On 9/15/2011 9:43 PM, Jesse Mazer wrote:
On Thu, Sep 15, 2011 at 9:05 PM, Stephen P. King
<[email protected] <mailto:[email protected]>> wrote:
On 9/15/2011 6:59 PM, Jesse Mazer wrote:
On Thu, Sep 15, 2011 at 6:30 PM, Stephen P. King
<[email protected] <mailto:[email protected]>> wrote:
On 9/15/2011 5:17 PM, meekerdb wrote:
On 9/15/2011 1:42 PM, Stephen P. King wrote:
On 9/14/2011 9:49 PM, meekerdb wrote:
snip
On the contrary, the singularity is in the
description. Which is why no physicist believes
the description (General Relativity) is valid.
Brent
Ummm, really? Let me see if I understand this
claim, no physicist believes that General Relativity
(GR) is valid or no physicists believe that there are
solutions to the field equations of GR that are
invalid? What about Roger Penrose and Stephen
Hawking? They wrote the paper that showed a proof
that the field equations of GR generate singularities
for relatively innocuous and plausible conditions and
yet they are still great proponents of GR. So... what
is the source of your opinion re "no physicist
believes ..."?
The importance of their paper was that it showed GR
predicted a singularity under very general conditions.
Before that,it had been widely assumed that the
singularity prediction was just an artifact of assuming
perfectly spherical 3-geometry with no rotation. Of
course I can't really vouch for what every physicist ever
believed. But I was in graduate school at the time
studying GR and nobody I knew, including Penrose whom I
met and my fellow students, drew any conclusion except
that GR breaks down and does not apply in those
circumstances. And no one was surprised by this. There
was already an active search for a quantum theory of
gravity, which it was assumed would avoid singularities.
Brent
Hi Brent,
AH! I understand and agree with you then. But we have to
deal with the observational evidence that space-time is
smooth down below scales that most forms of quantum gravity
theories, loop quantum gravity for example, predict a
granularity or foam or some other form of discontinuity.
What observational evidence are you referring to? There was
recently a paper by Philippe Laurent that was widely misreported
in the media as giving evidence that ruled out "granularity" at
the planck scale (see
http://www.esa.int/esaCP/SEM5B34TBPG_index_0.html for example),
but in fact if you look at the actual paper (at
http://arxiv.org/abs/1106.1068 ) it was specifically about ruling
out granular theories that predicted violations of the
Lorentz-symmetry of relativity. Most forms of string theory and
loop quantum gravity actually assume that Lorentz-symmetry is
*not* violated (see
http://books.google.com/books?id=dWmZb3uQWbQC&lpg=PA320&dq=supersymmetric%20string%20theory%20lorentz%20invariant&pg=PA320#v=onepage&q=supersymmetric%20string%20theory%20lorentz%20invariant&f=false
<http://books.google.com/books?id=dWmZb3uQWbQC&lpg=PA320&dq=supersymmetric%20string%20theory%20lorentz%20invariant&pg=PA320#v=onepage&q=supersymmetric%20string%20theory%20lorentz%20invariant&f=false>
and http://arxiv.org/abs/1012.1739 for instance), so the new
findings wouldn't be a problem for them.
For a number of physical arguments that general relativity is
likely to break down at the Planck scale, see
http://arxiv.org/abs/1001.1205
Jesse
--
See http://prl.aps.org/abstract/PRL/v97/i14/e140401
More soon..
The abstract says that the observations "constrain certain types of
relativity violations", which suggests they are again talking about
violations of Lorentz symmetry (the basic symmetry of relativity). The
preprint of the same paper at http://arxiv.org/abs/hep-ph/0607084
confirms this, saying on p. 2 that "A promising candidate effect is
relativity violations, which are associated with the breaking of
Lorentz symmetry, the invariance of the laws of physics under
rotations and boosts". So, the observations wouldn't rule out versions
of string theory and loop quantum gravity which preserve Lorentz
symmetry, which as I said is true of the most commonly discussed versions.
Jesse
--
Hi Jesse,
Any physically significant boost would act to alter the scale of
"Plankian effects", that is what general covariance basically tosses out
any physically real notion of space-time points what ever their size
might be. If our laws are physics are really invariant with respect to
coordinate transformations then those include boosts as well. So string
theory has its own set of problems with GR. Additionally, string and
brane theories *require* a fixed and flat space-time to act as a base
space for the fibration of compatified torii, orbifolds or what have
you. The same is true for all quantum field theories that depend of the
fiber bundle formulation.
You might wish to check the date of publication of the various
papers that we are throwing around. The ones you referenced are from
2006... Here is one published this year: http://arxiv.org/abs/1106.6053
A 'money quote' from page 8 of that paper:
"The observation of the highest energy gamma rays up to 31 GeV from
a distant gamma ray burst GRB 090510 also shows that there are no ob-
servable quantum effects of spacetime down to the Planck scale [15]. The
result therefore rules out those quantum gravity theories in which the speed
of light varies linearly with photon energy. There is no evidence of
violation
of Lorentz invariance down to the Planck length. Spacetime is continuous
and special relativity is right. The greatest mystery is why spacetime
is man-
ifestly so smooth and classical all the way to the smallest conceivable
level."
Here is a 2009 article that discusses GRB 090510:
http://www.nasa.gov/mission_pages/GLAST/news/first_year.html
Onward!
Stephen
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