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The single most interesting feature of attempts to replace dark matter with a modification of gravity is Milgrom’s discovery that in a wide variety of galaxies, there’s a unique place where ordinary gravity plus ordinary matter stops working: when the acceleration due to gravity (as Newton would have calculated it) drops below a fixed value a0 ≈ 10−10 m/s2. This is the basis of MOND, but the pattern itself is arguably more interesting than any current attempt to account for it. Very possibly it can be explained by the complicated dynamics of baryons and dark matter in galaxies — but in any event it should be explained somehow.

The existence of this feature gives a strong motivation for testing gravity in the regime of very tiny accelerations. Note that this isn’t even a statement that makes sense in general relativity; particles move on geodesics, and the “acceleration due to gravity” is always exactly zero. So implicitly we’re imagining some global inertial frame with respect to which such acceleration can be measured. That’s a job for a future theory to make sense of; for the moment we’re forgetting that we know GR and thinking like Newton would have.

So now Hernandez, Jimenez, and Allen have tried to test gravity in this weak-acceleration regime — and they claim it fails!

To read the rest of the article, click here.

"This is trivially explained in my paper in the Penrose-Hameroff issue of the Journal of Cosmology

http://journalofcosmology.com/SarfattiConsciousness.pdf


a0 ≈ 10−10 m/s2 ~ c^2(Cosmological Constant)^1/2

This is the smallest acceleration possible if we are hologram images projected from a 2D hologram screen which in this case is our future de Sitter event horizon. It is a kind of round-off error in the hologram cosmic computer.

"The existence of this feature gives a strong motivation for testing gravity in the regime of very tiny accelerations. Note that this isn’t even a statement that makes sense in general relativity; particles move on geodesics, and the “acceleration due to gravity” is always exactly zero. So implicitly we’re imagining some global inertial frame with respect to which such acceleration can be measured. That’s a job for a future theory to make sense of; for the moment we’re forgetting that we know GR and thinking like Newton would have."

The de Sitter metric in the observer-dependent static LNIF representation has

g00 = 1 - /\r^2

where we are at r = 0 and /\^-1/2 is the scale of our future event horizon.

a static LNIF has acceleration

g(r) = c^2/\^1/2(1 - /\r^2)^-1/2

the Unruh temperature is proportional to g(r).

so we have

g(0) = c^2/\^1/2

- Jack Sarfatti

See also

Thursday, April 13, 2006

Physics: Nagging Little Discrepancies

An interesting paper was posted yesterday on arXiv, “Is the physics within the Solar system really understood?” which summarises the following apparent anomalies for which there are varying degrees of experimental evidence:
Dark matter
Dark energy
The Pioneer anomaly
Excess velocity increase of spacecraft which fly-by Earth
Apparent secular increase in the astronomical unit (about 10 metres/century)
Quadrupole and octupole power in the cosmic background radiation correlated and aligned with the ecliptic
I was unaware of 4, and 5 and hadn't heard much about 6 recently although it was rumoured something interesting might be in the three year WMAP data. This paper does not cite that data release.
Wouldn't be interesting if all of these effects could be explained by the choice of too large a numerical integration step in a simulated universe? Note that items 3 and 4 both involve small apparent discrepancies in the motion of man-made objects which move more rapidly than most natural bodies on such geodesics—the creator (or, perhaps I should write, more reverently, “Creator”) of a simulation who chose a time step suitable for planetary motion (for example, the 1/100th day integration step I used in the Quarter Million Year Canon computation) might just find themselves caught out by pesky in-simulation sentients who made precision measurements of high-speed gravitational assist maneuvers or objects on hyperbolic trajectories.

Here are a few questions for physicists and numerical integration experts. Is the choice of an insufficiently fine integration step likely to produce discrepancies of the sign and magnitude observed in the Pioneer, fly-by, and astronomical unit anomalies? How large an integration step would be required to produce the deviations from general relativity in each case? Are they all the same, or related in a simple way? Are there observations of solar system bodies (for example, sun-grazing comets on hyperbolic escape trajectories or close asteroidal encounters with planets) which could exclude (or provide evidence for) this hypothesis?


Posted at April 13, 2006 14:30
http://www.fourmilab.ch/fourmilog/archives/2006-04/000683.html
Category: MyBlog

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