

Loop Quantum Gravity predicts discrete, not continuous spacetime topology - Allocator2008
http://www.sciam.com/article.cfm?id=big-bang-or-big-bounce

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river_styx
So where are we now with physics, anyway? Converging on some kind of
math/philosophy hybrid? Seems like as time goes by, these theories and
constructs become more elaborate and abstract, which makes it ever less likely
that we'll have the capability to test their predictions.

~~~
jerf
Check out this Google Talk, "Why Physicists Need the Large Hadron Collider":
<http://www.youtube.com/watch?v=M8xwSfuY8xA>

The problem is that particle physics has a model that perfectly predicts every
experiment we've done. However, it is aesthetically unappealing, and even more
damning, we _know_ it can't scale to explain everything in the universe.
However, until one of our experiments actually does something unexpected,
we're sort of twiddling our fingers.

In the video, the guy mentions that the worst case scenario for the LHC is to
precisely confirm the standard model with no surprises. One person asks him
something about what he'd do in that case (the question was muffled), and he
replied that his reaction to this conundrum was to develop a sideline in
quantum computing, since particle physics is really going nowhere until
someone runs an experiment that contradicts the Standard Model.

In the meantime, a lot of other physicists spend their time spinning various
weird, mutually-contradictory theories as a way to try to find an experiment
they can run that would actually generate usable data. The continuous-vs-
discrete reality question is scheduled to be investigated by a NASA satellite
experiment at some point in the indefinite future, but who knows when it'll
actually happen. In the meantime, we've got a lot of smart people stalled on
having no (useful) data.

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ars
More proof that we live in the matrix. Seriously <http://www.simulation-
argument.com/>

Also, the seems to say that black holes are impossible.

~~~
hhm
That's a problem for them I guess... we have empirical evidence of black
holes. Maybe what's impossible is singularities? Some physicist to correct me?

~~~
ars
A black hole is a singularity.

But we don't have empirical evidence of black holes - we have circumstantial
evidence of very dense blobs - they could be neutron stars or other things.

The reason they assume they are black holes is that if a neutron star was that
big it would be a black hole - but if loop quantum gravity is right, then it
would become a degenerate star instead and not a black hole.

Getting rid of black holes would help a lot BTW because they contradict a lot
of well established theories, plus they can not be reconciled with relativity.

~~~
Zaak
The defining feature of a black hole is the event horizon, not the
singularity.

Here is the current state of evidence for black holes, from
<http://chandra.harvard.edu/chronicle/0307/onbh/>

"Yet although thousands of bursts have been observed from neutron stars, not
one has been observed from a black hole candidate. The lack of X-ray bursts
from black hole candidates is a strong argument against the existence of
gravastars and other exotic alternatives to black holes."

What "well established theories" do black holes contradict?

Black holes are predicted by relativity. How could they not be reconciled with
it? <http://en.wikipedia.org/wiki/Schwarzschild_metric>

~~~
ars
An event horizon sort of implies a singularity doesn't it? Otherwise what is
it the horizon of?

The main theoretical problem with black holes is the information loss due to
the no hair theorem. Look up "Black hole unitarity".

Another problem is that hawking radiation can send out anything, with no
regard to the quantum numbers of what when in - so you put in a proton and
return photons, which normally is impossible (conservation of the
baryon/lepton/parity number).

No one really knows it that will really happen though, it's all theories and
ideas.

But a black hole is not so much a prediction of relativity as it's the point
where relativity fails. In that sense it was predicted by it, but relativity
can not explain what happens in a black hole.

~~~
hugh
_An event horizon sort of implies a singularity doesn't it? Otherwise what is
it the horizon of?_

Any clump of mass smaller than its own Schwarzchild radius will have an event
horizon, but that doesn't _necessarily_ mean that all that mass needs to wind
up as a point. Under what we currently think we know about gravity a clump
compressed to that radius will wind up as a singularity, but it's _possible_
that we might have something wrong about gravity which would result in black
holes with event horizons but no singularities -- instead you'd just have some
high-but-not-infinite density mass in the middle being prevented from
collapsing by... something we currently don't understand.

~~~
ars
I'm pretty sure that the singularity is not the fact that it's a point, but
that the gravity is so high that it wraps in on itself. i.e. that spacetime
has a hole in it.

Because once gravity is that high going any higher has no meaning by any
physics currently known. So by that definition once you crossed the event
horizon anything inside has no meaning - and that is the singularity (i.e. the
calculations of relativity have an infinity there, which is the singularity,
not the mass in a point).

Also I did sort of assume that if matter can not collapse to a point a black
hole is impossible. It does not appear that a quark star could be a black
hole. And nothing is known about the other candidates listed on wikipedia
(Preon star and Q star), but there sort of is the assumption that they could
not make a black hole.

The article referenced for Q stars says 1.5 Schwarzschild radius, so it looks
like to make a black hole you need a point mass.

~~~
hhm
_I'm pretty sure that the singularity is not the fact that it's a point, but
that the gravity is so high that it wraps in on itself. i.e. that spacetime
has a hole in it._

I know Wikipedia isn't a good source for physics, but:

"While in a non-rotating black hole the singularity occurs at a single point
in the model coordinates, called a "point singularity", in a rotating black
hole, also known as a Kerr black hole, the singularity occurs on a ring (a
circular line), defined as a "ring singularity". Such a singularity may also
theoretically become a wormhole."

<http://en.wikipedia.org/wiki/Gravitational_singularity>

It seems a singularity is either a point or a line, but never a sphere for
example.

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rkowalick
I find it interesting that the attempted theories to explain the makeup of
matter seem quite ridiculous and almost humorous now. One would hope the
theories about quantum gravity that are wrong will seem equally ridiculous in
the future.

~~~
jimbokun
I would say that the accepted theories seem quite ridiculous and humorous.

------
kirse
Does anyone else find this search to explain the creation of the universe
hilariously futile?

We will never find the answer in our lifetime. We can't even concretely
explain the process by which warm water sometimes freezes faster than cold
water (Mpemba effect).

Things just get more complex and the human capacity for understanding is so
limited in scope that when I read "The Big Bang may now be the Big Bounce" I
have to laugh at our utter lack of intelligence.

Of course, this is not to denounce the scientific process or suggest that we
scrap it altogether, maybe it should just serve as a good dose of humility to
scientists who love to make fun of those who choose to follow a religion.

~~~
schtog
Scientists observe reality and change the theory when the experimetns
contradict it.

Religious people don't observe reality, they have already decided and when
evidence suggests they are wrong they just don't take them into account(or
worse, kills whoever suggested it). So no, science and religion are not by any
means equal or comparable.

And how do you judge limited in scope of understanding? Compared to what? Do
you know fo another world where they figured out how their universe worked
much quicker and that universe was as complex as ours?

