Re: LQG vs. Minkowsky Spacetime (Was: The string theory dominance)

Marissa Kay

>From sci.physics.research, November 4, 2003:
http://groups.google.com/group/sci.p...1?dmode=source

Lubos Motl wrote:
> > on the basis of which, in turn, in conjunction with the laws of
> > thermodynamics, you can actually DERIVE Einstein's equations
> > Jacobsen, gr-qc/9504004
> > th4s also showing their equivalence, under the laws of
> > thermodynamics.
>
> But this is a reformulation of the laws of thermodynamics within the
> framework of general relativity. I totally missed what your post has to do
> with the topic of the discussion.

You also totally missed what the paper you're referring to said and
what it's actually about.

Jacobsen's paper is not a reformulation of the laws of thermodynamics
within the framework of GR, it's a *derivation* of Einstein's equations
and the Bekenstein Bound's proportionality constant from the
combination of (the 1st and 2nd law of thermodynamics) + (the existence
of some entropy bound proportional to surface area over certain types
of causal horizons). That's the very opposite of what you described.

It's worth pointing out that though Jacobson didn't specifically
advertise it as such, the 1/4 Planck area unit was also shown to be the
correct unit. That is, he understated his result: the Bekenstein Bound,
itself, emerges from the paper as output, not input.

As stressed by Jacobson, this result undercuts *all* approaches to the
unification of quantum theory and general relativity that posit a
fundamental field for gravitational interaction, whether it be LQG,
String Theory or otherwise.

To paraphrase him, there is no more need for gravity to be posed as a
fundamental interaction or the graviton as a fundamental particle than
there is for phonons to be regarded as quanta of a fundamental field.
Indirectly, he also brought up the notion of gravitons being "vacuum
phonons", though he never quite got to the point of uttering those two
words together in a single phrase, and related the notion of a
breakdown mode analogous to that seen for phonons in solid state
physics.

In particular for String theory, this means the occurrence of a
fundametal spin-2 field goes from being a kill-two-birds-with-one-stone
double positive back to being the
you-missed-the-bird-and-hit-the-pedestrian double negative it was
originally regarded as. With the bottom taken out by having the need
for a fundamental spin-2 field removed, the extra spin-2 field now
sticks out like a sore thumb.

But that should have been known all along. As anyone familiar with
strong gravity can tell you, just because something has spin-2 doesn't
mean it has anything to do with gravity.

The original post you were replying to, clearly, undercut and obviated
the prior discussion, thus making the appropriate question that of what
the rest of the discussion had to do with it, since this extra
contribution to the thread rendered moot the points contained therein
relating to whether LQG or string theory provides the more proper
foundation of any approach for reconciling quantum theory and general
relativity.