
What Does It Mean That Quantum Gravity Has No Symmetry? - espeed
https://www.forbes.com/sites/startswithabang/2019/06/29/ask-ethan-what-does-it-mean-that-quantum-gravity-has-no-symmetry/
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danbruc
For those that did not read to the end of the article, there is an important
update due to feedback from one of the authors. The result only applies to
global symmetries but the article talks mostly about gauge symmetries - which
are not really symmetries to begin with. More specifically in the standard
model it only applies to the difference of the lepton and baryon numbers.

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electrograv
Side note / question to real physicists: I am curious, for example, why
general relativity does not break conservation of momentum in 3D, given that
it seems like warped space time (to my lay perspective) violates 3D
translational symmetry. For example, how do wormholes or the Alcubierre drive
not violate conservation of momentum?

Edit: Removed my own confused interpretation of the article’s confused
interpretation and correction footnote.

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auntienomen
Energy and momentum are in general _not_ conserved in general relativity. This
is a well-known feature of the theory, not a mistake made by the article's
author.

In field theories like GR, energy is generator of time translations. It's
conserved if time translations leave the system invariant, but this only
occurs in very special cases in general relativity (static spacetimes). So in
general, in Gr, energy is not conserved. Same thing goes for momentum, which
generates spatial translations.

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electrograv
Thank you!

Another question on a tangentially related topic: Could you perhaps shed more
light on why there so much pessimistic skepticism towards any claims of
working reaction-less propulsion drives (violating conservation of momentum),
for example?

I understand the experience-based reasons for skepticism (since there have
been many claims that don’t replicate), but why do there also seem to be so
many physicists claiming with 100% confidence that no such thing could ever
work, according to current physics theory?

I’m referring for example to skepticism about things like “EMDrive” from a
theoretical perspective (e.g. the posts from physicists here:
[https://www.reddit.com/r/EmDrive/](https://www.reddit.com/r/EmDrive/)) as
opposed to focusing on experimental failures to replicate.

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auntienomen
I'm not an expert in these things. But my recollection is that the solutions
of general relativity that behave like warp drives all violate various energy
conditions and hence are only possible in the presence of what physicists
politely refer to as "exotic matter". The Alcubierre drive, for example,
requires matter with a negative energy density. We're reasonably certain that
the class of quantum field theories we use to describe matter (QED, QCD, the
Standard Model itself,..) are not compatible with these requirements.

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neonate
[http://archive.is/wo3JL](http://archive.is/wo3JL)

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rawoke083600
When you fall... you might not land on both feet at the same time...

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scotty79
I'm very happy that quantum gravity doesn't work.

Quantum describes matter and gravity as we know from GR is just shape of the
spacetime. It would be horrible if same math ruled two so different things.

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Retra
Math is abstract, which means it 'rules' many different things by its very
nature. You can describe the geometry of space-time with vectors just as well
as you can describe quantum state transitions. It's hard to make sense of what
your objection could be here; the world works the way it does, and it is not
good or bad whether we can describe it using unified mathematical frameworks
or not.

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scotty79
> You can describe the geometry of space-time with vectors just as well as you
> can describe quantum state transitions.

I don't think that's true. Quantum description of gravity that has any
aspirations to describe real world doesn't even get the math solid with
irreducible infinites popping up everywhere.

As they should.

I just don't think shape of the spacetime is something that should be possible
to quantize. It's just fundamentally different for me and expectation that it
should be ruled by similar equations as other forces seems misguided.

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Retra
Classical physics and quantum mechanics are already unified by shared
mathematical frameworks. So not only is it possible, it's already been done,
which is why people expect it to work for gravity as well.

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scotty79
Quantum mechanics is not so much unified with classical physics as it is more
precise description of part of physics that degrades to clasical physics when
you adjust parameters to get less detail.

Same way GR is more precise description of ANOTHER part of classical physics
that reduces to classical physics if you adjust some parameters.

There's really no reason that GR should share with quantum model much more
math than the simplest mathematical operations.

Why unify GR with quantum and not for example fluid dynamics?

GR and quantum field theory for me just describe completely different parts of
physics.

Sure it would be neat to know how world behaves when neither GR nor quantum is
sufficient to predict what will happen but I just think it's way more likely
that unified theory will be quantum model with relativistic terms not the
quantum description of gravity.

