
100 Years After Einstein's Relativity, We're Still Just Figuring Gravity Out - AliCollins
http://motherboard.vice.com/read/100-years-after-einsteins-relativity-were-still-just-figuring-gravity-out
======
vardump
Disclaimer: I am not a physicist.

Maybe gravity will turn out to be just a computational artefact. The more
energy and states in a volume, the "slower" progression of the [local] system.
An effect like this could appear as curved space at energy concentrations.
Perhaps GR could be derived from something like this?

~~~
danbruc
Ideas like that are often mentioned here when related topics come up but they
are almost surly totally wrong. Computing means doing something in several
steps and that is - as far as we can tell - absolutely not what the universe
is doing. Even if the universe did something like that it would just mean that
there is another layer below evolving in a more fundamental way.

Many people seem pretty confused when physicist talk about things like the
black hole information paradox. We think that all physical processes are
reversible which means that distinct physical states will always remain
distinct. If state A and state B could or would evolve into the same state C
this assumption would no longer hold because you could no longer tell whether
you arrived at state C from state A or B. The laws of physics would not be
time reversible.

And this is the sense in which physicist talk about information, the
description of the state space. If you throw something into a black hole the
only things that remain visible are things like the mass, the charge or the
(angular) momentum but you just lost all the details about the thing you threw
into the black hole which is a problem for our understanding of physics.

~~~
Filligree
> And this is the sense in which physicist talk about information, the
> description of the state space. If you throw something into a black hole the
> only things that remain visible are things like the mass, the charge or the
> (angular) momentum but you just lost all the details about the thing you
> threw into the black hole which is a problem for our understanding of
> physics.

Why is that a problem? The information could still exist, it'd just be
inaccessible...

This seems no different to me than a spaceship going beyond the Hubble
horizon.

~~~
danbruc
It is a problem in either case - if it really disappears then time
reversibility, a real corner stone of physics, is violated, if it does not
disappear the problem is that we don't understand (well) where it goes. As far
as I know the favourite theory is that the information is contained in the
Hawking radiation and I believe to remember that there was a paper not too
long ago that showed how to recover one (or a few) (qu)bit(s) from Hawking
radiation in a special case. Because time reversibility is such a deep concept
it seems likely that we just don't understand well where the information goes,
I doubt that many physicists believe the information really disappears.

~~~
AnimalMuppet
We believe that certain symmetries apply to the laws of physics. For example,
we believe that the laws of physics are independent of the observer's
position. I mean, look (visualize me stepping sideways here). So we can
demonstrate that one pretty easily. And they're independent of rotation.
Again, look (visualize me turning 90 degrees here). And exchanging left and
right. I mean, look... (visualize me pausing with a confused look on my face
as I try to figure out how to exchange left and right).

I can't actually do that experiment. And it turns out, when we got deep
enough, we found out that things actually behaved differently (parity non-
conservation).

When it comes to time reversal, we can't actually do the experiment, either,
so we don't really know whether physics behaves the same when time is
reversed. We expect, philosophically, that it will, but we don't _know_. But
it seems to me, given how relativity mixes time and space, if physics is not
the same with left and right reversed, then it can't be the same with time
reversed either.

~~~
danbruc
Saying we consider time reversibility true for more or less philosophical
reasons seems a bit of an understatement to me. Conserved quantities prevent
states differing in at least one conserved quantity from ever evolving into
the same state, i.e. conserved quantities partition the phase space and states
cannot cross those partition boundaries. This also has to hold for all
isolated subsystems. And conserved quantities arise from symmetries like
homogeneity and isotropy of space which are pretty solid empirical and
experimental facts. It's all interlinked and I imagine it would not be to easy
to eliminate time reversibility from the laws of physics without causing some
odd consequences.

------
timeu
PBS Spacetime has some really nice videos about Gravity (Newtonian vs GR)
[https://www.youtube.com/watch?v=NblR01hHK6U](https://www.youtube.com/watch?v=NblR01hHK6U)

------
AnimalMuppet
Warning: speculation follows.

If we understand correctly, the universe is about 13.7 billion years old. I
recall reading once that there is enough mass (matter plus dark matter) that
the Swarzchild Radius of that much mass is approximately 13.7 billion light-
years.

What if we're _inside_ a black hole? How would that affect how we interpret
what we see at large distances? (For example, does that explain the Hubble
redshift? Does it explain dark energy?)

And, if it's true, then what is a black hole? Can you have a black hole inside
the Swarzchild radius of another black hole?

------
bwooceli
Honest question. If the concept of dark matter flows from our current
understanding of gravity, and gravity is so admittedly poorly understood, how
confident are we that dark matter is "a thing"? I know an intuitive
sensibility is not the best foundation for an understanding of The Universe,
but every time I read about dark matter, I get a very "Aether"-ish vibe. I
just imagine comments section in the 2070 version of HN getting a good natured
laugh about dark matter.

~~~
chunky1994
> _gravity is so admittedly poorly understood_

Gravity as a force/phenomena in space on its own is actually very well
understood. In fact it's possibly one of the most well understood theories we
have in physics because of general relativity. However, at a particle scale we
lack the understanding of how gravity would work.

Dark matter is the collective name for whatever "stuff" exists in the universe
that we predict it should have, however because it is noninteractive in any
way other than exerting a gravitational pull (it accounts for a lot of
astrophysical observations that otherwise are bewildering given what we _see_
in the universe). We know it should exist, we don't know what it is and can
barely understand its current properties. So indeed, it is quite possible that
it might be Aether-ish if we end up finding something else that explains our
current observations or a drastic overhaul of how gravity works at large
scales (the latter being very unlikely because of how well GR has stood up to
experimentation so far).

------
kyberias
Ironically the tattoo in the image is the Newton's equation.

~~~
dogma1138
Which still is used and works even on cosmological scales.

Relativity is only really useful in certain scenarios, people still calculate
the motions of stars and galaxies with Newtonian physics:)

~~~
brianberns
Yes, but Newton's equation isn't relevant to the issues discussed in the
article.

