To introduce more physical ideas into this subject, in particular magic SUGRA I have been musing over this for a while. I think it is important to keep in contact with physical ideas. I do at the end here consider the prospect that this connects with J3 and E6. This has to do with the general uncertainty principle and the tension on the brane of our universe. If the tension is zero then the fluctuations in the spacetime are tiny. However, if we are observing spacetime under a high magnification of a Heisenberg microscope then we are imposing high transverse momenta and the tension is huge. Under those conditions the fluctuations are quite large.
We might think of the situation like this. Suppose the uncertainty principle is
О"pО"x = Д§ + F(x)О"xО"t.
This is a version of the uncertainty principle modified for string theory
О"pО"x = Д§ + ОјО"x^2,
where ОјО"x = F(x)О"x/c = F(x)О"t. The constant Ој has been replaced with a force that depends upon distance or a region in space/spacetime. I have though a term F(x) that is written to indicate force. This leads to the spread in the width of the uncertainty
О"x = О"pc/2F(x) +/- c sqrt{О"p^2 - 4Д§F(x)/c}/2F(x)
From О"pО"x = Д§ + F(x)О"xО"t we can divide by О"t with О"x/О"t ~ c to get
F(x)О"x = О"pc - Д§/О"x
where the LHS is the work equivalent due to this force. We may write this according to О"W = О"pc - Д§/О"x, so that the work is seen to increase with О"p and decrease with О"x. This may also be expressed to connect the momentum-position uncertainty with a "quantum work-time uncertainty" so that
О"WО"t = О"pО"x - Д§.
For F = 0 then О"pО"x = Д§ and the LHS is zero. This illustrates how О"WО"t is associated with an increase in uncertainty.
If you think this looks similar to the entropy force of gravity results you are not too far off! The entropy force of gravity concerns the transverse displacement of a holographic screen. Here we are concerned with the stretching or longitudinal motion along the screen. The entropy force of gravity F = T∇S determines work as
О"W = FО"x = T∇SО"x = TО"S.
We employ E = (1/2)NkT which equals Mc^2, We then see that temperature is T = Mc^2/Nk. This is inserted into the equation for the entropy force
FО"x = (2Mc^2/Nk)О"S
The motion of the holographic screen by a distance О"x = Д§/mc results in
F = (Mmc^3/Nk Д§)О"S
and we use N = A/4L_p^2 = ПЂR^2/(GД§/c^3). We then have that
F = 2GMm/(kR^2)О"S
Where we then assign the unit of entropy О"S = k/2 to get the result.
The NS5-brane has spacetime of four dimensions on a D-brane sandwiched between two other branes embedded in 5 dimensions. This sandwich is contained in an anti-de Sitter spacetime (AdS_5) of five dimensions. In four dimensions of spacetime a static gravity field in 3-dimensional space has the property that a two-dimensional surface bounding a mass will have the same number of line of flux through it independent of the radial size of the surface. Thus the force F multiplied by the surface area 4ПЂR^2 is a constant and equal to the gravity constant in 4-dimensions multiplied by the masses
GMm = 4ПЂFR^2,
Which leads to Newton's law of gravity
F = GMm/R^2
The 4ПЂ has been "absorbed into the definition of G. In five dimensions the surface bounding the mass is not two dimensional but three dimensional, which gives us the law
G_5Mm = 4ПЂFR^3
And so the force law in five dimensions is
F = G_5Mm/R^3.
The G_5 is the gravitational constant in five dimensions, and it reflects how the branes are "warped" in the embedding space by interacting with each other. G_5 in these units is Nm^3/kg^2 (N = Newton, m = meter, kg = kilogram). The relationship with the standard gravitational constant G with units Nm^2/kg^2 is that G_5 = GL, where L is a unit of length.
The brane tension, or equivalently the tension of strings attaching them together, is T = kL, for k = constant. The string length is L = 1/sqrt{R}, for R the curvature of the brane which we write as L = L_0^2/s, for L_0 the zero tension string with F(x) = 0 and s the "warp length," which is something that Randall worked out.
G_4 = (4ПЂ/3)О»G_5^2 = G_5/s
It is kind of neat. The big question I have is whether or not our four dimensional spacetime is not in fact on the conformal boundary of the AdS_5. This does go somewhat into the idea about changing gravity, which appeals to the physics of our brane in or on the AdS_5 and how gravity is related to the brane tension.
We also see that the tension depends upon the length L and for the tension at a minimum the Newton gravitational constant G_4 is small. For T or О» large the gravitational constant is huge. We then have a renormalization group flow for the gravitational constant. Also in addition G_4 = (Planck area)c^3/Д§, and for the tension large this is larger and the area of a Planck unit is larger. This is due to an impulse F(x)О"x = О"pc - Д§/О"x that "stretches" the horizon or Planck unit of area. This force is what adjusts the warp length.
Quantum gravity involves noncommutative geometry. In particular there are coordinates of spacetime that are not commensurate or which have an uncertainty relationship. This can be seen with the argument Bohr gave at the Solovay conference in 1930. Einstein argued that a box containing a photon could have a door opened and the mass of the box plus photon measured. This would get around the uncertainty principle. However, the uncertainty in momentum is О"p = TgО"m, for T an interval of time, g the acceleration of gravity and О"m the uncertainty in mass. The spread in distance is О"x = cО"t, and the product in О"pО"x = TgcО"mО"t = (Tg/c)О"mc^2О"t. With О"pО"x = Д§ then О"mc^2О"t = Д§ and the uncertainty in time is О"t = (Tg/c^2)О"x.
We may now consider this with the uncertainty in time and radial distance with the metric coefficients. We have then that t' = t sqrt{1 - 2GM/r'c^2}, and for О"t = t" - t' measured between two small radial distances in a Schwarzschild metric small enough then
О"t = t(sqrt{1 - 2GM/r"c^2} - sqrt{1 - 2GM/r'c^2}) ~ t(GM/c^2)(1/r" - 1/r').
For r" - r' = О"r then
О"t ~ -tGMО"r/(rc)^2,
We now consider the spread in the potential V = -GM/r, О"V = GMО"r/r^2 - GО"M/r. This leads to
О"t ~ (t/c^2)(GО"M/r + О"V)
We now let cО"t = О"x, for x a distance related to the radius. The time t is equated to GM/rc, as above. In addition the fluctuation in the radius and mass are for the Schwarzshild radius equal, so the uncertainty in the potential GО"M/r =~ О"V. This gives us
О"x ~ 2GО"M(t/rc) = 2GО"EV(t/rc^3).
A multiplication by О"t is
О"xО"t ~ 2GО"EО"t(t/rc^3) = 2GД§(t/rc^3).
The t and r are equated and this leads to
О"rО"t = 2GД§/c^4 = 8.5x10^{-75}ms.
This is a Planck unit of area divided by c. From О"pО"x = Д§ + F(x)О"xО"t it is easy to see that
О"pО"x = Д§ + (GД§/c^4)F(x)
= Д§(1 + GF(x)/c^4)
We then see that the uncertainty in the spacetime variables is due to this force. This force is what is responsible for the brane tension. If this tension is zero or minimal then the fluctuations on our brane are minimal. Measurements across huge distances are using a telescope to measure across vast distances. An extreme high energy experiment imparts huge transverse momenta/energy and increases the brane tension.
The entropy force of gravity involves motion of a holographic screen along a direction perpendicular to the screen. Here the concern is with the dynamics transverse to the motion derived by the entropy force of gravity. This means that we have a general problem with fluid dynamics. This seems to suggest that maybe the general form of fluid dynamics is given by the 3x3 matrix, maybe diagonalized by the E6.
LC