
An old theory of quantized gravity is gaining new life - bcaulfield
https://www.quantamagazine.org/why-an-old-theory-of-everything-is-gaining-new-life-20180108/
======
jpeg_hero
> The theory called asymptotically (as-em-TOT-ick-lee) safe gravity was
> proposed in 1978 by Steven Weinberg.

Would a person interested in an article about quantum gravity not be familiar
with the term asymptotically?

~~~
laretluval
Moreover knowing the pronunciation doesn't give you any help if you don't know
the meaning.

~~~
intrasight
That bothered me as well (pronouncing rather than defining)

~~~
sago
Particularly as the pronunciation is strange. Strange place to break
syllables, and I'm not sure the second vowel is a 'e' in most pronunciations.

/a-sim-TOT-i-klee/

~~~
gnulinux
This is how I pronounce it in IPA: /a.sim'to:.ti.kli:/ Why is TOT one
syllable?

~~~
jsmthrowaway
The weird thing about IPA is that the non-IPA pronunciation you’re replying to
seems easier to understand.

~~~
InternetOfStuff
Only to you.

I feel exactly the opposite way.

<Rant>

I wish people would use the IPA instead of their absurd ad-hoc systems. I
learned IPA at school, and it has served me well.

All the pronunciation systems based on some other language are unreliable and
idiosyncratic. And English, with its near-absence of reliable
pronunciation/spelling rules, is particularly ill-suited.

</Rant>

~~~
freehunter
>I learned IPA at school

That's why people use their ad-hoc systems. Because many many many people
don't know IPA. It's so far removed from English and uses so many letters that
don't occur in English that it's close to being a completely different
language.

The word "ɪn·tərˈnæʃ·ə·nəl" may make complete sense to someone who knows IPA
but to to the many people who don't, it's complete gibberish.

~~~
InternetOfStuff
> That's why people use their ad-hoc systems. Because many many many people
> don't know IPA.

I was disagreeing with parent's claim that ad-hockery was better than IPA.
Nothing more, nothing less.

However if ad-hockery is the best someone can do (I say this without snark),
then fine. Certainly better than nothing.

> It's so far removed from English and uses so many letters that don't occur
> in English that it's close to being a completely different language.

English is far removed from my native language, so either way I have to learn
a pronunciation system (and the English one is... special).

While we're at it, do we use pseudo-American or pseudo-British pronunciation
in our ad-hockery? Pseudo-US-East-coast or pseudo-Southern-US?

I think ad-hockery provides a trap in that it gives a false sense of security.
You _think_ you know to pronounce something, but if you actually try, chances
are your native speaker conversation partner will go ¿que?

> The word "ɪn·tərˈnæʃ·ə·nəl" may make complete sense to someone who knows IPA
> but to to the many people who don't, it's complete gibberish.

I agree that it looks like gibberish to those who don't know IPA. But I'm
honestly not sure "intaneshenel" (or "intönäschänäl", or whatever) is better.
For me at least, it's equally confusing.

As an aside: making up pronounciation if it's of little relevance to what
you're writing is one thing. What really grinds my gears is language books
(or, say, the language section of guide books) that make up their funny ways
of writing pronunciation. They might use that same amount of effort to teach
the basics of IPA, which is about the same amount of effort, and provides the
reader with something lastingly useful.

They even write multi-page explanations, complete with examples, on how to
interpret funny charecters in their script. I remember the absurd contortions
the Lonely Planet Germany guide went through to explain umlaut sounds to
people in terms of English words, and then coming up with ways that weren't
the IPA to type those sounds.

------
ars
> " _In other words, gravity would be asymptotically safe if the theory at
> high energies remains equally well behaved as the theory at low energies. In
> and of itself, this is not much of an insight. The insight comes from
> realizing that this good behavior does not necessarily contradict what we
> already know about the theory at low energies (from the early works of
> DeWitt and Feynman)._ "

But what are the actual expected changes at high energy that would differ from
what is seen at low energy?

I kept waiting for her to explain what differences would be observed if this
theory is correct, but she never did.

~~~
T-A
You would observe the theory not blowing up. :)

The key feature she never names is known as a "UV fixed point".

UV = UltraViolet, which in this context simply stands for "high energy".

Fixed point... for that you need to know that the effective values of coupling
"constants" aren't actually constant in quantum field theory; they depend on
the interaction energy (i.e. how hard you bang your particles together). So
they are called "running" coupling constants. Yes, it's contradictory.

That dependence on interaction energy is described by differential equations
(known as the renormalization group flow equations, which may sound impressive
but is actually a misnomer), which you can integrate to see how a coupling
measured in the lab at low energy will change when your descendants finally
get enough funding for a larger accelerator.

One interesting behavior is the coupling going to zero when you increase the
energy towards infinity. That happens in QCD: counterintuitively, the harder
you bang quarks together, the less they affect each other (via QCD, that is;
they also have other kinds of interactions which behave differently). So QCD
is called "asymptotically free": in the limit of infinite interaction energy,
as far as QCD is concerned, quarks are free particles, completely unaffected
by each other's presence.

In asymptotic safety, as opposed to freedom, the coupling does not go to zero
as you increase the energy; instead, the rate at which it _changes_ goes to
zero, and it does so at some finite value of interaction energy. So, as you
crank up the interaction energy toward that value, the coupling constant
"runs" slower and slower, and once you get there, it stops "running"
altogether. From there on, you can raise the interaction energy all you want,
and the coupling constant stays... constant.

And that is a UV fixed point.

~~~
ars
This helps, but doesn't say what the theory says _gravity_ will do at higher
energy levels.

To me it seems to imply gravity will get weaker at higher energies? Something
about that makes me feel like it violates conservation of energy, so maybe
that's not it.

~~~
goldenkey
Also begs the question of how that reaction could even take place if gravitons
are totally unreactive with anything other than their source - mass. How would
you accelerate a graviton. Gravitons always move at C just like light. Do we
have any idea what kind of production and annihalation products will give us
gravitons?

Just a guess - dark matter/energy might be the only way to create a graviton
without associated mass generating it. That is..the pair production of a
virtual dark photon and a graviton.

In that case what are our options?

~~~
virgilp
Are gravitons a thing - i.e.a generally accepted concept in the scientific
community? I thought their very existence was highly controversial...

~~~
TheOtherHobbes
I've always had the naive and hand-wavey understanding that gravity _is_
spacetime, while all the other quantum fields happen _in_ spacetime.

So there may be issues trying to use a particle theory to describe something
that may be more fundamental than the other fields.

But IANAP, so this could be completely wrong.

------
auntienomen
I'm not sure I see any real change in the status of asymptotic safety. It's a
hypothesis. Proving it directly is next to impossible. (Except in spacetime
dimension <= 3, when the theory is topological, making the assertion a
triviality.) Testing it experimentally is unlikely; the constraints it implies
are pretty weak. Nonetheless there are some people working on it, and while
their ideas have some merit, they haven't yet caused the physics world to pay
drastically more attention.

Also, it's unwarranted hype to call asymptotic safety 'an old theory of
everything'. It's explicitly not a theory of everything. It's only about
gravity. Couple it to the Standard Model, and you still have a theory which
breaks down at the hypercharge and Higgs Landau poles.

------
pixl97
I'm not a particle physicists by any means, but doesn't any theory of gravity
have to deal with 'real' infinities at high energies from the physics we
already know?

We have some idea that black holes are a type of singularity (something that
could possibly called an infinity). With E=MC2 we have an idea that if you
apply enough energy to a particle it can become massive enough that it becomes
a singularity itself.

~~~
Florin_Andrei
A singularity popping out of the math is almost always an indication that
something else is going on.

According to general relativity, at the center of a black hole there might be
some kind of "singularity" \- but in reality what that means is GR is probably
inadequate, when used alone, to describe the center of a BH.

"Singularity" really means "your math broke and you need to fix it" by
bringing in another theory that can fill in the gaps - or, rather, extending
the current theory in a way that eliminates the division by zero.

------
esahione
I think the author completely glosses over the fact that Loop Quantum Gravity
actually does make predictions, and could provide the explanation for fast
radio bursts.

Also, the theory is so intuitive and follows such a straightforward logical
path from Quantum Mechanics and Relativity, that I would be incredibly
surprised if it wasn't the theory closest to actual reality.

~~~
thicknavyrain
By any chance, we’re you ALSO in Carlo Rovelli’s lecture a few days ago? ;)

~~~
esahione
I've literally watched every lecture he has out there lol :)

------
perpetualcrayon
Has anyone (sane) ever theorized that the ripples in spacetime in some cases
may have existed before the masses of particles (ie. planets / stars) arrived
at those locations?

------
blackrock
Gravity is measurement of the distortion of space, caused by mass. It is an
emergent property. The more massive something is, the more its gravity will
distort the space around it.

So, a small piece of rock that you can hold in your hand, will cause a
gravitational ripple in the space surrounding it. This is similar to how the
Earth causes a gravitational ripple in the space around it, and it keeps us
grounded to the ground. But, the Earth causes a much bigger gravitational
ripple, than a small rock.

And we as humans, are grounded to the Earth, as if we are at the bottom of
this massive and invisible whirlpool. We cannot see it, but this whirlpool is
rotating all around us, and we cannot get out. It is easy for us to move
around horizontally, but it is very difficult for us to move vertically.
Unless we can move horizontally so fast, that we can reach escape velocity,
and then, we orbit the Earth.

That said. What is gravity? Why does mass cause a ripple in the space around
it?

I wonder if gravity is just some type of neutral electro-magnetic force. Kind
of like the magnetic field lines that you see with the North pole and South
pole. But instead of being magnetically charged, it is neutrally charged, and
it affects everything around it equally.

Or, instead of being magnetically charged like the North or South pole, it is
gravitationally-charged, and the direction points in only one way, into the
center of your massive object. And unless you are moving faster than that
massive object's escape velocity, then you will always be stuck in that
object's gravitational ripple.

If we can truly understand what is gravity, then I wonder if we can create a
counter gravity. Like, in your spaceship, if you can create a negative mass
above you, then it would cause your spaceship to act buoyant in the planet's
atmosphere. And you can use thrusters to move around vertically, as easy as
you can move around horizontally.

Or, maybe you can create a temporary blackhole above your spaceship, and
instead of your spaceship falling into the gravitational ripple caused by the
Earth, then it becomes attracted to the gravitational ripple caused by your
blackhole. And this causes your spaceship to rise, and defy the Earth's
gravity.

Anyways. I'm not a gravitational physicist. Maybe I should take this idea, and
write a novel. There has been much worse Sci-Fi ideas created, like Time
Travel, or the ridiculous many-worlds-interpretation theory of quantum
mechanics.

~~~
RuggeroAltair
From a physics perspective, the many-world, as ridiculous as it sounds, is a
more sound theory (believed to be correct by some top physicists) than
negative gravity.

The explanation of why gravity is only positive is a complex one, but
physicists believe it's almost axiomatic.

This doesn't mean that you can't have repulsive type of effects, similar to
one of the contributions that's believed to make the universe expand more
quickly than with the simple 4 fources (often called fifth force, maybe
related to dark energy, and to the rate of the universe expansion
acceleration).

PS. There are a number of Sci-Fi stories that use this idea, including a
famous one from Asimov, in which if I recall correctly the normal
gravitational force was working negatively when traveling in FTL speed.
Obviously that seemed fishy but the book was cool (Nemesis).

~~~
clauraform
> The explanation of why gravity is only positive is a complex one, but
> physicists believe it's almost axiomatic

Isn't that a consequence of relativity, the speed of light being constant by
definition? I'm not sure that makes any sense, though, what is the physical
unit of gravity N? Jf you mean classical mechanics, than there has to be a
negative counterforce by the three axioms of newtonian mechanics and it's just
a question of frame of reference.

BTW: Stress on _ax_ -iomatic, as in axis?!

~~~
ars
> Isn't that a consequence of relativity, the speed of light being constant by
> definition?

No, because if it was then there would not be negative electric charge, and
there is.

------
dang
Url changed from [https://www.wired.com/story/why-an-old-theory-of-
everything-...](https://www.wired.com/story/why-an-old-theory-of-everything-
is-gaining-new-life/), which points to this.

------
goldenkey
I think it is relevant to consider gravity from a more abstract standpoint as
well by looking at Le Sage's theory and particularly Hyugens' comments. [1]

Whether gravity is mediated by a graviton, and the intricacies that QM brings
to bear, one interesting idea I took from the above hundreds of years of
thought on the subject -- gravity may be a pushing force mediated by constant
spherical wave emission (quantized as the graviton if necessary) from all
mass/energy. Since every bit of mass-energy is releasing gravitons, the whole
of space is rather isometrically filled with gravitons. The reason two objects
would then appear to attract via Newton's inverse-square law is one of a
shadowing effect. The graviton would apply a pushing force normally to any
mass-energy it interacts with, and therefore any blockage/shadowing of one
object by another, would end up looking like attraction.

Le Sage's theory is more exacting and supposes gravitons are of similar
substance to regular matter. I don't agree with most of these notions but what
I said above is what I gained.

1]
[https://en.wikipedia.org/wiki/Le_Sage%27s_theory_of_gravitat...](https://en.wikipedia.org/wiki/Le_Sage%27s_theory_of_gravitation)

~~~
Taniwha
Dumb question: how do you exchange gravitons with a black hole?

~~~
ars
A graviton would have to be both massless and energyless. Thus it would be
unaffected by gravity.

It also means it can not have a "frequency" like light does (because lacking
any energy there is no frequency to change).

It must also always move at the speed of light, and could not exist at any
slower speed. (Because to slow down, would imply a change in its energy, which
it can not do.)

But that means it can't interact in a way that is detectable, other than
gravity.

Because you can not give it energy, you can also never create it artificially.
If you did, you would be creating gravitational force out of nothing, which is
impossible because it violates conservation of energy, and you can not pay for
that violation because you can not give it energy.

It's pretty much the ultimate impossible to study particle.

~~~
auntienomen
This comment is almost completely wrong. It muddles together classical and
quantum reasoning in any incoherent way.

Gravitons are best understood as the quanta of the effective qft governed by
the Einstein Hilbert action. This mathematical model is the universal low
energy approximation to any quantum theory that reduces to general relativity.

Gravitons need to be massless. They need not be zero energy. They can indeed
be associated with frequency. They do couple to gravitational backgrounds and
to other gravitons. You can crate them just like any other field quanta. The
barrier to studying gravitons the way we do photons is that they couple very
weakly to other systems.

~~~
ars
> They need not be zero energy.

How would that work exactly? First of all it means they could not escape a
black hole, since they would be carrying energy out of it. It also means they
would not travel in straight lines, since by carrying energy, they are subject
to gravitational force, and boy of boy what a mess that would make of things
if the gravity carrier was affected by its own gravity (or even the gravity of
the neighbors).

Second when the graviton "landed" it would have to transfer energy. If they
only act via gravity, how would you transfer energy to or from them?

And then if they carry energy, then what, energy is constantly streaming out
of every particle in the universe? Or are you proposing two kinds of
gravitons, those that carry energy and those that don't?

> They can indeed be associated with frequency.

Really? So how would their measured effects vary as their frequency varied?

Are you confusing gravitational waves with gravitons? The two are not the
same.

> You can crate them just like any other field quanta.

No you could not because if you did you would be creating gravitational force
out of nothing, which is impossible (violates a whole bunch on conservation
laws).

~~~
auntienomen
You need to learn some quantum field theory. You're trying to reason with a
popsci level understanding and failing miserably.

