Is LIGO just observing viscosity of the vacuum?

[Duda Jarek]

In experiments like LIGO we want to observe extremely weak gravitational waves from sources millions of light years away - we are assuming that their strength decreases like R^-3.

But because of this distance, even slightest interactions with the vacuum and other objects on the way, could diffuse/absorb them - and so the amplitude would decrease exponentially, making such observations completely hopeless.

 

In November 2005 LIGO has reached assumed sensitivity

"At its conclusion, S5 had achieved an effective range of more than 15 Mpc for the four-kilometer interferometers, and seven Mpc for the two-kilometer interferometer."

http://www.ligo.caltech.edu/~ll_news/s5_news/s5article.htm

But for these 3.5 years its only success is is a non-detection:

"During the intense blast of gamma rays, known as GRB070201, the 4-km and 2-km gravitational-wave interferometers at the Hanford facility were in science mode and collecting data. They did not, however, measure any gravitational waves in the aftermath of the burst.

That non-detection was itself significant. "

http://mr.caltech.edu/media/Press_Releases/PR13084.html

 

What is vacuum?

It definitely isn't just 'an empty space' - it for example is medium for many different waves, particles.

Nowadays many people believe that it can for example spontaneously create particle-antiparticle pairs...

Modern cosmological models says that there is required cosmological constant - additional density of energy of ... this vacuum ...

 

Anyway, even being only a medium for many kind of interactions - there is at least some field there - it has many internal degrees of freedom (like microwave radiation).

We usually believe that they can interact with each other, so there should be thermalization - all of them should contain similar amount of energy.

In physics there are usually no perfect mediums - there are always at least some very very very small interactions...

 

We observe more or less uniform 2.725K microwave radiation - it is believed to be created in about 3000K and then reduce the wavelength due to red shift in expanding universe.

But assume that the field of which vacuum is build is not perfectly transparent - for example that such photons interacts at average once per a million years - that would be already enough for thermalisation process.

So if the field of the vacuum is not perfectly transparent (there is interaction between different interactions), its internal degrees of freedom should have temperature 2.725K.

We observe only electromagnetic degrees of freedom (according to Wikipedia: about 6*10^-5 of total density of universe), but we know well that there is more types of interactions (weak, strong, gravitational ... ).

And their energies probably sum up to the cosmological constant...

 

Returning to the question from topic - general relativity theory says that vacuum is kind of fluid for gravitational waves.

It it already a field - it has some internal structure ... and there is QED, QCD, etc - I just don't believe we can assume that it's a perfect medium.

For fluids this kind of friction - converting macroscopic energy into internal degrees of freedom - is called viscosity (try to make waves on honey).

 

If there is some extremely small viscosity of vacuum (which has nonzero energy density/temperature), multiplying it by millions of light years, it could essentially reduce strength of gravitational waves reaching earth...

They are are already believed to be extremely weak...

 

Do You think it is why LIGO only success is a non-detection?

If not - why is that?