kea has noticed, Riofrio's theory leads to difficulties with fine structure constant alpha=e e^2/4*pi*hbar*c constant and Rydberg constant involving electron mass unless one makes additional assumptions (hbar, c, G and possible also electron mass vary as functions of cosmic time).

Solar system space-time sheet is predicted to not participate in expansion except possibly by rapid phase transitions and this is known to be true. This implies apparent reduction of c since the standard to which one compares increases slowly: the prediction for the rate of reduction is correct. In TGD framework the situation can therefore be said to be settled.

August 28, 2009 3:38 PM  
Blogger Kea said...

Anonymous, Lorentz invariance is broken in this quantum cosmology, in the sense that we talk about different values for c. It's a quantum cosmology, which implies that GR breaks down on large scales.

In order to refute this hypothesis one has two options:
1. travel to a different cosmic epoch, make measurements and communicate them (you will probably agree that this is impossible)
2. falsify some of Louise's (and others') predictions and demonstrate that the cosmological constant is a better explanation of the data

Although some visitors here don't seem to value GR, personally I have a great respect for it and believe that it must be recovered rigorously from the full quantum theory. But one cannot seriously expect it to apply to cosmology forever ... human progress is slow, but not that slow.

August 28, 2009 7:46 PM  
Blogger L. Riofrio said...

Again your tireless support is most appreciated! As we know, changing constants has become a subject of study with international astronomers. Some of them are still focused on changes in alpha, but the world is slowly seeing the light.

August 30, 2009 10:36 AM  
Blogger Kea said...

Hi Louise. I trust in the good sense of astronomers and honest physicists.

August 30, 2009 9:50 PM  

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