
The Contest Between Gravity and Quantum Physics - ThomPete
http://nautil.us/issue/29/scaling/will-quantum-mechanics-swallow-relativity
======
ISL
An important nitpick: the description of the equivalence principle [1] in the
article is incorrect.

A spinning astronaut in a universe without stars _can_ discern whether she or
her partner is spinning, particularly if they work together a little.

The astronaut can change her moment of inertia by curling into a ball or
spreading out: if the astronauts' relative rotation rate changes, she's
rotating. If it doesn't, she's not.

Things are more complicated if they're both rotating, but with mental math and
freshman/high-school physics, they'll be able to sort it out completely.

Even in a universe without any external reference, it's possible to place
limits on your rotation rate. If a centrifugal force is palpable when
extending an appendage, then you're rotating.

[1]
[https://en.wikipedia.org/wiki/Equivalence_principle](https://en.wikipedia.org/wiki/Equivalence_principle)

~~~
danbruc
IANAP, but that argument strikes me as circular - you are presupposing that
inertia works the way we are used to in order to figure out which astronaut is
rotating.

But imagine two astronauts in empty space and let's turn one upside down
stacking them head to head, or helmet to helmet. Now spin one of them around
his vertical axis [1].

The situation looks completely symmetric to both of them. Why should the
relative speed of rotation change if one of them extends or retracts his limbs
but not if the other one does the same thing? What breaks the symmetry of the
situation in absence of any other matter in the universe that could be used as
a preferred frame of reference for defining spinning versus being at rest?

[1] I am not sure if you could do that, but I tend to say no. One of the
astronauts would have to try to spin the other one which should make himself
spin in the opposite direction, conservation of angular momentum. Ad hoc it
seems like you could establish a preferred frame of reference, a frame of zero
angular momentum this way.

~~~
tbabb
One astronaut (the spinning one) will detect a centrifugal force, and the
other will not.

If the spinning astronaut took a yo-yo out of her pocket, she would observe it
accelerates radially away from her. The stationary astronaut's yo-yo would
drift freely with no acceleration.

Edit: See:
[https://en.wikipedia.org/wiki/Bucket_argument](https://en.wikipedia.org/wiki/Bucket_argument)

~~~
danbruc
The yo-yo is no different than extending and retracting limbs, it presupposes
inertia as we know it. Why should the two yo-yos behave differently if the
situation is totally symmetric?

~~~
tbabb
I am not sure what point you are trying to make or what you are asserting is
being "presupposed". What is described above are the results of an experiment.
That's the way physics works; that's what will happen if you do the
experiment. No suppositions needed.

The situation is _not_ totally symmetric: The yo-yo behaves differently for
the spinning astronaut than it does for the stationary one. That is why the
article is wrong; the two situations are measurably different, not equivalent.

~~~
danbruc
You are missing the point. This is not an experimental fact because we have no
empty universe at hand to do it. Two cylinders spinning relative to each other
around a common axis in otherwise empty three-dimensional Euclidean space are
totally symmetric. Or how would you tell which one is spinning? It is as
meaningless to speak about the rotation of one of the cylinders as it is
meaningless to speak about the velocity of a single object.

~~~
tbabb
Ah, I think I see what you are hung up on.

In newtonian mechanics, the equivalence principle says that any two inertial
coordinate frames look "the same". For example, if you have two spaceships
heading toward each other, the picture looks "the same" as if you pretend that
one spaceship is stationary and the other is moving twice as fast, or vice
versa.

This is not true only of _geometric_ relationships, but also for the _laws of
physics_ : If you choose to view things in coordinate frame A, and apply the
same laws of physics, you will still get the same predicted motion as if you
had done the calculation in coordinate frame B. There is no experiment that
can tell you which frame is "moving", because you always get the same answer.

With your example of two rotating bodies, it is true that the picture looks
_geometrically_ "the same" if we pick a different rotating reference frame.
The problem is that the laws of physics _do not_ look "the same" under this
transformation. If I choose a coordinate frame where the spinning astronaut is
stationary, and I try to predict what will happen using the laws of physics of
the original frame, I will get wrong results-- the motion of the yo-yo will
look "unphysical" for a stationary system. So when you change between rotating
and stationary coordinate frames, the laws of physics change too.

That means I can tell whether I am in an "absolutely rotating" coordinate
frame by doing tests to measure the laws of physics (i.e. are there coriolis
and centrifugal forces acting on me?), without any reference to some other,
external frame.

So the fundamental question is under what circumstances the picture looks "the
same". In both cases, geometric relationships are equivalent, but in only one
of the cases are the laws of physics _also_ equivalent. The original article
lied by saying the laws of physics are the same in the rotating case, and that
is simply not true.

Does that make sense?

~~~
danbruc
We are getting closer. You are essentially saying that we can have
geometrically indistinguishable states that are physically different. We can
have an astronaut with a yo-yo hanging down and one with a yo-yo extending
straight away while the world otherwise looks exactly the same to both of
them. Both astronauts have to go through the same change in angular velocity
just in opposite directions to get to the same state as the other one but one
would see the yo-yo dropping down while the other one will see it rising. How
does that make sense? What is so special about the one coordinate frame
without centrifugal forces? With respect to what is the astronaut not
rotating? Note that nothing looks unphysical here, just different but without
any obvious thing that justifies the difference. And I am definitely not
trying to say I am right and you are wrong, it just doesn't look consistent to
me to have centrifugal forces in an empty universe.

~~~
tbabb
The short answer (i.e. without teaching all of Phys 101 in this comment) is
that physics treats linear motion differently from curved motion. Objects that
are moving in a straight line will continue moving in a straight line unless
acted upon by a force. An inertial coordinate frame is by definition one where
this holds true.

We can define "absolute rotation" as rotation relative to that coordinate
frame. We will not have any trouble agreeing on who is rotating and who is
not, because only one coordinate frame will observe perfectly inertial
movement, and all the others will measure some coriolis and centrifugal
forces.

See this video for better intuition:
[https://www.youtube.com/watch?v=_36MiCUS1ro](https://www.youtube.com/watch?v=_36MiCUS1ro)

~~~
danbruc
Introducing a probe body does not really solve the problem, it still does not
explain what picks one frame of reference over the other. Why does the force
free motion of the probe body appear straight for on astronaut but not the
other one? I am pretty sure this entire topic is not nearly as straightforward
as you suggest, otherwise there would be no debate about Machian views and so
on. I will have a look at the papers referenced here [1], they seem to touch a
couple of interesting and related points.

[1] [http://physics.stackexchange.com/questions/1048/what-if-
the-...](http://physics.stackexchange.com/questions/1048/what-if-the-universe-
is-rotating-as-a-whole)

~~~
tbabb
I do not really see what the "problem" is. One can easily define a unique (up
to translation) "rotation-free" coordinate system that all observers will
agree on, without any reference to "fixed stars" or the rest of the universe.

Centrifugal force in a rotating frame is an inevitable consequence of a
universe in which conservation of momentum exists. Conservation of momentum is
an inevitable consequence (by Noether's theorem) of a universe in which
translational symmetry exists (i.e. the laws of motion are not different at
some displacement dx). By all observations, all three of those properties hold
for appropriate choices of frame.

You cannot suppose a universe with both translational symmetry and symmetry
under rotational movement; the two are in conflict. If conservation of
momentum holds in some coordinate frame, then it will not hold in coordinate
frames that are rotating relatively to it.

If you suppose a universe where a "rotating" reference frame is the one where
conservation of momentum holds, then I would be forced to ask "rotating
relative to what, if not an inertial frame?" And the inhabitants of such a
universe would still certainly find it meaningful to label that frame as
"inertial" and define rotation as relative to it.

If you are asking how we tell rotating and non-rotating reference frames
apart, the answer is trivial-- non-rotating reference frames obey inertia and
rotating frames do not; and there is no ambiguity or disagreement between
observers. If you are asking why there exists an inertial reference frame at
all, the answer is that this arises from translational symmetry. If you are
asking why the universe has chosen a _particular_ coordinate frame to be "non-
rotating" over any other, the question seems rather meaningless, because it
does not make sense to define rotation except with respect to an inertial
frame.

~~~
danbruc
Right now I am again convinced that there is no problem, but I have gone back
and forth between it makes sense and it makes no sense a couple of times. I
probably never looked at it with enough rigour and got things overlooked or
confused. But thanks for your effort and I will also read through "Mach's
principle and a relativistic theory of gravitation" which I came across
yesterday during this discussion, maybe I will finally reach a state of
lasting non-confusion.

------
nkrisc
I thoroughly enjoy reading and learning about this sort of physics, but I
still very much lack the understanding to discuss it.

So I'll just say that pondering the nature of the universe, that I am just a
region of low entropy, that anything I do will have no discernible effect on
the universe really makes any problems I have seem like small potatoes!

~~~
ThomPete
The completely over-simplistic uneducated TL;DR until someone way more
knowledgeable hopefully step in and correct my mistakes.

Classically Physics - Everything is local. Quantum Physics - Everything is
non-local

Both theories are proven to the extent anything can be proven both
mathematically and experimentally.

Problem is that they contradict each other and we have no way to combine them
in ways that make sense philosophically. All attempts have some sort of
missing element.

Article propose yet another attempt to combine them. My uneducated guess is
that somethings not going to be like we thought and we are back to two
incompatible models.

~~~
norea-armozel
My question is what's going to be 'lost' by the combination of GR/SR with QM?
I'm assuming that gravitational singularities will have to go or be re-
rationalized. So far, I can't see there's much chance of that happening as
long as blackholes retain their ever consistent behavior as we've observed so
far.

~~~
empath75
Space-time becomes an emergent phenomena, rather than fundamental.

~~~
cygx
Depends on your point of view: Arguably, the 'stringy' view of space-time is
more in line with the bimetric theories of days gone by, with the metric being
a field on top of a fixed background.

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peter303
A similar experiment is being used to ask whether we live inside a digital
computer simulation, AKA The Matrix. Both situations would be "pixelated". But
one would be due to the fundamental fabric of space-time and the other to The
Architect running the simulation. I wonder if there are experiments to
distinguish the two.

------
selimthegrim
Smolin lost all credibility with me after I read this blogpost by Jacques
Distler

[https://golem.ph.utexas.edu/~distler/blog/archives/002645.ht...](https://golem.ph.utexas.edu/~distler/blog/archives/002645.html#c044480)
(this is a comment, scroll up for the post)

