# Graininess of Space-Time found at GEO600?

Gold Member
"So would they be able to detect a holographic projection of grainy space-time? Of the five gravitational wave detectors around the world, Hogan realized that the Anglo-German GEO600 experiment ought to be the most sensitive to what he had in mind. He predicted that if the experiment's beam splitter is buffeted by the quantum convulsions of space-time, this will show up in its measurements (Physical Review D, vol 77, p 104031). "This random jitter would cause noise in the laser light signal," says Hogan.

In June he sent his prediction to the GEO600 team. "Incredibly, I discovered that the experiment was picking up unexpected noise," says Hogan. GEO600's principal investigator Karsten Danzmann of the Max Planck Institute for Gravitational Physics in Potsdam, Germany, and also the University of Hanover, admits that the excess noise, with frequencies of between 300 and 1500 hertz, had been bothering the team for a long time. He replied to Hogan and sent him a plot of the noise. "It looked exactly the same as my prediction," says Hogan. "It was as if the beam splitter had an extra sideways jitter."
Incredibly, the experiment was picking up unexpected noise - as if quantum convulsions were causing an extra sideways jitter

No one - including Hogan - is yet claiming that GEO600 has found evidence that we live in a holographic universe. It is far too soon to say. "There could still be a mundane source of the noise," Hogan admits."

http://www.newscientist.com/article/mg20126911.300-our-world-may-be-a-giant-hologram.html?full=true

## Answers and Replies

Gold Member
Isn't 10e-16m a little bit too big? That's not much smaller than a proton...

aceofspades
This is incredible! I've always believed space-time was composed of discrete points, but to actually be revealed in experiment is truly remarkable!

Gold Member
Well, even if that is true, it would show that the plack scale is extremely sensitive to non metrical parameters neither to G, c and hbar...

Gold Member
the blog
http://scienceblogs.com/catdynamics/2009/01/white_noise.php [Broken]
has some more conversation on this subject. I'd expect others will follow suit.

Really the article was published some months ago.
Received 6 June 2008; revised 23 September 2008; published 30 October 2008
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRVDAQ000078000008087501000001&idtype=cvips&gifs=yes [Broken]

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Science Advisor
If the indeterminacy in the transverse position of null geodesics goes as $\sqrt{L \, l_P}$, shouldn't this have been already detected in applications of optics? Why do gravitational waves have any special significance here?

Coin
"Shantanu" in the comments at Not Even Wrong links this claimed one-page refutation of Hogan's results by an Igor I. Smolyaninov. Smolyaninov argues that Hogan's original calculation was overly simplistic and neglected important effects, which when taken into account result in the expected "white noise" from holographic effects (if holographic effects exist) being orders of magnitude smaller than Hogan calculated them at. This would thus leave the holographic effects orders of magnitude too small to account for the "white noise" being seen at GEO600/LIGO. But GEO600/LIGO are seeing the white noise anyway, I guess?

Smolyaninov leaves a trap door:

In conclusion, Hogan’s method only works if self-focusing
effects are negligible in the effective theory of
holographic geometry.

...but despite Smolyaninov's best efforts to explain it I am not sure what "self-focusing" means in this context...

Science Advisor
Gold Member
Dearly Missed
Giovanni Amelino-Camelia just gave a seminar talk at Perimeter part of which was about GEO600. He is a quantum gravity phenomenologist and had several things to discuss.
http://pirsa.org/09030039/

JustinLevy
So even a "refutation" agrees that combining a holographic universe with quantum mechanics yields testable differences? Regardless of the order of magnitude of the effect, it amazes me that quantum mechanics somehow 'breaks' the analogy between the theory in N dimensions and the theory in N-1 dimensions ... where if it was a classical theory we could never determine for sure what was "real". How does quantum mechanics force a distinction?

JustinLevy
So even a "refutation" agrees that combining a holographic universe with quantum mechanics yields testable differences? Regardless of the order of magnitude of the effect, it amazes me that quantum mechanics somehow 'breaks' the analogy between the theory in N dimensions and the theory in N-1 dimensions ... where if it was a classical theory we could never determine for sure what was "real". How does quantum mechanics force a distinction?
No one has any insights here?
I'd really like to know how quantum mechanics allows a distinction to be made between a holographic N-1 dimensional universe and the analogous N dimensional universe.

Naty1
Justin,
I have the same question you do...

"Noise equals holographic universe" seems a stretch.

Seems like there are two separate issues (a) is space discrete, (b) does such discreteness, if actually confirmed experimentally, imply the holographic nature of our universe.

At http://scienceblogs.com/catdynamics/...hite_noise.php [Broken]

a comment is made

This would imply the universe is actually holographic

Maybe what they mean is that IF space is shown to be discrete in these current experiments, then the earlier and separate theoretical discovery of the apparent discrete holographic nature of a black hole horizon is now also implied by the experimental detection of such discreteness...

That would depend to a significant degree on how firm a theoretical foundation our understanding of black holes is...I'll leave that to the experts.

Seems a bit early to tell especially when the discreteness itself is apparently still uncertain...good speculation, however...

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Coin
Justin, I don't know if this helps but two observations.

So even a "refutation" agrees that combining a holographic universe with quantum mechanics yields testable differences?
First off what I'd note is that "I accept your premises in order to refute your conclusions" is a pretty standard method of arguing this sort of issue, and doesn't necessarily indicate that Smolyaninov himself personally accepts or endorses the holographic principle.

Second, as far as "how quantum mechanics allows a distinction to be made between a holographic N-1 dimensional universe and the analogous N dimensional universe"-- I am not qualified to comment really on either the holographic principle or how it connects mathematically with quantum mechanics, but let me mention something from mathematics that might make the idea seem a little less "weird". There is a thing in math called "Stokes' theorem" (or for a popular special case "Green's theorem") which states that for certain kinds of integrals over a region, you can find the answer just by doing an integral over the boundary of the region. In other words, what happens on the boundary of this region determines completely what is happening inside, for purposes of the integral.

So let's think about holography. Often the holographic principle is described as meaning that our three-dimensional universe is "really" two-dimensional. But as I understand things this isn't actually true. As I understand things what the holographic principle tells us is that for a region of space, its informational content is proportional to the area of the boundary of the region. In other words, holography would in function be like the precedent of stokes theorem, in a holographic universe what happens in a region is determined by what happens at its boundary.

(Note I am not suggesting here that Stokes' Theorem is a justification for, or applies in the case of, holography in physics, I am offering it only as an analogy.)

Gold Member
Seems like there are two separate issues (a) is space discrete, (b) does such discreteness, if actually confirmed experimentally, imply the holographic nature of our universe.

The rhetoric used in the article is a little troubling - equating discreteness with a bunch of little pixels. The more natural approach would seem to be to treat the grain as the limit of QM resolution.

Like talking about a fishing net. Is the net made of its holes or its mesh? The universe is a web of relationships and at a certain grain, stuff slips through. The fish become virtual particles so to speak.

And again, the whole hologram analogy is a little too concrete. The "woo woo" part is the implication that our 3D reality is somehow just a projection from some distant lower-D surface.

Instead, if the Hogan model is correct, it would seem we have been "looking through" a plumper Planck grain that is our actual world, and seeing its shrunken representation against a distant boundary. So instead of sharp points having a blurred projection, a grainier world has been modeled in more sharply drawn points.

Anyway, discreteness in the sense of a Planckscale limit to decoherence, to sieving of QM potential for the little fishes of spacetime events, does already seem "proved".

What would be new would be two tiers to the Planck grain - the larger blur we see close too and the smaller blur seen when measurements are projected to a far boundary.

You would have thought that if this two tier story applied, this extra observer effect, it would have shown up all over the place already though.

ExactlySolved
If the indeterminacy in the transverse position of null geodesics goes as $\sqrt{L \, l_P}$, shouldn't this have been already detected in applications of optics? Why do gravitational waves have any special significance here?

The holographic hypothesis says that spacetime is the boundary of a higher dimensional space, but matter as we know it is confined to the boundary, only gravitons can move through the higher dimensional space. The hypothesis that gravitons can spread out in more directions then photons, gluons, etc can explain why gravity is so much weaker then the other fundamental forces.

If photons, etc were ever to leave the boundary it would show up fairly obviously as an apparent violation of conservation of energy.